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UBC Theses and Dissertations

Seismoelectric responses from sulphide orebodies Kepic, Anton 1995

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S E I S M O E L E C T R I C R E S P O N S E S F R O M S U L P H I D E O R E B O D I E S by. A N T O N K E P I C B . S c , The Un ive r s i t y o f Western Aus t ra l i a , 1988 A T H E S I S S U B M I T T E D I N P A R T I A L F U L F I L L M E N T O F T H E R E Q U I R E M E N T S F O R T H E D E G R E E O F D O C T O R O F P H I L O S O P H Y i n T H E F A C U L T Y O F G R A D U A T E S T U D I E S (Department o f Geophysics and As t ronomy) W e accept this thesis as conforming to the required standard^ T H E U N I V E R S I T Y O F B R I T I S H C O L U M B I A September 1995 © A n t o n K e p i c , 1995 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. The University of British Columbia Vancouver, Canada Department DE-6 (2/88) Abstract In the ear ly 1980's a g r o u p of R u s s i a n researchers r epor t ed f i n d i n g a n e w se ismoelec t r ic p h e n o m e n o n assoc ia ted w i t h s u l p h i d e m i n e r a l s . T h e y asser ted that s t r o n g burs t s of b r o a d b a n d e lec t romagnet ic emiss ions appea red w h e n a se i smic w a v e passed t h r o u g h a s u l p h i d e o rebody . T h i s d i s c o v e r y has a roused cons iderab le c o m m e r c i a l interest because of the e c o n o m i c i m p o r t a n c e of m i n i n g s u l p h i d e m i n e r a l s for z i n c , c o p p e r , l e a d , g o l d , a n d o ther meta ls . H o w e v e r , there have been n o r e p o r t e d f o l l o w - u p s tud ies o f t h e i r f i n d i n g s , a n d no e v a l u a t i o n of the e x p l o r a t i o n po ten t i a l of this p h e n o m e n o n . T h i s thesis repor ts a n u m b e r of f i e l d e x p e r i m e n t s d e s i g n e d to e x a m i n e the R u s s i a n c l a i m s a n d e x p l o r e the p r o p e r t i e s o f the p h e n o m e n o n . These f i e l d t r i a l s h a v e c o n f i r m e d the existence of th is r e l a t i v e l y u n k n o w n se i smoe lec t r i c p h e n o m e n a o n a n d h a v e subs t an t i a l l y i nc reased o u r k n o w l e d g e of its charcter is t ics . In the tests 0.2-0.5 k g e x p l o s i v e charges w e r e de tona ted to p r o v i d e a s t rong source of s e i smic energy. W h e n the s e i s m i c d i s t u r b a n c e p a s s e d t h r o u g h a z o n e o f s u l p h i d e m i n e r a l i z a t i o n , h i g h f requency e lec t romagnet ic emiss ions were p r o d u c e d . T h e e lec t romagnet ic emiss ions appear i n the f o r m of b r i e f pu lses , 2 to 5 mic ro seconds i n d u r a t i o n . T y p i c a l peak a m p l i t u d e s , m e a s u r e d 80 to 120 m f r o m the z o n e s of s u l p h i d e m i n e r a l i z a t i o n , are 10 m V / m a n d 2 nT . T h e s p e c t r u m of the e lec t romagne t i c pu l ses spans a v e r y w i d e range of frequencies, f r o m 1 k H z to 3 M H z . F o u r i e r ana lys i s of the d i g i t a l r ecords f r o m one of o u r sites s h o w s a peak i n the e m i s s i o n s p e c t r u m , at 1.1-1.4 M H z . T h i s spec t ra l peak w a s cons i s t en t ly r e p r o d u c e d f r o m v a r i o u s p o r t i o n s of the s u l p h i d e o rebody . N e w m e a s u r e m e n t a n d i n t e r p r e t i o n t e c h n i q u e s w e r e d e v e l o p e d to s t u d y the e lec t romagnet ic s igna ls . These techniques were a p p l i e d to da ta f r o m each f i e l d t r i a l to d e m o n s t r a t e the p o t e n t i a l of se i smoe lec t r i c t echn iques for e x p l o r a t i o n . T h e resu l t s s h o w that the h i g h f requency seismoelectr ic p h e n o m e n o n can be u s e d to locate mass ive s u l p h i d e s i n u n d e r g r o u n d mines . A p h y s i c a l f r a m e w o r k to desc r ibe the f i e l d measu remen t s has been d e v e l o p e d . It i s p r o p o s e d that the s e i s m i c w a v e i n d u c e s e l ec t r i f i ca t i on v i a c r a c k f o r m a t i o n i n the o rebody . T h e e lec t r i f ied surfaces of the c rack r e c o m b i n e r a p i d l y i n a gas d i scha rge to p r o d u c e the obse rved range of e lectromagnet ic frequencies. i i Table of Contents Abstract • • i i Table of Contents i i i List of Tables v List of Figures • • v i Acknowledgements • i x Chapter 1: Introduction 1 1.1 Preface • 1 1.2 B a c k g r o u n d 6 Chapter 2: Field Instrumentation and Methods 12 2.1 O v e r v i e w of E q u i p m e n t a n d P r o c e d u r e 12 2.2 T h e Se i smic Source 15 2.3 O b t a i n i n g a Shot M o m e n t 17 2.4 M a g n e t i c F i e l d Sensors 18 2.5 Elect r ic F i e l d Sensors..... 21 2.6 S i g n a l T r a n s m i s s i o n a n d Cables , 27 2.7 A m p l i f i c a t i o n a n d F i l t e r i n g 28 2.8 A n a l o g to D i g i t a l C o n v e r s i o n 30 Chapter 3: Interpretation and Processing Methods 34 3.1 In t roduc t ion . . . . 34 3.2 Spec t rograms 35 3.3 P i c k i n g Even t s 39 3.4 Presen ta t ion of R a w S u r v e y D a t a 41 3.5 Sta t is t ica l A n a l y s i s 44 3.6 B o u n d a r y De l inea t i on 49 3.7 T o m o g r a p h i c Recons t ruc t ion 51 Chapter 4: Field Trials 57 4.1 F i e l d P r o g r a m Object ives 57 4.2 S u l l i v a n M i n e 58 i i i 4.3 M o b r u n M i n e 64 4.4 L y n x M i n e 1 75 4.5 L y n x M i n e II 83 4.6 L y n x M i n e HI 89 4.7 C e n t u r y 98 4.8 O t h e r F i e l d Tests 103 Chapter 5: A Physical Model for RPE 105 5.1 I n t r o d u c t i o n 105 5.2 P h y s i c a l At t r ibu tes of R P E a n d S u l p h i d e s 105 5.3 Po ten t i a l M e c h a n i s m s 108 5.4 Tr iboe lec t r ic i ty a n d C r a c k F o r m a t i o n I l l 5.5 E M F ie ld s f rom a n E x p a n d i n g C r a c k 118 Chapter 6: Thesis Conclusions 128 References 132 Appendix A: Seismic Source Characteristics 138 A . l E x p l o s i v e to R o c k C o u p l i n g 138 A . 2 Source Ene rgy Ca lcu la t ions 140 A . 3 Source Parameter Measuremen t s 143 Appendix B: Instrumentation Details 147 B . l T h e F ibe r O p t i c T i m e - B r e a k 147 B.2 E lec t romagne t i c N o i s e f r o m V a r i o u s T r i g g e r i n g M e t h o d s 148 B.3 O p t i m i z i n g S o l e n o i d a l M a g n e t i c Sensors 151 B.4 U B C M a g n e t i c Sensor D e s i g n 156 B.5 L o n g W i r e A n t e n n a : T h e o r y of O p e r a t i o n 158 B.6 N o i s e Character is t ics of the Pa ra l l e l Pla te D i p o l e 160 B.7 The Effects of R e d u c i n g S i g n a l B a n d w i d t h 161 B.8 A M D e m o d u l a t i o n i n O p e r a t i o n a l A m p l i f i e r s 163 B.8 D e m o d u l a t i o r s a n d Spect ra l D e c o m p o s i t i o n 165 Appendix C: Electrical Circuit Schematics 167 i v List of Tables 2.1 Charac ter i s t ics of the magne t ic sensors u s e d i n the f i e ld t r ials 33 2.2 P re -ampl i f i e r characterist ics 33 v List of Figures 1.1 Elec t r ic f i e ld a n d se ismic response to a nearby e x p l o s i o n (0.5 kg) 2 1.2 Schemat ic of a n u n d e r g r o u n d se ismoelect r ic m e t h o d o f e x p l o r a t i o n 5 2.1 Schemat ic of the in s t rumen ta t ion u sed i n se ismoelect r ic research at U B C 14 2.2 E x a m p l e of the se ismic pu lse p r o d u c e d b y a s m a l l exp los ive i n u n d e r g r o u n d rock , a n d the a m p l i t u d e s p e c t r u m of the se i smic d i s tu rbance 16 2.3 E q u i v a l e n t c i rcu i t of a d i p o l e antenna 22 2.4 I m p u l s e response of the U B C I V magnet ic a n d pa ra l l e l plate d i p o l e an tennas 25 3.1 E x a m p l e of a spec t rogram f rom a h i g h frequency record 36 3.2 T h e segmenta t ion scheme u s e d i n d i v i d i n g t ime-series da ta in to o v e r l a p p i n g estimates of the a m p l i t u d e s p e c t r u m vs. t i m e 37 3.3 Spec t rog ram a n d an a m p l i t u d e vs . t ime trace c o m p a r i s o n 38 3.4 E x a m p l e s of scatter a n d wiggle- t race plots. 43 3.5 H i s t o g r a m , example 45 3.6 A n i l l u s t r a t i o n of the b o u n d a r y de l i nea t i on m e t h o d 50 3.7 T o m o g r a p h i c recons t ruc t ion of po in t sources 53 3.8 Synthe t ic s u r v e y e x a m p l e 54 3.9 T o m o g r a p h i c recons t ruc t ion of the synthet ic o rebody 56 4.1 M a p of the S u l l i v a n M i n e expe r imen t 59 4.2 A r e c o r d f r o m SP2 , S u l l i v a n M i n e 62 4.3 C o m p a r i s o n be tween d r i l l core results a n d recons t ruc ted wavef ronts , S u l l i v a n M i n e 63 4.4 M a p of the M o b r u n M i n e exper iment 65 4.5 A record of a shot at SP7, M o b r u n M i n e 67 4.6 E M ac t iv i ty vs. offset, M o b r u n M i n e 70 4.7 C o m p a r i s o n be tween s i g n a l a r r i v a l t imes a n d es t imated t r ave l t imes to the nearest por t ions of the A a n d B lens features 73 4.8 A p p l i c a t i o n of the b o u n d a r y de l inea t ion m e t h o d to the M o b r u n data-set 74 4.9 M a p of the exper iment o n l e v e l 14, L y n x M i n e 76 v i 4.10 E x a m p l e s of the three types of s i gna l observed at l e v e l 14 79 4.11 H i s t o g r a m of sp ike events, l eve l 14, L y n x M i n e 80 4.12 N o r t h w a r d project ion of a r r i v a l data , l eve l 14 L y n x M i n e 82 4.13 M a p of exper iments o n l eve l 10, L y n x M i n e 84 4.14 A n example of a r ecord a c q u i r e d f rom l e v e l 10, L y n x M i n e 86 4.15 H i s t o g r a m of the s i g n a l a r r i v a l t imes, l e v e l 10, L y n x M i n e 87 4.16 A n a p p l i c a t i o n of the t o m o g r a p h i c recons t ruc t ion m e t h o d , l e v e l 10, L y n x M i n e 88 4.17 T w o examples of the type of pu lse recorded by the h i g h b a n d w i d t h sys tem, l e v e l 10, L y n x M i n e 92 4.18 Spec t rog ram of the electric f ie ld dis turbances created by a 0.5 k g shot, l eve l 10, L y n x M i n e 93 4.19 D e m o n s t r a t i o n of a n o rebody t i r i ng out, l eve l 10, L y n x M i n e 94 4.20 F o u r records of the electric f i e ld after a 0.5 k g shot at SP1 95 4.21 Spect rograms of repeat shots at SP1 96 4.22 H i s t o g r a m of event a r r i v a l t imes, C e n t u r y 102 5.1 Tr iboe lec t r ic charge genera t ion I l l 5.2 Tens i le s t ra in relief by a crack 116 5.3 Co-o rd ina t e sys tem used i n desc r ib ing the electric d i p o l e f ie lds 119 5.4 M o v e m e n t of a free electron i n a cha rged gap 123 5.5 A n i l l u s t r a t ion of the process of b remss t r ah lung 125 A . l V e r t i c a l g r o u n d m o t i o n due to a r ad i a l P - w a v e 144 B . l E lec t r ic f i e l d f rom detonator in i t i a t ion a n d blast 149 B.2 E lec t r i ca l noise verses t ime-break m e t h o d 150 B.3 P rac t i ca l magne t ic sensor topologies u s i n g a c o i l 152 B.4 E q u i v a l e n t c i rcu i t of the l o n g w i r e a n d charge a m p l i f i e r 159 B.5 P a r a l l e l d i p o l e antenna c i rcu i t a n d in t r ins ic noise sources 160 B. 6 R F I d e m o d u l a t i o n effects i n four c o m m o n O p - A m p s 164 C l F ibe r opt ic c i rcu i t schematics 167 C 2 U B C V magnet ic sensor c i rcu i t schematic 168 C. 3 H B W pre-ampl i f ie r c i rcu i t schematic 169 v i i C.4 D e m o d u l a t o r c i r cu i t schemat ic 170 C.5 N o t c h f i l ter c i r cu i t schemat ic 171 C 6 T - B o x p re -ampl i f i e r c i r cu i t schemat ic 172 C.7 Spect ra l D e c o m p o s i t i o n Schematic 173 v i i i A c k n o w l e d g m e n t s F i rs t a n d foremost , I w o u l d l i k e to t h a n k R . D . R u s s e l l , M . M a x w e l l , a n d K . E . B u t l e r for the i r he lp . T h e i r con t r ibu t ions i n the f i e ld , a n d i n c r i t i c a l l y e x a m i n i n g the resul ts have been essential . In a d d i t i o n , i t has been a p leasure to w o r k w i t h this g r o u p . A s p e c i a l t hanks to the opera to rs a n d w o r k e r s of the v a r i o u s m i n e s w h e r e I h a v e ga thered data . F r o m the S u l l i v a n M i n e : M a r c i a K n a p p , a n d G r a n t Scot "Scottie". F r o m the M o b r u n M i n e ,P-J Laf leur . F r o m o u r A u s t r a l i a n exper iments : A n d r e w M u t t o n , B o b S m i t h , a n d G r a h a m Creer . A spec ia l thanks to the f r i end ly c r e w of the L y n x M i n e : M i k e Becher , C l i f f Pea r son , R i c h a r d W a l k e r , a n d m a n y others. M y research w a s s u p p o r t e d b y N S E R C a n d f ive i n d u s t r i a l par tners i n the f o r m of a C o -opera t ive Research a n d D e v e l o p m e n t G r a n t to R . D . R (grant n u m b e r CRD-0094237) . The p a r t n e r s w e r e B H P - U t a h M i n e s L t d , C o m i n c o L t d . , C R A E x p l o r a t i o n P t y . L t d . , L a m o n t a g n e G e o p h y s i c s L t d , a n d Placer D o m e Inc. A d d i t i o n a l suppo r t w a s p r o v i d e d b y Science C o u n c i l of B r i t i s h C o l u m b i a (grant n u m b e r s 118 ( T - l ) a n d 65 (T-3) ) a n d N S E R C ope ra t ing grant to R. D . R u s s e l l (grant n u m b e r O G P 0000720). T h e first au tho r ( A . W . K . ) w i shes to a c k n o w l e d g e the pe r sona l suppo r t b y the Science C o u n c i l of B . C . a n d a U . B . C . G r a d u a t e Research F e l l o w s h i p L a s t l y , I w o u l d l i k e to t hank m y g i r l f r i e n d a n d e v e n t u a l w i f e , D e l p h i n , for her m o r a l suppor t d u r i n g this p e r i o d of m y life. i x C H A P T E R 1 INTRODUCTION 1.1 Preface T h i s thesis is a n accoun t of m y i n v e s t i g a t i o n of a n u n u s u a l a n d p o t e n t i a l l y u s e f u l p h e n o m e n o n that can be i n d u c e d i n n a t u r a l s u l p h i d e s . T h e aspect of the p h e n o m e n o n that has s t i r r e d the mos t interest is the ab i l i t y to conver t s e i smic ene rgy in to e lec t r i ca l energy. There are m a n y other p h y s i c a l processes that d o this , s u c h as p iezoe lec t r ic i ty a n d e lec t rok ine t i c effects. H o w e v e r , these processes p r o d u c e a n e lec t r ica l s i g n a l (current , charge o r vol tage) that is l i n e a r l y re la ted to the stress a p p l i e d to the m a t e r i a l , whereas the p h e n o m e n o n u n d e r s t u d y does not . A n e x a m p l e of th is n o n - l i n e a r r e sponse is s h o w n i n F i g u r e 1.1. It is i m m e d i a t e l y o b v i o u s that the e lec t r ica l response i n F i g u r e 1.1 differs f r o m the se i smic response F u r t h e r m o r e , the s p e c t r u m of the e lec t r ica l response spans a range of 1 to 2000 k H z , w h i c h is three orders of m a g n i t u d e greater than the s e i s m i c d i s t u r b a n c e that p r o d u c e d it . T h e n o n - l i n e a r na tu re of th is c o n v e r s i o n is u n c o m m o n , bu t not u n k n o w n , as s i m i l a r conve r s ions are o b s e r v e d i n l abo ra to ry r o c k fa i lure exper iments (N i t s an , 1977; Y a m a d a , et a l , 1989). T h e d i s c o v e r y a n d i n i t i a l s t u d y of h i g h f requency se ismoelec t r ic effects f r o m s u l p h i d e s w a s b y a g r o u p of Sovie t geophysic is t s l e d b y G . A . Sobo lev (1980). T h i s g r o u p cons i s ted of G . A . Sobo lev , V . M . D e m i n , Y . Y . M a y b u k , a n d V . F . L o s ; mos t references i n this thesis to Sobolev et a l . refer to the efforts of this g roup . A p a r t f r o m the extens ive efforts of the a b o v e - m e n t i o n e d Sovie t scientists the o n l y sys temat ic research of w h i c h I a m a w a r e o n this p h e n o m e n o n has been c o n d u c t e d b y the G e o p h y s i c a l I n s t r u m e n t a t i o n G r o u p , headed b y R . D . R u s s e l l , at the U n i v e r s i t y of B r i t i s h C o l u m b i a . T h e p h e n o m e n o n w a s d i s c o v e r e d i n the late 1970's b y S o b o l e v a n d h i s g r o u p w h i l s t i n v e s t i g a t i n g the p iezoe lec t r ic effects of rocks i n s i tu . P iezoe lec t r i c s igna ls l o o k s i m i l a r 1 Chapter 1: Introduction 2 Figure 1.1 T h e response of a n electric f ie ld antenna (above) a n d a geophone to a n e a r b y e x p l o s i o n (0.5 k g of pen to l i t e ) . T h i s r e c o r d is f r o m a n u n d e r g r o u n d exper iment i n the L y n x M i n e , V a n c o u v e r Is., B C . Chapter 1: Introduction 3 to g e o p h o n e s igna l s , that i s , they c o n t a i n a p p r o x i m a t e l y the s ame f requency content (near ly a l l of the energy is b e l o w 2 k H z ) . F i e l d t r ials c o n d u c t e d near s u l p h i d e mine ra l s r evea l ed another type o f s i g n a l . T h e n e w type of s i gna l was a r e l a t ive ly h i g h a m p l i t u d e p u l s e ( c o m p a r e d to p i ezoe l ec t r i c s igna l s ) of shor t d u r a t i o n (e.g. F i g u r e 1.1). P u l s e b a n d w i d t h s were t y p i c a l l y i n the M H z range (Sobolev et a l . , 1980). Sobo lev et a l . i n i t i a l l y l abe l ed th is p h e n o m e n o n P R R E R , p u l s e d r a d i o range e lec t romagnet ic r a d i a t i o n (1982), b u t h a v e s u b s e q u e n t l y r e n a m e d i t as R P E , the r a d i o p u l s e d effect ( p r i v a t e c o m m u n i c a t i o n w i t h M . M a x w e l l d u r i n g a v i s i t to M o s c o w , 1992). In deference to the d i scove re r s , the h i g h f requency se i smoelec t r ic p h e n o m e n o n assoc ia ted w i t h s u l p h i d e s w i l l be ca l l ed R P E th roughou t this thesis. A n at t ract ive p rope r ty of R P E is that it appears to be d i s t i nc t l y associa ted w i t h s u l p h i d e mine ra l s . Base meta ls , s u c h as t i n , l ead , z i n c , n i c k e l a n d often coppe r , c o m e f r o m the m i n i n g of s u l p h i d e orebodies . A l s o , s ign i f ican t a m o u n t s of g o l d a n d s i l v e r are m i n e d f r o m s u l p h i d e ores. S u l p h i d e s are meta l l i c salts of s u l p h u r , for example : c inn iba r , H g S ; p y r i t e , FeS; spha l e r i t e , F e / Z n S ; c a l c o p y r i t e , C u S ; ga lena , P b S . C o n s e q u e n t l y , a n y p h e n o m e n o n as soc ia t ed w i t h s u l p h i d e m i n e r a l s is o f great in teres t to the m i n i n g i n d u s t r y because of the economic s igni f icance of su lph ides . A n u m b e r of g e o p h y s i c a l techniques have been d e v e l o p e d to detect s u l p h i d e orebodies . A m o n g the mos t p o p u l a r are ae romagne t i c , i n d u c e d p o l a r i z a t i o n , a n d v a r i o u s E M methods . H o w e v e r , m a n y orebodies d o not r e s p o n d w e l l to these es tabl i shed me thods , or the response is m a s k e d b y the p h y s i c a l proper t ies of the host rock. N e w methods that can r e l i ab ly detect these orebodies are eager ly sough t by the m i n i n g i n d u s t r y . A n o t h e r p r o b l e m w i t h m a n y es tabl ished me thods is that they cannot be u s e d i n ope ra t ing mines because the m i n e infras t ructure p roduces too m u c h interference. T h i s restricts the m i n e opera tor ' s a b i l i t y to r educe costs: c o r e - d r i l l i n g costs far m o r e than m o s t g e o p h y s i c a l s u r v e y s . Chapter 1: Introduction 4 Shot Electromagnetic Sensors Target Figure 1.2 Schemat i c of an u n d e r g r o u n d se i smoelec t r ic m e t h o d of e x p l o r a t i o n . A n e x p l o s i o n generates a se ismic w a v e w h i c h propagates o u t w a r d . A s the se ismic w a v e passes t h rough regions of suph ides E M emiss ions are detected by an array of antennas. T h e p rob l ems w i t h cur ren t me thods of exp lo ra t i on have p r o v i d e d an impe tus to s t udy the pos s ib l e use of R P E for e x p l o r a t i o n . Se i smoe lec t r i c effects have been s t u d i e d spo rad i ca l l y i n the W e s t s ince 1936 ( T h o m p s o n , 1936; M a r t n e r a n d Sparks , 1959, Bu t l e r et a l . , 1994), bu t not u s e d for e x p l o r a t i o n . In contras t , s e i smoe lec t r i c e x p l o r a t i o n me thods were a c t i v e l y p u r s u e d by Sovie t scientis ts f rom the 1950's ( V o l a r o v i c h et a l . , 1959, V o l a r o v i c h a n d Sobo lev , 1969, N e y s h t a d t et a l . , 1972) u p u n t i l the b reak-up of the Sov ie t U n i o n i n the 1990's. M o s t of these efforts were i n the area of p i ezoe l ec t r i c phenomena . A s of 1992 ( in fo rmat ion f rom M . M a x w e l l a n d R. D . R u s s e l l after a v is i t to M o s c o w a n d St. Pe te rsburg) , the same scient is ts were s t i l l r e sea rch ing a n d a p p l y i n g se ismoelec t r ic methods . F i g u r e 1.2 i l lustrates s chemat i ca l ly the se i smoelec t r ic m e t h o d Chapter 1: Introduction 5 of e x p l o r a t i o n . A n e x p l o s i v e charge (or a n a l t e rna t ive s e i s m i c source) p r o d u c e s a s e i smic p u l s e , w h i c h p ropaga te s o u t w a r d . A n t e n n a s r ece ive e l ec t r i ca l d i s t u r b a n c e s c rea ted b y the pu l s e , a n d t r ansmi t the s i g n a l to a r e c o r d i n g d e v i c e . T h e d e l a y the be tween blast a n d the recep t ion of a n electr ical d i s tu rbance p r o v i d e s i n f o r m a t i o n about the loca t ion of the target. N e a r l y a l l of this de lay is due to the t ime taken for the se i smic energy to reach the r e g i o n of c o n v e r s i o n . Therefore , i f the s e i smic v e l o c i t y is k n o w n then the d is tance be tween the se ismic source a n d the target can be ca lcu la ted . In 1990, as par t of the G e o p h y s i c a l Ins t rumen ta t ion G r o u p at the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I was g i v e n the task of inves t iga t ing the proper t ies of R P E a n d its poss ib le use as a n e x p l o r a t i o n too l . T h e p r i n c i p a l hypo thes i s to be tested i n m y research is that R P E can be u s e d to explore for mass ive su lph ides . T w o major tasks needed to be under t aken : to s h o w that a p h e n o m e n o n f i t t i ng the d e s c r i p t i o n of R P E exists , a n d to d e v i s e a n d evaluate a n exp lo ra t ion m e t h o d based u p o n R P E . N o t e that i n 1990 there w a s s t i l l d o u b t abou t the exis tence of R P E because of the l a c k o f i n d e p e n d e n t c o n f i r m a t i o n . T h e Ins t rumen ta t ion G r o u p at U . B . C . d e c i d e d that f i e ld tests near mass ive s u l p h i d e s w o u l d g ive the mos t dec i s ive results. These f i e ld tr ials are the corners tone of this thesis. Is the h y p o t h e s i s t rue o r false? It is t rue. P o s i t i v e resul ts f r o m f i e l d tests i n fou r u n d e r g r o u n d l o c a t i o n s s h o w that R P E does exis t , a n d that it c a n be u s e d as a n e x p l o r a t i o n too l . De ta i l s of the f i e ld t r ia ls a n d e x a m i n a t i o n of the resul ts are f o u n d i n C h a p t e r 4. E a c h f i e ld t r i a l a i m e d to s h o w that p u l s e d E M s ignals w e r e s t i m u l a t e d b y the passage of s e i s m i c w a v e s t h r o u g h the o r e b o d y , a n d cha rac t e r i ze these s igna l s . I n a d d i t i o n , v a r i o u s aspects of the e x p l o r a t i o n m e t h o d , s u c h as range , n e w in s t rumen t s , charge s ize a n d charge placement , were tested i n each t r ia l . A r r i v a l s f r o m the records ob t a ined f r o m these f i e l d t r ia ls n e e d e d to be p i c k e d (event se lec t ion) a n d in t e rp re t ed . D e s p i t e m a n y s i m i l a r i t i e s , a d a p t a t i o n o f s i m p l e s e i s m i c t echn iques p r o v e d to be inadequa te because of f u n d a m e n t a l differences b e t w e e n R P E Chapter 1: Introduction 6 a n d s e i s m i c m e t h o d s . In a d d i t i o n , the c o m p l e x s t ruc tu re o f m o s t m i n e s presents d i f f i cu l t i e s i n d i s p l a y i n g the da ta . M e t h o d s a n d ru les for p i c k i n g , d i s p l a y i n g a n d in te rpre t ing were d e v i s e d a n d are de ta i l ed i n C h a p t e r 3. In that chapter I descr ibe h o w to recogn ize an R P E s igna l , a n d descr ibe me thods u s e d to ana lyze the dataset or i nve r t for s t ruc tu ra l i n fo rma t ion . These techniques f o r m a basic tool-set for the p rocess ing aspects of an e x p l o r a t i o n m e t h o d . T h e p rac t i ca l s ide of an exp lo ra t i on m e t h o d requires the co l l ec t i on of data . O n e a i m of the U . B . C . research p r o g r a m was to p r o v i d e a comple t e d e s c r i p t i o n of the e q u i p m e n t a n d speci f ica t ions necessary for success i n se ismoelec t r ic w o r k . S p e c i a l care is n e e d e d because the e lec t romagnet ic s igna l s are s m a l l , m i l l i v o l t s a n d nano tes la , a n d are often m e a s u r e d i n the presence of s t r ong e lec t r i ca l in ter ference . F u r t h e r m o r e , the w i d e b a n d w i d t h of R P E s igna l s presents cha l lenges i n the d e s i g n of l o w no i se sensors to acqui re these s ignals . Elec t r ic a n d magnet ic f i e ld data were r eco rded i n o u r f i e l d tr ials , as w e l l as some geophone data. A l l of this data was r e c o r d e d i n d i g i t a l format , a first for b o t h U . B . C . a n d o u r R u s s i a n counterpar ts . C h a p t e r 2 de ta i l s the i n s t r u m e n t a t i o n a n d me thods u s e d for data acqu i s i t i on . In a d d i t i o n , i m p r o v e m e n t s i n v a r i o u s aspects of the ins t ruments a n d m e t h o d are suggested. A rea l i s t i c m o d e l of the processes that p r o d u c e R P E is n e e d e d to f u l l y e x p l o i t a n d u n d e r s t a n d R P E . Sobo lev et a l . (1982a) have p r o v i d e d a n e x p l a n a t i o n , bu t the i r m o d e l cannot e x p l a i n some aspects of the p h e n o m e n o n . R P E is d i f f i cu l t to e x p l a i n because the E M f ie lds are large, a n d the pu lses are ve ry shor t i n d u r a t i o n . I p r o v i d e a n a l te rna t ive p h y s i c a l m o d e l of R P E i n C h a p t e r 5 that is based u p o n tr iboelectr ic effects. T h i s m o d e l of R P E exp la ins m a n y effects seen i n the f i e ld a n d labora tory , a n d exp la ins w h y s u l p h i d e s i n p a r t i c u l a r s h o u l d exh ib i t this p h e n o m e n o n . 1.2 Background R P E differs f r o m other se ismoelec t r ic p h e n o m e n a i n t w o k e y areas: it is not repeatable, Chapter 1: Introduction 7 a n d a th resho ld of se ismic p o w e r is necessary to ini t ia te the process (Sobolev et a l . , 1983). T h e first c l a i m needs some qua l i f i ca t ion ; the s ignals are not exact ly repeatable i n a r r i v a l t i m e o r a m p l i t u d e , bu t o n repeat expe r imen t s R P E s igna l s w i l l g e n e r a l l y a p p e a r at a p p r o x i m a t e l y the same t i m e w i t h s i m i l a r a m p l i t u d e s . T h e i m p l i c a t i o n o f the s e c o n d c l a i m is that n o t h i n g shor t of a n e x p l o s i v e source w i l l exci te the m e c h a n i s m . R P E is c l ea r ly non- l inea r i n its response to the se i smic inpu t . S o b o l e v et a l . (1982) r e p o r t e d that o ther p h y s i c a l effects are a s soc ia t ed w i t h R P E . U l t r a s o n i c t ransducers p l a c e d i n the s u l p h i d e o r e b o d y m e a s u r e d v i b r a t i o n s c o i n c i d e n t w i t h R P E emiss ions . A l s o , b o t h l igh t a n d X - r a y s w e r e reg is te red o n p h o t o g r a p h i c f i l m inser ted in to the o rebody , but none f r o m f i l m pu t i n the host rock. A conta iner m a d e of w o o d p reven ted l i gh t f r o m reach ing the f i l m i n the tests for X - r a y s . In ano ther paper , Sobo lev et a l . (1984b) descr ibe a n exper iment w h e r e a g a m m a ray spec t rometer detected g a m m a rays , w i t h energies i n the 100 to 700 k e V range, that w e r e c o i n c i d e n t w i t h the se i smic pu lse . U . S . a n d w o r l d - w i d e patents o n a n e x p l o r a t i o n m e t h o d b a s e d u p o n R P E have been g ran t ed to S o b o l e v et a l . (1986). T w o in te res t ing de ta i l s c o m e f r o m these patents: the p o w e r s p e c t r u m of the s i g n a l depends u p o n the mine ra l s c o m p r i s i n g the o r ebody , a n d better s igna l s are ob ta ined i f t w o successive shots are u s e d ins tead of one shot. In the patent i t is m e n t i o n e d that the first shot "charges u p " the o r e b o d y a n d s e c o n d shot , s m a l l e r t h a n the first , is u s e d to extract the s igna ls . T h e charge is m o n i t o r e d b y a n electrometer, w h i c h is p r e s u m a b l y connected to the o rebody u n d e r s tudy , a n d the second blast is i n i t i a t ed w h e n the electrometer has reached a peak va lue . T h e p h e n o m e n o n of se i smica l ly i n d u c e d static charge is desc r ibed i n a short paper b y Sobo lev et a l . (1982b), i n w h i c h a b u i l d - u p of charge appears o n the surface of the o r ebody , w i t h r e s i d u a l decay t imes of seconds. T h i s effect appears to be qui te impor t an t i n the i r i m p l e m e n t a t i o n of a se i smoelec t r ic e x p l o r a t i o n m e t h o d u t i l i z i n g R P E . Chapter 1: Introduction 8 These d e v e l o p m e n t s came to the a t ten t ion of R . D . R u s s e l l w h e n he met S o b o l e v at a n I A S P E I m e e t i n g i n L o n d o n , O n t a r i o , d u r i n g 1981. Sobo lev offered to demons t ra te the i r e q u i p m e n t a n d techniques , a n d i n 1983 bo th Sobo lev a n d D e m i n a r r i v e d i n V a n c o u v e r to d o t w o u n d e r g r o u n d tests (Sobolev et a l . , 1984a). T h e t w o si tes w e r e the G i a n t -Y e l l o w k n i f e M i n e i n the N o r t h - W e s t Ter r i to r i e s , a n d the S u l l i v a n M i n e at K i m b e r l y , B r i t i s h C o l u m b i a . A t S u l l i v a n the tests were for R P E o n l y , a n d at the G i a n t - Y e l l o w k n i f e b o t h p i ezoe l ec t r i c a n d R P E t y p e responses w e r e sought . S e v e r a l geophys i c i s t s f r o m U . B . C , M . M a x w e l l , B . N a r o d , a n d P . W h a i t e , also pa r t i c ipa ted i n these exper iments . T o enhance p o r t a b i l i t y , a l l of the Sov ie t e q u i p m e n t w a s ba t te ry p o w e r e d . P r i n c i p a l i n s t rumen t s w e r e a H i t a c h i reel- to-reel V C R for r e c o r d i n g the da ta , a n d i n d u c t i o n c o i l magne t i c antennas. A cu r r en t - ca r ry ing w i r e w r a p p e d a r o u n d the e x p l o s i v e p r o v i d e d a t ime break for the data. D a t a f r o m the V C R w a s d i v i d e d in to h i g h f requency da ta o n the v i d e o c h a n n e l , a n d l o w f requency p l u s vo ice da ta o n the a u d i o channe l s . T h e tapes were a n a l y z e d b y r e p l a y i n g the s ignals to a t r iggered osci l loscope, a n d pho tographs of the osc i l loscope screen were u s e d to gather t i m i n g in fo rma t ion . These c o l l a b o r a t i v e tests s h o w e d e n c o u r a g i n g r e su l t s , b u t w e r e no t c o m p l e t e l y c o n v i n c i n g . A t this stage, o n l y a few years after d i s cove ry , the Sovie ts were p r o b a b l y s t i l l per fec t ing the m e t h o d . A l s o , the me thods of in te rpre ta t ion a n d presen ta t ion w e r e v e r y d i f ferent f r o m those r o u t i n e l y u s e d b y W e s t e r n na t ions . In h i n d s i g h t , the s ty l e of p resen ta t ion w a s app rop r i a t e g i v e n that the s u r v e y l ines r a n p a r a l l e l to the ore-zones a n d the s m a l l s i ze of the data-set (see C h a p t e r s 3 a n d 4). T h e Sov ie t s w e r e a l r e a d y c o n v i n c e d that the b l i p s o n the o s c i l l o s c o p e traces w e r e not ar t i facts , as they w e r e f a m i l i a r w i t h th is type of s i g n a l a n d i m m e d i a t e l y r e c o g n i z e d R P E , bu t m a n y W e s t e r n obse rve r s w e r e not p r e p a r e d to share the i r con f idence u n t i l p r e sen t ed w i t h m o r e c o n v i n c i n g data . In 1986, B . N a r o d a n d M . M a x w e l l d u p l i c a t e d the tests at the S u l l i v a n M i n e u s i n g loca l l y Chapter 1: Introduction 9 a v a i l a b l e e q u i p m e n t (see C h a p t e r 4 for de ta i l s o f the S u l l i v a n site). A R a c a l tape recorder a l l o w e d the r e c o r d i n g of four channels at a b a n d w i d t h of 300 k H z . M a g n e t i c f i e l d sensors cons i s ted of a U T E M c o i l b o r r o w e d f r o m C o m i n c o , a n d a c u s t o m d e s i g n e d c o i l m a d e b y B . N a r o d , c a l l e d U B C I. Blas t t ime-breaks w e r e p r o d u c e d b y the same m e t h o d u s e d b y the Soviets , a current c a r r y i n g w i r e w r a p p e d a r o u n d the exp los ive . Bu r s t s of shor t d u r a t i o n E M pu l ses , s i m i l a r to those o b t a i n e d b y the Sov ie t s , w e r e p r o d u c e d d u r i n g these tests. T h e s i g n a l to no i se ra t io w a s l o w , a r o u n d 2 to 4. F u r t h e r m o r e , the best rece iver t u r n e d out to be the geophone : the U T E M c o i l b e h a v e d e r ra t i ca l ly , a n d U B C sensor h a d a h i g h e r noise l e v e l . D e s p i t e these i n s t r u m e n t faul ts a n d q u i r k s , the records s h o w e d bursts of s ignals at a p p r o x i m a t e l y the same t imes f o u n d b y Sobo lev a n d D e m i n . T h e exper iment was d e e m e d to be successful i n d u p l i c a t i n g the w o r k of the Sov ie t scient is ts . H o w e v e r , a p r o f u s i o n of s igna l s i m m e d i a t e l y after the blast d i d cast some d o u b t u p o n the source of the s ignals . E M pu l ses w e r e expec ted to start 5 to 6 m i l l i s e c o n d s after the blast because the m a i n o r e b o d y w a s abou t 30 meters f r o m the shots , bu t the s igna ls often s tar ted earl ier . T h e cause w a s n a r r o w e d to t w o poss ib i l i t i e s : s m a l l a m o u n t s o f s u l p h i d e s near the sho tpo in t , o r in ter ference f r o m the w i r e - b r e a k c i rcu i t . S u l p h i d e s near the sho tpo in t w e r e c o n s i d e r e d to be the d o m i n a n t m e c h a n i s m because of the c o m m o n t imes of s t rong s igna ls seen b y b o t h f i e l d t r ia ls , a n u n l i k e l y event for w i r e -b reak interference. L a b o r a t o r y tests o n s u l p h i d e samples by Sobolev et a l . (1982) r evea l that R P E is not easy to p r o d u c e o n a s m a l l scale. T h e s imples t test is to u n i a x i a l l y compress a s a m p l e a n d e x a m i n e the response . T h i s t y p e of test d i d no t s h o w the s u l p h i d e s a m p l e s to be p a r t i c u l a r l y act ive. In fact, s amples c o n t a i n i n g quar tz gave the greatest s ignals . Sobo lev et a l . f o u n d that i f a l o w f requency A C vo l t age w a s a p p l i e d to the s a m p l e then the s u l p h i d e s p r o d u c e d m u c h s t ronger s ignals ; the responses f r o m other types of r o c k were u n c h a n g e d . It w o u l d a p p e a r that the a p p l i e d v o l t a g e serves the s ame ro l e as the r epor t ed charge b u i l d - u p o n the o rebody (Sobolev et a l . , 1982b). H o w e v e r , a later s amp le Chapter 1: Introduction 10 tes t ing m e t h o d (1992, p r i v a t e c o m m u n i c a t i o n be tween M . M a x w e l l a n d R. D . R u s s e l l w i t h G . A . Sobo lev ) i n v o l v e d p r e s s i n g the s a m p l e onto a k n i f e edge to f racture the s a m p l e w h i l e a cur rent is pas sed t h r o u g h it. T h e l abora to ry m e t h o d s of S o b o l e v et a l . ind ica tes that they cons ide r b o t h fracture a n d l o w f requency electr ic f ie lds v i t a l for the p r o d u c t i o n of R P E . T h e i m p o r t a n c e o f f rac ture a n d l o w f r equency e lec t r ic f i e ld s i s r e f l ec ted i n t he i r exp lana t ion of R P E . In 1982 (Sobolev et a l , 1982a) they p r o p o s e d that cracks generate the R P E pu l se , a n d the pu lses are a m p l i f i e d a n d s h a p e d fur ther b y n a t u r a l s e m i c o n d u c t o r c i rcu i t s w i t h i n the o rebody , w h i c h are p o w e r e d b y nearby p iezoe lec t r ic m ine ra l s s u c h as q u a r t z o r spha le r i t e . I f i n d this e x p l a n a t i o n i nadequa t e as it r equ i r e s p i ezoe l ec t r i c mate r ia l s , a n d seems too c o m p l e x for a n a t u r a l p h e n o m e n o n . T h e i r e x p l a n a t i o n is not m e n t i o n e d i n later pub l i ca t ions , a n d I suspect it has fa l len out of favor . In 1992 R . D . R u s s e l l a n d M . M a x w e l l v i s i t ed M o s c o w a n d L e n i n g r a d ( n o w St. Petersburg) to see w h a t a v e n u e s the e x - S o v i e t s c i en t i s t s w e r e p u r s u i n g i n the a r e a o f se i smoelec t r i c i ty . O t h e r than a shor t p a p e r b y D e m i n a n d S o b o l e v i n 1988, there h a d been n o recent repor ts . M . M a x w e l l a n d R . D . R u s s e l l d i s c o v e r e d that the f i e l d of p i e z o e l e c t r i c i t y h a d m o v e d f r o m research a n d d e v e l o p m e n t to a p p l i c a t i o n a n d f i e l d su rveys (under the auspices of a c o m p a n y ca l l ed R u g i d f i z i k a ) . D e m i n a n d M a y b u k w e r e c o n t i n u i n g the s t u d y of R P E ( in M o s c o w ) , b u t w e r e at a less a d v a n c e d s tage t h a n R u g i d f i z i k a i n a p p l i c a t i o n . H o w e v e r , they were also b u s y p r o d u c i n g case his tor ies . In fact, the ex-Sovie ts h a d q u i e t l y co l l ec t ed a n i m p r e s s i v e set of case h i s to r ies , none of w h i c h have a p p e a r e d i n W e s t e r n l i terature (or p r o b a b l y i n Sov ie t l i terature) . W i t h the b r e a k - u p of the Sov ie t U n i o n in to v a r i o u s count r ies a n d the r e s u l t i n g e c o n o m i c chaos there is some uncer ta in ty as to con t inua t ion of these p rog rams . B y 1990 there w a s a s t r o n g des i re for i n d e p e n d e n t c o n f i r m a t i o n of the v a l i d i t y of Sobolev ' s w o r k . A n u m b e r of m i n i n g compan ie s h a d h e a r d o r r ead of the Sov ie t w o r k Chapter 1: Introduction 11 a n d w a n t e d to k n o w i f s u c h a p h e n o m e n o n w a s real . If so, w o u l d i t be feasible to use it for m i n e r a l e x p l o r a t i o n ? A s par t of a b r o a d p r o g r a m to d i s c o v e r a n d d e v e l o p se i smoe lec t r i c p h e n o m e n a for g e o p h y s i c a l e x p l o r a t i o n , the I n s t r u m e n t a t i o n G r o u p of U . B . C . w a s g i v e n the resources a n d funds to p r o v i d e th is i n f o r m a t i o n . M y thesis is the e m b o d i m e n t of the research effort i n R P E at U . B . C d u r i n g 1990 to 1995. C H A P T E R 2 FIELD I N S T R U M E N T A T I O N A N D M E T H O D S 2.1 Overview of Equipment and Procedure M o s t o f m y f i e l d s tud ie s w e r e c a r r i e d ou t i n a n u n d e r g r o u n d e n v i r o n m e n t . A n advan tage to u n d e r g r o u n d w o r k is that the E M noise b a c k g r o u n d can be v e r y l o w , d u e to the s h i e l d i n g of the o v e r l y i n g rock. A great d i sadvan tage is the l i m i t e d choice i n shot a n d i n s t r u m e n t p lacement . F u r t h e r m o r e , the u n d e r g r o u n d e n v i r o n m e n t is h a r s h o n e q u i p m e n t because o f the m o i s t u r e , m u d , dus t , a n d , i n m a n y s u l p h i d e m i n e s , t h e ac id i ty . U n l e s s o the rwi se stated, this sec t ion of the thesis i m p l i c i t l y assumes that the d e s c r i b e d e q u i p m e n t a n d m e t h o d s are for u n d e r g r o u n d w o r k , as th is aspect is mos t re levant i n u n d e r s t a n d i n g the case his tor ies i n C h a p t e r 4. T h e basic a i m of o u r f i e l d t r ia ls was to measu re the e lec t romagne t i c response of t he ear th to a s e i smic s t i m u l u s . C o l l e c t i n g this da ta i n v o l v e s se t t ing off a n e x p l o s i o n to p r o d u c e the se i smic d i s turbance , w h i l s t r e c o r d i n g the response f r o m va r ious electric a n d m a g n e t i c f i e l d an tennas p l a c e d near the area of interest . T h i s p r o c e d u r e i s b r o a d l y s i m i l a r to se ismic methods . H o w e v e r , the p lacement of sensors is not as c r u c i a l because e lec t romagnet ic d is turbances t r ave l at least four orders of m a g n i t u d e faster than se i smic waves , a n d a l l of the E M sensors receive the s i g n a l w i t h i n m i c r o s e c o n d s of each other. H e n c e , the t ime l ag seen on the records is p r i n c i p a l l y d u e the p r o p a g a t i o n t ime for the se ismic w a v e to reach the target. U n l i k e se ismic me thods , m u l t i c h a n n e l r e c o r d i n g does not g i v e fur ther i n f o r m a t i o n about target l o c a t i o n , bu t offers r e d u n d a n c y a n d better d i s c r i m i n a t i o n . S t i m u l a t i n g R P E requi res a fa i r ly p o w e r f u l source , w h i c h is p r o v i d e d b y an exp los ive . T o ob ta in g o o d s i gna l recep t ion w e w a n t to be near the sho tpo in t , bu t not so c lose as to endanger the c r e w or equ ipmen t . W e have gene ra l ly kep t o u r sensors a n d ourse lves 12 Chapter 2: Field Instrumentation and Methods 13 be tween 50 a n d 200 meters f r o m the blast. T h i s dis tance is suf f ic ien t ly c lose to receive s igna l s r e l i ab ly . In a n u n d e r g r o u n d m i n e there is l i t t l e cho ice i n sites to p lace shots , e q u i p m e n t , a n d peop le , so shot p lacement is d o n e w i t h a lot of thought . A r e a s w h e r e there is c l ea r ly uns tab le r o c k format ions , o r w h e r e d a m a g e to m i n e inf ras t ruc ture m i g h t o c c u r are a v o i d e d . V e n t i l a t i o n is a n i m p o r t a n t c o n s i d e r a t i o n because the e x p l o s i o n p r o d u c e s dus t a n d fumes, w h i c h m a y con ta in h a r m f u l gases s u c h as c a r b o n - m o n o x i d e . E q u i p m e n t a n d c r e w are best d e p l o y e d u p w i n d of the blast , p re fe rab ly i n a n a l cove or short d r i f t / c ros scu t , a n d out of the direct pa th of the blast a i r w a v e a n d other m i n e traffic. E a c h e x p l o s i v e charge is p l a c e d i n a shor t (1 to 3m) d r i l l ho le . S o b o l e v a n d D e m i n (p r iva te c o m m u n i c a t i o n w i t h R . D . R u s s e l l ) of ten p l ace t he i r charges o n the t u n n e l surface because it is m o r e c o n v e n i e n t t h a n d r i l l i n g n u m e r o u s s m a l l ho les . Surface charges d o not efficiently p r o d u c e se ismic energy, therefore, a la rger charge is necessary. A t y p i c a l charge s ize for o u r w o r k ranges f r o m 0.2 to 0.5 k g , for the Russ i ans 2 to 5 k g is m o r e t yp i ca l . I feel that the extra damage done to m i n e infras t ructure b y this technique m o r e t h a n offsets the t i m e s a v e d b y no t d r i l l i n g shot ho les . O n e effective w a y to increase p r o d u c t i v i t y is to fire the shots i n salvoes. Because it is not safe to a p p r o a c h the area of a n e x p l o s i o n for a p e r i o d of ten to fifteen m i n u t e s after the b las t (gases a n d s p a l l i n g / f a l l i n g rock ) f i r i n g a sequence of f ou r shots at t w o m i n u t e i n t e r v a l s saves a p p r o x i m a t e l y 30 to 40 m i n u t e s that w o u l d o the rwise be spent w a i t i n g for the dus t to settle. T h e r equ i r emen t for b r o a d b a n d , l o w - n o i s e E M measurements necessi ta ted cus tom-bu i l t sensors a n d p re -ampl i f i e r s . Sensor b a n d w i d t h s were t y p i c a l l y 1 k H z to 100 k H z , w i t h some a l l o w i n g measurement u p to 5 M H z . T h e u p p e r f requency bands are not t y p i c a l l y u s e d i n geophys i c s , therefore, mos t c o m m e r c i a l l y a v a i l a b l e sensors l ack the a b i l i t y to m a k e b r o a d b a n d measurements i n this range. A n o t h e r factor is that mos t g e o p h y s i c a l w o r k is d o n e o n the surface a n d E M measu remen t is l i m i t e d b y a m b i e n t noise . T h e u n d e r g r o u n d e n v i r o n m e n t is gene ra l ly quie t at these frequencies , w i t h mos t t ransients Chapter 2: Field Instrumentation and Methods 14 occurring in sync with the mine grid (e.g.. a transient every half cycle, or 8.33 ms). Thus, the ability to detect signals is often limited by the intrinsic noise properties of the sensor, which can be optimized by design and component selection. Transducer and Pre-Amplifier 20 to 100 m of Shielded Twisted Pair Cable 60 Hz Notch Filter > 3COOOOOOOOOC V I <7 Hi-Pass Lo-Pass Gain Computer based Amplification and Filtering Digitizer Tektronix AM502 Figure 2.1 Schematic of the instrumentation used in seismoelectric research at UBC. The E M sensors and geophones were remotely set, and linked to a central instrumentation site via shielded cable; an arrangement similar to many seismic surveys. A flow diagram of the signal path is shown in Figure 2.1. This arrangement assures flexibility in sensor placement, as each has a separate cable, and reduces the possibility of E M interference from the computer/digitizer system. All of the E M sensors have integral pre-amplifiers so that the cables do not alter the signals significantly. Signal conditioning electronics, the computer based digitizer, test equipment and batteries are located in one area, the instrumentation site. This centralized arrangement Chapter 2: Field Instrumentation and Methods 15 a l l o w s one p e r s o n to m o n i t o r a n d adjust the i n c o m i n g s igna l s , c h e c k sensors a n d batteries for p r o p e r ope ra t ion , a n d to save the d i g i t i z e d da ta to a por t ab le c o m p u t e r . Ba t t e r i e s p o w e r a l l o f the e q u i p m e n t i n o r d e r to e n h a n c e p o r t a b i l i t y w i t h o u t c o m p r o m i s i n g the E M e n v i r o n m e n t . D C p o w e r a v o i d s the p r o b l e m s of s w i t c h i n g t rans ients often f o u n d i n A C e q u i p m e n t a n d i n m a i n s inve r t e r s . T h e bat ter ies a lso s u p p l y p o w e r to the E M sensors t h r o u g h extra w i re s i n the cables. 2.2 The Seismic Source V a r i o u s types of s e i smic sources have been tested a n d o n l y one s tands out: pen to l i t e ( P E T N / T N T mix ture ) boosters. T h i s type of exp los ive i n sizes 0.18 to 0.5 k g has w o r k e d v e r y w e l l i n the f i e ld t r ials desc r ibed i n C h a p t e r 4. D u r i n g m y first f i e l d t r i a l ( S u l l i v a n M i n e ) I tested a n u m b e r of s m a l l sources , i n c l u d i n g a s ledge h a m m e r , s h o t g u n b l anks , a n d d e t o n a t o r s , a n d o b s e r v e d n o e l e c t r o m a g n e t i c r e sponse . B l a s t i n g agen ts or e m u l s i o n - t y p e e x p l o s i v e s ( a m m o n i u m - n i t r a t e / f u e l o i l m ix tu re s ) of 0.1 to 0.3 k g s izes were a lso t r i ed i n th is test (and i n s izes u p to 10 k g i n later surface t r ia ls i n A u s t r a l i a ) w i t h l i t t le success. T h i s was p robab ly d u e to p o o r c o u p l i n g to the host rock rather than a lack of energy. T h e pentol i te boosters were v e r y s t rong ly r e c o m m e n d e d b y G r a n t Scott, a m i n e r / p o w d e r m a n o n l o a n to us f r o m the S u l l i v a n M i n e w o r k f o r c e . A f t e r h e a r i n g our needs Scott sugges ted that the boosters were the correct e x p l o s i v e for the task, a n d he in s i s t ed o n ga the r ing s o m e for o u r exper iment . Scott w a s v i n d i c a t e d as the pen to l i t e exp los ives succeeded where the other sources fa i led . Pen to l i t e di f fers f r o m mos t c o m m o n l y a v a i l a b l e e x p l o s i v e s i n that i t has a v e r y h i g h v e l o c i t y of d e t o n a t i o n (7500 m / s ) a n d it is r e l a t i ve ly dense (1600 k g / m 3 ) . N o r m a l l y , pen to l i t e is u s e d to boost the exp lo s ive process i n large charges m a d e of a m m o n i u m -n i t r a t e / f u e l o i l m ix tu r e s (Tour , 1992). H i g h ve loc i ty exp los ives , s u c h as pen to l i t e , are efficient i n p r o d u c i n g se i smic pu l ses i n u n d e r g r o u n d r o c k because the charac ter i s t ic i m p e d a n c e of these e x p l o s i v e s a n d the i m p e d a n c e of u n d e r g r o u n d r o c k are w e l l ma t c hed ( N i c h o l l s , 1962; see A p p e n d i x A . l ) . Chapter 2: Field Instrumentation and Methods 16 Frequency (Hz) Figu re 2.2 E x a m p l e of the se ismic pu lse (above) p r o d u c e d by a s m a l l exp los ive i n u n d e r g r o u n d rock , a n d the a m p l i t u d e s p e c t r u m of the s e i s m i c d is turbance (below). Chapter 2: Field Instrumentation and Methods 17 T h e resul ts f r o m m y u n d e r g r o u n d f i e l d - w o r k ind ica t e that a p p r o x i m a t e l y 0.5 k g of p e n t o l i t e e x p l o s i v e is n e e d e d to r e l i a b l y i n d u c e R P E i n o re -zones 75 m f r o m the shotpoin t . T h i s c o m b i n a t i o n of charge s ize a n d range co r re sponds to a peak pressure of abou t 150 k P a at the o r e b o d y (see A p p e n d i x A . 2 a n d A . 3 ) . There fo re , the s i ze of exp lo s ive charge s h o u l d be adjusted so that a se i smic stress of a p p r o x i m a t e l y 100 to 200 k P a is d e l i v e r e d to the target ( A p p e n d i x A . 2 g ives the sca l ing laws) . A n example of the se i smic wave le t a n d s p e c t r u m obta ined b y m y m e t h o d o l o g y is s h o w n i n F i g u r e 2.2. The pu l se is ve ry b road -band for a h i g h energy se i smic pu l se , a n d contains subs tan t ia l energy f r o m 200 to 1000 H z (a 1000 H z low-pass f i l ter was u s e d i n a c q u i r i n g this data). 2.3 Obtaining a Shot Moment T h e shot m o m e n t or t ime-break is an e lec t r ica l s i g n a l i n d i c a t i n g that the e x p l o s i o n has b e g u n . T h i s is a v e r y impor t an t s i gna l as it is the reference for c o m p a r i n g events f r o m other shots. In this case, a s i m p l e concept does not translate into a n easi ly rea l izable one. It is r e l a t ive ly easy to dev ise a m e t h o d to obta in a shot m o m e n t s igna l ; the p r o b l e m is i n d e v i s i n g one that does not p o l l u t e the E M e n v i r o n m e n t d u r i n g measurement . I n i t i a l l y , t w o so lu t i ons were p u r s u e d to r e m e d y the p r o b l e m of e lec t r i ca l noise . O n e m e t h o d was based u p o n a m o d i f i e d b l a s t i ng box that q u i c k l y d i sconnec t s the cur ren t source ( in less that 0.01 ms) once the de tona t ion has been in i t i a t ed . T h i s e l i m i n a t e d the p o s s i b i l i t y of fur ther w i r e contacts i n d u c i n g false s igna l s , h o w e v e r , the cu r ren t p u l s e in to the de tona tor p roduces a large E M transient. A n o t h e r m e t h o d is to w r a p a l o o p of w i r e a r o u n d the exp los ive , pu t a s m a l l current (0.2 m A ) t h r o u g h the l o o p , a n d m o n i t o r the resistance change b y m e a s u r i n g the vol tage across the l o o p as i t is b r o k e n ; w e c a l l t h i s m e t h o d a w i r e - b r e a k . Because the w i r e - b r e a k is i n d e p e n d e n t of the t y p e of detonator u s e d a se ismic detonator is not necessary. The ab i l i t y to use o r d i n a r y e lect r ical detonators a n d fused caps (detonators in i t i a t ed b y a f lame at the e n d of a s l o w - b u r n i n g Chapter 2: Field Instrumentation and Methods 18 fuse) is a des i rab le feature as it a l l o w s the use of de tonators f r o m the m i n e m a g a z i n e . U n f o r t u n a t e l y , ne i ther m e t h o d met o u r needs as b o t h created too m u c h e lec t r ica l noise . A fur ther d i s advan t age for the w i r e -b r eak is that the a d d i t i o n a l c o n d u c t o r a t tached to the exp los ive package m a y c o m p r o m i s e safety. M y s o l u t i o n to the p r o b l e m of e l ec t r i ca l ly i s o l a t i n g the b las t f r o m the an tennas a n d other e lect r ical equ ipmen t w a s to use a f iber opt ic cable a n d fuse b l a s t i ng caps. The i dea b e h i n d the f iber opt ic t ime-break is that the intense l igh t emi t t ed b y h i g h exp los ives can be u s e d to o b t a i n a n accurate t i m e break. In pract ice , a s m a l l l e n g t h o f u n s h e a t h e d o p t i c a l f iber is a t t ached to the de tona to r or e x p l o s i v e . S o m e of the l i g h t f r o m the e x p l o s i o n is co l lec ted b y the fiber opt ic s t rand , a n d is t r ansmi t t ed t h r o u g h the l eng th of the f iber op t i c cable to a n o p t i c a l receiver , w h i c h translates the l i g h t in to a n e lec t r ica l s i gna l ( K e p i c et a l . , i n press; detai ls are i n A p p e n d i x B . l ) . A f t e r the blast, the b u r n t e n d of the fiber op t ic cable is t r i m m e d a w a y a n d the cable is reused. The fiber op t i c t ime break has e l imina t ed the s t rong E M ac t iv i ty , 0 to 3 m s after the blast, that f requent ly o c c u r r e d i n p r e v i o u s expe r imen t s (see A p p e n d i x B .2 , a n d the S u l l i v a n M i n e a n d M o b r u n M i n e f i e ld tr ials) . T h i s blast-related ac t iv i ty m a s k e d ear ly s ignals , a n d p l a c e d some d o u b t u p o n the source of later a r r iva l s . T h e f iber op t i c t r igger is also v e r y accurate . A s ide -by- s ide c o m p a r i s o n of the f iber op t i c a n d w i r e - b r e a k m e t h o d s o n a de tona tor s h o w e d that the f iber op t ic t r igger o c c u r r e d first , a n d the w i r e - b r e a k s i g n a l f o l l o w e d 10-20 us later. T h i s demonstrates the great accuracy of the f iber op t ic t ime-break because the w i r e -b r eak is k n o w n to be a ve ry accurate m e t h o d of d e t e r m i n i n g t ime of de tona t ion ( B u r r o w s , 1936). 2.4 Magnetic Field Sensors A m a g n e t i c f i e l d t r ansduce r (or magne tomete r ) p r o d u c e s a n e l ec t r i ca l s i g n a l that is s i m p l y re la ted to the magnet ic f ie ld . There are m a n y w a y s to ach ieve th is resul t , but the s imples t a n d mos t effective m e t h o d for the f requency range of 1 k H z to 5 M H z is to use Chapter 2: Field Instrumentation and Methods 19 wire coiled onto a ferrite rod. Faraday's law of induction describes the coil's behavior in a magnetic field V = -NA — (2.1) at where V is the voltage across the coil. The voltage is proportional to the rate of change in the magnetic flux enclosed by the coil, which is equal to number of turns QV) x cross-sectional area (A) x magnetic flux density parallel to the coil axis (B). A simple electrical circuit representation of the coil is an inductor in series with a resistor. The inductor represents the self-inductance (L) of the coil and the resistor represents the resistance of the wire comprising the coil. The effects of coil resistance can be ignored for most purposes because it does not play a significant role until very low frequencies (sub 10 Hz), where the impedance of the inductor is less than that of the resistor. A shunt capacitor needs to be added to complete the electrical representation of the coil. This capacitor approximates the behavior of the distributed capacitance between coil windings. The effect of the magnetic field may be represented as either a voltage source in series with the inductor using equation 2.1, or as a current source in parallel with the inductor. The current source has a current (z) of . NAB > = — (2.2) Note that the current is proportional to the magnitude of the magnetic field (H). Shunting the coil with a resistor and measuring the voltage across the resistor provides a passive means to measure the current produced by the coil. This type of magnetic sensor is electrically equivalent to a damped parallel-resonant circuit, and I have labeled this arrangement as the "damped resonator" arrangement to distinguish it from the more common inductive pick-up, which is an undamped coil. The transfer function of the damped resonator design is obtained by considering the voltage (V) across the parallel combination of current source (z), inductance (L), resistance (R), and capacitance (C). Conservation of current gives: Chapter 2: Field Instrumentation and Methods 20 NAB „( 1 1 O = V — + — + — In the f requency d o m a i n the transfer func t ion is (2.3) Yl^=NA m (24) If the c o i l is m o r e than c r i t i ca l ly d a m p e d , R<RC where R=-J— (2.5) then the t ransfer f u n c t i o n has l o w a n d h igh -pas s c o r n e r f requenc ies (fL a n d fH r e spec t ive ly ) . W h e n the c o i l is u n d e r - d a m p e d , R> Rc , the response is p e a k e d at the resonance frequency (f0). 1 R 1 I n / / = a n d fo r = (2 6) F o r a w e l l d a m p e d c o i l , R « Rc, the d a m p e d resonator p roduces a vo l tage p r o p o r t i o n a l to the magnet ic f ie ld be tween frequencies fL a n d fH . T h i s is o n l y s t r ic t ly t rue near fg, but i f the resonance is w e l l o v e r - d a m p e d then w e can neglect the ro l l -o f f near fL a n d fH for mos t pass -band frequencies. T h e p r i n c i p a l benefits of the d a m p e d resonator de s ign are that the f requency response is flat o v e r a r ange of u se fu l f requencies , a n d this response ex tends b e y o n d the co i l ' s resonance. I n d u c t i o n c o i l sensors gene ra l ly measure f requencies b e l o w the co i l ' s self-resonance, or at resonance, w h i c h l im i t s their use as b r o a d b a n d sensors. The pass -band responses of the magnet ic sensors u sed i n m y research were chosen to be be tween 1 k H z a n d 300-3000 k H z , as this r eg ion encompasses mos t of the observable R P E spec t rum, a n d it a v o i d s the large a m o u n t s of p o w e r - l i n e noise i n the 10 to 1000 H z b a n d . In a d d i t i o n , these sensors w e r e r e q u i r e d to detect v e r y s m a l l s igna l s i n a l o w noise e n v i r o n m e n t , therefore, cons ide rab le effort was m a d e i n o p t i m i z i n g b o t h the sensor no i se l e v e l a n d the f requency response ( A p p e n d i x B.3). Chapter 2: Field Instrumentation and Methods 21 T h e m e a s u r e d proper t ies of the U B C I, U B C I V a n d U B C V sensors, w h i c h w e r e u s e d i n m y f i e l d t r ia ls , are g i v e n i n table 2.1 (see A p p e n d i x B.4 for detai ls) . O v e r a l l sens i t iv ies were adjusted i n the U B C I V a n d V des igns based u p o n the exper ience g a i n e d f r o m t w o p r e v i o u s f i e l d t r ia ls w i t h U B C I, w h i c h w a s f o u n d to be set too l o w . A sens i t i v i t y of about 0.1 V / n T w a s f o u n d to be su i tab le for the measurement of R P E s igna l s 50 to 200 meters f r o m the source . 2.5 Electric Field Sensors U n l i k e the magne t i c sensors, there w e r e severa l techniques u s e d to measure the electric f i e l d . T h e three m a i n t echn iques were : the d i p o l e , m e a s u r e m e n t of the p o t e n t i a l b e t w e e n a p a i r of s tainless steel stakes d r i v e n in to the ear th some d i s tance apart ; the l o n g w i r e an tenna , measuremen t of the po ten t i a l difference be tween a n i n s u l a t e d w i r e o v e r l y i n g the ear th a n d a g r o u n d stake; a n d the p a r a l l e l p la te d i p o l e , the measu remen t of the po ten t i a l be tween t w o freely s u s p e n d e d , pa r a l l e l , e lec t rode plates . E a c h has its p a r t i c u l a r advan tages a n d charac te r i s t ics , w h i c h w i l l be d i s c u s s e d i n the f o l l o w i n g sect ions . Grounded Dipole G r o u n d e d d i p o l e refers to the measurement of the po ten t i a l be tween t w o po in t s i n the earth. T h i s is a c h i e v e d w i t h a d i f fe ren t ia l p r e - ampl i f i e r connec ted b e t w e e n t w o stakes set in to the earth. The stakes are u sed to ensure that a l o w resistance e lect r ical contact is m a d e w i t h the earth; the resistance be tween stakes (2-10 meters apart) is u s u a l l y of the o rde r of 10 k Q . In m a n y g e o p h y s i c a l app l i ca t i ons p o r o u s pot e lectrodes are u s e d , bu t these are p r i m a r i l y u s e d to r emove l o w frequency effects (Tel ford et a l . , 1986) that are not i m p o r t a n t i n m y studies . E a c h p re -ampl i f i e r is h o u s e d i n a m e t a l die-cast b o x to protect the electronics f rom the elements a n d has banana jacks to connect w i r e s to the electrode stakes. The p re -ampl i f i e r s u s e d i n m y w o r k are based u p o n the three o p - a m p i n s t r u m e n t a t i o n Chapter 2: Field Instrumentation and Methods 22 ampl i f i e r de s ign (see a p p e n d i x C for schematic) , w h i c h p r o v i d e s a h i g h i n p u t i m p e d a n c e a n d good c o m m o n - m o d e s igna l rejection ( H o r o w i t z a n d H i l l , 1989). In a d d i t i o n , the use of a di f ferent ia l ampl i f i e r prevents g r o u n d loops . I const ructed two m o d e l s of ampl i f i e r , the T-box (named after the pat tern of e lec t r ica l tape aff ixed o n the hous ing ) a n d h i g h b a n d w i d t h p re - ampl i f i e r ( H B W ) , for b o t h m y use a n d for genera l use for the research g roup . Speci f ica t ions for these ampl i f i e r s are tabula ted i n table 2.2. T h e T-box type of p re -ampl i f i e r is p r i n c i p a l l y u sed b y the U . B . C . research g r o u p i n tests for l o w frequency seismoelectr ic phenomena , a n d the frequency response, 0.1 H z to 30 k H z , extends l o w e r than is necessary for R P E w o r k . H i g h b a n d w i d t h pre -ampl i f ie r s have either a 1 k H z or 0.1 H z low-cut . Dipole C. R Pre-Amplifier Input • c R-Figure 2.3 E q u i v a l e n t c i rcu i t of a s m a l l d i p o l e antenna. P r i n c i p a l weaknesses of the g r o u n d e d d i p o l e are a s u s c e p t i b i l i t y to m a g n e t i c f ie lds because of the l o o p f o r m e d by the w i r e s to the stakes ( W u a n d T h i e l , 1989), a n d an uncer ta in frequency response b e y o n d 100 k H z . K e e p i n g the leads to the stakes short and straight reduces the former p r o b l e m . The latter p r o b l e m is due to stray inpu t capaci tance to the p re -ampl i f i e r , a n d the i n p u t capaci tance of the ampl i f i e r . T h e equ iva len t c i rcu i t of a short d i p o l e antenna ( smal l c o m p a r e d to E M wavelengths ) is s h o w n i n F i g u r e 2.3 (Casey a n d Bansa l , 1991). The inpu t resistance a n d capaci tance of the pre-ampl i f ie r , /? ; Chapter 2: Field Instrumentation and Methods 23 and C- ,are included in the equivalent circuit (Figure 2.3) because of their influence on the sensor response. A low-pass filter is formed by the source resistance Rs, and input capacitance C, with a corner frequency of f 1 The source resistance, Rs, is the resistance between the two ground stakes. Typical values of Rs and C, are 10 kQ. and 50 pF respectively, which results in a 300 kHz low-pass response. This effect precludes the use of grounded dipoles for wide-band measurements. Note that at higher frequencies the dielectric currents of the earth predominate, and provide the voltage across the wires. At very high frequencies the frequency response is flat (if limitations on amplifier bandwidth are neglected) because the capacitors (Figure 2.12) are the only significant circuit elements at high frequencies. However, the input voltage is lowered by the voltage dividing action of the stray capacitance verses the source capacitance Ca at high frequencies. Long Wire Antenna The long wire antenna (LWA) proved to be very useful in my early experiments because of its large effective height (effective height equals the open circuit voltage produced by an antenna divided by the electric field parallel to it) and ease of implementation. The long wire antenna is a long straight insulated wire placed near the ground with one end connected to an differential amplifier. The other input to the amplifier is connected to a ground stake at one end of the wire. The long wire antenna arrangement was introduced to the U.B.C. instrumentation group by A. Boyle in 1990, then at C.R.A. Group Special Equipment, Melbourne, Australia. A debate about what the long-wire was actually measuring arose between us. Further controversy was prompted by a series of letters between Wait (1989) and Wu and Thiel (1989b) over a paper by the latter (1989a), which was about the improved magnetic field immunity of the long wire antenna over grounded dipoles. R.D. Russell and I Chapter 2: Field Instrumentation and Methods 24 be l i eve w e have set t led the ques t ion of w h a t is m e a s u r e d (1991 I E E E / U R S I m e e t i n g i n L o n d o n , O n t a r i o ; see A p p e n d i x B.5) . T h e l o n g w i r e an t enna c a n be m o d e l e d as a s u m m i n g a m p l i f i e r w i t h each segment of w i r e ac t ing as a l o c a l capac i t ive p i c k - u p . T h i s ac t ion of d i s t r i b u t e d measurement contrasts great ly w i t h the measu remen t m a d e w i t h a g r o u n d e d d i p o l e , w h i c h infers electr ic f i e l d b e h a v i o r f r o m the m e a s u r e m e n t o f the electric po ten t ia l be tween t w o points . T h e l o n g w i r e an tenna w a s o r i g i n a l l y u s e d as a l o n g s t ra igh t i n s u l a t e d w i r e o n the g r o u n d w i t h one e n d connec ted to a charge a m p l i f i e r . T h e " g r o u n d reference" of the charge a m p l i f i e r c i rcu i t was connected to a stake at one e n d of the w i r e . A p r o b l e m w i t h u s i n g the l o n g w i r e an tenna w i t h a charge a m p l i f i e r is that the capaci tance of the w i r e m u s t be k n o w n , o r m e a s u r e d , to a c c u r a t e l y d e t e r m i n e the ave rage f i e l d s t r eng th . H o w e v e r , a m u c h greater p r o b l e m is that i f m o r e t h a n one an t enna is set ou t then m u l t i p l e g r o u n d po in t s d u e the g r o u n d e d stake of each an tenna alters the electr ic f i e ld . B o t h p r o b l e m s c a n be c i r c u m v e n t e d b y the use of a h i g h i n p u t i m p e d a n c e d i f fe ren t ia l ampl i f i e r . T h e transfer func t ion of this a r rangement ( in the f requency d o m a i n ) is T/ jO)RCw u V =— V—V (2 8) l + jcoRCw u - o ; w h e r e Cw is the capaci tance of the l o n g w i r e , a n d R is the i n p u t i m p e d a n c e of the a m p l i f i e r ( t reatment of the p r o b l e m is s i m i l a r to the cha rge a m p l i f i e r a r r a n g e m e n t except that the cur ren t co l lec ted b y the l o n g w i r e f l o w s t h r o u g h the i n p u t i m p e d a n c e of the ampl i f i e r , R , to the g r o u n d stake; see A p p e n d i x B.5 for detai ls) . A t frequencies above the R C po le the response is equa l to the m e a n po ten t i a l difference u n d e r the w i r e , V, referenced to the g r o u n d stake. E q u a t i o n 2.8 i m p l i c i t l y assumes that the capaci tance per un i t l eng th of the w i r e is constant, o therwise the potent ia ls are w e i g h t e d b y the capac i ty per u n i t l eng th ( A p p e n d i x B.5). T h e pole is t y p i c a l l y b e l o w 100 H z (R>5 M f l a n d C>1 n F for 50 metres of w i r e ) a n d is not of consequence i n m y measurements . I d o not use the l o n g w i r e a n t e n n a for h i g h b a n d w i d t h m e a s u r e m e n t s because i t suffers the same uncer ta in ty i n h i g h - e n d f requency response as its g r o u n d e d counterpar t . Chapter 2: Field Instrumentation and Methods 25 Para l l e l Plate D i p o l e The parallel plate dipole is indispensable for measuring vertical fields and high-bandwidth signals. It has a flat response from 1 kHz to 4.5 MHz (upper end limited by the pre-amplifier; Figure 2.4) and it is very portable (Baum, 1980). This is a non-contact antenna consisting of a very high input impedance amplifier connected to a pair of freely suspended conductors. The electrical equivalent circuit is the same as the grounded dipole (Figure 2.3), but the source resistance term Rs can be neglected as it is near-infinite. Figure 2.4 Test of the impulse response of the UBC IV magnetic antenna and parallel plate dipole antenna. The plot is an FFT of a 20 Msample/s time-domain record. Chapter 2: Field Instrumentation and Methods 26 T h e t ransfer f u n c t i o n of the p a r a l l e l p la te d i p o l e s y s t e m i n the f r equency d o m a i n ( M a t s u i , 1991) is V = J*0™* EI (2.9) l + jaRiC. + Q) w h e r e / is the d i s tance b e t w e e n plates , E the e lec t r ic f i e l d s t r eng th , R the i n p u t i m p e d a n c e of the p re -ampl i f i e r , C, the i n p u t capaci tance of the p reampl i f i e r , a n d Ca the capaci tance of the antenna (i.e. the a m o u n t of d ie lec t r ic c o u p l i n g ) . T h e i n p u t resis tance a n d the to ta l capaci tance of the s y s t e m fo rms a h igh -pas s f i l ter . B e y o n d this R C po le the ou tpu t of the pa ra l l e l plate d i p o l e is flat u n t i l the p re -ampl i f i e r starts to r o l l - o f f (at abou t 5 M H z for the h i g h b a n d w i d t h p r e - a m p s ) . T h e c o r n e r f requency of the h igh-pass f i l ter is about 100 to 200 H z i n m y i m p l e m e n t a t i o n s of the p a r a l l e l p la te d i p o l e (R=100 M Q a n d C=20 pF) . In a d d i t i o n , i t c an be seen f r o m 2.9 that the effective he igh t of the an tenna is s t r ong ly i n f l u e n c e d b y the i n p u t capaci tance Cr T h e h i g h b a n d w i d t h series of p r e - ampl i f i e r s (speci f ica t ions are i n the d i p o l e sect ion) were d e s i g n e d w i t h this a p p l i c a t i o n i n m i n d , a n d have o n l y 8 p F i n p u t capacitance. T h i s is necessary as mos t p rac t i ca l d ipo les for i n - m i n e use have o n l y 5 to 15 p F of capaci tance. H e n c e , the effective he ight of the antenna is u s u a l l y o n l y ha l f o f it 's p h y s i c a l s ize . T o ob ta in a g o o d s ignal- to-noise rat io the plates s h o u l d be p l a c e d far apar t a n d m a d e as large as poss ib le ( A p p e n d i x B.6). F o r reasons of p o r t a b i l i t y m y i m p l e m e n t a t i o n of the an tenna is l i m i t e d to a m a x i m u m d i m e n s i o n of about 2 meters . V e r y l a rge plates are c l u m s y so I se t t led o n plates s i z e d 0.4 m x 0.4 m . T h i s d e s i g n r e su l t ed i n a n effective antenna he ight of about 1.2 m a n d a n antenna capaci tance of 12 p F . A n u n e x p e c t e d a d v a n t a g e of the p a r a l l e l p la te d i p o l e i n u n d e r g r o u n d use is the enhancement of the effective he igh t of the an tenna b y the d ie lec t r i c p rope r t i e s of the earth. I i n i t i a l l y f o u n d that the electr ic f i e l d s t rengths m e a s u r e d b y the p a r a l l e l p la te d i p o l e w e r e cons is ten t ly h i g h e r than the g r o u n d e d d i p o l e measurements , b y a factor of 10 o v e r the 1 to 30 k H z b a n d . A n e x p l a n a t i o n for th i s d i s c r e p a n c y is that i n Chapter 2: Field Instrumentation and Methods 27 u n d e r g r o u n d use the an t enna is i n a a i r -space f o r m e d b y the t u n n e l , a n d that the c o n t i n u i t y of D = eE, c o m b i n e d w i t h the h i g h d ie lec t r i c cons tan t of the s u r r o u n d i n g r o c k , increases the e lec t r ic f i e l d i n the t u n n e l . T h e t u n n e l g e o m e t r y l o w e r s th i s enhancement b y a factor of t w o (for a c y l i n d r i c a l v o i d , K i t t e l , 1986) w h e n m e a s u r i n g across the t u n n e l . N o t e that there is n o increase of the electr ic f i e l d a l o n g the t u n n e l d i r e c t i o n . 2.6 Signal Transmission and Cables There are t w o p r i n c i p a l concerns about cab l ing : E M interference, a n d capac i t ive l o a d i n g of the p re - ampl i f i e r . T r a n s m i t t i n g b r o a d - b a n d s igna l s is d i f f i cu l t because m o s t cables p e r f o r m d i f fe ren t ly for a u d i o verses r a d i o f requencies (Ott,1988). F o r l o w f requency measu remen t s s h i e l d e d t w i s t e d p a i r cables w i t h d i f fe ren t i a l t r a n s m i s s i o n of the s i g n a l has w o r k e d w e l l . The s h i e l d is connected to g r o u n d at o n l y one e n d to p reven t g r o u n d l o o p s . D i f f e r e n t i a l t r a n s m i s s i o n a n d r ecep t ion r e m o v e s c o m m o n - m o d e no i se s u c h as i n d u c t i v e p i c k - u p . O t t (1988) r e c o m m e n d s th is t ype of s y s t e m o v e r c o a x i a l cable for a u d i o frequencies. Unfo r tuna t e ly , the tw i s t ed p a i r cable u s e d i n m y f i e ld t r ia ls does not p e r f o r m w e l l b e y o n d 50 k H z . T h i s m a y bedue to imperfec t ions i n the s h i e l d of the cable (i.e. breaks) a n d a s y m m e t r y i n the t w i s t i n g of the w i r e pa i r s . T h e s h i e l d is m a d e of a l u m i n u m f o i l a n d has 100 % coverage , bu t after cons ide rab l e f i e l d use the s h i e l d is b r o k e n a n d s e g m e n t e d because of cable b e n d i n g a n d u n b e n d i n g d u r i n g use. A n u n i n s u l a t e d d r a i n w i r e p reven t s the s h i e l d f r o m b e c o m i n g d i s c o n n e c t e d , bu t cannot p reven t u n e v e n coverage. In h inds igh t , a c o m b i n a t i o n of b r a i d e d a n d f o i l s h i e l d w o u l d be better for f ie ld use. A t h i g h frequencies c o a x i a l cable w o r k s v e r y w e l l . B e l o w 10 k H z , c o a x i a l cable w o r k s p o o r l y because the resis tance i n the s h i e l d i n h i b i t s the m u t u a l i n d u c t a n c e b e t w e e n centre c o n d u c t o r a n d s h i e l d that n o r m a l l y cancels i n d u c t i v e l y c o u p l e d s igna l s (Ott , 1988). In a d d i t i o n most c o a x i a l cables are o n l y s h i e l d e d b y b r a i d e d w i r e . T h i s t ype of c o v e r i n g results i n a 80 to 90% p h y s i c a l coverage, w h i c h reduces s h i e l d effectiveness. A s Chapter 2: Field Instrumentation and Methods 28 w i t h the t w i s t e d pa i r , a better s o l u t i o n is the use of b r a i d a n d f o i l s h i e l d , w h i c h I have u s e d w i t h the p a r a l l e l plate d i p o l e antenna. W h e n the w a v e l e n g t h of the s i g n a l becomes comparab l e to the l eng th o f the cable t hen the cable behaves l i k e a t r an smi s s ion l ine (Ott, 1988). T o p reven t ref lect ions the cable s h o u l d be t e r m i n a t e d w i t h a resistance e q u a l to the character is t ic i m p e d a n c e . F o r the t w i s t e d s h i e l d e d pa i r i t is about 100 Q , for s t anda rd i n s t rumen ta t i on c o a x i a l cable it is 50 Q . T e r m i n a t i n g the cable places a n extra l o a d o n the p r eamp l i f i e r , w h i c h m u s t d e l i v e r suff icient cur ren t to p r o d u c e the r e q u i r e d vol tage . F o r a 5 v o l t s i g n a l th is can translate i n to 100 m A of cur ren t , w h i c h is b e y o n d the capab i l i t i e s of m o s t p r e - a m p l i f i e r s . In a d d i t i o n , at l o w frequencies the cable acts as a large capaci tor (10 n F for 100 m of s h i e l d e d t w i s t e d pa i r ) . C h a r g i n g a n d d i s c h a r g i n g this capaci tance can r equ i re u p to 100 m A of cur rent for fast s l e w i n g s ignals . These capac i t ive loads can al ter the f requency response of the p re -ampl i f i e r s . T o o v e r c o m e the l o a d i n g p r o b l e m s of cables a buffer s h o u l d be p l a c e d after the p re -ampl i f i e r . 2.7 Amplification and Filtering W h e n a s i g n a l is d i g i t i z e d i t is v e r y i m p o r t a n t to m a x i m i z e the d y n a m i c range ; o the rwise , the s m a l l e r s igna l s w i l l "staircase" because of the l i m i t e d represen ta t ion of the w a v e f o r m . T h e role of a n a m p l i f i e r is to adjust the s i g n a l a m p l i t u d e so that g o o d use of the ava i l ab le d y n a m i c range is made , a n d to a v o i d c o r r u p t i o n f r o m externa l noise sources. In the U . B . C . sys tem (see f ig 2.1), f ixed ga in pre -ampl i f ie r s are u s e d to boost the s igna l s to l eve ls w h e r e the cables w i l l not u n d u l y affect the s igna l - to -no ise ra t io , t hen the s i g n a l is fur ther a m p l i f i e d a n d f i l t e red before d i g i t i z a t i o n b y a b a n k of T e k t r o n i x A M 5 0 2 ampl i f i e r s . Because the E M noise var ies so great ly f r o m site-to-site the last factor of 10-100 of ga in is ach i eved w i t h pos t -ampl i f ie rs , w h i c h are adjustable i n a 1-2-5-10 type g a i n sequence. The A M 5 0 2 is a p l u g - i n m o d u l e , fou r of w h i c h share a spec i a l h o u s i n g ( m o d e l T M 5 1 5 that s u p p l i e s the p o w e r a n d p r o v i d e s a m o u n t i n g s t ruc ture for por tab le use. A n extra c u s t o m b u i l t m o d u l e was u s e d to s u p p l y D C p o w e r d i r ec t l y f r o m batteries; Chapter 2: Field Instrumentation and Methods 29 the u n i t is n o r m a l l y p o w e r e d f rom the ma ins . T o p reven t a l i a sed records , the s igna l s are f i l tered b e l o w the l i m i t set b y the N y q u i s t f requency of the d ig i t i ze r . A m u l t i - p o l e anti-al ias f i l ter w a s a v o i d e d because these filters t end to d i s to r t the pu l se shape, a n d s i m p l e one or t w o po le filters w e r e used . T h e pre-ampl i f i e r s are of f ixed b a n d - w i d t h for s i m p l i c i t y a n d ruggedness , a n d the pos t -ampl i f ie r , a T e k t r o n i x A M 5 0 2 , shapes the f ina l pass-band characterist ics. T h e A M 5 0 2 has b o t h l o w -pass filters (6 d B / octave w i t h 1 M h z , 300 k H z , 100 k H z , 30 k H z , 10 k H z , 3 k H z , 1 k H z , 300 H z settings) a n d high-pass filters (12 d B / O c t a v e w i t h 0.1 H z , 1 H z , 10 H z , 100 H z , 1 k H z , 10 k H z sett ings). I gene ra l ly set the pass -band to 1 k H z to 30 k H z w h e n u s i n g the R C E d i g i t i z e r at 125 k s a m p l e s / s . F o r b r o a d b a n d m e a s u r e m e n t s the A M 5 0 2 u n i t s w e r e bypas sed because of the 1 M H z m a x i m u m b a n d w i d t h of the A M 5 0 2 . T h e s y s t e m b a n d w i d t h s h o u l d be m a x i m i z e d because the e lec t romagnet ic s igna l s f r o m R P E consis t of ve ry b r i e f (<5 ps) pulses , a n d l ow-pas s f i l t e r ing of the s i g n a l b e l o w it 's na tu r a l b a n d w i d t h w i l l l o w e r the s ignal- to-noise of the s igna l . T h e a m p l i t u d e of a pu l se is p r o p o r t i o n a l to the b a n d w i d t h , bu t the R M S a m p l i t u d e of G u a s s i a n or w h i t e no ise (the p r i n c i p a l type of in t r ins ic noise i n m y system) is p r o p o r t i o n a l to the square root of the b a n d w i d t h . H e n c e , the s ignal - to-noise rat io of a b a n d - w i d t h l i m i t e d R P E pu l se falls w i t h the square root of b a n d w i d t h , a n d it is o p t i m a l to preserve the na tu r a l b a n d w i d t h of the pu l se (see A p p e n d i x B.7 for details) . A n i n s i d i o u s p r o b l e m w i t h m a n y l o w noise p re -ampl i f i e r s is the d e m o d u l a t i o n of A M r a d i o s igna l s ( H o r o w i t z a n d H i l l , 1989). T h i s p r o b l e m can p reven t successful surface w o r k un le s s r e m e d i e d . D e m o d u l a t e d A M r a d i o s igna l s are m o s t l y m u s i c a n d vo i ce , therefore, m o s t of the in ter ference is f i l t e r ed b y the 1 k H z h i g h - p a s s o n the post-a m p l i f i e r . H o w e v e r , a suff ic ient a m o u n t of energy r ema ins to g i v e less than o p t i m a l s ignal - to-noise . M o s t l o w - n o i s e amp l i f i e r s d e s i g n e d for a u d i o f requencies suffer f r o m this b e h a v i o r to some degree. Unfo r tuna t e ly , m a n y p o p u l a r l o w - n o i s e o p - a m p s (such as the OP-27) suffer m o r e than w e c a n t y p i c a l l y tolerate ( A p p e n d i x B.8) . T o a v o i d A M Chapter 2: Field Instrumentation and Methods 30 d e m o d u l a t i o n an a m p l i f i e r s h o u l d be chosen for h i g h s lew-ra te pe r fo rmance , o r a f u l l p o w e r b a n d w i d t h of a p p r o x i m a t e l y 1 M H z or better (the t w o are essen t ia l ly the same specif icat ion) . 2.8 A n a l o g to D i g i t a l C o n v e r s i o n R C E D i g i t i z e r M o s t of m y da ta has c o m e f r o m e lec t r ica l s igna ls that are c o n v e r t e d f r o m the i r a n a l o g f o r m to a d i g i t a l representa t ion b y a c o m p u t e r based d i g i t i z e r m a d e b y R C E Elec t ron ics (Santa Barbara , Ca l i fo rn i a ) . T h e ac tua l d i g i t i z e r is a p r i n t e d c i rcu i t b o a r d that fits i n s ide I B M c o m p a t i b l e pe r sona l compute r s . In this case, the c o m p u t e r is a por t ab le m a d e b y E p s o n . T h e advan tage of this sys t em is that the por table c o m p u t e r (and d ig i t i ze r ) can be p o w e r e d b y a 12 V battery, r e m o v i n g the dependence o n m a i n s electr ic i ty . T h i s d i g i t i z e r sy s t em w a s a c q u i r e d b y R . D . R u s s e l l i n the late 1980's to observe re la t ive ly l o w frequency s e i s m o e l e c t r i c p h e n o m e n a , s u c h as p i e z o e l e c t r i c i t y , a n d s o m e aspects o f the R P E p h e n o m e n o n . T h e R C E - b a s e d s y s t e m is not i d e a l for s tud ies of R P E because of it 's l i m i t e d speed a n d buffer dep th , bu t i t was ava i l ab le i n ea r ly 1991 w h e n affordable h i g h speed d ig i t i ze r s were s t i l l o n the d r a w i n g b o a r d a n d the research g r o u p h a d m a n y cables, a m p l i f i e r s a n d fi l ters for the un i t . Therefore , i t w a s a l o g i c a l cho ice at the t ime . I n a d d i t i o n , the un i t ' s po r t ab i l i t y is a great asset i n the u n d e r g r o u n d env i ronmen t . The basic opera t ion of the d i g i t i z e r is that the a n a l o g s i g n a l is fed to a n ana log- to -d ig i t a l conver te r a n d the r e s u l t i n g d i g i t a l representa t ion is s to red i n a m e m o r y buffer o n the boa rd . A n a l o g s ignals are w i r e d to the b o a r d v i a a n interface b o x a n d r i b b o n cable to a spec ia l receptacle o n the boa rd . W e rep laced the s t anda rd cable w i t h a t w i s t e d p a i r type to m i n i m i z e cross- ta lk o n the r i b b o n cable. Sof tware for the host c o m p u t e r is p r o v i d e d so that the b o a r d m a y be opera ted i n manne r s i m i l a r to a d i g i t a l osc i l loscope . K e y features of the R C E d i g i t i z e r are: the a b i l i t y to d i g i t i z e 1 to 16 i ndependen t e lect r ical i npu t s (or channels) ; conver t the ana log s i g n a l to a 12 bi t representa t ion ( i n c l u d i n g s ign) ; Chapter 2: Field Instrumentation and Methods 31 a f a i r l y la rge o n - b o a r d buffer of 64 ksamples ; a v e r y f lex ib le t r i g g e r i n g sys t em; a n d a m a x i m u m s a m p l i n g rate of 1 M s a m p l e / s . In a d d i t i o n , the d i g i t i z e r is r e l a t i ve ly easy to conf igu re v i a c u s t o m p r o g r a m s o n the host compute r . Because the R C E b o a r d has o n l y one ana log- to -d ig i t a l conver te r there is a trade-off i h s a m p l i n g speed a n d i n the n u m b e r of channels used . The b o a r d samples each electr ical channe l i n a sequen t ia l o rde r (e.g. 1-2-3-4-1-2-etc. for four channels) , so the m a x i m u m speed for a g i v e n n u m b e r Of channels is equa l to 1 M s a m p l e / s d i v i d e d b y the n u m b e r of channels ( w h i c h has to be a p o w e r of t w o because of the board ' s architecture). In general , I operate the b o a r d w i t h 8 channels at a rate of 125 k s a m p l e s / s . T h i s se t t ing g ives a 65 m s t ime w i n d o w before the on-board buffer is exhausted , w h i c h is appropr i a t e for u n d e r g r o u n d tests as b y this t ime the se i smic w a v e has t r ave l ed abou t 250 meters a n d is too w e a k to i n d u c e a response. T h e r e su l t ing N y q u i s t f requency is about 62 k H z a n d I gene ra l ly set a 30 k H z h igh-cu t o n the (Tekt ronix) amp l i f i e r s that feed the s igna ls in to the boa rd . W i t h eight channels I a m able to dedica te one to the t r igger s i g n a l , have a three c o m p o n e n t a r ray of magne t i c a n d electr ic sensors , p l u s a spare c h a n n e l for a geophone . Gage Digitizer In late 1992 the g e o p h y s i c a l i n s t r u m e n t a t i o n g r o u p at U . B . C . p u r c h a s e d a h i g h speed d i g i t i z e r f r o m G a g e Elec t ronics (Mont rea l ) , a m o d e l C S 2 2 0 - 1 M , for m y R P E s tudies . The G a g e d i g i t i z e r is s i m i l a r to the R C E b o a r d i n that it needs a n I B M c o m p a t i b l e host, a n d has a la rge o n - b o a r d buffer. P r i n c i p a l differences are i n speed , 40 M s a m p l e s / s vs . 1 M s a m p l e / s , a n 8 bi t A / D , a n d a m a x i m u m of t w o channels . T h i s d i g i t i z e r needs a large buffer (1 M s a m p l e ) because at h i g h sample rates the buffer m e m o r y is q u i c k l y c o n s u m e d . I h a v e u s e d the b o a r d i n the t w o c h a n n e l c o n f i g u r a t i o n at a s a m p l e ra te o f 10 M s a m p l e s / s . W i t h the 1 M s a m p l e buffer this g ives a 51 m s t ime w i n d o w a n d a 5 M H z N y q u i s t f requency. A n o t h e r v e r y usefu l feature is the ab i l i t y to a m p l i f y or attenuate the s i gna l before d i g i t i z a t i o n , g i v i n g s i gna l i n p u t (or d y n a m i c ) ranges of +/-5 V d o w n to + / -Chapter 2: Field Instrumentation and Methods 32 200 m V . W i t h the extra g a i n adjustment the use of ex terna l post ampl i f i e r s , s u c h as the T e k t r o n i x A M 5 0 2 , are unnecessary . T h e 8 b i t r ep resen ta t ion does not leave m u c h r o o m for e r ro r i n se t t ing the g a i n of a m p l i f i c a t i o n before d i g i t i z a t i o n ; a factor of 10 too l i t t l e w i l l r esu l t i n a s t a i rcased recons t ruc t ion of the s i gna l , too m u c h g a i n results i n c l i p p i n g a n d a to ta l loss of s i g n a l s t ruc tu re b e y o n d the d y n a m i c range of the sys t em. U n f o r t u n a t e l y , there is a ce r ta in inhe ren t v a r i a b i l i t y i n a m p l i t u d e of the R P E response . In m y o n l y f i e l d test o f th is d i g i t i z e r I became exper ienced e n o u g h to guess w i t h i n a factor of t w o the r igh t ga in , a n d after a few shots I h a d the sys tem at a n o p t i m u m sett ing: just e n o u g h g a i n for the largest s ignals to p u s h the d y n a m i c range of the sys tem. Chapter 2: Field Instrumentation and Methods 33 U B C I U B C I V U B C V Pass -Band ( k H z ) 0.7 - 300 1.5-3500 4 - 9 5 0 S e n s i t i v i t y ( m V / n T ) 1.67 100 100 N o i s e ( f T / H z 1 / 2 ) 360 12 8 N o i s e p - p i n p T 1600 225 60 D y n a m i c R a n g e n T 6000 100 100 Sensor S i ze 40 c m x 5 c m D i a . 80 c m x 5 c m D i a . 80 c m x 5 c m D i a . Table 2.1 Charac ter i s t ics of the magne t ic sensors u s e d i n the f i e ld t r ials . T - B o x H B W Pass -Band ( k H z ) 0 .1 -30 1 - 5000 Input Impedance ( M Q ) 4 100 G a i n 28 25 N o i s e ( n V / H z 1 / 2 ) 90 8 N o i s e p -p i n u,V 160 160 Table 2.2 P r e - a m p l i f i e r charac te r i s t i c s . These p r e - a m p l i f i e r s w e r e u s e d to a m p l i f y a n d buffer the measurements of electric po ten t ia l . C H A P T E R 3 INTERPRETATION A N D P R O C E S S I N G M E T H O D S 3.1 I n t r o d u c t i o n P r i o r to this thesis the o n l y i n f o r m a t i o n ava i l ab le o n the in t e rp re t ion of R P E da t a was a b r i e f o u t l i n e f r o m the patent of Sobo lev et a l . (1986). M o s t of t he i r p u b l i s h e d w o r k p r o v i d e s de ta i l s abou t the E M s igna tu re a n d assoc ia ted p h e n o m e n a of R P E . S o m e u s e f u l h in t s abou t l o c a t i n g targets w i t h se i smoe lec t r i c m e t h o d s c a n be f o u n d i n the m o n o g r a p h abou t p iezoe lec t r i c m e t h o d s b y S o b o l e v a n d D e m i n (1980); i t conta ins the b o u n d a r y d e l i n e a t i o n m e t h o d (descr ibed sect ion 3.5). T h i s chapter a d d s subs tan t ia l ly to the cu r ren t p o o l of k n o w l e d g e b y g i v i n g de ta i l s about a su i te of t echn iques u s e d to ana lyze a n d interpret data f r o m the f i e ld tr ials desc r ibed i n C h a p t e r 4. The basic steps that I use to ana lyze the results of a s u r v e y are: (1) p lo t each shot record ; (2) p i c k the R P E events w i t h i n each record ; (3) const ruct a h i s t o g r a m of the n u m b e r o f events verses a r r i v a l t ime , a n d obta in a s tat is t ical measure of the su rvey ' s conf idence i n loca t ing su lph ides ; (4) p lo t the event da ta f r o m the w h o l e s u r v e y i n a scatter-plot format ( a r r i v a l t i m e vs . o f f s e t / s h o t p o i n t loca t ion) a n d l o o k for pat terns; (5) t r y to de l inea te g e o l o g i c a l features or p r o v i d e a t o m o g r a p h i c i m a g e f r o m the pa t te rn of events . T h i s m e t h o d o l o g y has e v o l v e d ove r t i m e w i t h the advances i n da ta co l l e c t i on me thods , a n d w i t h greater u n d e r s t a n d i n g of the R P E p h e n o m e n o n . A n a l y s i s o f the earl iest f i e l d t r i a l , at the S u l l i v a n M i n e , w a s f a i r ly c r u d e a n d p r i n c i p a l l y i n v o l v e d steps 1,2, a n d 4 w i t h v e r y l i t t le in te rpre ta t ion . In contrast , da ta co l l ec ted f r o m the L y n x M i n e w a s p rocessed a n d a n a l y z e d u s i n g a l l of the above m e n t i o n e d steps because of the a m o u n t a n d q u a l i t y of da ta obta ined. 34 Chapter 3: Interpretation and Processing Methods 35 3.2 Spectrograms W i t h the a r r i v a l of the G a g e d i g i t i z e r a p r o b l e m arose, the p r o b l e m of p l o t t i n g a n d a n a l y z i n g s i gna l traces w i t h over 500 000 points . There are too m a n y po in t s to p lo t as an a m p l i t u d e vs. t ime trace; even p l o t t i n g at 100 po in t s per m m w o u l d resul t i n a trace 5 m l o n g . T h e p r i n c i p a l pu rpose of p lo t t i ng the shot r ecord is to present the da ta so that the R P E s ignals a n d their a r r i v a l t imes are c lea r ly observable. Segmen t ing the da ta (e.g. 2000 segments of 250 poin ts ) a n d p l o t t i n g the peak- to-peak v a l u e o f each segment p r o v i d e s a poss ib le s o l u t i o n , bu t d i s c a r d i n g mos t of the data i n this w a y negates the benefi ts of a h i g h - b a n d w i d t h d ig i t i ze r . O n e of the p r i n c i p a l benefits of the G a g e d i g i t i z e r is the ab i l i t y to capture the fu l l p o w e r s p e c t r u m of R P E s ignals (1-5000 k H z ) . A s o l u t i o n to o b t a i n i n g b o t h h i g h r e s o l u t i o n t i m e a n d f requency i n f o r m a t i o n is to p lo t a s p e c t r o g r a m of the data. A spec t rog ram plots the e v o l u t i o n of the p o w e r s p e c t r u m w i t h t ime (Cohen , 1995). It is a n a m p l i t u d e vs . f requency vs . t ime plot . T h e a m p l i t u d e is t y p i c a l l y represen ted b y a c o l o u r or is c o n t o u r e d , a n d the o ther t w o quan t i t i e s f o r m a n o r t h o g o n a l co -o rd ina te sy s t em. A n e x a m p l e is s h o w n i n F i g u r e 3.1. There are m a n y s o p h i s t i c a t e d w a y s to process the da ta so that the f r e q u e n c y / a m p l i t u d e e s t ima t ion is o p t i m a l ( C o h e n , 1995), bu t I use the m o s t bas ic f o r m : a m o v i n g w i n d o w or s e g m e n t w i t h a w i n d o w e d p e r i o d o g r a m est imate of the spec t rum. The trace is d i v i d e d in to 512 segments of 1024 p o i n t s , a n d a spec t r a l es t imate is m a d e o n each of the segmen t s . E a c h segmen t represents a 0.1 ms t ime w i n d o w , w h i c h g ives prec ise t i m i n g o f a n event (the P - w a v e t rave ls abou t 0.6 meters i n th is t ime) . Ra the r t h a n c o m p u t i n g a 1024 p o i n t F F T , I p r o d u c e 8 o v e r l a p p i n g segments of 256 po in t s a n d c o m p u t e eight 256 po in t F F T ' s . F r o m the 256 p o i n t F F T ' s an average p o w e r s p e c t r u m is cons t ruc ted . F i g u r e 3.2 i l lus t ra tes the w a y these sub-segments are f o r m e d f r o m the 1024 po in t segments o f data . T h e start of the da ta is z e r o - p a d d e d because of the o v e r l a p p i n g of sub-segments . T h i s scheme of a n ave rage s p e c t r a l es t imate sacr i f ices r e s o l u t i o n i n f r equency for a r e d u c e d spec t r a l Chapter 3: Interpretation and Processing Methods 36 Chapter 3: Interpretation and Processing Methods 37 var iance (Press et a l . , 1989). A P a r z e n w i n d o w is used for the F F T calcula t ions(Press et a l , 1989). I u s e d the Press et a l . ( N u m e r i c a l recipes, 1989) spectral est imate scheme to reduce the graininess of the noise. R P E pulses do not benefit s ign i f ican t ly f rom increased f requency r e so lu t ion , but the contrast be tween s i g n a l a n d noise can i m p r o v e m a r k e d l y w i t h a n ave raged spec t ra l est imate. In a d d i t i o n , the smoother est imate a l l o w s the r e m o v a l of c o n t i n u o u s ( in t ime) bands of no i se i n the s p e c t r o g r a m by s u b t r a c t i n g a reference spec t rum. T h e reference s p e c t r u m is gathered f rom a noise r e co rd ing (no shot) ga thered i n the f ie ld . It can a lso be ob ta ined f r o m sections of the shot r ecord where there is no apparent s i gna l , bu t I u sed the fo rmer m e t h o d because it w o r k s w e l l a n d it is a l i t t le easier to i m p l e m e n t . The re is s t i l l s ome va r i ance i n the a m p l i t u d e of the spec t r a l est imate so I n o r m a l l y subtract a l i t t le m o r e than necessary (about 5% m o r e than the reference) to c o m p l e t e l y e l imina te the bands . A cost of r e m o v i n g the b a n d s b y over -sub t r ac t i ng is that some "hole b u r n i n g " i n the pu l se s p e c t r u m can occur for w e a k signals Time Series 1024 pts FFT 1 FFT 3 FFT 5 FFT 7 Segment n-1 Segment n Segment n+1 FFT 2 FFT 4 FFT 6 FFT 8 256 pts Figure 3.2 A n i l l u s t a r t i on of the segmenta t ion scheme used i n d i v i d i n g the t ime-series da ta in to o v e r l a p p i n g estimates of the a m p l i t u d e s p e c t r u m vs. t ime . Chapter 3: Interpretation and Processing Methods 38 Figure 3.3 A comparison between a spectrogram and an amplitude vs. time trace. The signal is clearly observable in the spectrogram, but the trace shows no evidence of a signal. Chapter 3: Interpretation and Processing Methods 39 Spec t rograms have o n l y been a p p l i e d to data f r o m one f i e ld t r i a l (see C h a p t e r 4, L y n x LTI), bu t the h i g h b a n d w i d t h a c q u i s i t i o n s y s t e m p r o v i d e d a s t o u n d i n g i m p r o v e m e n t s ove r the p r e v i o u s s y s t e m as j u d g e d b y s ide -by-s ide c o m p a r i s o n w i t h the n e w sys t em. T h e a b i l i t y to d i s t i n g u i s h s ignals f r o m noise i m p r o v e d d r ama t i ca l l y ; an e x a m p l e is s h o w n i n F i g u r e 3.3. A large part of this i m p r o v e m e n t is d u e to the ab i l i t y to see concentra t ions of s i g n a l e n e r g y i n c e r t a i n f r e q u e n c y b a n d s w i t h o u t the p r i o r k n o w l e d g e o f w h i c h f requency bands w i l l be mos t useful . There are other benefits o f u s i n g t ime-f requency ana lys i s . F o r example , n a r r o w - b a n d s ignals u s u a l l y need to be r e m o v e d i n o rde r to see l o w a m p l i t u d e a r r i v a l s o n a m p l i t u d e - v s - t i m e traces, bu t i n a s p e c t r o g r a m p l o t the n a r r o w - b a n d s igna l s are v i s u a l l y d i s t i n g u i s h a b l e f r o m the b r o a d - b a n d pu l se s of R P E ( h o r i z o n t a l l ines verses v e r t i c a l l ines) . A s m e n t i o n e d p r e v i o u s l y , these n a r r o w b a n d s ignals can be r e m o v e d effectively by subt rac t ing a noise template . If the spectra l peaks of R P E s ignals are d i s t i nc t ly re la ted to m i n e r a l c o m p o s i t i o n (Sobolev et a l , 1986a) then it m a y be poss ib le to note c o m p o s i t i o n a l differences w i t h i n the o rebody f r o m changes i n the s p e c t r u m as the se i smic w a v e t ravels t h o u g h the o r e b o d y . M a n y s u l p h i d e orebodies have s t r ingers o r fr inge zones that are different i n c o m p o s i t i o n f r o m the i n t e r i o r of the o r e b o d y . T h i s concep t has not been tes ted, bu t i t w o u l d be o f i m m e n s e v a l u e for in te rpre ta t ion purposes . 3.3 Picking Events E a c h shot f r o m a f i e ld t r i a l has a r eco rd that conta ins the response of va r i ous sensors to effects of the blast. T y p i c a l l y 10-20 ms of pre-blast a n d 40-50 ms post blast da ta is co l lec ted o n each record . A record contains the response f r o m at least t w o to s ix E M sensors. E a c h of these records is p lo t t ed a n d events that m i g h t be due to R P E are p i c k e d f r o m the E M sensor traces o n the p lo t . T h e E M s igna tu re of R P E is a b r i e f pu l se , o r g r o u p of pu lses of a n o m a l o u s a m p l i t u d e (Sobo lev et a l . , 1982a). O t h e r t han th is g e n e r a l d e s c r i p t i o n there are f e w u s e f u l Chapter 3: Interpretation and Processing Methods 40 charac te r i s t ics to h e l p i n the sea rch for R P E s i g n a l s w i t h i n shot r ecords . E l e c t r i c a l d i scha rges a n d p o w e r s w i t c h i n g t ransients share s i m i l a r character is t ics , a n d are often i n d i s t i n g u i s h a b l e f r o m R P E . H o w e v e r , s i gna l s f r o m the la t ter a re of ten r e l a t ed to p e r i o d i c s i gna l s , s u c h as the p o w e r g r i d , a n d m a y be i d e n t i f i e d b y the i r p e r i o d i c occurrence . T h i s was the case w i t h records ob ta ined f r o m the S u l l i v a n M i n e , a n d some f r o m the L y n x M i n e , w h i c h con ta ined transients i n sync w i t h the 120 V , 60 H z , m a i n s p o w e r . I n genera l , a n y short d u r a t i o n p u l s e f o u n d i n the da ta f o l l o w i n g the blast is cons ide red to be p o s s i b l y d u e to R P E , unless there is an o b v i o u s a l ternat ive exp lana t ion . A n o c c a s i o n a l e x c e p t i o n to th is r u l e is the case w h e r e the pre -b las t da t a con ta ins s ign i f ican t a m o u n t s of " R P E - l i k e " s ignals ; i n this case the r e c o r d is d i s c a r d e d because a na tu r a l or m a n - m a d e d i s tu rbance is i n progress a n d the post-blast s igna ls are p r o b a b l y d u e to the same process. O n c e a s i g n a l is i den t i f i ed w h a t are the impor t an t character is t ics? F o r e x a m p l e , does a b i g g e r s i g n a l de se rve m o r e a t t en t ion t h a n a s m a l l e r one? Because the m e c h a n i s m respons ib le for R P E is not cer ta in , a n y in te rpre ta t ion based u p o n the peak a m p l i t u d e of the s i g n a l is a gamble . Fu r the rmore , peak a m p l i t u d e s are p o o r l y r e p r o d u c e d i n repeat shots. Therefore, I t end to place l i t t le impor t ance u p o n the peak a m p l i t u d e o f the s igna l , a n d g i v e each i d e n t i f i e d s i g n a l an e q u a l w e i g h t i n g . T h e a r r i v a l t ime is a n i m p o r t a n t charac te r i s t i c because i t g ives the d i s t ance the s e i smic w a v e has t r a v e l e d f r o m the sho tpo in t . F u r t h e r m o r e , the a r r i v a l t ime is genera l ly r e p r o d u c i b l e , w i t h i n the l i m i t s set b y the d u r a t i o n of m a x i m u m fo rc ing of the se i smic w a v e . T h e p o w e r s p e c t r u m of the s i g n a l m a y be another usefu l character is t ic (Sobolev et a l . , 1986). If the type of m i n e r a l s t r o n g l y inf luences the R P E p o w e r s p e c t r u m then the v a r i o u s m i n e r a l o g i c a l differences w i t h i n the o rebody w o u l d p r o d u c e different classes of s igna l . A s m e n t i o n e d p r e v i o u s l y , the basic process of p i c k i n g events is to l o o k for a n o m a l o u s pu l se s amongs t p o w e r - g r i d - r e l a t e d sp ikes a n d other interference. Spec t rog rams , h i g h b a n d w i d t h r e c o r d i n g , a n d f iber-opt ic blast sens ing have m a d e this job m u c h easier than Chapter 3: Interpretation and Processing Methods 41 it w a s o r i g i n a l l y . H o w e v e r , de f in ing a n event is not a l w a y s easy. R P E s igna ls w i l l often appea r as burs ts of s p i k e s / p u l s e s that con t i nue for m a n y m i l l i s e c o n d s . S o b o l e v et a l . (1986) c l a i m that this is an i n d i c a t i o n of the d i m e n s i o n s of the o r e b o d y ( m u l t i p l e s igna ls as the s e i s m i c w a v e s w e e p s t h rough ) . Therefore , i t is i m p o r t a n t for i n t e r p r e t a t i o n purposes to represent the d u r a t i o n of this ac t iv i ty . S h o u l d a burs t of pu l ses be w e i g h t e d m o r e h e a v i l y t h a n a so l i t a ry pu l se , a n d h o w s h o u l d the w e i g h t i n g be done? If each r e s o l v e d pu l se is g i v e n equa l w e i g h t then one or t w o shot records can d o m i n a t e further i n t e rp re t a t ion because one burs t m a y consis t of h u n d r e d s o f pu l ses . In a d d i t i o n , the pu l ses w i t h i n a burs t m a y not be r e a d i l y r e s o l v e d . A f t e r c o n s i d e r i n g these issues , I d e c i d e d to def ine an event as any pu lse or g r o u p of pulses w i t h i n 0.3 m s to 1 m s of the t i m e of the larges t pu l se w i t h i n the g r o u p . F o r e x a m p l e , a l o n g burs t of pu l se s is represented as a sequence of events spaced e q u a l l y apart i n t ime (say 0.3 ms) . T h e t ime i n t e r v a l of 0.3-1 m s is s o m e w h a t a rb i t r a ry ; h o w e v e r , th i s t i m e w i n d o w reflects the accuracy i n r e p r o d u c i n g a r r i v a l t imes i n repeat exper iments . T h i s arbi t rar iness can be r e m e d i e d b y d e f i n i n g a start a n d e n d t ime to a n event , bu t this s o l u t i o n r equ i r e s subs t an t i a l changes i n s o m e of the m e t h o d s a n d a l g o r i t h m s to be desc r ibed later i n this chapter. These a lgo r i thms were de s igned for s ing le events because th is w a s the p r e d o m i n a n t t y p e o f s i g n a l seen i n r e c o r d s p r o d u c e d b y the l o w e r b a n d w i d t h R C E d ig i t i z e r , w h i c h has been the w o r k h o r s e of m y f i e ld t r ials . Spec t rograms of da ta a c q u i r e d b y the (newer) G a g e d i g i t i z e r s h o w pe r iods of a c t i v i t y i n nea r ly eve ry r ecord , a n d an R P E event s h o u l d be de f ined w i t h a start a n d end t ime to ga in m a x i m u m benefit f r o m this type of h i g h b a n d w i d t h sys tem. 3.4 Presentation of Raw Survey Data It is s t a n d a r d pract ice by m a n y geophys ic i s t s to present s e i smic da ta b y p l o t t i n g m a n y a m p l i t u d e - v s - t i m e traces next to each other (i.e. the wiggle- t race format a n d its var iants) . Trace p o s i t i o n o n the p lo t m a y d e p e n d u p o n shot- to-receiver d i s tance (a gather), o r shot to rece ive r m i d p o i n t p o s i t i o n o n the s u r v e y l ine . T h i s type of p r e sen t a t i on has been Chapter 3: Interpretation and Processing Methods 42 e n o r m o u s l y success fu l s ince the 1950's i n i n t e r p r e t i n g 2 - D s u r v e y s to p i c k g e o l o g i c a l features. G r o u n d penet ra t ing radar da ta is also presented i n the same manner . G i v e n the success of "wiggle- t race" sys t em of presenta t ion , a n d the s i m i l a r i t y b e t w e e n se ismoelect r ic me thods a n d se ismic methods , it w o u l d appear to be a g o o d cand ida te for d i s p l a y i n g f i e l d data . H o w e v e r , I f i n d that "wiggle - t race" p lo t s are i n a p p r o p r i a t e for p re sen t ing R P E s u r v e y results . F i r s t l y , the r a w or p rocessed a m p l i t u d e s o n the se i smic a n d r ada r traces are representat ive of a p h y s i c a l measurement of a w e l l de f ined p roper ty . F o r e x a m p l e , i n the se i smic ref lec t ion m e t h o d a m p l i t u d e changes represent a n ab rup t change of m a t e r i a l p rope r t i e s w h e r e the a m p l i t u d e is p r o p o r t i o n a l to the contras t i n impedance . A s p r e v i o u s l y m e n t i o n e d (see sect ion 3.2, P i c k i n g Even t s ) , w e have n o r ea l basis w i t h w h i c h to j udge R P E a m p l i t u d e s . "Wiggle- t race" p lo t s are d e s i g n e d to d r a w at tent ion to a m p l i t u d e anomal ies be tween traces, a p r o p e r t y that R P E presenta t ion c o u l d d o w i t h o u t g i v e n the great va r i a t i on i n a m p l i t u d e of R P E a r r iva l s . In shor t , I f i n d that "wiggle- t race" p lo t s are ineffect ive because they t end to e m p h a s i z e u n k n o w n proper t ies i n R P E data . I prefer to use a s y m b o l to represent an a n o m a l o u s pu l se , w h i c h I refer to as a n event. W i t h th is s cheme each event has e q u a l i m p o r t a n c e , a n d i t is the r e l a t i ve t i m i n g of events that domina t e s the presenta t ion . D a t a f r o m the ear l ie r f i e l d t r ia ls ( S u l l i v a n a n d M o b r u n , see C h a p t e r 4) was co r rup ted b y blast E M , a n d in te rpre t ing the da ta b y a n a l y z i n g "wiggle- t race" plots w a s a lmost imposs ib l e . It p r o v e d to be easier to p i c k the R P E events f r o m each shot r e c o r d a n d then interpret f r o m a scatter-plot (o f f se t / sho tpo in t vs. t ime) . T h i s t echn ique w o r k s w e l l w i t h data f r o m shot loca t ions that are f i r ed m o r e than once (to i m p r o v e the chances of r e c e i v i n g s igna l s ) because R P E da t a canno t be u s e f u l l y s tacked d u e to a m p l i t u d e va r i a t i on a n d jitter of the n a r r o w pulses . E x a m p l e s of b o t h the w igg le - type a n d scatter-plot type of representat ion can be seen i n F i g u r e 3.4. Chapter 3: Interpretation and Processing Methods 43 CP - Q E o CL *-> O oo 10 15 a n o • • D a • • • • • • • n • • • • • ,na a • a ta na a a o El a • • D O B a • D • a • • • o • n o > a a o • • • 10 15 20 Time (ms) 25 S P 2 S P 3 S P 4 S P 5 S P 6 SP7 S P 8 S P 9 S P 1 0 SP11 f* >'*^n'W»<tr~+'~.^" " —~ - . • ' • 12 16 Time ms Figu re 3.4 E x a m p l e s of scatter (above) a n d wiggle- t race plots (below). These are f r o m the same data-set, a n expe r i men t at the M o b r u n M i n e . G r e y areas indica te where I p i c k e d s ignals . N o t e that the interpreta t ions are s l i g h t l y different i n the p lacement of the second a n d t h i r d g rey areas. The scatterplot i nc ludes i n f o r m a t i o n f r o m repeat shots. Chapter 3: Interpretation and Processing Methods 44 3.5 Statistical Analysis Sta t i s t i ca l a n a l y s i s of the data-set has been of i m m e n s e v a l u e i n r e c o v e r i n g u s e f u l i n f o r m a t i o n f r o m su rveys p l a g u e d w i t h excessive E M noise a n d interference. It can a lso serve as a q u a l i t y con t ro l measure for a su rvey . W h e n p i c k i n g pre-blast events d i g i t i z e d f r o m the l o w e r b a n d w i d t h R C E s y s t e m I f o u n d the presence of r a n d o m l y o c c u r r i n g sp ikes that are i n d i s t i n g u i s h a b l e f r o m R P E ; b o t h types of events appear as the i m p u l s e response of the sys tem. A lot of m a n - m a d e noise has th is character , a n d so d o nearby e lec t r ica l d i scharges (e.g. t h u n d e r s t o r m s ) . T h i s r a n d o m b a c k g r o u n d can obscure s e i s m i c a l l y i n d u c e d p h e n o m e n a . F o r e x a m p l e , b o t h surface f i e ld tr ials i n Q u e e n s l a n d , A u s t r a l i a (see C h a p t e r 4, C e n t u r y ) , w e r e h a m p e r e d b y large a m o u n t s of a tmospher i c noise (spherics) a n d an u n r e s p o n s i v e o rebody . Sta t is t ica l ana lys i s was the o n l y usable too l i n these cases. If the b a c k g r o u n d of no i se sp ikes d i s p l a y s no d i s ce rn ib l e pa t t e rn o r p r e d i c t a b i l i t y i n a r r i v a l t ime then the noise can be treated as a P o i s s o n process ( M o n t g o m e r y a n d H i n e s , 1980; B a r l o w , 1994). The P o i s s o n p robab i l i t y d i s t r i b u t i o n is P ( x ) = ^ _ i _ and a = Xt (3.1) w h e r e A is the rate of events pe r un i t t ime, t is the t i m e - i n t e r v a l i n w h i c h events are coun ted , a n d x is the n u m b e r of events i n the t ime i n t e r v a l t . M e a n a n d va r i ance of the P o i s s o n d i s t r i b u t i o n are the same, a. As a, the e x p e c t e d n u m b e r o f events , increases the P o i s s o n d i s t r i b u t i o n a p p r o a c h e s the n o r m a l d i s t r i b u t i o n . W i t h th i s d e s c r i p t i o n of the noise characteris t ics a m o d e l of the d i s t r i b u t i o n o f events w i t h i n shot records can be cons t ruc ted , a n d c o m p a r e d w i t h the ac tua l d i s t r i b u t i o n p r o d u c e d b y the f i e l d da ta . If the there is c o n s i d e r a b l e d i s p a r i t y b e t w e e n the t w o t h e n th i s is a n i n d i c a t i o n that the dataset conta ins events that are not r a n d o m l y generated. Chapter 3: Interpretation and Processing Methods 45 30 i . 25 -20 - | 1 I 15 ] Q f i.i g I.' I,I I,I i.i g g g g q -1 5 -10 -5 0 5 10 15 20 25 30 35 40 45 Time After the Blast (ms) Figure 3.5 A example of a h i s togram. O n c e the events have been p i c k e d f rom the v a r i o u s shot records a h i s t o g r a m of the n u m b e r of p i c k e d events per t ime i n t e r v a l after the blast is p lo t t ed . N o t e that it is i m p o r t a n t to p i c k b o t h pre-blas t a n d post -blas t events for this p u r p o s e . T h e p i c k e d events of a s u r v e y are g r o u p e d a c c o r d i n g to a r r i v a l t ime after the blast a n d coun ted . F i g u r e 3.5 d i s p l a y s a n e x a m p l e f r o m the L y n x M i n e . I u s u a l l y b i n the da ta in to 5 ms t ime in te rva l s . B i n s w i t h nega t ive i n t e r v a l l i m i t s represent pre-blas t events . If the h i s tog ram is re la t ive ly flat then it is an i n d i c a t i o n that R P E has not been i n d u c e d , or the response is not recognizable . H o w e v e r , i f the h i s tog ram s h o w s large peaks after the blast then it is p robab le that there is an o r e b o d y nearby. T h e p o s i t i o n of the peak i n the h i s tog ram indicates the a p p r o x i m a t e dis tance of the target f rom the shotpoin ts . T h i s is v e r y use fu l i n f o r m a t i o n w h e n no other o b v i o u s pat terns emerge f rom the da ta ; it indicates the presence a n d app rox ima te dis tance of an orebody. Of ten it is in tu i t ive ly obv ious , as i n fig 3.5, that R P E responses have been p r o d u c e d , but somet imes the peak is s m a l l o r the noise is ve ry h i g h , a n d a p rocedu re to n u m e r i c a l l y evaluate the l i k e l i h o o d that a peak i n the h i s t o g r a m is due to a f luc tua t ion i n the noise Chapter 3: Interpretation and Processing Methods 46 is needed . H y p o t h e s i s test ing (Hines a n d M o n t g o m e r y , 1980) p r o v i d e s s u c h a p rocedure . It w o r k s b y cons t ruc t ing a hypothes i s about the data-set, then tests are p e r f o r m e d o n the da ta to see i f th is hypo thes i s can be rejected w i t h reasonable conf idence . T h e hypo thes i s to be tested is that a n y peak i n the h i s t o g r a m is a n a t u r a l f l u c t u a t i o n o f the r a n d o m process genera t ing the b a c k g r o u n d noise . If the hypo thes i s is t rue t hen pre-blas t a n d pos t -b l a s t b i n s s h o u l d c o n t a i n a p p r o x i m a t e l y the s a m e c o u n t w i t h i n s t a t i s t i c a l uncer ta in t i t i es . A nearby o r e b o d y w o u l d raise the coun t of the post -blas t b i n b e y o n d reasonable expectat ions, a n d n u m e r i c a l tests w o u l d s h o w the hypo thes i s to be false. There are t w o types of e r ror that m a y occur w h e n hypo thes i s test ing: t y p e I a n d type II ( M o n t g o m e r y a n d H i n e s , 1980). A type I error occurs w h e n the hypo thes i s is i n fact true, bu t the test rejects the hypothes is . T y p e II errors are the converse: the hypo thes i s is false a n d the test accepts it. The prac t ica l consequence of a type I e r ror is that a false target is iden t i f i ed . A type II error w o u l d result i n a real target b e i n g o v e r l o o k e d . B o t h errors can be cos t ly , a n d need to be s m a l l for the tests to have a n y s ignif icance. T y p e I errors can be con t ro l l ed b y inc reas ing the s a m p l i n g (i.e. shoot more) . T h e other type of er ror depends u p o n b o t h target contras t a n d s a m p l e s ize , therefore, a s u r v e y is d e s i g n e d to detect targets that p r o v i d e a sufficient response. A n o t h e r p r o p e r t y of this type of tes t ing is that a "s t rong" c o n c l u s i o n about the existence of an a n o m a l y can be m a d e ; h o w e v e r , w h e n the test suppor t s the hypo thes i s then it is "weak" c o n c l u s i o n , a n d w e c a n o n l y c o n c l u d e that w e have fa i led to ident i fy a target. T h e T-test is a hypo thes i s tes t ing m e t h o d w h i c h can be u s e d to c o m p a r e means f r o m t w o different popu la t i ons , w h i c h are n o r m a l l y d i s t r ibu ted . In m y case I w a n t to c o m p a r e the m e a n rate of events before a n d after the blast at cer ta in t ime in te rva ls . T h e f o r m of the t-statistic is where (AZ1-1)512 + (» 2-1)5 2 2 rtj + n2 - 2 (3.2) Chapter 3: Interpretation and Processing Methods 47 w h e r e the subsc r ip t s denote the p o p u l a t i o n (pre- a n d post-blas t ) , x the m e a n , 5 the var iance , a n d n the n u m b e r of samples f r o m each p o p u l a t i o n . E v a l u a t i n g equa t ion 3.2 g i v e s a n u m b e r e x p r e s s i n g the a m o u n t of d e v i a n c e b e t w e e n the m e a n s of the t w o popu la t i ons . T h e t-statistic is ana logous to the d is tance f r o m the expected m e a n i n un i t s of s t a n d a r d d e v i a t i o n . T h i s n u m b e r is u s e d w i t h Student 's t -d i s t r i bu t ion to calcula te the p r o b a b i l i t y that the hypothes i s is false (tabulated i n mos t statistics texts). T h e use of a n o r m a l a p p r o x i m a t i o n to a P o i s s o n process is reasonable i f the n u m b e r of events i n each b i n is greater t han o r e q u a l to 5 ( B a r l o w , 1994), a n d is accura te for b a c k g r o u n d coun ts greater than 10 events per b i n . T h e n o r m a l d i s t r i b u t i o n a l l o w s the p o s s i b i l i t y of nega t ive n u m b e r s i n the coun t n u m b e r , w h i c h is not poss ib le , bu t as the expec ted n u m b e r of coun t s becomes greater the p o r t i o n of the n o r m a l d i s t r i b u t i o n g i v i n g nega t ive coun ts becomes v a n i s h i n g l y s m a l l . T h e s i tua t ion of c o m p a r i n g events f r o m different b ins is ana logous i f w e subst i tu te the f o l l o w i n g i n to equa t ion 3.2: * = Qt <7( is the number o f events i n b in i Sf = qi where m is the number o f records n{ = m N o t e that for P o i s s o n d i s t r i bu t ions the va r iance is e q u a l to the m e a n . T h i s subs t i tu t ion results i n the f o l l o w i n g express ion for the t-statistic ma, - mq0 t= I ' (3.3) ^]mq} + mq2 li a n e w va r i ab le T is de f ined as the total n u m b e r of events a p p e a r i n g i n a cer ta in b i n p o s i t i o n (e.g. 10-15 m s after the blast) s u m m e d over a l l the records then 3.3 becomes Tx-T2 a n d w e n o w have a m e t h o d to c o m p a r e pre-blast a n d post-blast b i n counts . T o use the Chapter 3: Interpretation and Processing Methods 48 tables i n v a r i o u s books w e need to use the r igh t cu rve , w h i c h depends o n the n u m b e r of degrees of f r e e d o m . T h i s n u m b e r is the n u m b e r of r a n d o m v a r i a b l e s m i n u s t w o , T1+T2 — 2 ( H i n e s a n d M o n t g o m e r y , 1980). W i t h v e r y la rge degrees of f r e e d o m the t-d i s t r i b u t i o n is near ly i den t i ca l to the n o r m a l c u r v e a n d the t-statistic is the d is tance f r o m the m e a n i n s t anda rd dev ia t ions (Hines a n d M o n t g o m e r y , 1980). A n i m p r o v e m e n t i n o u r ana lys i s can be m a d e i f the pre-blast b ins are g r o u p e d together a n d equa t ion 3.4 is w e i g h t e d for the greater pre-blast t ime in te rva l . U = f f 1 " * 1 ? ' 2 ' (3.5) T h i s f o r m u l a is no t s t r i c t l y a t-s tat is t ic ( H i n e s a n d M o n t g o m e r y , 1980), b u t i t i s suf f ic ien t ly close for mos t purposes . T h e t ime i n t e r v a l c o v e r e d b y the g r o u p of b ins for each p o p u l a t i o n is d e n o t e d by k. F o r example , a coun t of events 0 to 10 m s before the blast = 1 0 ms ) c o u l d be c o m p a r e d to the n u m b e r of events 20 to 25 m s after the blast(k2 = 5 ms) . E q u a t i o n 3.4 a n d 3.5 are v a l i d for f i n d i n g the p r o b a b i l i t y that a pa r t i cu l a r b i n w i l l reach it 's v a l u e f r o m a n a t u r a l f l uc tua t ion ; that is , they have s ign i f i cance o n l y for the t ime i n t e r v a l tested. T h i s is not equ iva len t to s cann ing across the h i s t o g r a m a n d tes t ing u n t i l a s ign i f i can t ly dev i an t peak is f o u n d ( B a r l o w , 1989). T h e correct in te rpre ta t ion o f a peak ( w i t h j counts) w h e n scann ing a h i s tog ram (of n post blast bins) for s ign i f icant peaks is / ^ ( h i s t o g r a m has a b in > j) = 1 - (1 - Prob{bm > j))n (3.6) E q u a t i o n 3.6 can be t rans la ted as f o l l o w s : the p r o b a b i l i t y that a h i s t o g r a m has a p e a k greater than is equa l to u n i t y m i n u s the p r o b a b i l i t y that none of the peaks w i l l exceed j-1. T h u s , the p r o b a b i l i t y that a h i s t o g r a m conta ins a no i se f l u c t u a t i o n greater t han o r e q u a l to j is h i g h e r than that for a s ing le t ime i n t e r v a l . T h e v a l u e of Prob(bin > j) is f o u n d f r o m c u m u l a t i v e p robab i l i t y tables or c o m p u t e d once the t-statistic is ca lcu la ted b y equa t ion 3.4 or 3.5 is obta ined. Chapter 3: Interpretation and Processing Methods 49 In pract ice , I cons t ruc t a h i s t o g r a m f r o m events p i c k e d f r o m the records , a n d v i s u a l l y inspect for peaks. If a post-blast b i n appears to be s ign i f i can t ly greater than pre-blast b ins then equa t i on 3.4 or 3.5 i s u s e d w i t h the event coun t s f r o m the d e v i a n t a n d pre-blas t b ins to ob ta in a t-statistic (t). The t-statistic is u s e d to p r o v i d e a p r o b a b i l i t y that the b ins u n d e r c o m p a r i s o n share the same m e a n ( H i n e s a n d M o n t g o m e r y , 1980). If I a m not expec t ing to see a peak i n a pa r t i cu la r b i n p o s i t i o n then equa t ion 3.6 is u s e d to l o w e r the s igni f icance o f the peak, w h i c h increases the p r o b a b i l i t y that the t w o sample s share the same m e a n . T h i s p r o b a b i l i t y a l l o w s m e to test the h y p o t h e s i s that the p e a k c a n be a t t r i bu ted to the b a c k g r o u n d of r a n d o m pu lses . If the h y p o t h e s i s is rejected, then I c o n c l u d e that the peak is d u e to R P E because this is the o n l y p h e n o n m e n o n that I k n o w of that can p r o d u c e s u c h a peak. 3.6 B o u n d a r y D e l i n e a t i o n B o u n d a r y d e l i n e a t i o n is an in te rp re ta t ion m e t h o d based u p o n the a s s u m p t i o n that the r e c e i v e d s i g n a l s c o m e f r o m edges or b o u n d a r i e s of a n o r e b o d y . T h e r e is s o m e e x p e r i m e n t a l ev idence ( M o b r u n M i n e expe r imen t , chapter 4) that R P E is m o r e eas i ly p r o d u c e d f r o m the o re - zone p e r i p h e r y . W i t h th i s a s s u m p t i o n a n d s o m e s i m p l e geometr ic concepts the boundar ie s of the o rebody can be m a p p e d . T h e m e t h o d is based u p o n the p remise that w h e n a n E M s i g n a l is r ece ived the source of the s i g n a l is loca ted s o m e w h e r e o n the se i smic wavefront . In the u n d e r g r o u n d context , this wave f ron t is a p p r o x i m a t e d b y a sphere centered u p o n the sho tpo in t . N o t e that i f the m e d i u m is v e r y i n h o m o g e n e o u s then the w a v e f r o n t has a c o m p l i c a t e d s t ruc ture , a n d the s i m p l e geomet ry w o u l d be a p o o r a p p r o x i m a t i o n . L u c k i l y , m a n y u n d e r g r o u n d e n v i r o n m e n t s are r e l a t i ve ly h o m o g e n e o u s (a 5% v a r i a t i o n i n l o c a l s e i smic ve loc i t i e s is the mos t I have measured) a n d de ta i l ed se i smic m o d e l i n g is unnecessary . B o u n d a r y d e l i n e a t i o n i n it 's present f o r m i s . a t w o d i m e n s i o n a l g r a p h i c a l m e t h o d of in terpre ta t ion . E a c h event has an a r r i v a l t ime (r,) a n d a sho tpo in t loca t ion (x,y). F r o m Chapter 3: Interpretation and Processing Methods 50 CD E CO > Shotpoint Number Figure 3.6 A graphic illustration of the boundary delineation method. In this method the seismic wavefronts at the time of signal reception are reconstructed to form a picture of the interface. Chapter 3: Interpretation and Processing Methods 51 o u r k n o w l e d g e of the l o c a l se i smic ve loc i ty ( v p ) a c i r cu la r arc of r ad iu s r = vp ti is d r a w n a r o u n d (x,y) to s h o w the w a v e f r o n t p o s i t i o n at t i m e /,. A n e x a m p l e of th i s s tep is i l l u s t r a t e d i n F i g u r e 3.6. In genera l , I use a scat ter -plot to t ry to p i c k re f l ec to r - l ike g r o u p i n g of events (i.e. a sequence of events s l o w l y c h a n g i n g i n a r r i v a l t i m e f r o m one sho tpo in t to the next) a n d then I use this l i s t of events to p l o t the c i r cu l a r arcs. P l o t t i n g a l l of the su rvey ' s events at once w i l l often p r o d u c e a v e r y confus ing p ic ture . A f t e r the c i r cu l a r arcs are d r a w n then c o m m o n tangents are d r a w n be tween adjacent arcs to f o r m a p rof i l e of the b o u n d a r y (Figure 3.6). There are some p r o b l e m s w i t h the 2 - D a l g o r i t h m (a 3 - D a l g o r i t h m is poss ib le , bu t i t is v e r y d i f f icu l t to interpret a n d present 3 - D results). T h e greatest p r o b l e m is the choice of p l a n e to w o r k w i t h . F o r e x a m p l e , the p l a n e m i g h t be h o r i z o n t a l a n d i n c l u d e the sho tpo in t s , o r v e r t i c a l a n d a l i g n e d w i t h a r e g i o n a l geo log i ca l s t ruc ture . T h e cho ice of w o r k i n g p lane w i l l inf luence the type of p ic ture seen by the interpreter , a n d needs to be chosen (or guessed) w i t h care. A n o t h e r d i f f i cu l ty is that there are t w o l ines p r o d u c e d b y the m e t h o d : one o n ei ther s ide of the l ine of shotpoin ts (F igure 3.6). D r i l l da ta o r other g e o l o g i c a l cons t ra in ts m a y be u s e d to reject one of the b o u n d a r i e s , bu t i f th is da ta is u n a v a i l a b l e then bo th bounda r i e s have to be cons ide red . 3.7 T o m o g r a p h i c R e c o n s t r u c t i o n T h e p r i n c i p a l i d e a b e h i n d t o m o g r a p h i c r econs t ruc t ion is that a r e g i o n of g e o l o g y that r e s p o n d s to one shot w i l l a lso r e s p o n d to other shots. A r r i v a l t i m e i n f o r m a t i o n f r o m shots at v a r i o u s pos i t i ons a l l o w s the t r i a n g u l a t i o n of the source p o s i t i o n (Sobolev a n d D e m i n , 1980). T h e m e t h o d of t r i a n g u l a t i o n is perfect i f the source of the s i g n a l c a n be r e l i a b l y t r i g g e r e d a n d i d e n t i f i e d . U n f o r t u n a t e l y , R P E is less t h a n perfect i n b o t h r e l i ab i l i t y a n d un iqueness of s ignature . T o o v e r c o m e the imper fec t b e h a v i o r of R P E , I have d e v e l o p e d a a l g o r i t h m that a t tempts to recons t ruc t the s t r u c t u r a l f o r m of R P E -p r o d u c i n g reg ions . T h i s t o m o g r a p h i c r e c o n s t r u c t i o n a l g o r i t h m appea r s to be f a i r l y Chapter 3: Interpretation and Processing Methods 52 robus t , a n d has w o r k e d w e l l i n at least one ins tance w h e n a p p l i e d to "real" da t a (see C h a p t e r 4, L y n x M i n e III). T h e a l g o r i t h m for r econs t ruc t i on is r e l a t i v e l y s i m p l e . F i r s t l y , the v o l u m e is d i v i d e d in to v o l u m e elements v i a r egu la r gr ids . E a c h v o l u m e element is a cube 4 to 10 meters a s ide a n d has a n u m b e r associated w i t h it , w h i c h is i n i t i a l l y set to zero. It is a s s u m e d that the s e i s m i c v e l o c i t i e s are r e l a t i v e l y h o m o g e n o u s a n d i s o t r o p i c so that the s e i s m i c wave f ron t can be accura te ly represented b y a s p h e r i c a l she l l s . A l is t of event a r r i v a l t imes a n d the co-ordinates of the sho tpo in t that generated each event is p r epa red . T h e a l g o r i t h m scans the a r r a y of v o l u m e e lements for v o l u m e e lements that c o u l d be respons ib le for a pa r t i cu l a r event o n the l is t . Tha t is , the (Cartesian) d i s tance be tween a v o l u m e element a n d the sho tpo in t respons ib le for the event is c o m p a r e d to the d is tance of the event (i.e. a r r i v a l t ime x acoust ic ve loc i ty ) . If there is a m a t c h then the n u m b e r associa ted w i t h the v o l u m e element is i nc remen ted . Therefore , the n u m b e r associa ted w i t h each v o l u m e e lement co r re sponds to the n u m b e r of c i r c u l a r arcs p a s s i n g t h o u g h that v o l u m e e lement (see F i g u r e 3.7 for an example ) , w h i c h I s h a l l c a l l the count . If a r e g i o n is a c t i ve ly p r o d u c i n g R P E s igna ls then the v o l u m e e lements of this r e g i o n w i l l have a large n u m b e r of counts . T h i s is the basis of in te rpre ta t ion for m y t o m o g r a p h i c r econs t ruc t i on a l g o r i t h m . T h i s t o m o g r a p h i c a l g o r i t h m is not s t r ic t ly a n i n v e r s i o n process because it does not t ry to f i n d a pa r t i cu l a r s o l u t i o n . There is no m i n i m i z a t i o n of a n o r m . Instead, a l l poss ib i l i t i e s are represented w i t h the expecta t ion that the rea l s t ructure w i l l d o m i n a t e the p ic tu re i f there is sufficient data . There are t w o m a i n depar tu res f r o m i d e a l c o n d i t i o n s for the r econs t ruc t i on a l g o r i t h m i n p r ac t i ca l f i e ld da ta co l l ec t ion . O n e is that w e are often i m a g i n g a three d i m e n s i o n a l s t ructure w i t h a l i m i t e d a m o u n t of f reedom i n shot p lacement . T o locate a p o i n t source i t takes at least four shot locat ions (Sobolev a n d D e m i n , 1980). T h e four po in t s cannot be co-p lanar , o the rwise , a n i m a g e of the source w i l l appea r o n the other s ide of the p lane . Chapter 3: Interpretation and Processing Methods 53 In underground experiments this means collecting data from different elevations, which I have been unable to do because of the potential difficulty in co-ordinating shot timing and data acquisition between two mine levels. Constraints from regional geology and drill data can be used to reject these image sources. The worst case scenario for tomographic imaging is a survey where the shotpoints are in a line. Because there is only one degree of freedom in shot placement (along the tunnel) the tomographic image will be a series of toroids and rings sharing the same axis as the tunnel. 0 20 40 60 80 100 Count • - Target X - Shotpoint Figure 3.7 A demonstration of tomographic reconstruction. In this example there are four targets and four shotpoints. Note how the poor resolution of the targets outside the region encompassed by the shotpoints. The other significant problem is that most shot records contain more than one signal. Trying to match signals across records is impossible as they are usually indistinguishable (apart from amplitude, in which no two are alike). This can result in non-uniqueness in the problem of locating the source. An example is shown in Figure 3.7, where there is more than the three shotpoints necessary to locate a source in 2-D, but there are four indistinguishable signals from each location. The net result is that one or two other Chapter 3: Interpretation and Processing Methods 54 poss ib i l i t i e s ar ise for the source locat ions . Source loca t ion degeneracy is r e m o v e d b y h a v i n g m o r e shotpoin ts locat ions than is r equ i r ed for one s igna l . G i v e n the above m e n t i o n e d p r o b l e m s a n d proper t ies of R P E w h a t is the best s u r v e y geometry? The idea l s u r v e y w o u l d be to s u r r o u n d the target of interest w i t h a n u m b e r of s h o t p o i n t s o n at least t w o di f ferent e leva t ions . T h i s is i m p r a c t i c a l for a lot of u n d e r g r o u n d w o r k (if w e confine ourselves to the tunnels a n d cross-cuts, bu t not i f w e use d r i l l - h o l e s ) . F o r m o s t s u r v e y s the m i n i m u m is to have at least t w o r o u g h l y o r thogona l shot-l ines, a n d to try a n d shoot f r o m as m a n y d i rec t ions as the va r ious dr i f t s , cross-cuts , a n d shafts a l l o w . A t least t w o degrees of f r eedom i n shot p l acemen t is necessary for t o m o g r a p h i c recons t ruc t ion to w o r k . Synthetic Survey SP1 jtr-& ft ft flr-i*r SPIO: J SP9 ft ft ft SP16 -ft ft ft ft ft ft ft SP17 25 metres ft ft, JJ JJ ft ft ft SP30 Figure 3.8 A m a p of the synthet ic su rvey example . The Ore-zones are the s h a d e d areas, a n d the shotpoints of the su rvey are m a r k e d b y stars. Chapter 3: Interpretation and Processing Methods 55 A syn the t i c s u r v e y demons t ra tes the capab i l i t i e s of this a l g o r i t h m . In F i g u r e 3.8 w e have a t w o d i m e n s i o n a l p r o b l e m i n loca t ing a l o n g t h i n lens that is sp l i t b y a fault . T h i s is not a n easy test for the a l g o r i t h m , bu t the o rebody ' s shape a n d the s u r v e y geomet ry represents a f a i r l y real is t ic cha l lenge . I generated 145 events f r o m the 30 sho tpo in t s b y a s s u m i n g a n e v e n (but r a n d o m w i t h i n ) d i s t r i b u t i o n of R P E events f r o m the o r e b o d y . E a c h par t of the o r e b o d y reacts i n the same manner , a n d host r o c k does not react at a l l . In the m o c k da ta no events w e r e genera ted m o r e than 100 meters f r o m the shot (the effective range of the charges), a n d a h ighe r p r o b a b i l i t y w a s g i v e n for genera t ing R P E at dis tances of 50 meters or less (a factor of two) . I u s u a l l y shoot t w o shots pe r sho tpo in t i n a s u r v e y . A p p l y i n g this r u l e to the m o c k dataset g ives a p r o d u c t i v i t y of t w o o r three events pe r shot; a f a i r l y t y p i c a l n u m b e r for m y su rveys . T o m o g r a p h i c r econs t ruc t i on p roduces a fa i r ly g o o d image f rom this dataset (Figure 3.9 ). P lacement of the a n o m a l y is accura te , bu t i t de te r io ra tes o u t s i d e the a rea b o u n d e d b y the s h o t p o i n t s . T h i s a consequence of fewer shots c o n t r i b u t i n g cons t ruc t ive ly to the coun t i n the outer reg ions of the su rvey . The s m a l l lens is p o o r l y i m a g e d because of the lack of shots to the no r th , bu t i t is c l e a r l y de tec ted nonetheless . Spa t i a l r e s o l u t i o n of the t o m o g r a m is s o m e w h a t p o o r because of the 10 meter sho tpo in t spac ing a n d the c o m p a r a t i v e l y n a r r o w features to be i m a g e d . In pract ice , 5 meter spac ing w o u l d be preferable, a n d u s e d i f c i rcumstances ( t ime /cos t ) pe rmi t t ed . A p r a c t i c a l d i s a d v a n t a g e of the r econs t ruc t i on a l g o r i t h m is that i t does no t p r o v i d e quan t i t a t ive i n f o r m a t i o n about ma te r i a l proper t ies . T h i s can be a t t r ibu ted to o u r l a c k of k n o w l e d g e about the R P E m e c h a n i s m . H o w e v e r , the coun t f r o m m y a l g o r i t h m appears to g i v e a g o o d i n d i c a t i o n of the presence of s u l p h i d e s , w h i c h is a v e r y des i rab le result . S e i s m i c a n d e lec t r i ca l m e t h o d s (at present) d o no t g i v e a q u a n t i t a t i v e v a l u e o n the a m o u n t of s u l p h i d e . The presence a n d a m o u n t is in fe r red f r o m the p h y s i c a l parameters (e.g. p - w a v e ve loc i ty or conduc t iv i t y ) , often w i t h p o o r accuracy. Chapter 3: Interpretation and Processing Methods C H A P T E R 4 F IELD T R I A L S 4.1 F i e l d P r o g r a m Objec t ives T h e p r i n c i p a l a i m of the f i e ld p r o g r a m was to c o n f i r m the existence of the p h e n o m e n o n d e s c r i b e d b y Sobo lev et a l . (1980, 1982). In essence, ver i fy that pu lses of E M energy are p r o d u c e d f r o m s u l p h i d e m i n e r a l s w h e n s e i s m i c a l l y exc i ted . P u b l i s h e d m a t e r i a l f r o m Sobolev ' s g r o u p is r i c h w i t h the descr ip t ions of the character of the emiss ions (1982) a n d re la ted proper t ies (1984), but they g ive few detai ls about the exper iments . W h y are they so sure that the s igna l s come f r o m the s u l p h i d e s ? A l t e r n a t i v e exp l ana t i ons , s u c h as t r iboelectr ic effects f r o m the blast, w o u l d appear to be equa l ly v iab le . F u r t h e r m o r e , w h a t are the i m p l i c a t i o n s of the i n a b i l i t y to exac t ly rep l ica te the s e i s m o e l e c t r i c r e sponse (Sobolev et a l . , 1982), a n d h o w ser ious is the p r o b l e m of r ep l i ca t i ng resul ts? It is d i f f icu l t to u n d e r s t a n d s o m e of t h e i r c l a i m s w i t h o u t s o m e d i r e c t e x p e r i e n c e w i t h the measu remen t of R P E . A n o t h e r p u r p o s e of the f i e l d p r o g r a m w a s to i nves t i ga t e the p o s s i b l e use o f th is p h e n o m e n o n as a n e x p l o r a t i o n too l . D i s c u s s i o n s be tween the Russ i ans , ourse lves ( R . D . R u s s e l l a n d M . M a x w e l l of the U . B . C . Ins t rumenta t ion G r o u p ) , a n d B o b S m i t h , the head geophys ic i s t of C . R . A . (an A u s t r a l i a n m i n i n g c o m p a n y ) l e d us to be l i eve that D e m i n a n d M a y b u k w e r e u s i n g R P E for exp lo r a t i on . In fact, they w e r e o f fe r ing the i r services to W e s t e r n compan ie s . S o m e m i n i n g compan ie s are k n o w n to be in teres ted i n R P E , bu t d o not w a n t to c o m m i t themselves to a m e t h o d that is ne i ther u n d e r s t o o d o r p r o v e n . T h e techniques u s e d b y the Russ i ans to acqui re a n d in terpret R P E is u n c e r t a i n despi te i n f o r m a t i o n f r o m patents g ran ted to Sobo lev et a l . (1986). F u r t h e r m o r e , no i n f o r m a t i o n o n the effectiveness of the i r techniques exists. In shor t , there w a s an aura of mystery about the w h o l e business of R P E - b a s e d exp lora t ion . 57 Chapter 4: Field Trials 58 T o s u m m a r i z e , the a ims of the f i e l d p r o g r a m w e r e to assuage a n y d o u b t s about the existence of the R P E p h e n o m e n o n a n d to evaluate i ts po t en t i a l for m i n e r a l ex p l o r a t i on . W h i l s t a c h i e v i n g these b r o a d a ims I i n t e n d e d to l ea rn m o r e about the p h e n o m e n o n a n d d e v e l o p a sy s t em that c o u l d r e l i ab ly measure the E M attr ibutes of R P E i n the f i e ld . T h e measuremen t a n d ana lys i s tools d e v e l o p e d w o u l d also p r o v i d e a basis to eva lua te R P E as a n e x p l o r a t i o n m e t h o d . 4.2 S u l l i v a n M i n e T w o p r e v i o u s se ismoelect r ic exper iments w e r e c o n d u c t e d at this site, i n 1983 (Sobolev et a l . , 1984a) a n d 1986. B o t h i n v o l v e d m e m b e r s f r o m the U . B . C . research g r o u p , bu t not myse l f . D u e to the encourag ing resul ts f r o m these t w o exper iments , a n d a w e l l de f ined geo logy , the site was chosen to test v a r i o u s se i smic sources a n d n e w in s t rumen ta t i on . T h e objectives w e r e to c l ea r ly observe se ismoelec t r ic emiss ions , i f a n y exist, u s i n g o u r mos t recent i n s t rumen ta t ion , a n d to de te rmine the best source for a c c o m p l i s h i n g this . E x p e r i m e n t D e t a i l s T h e S u l l i v a n m i n e is a l e ad -z inc s u l p h i d e m i n e loca ted i n K i m b e r l y , B . C . , opera ted b y C o m i n c o L t d . It's o r e b o d y is a 160 m i l l i o n ton, gent ly d i p p i n g , i r o n - l e a d - z i n c s u l p h i d e lens w h i c h l ies c o n f o r m a b l y i n P r o t o z o i c c las t ic m e t a - s e d i m e n t a r y r o c k ( H a m i l t o n , 1982). F o o t w a l l rocks are g r a d e d q u a r t z w a k e a n d m u d s t o n e beds 10 to 30 c m th ick . P r i m a r y m i n e r a l c o n s t i t u e n t s a re q u a r t z , s e r i c i t e , b i o t i t e a n d s o m e p y r r h o t i t e l a m i n a t i o n s u p to 1 m m th i ck o c c u r r i n g 5 to 15 m b e l o w the " m a i n b a n d " of the ore b o d y . T h e m a i n b a n d is 3 to 24 m th i ck a n d consis ts of a success ion of f ine g r a i n e d pyr rho t i t e , sphaler i te a n d galena beds. T y p i c a l grades of the m a i n b a n d are about 10% P b a n d 15% Z n , w i t h 25% Fe o c c u r r i n g m a i n l y as pyr rho t i t e . T h e m a i n b a n d is o v e r l a i n b y a s u c c e s s i o n abou t 20 m t h i c k c o m p r i s i n g 35% s u l p h i d e - r i c h l aye r s i n f ou r b a n d s in t e rca l a t ed w i t h three s u l p h i d e p o o r in te rbeds of m u d s t o n e a n d q u a r t z w a k e . T h e h a n g i n g w a l l s are g raded q u a r t z w a k e a n d muds tone . Chapter 4: Field Trials 59 Our experiments were conducted over a three day period, 16-18 April, 1991, at level 2700, in the eastern part of the mine (Figure 4.1). Participants in these experiments were R. D. Russell, B. B. Narod, and myself from U.B.C, and a blaster/loader (Grant Scott) loaned to us from the mine workforce. Shot points were placed in opening 2713 at 10 m intervals. The orebody is approximately 30 m above opening 2713 and dips 20 degrees north-west. Instrumentation and the portable computer containing the digitizing card were placed on a powder-car in opening 2711 near the triple junction with 2713, and the opening to the hoist. The nearest shot point was 40 m distant, and the furthermost was 140 m distant. Measurements of the P-wave velocity were on average 5700 m/s for this area, which is similar to the 5800 m/s measured in previous experiments at this site. Figure 4.1 Map of the Sullivan Mine experiment. The orebody lies approximately 20-30 meters above the shotpoints, and dips north-east. Chapter 4: Field Trials 60 N u m e r o u s types of s e i smic sources were e x p e r i m e n t e d w i t h at sho tpo in t S P 2 (F igure 4.1). N o n - e x p l o s i v e sources were stacks of 20 b l o w s f r o m a s l edge -hammer , a n d 1000 b l o w s f r o m a p n e u m a t i c rock d r i l l . E x p l o s i v e sources were : detonator , de tona tor w i t h a s m a l l p r i m e r jacket, 1 /2 a n d f u l l s t i ck of e m u l s i o n type e x p l o s i v e (<0.4 k g ) , pen to l i t e p r i m e r (0.17 k g Tro jan m a d e b y A u s t i n E x p l o s i v e s ) , a n d 12 gauge b l a n k s h o t g u n she l l s f i r ed f r o m a t r i p o d m o u n t e d "buffalo" g u n . L i t t l e or no damage resu l t ed f r o m the use of these sources , a n d the same ho le (SP 2) w a s u s e d for s eve ra l shots . E x p l o s i v e s w e r e de tona ted w i t h electric b l a s t i ng caps f r o m a spec ia l b l a s t i ng c i r cu i t m a d e at U . B . C . that d i s c o n n e c t s the e l e c t r i c a l c i r c u i t o n c e d e t o n a t i o n b e g i n s to p r e v e n t e l e c t r i c a l interference. T h e de l ay be tween the current pu l se a n d the blast was set to a p p r o x i m a t e l y 0.6 seconds b y choice of a #25 b las t ing cap. A t least t w o magne t i c sensors were d e p l o y e d d u r i n g each of the expe r imen t s , a n d a n electric f i e ld sensor ( L o n g W i r e An tenna ) was a d d e d o n the last day . These sensors w e r e p l a c e d 30 m no r thwes t in to o p e n i n g 2711. M a g n e t i c sensors cons i s ted of three m o d e l s , U B C 1, U B C 2 a n d s o m e U B C 3 types. T h e U B C 3's p r o v e d to be too insens i t ive for this a p p l i c a t i o n , a n d U B C 2 w a s too n a r r o w i n b a n d w i d t h (1-10 k H z ) . Because U B C 1 is capable of a 300 k H z of b a n d w i d t h an A M d e m o d u l a t o r was connec ted to this sensor (see A p p e n d i x B.8 for d e s c r i p t i o n of d e m o d u l a t o r ) . E a c h of the s enso r s i g n a l s w e r e t r ansmi t t ed to one of the T e k t r o n i x A M 502 ampl i f i e r s at the i n s t rumen t site, a n d b a n d -l i m i t e d to 30 k H z before b e i n g r eco rded . T h e A M 5 0 2 ' s w e r e recent ly p u r c h a s e d , a n d o n l y four were ava i l ab le for this exper iment . T h i s expe r imen t tested the n e w a m p l i f i e r s y s t e m ( A M 502) for the first t ime . A n o t h e r first for R P E inves t iga t ion was the use of a d i g i t i z e r i n the f i e ld . P r e v i o u s l y , da ta w e r e r eco rded o n a n a n a l o g magne t ic tape sys tem a n d then d i g i t i z e d later. The R C E d i g i t i z e r c a r d r e c o r d e d 8 channels at 8 mic roseconds pe r sample . In general , a geophone close to the source w o u l d t r igger the start of the acqu i s i t i on process. Shots o n the 18th o f A p r i l u s e d the E M emiss ions f r o m the e x p l o s i o n i tself to enable the tr igger. Chapter 4: Field Trials 61 A l l of the e q u i p m e n t was bat tery p o w e r e d w i t h the excep t ion of a po r t ab le T e k t r o n i x o sc i l l o scope , w h i c h w a s o n l y u s e d for d i a g n o s i n g p r o b l e m s w i t h the e q u i p m e n t a n d e x a m i n i n g no i se f r o m the sensors . A n e m e r g e n c y she l te r a n d l u n c h r o o m 180 m northeast f r o m the i n s t rumen t site p r o v i d e d m a i n s p o w e r for the o sc i l l o scope a n d a 60 W safety l igh t . E x p e r i m e n t s w i t h the m a i n s o n a n d off s h o w e d no d i s ce rn ib l e difference i n ext raneous noise . D i s c u s s i o n of Resu l t s O n l y the pen to l i t e exp los ives p r o d u c e d a n y ev idence of se i smoelec t r ic s igna l s . N o t a h in t of s i g n a l w a s obse rved w i t h the o ther sources , w h i c h con t ras ted g rea t ly w i t h the c o p i o u s a m o u n t s of E M a c t i v i t y after the b las t f r o m pen to l i t e sources . N o d i s tan t g e o p h o n e r e c o r d s are a v a i l a b l e f r o m these tests, so the re are n o q u a n t i t a t i v e c o m p a r i s o n s be tween the sources A geophone p l a c e d near the shot p r o d u c e d n o use fu l i n f o r m a t i o n because the g r o u n d m o t i o n w a s m a s k e d b y a i r - w a v e effects. H o w e v e r , it w a s o b v i o u s that the pen to l i t e e x p l o s i v e s p r o d u c e d a s e i smic i m p u l s e w i t h h i g h e r a m p l i t u d e a n d f requency content t han the other sources . T h e di f ference b e t w e e n the pento l i te exp los ives a n d the e m u l s i o n type explos ives (Forcite) c o u l d be felt t h r o u g h o u r feet: pento l i te exp los ives gave a d i s t inc t ive crack, whereas the other exp los ives p r o d u c e d a m u f f l e d t h u m p (an effect not iceable a fract ion of a second before the a i r - w a v e arr ives) . A 100 m prof i l e a l o n g 2713 be tween SP2 a n d SP12 (Figure 4.1) y i e l d e d 7 use fu l records f r o m 6 sho tpo in t locat ions . A l l of these records s h o w se ismoelec t r ic events, a n e x a m p l e of one o f these records is s h o w n i n F i g u r e 4.2. T r a i n s of E M pu l ses appea r 4 to 10 m s after the e x p l o s i o n (F igure 4.2), w h i c h is consis tent w i t h exc i ta t ion of the ore zones b y the se i smic w a v e . In some records the s ignals were c l i p p e d b y the ampl i f i e r s because of the u n e x p e c t e d l y h i g h a m p l i t u d e of the p h e n o m e n o n (>10 n T a n d >4 m V at the sensor). S igna l s f r o m the A M d e m o d u l a t o r (connec ted to U B C I) a n d the l o n g w i r e an tenna e x h i b i t e d greatest a m o u n t s of s i gna l . T h e fo rmer ind ica tes s ign i f ican t a m o u n t s of h i g h f requency energy not accessible to the l o w e r b a n d w i d t h sensors , the lat ter demonst ra tes Chapter 4: Field Trials 62 the mer i t s of m e a s u r i n g electric f ields. A s the shot po in t m o v e d a w a y f r o m the sensors sma l l e r s i g n a l a m p l i t u d e s were recorded . F igu re 4.2 A r e c o r d f r o m S P 2 , S u l l i v a n M i n e . T h e u p p e r m o s t trace is f r o m a geophone near the shot , a n d the others are f r o m v a r i o u s E M sensors. It w a s expected that the s ignals w o u l d start at about 4 ms after the blast, the t i m e to reach the m a i n o rebody . H o w e v e r , there is s u b s t a n t i a l a m o u n t s of s i g n a l ear l ier than 4 ms. These ear ly s ignals m a y be d u e to s m a l l a m o u n t s of s u l p h i d e near the sho tpo in t , o r emi s s ions f r o m the blast. T h i s burs t of early ac t iv i ty appears to cease after a p p r o x i m a t e l y 3 ms , a n d is characterist ic of a l l of the records f r o m the S u l l i v a n M i n e . There is 3-5 ms of blast-associated E M i n a l l of the S u l l i v a n M i n e records. The d u r a t i o n of the blast-associated E M masks any seismoelectr ic s ignals less t han 20-25 m a w a y f r o m the shot po in t , a n d it m a k e s it d i f f i cu l t to de t e rmine w h e t h e r a s i g n a l is f r o m the s u l p h i d e zones , or f r o m s o m e p h e n o m e n a near the shot-hole cav i ty (O 'Keefe a n d T h i e l , Chapter 4: Field Trials 63 1991). Two explosives were freely suspended 1-2 m from the tunnel ceiling and set off, to measure the intensity and duration of noise from the explosion plasma. Both shots gave less than 2 ms of EM impulses with much less amplitude than the shots in the drill-holes. This test indicates that the explosive plasma is not the source of the (seismoelectric) signals on other records. Host Rock Mudstone and Quartzwacke Sulphide Ore Fe-Zn-Pb Interpolated from Drill Data Ore Intersection Drill Data Possible Location of Seismoelectric Source Shotpoint 0.18 kg of High Explosive 0 20 40 60 m Figure 4.3 A comparison between drill core results and reconstructed wavefronts, Sullivan Mine. The arcs in this diagram represent the seismic wavefront at times of significant E M activity. The burst of signal activity immediately after the blast was not included in this interpretation, as these emissions were thought to be possibly related to processes from the blast itself. A comparison of signal arrival data and the drill data is shown in Figure 4.3. From Figure 4.3 it can be seen that most the arcs will intersect the orebody at some point. This shows that it is quite plausible that the orebody produces these signals. However, the elimination of events before 3 ms and the proximity of the main orebody makes it fairly easy for the data to fit this scenario. A previous seismoelectric experiments in this area (Sobolev et al., 1984a) obtained very similar arrival times, and also saw many early arrivals. Comparison Between Drill Core Results and Interpreted Data Sullivan Mine B.C. SP2 SP3 SP5 SP8 S P 1 1 S P 1 2 C o n c l u s i o n s Chapter 4: Field Trials 64 N a r r o w p u l s e s of E M ene rgy w e r e o b s e r v e d at t imes after the e x p l o s i o n that are consis tent w i t h se i smogenic effects o c c u r r i n g w i t h i n the o rebody . Effects f r o m the blast cannot be u n e q u i v o c a l l y e x c l u d e d as the source of the s igna ls . H o w e v e r , the s i m i l a r i t y of m y results to the 1983 a n d 1986 exper iments p r o v i d e s fur ther suppo r t for the existence of R P E . T h e i n s t r u m e n t a t i o n p e r f o r m e d w e l l i n the u n d e r g r o u n d e n v i r o n m e n t w i t h the d e m o d u l a t o r a n d L W A p e r f o r m i n g ve ry w e l l i n r e c e i v i n g these s ignals . A n e w type of exp los ive source , the pentol i te p r imer , was d i s cove red to be v e r y effective i n exc i t ing the o r e b o d y w i t h o u t d a m a g i n g the m i n e infrastructure . 4.3 M o b r u n M i n e T h e p r i n c i p a l a i m of this exper iment (Kep ic et a l . , 1995) w a s to s h o w that R P E or iginates f r o m s u l p h i d e mine ra l s . T h e S u l l i v a n da ta d i d not p r o v e this hypo thes i s because the da ta m a y have been con tamina ted w i t h blast-associated effects. A t the M o b r u n M i n e the p l a n w a s to m o v e the shots a w a y f r o m the o r e b o d y a n d observe the r e s u l t i n g a r r i v a l t imes of the s igna ls . If the s igna l s a r r i v e d p r o g r e s s i v e l y later, a n d at t imes consis tent w i t h convers ions w i t h i n the o rebody , then this w o u l d demons t ra te that the o r e b o d y w a s respons ib le . E x p e r i m e n t D e t a i l s T h e M o b r u n M i n e is ope ra t ed b y A u d r e y Resources , a n d i s l o c a t e d nea r R o u y n -N o r a n d a , Quebec . R i o A l g o m d i s c o v e r e d the m a i n lens i n 1956 b y means of a m o b i l e E M r o a d s u r v e y (Seigel, 1957). A recent ly d i s cove red i ron -coppe r - z inc o rebody , the 1100 lens , was the object of o u r i nves t i ga t i on (F igure 4.4). It is l oca ted 250 m s o u t h of the m a i n lens, a n d is 360 m b e l o w the surface. The 1100 lens is hos ted i n a large pyroc las t i c un i t b o u n d e d to the s o u t h b y rhyo l i t e a n d andesi te f lows , a n d by a th ick rhyo l i t e u n i t to Chapter 4: Field Trials Plow ^nnn Instrument Site \ ^ Level 6 Vertical Projection - \ Elev 4900 Sensor Sites / Shotpoints 1100 Lens Fe-Cu-Zn 10900 E / 11000 E 11100 Ej 11200 E Figu re 4.4 A m a p of the M o b r u n M i n e exper imen t . M a p co-ord ina tes are i n meters . Chapter 4: Field Trials 66 the n o r t h ( C a u m a r t i n a n d C a i l l e , 1990). It is a ve r t i ca l ly -o r i en ted mass ive su l f ide b o d y e x t e n d i n g 300 m east-west l a te ra l ly , a n d f r o m 360 to at least 740 m b e l o w the surface, w i t h a m a x i m u m of 50 m th ickness . T h e o r e b o d y is sp l i t a l o n g its l o n g i t u d i n a l ax i s f o r m i n g t w o z i n c - r i c h la te ra l fr inges. T h e su l f ides consis t of f ine to m e d i u m g r a i n e d , p a r t i a l l y g r a n u l a t e d p y r i t e m a t r i x , c o n t a i n i n g 5 to 15 percent spha le r i t e as i r r e g u l a r sha rds a n d c lus ters , a n d 1 to 5 percent d i s s e m i n a t e d , v e r y f ine g r a i n e d c h a l c o p y r i t e ( C a u m a r t i n a n d C a i l l e , 1990). O u r tests were p e r f o r m e d i n a r a m p that descends south-east f r o m l e v e l 6 to the top of the 1100 lens (Figure 4.4). Par t ic ipants i n the exper iments were M . M a x w e l l , G . M e l l e m a , a n d myse l f . O n e of the r e g u l a r m i n e r s ass is ted w i t h d r i l l i n g of the shot ho les a n d b las t ing . T h e se ismic source u sed i n o u r tests was a 0.22 k g booster charge m a d e o f pentol i te . E a c h charge w a s p l a c e d u n t a m p e d in to a 1.5-2 m d r i l l ho le , a n d de tona ted w i t h a four -second electric b l a s t ing cap. The four second de lay a l l o w e d the sensors to fu l l y recover f r o m a m o m e n t a r y o v e r l o a d i n d u c e d b y large currents f r o m the b l a s t i ng box . Sho tpo in t s were p l a c e d at 5 m in te rva ls a l o n g the r a m p , s tar t ing w i t h sho tpo in t zero at the in tersect ion of the r a m p a n d the o r ebody , a n d f i n i s h i n g w i t h sho tpo in t s ix teen , 80 m fur ther u p the r a m p a n d 50 m a w a y f r o m the nearest ore-zone (F igure 4.4). Sho tpo in t s 12, 13, a n d 15 were not u s e d because of the p o s s i b i l i t y of d a m a g e to m i n e inf ras t ruc ture . A f t e r the i n i t i a l t raverse, the shot holes genera l ly s h o w e d l i t t le damage , a n d were reused ; s o m e holes w e r e u s e d three o r four t imes w i t h no s igni f icant difference i n se i smic ou tpu t . A n ar ray of e lectromagnet ic sensors a n d the r eco rd ing site were loca ted 100-130 m u p the r a m p f r o m sho tpo in t zero. T h e electric f i e ld was m e a s u r e d b y l o w noise (30 n V / V H z ) p r e - a m p l i f i e r s c o n n e c t e d to e i ther t w o s ta in less s teel s takes set i n t o the g r o u n d ( g r o u n d e d d ipo l e ) , or one stake a n d a l o n g in su l a t ed w i r e ac t ing as a capac i t ive p i c k u p ( long w i r e an tenna o r L W A ) . M a g n e t i c sensors cons is ted of an in tegra ted p r e a m p l i f i e r Chapter 4: Field Trials 67 fed b y b r o a d b a n d co i l s w o u n d o n a ferrite r o d ( A p p e n d i x B.4). T h e b a n d w i d t h s of the electr ic f i e ld sensors w e r e 1 H z to 30 k H z ( long w i r e antenna) a n d 1 H z to 10 k H z ( g r o u n d e d d ipo l e ) , a n d of the magne t ic sensors were 1-300 k H z a n d 2-7 k H z (the U B C I a n d U B C II m o d e l s r e spec t i ve ly ) . A b a n k of T e k t r o n i x A M 5 0 2 a m p l i f i e r s at the r e c o r d i n g site a m p l i f i e d a n d f i l te red (general ly w i t h to a bandpass o f 1 to 30 k H z ) the s ignals f r o m the sensors before they were recorded b y the R C E data acqu i s i t i on sys tem at 62.5 k i l o s a m p l e s pe r second . T o r eco rd the presence of frequencies b e y o n d the N y q u i s t f requency o f the r e c o r d i n g sys t em, a w i d e - b a n d , p r e c i s i o n rect i f ier ( s im i l a r to a n A M d e m o d u l a t o r ) w a s connec ted to the 1-300 k H z magnet ic antenna (See A p p e n d i x B.8 for de t a i l s ) . A l l o f the e q u i p m e n t w a s ba t t e ry p o w e r e d to r e d u c e e l e c t r o m a g n e t i c interference a n d enhance por t ab i l i t y . A geophone at the ins t rument site m o n i t o r e d the se i smic f i e ld near the sensors , a n d w a s u s e d to calculate P - w a v e ve loc i t ies , w h i c h were f o u n d to be a p p r o x i m a t e l y 5400 ± 200 m / s. 9 V 4 0 m V 4 m V / m 6 m V / m 6 nT 9 n T 5 1 0 T i m e (ms) 15 2 0 F igure 4.5 A r eco rd of a shot at SP7 , M o b r u n M i n e . The four l o w e r m o s t traces are f r o m E M sensors. These traces s h o w s igna ls a t t r ibu tab le to the o rebody , labe led as R P E , a n d the same type of near shot E M seen i n the S u l l i v a n e x p e r i m e n t . Chapter 4: Field Trials 68 Discussion of Results A n e x a m p l e of the data a c q u i r e d d u r i n g a shot is s h o w n i n F i g u r e 4.5. M o s t of the E M ac t iv i ty 0 to 3 m s after the blast is thought to be caused b y s m a l l a m o u n t s o f su l f ides near the shot . C l o s e e x a m i n a t i o n of the ear ly po r t i ons of the records s h o w s that the "blast-associa ted E M " does not a l w a y s start w i t h the blast, a n d i t is often d e l a y e d . I f o u n d that the de l ay c o u l d be a t t r ibuted to the t ime for the se i smic w a v e to reach a n a r r o w zone of s m a l l p o d s o f s u l p h i d e mine ra l s . T h i s zone cuts t h r o u g h the shot l i ne at S P 3 a n d SP9; records f r o m these shotpoin ts t end to have blast E M s tar t ing at 0 ms . In a d d i t i o n , at SP3 a n d SP4 the s m e l l of s u l p h u r d i o x i d e was d i s t i nc t i ve ly present after the blast; i n d i c a t i n g that o u r shots h a d been i n contact w i t h s u l p h i d e s . I b e l i e v e that r e l a t i v e l y s m a l l a m p l i t u d e emiss ions f r o m the blast area were re-radia ted b y the b la s t ing leads connec ted to the exp los ive . These w i r e s pass near the E M sensors u s e d i n o u r exper imen t s , thus , i n d u c e d c u r r e n t s i n the b l a s t i n g l e a d s m a y h a v e c o m m u n i c a t e d the nea r -b la s t e m i s s i o n s . Pu lses o n the E M sensor traces i n F i g u r e 4.5 occu r r i ng later than 3 m s after the blast are examples of R P E . The L W A sensor trace i l lus t ra tes best the u n i q u e p roper t i es of these s ignals : t y p i c a l l y , a short d u r a t i o n pu l se or a s m a l l g r o u p of pulses o f one po l a r i t y . The ear l ier E M ac t iv i ty , b y contrast, exhibi ts no preference i n po l a r i t y a n d is m o r e osc i l l a to ry i n character. In a d d i t i o n , it is expected that the r a n d o m l y p o l a r i z e d emiss ions f r o m the shot area w i l l be r ece ived mos t s t r ong ly by the sensor closest to the sho tpo in t , w h i c h is the L W A , because E M f i e ld a m p l i t u d e s decay r a p i d l y w i t h d i s tance f r o m the source . It can be seen i n F i g u r e 4.5 that the L W A does i n fact r e s p o n d m o s t s t r ong ly to the ea r ly , near-shot E M . These d i f f e r ing character is t ics were u s e d to p i c k the R P E s igna l s . T h e a p p e a r a n c e o f R P E s igna l s a n d the presence o f ea r ly E M a p p e a r to be u n r e l a t e d ; somet imes there were no R P E s ignals a n d o n l y ear ly E M , a n d somet imes o the rwise , bu t i n mos t cases bo th appea red o n o u r records. T h e L W A response i n F i g u r e 4.5 does pu t some d o u b t u p o n the second (and later) g r o u p of a r r iva l s l abe led as R P E because they Chapter 4: Field Trials 69 appear w e a k l y o n a l l an tenna except the L W A , but the consistent negat ive p o l a r i t y of the pu lses suggests that these s ignals are s t rong ly p o l a r i z e d a n d i t m a y be that the L W A is f avorab ly or ien ted . N o t e that the a r r i v a l of the se i smic p u l s e at the in s t rumen t site ( K G trace i n F i g u r e 4.5), about 18 ms after the blast, is w e l l after most of E M signals have been r e c o r d e d , d e m o n s t r a t i n g that the E M s igna l s are u n r e l a t e d to a n y se i smic - to -an tenna c o u p l i n g . Sobo lev et a l . (1982) m e n t i o n that a characterist ic of R P E is the w e a k repea tabi l i ty of s ize , shape , a n d a r r i v a l t i m e of e m i s s i o n s , w h i c h t hey a t t r ibute to i r r e v e r s i b l e processes i n v o l v e d i n the s i g n a l genera t ion (e.g. the c rea t ion of c racks) . T o test th i s c l a i m w e re t i r ed m a n y of o u r sho tpoin ts . In terpre ta t ion w a s often d i f f i cu l t because i t is h a r d to t e l l the di f ference be tween the R P E s p i k e a n d the ear ly no i se f r o m shots c lose to the o rebody . A l t h o u g h the traces were not iden t i ca l i n appearance, I f o u n d that mos t repeat shots c o n t a i n e d the same R P E s igna l s w i t h o n l y s l i g h t l y different a r r i v a l t imes ( w i t h i n ± 0 . 5 m s ) a n d d i f fe ren t a m p l i t u d e s t h a n the o r i g i n a l ( K e p i c et a l . , 1995). S o m e i r r e p r o d u c i b i l i t y appears to be an inheren t par t of this se i smic- to -e lec t r i c c o n v e r s i o n process because the se ismic pu l se shape was fa i r ly consistent for each shotpoin t . Desp i te the less - than- idea l r e p r o d u c i b i l i t y , o u r repeat shots s h o w that the cons i s t ancy of s i g n a l a r r i v a l t imes is sufficient to ident i fy targets. T h e r a w da t a d i s p l a y e d i n F i g u r e 4.6 s u p p o r t s the c l a i m that the s igna l s w i t h R P E character is t ics come f r o m the o rebody . In pane l (a) the best l o o k i n g traces w e r e chosen f r o m each sho tpo in t ; best l o o k i n g means that the first R P E s i g n a l domi na t e s a n y ea r ly E M . T h i s trace was genera l ly f r o m the H F d i p o l e , w h i c h w a s furthest f r o m the shots, bu t w a s no t u s e d t h r o u g h o u t the tests so s o m e s h o t p o i n t s w e r e not r e c o r d e d w i t h th is sensor . P a n e l (b) is f r o m the A M d e m o d u l a t o r , w h i c h tends to e m p h a s i z e o s c i l l a t o r y s igna l s o v e r pu l se s , bu t w a s u s e d e x t e n s i v e l y d u r i n g the tests a n d s h o w s the la ter a r r iva l s better than p a n e l (a). Sho tpo in t to ore-zone dis tances, to a n accuracy of about 5 m , w e r e o b t a i n e d f r o m d r a w i n g s p r o v i d e d b y the m i n e . D i s t a n c e es t imates f r o m Chapter 4: Field Trials 70 E o Q- 0_ CO CO c\j co in ID CL D- CL CL CL CL CO CO CO CO CO CO Q. Q CO Q O 7 OL CO E CD E h-CL Q- CL Q_ CO CO CO CO CL CO cn QJ c .5 cc co cu cc CO > QJ CD U HC Ol CA) 3 cn co O >-c U H Xi QJ > -»-» u cu QJ t j ' Q J g cn i ! QJ C Q >H ^ £ T3 £ O cn 6 QJ QJ CO >H Ol CO 1 § He c . co QJ CL, I « CO H O "C ° £ QJ t i QJ o £ cn ^ > to CO QJ U H ° -o <-> 7? QJ < £ S o CM o o (LU) ApoqajQ eiu IUOJJ aoueisiQ QJ >H bp Chapter 4: Field Trials 71 s h o t p o i n t to o re -zone , to a n accu racy of abou t 5 m , w e r e o b t a i n e d f r o m d r a w i n g s p r o v i d e d b y the m i n e . G i v e n the d i s t ance to the nearest o re -zone a n d the P - w a v e v e l o c i t y w e c a n c a l c u l a t e the e x p e c t e d a r r i v a l t imes , w h i c h are m a r k e d b y the in tersect ion of the traces w i t h a do t t ed l ine . E i g h t traces (SP 3 to 6, S P 9 to 11, S P 14, a n d p o s s i b l y S P 1 a n d S P 2) c o n t a i n a s p i k y s i g n a l that m o v e s out w i t h a s l o p e consis tent w i t h convers ions at the nearest por t ions of the orebody . T h i s t r end is v e r y c lear i n p a n e l (a), e l i m i n a t i n g the pos s ib i l i t y that E M emiss ions f r o m the e x p l o s i o n are respons ib le : the h i g h c o r r e l a t i o n w i t h the expec ted a r r i v a l t imes m a k e s a n e x p l a n a t i o n ba sed u p o n a r a n d o m process i m p l a u s i b l e . S u c h q u a s i - r a n d o m E M emiss ions have been r e p o r t e d b y O ' K e e f e a n d T h i e l (1991) i n the i r obse rva t ion of q u a r r y blasts a n d they have p r o p o s e d that s p a l l i n g a n d stress releases f r o m the sha t t e red r o c k nea r the s h o t p o i n t are respons ib le . T h i s type of process m i g h t e x p l a i n the presence of the ea r ly o r near-shot s ignals (see F igures 4.5 a n d 4.6), but these s ignals do not start at the same t ime as the blast a n d i n s o m e cases are no t iceab ly late ( S P M for instance), u n l i k e the emiss ions obse rved b y O ' K e e f e a n d T h i e l . T h e offset b e t w e e n the d o t t e d l i n e a n d the R P E a r r i v a l s is p r o b a b l y d u e to the t i m e t a k e n for the s e i s m i c e n e r g y to r e a c h p e a k a m p l i t u d e ( a p p r o x i m a t e l y ms , see sho tpo in t geophone trace i n F i g u r e 4.5), o r f r o m errors i n the estimates of a r r i v a l t imes f r o m the m i n e d r a w i n g s . T h i s dataset p r o v i d e s f i r m ev idence that the i m p u l s i v e E M s ignals iden t i f i ed as R P E o n o u r records arise f r o m se ismoelec t r ic convers ions b y mass ive sul f ides w i t h i n the 1100 lens because no other nea rby geo log i ca l s t ructures or k n o w n p h y s i c a l process c o u l d p l a u s i b l y p r o d u c e the same m o v e o u t . The first a r r iva l s appear to del ineate the near edge of the o rebody , as expected , bu t i f the rate of s i g n a l gene ra t ion is p r o p o r t i o n a l to the d e n s i t y of s u l f i d e m i n e r a l s t h e n w e w o u l d expect sus ta ined bursts of s ignals as the se ismic w a v e sweeps t h r o u g h the o r e b o d y u n t i l there is insuff ic ient energy to excite the R P E m e c h a n i s m . A g lance at F i g u r e 4.6 s h o w s that this is c lear ly not the case. T h e most r e spons ive por t ions of the 1100 o r e b o d y appear to be the edges n o r m a l to the wavef ron t , o r nearest to the shot. It m a y be that R P E is s t i m u l a t e d mos t eff iciently at the interfaces be tween the host ma te r i a l a n d su l f ide Chapter 4: Field Trials 72 m i n e r a l s , o r c o n d u c t i v e o r e b o d i e s s u c h as the 1100 l ens m a y a t t enua te s i g n a l s emana t ing f r o m in te r io r po in t s . G i v e n that it is the edges of the o r e b o d y that r e s p o n d best, w h a t is the s igni f icance of the late R P E s ignals , w h i c h can be seen i n F i g u r e 4.6 o n traces f r o m S P 2 to SP10? T h e p l a n v i e w of the o r e b o d y i n F i g u r e 4.4 offers a c lue : the w e s t e r n sec t ion of the o r e b o d y sp l i t s in to t w o r o u g h l y p a r a l l e l lenses. If the es t imated t r ave l t imes to the m o r e dis tant lens are c o m p a r e d w i t h the a r r i v a l t imes o f these later s igna ls a g o o d ma tch is obta ined . In F i g u r e 4.7 o u r i den t i f i ed R P E a r r i v a l t imes a n d the ca l cu la t ed t r ave l t imes to the nearest p o r t i o n of the t w o lenses, A a n d B (the B lens is n o r t h of A ) , f r o m each sho tpo in t are p lo t t ed for c o m p a r i s o n . O u r p i c k s f r o m the data c lose ly ma tch the expected a r r i v a l t imes, too w e l l to at t r ibute to chance. T h e p o s s i b i l i t y that the s e c o n d a r r i v a l pa t te rn is d u e to a s e i smic re f lec t ion is u n l i k e l y as there is no o b v i o u s re f l ec t ing s t ruc tu re w i t h the r i g h t geome t ry , a n d l i t t l e i n d i c a t i o n o f s t r o n g reflectors o n o u r geophone records. N o r is i t conce ivab le that S-wave c o n v e r s i o n at the nearest ore-zone is respons ib le for the second g r o u p of a r r iva l s because the rat io of first a r r i v a l t ime to second a r r i v a l t ime varies too greatly. T h e data s h o w s t w o g roups of R P E s ignals that appear to or ig inate f r o m seismoelectr ic c o n v e r s i o n at the near edges of the A a n d B lens structures of the 1100 orebody . T h e d i s c u s s i o n of resul ts so far has s h o w n the g o o d c o r r e l a t i o n b e t w e e n the s i g n a l a r r i v a l t imes w i t h the k n o w n geo logy . S ince w e feel cer ta in that the s p i k y s igna ls are R P E , a n d that they o r ig ina t e f r o m zones of e c o n o m i c interest , w h a t c o u l d w e have d e d u c e d about the o rebody u s i n g a m i n i m u m of p r i o r geo log ica l i n fo rma t ion? A su i tab le m e t h o d of in te rpre t ing the data is to l o o k for b o u n d a r i e s b y f i n d i n g c o m m o n tangents to the c i r cu l a r arcs i n a sect ion (descr ibed i n C h a p t e r 3, B o u n d a r y De l inea t ion) . Because b o u n d a r y de l i nea t i on is a 2 - D m e t h o d a p l ane o r i en ta t ion m u s t be selected for the in terpre ta t ion . F o r the interface to be m a p p e d accura te ly i t m u s t l i e near the sect ion plane. D r i l l results , at 50 m spac ing , indicates the o rebody is a p p r o x i m a t e l y lens shaped , a n d w h e n projected onto the surface s t r ikes east-west a l o n g 10 0 0 0 N (F igure 4.4); thus, Chapter 4: Field Trials 73 the 10 0 0 0 N p lane is a p lane l i k e l y to be successful . Fur the rmore , the r a m p intersects the o r e b o d y at e l eva t ion 4970 meters so a h o r i z o n t a l sect ion i n that p lane w o u l d a lso be a g o o d candida te . F i g u r e 4.8 s h o w s the r e su l t i ng interpretat ions m a d e f r o m these cross-sect ions. S h a d e d areas i n F i g u r e 4.8 m a r k w h e r e I have p i c k e d interfaces f r o m the wavef ron t pos i t ions , w h i c h are s h o w n as arcs i n the d i a g r a m . The arcs w e r e shor tened for c la r i ty , a n d others for w h i c h no c o m m o n tangents c o u l d be f o u n d are p lo t t ed i n the same d i r ec t i on as those that were successful ly connected. In F i g u r e 4.8a the in te rpre ted interfaces correlate w e l l w i t h the es t imated pos i t ions of the r idges associated w i t h the A a n d B lens features. T h e first a r r i v a l s i n F i g u r e 4.8b cor rec t ly de l inea te the nearest interface, the top of the B lens , bu t the second interface has been m a p p e d to a l o w e r p o s i t i o n (13-15 m) . In fact, it is loca ted at a p p r o x i m a t e l y the same e l eva t ion as the nearest interface, bu t l ies i n the 9 9 7 0 N p lane as is apparent i n F i g u r e 4.8a. C l e a r l y , the select ion of the 10 000 N p lane (Figure 4.8b) was not a g o o d choice for de l inea t ing the A lens. 1 5 10 E CD E O Expected Time B-Lens • First Arrival Pick A Expected Time A-Lens A Second Arrival Pick A A * i  A A & A A -0- O O • 8 • o • o o —r-1 2 1 5 Shotpoint F i g u r e 4.7 A c o m p a r i s o n be tween s i g n a l a r r i v a l t imes a n d es t imated t r ave l t imes to the nearest po r t i ons of the A a n d B lens features. T h e p i c k s are f rom the traces d i s p l a y e d i n F igu re 4.6. Zi Chapter 4: Field Trials 74 (A) (B) F i g u r e 4.8 10 050 N 10 000 N 10 950 E 10 950 E + Marks the projected position of each shotpoint 11 000 E + Marks the projected position of each shotpoint 11 000 E Elev 5000 Elev 4900 A p p l i c a t i o n of the b o u n d a r y de l inea t ion m e t h o d to the M o b r u n data-set. A r r i v a l s f r o m repeat shots were i n c l u d e d i n the ana lys i s . A r c s i n this d i a g r a m represent the se i smic wavef ron t at t imes of s i g n i f i c a n t E M ac t iv i ty . T h e m e t h o d has succes s fu l l y o u t l i n e d t w o p r o m i n e n t interfaces associated w i t h the A a n d B ore lenses. C o n c l u s i o n s Chapter 4: Field Trials 75 T h e da t a o b t a i n e d f r o m the M o b r u n M i n e s h o w s c o m p e l l i n g e v i d e n c e that R P E or ig inates f r o m s u l p h i d e mine ra l s . It appears that the fr inges of the o r e b o d y p r o d u c e d mos t of the s igna ls . T h i s m a y be d u e to the c o n d u c t i v e i n n e r po r t i ons of the o r e b o d y a t tenuat ing the s ignals , o r favorable m i n e r a l o g y w i t h i n the fr inges. T h e incons i s ten t na tu re of R P E a r r i v a l t imes a n d a m p l i t u d e s h i n d e r e d in t e rp re t a t ion , bu t it w a s f o u n d to be su f f i c i en t ly consis tent to d e t e r m i n e the source of the s igna l s . I n c l u d i n g s o m e s i m p l e g e o l o g i c c o n s t r a i n t s e n a b l e d the b o u n d a r y d e l i n e a t i o n in te rp re ta t ion m e t h o d to m a p the nearest interface accura te ly , a n d to d o fa i r ly w e l l o n the m o r e d is tan t interface. These interfaces w e r e 0 to 60 m d is tan t f r o m the sho tpo in t , thus demons t r a t i ng the ab i l i t y of an R P E - b a s e d m e t h o d to i m a g e a target to 60 m . 4.4 L y n x M i n e I T h e p r i n c i p a l p u r p o s e of this exper iment was to de t e rmine i f the se ismoelec t r ic m e t h o d c o u l d detect s u l p h i d e ore that w a s p r i m a r i l y sphaler i te . A n o t h e r k e y a i m was to a t tempt to de l inea te the b o u n d a r y of the o r e b o d y , or p r o d u c e an i m a g e o f the ore s t ruc ture . O t h e r a ims were to ex tend shot - to-orebody a n d sensor- to-orebody dis tances , to evaluate n e w ins t ruments , a n d to genera l ly i m p r o v e p r o d u c t i v i t y . E x p e r i m e n t D e t a i l s T h e L y n x m i n e , opera ted b y W e s t m i n Resources , is l oca ted i n the centre of V a n c o u v e r I s land , B . C . T y p i c a l ore grades f rom the L y n x m i n e are 7.8% Z n , 1.3% P b , 1 .2%Cu, 135g / t A g a n d 2.8 g / t A u . In decreas ing order of abundance , the m a i n s u l p h i d e m i n e r a l s are pyr i t e , sphaler i te , cha lcopyr i t e , galena, tennanti te a n d born i te (Pearson, 1993). T h e l y n x o r e b o d y is a fau l ted a n d f o l d e d a r ray of i n d i v i d u a l lenses a l o n g a 2700 m s t r ike length . These ore lenses occur w i t h i n a s ta t igraphic un i t k n o w n as the L y n x - M y r a - P r i c e H o r i z o n (Pearson , 1993). T h i s h o r i z o n fo rms a n a s y m m e t r i c a l an t i c l i ne , w i t h the ore o n the Chapter 4: Field Trials 77 s o u t h s ide of the an t i c l ine ca l l ed the S-zone, a n d the o rebodies o n the n o r t h s ide ca l l ed the G - z o n e . O u r i n i t i a l tests were c o n d u c t e d near lenses b e l o n g i n g to the S-zone o f ore lenses. S-zone lenses are severa l meters th ick, a n d d i p s teeply at about 70-80 degrees. G -zone lenses t end to be th icker , a n d d i p at angle of a p p r o x i m a t e l y 40-50 degrees. In b o t h regions h a n g i n g w a l l rocks are andesi te , a n d the f o o t w a l l rocks are rhyo l i t e . T h e tests w e r e o n a p o r t i o n of the S-zone o n l e v e l 14 i n the L y n x M i n e i n a passage r u n n i n g r o u g h l y p a r a l l e l w i t h the o r e b o d y a l o n g 1 1 1 0 0 N b e t w e e n 2000E a n d 3000E ( M i n e co-ord ina tes , F i g u r e 4.9). M e a s u r e m e n t s were c a r r i e d out d u r i n g M a y 11 to 15, a n d J u l y 15 to 19, 1992. Pa r t i c ipan t s i n the M a y expe r imen t w e r e M . M a x w e l l , B . B . N a r o d , a n d mysel f . In the Ju ly exper iment R. D . R u s s e l l , M . M a x w e l l , K . E . B u t l e r j o ined m e i n the f i e l d w o r k . A p p r o x i m a t e l y 100 shots w e r e f i red , w i t h mos t f i r ed be tween 2100E a n d 2600E (F igure 4.9). E a c h shot cons is ted of a 0.22 o r 0.45 k g p r i m e r charge (pentolite) p l a c e d i n a 6 ft or 8 ft d r i l l - ho l e . T h e M a y tests u sed the smal le r charge size, la rger charges were u s e d i n J u l y tests because I w a s c o n c e r n e d abou t the l a c k of o b s e r v e d s i g n a l s f r o m the M a y exper iments . T h e fiber opt ic t r igger m a d e its debu t d u r i n g the M a y exper iments . Safety fuse detonators were u s e d w i t h the fiber opt ic t r igger to e l imina te a n y w i r e s l e a d i n g in to the exp los ive . These tests i n c l u d e d s o m e n e w l y d e v e l o p e d E M sensors ( U B C V a n d the H i g h B a n d w i d t h preampl i f ie r s ) . The w i d e r b a n d w i d t h of these sensors w a s not exp lo i t ed , but the l o w e r no ise p r o v e d to be v e r y usefu l . E lec t r i c a n d magne t i c f i e l d an tennas w e r e p l aced at three locat ions a n d m o n i t o r e d E M i n the 1 k H z to 60 k H z b a n d . In genera l , t w o o r t h o g o n a l h o r i z o n t a l d i p o l e s m e a s u r e d the electr ic f i e ld , a n d three o r t h o g o n a l U B C V m a g n e t i c sensors m e a s u r e d the magne t i c f i e ld . In the M a y tests the U B C V sensors exh ib i t ed pos i t i ve feedback p rob lems , a n d t w o were rep laced w i t h the o l d e r U B C I sensor a n d a l o n g w i r e an tenna a r rangement . Three c o m p o n e n t m a g n e t i c a n d electr ic f i e l d Chapter 4: Field Trials 78 d a t a w e r e a c q u i r e d i n the J u l y tests w i t h three i m p r o v e d U B C V an tennas , t w o h o r i z o n t a l d ipo l e s , a n d a s m a l l ve r t i ca l ly o r ien ted pa ra l l e l plate d i p o l e . A geophone w a s p l a c e d w i t h the sensors to m o n i t o r g r o u n d m o t i o n , a n d ob ta in v e l o c i t y i n f o r m a t i o n . P -w a v e veloci t ies were measu red to be about 5800 m / s . A l l of the sensors were g r o u p e d together, bu t m o v e d occas iona l l y because of concerns abou t s i g n a l q u a l i t y . In the M a y e x p e r i m e n t the sensors w e r e g r o u p e d n e a r the i n s t r u m e n t a t i o n site at 2800 E ( F i g u r e 4.9), w h i c h is u p to 300 m f r o m the exc i t ed po r t i ons of the o rebody . It was d e c i d e d to m o v e the sensors c loser to 2100 E i n the J u l y tests, bu t ha l f -way t h r o u g h the exper iments w e r ea l i zed that the t u n n e l w a l l s i n this area w e r e c o v e r e d w i t h a r e in fo rc ing m e s h of steel. T h e sensors were m o v e d fur ther s o u t h to reduce the s h i e l d i n g effects f rom the nearby c o n d u c t i v e structures. Discussion of Results I i den t i f i ed three types of se ismoelec t r ic response, w h i c h I have l abe l ed as f u z z , sp ike , a n d b u r s t t y p e s ( F i g u r e 4.10). F u z z s i g n a l s a p p e a r as a n i nc rea se i n r a n d o m e lec t romagne t ic ac t i v i t y , w i t h no preference i n p o l a r i t y o r i n f requency. A s p i k e is a so l i t a ry pu l se l as t ing less than 0.1 ms. Burs ts l o o k l i ke a sequence of sp ikes o n the h i g h f requency antennas (1 k H z low-cu t ) , bu t subs tan t ia l a m o u n t s of l o w e r f requency energy was ev ident o n a d i p o l e set w i t h a 10 H z low-cut . O f the three types of response o n l y the sp ike s ignals can be c lear ly a t t r ibuted to the orebody. The fuzz s ignals were seen d u r i n g o u r first test, w h e n the antennas were p l a c e d at 2800E, bu t were not seen o n a n y of the second test records w i t h the antennas p l a c e d near 2100E. A l s o , o n l y the shots s i ted f r o m 2400E to 2600E i n d u c e d these s ignals . T h e a r r i v a l t imes of the f u z z s igna ls are before the se i smic w a v e c o u l d have reached the m a i n o rebody ; therefore, these s igna l s appea r to o r ig ina te i n the host r o c k near the sho tpo in t s . It is poss ib le that s igna ls c o u l d be d u e to the presence of s m a l l a m o u n t s of s u l p h i d e s i n the host rock , or a n u m b e r of nearby ore p o d s (a c o n c l u s i o n reached after d i scuss ions w i t h Chapter 4: Field Trials the L y n x M i n e geologis t , M i k e Becher). 79 cu c CO CJ 0) LU 5 mV/m 1 mV/m 800 uV/m 2 4 6 Time (arbitrary origin) ms Figu re 4.10 E x a m p l e s of the three types of s igna l r ecorded f r o m shots at l eve l 14. T h e burs t s ignals have m a n y characterist ics desc r ibed b y Sobolev et a l . (1982, 1984) a n d are c lear ly i n d u c e d b y the blast, but the s ignals come p r e d o m i n a n t l y f r o m shots i n o n l y one reg ion , 2100E to 2200E. Records c o n t a i n i n g this type of s i gna l u s u a l l y have f ive or s ix burs ts a r r i v i n g f r o m apparen t dis tances of 10 meters to m o r e than 200 meters f r o m the shotpoin t . If the burs t s are d u e to s u l p h i d e s then, a c c o r d i n g to Sobo lev a n d D e m i n (1986), the n u m b e r of d i s t inc t ore packets i n the r eg ion is equa l to the n u m b e r of bursts , a n d the app rox ima te s ize of the o rebody can be d e d u c e d f r o m the d u r a t i o n of the burst . Because m a n y burs t s ignals consis tent ly a r r ive at t imes w h e n the acoust ic w a v e is m o r e than 200 meters f r o m the sho tpo in t w e w o u l d expect that m a n y of the other shotpoin ts w o u l d be able to i n d u c e these s ignals . Unfo r tuna te ly , w e were not able to detect e n o u g h of these s ignals at other shotpoin ts to ident i fy the source. T h e burs t type of s i gna l m a y Chapter 4: Field Trials 80 c o n t a i n va luab l e i n f o r m a t i o n about the geo logy , bu t I was u n a b l e to c o n f i r m w h i c h geo log ica l un i t is responsib le ; a poss ib le c lue is that this type of s i g n a l was not seen i n later tests o n l e v e l 10 o f the L y n x M i n e . T h e s p i k e s igna ls appea r i n records f r o m a l l of the shotpoin ts , a n d m a n y large s igna l s a r r ive at t imes w h e n the se i smic wavef ron t has just entered the ore-zones. A p r o b l e m w i t h the s p i k e s igna l s is that they are v e r y s i m i l a r to m a n - m a d e interference. N o i s e records ( r a n d o m records m a d e i n the absence of the se i smic source) ga the red i n the m i n e a lso s h o w this type of t ransient noise , a n d it w a s f o u n d that the a r r i v a l t imes o f these sp ikes w e r e r a n d o m . T o a n a l y z e the s ign i f i cance of the s p i k e a r r i v a l t imes I cons t ruc t ed a h i s t o g r a m (F igu re 4.11) that p lo t s the n u m b e r sp ike s i n a 5 m s t i m e i n t e rva l at different de lays f r o m the blast. T w o records con ta ined a n o m a l o u s a m o u n t s of noise (i.e. fu l l of spikes) a n d were not i n c l u d e d i n this analys is . 40 cu Q . CO -O E 30 20 H 10 4 -15 Probability that peak is > < 0 . 1 % due to noise fluctuation Time of Blast 1 B 8 8 B i l l l l l f Mean Noise Level -10 0 5 10 15 20 25 30 Time Window (ms after shot fired) 35 Figu re 4.11 H i s t o g r a m of s p i k e events, l e v e l 14, L y n x M i n e . The h i g h coun t 10 to 15 m s after the blast is cons is tent w i t h the k n o w n l o c a t i o n of the orebody. Chapter 4: Field Trials 81 Seismoelec t r ic s igna ls f r o m the mass ive s u l p h i d e s s h o u l d t y p i c a l l y a r r i v e at t imes 5-20 m s after the blast because the se ismic w a v e w i l l be p r o p a g a t i n g w i t h sufficient energy to excite R P E i n the o r e b o d y at these t imes. There is a clear increase i n the s p i k e coun t i n the h i s t o g r a m 10-15 m s after the blast (F igure 4.11 T h e 10-15 ms d e l a y co r re sponds to a d is tance o f 60-90 m f r o m the shotpoin ts (5800 m / s acoust ic ve loc i ty ) , w h i c h is consistent w i t h the k n o w n o r e b o d y l oca t i on re la t ive to the sho tpoin ts . T o test the s ign i f i cance of the peaks i n the h i s t o g r a m a c o m p a r i s o n b e t w e e n coun t s at nega t ive t i m e i n t e rva l s , w h i c h are a measure of the b a c k g r o u n d noise (man-made a n d na tura l ) , a n d peaks are m a d e . A t-test p e r f o r m e d o n the count n u m b e r s ind ica tes that the largest p e a k (10-15 ms) has a s m a l l e r than 1 i n 10 000 chance of b e i n g d u e to r a n d o m f luc tua t ions i n the b a c k g r o u n d noise d u r i n g the shoo t ing . T h e chance that a n y of the c o u n t b i n s w e r e to reach this coun t is less than 1 i n 1000. Therefore , the peak is s ign i f icant a n d cannot be a t t r ibu ted to a f luc tua t ion i n noise. T o locate a target p r o p e r l y , a n u m b e r of shots are n e e d e d to t r iangula te the p o s i t i o n of convers ions . I w a s unab le to d o this w i t h da ta f r o m l e v e l 14 because the sho tpo in t s are p o s i t i o n e d i n a l i ne r o u g h l y p a r a l l e l to the o r e b o d y (see C h a p t e r 3, T o m o g r a p h i c Recons t ruc t ion) . H o w e v e r , w i t h the k n o w l e d g e that the o r e b o d y runs r o u g h l y east to west , a n d is n o r t h of the shotpoin ts , the data can be p lo t t ed i n a d i r e c t i o n (the m a p p i n g of the a r r i v a l t ime to d is tance a l o n g a pa r t i cu la r d i r e c t i o n f r o m each shotpoin t ) that w i l l s h o w t rends i n the da ta a n d ind ica te some of the o r e b o d y s t ructure . T h e best d i r e c t i o n for the l e v e l 14 da ta is d u e n o r t h because the earliest s igna ls f r o m the o r e b o d y w i l l be d u e n o r t h of the shotpoin t , later s igna ls m a y c o m e f r o m other d i r ec t ions as the se i smic w a v e spreads a l o n g the h o r i z o n . A p lo t of the s p i k e data projected n o r t h (F igu re 4.12) s h o w s the c o r r e s p o n d e n c e b e t w e e n la rge s p i k e s igna l s a n d ore-zones r u n n i n g a l o n g 11200N. The s ignals that appear to come f r o m distances further n o r t h are p r o b a b l y f r o m r e g i o n s h i g h e r o r l o w e r i n e l e v a t i o n , o r east a n d wes t of the eas t ing s h o w n . T h e p u r p o s e of the d i a g r a m is to s h o w the s igni f icant increase i n s igna ls as the se i smic w a v e s t r ikes the ore-zones. A n in te rp re ta t ion of the d i a g r a m is that there appears to be a n Chapter 4: Field Trials 82 a n o m a l y be tween 11150 a n d 11300N, 2100E to 2600E, and at an e leva t ion s i m i l a r to l e v e l 14 (10650). A l s o , there is s o m e ev idence of a sma l l e r a n o m a l y s t r i k i n g a l o n g 1 1 0 2 5 N near 2500E to perhaps 2800E. The lack of s ignals f r o m shots west of 2100E is p r o b a b l y d u e to the s m a l l n u m b e r of shots f i r ed at these locat ions ; therefore, l i t t le can be s a i d about extent of the o rebody west of 2100E. 11800 c 11600 = 11400 10800 11200 - + ++++ + Shotpoint 11000 - • Small • Medium • • Large : 4v +++. 1600 1800 2000 2200 2400 Mine Easting 2600 2800 Figu re 4.12 N o r t h w a r d pro jec t ion of a r r i v a l data , l e v e l 14 L y n x M i n e . T h i s p lo t h i g h l i g h t s the sha rp increase i n s igna ls once the se i smic w a v e enters the ore-zones, w h i c h s t r ike east-west o n 11150 a n d 11200 N . C o n c l u s i o n s T h e sphaler i te deposi t s at the L y n x M i n e ( level 14) are able to p r o d u c e R P E . I w a s not able to accurate ly del ineate the o rebody d u e to the unfavorable shot- l ine geomet ry a n d a l ack o f data, bu t I c o u l d d e d u c e the a p p r o x i m a t e dis tance of the o rebody a n d , w i t h some k n o w l e d g e about the o r e - h o r i z o n , gather i n f o r m a t i o n about the l a te ra l extent o f the orebody. Chapter 4: Field Trials 83 T h e s igni f icance of the different species of se ismoelect r ic s igna l is a m b i g u o u s . H o w e v e r , the sp ike type of s ignals have the characterist ics of R P E , a n d they appear to emina te f r o m the orebody . N o clear difference was f o u n d i n the response f r o m the different charge sizes u s e d , 0.22 or 0.45 k g . The large n u m b e r of events 10-15 ms after the blast i n the h i s t o g r a m indica tes that the range of de tec t ion is at least 60-90 m . A n e w m e t h o d of o b t a i n i n g a t ime-break, the fiber opt ic t r igger , was successful ly i m p l e m e n t e d . N o blast E M w a s obse rved i n these e x p e r i m e n t s . I a t t r ibu te th is to the use of the f iber o p t i c t r i g g e r a n d safety fuse detonators . T h e n e w magne t i c sensors ( U B C V ) p r o v e d to be better at de tec t ing s m a l l s ignals that w e r e o therwise m i s s e d b y the o lde r genera t ion of sensors. 4.5 Lynx Mine II T h e p r i n c i p a l a i m w a s to p r o d u c e a t o m o g r a p h i c i m a g e of the o r e b o d y , a n d to exper imen t fur ther i n a p p l y i n g R P E for exp lo ra t ion . Experiment Details T h e o r e b o d y i n th is area be longs to the G - z o n e , a n d is near the apex of the an t i c l ine s t ruc ture of the L y n x - M y r a - P r i c e H o r i z o n (see L y n x I for s o m e g e o l o g i c a l de ta i l s , o r Pea r son , 1993). W o r k o n l e v e l 10 was ca r r i ed out o n 19 to 21 J u l y 1992 i n a r e g i o n near 5200E a n d 11500N i n passages 10-581Dr, 10-501 D r , 10-501X-C, a n d 10-531 D r (F igure 4.13), d u r i n g a f te rnoon a n d n igh t w h e n the m i n e w a s quiescent . R . D . R u s s e l l , M . M a x w e l l , a n d K . Bu t l e r assis ted m e i n these tests. T h e sho tpo in t s w e r e a r r a n g e d so that w e w o u l d be able to t r i angu la te the p o s i t i o n of r e g i o n s that cons i s t en t l y p r o d u c e d s igna l s a n d p r o d u c e a n i m a g e o f the o re -zone . A p p r o x i m a t e l y 50 shots were f i red i n this s u r v e y f r o m 18 locat ions u s i n g a 0.45 k g charge of pentol i te exp los ive i n a 8 ft d r i l l hole for each shot. A l l of the shots w e r e f i r ed at least once w i t h a f iber opt ic t ime break. T o speed the s h o o t i n g process the remote geophone Chapter 4: Field Trials 85 w a s u s e d to t r igger the data acqu i s i t i on sys tem for m a n y of the later shots , a n d the fiber op t ic w a s not u sed . T h i s scheme w o r k e d w e l l because of the excel lent r e p r o d u c t i o n of the se i smic w a v e ob ta ined b y u s i n g pentol i te i n a borehole . T h e type of sensors u s e d a n d the a r rangement was i den t i c a l to the J u l y tests o n l e v e l 14 ( L y n x M i n e I). Three U B C V magne t i c sensors , t w o h o r i z o n t a l d i p o l e s , a n d a v e r t i c a l p a r a l l e l plate d i p o l e were p l aced at 5450 E a n d 11600N in to b l i n d cross-cut. A geophone w a s p l a c e d a m o n g s t the sensors for t r i gge r ing a n d P - w a v e v e l o c i t y measu remen t . P -w a v e veloci t ies were ca lcu la ted to be app rox ima te ly 5500 m / s . D i s c u s s i o n of Resu l t s A n a l y s i s of the da ta f r o m l e v e l 10 was m u c h s i m p l e r t han l e v e l 14 because there w a s o n l y one va r i e ty of s igna l , the sp ike . A n e x a m p l e of a t y p i c a l shot r eco rd i s s h o w n i n F i g u r e 4.14. A d d i t i o n a l l y , w e rece ived twice as m a n y s igna ls pe r shot. A h i s t o g r a m of the n u m b e r of sp ikes vs. de l ay (F igure 4.15) s h o w s that most of the s igna l s occur 5 to 10 m s after the blast , o r a p p r o x i m a t e l y 30 to 65 meters f r o m the sho tpo in t s . It is v e r y u n l i k e l y that this peak is d u e to count f luctuat ions (less than 1 i n 8000). R e p r o d u c i b i l i t y of da ta w a s a lso m u c h better; for example , of four shots f i r ed at SP6 three con t a ined a s i gna l at 10.6 ms . The q u a l i t y of data a n d the degree of f reedom i n shot p lacement a l l o w e d us to inver t the data to p r o d u c e a p seudo- tomograph ic image u s i n g the technique d i scussed i n C h a p t e r 3. Because a l l o f the shots l i e o n a p l a n e ( leve l 10) a n o m a l i e s a b o v e the sho tpo in t s w i l l have m i r r o r images b e l o w the sho tpo in t p lane; therefore w e cannot ascer ta in whe the r a n a n o m a l y is above o r b e l o w l e v e l 10 w i t h o u t u s i n g r e g i o n a l g e o l o g y o r cons t ra in ts f r o m d r i l l da ta . A s m a l l n u m b e r of p a r a l l e l s l ices ( t omograms) w e r e genera ted a n d s t a c k e d to p r o d u c e the f i n a l i m a g e because of u n c e r t a i n t y i n sou rce e l e v a t i o n a n d p o s i t i o n d u e to the d i p p i n g na ture of the orebody . T h i s p roduces a smoo the r a n d m o r e diffuse p ic tu re , bu t i t suppresses the p r o d u c t i o n of artifacts d u e to p ro jec t ing the data Chapter 4: Field Trials 86 CD LL 2 o o ~ C O D ) CO CO ^ o O CO !Z C O 0 L U Time of • Blast -10 H»W>illl -4 10 Time (ms) ' ii 20 30 OPT TRIG B GREEN B RED B BLUE REM G E O DIP1 DIP2 E V E R T F i g u r e 4.14 A n e x a m p l e of a r e c o r d a c q u i r e d f r o m l e v e l 10, L y n x M i n e . T h e topmos t trace is f r o m the f iber-opt ic blast sense c i r cu i t , a n d the fifth trace is f r o m a geophone at the sensor site. B o t h electric a n d magne t ic f i e ld antennas s h o w R P E - l i k e s igna ls f r o m 3 to 10 ms . T h e use of a f i be r -op t i c b las t sense a n d fuse b l a s t i n g caps r e s u l t e d i n a c lea r r e co rd ing of the onset of s i gna l ac t iv i ty . onto the w r o n g p l ane (a ve r t i ca l d i s t r i b u t i o n of shots w o u l d reso lve this p r o b l e m , see C h a p t e r 3). F i g u r e 4.16 is a p l a n v i e w of a stack of 10 h o r i z o n t a l sl ices be tween 0 a n d 40 meters above l e v e l 10. There is c lear ly an a n o m a l y s t r i k i n g east-west a l o n g 11430N. The a n o m a l y shape a n d p o s i t i o n is v e r y c lose to the expec ted p o s i t i o n of the ore-zones cove red by the shot pat tern. Ex t r apo la t i ng the ava i lab le d r i l l da ta puts the expected ore-zones a l o n g 1 1 4 0 0 N ra ther than a l o n g 11430, but this m a y be d u e to the d i p of the Chapter 4: Field Trials 87 o r e b o d y ( towards the north) . A l s o , note that there are t w o w e a k anomal ie s , one a l o n g 11360 (5100E to 5100E) a n d ano the r at about 11550N a n d 5150E. The first m i g h t be associa ted w i t h the u n m i n e d ore h o r i z o n , a n d the second to p i l l a r s left f r o m p r e v i o u s m i n i n g ac t iv i ty . 50 -, 1 40 A c -15 -10 -5 0 5 10 15 20 25 30 35 Time Window (ms after shot fired) Figu re 4.15 A h i s t o g r a m of the s i g n a l a r r i v a l t imes, l e v e l 10, L y n x M i n e . T h e large peak at 5 to 10 ms is consis tent w i t h the k n o w n loca t i on of the ore-zones . C o n c l u s i o n s Because of the sho tpo in t p o s i t i o n i n g a n d g o o d s i gna l p r o d u c t i o n o n l e v e l 10 w e were able to p r o d u c e a fair i m a g e of the ore-zones i n the area c o v e r e d i n the exper iment . T h e p r i n c i p a l a n o m a l y i n the t o m o g r a m is an e longa ted shape w i t h the l o n g i t u d i n a l ax i s s t r i k i n g a l o n g 11430 a n d e x t e n d i n g be tween 5000E to 5200E. T h i s i m a g e agrees ra ther w e l l w i t h the app rox ima te p o s i t i o n a n d shape of the o rebody in fe r red f r o m d r i l l data . Chapter 4: Field Trials 0 25 50 75 100 Count Figure 4.16 A n application of the tomographic reconstruction method, level 10, Lynx Mine. The data-set was approximately 100 events picked from 40 shot records. A n outline of the approximate shape and location of the orebody is superimposed upon the image. The question marks indicate a lack of drill core data. Chapter 4: Field Trials 89 4.6 L y n x M i n e III T h e 1994 f i e l d t r i a l at l e v e l 10 tested the concept of in ject ing a D C bias cur ren t in to the o r e b o d y i n the hope that i t m i g h t p r o d u c e m o r e s ignals , or alter the response suff ic ient ly to observe the ro le that t e l lu r ics p l a y i n R P E . A n o t h e r i m p o r t a n t aspect of these tests w a s to use a v e r y h i g h b a n d w i d t h d i g i t i z e r w i t h w i d e - b a n d sensors to l ea rn m o r e about R P E . T h e first objective was not ach ieved , bu t the second was spec tacu la r ly successful . E x p e r i m e n t D e t a i l s A p p r o x i m a t e l y 30 shots were f i r ed be tween 22 a n d 24 N o v e m b e r , 1994. A f e w ear ly shots w e r e f i r ed w i t h 2-3 sma l l -d i ame te r s t icks (about 1" i n d i ame te r ) , bu t these shots l a c k e d se i smic energy, a n d d i d not s t imula te s ignals f r o m the o rebody . M o s t shots were f i r ed w i t h 16 oz p r i m e r charges, w h i c h were squeezed w i t h d i f f i c u l t y in to the 2" holes w e w e r e u s i n g , but pe r fo rmed v e r y w e l l . O n l y t w o areas were u s e d for f i r i ng . F i r s t l y , w e f i r ed 17 shots i n loca t ions SP1 to SP4 (Figure 4.13), then w e s w i t c h e d to loca t ions SP10 a n d S P U for shots 18 to 24, a n d the last t w o da ta co l l ec t i on shots (numbers 25 a n d 26) were back i n the area SP1 to SP4. Elec t r ic current w a s injected, v i a a c u s t o m borehole probe , 100 ft in to a p re -ex i s t ing bore-ho le at 52E50 a n d 112N00. T h e current was a p p r o x i m a t e l y 5 to 10 m A at about 160 V . O r i g i n a l l y w e h a d p l a n n e d to use a P h o e n i x IP t ransmit ter , bu t resor ted to w i r i n g spare 12V cel ls a n d the batteries f r o m an electric l o c o m o t i v e i n series w h e n i t w a s d e t e r m i n e d that the IP t ransmi t te r p r o d u c e d large a m o u n t s of E M noise . Tests w i t h the cu r ren t a l te rna te ly off a n d o n w e r e c o n t i n u e d u n t i l shot 15. N o cur ren t w a s injected in to the o rebody after shot n u m b e r 15. O u r n e w (Gage) a n d o l d ( R C E ) d i g i t i z e r sy s t ems w e r e c o m p a r e d b y u s i n g b o t h s i m u l t a n e o u s l y , a l t h o u g h , each sy s t em h a d i n d e p e n d e n t antennas. T h e G a g e d i g i t i z e r requires a s t anda rd c o m p u t e r sys tem to act as a host because of p o w e r , bus , a n d m o n i t o r Chapter 4: Field Trials 90 requi rements . A desk top (286 A T style) c o m p u t e r w i t h a c o l o u r m o n i t o r w a s u s e d as a host. T h e d e s k t o p c o m p u t e r w a s p o w e r e d w i t h a s ine w a v e inve r t e r (12 V D C to 120 V A C ) to p r o v i d e m i n i m a l e lec t r ica l interference. T h e m o n i t o r w a s t u r n e d off d u r i n g s h o o t i n g to e l i m i n a t e the p o s s i b i l i t y of l i n e a n d v i d e o f requencies a p p e a r i n g o n the sensors. The G a g e d ig i t i z e r w a s opera ted w i t h t w o channels d i g i t i z i n g ana log s ignals at a rate of 10 M s a m p l e s / s . B o t h magne t ic a n d a n electric f i e ld antenna were u s e d w i t h each d i g i t i z e r sys tem. A l l of the E M sensors w e r e p l a c e d i n the area at 5200E a n d 1 1 2 0 0 N (F igure 4.13). T h e l o w b a n d w i d t h sy s t em ( R C E d ig i t i z e r ) u s e d t w o d i p o l e s a n d three magne t i c antennas ( U B C V ) w i t h a n o p e r a t i n g b a n d w i d t h of 1 to 30 k H z (set b y the A M 5 0 2 a m p l i f i e r s ) . I n a d d i t i o n , the l o w b a n d w i d t h s y s t e m r e c e i v e d s i g n a l s f r o m a s t a n d a r d e x p l o r a t i o n g e o p h o n e s i t ed w i t h the sensors , a n d f r o m a g e o p h o n e e lement m o u n t e d w i t h i n the cu r ren t in jec t ion p robe . T h e G a g e s y s t e m u s e d a p a r a l l e l p la te d i p o l e w i t h a h i g h b a n d w i d t h p r e a m p l i f i e r (0.1 to 5000 k H z ) for m e a s u r i n g electr ic f ie lds , a n d a U B C I V magne t ic sensor (1.5 to 3000 k H z ) . T e k t r o n i x A M 5 0 2 pos t - ampl i f i ca t ion w a s not u s e d o n these sensors because these amp l i f i e r s are l i m i t e d to a b a n d w i d t h of 1 M H z . C o a x i a l cab le w a s u s e d for the h i g h b a n d w i d t h sensors w i t h n o e v i d e n c e of c ross - t a lk o r interference. The coax ia l cable was cons t ruc ted w i t h bo th f o i l a n d b r a i d s h i e l d i n g , a n d a 50 Q. i m p e d a n c e . B o t h cables were t e rmina ted b y a 50 Q t e rmina to r at the d i g i t i z e r to reduce reflections. D i s c u s s i o n of Resu l t s T h e r e su l t s f r o m the c u r r e n t i n j e c t i o n are i n c o n c l u s i v e as the re w e r e n o c l e a r i m p r o v e m e n t s i n r e p r o d u c i b i l i t y o r s i g n a l q u a l i t y t raceable to the cu r r en t in jec t ion . C h a n g i n g the current f r o m shot-to-shot d i d not appear to d i r ec t ly al ter the se ismoelect r ic response . H o w e v e r , da ta f r o m shots after the cur ren t in jec t ion tests w e r e not as w e l l r e p r o d u c e d . T h i s m a y be d u e to the loca t ion of shots (SP10 a n d 11 mos t l y ) , or s i m p l y b y chance. I feel that the cur ren t in jec t ion d i d not have m u c h effect because the s m a l l Chapter 4: Field Trials 91 a m o u n t s of cu r ren t injected c o u l d not s i gn i f i c an t l y a l ter the c o n d i t i o n s near the ore-zone . La rge r currents at the ore-brear ing areas are needed to test the idea p r o p e r l y . T h e G a g e d i g i t i z e r w o r k e d ex t remely w e l l . W e w e r e able to see the E M s igna l s m o r e c l e a r l y t h a n i n p r e v i o u s expe r imen t s ; the best r eco rds w e r e f r o m the e lec t r ic f i e l d (para l le l plate d ipo le ) channe l of the G a g e d ig i t i z e r . The f u l l a m p l i t u d e of the s ignals are p rese rved b y the greater b a n d w i d t h of the G a g e d i g i t i z e r (5 M H z vs. 60 k H z ) , so, rather than just c a p t u r i n g o n l y the v e r y largest s ignals (usua l ly one o r t w o per record) w e were able to observe m a n y more ; for instance, most large transients are a c c o m p a n i e d b y m a n y s m a l l e r t ransients . W e have c o n f i r m e d the v e r y h i g h f requency na tu re of the E M s igna l s . F i g u r e 4.17 d i s p l a y s t w o examples of the s igna l s w e were r e c o r d i n g . T h e top e x a m p l e is a shor t p u l s e w i t h s o m e o s c i l l a t o r y c o m p o n e n t s (the b u m p s o n top of the pu l se ) , a n d the e x a m p l e b e l o w is p r i m a r i l y osc i l l a to ry i n nature. B o t h last for less than 5 ps , w i t h r ise-t imes of less t han 1 ps . T h e osc i l l a to ry s i g n a l a p p e a r e d mos t often. O n e of the mos t in te res t ing features is that the frequency of the osc i l la t ions (approx . 1.3 M H z ) w a s qu i te r e p r o d u c i b l e f r o m shot-to-shot, a n d w i t h i n each record . A n e x a m p l e o f this cons is tency can be seen i n a spec t rog ram (Figure 4.18) of the electric f i e l d f r o m a shot at SP4 . T h i s b e h a v i o r is consis tent w i t h a c l a i m b y R u s s i a n researchers that each type of o r e / m i n e r a l has d i s t i nc t i ve spec t ra l peaks (Sobolev et a l . , 1986). T h e m i n e r a l assemblages that m a k e u p the ore i n the l e v e l 10 area are fa i r ly homogeneous , hence, the spec t r a l response is not expected to v a r y s ign i f ican t ly . Unfo r tuna t e ly , they d o not m e n t i o n w h a t the spec t ra l peaks are for different minera l s . T h e h i g h - b a n d w i d t h tests p r o v i d e d have p r o v i d e fur ther i n s igh t s o n the p h e n o m e n o n of e x h a u s t i n g the o r ebody , a n d the p r o b l e m of r e p r o d u c i n g da ta i n these tests. B o t h aspects have been m e n t i o n e d b y o u r R u s s i a n forerunners (Sobolev et a l . 1982). W h a t I m e a n by exhaus t ing the o rebody is that the o rebody appears to p r o d u c e fewer s igna ls as shoo t ing progresses. F o r example , w h e n w e last c o n d u c t e d tests o n l e v e l 10 ( in 1992) w e Chapter 4: Field Trials 92 0.8 -0.2 J vy.o - | 1 1 , 1 1 1 . 1 5 10 15 20 25 Time (us) F i g u r e 4.17 T w o e x a m p l e s of the type of pu l se r e c o r d e d b y the h i g h b a n d w i d t h sys tem, l e v e l 10, L y n x M i n e . B o t h pu l ses are shor t i n d u r a t i o n , less than 5 LIS. A c o m m o n characterist ic of pulses f r o m the l eve l 10 data-set is the cons is tency of energy i n the 1.2-1.5 M H z b a n d , w h i c h is ev iden t i n the osc i l la t ions i n b o t h pulses . Chapter 4: Field Trials 93 Chapter 4: Field Trials 94 f o u n d that after a p a r t i c u l a r l y l o n g sess ion of s h o o t i n g the o r e -body w a s n o l onge r r e s p o n d i n g - that is , w e c o u l d not see any recognizab le s ignals f r o m the last e ight shots. A g a i n w e have o b s e r v e d this p h e n o m e n o n . F i g u r e 4.19 i l lustrates this po in t . N o t e that of the seven shots that were sequent ia l ly f i red f rom locat ions SP10 a n d 11 ( w h i c h are less t han 10 meters apart) there are v e r y f ew r e m a r k a b l e h i g h f requency responses f r o m shots 21 a n d 22 (ignore the l o w frequency r o l l o n shot 22). Th i s behav io r w a s a lso seen o n a series of shots at locat ions SP1 to 4. It appears that the response of the o r e b o d y can be exhaus ted after severa l shots. Af t e r o b s e r v i n g the g r a d u a l decrease i n a c t i v i t y f r o m o u r w o r k i n the area of SP1 to 4 (shots 4 to 15) w e d e c i d e d to i m m e d i a t e l y t ry shoo t i ng at Location SP10 SP11 SP10 SP11 SP10 SP11 SP10 Shot 16 at 2:13 pm Shot 17 at 223 pm Shot 18 at 2:45 pm Shot 19 at 2:52 pm if" Shot 20 at 3:19 pm Shot 21 at 326 pm Shot 22 at 3:37 pm Illlfllp1'1 "II Amplitude 30 mV 9 mV 40 mV 20 mV 50 mV 10 mV 10 mV Time (ms) Figu re 4.19 D e m o n s t r a t i o n of an o r e b o d y t i r i n g out , l e v e l 10, L y n x M i n e . These are r ecord ings of the h o r i z o n t a l E - f i e ld after a 0.5 k g shot. Sho tpo in t s 10 a n d 11 are a p p r o x i m a t e l y 10 meters apart. Af t e r f ive shots i n th is area the o r e b o d y ceased to p r o d u c e s ign i f i can t h i g h f r equency E M s igna l s . H o w e v e r , the o r e b o d y r e s p o n d e d to o u r shots after w e re tu rned to this area severa l hours later. Chapter 4: Field Trials 95 locat ions SP10 a n d 11 to see i f the w e h a d exhausted the o rebody , or whe the r it was no longer r e s p o n d i n g to shots f r o m a pa r t i cu la r d i r ec t ion . The ac t iv i ty o n the ear l ier traces i n F i g u r e 4.19 demons t ra te that it was the latter. Fur the rmore , the o r e b o d y appea red to recover after a short p e r i o d of t ime. Af t e r shot 22 w e ex i ted the m i n e for a few hour s a n d then r e t u r n e d to shoot b o t h areas aga in . B o t h areas r e s p o n d e d a n e w , w i t h large number s of s ignals . Shot 9 Shot 13 I ^ M M . Shot 25 10 T ime (ms) 15 20 Figu re 4.20 F o u r records of the electric f ie ld after a 0.5 k g shot at S P 1 . These shots w e r e f i r ed u n d e r n e a r l y i d e n t i c a l c o n d i t i o n s . H o w e v e r , the traces appear to be d i s s imi l a r . Pe rhaps it is o n l y the large a m p l i t u d e s ignals that d isappear ; these are the easiest to see o n the vol tage vs . t ime traces, bu t excep t iona l c i rcumstances w i t h i n the o r e b o d y m a y be needed to d e l i v e r these large s ignals . I can e n v i s i o n c i rcumstances where this behav io r m i g h t occur , s u c h as the o p e n i n g of cracks or joints b y the ac t ion of the se i smic w a v e . Chapter 4: Field Trials 97 T h e spec t rograms, w h i c h are better at r e s o l v i n g s m a l l a m p l i t u d e s igna l s , appea r to s h o w less ac t iv i ty too, but the sma l l e r s ignals are m u c h m o r e persistent. Therefore , i t appears that the e x h a u s t i o n of the o r e b o d y is p r i n c i p a l l y a p r o p e r t y of the l a rge a m p l i t u d e s igna l s . T h i s effect is less p r o n o u n c e d for the (more n u m e r o u s ) s m a l l a m p l i t u d e s ignals . The other, a n d p o s s i b l y related, p r o b l e m is that of r e p r o d u c i n g o u r data . In the past, w e have ra re ly been able p r o d u c e i den t i c a l copies of o u r da ta b y s h o o t i n g ano the r shot i n the same l o c a t i o n . In genera l , the s igna l s are f a i r l y c lose i n t i m e of a r r i v a l , bu t i n a m p l i t u d e a n d shape they are v e r y different. T h i s p r o p e r t y u n d e r m i n e s the c r e d i b i l i t y of the se ismoelect r ic m e t h o d for su lph ides . F o r an example l o o k at F i g u r e 4.20, w h i c h is c o m p o s e d of traces f r o m shots at p o s i t i o n SP1 ove r the t w o d a y p e r i o d . T h e traces i n F i g u r e 4.20 a p p e a r to have l i t t le i n c o m m o n because the largest s igna l s are scat tered about i n a r r i v a l t ime . H o w e v e r , the spec t rograms of these traces (F igure 4.21) s h o w a different s tory , the four panels are s i m i l a r i n appearance . T h e p r i n c i p a l area that the spec t rograms are different are i n the l oca t ion ( in t ime) of la rge a m p l i t u d e s igna ls ( look for the r ed -b lue ve r t i ca l l ines) , w h i c h are the o n l y basis for c o m p a r i s o n s i n the vo l t age vs . t i m e traces. T h e a p p a r e n t l y f ick le na tu re of the largest s igna l s contrasts w i t h the persis tence of the m o r e n u m e r o u s sma l l e r s ignals , e spec ia l ly those i n the 1.1 to 1.4 M H z b a n d . It seems that i n the past w e have been u s i n g the w r o n g y a r d s t i c k for c o m p a r i n g r eco rds for r e p r o d u c i b i l i t y . T h e s p e c t r o g r a m p r o v i d e s a bet ter measu re to c o m p a r e records. C o n c l u s i o n s T h e p r o b l e m of r e l i ab ly p r o d u c i n g s ignals is better i f the h i g h a m p l i t u d e s igna ls are not g i v e n great impor t ance . A n a l y s i s of the da ta w i t h spec t rograms s h o w s that the l o w e r a m p l i t u d e s ignals are reproduc ib le . The largest s ignals are ei ther changed b y the process of R P E , or requi re a l eng thy t ime to recover. Chapter 4: Field Trials 98 T h e p h e n o m e n o n of o r e b o d y e x h a u s t i o n w a s d e m o n s t r a t e d . H o w e v e r , the apparen t effects of e x h a u s t i o n of the o r e b o d y can be c o u n t e r e d b y a l t e rna t ing shot l o c a t i o n o r w a i t i n g for the o r e b o d y to recover . T h i s indica tes that angle of i nc idence a n d t ime after the last exci ta t ion are impor t an t i n the R P E process. G a t h e r i n g h i g h e r f requency c o m p o n e n t s b e y o n d 100 k H z is a de f in i t e i m p r o v e m e n t . T h e benefits i n c l u d e better s i gna l d i s c r i m i n a t i o n , poss ib le i den t i f i ca t ion of ore- type, a n d greater r e l i ab i l i t y i n o b t a i n i n g s ignals . W i t h l o w noise , w i d e - b a n d sensors I w a s able to p r o d u c e a response f r o m greater por t ions of o rebody; ins tead of so l i t a ry sp ikes , pe r iods of a c t i v i t y c o i n c i d e n t w i t h the passage of the se i smic w a v e t h r o u g h the ore-zones w e r e obse rved . T h e mos t usefu l sensor for b r o a d b a n d use was the p a r a l l e l p la te d i p o l e . It ga the red b r o a d - b a n d (<5 M H z ) s igna l s w i t h g o o d s igna l - to -no i se , a n d p r o v e d to be c o n v e n i e n t to use. T h e r e is n o f i r m e v i d e n c e that i n j ec t i ng c u r r e n t i n to the o r e b o d y i m p r o v e s the se ismoelect r ic m e t h o d . I feel that idea has mer i t , bu t the a m o u n t of cur rent injected was too s m a l l , a n d the p r o b e too far f r o m the o r e b o d y , to a l ter the c o n d i t i o n s near the o rebody suff ic ient ly . 4.7 C e n t u r y In ear ly 1992 the U . B . C . G e o p h y s i c a l Ins t rumenta t ion G r o u p was i n v i t e d b y C . R . A . to test a R P E - b a s e d e x p l o r a t i o n m e t h o d u p o n a recen t ly d i s c o v e r e d s u l p h i d e o r e b o d y . T h e o r e b o d y h a d e l u d e d de tec t ion by g r a v i t y , magne t i c a n d T E M s u r v e y s ( T h o m a s et a l . , 1992) because i t is p r i m a r i l y c o m p o s e d o f s p h a l e r i t e , w h i c h has n o d i s t i n c t i v e g e o p h y s i c a l trait.. T h i s exper iment was o u r first a t tempt to p e r f o r m R P E measurements o n the surface. P r e v i o u s tr ials were loose ly based u p o n the Sov ie t s ty le of u n d e r g r o u n d w o r k . The a i m of the f i e ld t r ia l was to detect the presence of a large depos i t o f sphaler i te f r o m surface measurements . If the o r e b o d y p r o v e d to be detectable then an a t tempt to image or del ineate it was to be made . Chapter 4: Field Trials 99 E x p e r i m e n t D e t a i l s T h e C e n t u r y depos i t is a large (116 M t ) s u l p h i d e o r e b o d y loca ted a p p r o x i m a t e l y 250 k m n o r t h of M t . Isa i n Q u e e n s l a n d , A u s t r a l i a . It consis ts of a s m a l l e r s h a l l o w s o u t h e r n b lock , w h i c h subcrops i n the sou thwes te rn m a r g i n of the o rebody , a n d a larger , deeper , n o r t h e r n b l o c k c o m p l e t e l y concea led beneath C a m b r i a n l imes tone a n d recent a l l u v i u m ( T h o m a s et a l . , 1992). S u l p h i d e m i n e r a l i z a t i o n is h o s t e d i n d o l o m i t i c s i l t s tones a n d carbonaceous shales i n a sequence about 40 meters th ick , a n d consis ts of four p r i n c i p a l l a t e ra l ly c o n t i n u o u s s u b - d i v i s i o n s . F o o t w a l l rocks consis t of shales a n d s i l t s tones , a n d h a n g i n g w a l l r ocks are d o l o m i t i c s i l t s tones a n d shales. M o s t of the m i n e r a l i z a t i o n occurs as s t r a t abound , b a n d e d sphaler i te , galena, a n d p y r i t e w i t h i n b l a c k carbonaceous shales. T y p i c a l grades are 10.3% z i n c , 1.5% lead , a n d 35 g / t o n n e s i lver . T h e o rebody d i d not p r o d u c e a n a n o m a l o u s response to g rav i ty , T E M , a n d magne t i c su rveys , a n d d r i l l i n g was p r i n c i p a l l y based u p o n geochemica l anomal i e s f o u n d o n the surface near w h e r e the o rebody ou tc rops ( T h o m p s o n et a l . , 1992). T h e f i e l d t r i a l w a s c o n d u c t e d d u r i n g the p e r i o d of Sep tember 14 to 23, 1992, b y R . D . R u s s e l l , M . M a x w e l l , K . E . But le r , a n d myself . E x p e r i m e n t s w e r e p e r f o r m e d i n t w o areas ove r the s o u t h e r n p o r t i o n of the o r e b o d y w h e r e i t is r e l a t i ve ly s h a l l o w . T h e first area w a s loca ted s o u t h of a n e x p l o r a t i o n shaft. T h e sho tpo in t s f o r m e d a T shape w i t h the a rms ex t end ing east, west a n d s o u t h w i t h the e x p l o r a t i o n shaft o n the no r th - sou th l ine . The second area h a d a cross-shaped s u r v e y l ine that was centered o n d i s c o v e r y h i l l . The t w o locat ions are referred to as the T a n d X surveys . Sho tho l e s w e r e d r i l l e d to a d e p t h of 6 meters i n t o b e d r o c k to g i v e bet ter s e i s m i c c o u p l i n g . T w o types of e x p l o s i v e w e r e u s e d , A n z o m e x (a p en t o l i t e e x p l o s i v e ) a n d P o w e r g e l (an a m m o n i u m n i t r a t e / f u e l o i l m i x ) . T y p i c a l l y , 0.7 k g of A n z o m e x w a s u s e d , w i t h some charges u p to 2 k g i n size. P o w e r g e l w a s tested i n sizes r a n g i n g f r o m 1 to 6 k g w i t h 0.5 k g of the pen to l i t e exp lo s ive to ensure f u l l de tona t ion . N e a r the e n d of f i e ld t r i a l some exp los ives were pu t in to deeper d r i l l -ho le s (up 60 m ) near the X - s u r v e y i n a n Chapter 4: Field Trials 100 effort to get closer to the orebody. A total of 97 shots were recorded . T w o por tab le compu te r s , each w i t h a R C E d i g i t i z e r , w e r e ava i l ab le for th is exper iment . In genera l , one w a s u s e d w i t h one or t w o antennas to ob ta in a 500 o r 250 k H z N y q u i s t f r e q u e n c y . T h e o the r c o m p u t e r w a s u s e d i n the s a m e m a n n e r as i n p r e v i o u s u n d e r g r o u n d exper iments : 8 channels w i t h a N y q u i s t of 62 k H z w i t h s i x channels u s e d for E M s igna l s a n d the o ther t w o for t r i g g e r i n g a n d a geophone . D i p o l e s , l o n g w i r e antennas a n d the ferrite-core magne t ic sensors were used . In a d d i t i o n , a sma l l e r v e r s i o n of the U B C V type magne t i c antenna was cons t ruc ted be fo rehand for use i n boreholes . M o s t of the sensorss were d e p l o y e d w i t h i n 50 meters of the sho tpo in t . A remote (<200 m a w a y ) d i p o l e was often d e p l o y e d i n an effort to d i sc r imina t e be tween loca l l y generated s ignals a n d spher ics . T h e T s i te w a s s u r v e y e d f irs t because o f the p r o x i m i t y to the su r face o f h i g h c o n c e n t r a t i o n ore. S e v e r a l deep boreho les i n the area a l l o w e d the l o w e r i n g of the sensors to o r e b o d y depths a n d ve r t i ca l c o m p o n e n t measuremen t of electric f ie lds v i a the l o n g w i r e antenna. B o t h h i g h frequency (1 k H z to 30 k H z ) a n d l o w frequency (10 H z to 1 k H z , to l o o k for poss ib l e p iezoe lec t r i c responses) da ta w e r e co l l ec t ed o n the first t w o days . A f t e r this p e r i o d o n l y h i g h frequency data was co l lec ted because of the absence of audio- f requency s ignals . F o u r days were spent i n this area t r y i n g to ob ta in s ignals f r o m a 40 to 120 m deep orebody. T h e o r e b o d y is s h a l l o w e r at the X site, bu t the ore grades are poore r d u e to l each ing . A greater va r i e ty of antennas a n d r e c o r d i n g me thods were u s e d o n this site because of the l a c k of success d u r i n g the T s u r v e y . In genera l , there w a s a grea ter e m p h a s i s i n o b t a i n i n g s igna ls w i t h a b a n d w i d t h b e y o n d 200 k H z . T h e spec t ra l d e c o m p o s i t i o n un i t ( A p p e n d i x B.9) was i m p l e m e n t e d i n this s u r v e y to h e l p d i s c r i m i n a t e b e t w e e n spher ics a n d V L F noise , a n d b r o a d b a n d s ignals . F r e q u e n c y bands f r o m 50 k H z to 5 M H z were m o n i t o r e d b y th is un i t , h o w e v e r , the use of 100 m of t w i s t e d p a i r cab le b e t w e e n the d i p o l e p re -ampl i f i e r a n d the ins t rument site l i m i t e d the response to 50 k H z to 1.5 M H z . Chapter 4: Field Trials D i s c u s s i o n o f Resu l t s 101 T h e resul ts f r o m b o t h su rveys w e r e d i s a p p o i n t i n g . N o o b v i o u s se i smoelec t r i c s i gna l s appeared . A l l of the records are severe ly c o n t a m i n a t e d b y a c o n t i n u o u s b a c k g r o u n d of V L F - t y p e sources a n d spher ics . T h e s i n u s o i d a l noise ( V L F ) c o u l d be r e m o v e d w i t h post-p rocess ing (na r row n o t c h filters), bu t the spher ics were s i m i l a r i n appearance to m a n y of the s igna ls seen i n p r e v i o u s u n d e r g r o u n d exper iments a n d c o u l d not be r e m o v e d easi ly . Spher ics are t y p i c a l l y l i m i t e d to 10 to 30 k H z frequencies ( M c C r a c k e n et a l . , 1984), w h i c h are d e t e r m i n e d b y the a tmosphe r i c p r o p a g a t i o n modes . It w a s h o p e d that the h i g h e r f requency bands w o u l d s h o w some seismoelect r ic responses. A r e l a t i ve ly s m a l l n u m b e r of b r o a d b a n d s igna ls were detected i n these h ighe r bands o n the spec t ra l d e c o m p o s i t i o n u n i t a n d o n some h i g h f requency electr ic f i e ld measurements , bu t w e w e r e not able to cons is ten t ly r ep roduce a n y of these s ignals . The use of a r emote ly set d i p o l e , separa ted f r o m the o ther antennas b y a p p r o x i m a t e l y 200 meters , d i d not p r o v i d e d i s c r i m i n a t i o n be tween spher ics a n d l o c a l l y generated s ignals . In h inds igh t , the an tenna s h o u l d have been at least 0.5 to 1 k m a w a y to w o r k effectively as a remote detector. T o a n s w e r the q u e s t i o n of whe the r a response m i g h t h a v e been i n d u c e d a s ta t i s t ica l ana lys i s o n the occurrence of sp ikes vs. t ime of blast w a s pe r fo rmed . T h e da ta f r o m the X s u r v e y can be s u r m i s e d as b e i n g of no s ignif icance: w e fa i led to p r o d u c e a recognizab le response. D a t a f r o m the T s u r v e y is m o r e in teres t ing. F i g u r e 4.22 s h o w s a h i s t o g r a m of s p i k e a r r i v a l t imes f r o m the T s u r v e y . T w o peaks i n the h i s t o g r a m are s o m e w h a t s ignif icant , one at 30 to 40 ms a n d the other at 90 to 100 ms. D a t a f r o m p r e v i o u s se i smic w o r k g ives a n average P - w a v e ve loc i ty of about 2700 m / s for s h a l l o w depths ; therefore, the anoma l i e s c o m e f r o m apparen t dep ths of a p p r o x i m a t e l y 100m a n d 300m. T h e first peak c o u l d c o n c e i v a b l y c o m e f r o m the o r e b o d y b e l o w the exper iments . H o w e v e r , it is u n l i k e l y that other peak comes f r o m sources 300 m at d e p t h because the se i smic v e l o c i t y u s e d i n this est imate is o n l y v a l i d for r e l a t ive ly s h a l l o w sources (less than 100 m) , a n d is too l o w for a deeper source. A de l ay of 90 ms w o u l d s h o u l d c o r r e s p o n d to a d e p t h of Chapter 4: Field Trials 102 m o r e than 400 m . T h e se i smic d i s tu rbance w o u l d be very weak at this d e p t h , a n d ve ry u n l i k e l y to i n d u c e R P E . If the late peak is f r o m a seismoelectr ic c o n v e r s i o n then i t m a y be f r o m a s h a l l o w source adjacent to the s u r v e y area. 40 30 E Shot Fired \ t t I CVf Jl 4 \ \ - i 1 r * '** , •< I Bill i > r A t SB I K -10 0 10 20 30 40 50 60 70 80 90 100 110 120 Time (ms after the blast) F i g u r e 4.22 H i s t o g r a m of event a r r i v a l t imes, C e n t u r y . T h e m e a n noise l e v e l w a s e s t ima ted f r o m a co l l ec t i on of no ise records a n d pre-blas t t ime in te rva l s , g i v i n g a m e a n rate of 47 sp ikes per s econd (95% conf idence i n t e r v a l o n this estimate i s 42-52). There is a less than 2% chance that a count f luc tua t ion (due to the P o i s s o n type process) w o u l d g i v e a count above 28 at a pa r t i cu la r t ime in t e rva l . T h e chance that s u c h a peak w o u l d occu r s o m e w h e r e i n the h i s t o g r a m is m u c h h i g h e r at 25%. A f igure o f 25% w o u l d not be cons ide red s ignif icant i n m a n y scient if ic tests, 1-5% is t y p i c a l l y used . H o w e v e r , m o s t of the b ins are h ighe r than the es t imated noise l e v e l , w h i c h is a n u n l i k e l y event i n i tself . In a d d i t i o n to f o r m i n g a n o v e r a l l h i s t o g r a m , h i s tog rams of data f r o m each a r m of the T were c o m p i l e d a n d c o m p a r e d . E a c h sub-g r o u p shared r o u g h l y the same pattern: t w o larger than n o r m a l peaks at about 10-30 ms a n d 90-100 ms. The sub-g roups were too s m a l l i n s ize to p r o d u c e m e a n i n g f u l statistics, but the resemblance of each s u b - g r o u p h i s t o g r a m to the s u r v e y h i s t o g r a m suppor t s the Chapter 4: Field Trials 103 n o t i o n of a consistent response rather than a co l l ec t ion of r a n d o m events. If the estimate of the of the b a c k g r o u n d noise is w r o n g (say it r ea l ly w a s 51 ins tead o f 47) t h e n the peaks h a v e n o r e l evance a n d the h i s t o g r a m is w e l l m o d e l e d b y a r a n d o m b a c k g r o u n d of spher ics . T h i s is poss ib le , bu t u n l i k e l y because a fa i r n u m b e r of no i se records were ava i l ab le to a d d to the pre-blast estimate. O f greater conce rn is the l ack of iden t i f iab le s ignals ; mos t of the s ignals l o o k v e r y s i m i l a r to spher ics r e c o r d e d b y L a b s o n et a l . (1985) a n d M c C r a c k e n et a l . , (1984). T h i s m a y be d u e to the r e l a t i v e l y n a r r o w b a n d w i d t h of the f ina l p rocessed records; mos t of the records were r e c o r d e d w i t h 1-30 k H z b a n d w i d t h s , a n d then no tch f i l tered at severa l frequencies i n the 20-40 k H z range to r e m o v e d o m i n a t i n g n a r r o w b a n d s ignals . That there is no apparent sub-species o f s i gna l a t t r ibutable to these peaks i n the h i s t o g r a m is pe rhaps not s u r p r i s i n g g i v e n the s i m i l a r b a n d w i d t h s of spher ics a n d the f ina l records. C o n c l u s i o n s T h e C e n t u r y o rebody d i d not r e spond w e l l to our at tempts to i n d u c e R P E . H o w e v e r , the balance of a rguments based u p o n stat ist ical ana lys is of s p i k e a r r i v a l t ime da ta f r o m the T s u r v e y a rea ind ica te s that a response was l i k e l y i n d u c e d . I a m u n s u r e abou t the s igni f icance of the late a r r i v a l peak (90-100 ms) f r o m this dataset. It is poss ib l e that the s igna ls c o m e f r o m a re la t ive ly s h a l l o w area, bu t offset f r o m the sho tpo in t b y 100 to 150 m . There is also the p o s s i b i l i t y that this peak comes f r o m a deeper target, o r is s i m p l y not s igni f icant . 4.8 O t h e r F i e l d Tests I m m e d i a t e l y after the C e n t u r y f i e l d t r i a l another surface f i e ld t r i a l w a s c o n d u c t e d at a recent ly d i s c o v e r e d mass ive s u l p h i d e o r e b o d y (also i n N o r t h Q u e e n s l a n d , A u s t r a l i a ) o n a p r o p e r t y o w n e d b y B . H . P . T h i s test w a s v e r y s i m i l a r to the C e n t u r y test i n that the same ins t ruments a n d p rocedures were u s e d . Resu l t s f r o m this t r i a l w e r e a lso s i m i l a r : no o b v i o u s s igna ls , bu t an i n d i c a t i o n that a s ign i f i can t ly h i g h e r than expec ted coun t of Chapter 4: Field Trials 104 sp ikes o c c u r r e d after the blast. M i c h a e l M a x w e l l (of the U . B . C Ins t rumen ta t ion G r o u p ) d i d the s p i k e coun t a n d ana lys i s of a m p l i t u d e vs . t ime traces o n this dataset. Spher ics a n d V L F noise w e r e aga in a h indrance . A q u e s t i o n that m i g h t be a s k e d is d o w e see R P E s ignals e v e r y w h e r e ? T h e p r e v i o u s l y d e s c r i b e d f i e l d t r ia ls have one t h i n g i n c o m m o n : a l l c o n c l u d e that R P E w a s i n d u c e d . W h a t about sites that d o not con ta in s u l p h i d e s ? T h e Ins t rumen ta t ion G r o u p at U . B . C . has c o n d u c t e d m a n y other se i smoelec t r ic tests, s ea rch ing for p i ezoe lec t r i c p h e n o m e n a , i n a reas k n o w n to h a v e v e r y l i t t l e s u l p h i d e m i n e r a l i z a t i o n . A t t w o o f these p iezoelec t r ic f i e ld t r ials , at the Paymas te r site i n O n t a r i o (1991) a n d at H u m b o l d t (1992) i n A u s t r a l i a , 1-30 k H z records w e r e m a d e to check for R P E . N o R P E - l i k e s i g n a l s were obse rved . I have no t s t u d i e d a b a r r e n site ex tens ive ly , bu t there appea r s to be l i t t l e i n d i c a t i o n of N y q u i s t - l i m i t e d ( typ ica l r e co rd i n g b a n d w i d t h s are i n the 5 to 10 k H z range) p u l s e - l i k e s i g n a l s at the v a r i o u s p i ezoe l ec t r i c f i e l d t r i a l s . In s u m m a r y , there is n o e v i d e n c e o f R P E at si tes l a c k i n g s u l p h i d e m i n e r a l s , w h i c h is cons i s t en t w i t h the conc lus ions m a d e b y Sobolev et a l . (1982a). CHAPTER 5 A PHYSICAL MODEL FOR RPE 5.1 Introduction Before I p r o v i d e de ta i l s about a pa r t i cu l a r m e c h a n i s m to e x p l a i n m y observa t ions , a n d those of S o b o l e v et a l . , I w i l l d i scuss the i m p o r t a n t at t r ibutes of R P E so that i t w i l l be c lear w h y pa r t i cu la r candida te mechan i sms are rejected. A n u m b e r of m e c h a n i s m s w e r e p r o p o s e d to m e , a n d I searched ex tens ive ly for po t en t i a l candida tes . M o s t c a n d i d a t e m e c h a n i s m s f a i l e d to d e l i v e r ha l f of the k n o w n p rope r t i e s o f R P E a n d w e r e q u i c k l y d i s m i s s e d . A few of the rejected m e c h a n i s m s w i l l be a n a l y z e d to i l lus t ra te the issues i n v o l v e d i n e x p l a i n i n g the processes of R P E . T h e rest o f the chap te r desc r ibes a m e c h a n i s m based u p o n the f o r m a t i o n o f c r acks w i t h i n s u l p h i d e mater ia l s . M y m o d e l addresses the issues of mechan ica l - to -e l ec t r i ca l c o n v e r s i o n a n d the p r o d u c t i o n of the d i s t i n c t i v e E M e m i s s i o n s o f R P E . P h y s i c a l p a r a m e t e r s of th i s c r a c k - b a s e d m o d e l are e s t i m a t e d , a n d c o m p a r e d w i t h f i e l d measurements w h e n poss ib le . A t present there is no accepted m e c h a n i s m for R P E , a n d w h a t f o l l o w s is l a rge ly specula t ive . 5.2 Physical Attributes of RPE and Sulphides T h e resul ts f r o m m y f i e ld t r ia ls a n d those of Sobo lev ' s research g r o u p p r o v i d e a m p l e d e m o n s t r a t i o n that s u l p h i d e s are a n i m p o r t a n t , i f not e x c l u s i v e , class o f ma te r i a l s to host R P E . In a d d i t i o n , the se ismic f lux m u s t be greater than a cer ta in t h r e s h o l d to create the obse rved e lect romagnet ic s ignals . T h e f i e ld s tudies s h o w that th is t h r e sho ld is of the o r d e r of 200 k P a . B e l o w this l e v e l of stress v e r y f e w s igna l s appea r . F u r t h e r m o r e , greater stresses d o not increase the a m p l i t u d e of the s igna l s , bu t i n s t ead p r o d u c e a greater n u m b e r of s ignals (Sobolev et a l . , 1982a). 105 Chapter 5: A Physical Model for RPE 106 So w h y are s u l p h i d e s so specia l? T h i s is a n impor t an t ques t i on to a n s w e r i f ins igh t in to the process of R P E is to be ga ined . O n e d i s t inc t ive p r o p e r t y of s u l p h i d e mine ra l s is that they are f a i r ly react ive c h e m i c a l l y c o m p a r e d to other rock types , a n d t end to o x i d i z e o r r e d u c e i n the p r e s e n c e o f a i r a n d w a t e r (Sato a n d M o o n e y , 1960). T h e s e o x i d a t i o n / r e d u c t i o n potent ia ls w i l l often p r o d u c e te l lur ic currents , w h i c h can be u s e d to detect the nea rby presence of an o r e b o d y (Te l ford et a l . , 1986). A n o t h e r character is t ic p r o p e r t y o f m a n y s u l p h i d e s is that e lec t ronic c o n d u c t i o n is the d o m i n a n t cur ren t f l o w m e c h a n i s m . M o s t pu re s u l p h i d e mate r i a l s are s emi -conduc to r s , bu t w i t h the a d d i t i o n of i m p u r i t i e s c o n d u c t i v i t y increases great ly (K i t t e l ,1986). In genera l , host rocks consis t of a s i l icate ma t r i x , w h i c h is n o n - c o n d u c t i n g , a n d the currents are ca r r i ed b y e lect rolyt ic i ons i n the p o r e f l u i d s . T h i s e lec t ronic c o n d u c t i o n trai t i s e x p l o i t e d i n the I n d u c e d P o l a r i z a t i o n m e t h o d of e x p l o r a t i o n (Te l fo rd et a l . , 1986). A c o u s t i c a n d m e c h a n i c a l p r o p e r t i e s of s u l p h i d e ores are no t r e m a r k a b l y d i f fe ren t f r o m m a n y o the r r o c k s ( C a r m i c h a e l , 1989), bu t I have no t i ced ( f rom m y labora tory tests) that s u l p h i d e samples are p rone to fracture a n d s p a l l i n g w h e n u n i a x i a l stresses greater than (approx.) 2 M P a are a p p l i e d . O t h e r t h a n s o m e s h a l e s a m p l e s , the hos t r o c k s ( m o s t l y i g n e o u s o r m e t a m o r p h i c ) w e r e m u c h s t ronger . A l l o r some of these proper t ies m a y be impor t an t , a n d there m a y be others; I have m e n t i o n e d the proper t ies that I feel are impor tan t . Sobo lev et a l . (1982a) c l a i m to have m e a s u r e d u l t r a son ic v ib ra t ions , c o i n c i d e n t w i t h the E M r a d i a t i o n , w i t h i n the o r e b o d y u n d e r test. T h e presence of u l t r a s o u n d is a k e y i n d i c a t i o n of m i c r o - c r a c k i n g ( Y a m a d a et a l . , 1989). Fu r the r ev idence for the i m p o r t a n c e of cracks is p r o v i d e d b y the c l a i m b y Sobo lev a n d D e m i n (pr ivate c o m m u n i c a t i o n w i t h R. D . R u s s e l l a n d M . M a x w e l l o n a v i s i t to M o s c o w , 1991) that they c a n r e p r o d u c e the s igna l s b y f r ac tu r ing a s a m p l e of s u l p h i d e ore o n a kn i fe -edge w h i l s t p a s s i n g cu r r en t t h r o u g h the sample . M y at tempts at r e p r o d u c i n g the e lec t romagnet ic s igna l s b y s i m p l y s t ress ing the s a m p l e u n i - a x i a l l y were unsuccess fu l ; the R u s s i a n s a l so e x p e r i e n c e d this p r o b l e m . T h i s lack of success i n s i m p l e labora tory tests indicates that the b u l k proper t ies of the ma te r i a l are not the o n l y factor i n p r o d u c i n g R P E , the s u r r o u n d i n g e n v i r o n m e n t Chapter 5: A Physical Model for RPE a n d s t ructure of the ma te r i a l mus t also be cons ide red . 107 T h e e lec t romagne t ic responses f r o m R P E range across the f u l l s p e c t r u m , f r o m V L F to H F , l i gh t a n d X - r a y s (Sobolev et a l . , 1982a; S o b o l e v et a l , 1984b). I n the V L F to R F p o r t i o n of the s p e c t r u m the s i g n a l is i n the f o r m of a b r i e f pu l se , s eve ra l m i c r o s e c o n d s i n d u r a t i o n . T h i s p u l s e m a y be o s c i l l a t o r y , w i t h t he f r e q u e n c y o f o s c i l l a t i o n charac te r i s t i c of the t ype of m i n e r a l r e s p o n s i b l e for p r o d u c i n g the e l ec t romagne t i c r a d i a t i o n (Sobolev et a l . , 1982a, Sobo lev et a l . , 1986). X - r a y s often resu l t f r o m co l l i s i ons be tween h i g h energy electrons a n d a tomic n u c l e i ( D y s o n , 1973). T y p i c a l energies for X -r ay p r o d u c t i o n l i e i n the range 10 k e V to 100 k e V (Potts, 1993); thus , e lectr ic f ie lds r a n g i n g f r o m 10 to 1000 M V / m are n e e d e d (the r e g i o n of acce le ra t ion m u s t be f a i r l y s m a l l o the rwi se the electrons w i l l lose too m u c h energy v i a i o n i z i n g co l l i s i ons ) . N o t e that the r a p i d d i s c h a r g e of ene rgy i n the host m e d i u m b y these e lect r ic f i e lds c a n v a p o r i z e par t s o f the m e d i u m a n d cause acous t i c d i s t u r b a n c e s . C o n s e q u e n t l y , a se i smogen ic process that can create eno rmous electric f ie lds o r potent ia ls w o u l d e x p l a i n a n u m b e r of i m p o r t a n t proper t ies of R P E . A n o t h e r s i g n i f i c a n t aspect o f R P E is that the e l ec t romagne t i c s i g n a l s a r i s i n g f r o m iden t i ca l tests are not the same, a n d often bear no resemblance to each other ( K e p i c et a l . , 1995; S o b o l e v et a l . , 1982). Spec t rograms (of the E M signals) f r o m the L y n x M i n e often d i s p l a y e d r emarkab le s i m i l a r i t y i n iden t i ca l tests, bu t i n genera l there is a un iqueness to each test that is i nhe ren t ly part of the process. It w a s f o u n d i n the L y n x m i n e tests that the largest s ignals are the least r ep roduc ib le . In s u m m a r y , I a m l o o k i n g for a p h e n o m e n o n that can be i n i t i a t e d b y a s t rong se i smic d i s t u r b a n c e , p r o d u c e s a l a rge E M pu l se , a n d c a n be p r o d u c e d i n n a t u r a l s u l p h i d e mater ia ls . In a d d i t i o n , l igh t , X - r a y s , a n d m i c r o - c r a c k i n g m a y be a b y - p r o d u c t . I expect that the process alters its e n v i r o n m e n t suff ic ient ly to i n h i b i t the success o f r epea t ing a n y e x p e r i m e n t s . Chapter 5: A Physical Model for RPE 5.3 P o t e n t i a l M e c h a n i s m s 108 Seve ra l pos s ib l e exp lana t ions of R P E w i l l be e x a m i n e d i n this sec t ion . U l t i m a t e l y , a l l bu t the last w i l l be rejected because they fa i l to p r o v i d e an adequate d e s c r i p t i o n of the obse rved characterist ics of R P E . A n ea r l y s u g g e s t i o n p u t to m e w a s the p o s s i b i l i t y of p i e z o e l e c t r i c c h a r g i n g o f the s u l p h i d e m a t e r i a l f o l l o w e d b y a v e r y q u i c k d e p o l a r i z a t i o n . T h i s is i n essence the hypo thes i s that Sobo lev et a l . (1982a) i n i t i a l l y pu t f o r w a r d . T w o ques t ions arise: w h a t is the m e c h a n i s m of d e p o l a r i z a t i o n , a n d are the r e s u l t i n g e lec t r ic f i e lds suf f ic ien t to account for associa ted p h e n o m e n a s u c h as X - r a y s . S o b o l e v et a l . (1982a) p r o p o s e d that na tu r a l thyr i s tors , t ransistors a n d other non- l inea r c i rcu i t e lements are p r o d u c e d b y the i m p u r i t i e s a n d c rys t a l g r a i n bounda r i e s w i t h i n the s u l p h i d e mine ra l s ; these w i l l break-d o w n o n c e the p o t e n t i a l i s s u f f i c i e n t , a n d a l l o w free e l e c t r o n s w i t h i n the s e m i c o n d u c t o r / s u l p h i d e s to r a p i d l y neu t ra l i ze the p o l a r i z a t i o n . F i r s t l y , I f i n d i t d i f f i cu l t to b e l i e v e that s u c h a n u n l i k e l y n a t u r a l c i r c u i t c o m b i n a t i o n s h o u l d be so c o m m o n t h r o u g h o u t the o rebodies that I have inves t iga ted . A l s o , e lec t r ica l c i r cu i t s , n a t u r a l o r o the rwise , t end to r e p r o d u c e resul ts w h e n i d e n t i c a l l y tested; R P E does no t share th is p r o p e r t y . A s to the s e c o n d q u e s t i o n , X - r a y p r o d u c t i o n is u n l i k e l y because the p i ezoe lec t r i c coefficients of n a t u r a l ma te r i a l s are too s m a l l (of the o r d e r of p C / N ) to p r o d u c e suff icient p o l a r i z a t i o n (under t y p i c a l f ie ld cond i t ions ) for X - r a y p r o d u c t i o n . A s i m p l e r m e c h a n i s m w o u l d have a crack f o r m i n a cha rged p iezoe lec t r ic ma te r i a l . T h e e l ec t r i ca l b r e a k - d o w n of the a i r gap i n the c r ack w o u l d p r o v i d e the i m p u l s i v e E M s ignals . T h i s m e c h a n i s m can p r o d u c e l igh t , X - r a y s a n d u l t r a s o u n d ( N i t s a n , 1977; Cres s et a l . , 1987; Y a m a d a et a l . , 1989, E n i m o t o a n d H a s h i m o t o , 1990). It is not a p a r t i c u l a r l y exot ic effect; m a n y h a n d - h e l d gas l ighters are based u p o n the e lec t r ica l gas -d i scharge f r o m a stressed piezoelect r ic . A s ignif icant d r a w b a c k to a p iezoe lec t r ic based hypotheses is the r equ i remen t of a p iezoe lec t r ic mate r ia l . T h e o n l y c o m m o n p iezoe lec t r i c s u l p h i d e is spha le r i t e , bu t m a n y o ther n o n - p i e z o e l e c t r i c s u l p h i d e s p u r p o r t e d l y p r o d u c e R P E Chapter 5: A Physical Model for RPE 109 (Sobolev et a l . , 1986). Therefore, s igni f icant quant i t ies of sphaler i te , qua r t z , o r the m u c h less abundan t s u l p h i d e s of c a d m i u m or arsenic w o u l d be necessary for these processes to w o r k . A n o t h e r p h e n o m e n o n that I have cons ide red is the acoustoelectr ic effect f o u n d i n s e m i -c o n d u c t i v e p iezoe lec t r i c mater ia l s ( H u t s o n a n d W h i t e , 1962). H u t s o n a n d W h i t e (1962) f o u n d that i f a D C bias cur ren t w e r e pu t across a s a m p l e of c o n d u c t i v e p iezoe lec t r i c m a t e r i a l ( C d S t y p i c a l l y ) then acous t ic w a v e s c o u l d be a m p l i f i e d as they p r o p a g a t e d t h r o u g h the s amp le . A m p l i f i c a t i o n o n l y o c c u r r e d once the cur ren t exceeded a c r i t i c a l v a l u e . T h e c o n d i t i o n for a m p l i f i c a t i o n is that the dr i f t v e l o c i t y of the charge car r ie rs m u s t be greater t han the acous t ic v e l o c i t y (Whi t e , 1962). T h e D C bias cur ren t sets a m e a n ve loc i ty to the charge carr iers (the dr i f t ve loc i ty ) . If a n e l ec t romot ive force causes the charge carr iers to b u n c h o r d isperse l oca l l y then this pe r tu rba t ion m o v e s at the dr i f t ve loc i ty , vd (Ki t t e l , 1986), vd=liE (5.1) w h e r e fx is the m o b i l i t y of the charge carriers (/x=500 c m ^ V " 1 s " l is a t y p i c a l v a l u e for m a n y p u r e s e m i c o n d u c t o r s ) a n d E is the e lec t r ic f i e l d . A n a c o u s t i c w a v e i n a c o n d u c t i v e p iezoe lec t r ic creates a spa t ia l pe r tu rba t ion of the charge carr iers , w h i c h dr i f t v i a the bias current . If the dr if t ve loc i ty is sufficient then pos i t ive feedback ampl i f i e s the acoust ic w a v e . N o r m a l l y the space charge acts to further stiffen the s o l i d (due to energy s to red i n the e lectr ic p o l a r i z a t i o n ) , bu t r e in fo rcement of the acous t i c m o t i o n occu r s w h e n the space charge of one acoust ic phase is t ransferred to a n area of oppos i t e phase. If one of these mate r ia l s is s tressed grea t ly then the r e s u l t i n g p i ezoe lec t r i c f i e lds m a y b o o t s t r a p the p rocess a n d the n a t u r a l ( p h o n o n ) m o d e s of the c r y s t a l / s a m p l e are a m p l i f i e d u n t i l sa tura t ion ( W a n g , 1965; Pus tovo i t , 1969; P o z h e l a , 1981). T h e acous toe lec t r ic effect generates acous t ic shock w a v e s , a n d v e r y h i g h electr ic f ie lds ( R i d l e y a n d W i l k i n s o n , 1969). In a d d i t i o n , pa r t i c l e e m i s s i o n , l i g h t a n d m e c h a n i c a l Chapter 5: A Physical Model for RPE 110 f a i l u r e h a v e b e e n o b s e r v e d ( R i d l e y a n d W i l k i n s o n , 1969). In m a n y w a y s the acoustoelect r ic effect meets the requ i rements for R P E , but i t fai ls i n some c r i t i c a l areas. Because i t relies o n p iezoelec t r ic mater ia ls it does not comfor t ab ly e x p l a i n w h y orebodies c o n t a i n i n g p r e d o m i n a n t l y non-p iezoe lec t r i c s u l p h i d e s r e s p o n d . Spha le r i t e , qua r t z o r a less abundan t c o n d u c t i v e p iezoelect r ic needs to be present. A l s o , it is debatable whe the r the se i smic w a v e can generate e n o u g h stress m a n y tens of meters f r o m the sho tpo in t to in i t i a te the process. G i v e n the p iezoe lec t r ic coefficient a a n d se i smic stress T w e can ca lcula te the r e su l t ing effective electric f i e ld , a n d subst i tute this in to equa t ion 5.1 to g ive the dr i f t ve loc i ty ( £ is the d ie lec t r ic p e r m i t i v i t y of the mater ia l ) : S u b s t i t u t i n g h i g h va lues of stress (1 M P a ) , p i ezoe l ec t r i c coeff ic ient (2 p C / N ) , a n d m o b i l i t y (400 c m ^ V " 1 s" 1) g ives vd ~ 1 300 ms w h i c h is i n the r igh t b a l l p a r k (P -wave ve loc i ty ) , bu t a l l of these n u m b e r s are o p t i m i s t i c a l l y h i g h . T h e mos t o p t i m i s t i c f igure is the m o b i l i t y , w h i c h is for perfect a n d pu re crystals ; i m p u r i t i e s , d i s l o c a t i o n s a n d other defects f r o m other mine ra l s a n d metals i n t roduce further scat ter ing, a n d w i l l r educe the m o b i l i t y b y at least one to t w o orders of m a g n i t u d e (Ki t t e l , 1986). T h e a b o v e - m e n t i o n e d m o d e l s for the R P E process can satisfy m a n y of the requ i rements for R P E , bu t d o not f u l f i l l a l l the r e q u i r e m e n t s un l e s s c o m p l e x a n d q u e s t i o n a b l e ad jus tments are m a d e . M y p re f e r r ed m o d e l is o u t w a r d l y the s imp le s t : a l l that is r e q u i r e d is that a c rack o p e n i n the s u l p h i d e mate r ia l . C r a c k s are a v i a b l e e x p l a n a t i o n because v e r y h i g h t ransfers of m e c h a n i c a l to e l e c t r i c a l e n e r g y are p o s s i b l e v i a t r i b o e l e c t r i c i t y , a n d the a i r - g a p / v a c u u m f o r m e d b y the c r a c k a l l o w s the en t i r e e lec t romagnet ic s p e c t r u m of R P E observat ions to be e x p l a i n e d i n a fa i r ly s i m p l e manner . Three p h y s i c a l effects are needed to p r o d u c e R P E a c c o r d i n g to the c rack m o d e l : m i c r o -c r a c k i n g , t r iboe lec t r ic i ty , a n d E M r a d i a t i o n f r o m a gas d i scharge . T h e first t w o ( m a i n l y mechanica l ) effects w i l l be cons ide red f o r t h w i t h , the latter is t reated i n sec t ion 5.5. Chapter 5: A Physical Model for RPE 111 5.4 T r i b o e l e c t r i c i t y a n d C r a c k F o r m a t i o n T r i b o e l e c t r i c i t y is the p h e n o m e n o n of charge separa t ion a r i s i n g f r o m the m e c h a n i c a l a l te ra t ion of surfaces ( H e i n i c k e , 1984), a n d is the source of m a n y n a t u r a l l y o c c u r r i n g static charges (Hays , 1991). Tr iboelec t r ic effects are p r i n c i p a l l y d u e to the creat ion of fresh surfaces (F igure 5.1). T h e f u n d a m e n t a l process b e h i n d these effects is the transfer o f charge across a b o u n d a r y p r i o r to separat ion; i n essence a d o u b l e electric l aye r is f o r m e d at the b o u n d a r y (He in i cke , 1984). W h e n the t w o n e w surfaces are created b y sph t t i ng the ma te r i a l across the b o u n d a r y some of the transferred charges are not r e tu rned , a n d the t w o surfaces become oppos i t e ly charged. F igu re 5.1 A n i l l u s t r a t i o n of the process of t r iboelect r ic charge genera t ion . A s t w o mate r ia l s are p u l l e d apar t a l o n g a f l aw or jo in t r e s i d u a l charges r e m a i n on the surfaces. T h e s t i m u l u s for charge transfer can be p r o v i d e d i n t w o ways : b y differences i n the b u l k proper t ies of the mater ia ls o n ei ther s ide of the b o u n d a r y , or b y e lec t ro-chemica l means ( H o r n et a l . , 1993; He in icke ,1984) . If t w o mater ia l s possess ing different w o r k funct ions are pu t together then charge w i l l be t ransferred across the b o u n d a r y u n t i l the surface potentials are equa l (Hays , 1991). The w o r k funct ion is def ined as the po ten t i a l energy to free an e lect ron f r o m the surface of the mate r i a l , a n d is character is t ic of the e lec t ronic p r o p e r t i e s of the m a t e r i a l . W h e n the t w o m a t e r i a l s are s e p a r a t e d s o m e o f the Chapter 5: A Physical Model for RPE 112 t ransferred charge m a y not re tu rn because these charges are r e l a t ive ly i m m o b i l e . M e t a l s d o not exh ib i t s ign i f ican t t r iboelect r ic effects because the charges are v e r y m o b i l e , bu t insu la to r s a n d semi -conduc to r s often p r o d u c e notable t r iboelec t r ic effects ( H a y s , 1991). M o s t t r iboelect r ic effects are d u e to this m e c h a n i s m . Because s u l p h i d e ores t end to be ra ther he terogeneous i n nature , w i t h different s u l p h i d e m i n e r a l s c r y s t a l l i z i n g amongs t others, I expect that the l i k e l i h o o d for this type of t r iboelect r ic process to be f a i r ly large. A p r o b l e m w i t h this m e c h a n i s m is that the heterogenei ty has to be f a i r ly la rge i n s ize (of the o rder of 10 cm) to p r o d u c e the large E M fields obse rved i n m y f i e ld t r ials . T h e s e c o n d m e c h a n i s m occurs w i t h o u t d i f ferences i n the subs t ra te ma te r i a l s . A n e lect r ica l o r c h e m i c a l process d i r ec t ly p r o v i d e s the charge transfer m e c h a n i s m across the b o u n d a r y ( H e i n i c k e , 1984; H o r n et a l . , 1993). A s i tua t ion w h e r e this t ype of process m a y be i m p o r t a n t i n the s u l p h i d e o r e b o d y is the passage of t e l l u r i c c u r r e n t across the b o u n d a r y of a large f l a w or joint , whe re there is s t i l l s o l i d contact across the joint , bu t some m i c r o - c r a c k i n g has a l ready occur red . Elect r ic potent ia ls across the b o u n d a r y causes i o n s / e l e c t r o n s to m i g r a t e to the o p p o s i t e l y c h a r g e d surface a n d a c c u m u l a t e . If the surfaces are p u l l e d apar t then s o m e of the d i s p l a c e d charges are too i m m o b i l e to neu t r a l i ze the oppos i t e charges before a gap opens. T h i s m e c h a n i s m p r o v i d e s a g o o d e x p l a n a t i o n for the d o m i n a n c e of a s ing le p o l a r i t y i n the E M s igna l s r e c o r d e d i n o u r (and Sobo lev ' s g r o u p ) f i e ld exper iments because the d i p o l e o r i en ta t ion is c o n t r o l l e d b y the d i r e c t i o n of the t e l lu r i c cur ren t , w h i c h is f a i r l y u n i f o r m o v e r d is tances of s eve ra l meters to tens of meters ( d e p e n d i n g o n the s i ze of the o r e b o d y a n d the source of the te l lur ics) . T h e or ien ta t ion of c o m p o s i t i o n a l differences are u n l i k e l y to be as u n i f o r m . T h e m a x i m u m a m o u n t of charge that can be left o n the c rack surfaces is l i m i t e d b y t w o p h y s i c a l constraints : the n u m b e r of a tomic sites for the charges a n d f i e l d emi s s ion . T h e latter effect n o r m a l l y domina t e s , l i m i t i n g the o c c u p a n c y to about 10% of the a v a i l a b l e sites (Hays , 1991). F i e l d emi s s ion of electrons occurs w h e n the electrostatic r e p u l s i o n of the excess nega t ive charges is suff icient to ove rcome the b i n d i n g energy to the surface Chapter 5: A Physical Model for RPE 113 (Nasser , 1971). T h u s , electrons w i l l f ly off the surface i f there are too m a n y of them. T h e electric f ie lds near the charges mus t be suff ic ient ly great that a n e lec t ron w i l l g a i n m o r e energy than the b i n d i n g energy. T h i s b i n d i n g energy is equ iva len t to the w o r k func t ion . Therefore , for f i e l d e m i s s i o n to occu r the electr ic f i e l d m u s t be greater t h a n the w o r k f u n c t i o n d i v i d e d b y the d i s tance the e lec t ron m u s t t r ave l before e s c a p i n g the surface forces ( H a y s , 1991). N o t e that th is is a n o v e r s i m p l i f i c a t i o n . A n exact c a l c u l a t i o n i n v o l v e s the p h e n o m e n a of e lec t ron t u n n e l i n g t h r o u g h the F e r m i / w o r k po ten t i a l , a n d the ro le of the electr ic f i e ld is to l o w e r the ba r r i e r po ten t i a l a n d increase the p r o b a b i l i t y of escape (Nasser , 1971); h o w e v e r , bo th me thods g ive a p p r o x i m a t e l y the same answer . T h e w o r k func t ion (W) for mos t metals a n d semi-meta ls is about 4 e V , a n d the d is tance the e lec t ron m u s t t r ave l (d) is a p p r o x i m a t e l y the th ickness of one a t o m i c l a y e r or less (approx . range of surface forces), w h i c h is a p p r o x i m a t e l y 0.7 n m . These n u m b e r s g i v e f i e l d e m i s s i o n at about 5 G V / m (Nasser , 1971, H a y s , 1991). In prac t ice , the surface roughness l i m i t s average f ie lds to about 1 G V / m . O n c e the process of f i e l d e m i s s i o n beg ins e lec t rons w i l l l eave the surface u n t i l the m u t u a l r e p u l s i o n of the r e m a i n i n g electrons is insuf f ic ien t to o v e r c o m e the w o r k func t ion . If the surface is r e l a t i ve ly flat then Gauss ' s L a w of electrostat ics can be u s e d to f i n d the m a x i m u m surface charge dens i ty before f i e ld emi s s ion occurs . ^ We ° - m a x ^ — (5.3) a U s i n g the p r e v i o u s va lues for W a n d d, g ives a l i m i t of about 30 m C m ~ 2 . T h i s n u m b e r is a p p r o x i m a t e l y equa l to the largest t r iboelectr ic surface charges r epo r t ed ( H a y s , 1991; H o r n a n d S m i t h , 1992). H e n c e , f i e l d e m i s s i o n l i m i t s t r i boe l ec t r i c su r face cha rge densi t ies to less than 30 m C m ~ 2 . La rge va lues of t r iboelectr ic surface charges are i n the range of 1 to 10 m C m ~ 2 (Hays , 1991). I expect that i f a rather u n i q u e p h e n o m e n a l i k e R P E is based u p o n t r iboelec t r ic effects then excep t iona l ly la rge a m o u n t s of charge are p r o d u c e d , a n d that 1 to 10 m C m ~ 2 is a g o o d s tar t ing po in t for further inves t iga t ion . Chapter 5: A Physical Model for RPE 114 T h e r e s i d u a l surface charges p l a y a n i m p o r t a n t r o l e i n the e v o l u t i o n of the c rack . A t t r a c t i v e electrostatic forces f r o m the oppos i t e ly cha rged surfaces resist further o p e n i n g of the c rack gap (Hays , 1991; H o r n a n d S m i t h , 1992). T h i s electrostatic stress is equa l to 2 Subs t i t u t i ng surface charge densi t ies of 1 to 10 m C m - 2 in to 5.4 g ives o p e n i n g pressures of 55 to 5500 k P a . If the gap is to w i d e n then tens ion greater than the stress ca lcu la ted b y e q u a t i o n 5.4 m u s t be a p p l i e d . A n i m p o r t a n t i m p l i c a t i o n of e q u a t i o n 5.4 is that the surface charge dens i ty that can appear o n the fresh surface is restr ic ted b y the a m o u n t of external stress ava i l ab le to p u l l the crack open , o therwise , the c rack w i l l not open . T h e se i smic d i s tu rbance f r o m a s m a l l exp los ive charge is capable of p r o v i d i n g forces of u p to 200 k P a tens of meters f r o m the source (Chapter 2). A stress o f 200 k P a l i m i t s the surface charge densi t ies o n the crack to less than 2 m C m - 2 . A p l a u s i b l e scenar io is that the se i smic d i s tu rbance creates a crack d u r i n g rarefact ion (rocks are m u c h w e a k e r u n d e r t ens ion than u n d e r compres s ion : Jaeger a n d C o o k , 1976), a n d then proceeds to o p e n the gap fur ther b y t rans fe r r ing the se i smic s t ra in f r o m the t ens ion to the gap . T h e gap is effect ively a soft s p r i n g be tween t w o stiff sp r ings ; mos t of the s t r a in is t aken u p b y the softest s p r i n g w h e n tens ion o r c o m p r e s s i o n is a p p l i e d . The p r e c e d i n g scenario has a g l a r i n g weakness : the total s t ra in ob ta ined f r o m the se i smic w a v e is d e l i v e r e d ove r a p e r i o d of about 1 ms . T h i s poses a cons ide r ab l e p r o b l e m because mos t of the surface charge w i l l be lost b y 1 m s , un le s s the m e d i u m is v e r y resis t ive. A consequence of the charge leakage is that the d i p o l e m o m e n t f o r m e d b y the charges a n d gap w i l l not be suf f ic ien t ly large to e x p l a i n the E M f ie lds f r o m R P E (this is d i scussed i n de t a i l i n the next sect ion, 5.4). If the t ime constant of leakage is 100 us then the gap w i l l o n l y be about 2 p m w i d e at the t ime of m a x i m u m d i p o l e m o m e n t . T h i s gap s ize is far too s m a l l to r ep roduce the measu red E M fields i n m y f i e ld t r ials . In a d d i t i o n , o n l y s u b - k e V energies are poss ib le for e lectrons t r ave r s ing the gap; the gap p o t e n t i a l Chapter 5: A Physical Model for RPE 115 m u s t reach at least 10 k V for electrons to p r o d u c e a measurab le a m o u n t o f X - r a y s (Potts, 1993). M y p re fe r red m o d e l of the c rack process is that the se i smic d i s t u r b a n c e p r o v i d e s the extra t ens ion to rock that is a l r eady u n d e r l o c a l i z e d extens ive stress to f o r m cracks , bu t the p re -ex i s t ing stress p r o v i d e s mos t of the w o r k necessary to ob ta in large surface charge dens i t i e s , l a rge r gaps , a n d r e l a t i v e l y q u i c k o p e n i n g t imes . M o s t u n d e r g r o u n d r o c k e n v i r o n m e n t s are u n d e r a n u m b e r of i n h o m o g e n o u s stresses d u e to o v e r b u r d e n forces a n d de fo rma t ive forces (Herget , 1988; S h o l z , 1990). T y p i c a l h o r i z o n t a l stress m a g n i t u d e s i n u n d e r g r o u n d rock range f r o m 1 to 20 M P a for rocks 10 to 500 m deep (Roberts , 1981; H e r g e t , 1988; S h o l z , 1990; p r i v a t e c o m m u n i c a t i o n w i t h T . U r b a n i c o f E n g i n e e r i n g S e i s m i c G r o u p i n K i n g s t o n , Ont . ) . M y conjecture is that the s e i smic w a v e g e n e r a l l y needs to a d d a n extra 5 to 10% to a p re -ex i s t ing stress to cons i s ten t ly generate R P E . I h a v e m a n y r e c o r d s w i t h s i g n a l s that a r r i v e m o r e t h a n 30 m s after the b la s t , c o r r e s p o n d i n g to apparent distances f r o m the shot of m o r e than 170 m . It is v e r y h a r d to e x p l a i n the E M s i g n a l a m p l i t u d e s i f the se i smic w a v e has to d o a l l of the w o r k ; even at d is tances of 50 m it is a d i f f i cu l t p r o b l e m . A n o t h e r advan tage of r e l y i n g u p o n pre -ex i s t ing stresses is that the ava i lab le surface charge dens i ty is not g rea t ly affected b y the d i m i n i s h i n g force of the se i smic w a v e w i t h d is tance f r o m the shot. The effect of greater dis tances f r o m the shot i n this m o d e l is to reduce the l i k e l i h o o d of o p e n i n g n e w cracks. M a x i m u m surface charge dens i t y is g o v e r n e d b y the a m o u n t of stress w i t h i n the r o c k a n d equa t i on 5.4, a n d is i n d e p e n d e n t of s e i smic b e h a v i o r i f the p r e - e x i s t i n g stress is m u c h greater than se ismic stresses. I a m a s s u m i n g that ex tens iona l forces are r e spons ib le because the tens i le s t r eng th of rocks is an o rde r of m a g n i t u d e l o w e r than the compres s ive s t rength, a n d is t y p i c a l l y 3 to 10 M P a for m a n y u n d e r g r o u n d rock types (Ca rmichae l , 1989; Jaeger a n d C o o k , 1976). In a d d i t i o n , t ens i l e c r a c k s p r o v i d e a n a t u r a l w a y to f o r m a n e lec t r i c d i p o l e f r o m tr iboelectr ic effects. A poten t ia l d i sadvan tage of r e l y i n g u p o n pre -ex i s t ing stresses is that Chapter 5: A Physical Model for RPE 116 not all of the orebody may be under tensile stresses, therefore, not all of the orebody will respond. There is no proof from my field trials that every section of the orebody does respond. In fact, the field evidence might tend to show otherwise (Chapter 4, Mobrun Mine). This property would be a liability for RPE-based exploration. Nevertheless, I cannot produce a convincing model that does not involve pre-existing stresses. It appears that pre-existing stresses must be included in a crack-based explanation of RPE. However, there is an abundance of evidence that tensile cracks exist in most geological formations (Herget, 1988; Sholz, 1990). These cracks are often associated with shear faults, folds, and joints (Sholz, 1990). My picture of an orebody is a collection of blocks separated by joints and full of cracks oriented in many directions (Herget, 1988), some of these cracks are close to failure due to the local stresses in the rock mass. Figure 5.2 An illustration of tensile strain relief by a crack. The crack is roughly ellipsoidal in cross-section, and the air-gap is formed by the relief of strains around the material surrounding the gap. The volume of material that gives up strain to the gap is approximated by a sphere enclosing the crack. Chapter 5: A Physical Model for RPE 117 Two principal parameters determine most of the crack properties. These parameters are the pre-existing stress, T, and the final diameter of the failed region, /. The crack is modeled as an expanding disk that grows to a diameter / and with a gap of s. Strains from the surrounding medium, to a distance 1/2 from the crack centre, are taken up by the gap (Figure 5.2). The maximum gap size , s, is IT s = ~Y~ (5.5) where Y is the Young's modulus, and T is the tensile stress (Jaeger and Cook, 1976). A value of 50 GPa is a representative value of Young's modulus for many igneous rocks, such as andesite and rhyolite (Carmichael, 1989; Young's modulus will change at great depths, >1000 m, due the closing of cracks). To progress further with this model numbers for T and / must be found. My chosen numbers are, T=5 MPa and /=0.3 m. The former represents a median value of tensile strength for many underground rocks (Carmichael, 1989; Jaeger and Cook, 1976), and the latter is based upon the observation of cleavage patterns in the tunnels of mines that I have conducted field trials. A crack area of about 30 cm x 30 cm is fairly large. However, it may well represent a collection of smaller fractures that connect to form a rupture. Substitution of these numbers and the various rock parameters gives T = 5 MPa / = 0.3m Y = 50 GPa S = 3 ° " m , (5.6) a = 10 mCm"2 At = 30 Where s is the maximum gap size, a is the upper bound upon surface charge density, and At is the time taken to relieve the strains around the crack. The time to form the crack is sufficiently short to preserve most of the original charge. I will assume that electrostatic forces provided a substantial amount of the adhesive forces supporting the flaw before rupture, and this will be reflected in the amount of charge that is left on the surfaces. Hence, a value of 10 mC m"2 for the surface charge density will be used for calculating the magnitude of the EM pulse. Chapter 5: A Physical Model for RPE 118 In my model of RPE, the role of the seismic wave is to push a rock that is already under about 5 MPa of tension to the point of rupture. Subsequent cracking relieves strains from regions up to 30 cm from the centre of the crack, thus producing a large crack with a gap of 30 pm in about 30us. The fresh surfaces have about 10 mC m - 2 of surface charge distributed over them, which will provide the source of the EM signals observed from RPE. 5.5 EM Fields from an Expanding Crack I propose that the EM radiation seen in my field trials and those conducted by Sobolev's group comes from a gas discharge. As the gap widens and fills with desorbed gases then the surface charges will recombine due to electrical breakdown of the gases under the high electric fields within the gap. This electrical model of the EM process gives an intense and very short pulse of current, and can account for the brief nature of the EM pulse. However, explaining the amplitudes of the EM fields measured in my field experiments is not trivial. This is not a problem limited to my crack model. Al l of the rejected mechanisms share this problem because the observed EM fields from RPE are large. To estimate the EM fields the crack is modeled as a small electrical dipole in an isotropic and homogenous medium. The crack will be treated as a parallel plate capacitor with equal and opposite charges Q = a A, where A is the area of the plates. At the time of the gas discharge a large amount of current flows briefly. This current (P) provides the strong EM fields seen in the field trials. P=^r (5-7) The solution to the problem of the EM fields from a small current dipole is well known (Jackson, 1975; Kraichmann, 1976), and for a given dipole moment the fields are Chapter 5: A Physical Model for RPE 119 y Anr I yr exp(-yr) Eg(co). jcofi Psin6 4nr yfi Psind 1 1 1 1 + — + 2^2 yr y r ex •p(-y) f Anr 1 1 + exp(-yr) (5.8) where yis the propagation constant (complex wave-number), y = jk . and y2 = -co2jie + (5.9) and the co-ordinate system is shown in Figure 5.3. These equations are valid if the distance from the source (r) is large compared with size of the dipole, which is the case in the field trials. Typical resistivities (p) of igneous underground rocks are about 1000 Q. m (Telford, 1988). Figure 5.3 Co-ordinate system used in describing the electric dipole fields. The dipole consists of charges +Q and -Q centered at the origin, but separated by a distance of s. The measurement of the field is at co-ordinates (0, <}>, r). Chapter 5: A Physical Model for RPE 120 M o s t of m y E M measurements con ta ined frequencies f r o m 1 to 50 k H z . A t this range of f requencies c o n d u c t i o n currents p r e d o m i n a t e (for t y p i c a l u n d e r g r o u n d res i s t iv i t i es a n d d ie lec t r ic pe rmat iv i t i e s ) , a n d the equat ions i n 5.7 s i m p l i f y to a f o r m k n o w n as the near-f i e l d a p p r o x i m a t i o n ( K r a i c h m a n n , 1976). _ pPcosO 4nr 3 exp(/r) ^ ^ e x p ( r r ) (5.10) Ee = —A——exp(yr) Anr 2 _ jCOfl a n d y ~ „ N o t e that i f the a rgumen t of the exponen t i a l is s m a l l , as is a s s u m e d here, then the f o r m of these equat ions is the same i n bo th the t ime a n d f requency d o m a i n . T o est imate the E M f ie lds f r o m the gap a v a l u e for the d i p o l e m o m e n t is needed . If I assume that the gas d ischarge occurs soon after the c rack has f u l l y f o r m e d , a n d that a l l of the surface charges r ecombine then the number s f r o m the crack m o d e l can be u s e d (5.6). T h u s , equa t ion 5.7 a n d the number s f rom 5.6 can be u s e d to calcula te the current d i p o l e . H o w e v e r , the gas d i scharge occurs i n nanoseconds (Nasser , 1971), a t ime i n t e rva l that is too shor t for the detec tors to measu re the t rue peak a m p l i t u d e . H e n c e , the p e a k m a g n i t u d e of the d i p o l e m o m e n t m u s t be adjus ted for the l i m i t e d b a n d w i d t h of the r e c o r d i n g sys tem, a n d equa t ion 5.7 becomes P « A(-^2nAf A t =2nAfQs (5.11) T h e equat ions i n 5.10 are not p a r t i c u l a r l y use fu l i n thei r present f o r m because it is the m a g n i t u d e s of the E M fields that I w i s h to c o m p a r e to the resul ts f r o m m y f i e l d t r ia ls . Therefore , I w i l l a ssume that a l l or ientat ions are poss ib le , a n d integrate the square of the Chapter 5: A Physical Model for RPE 121 f i e ld m a g n i t u d e to obta in the R M S . va lues of E a n d B \ E \ ~ ~ 3 (5.12) 3r 6r The exponent ia l , or s k i n dep th , t e rm is omi t t ed because it is close to u n i t y at 1 to 50 k H z frequencies . W i t h equat ions 5.11 a n d 5.12 a n estimate of the E M fie lds f r o m the gas d i scha rge current can be made . Subs t i tu t ing the f o l l o w i n g crack m o d e l A = 350 c m 2 o - = 1 0 m C r n 2 Q = 350 pC s = 30 pm (5.13) (5.14) a n d f i e ld exper imen t parameters A f = 50 k H z p = 1000 i l m r = 8 0 m p. = 1 .26x10-* H m 1 gives these E M f ie ld magn i tudes \E\ = 340 n V m" 1 and \B\ = 17 f T B o t h of these n u m b e r s are four to f ive orders of m a g n i t u d e too s m a l l to e x p l a i n t y p i c a l f i e ld measurements (5 m V / m a n d 1 nT) . It is t e m p t i n g to d i s c a r d the crack m o d e l because of these l o w E M fields . H o w e v e r , none of the p r e v i o u s l y d i scussed mode l s w i l l fit the data ei ther because i t is v e r y h a r d to f o r m a s u f f i c i e n t l y l a rge d i p o l e m o m e n t f r o m the n a t u r a l p rocesses w i t h i n r o c k s . T h e p r o b l e m of d e s c r i b i n g the E M fields m a y he w i t h the s i m p l e cur ren t d i p o l e m o d e l of a gas d i s cha rge . T o m y k n o w l e d g e , there is n o accep ted q u a n t i t a t i v e m o d e l o f a gas d i s cha rge that exp la ins the large, a m o u n t s of R F e m i s s i o n that a c c o m p a n y a s p a r k or c o r o n a l d ischarge . Chapter 5: A Physical Model for RPE 122 A poss ib le s o l u t i o n to th is p r o b l e m is to examine the con t r i bu t ions f r o m the charges o n a m i c r o s c o p i c scale. I w i l l use the express ion for the magne t ic f i e ld i n 5.8 as an example . In the t ime d o m a i n the magnet ic f i e ld is ( L o r r a i n 2 a n d C o r s o n , 1988; Jackson, 1975) Ancr ' c -A P + -P r (5.15) T h e te rms associa ted w i t h the t i m e d e l a y a n d phase d i s t o r t i o n are no t s ign i f i can t for frequencies b e l o w a h u n d r e d k i l ohe r t z for the f i e ld cond i t i ons s t ipu la t ed i n 5.14, a n d are not i n c l u d e d i n 5.15. In a d d i t i o n , I w i l l cons ide r c, the v e l o c i t y of e lec t romagnet ic w a v e s to be cons tant . In fact, at l o w f requencies th is v e l o c i t y is f r equency d e p e n d e n t i n c o n d u c t i v e m e d i a , bu t the v e l o c i t y change w i t h f r equency does no t a l te r the bas ic a rguments to be presented here. There are t w o sources of the magne t i c f i e ld i n equa t ion 5.15: r a d i a t i o n (P) a n d i n d u c t i o n (P) f ie lds . T h e lat ter w a s u s e d i n 5.10 because the r a d i a t i o n c o n t r i b u t i o n is m u c h sma l l e r at these frequencies. H o w e v e r , the use o f these equa t ions a s sumes that a r e sponse c a n be p r o d u c e d f r o m a l i n e a r s u p e r p o s i t i o n of frequencies, w h i c h is reasonable i f M a x w e l l ' s equat ions h o l d . I f i n d i t i n s t ruc t ive to l o o k at the m i c r o s c o p i c scale of e l e c t r o d y n a m i c s because c l a s s i ca l f i e l d t heo ry ( M a x w e l l ' s equat ions i nc luded ) does not a l w a y s correct ly descr ibe the behav io r of cha rged part icles. F i r s t l y , cons ide r a free electron near the surface of a nega t ive ly cha rged pla te of a p a r a l l e l plate capacitor . E a c h plate has a surface charge dens i ty of r j , a n d the plates are separated by a gap of distance s. T h e v o l u m e be tween the plates conta ins an electric f i e ld , E = aje, w h i c h w i l l accelerate the e lect ron t o w a r d the other plate. A c c e l e r a t i o n s tops w h e n the o ther p la te is r eached , a n d s o o n the e lec t ron s tops too because of c o l l i s i o n s w i t h the mo lecu le s that m a k e u p the pla te (F igure 5.4). F o r the m o m e n t , cons ide r the E M f ie lds generated b y the e lect ron as it traverses the gap be tween the plates. T h e average va lues for P a n d P are g i v e n b y (where e a n d m is the charge a n d mass of the electron) P = ^ (5.16) me Chapter 5: A Physical Model for RPE 123 2ms (5.17) F o r s m a l l gaps the r ad ia t ion w i l l domina te . Subs t i tu t ing the es t imated va lues for o,e, a n d s (10 m C m " 2 , 8.8 x 10" 1 2 F m " \ and 30 jim) f r o m the crack m o d e l in to 5.16 a n d 5.17 s h o w s that r ad ia t ion is s ix orders of magn i tude greater than the i n d u c t i v e f ields. ~ electron + + + + + + + t Accelerat ion v Velocity time Figu re 5.4 A n i l lus t r a t ion of the m o v e m e n t of a free electron i n a cha rged gap. If the electric f ie ld is u n i f o r m then the e lect ron accelerates at a constant rate u n t i l it reaches the other s ide of the gap, w h e r e u p o n , it decelerates v e r y r a p i d l y i n a series of c o l l i s i o n s w i t h the a toms of the s o l i d ma te r i a l . So w h y d o i n d u c t i o n f ie lds d o m i n a t e the near- f ie ld? In genera l , w h e n the e lec t ron smacks in to the other plate it decelerates, a n d the f i e ld seen by a n externa l sensor is an e q u a l a n d oppos i t e f i e ld to the acce le ra t ion phase (note that the f ie lds c lose to the e lec t ron m a y change i n m o v e m e n t d u e to l o c a l p o l a r i z a t i o n f r o m the s u r r o u n d i n g dielectr ic , ) . In l o w energy co l l i s i ons the r ad ia t ion is of the f o r m of a b r o a d b a n d pulse , Chapter 5: A Physical Model for RPE 124 the b a n d w i d t h of the r a d i a t i o n p r o d u c e d is the F o u r i e r t r ans fo rm of the c o l l i s i o n event (Jackson, 1975). If w e were to t ry a n d measure the E M f i e ld w i t h a l o w l a n d w i d t h sensor w e w o u l d see l i t t l e of the r a d i a t i o n as there is n o net acce le ra t ion o v e r the r e l a t i v e l y l o n g t i m e p e r i o d s that the sensor integrates the f ie lds . In th is case the nea r - f i e ld o r i n d u c t i o n a p p r o x i m a t i o n h o l d s . If the e lect ron is accelerated so that it reaches energies of m o r e than 10 k e V then c lass ica l f i e l d t heo ry (i.e. M a x w e l l ' s equat ions) does no t adequa t e ly desc r ibe the E M b e h a v i o r w h e n c o l l i s i o n s w i t h o ther par t ic les are m a d e (E i sbe rg a n d R e s n i c k , 1974). In a h i g h energy c o l l i s i o n be tween a fast e lectron (speeds w i t h an apprec iab le f ract ion of the speed of l igh t ) a n d a n a tomic nuc leus the c o l l i s i o n is so q u i c k that, a c c o r d i n g to c lass ica l f i e ld theory, the b a n d w i d t h covers the E M s p e c t r u m u p to g a m m a rays (Jackson, 1975; E i s b e r g a n d R e s n i c k , 1974). A s ing le g a m m a ray w o u l d have m o r e ene rgy than the i n c i d e n t par t i c le i n m a n y cases. In this s i t ua t ion a p r o b l e m arises because c lass ica l f i e l d theory does not adequa te ly desc r ibe the character is t ics of h i g h energy E M w a v e s o r p h o t o n s (e.g.. X - r a y s a n d g a m m a rays ) . P h o t o n s h a v e p a r t i c l e - l i k e p r o p e r t i e s , s u c h as m o m e n t u m , w h i c h are not e x p l a i n e d b y M a x w e l l ' s equa t ions . A c c o r d i n g to c l a s s i ca l f i e ld theory the g a m m a ray m a y have a n y a m p l i t u d e , bu t q u a n t u m processes s t ipu la te there is a s i ng l e energy for a g a m m a p h o t o n , a n d that w a v e s cons is t of a n u m b e r of p h o t o n s . W h e n an e lec t ron co l l ides w i t h an a tomic nuc leus p h o t o n s are e m i t t e d w i t h a n energy equa l to, o r less than , the energy lost by the e lectron i n the c o l l i s i o n (F igu re 5.5). T h i s p rocess is c a l l e d b r e m s s t r a h l u n g , or b r a k i n g r a d i a t i o n (Jackson, 1975; E i s b e r g a n d R e s n i c k , 1974). In mos t co l l i s i ons a large n u m b e r o f soft pho tons of different energies are emi t t ed a n d q u a n t u m theory agrees w i t h c lass ica l results i f the c lass ica l s p e c t r u m is l i m i t e d to frequencies less than the m a x i m u m a l l o w a b l e p h o t o n energy (Jackson, 1975). H o w e v e r , i n a s m a l l n u m b e r of co l l i s i ons a s ing le p h o t o n w i l l take mos t o f the energy, a n d a n X - r a y results . In this s i tua t ion the m o m e n t u m absorbed b y the p h o t o n m u s t be Chapter 5: A Physical Model for RPE 125 t aken in to account , a n d the c o n t r i b u t i o n to the l o w e r f requency par t o f the s p e c t r u m (soft pho tons ) is a l te red (Jackson, 1975). A s i m p l e p ic tu re of the process is that the e lec t ron is dece le ra ted , bu t a p h o t o n ra ther than an o p p o s i n g (b roadband) f i e l d is p r o d u c e d . Therefore , the use of the c lass ica l f i e l d equat ions is no t a p p l i c a b l e i n some cases even t h o u g h the par t ic le changes ve loc i ty . Photon Electron Path F i g u r e 5.5 A n i l l u s t r a t i on of the process of b remss t rah lung , or the inverse pho to -electr ic effect. V e r y fast e lectrons often pass t h r o u g h the c l o u d of e l ec t rons s u r r o u n d i n g a n a t o m i c n u c l e u s a n d in te rac t w i t h the nuc leus d i rec t ly . Electrostat ic forces be tween the e lectron a n d nuc leus deflect the electron, but bare ly alter the m o t i o n of the nuc leus because o f the latter 's great mass . T h e def lec t ion of the e lec t ron p r o d u c e s . a n E M d is tu rbance , a p h o t o n , o r a n u m b e r of photons . O c c a s i o n a l l y , the p h o t o n carr ies mos t of the m o m e n t u m f r o m the c o l l i s i o n , p r o d u c i n g an X - r a y or g a m m a ray. T h e i m p l i c a t i o n of b r e m s s t r a h l u n g is that i f the electrons t ravers ing the gap reach h i g h energies , then s o m e w i l l not p r o d u c e (broadband) E M fields w h e n they s l o w d o w n i n the s o l i d m e d i u m . C o n s e q u e n t l y , a net a m o u n t of r a d i a t i o n f r o m the acce le ra t ion of these electrons can be seen by o u r detectors. Because the r ad i a t i on f ie lds are so m u c h greater t han the i n d u c t i o n f ie lds i n the s m a l l gaps of the c rack m o d e l o n l y a s m a l l percentage of electrons need to par t ic ipa te i n b remss t r ah lung for r ad i a t i on to d o m i n a t e . The fract ion of electrons that p roduce X- rays is Chapter 5: A Physical Model for RPE 126 77 = l . l x l ( T 9 Z V = BZ— (5.18) w h e r e Z is the a tomic n u m b e r of the target ma te r i a l a n d V is the energy of the e lect ron i n k e V . T h i s express ion was d e r i v e d e m p i r i c a l l y b y C o m p t o n a n d A l l i s o n (Massey a n d B u r h o p , 1952) to express the efficiency of X - r a y p r o d u c t i o n verses a t o m i c n u m b e r a n d acce le ra t ing vo l tage . T h e s e c o n d exp re s s ion i n 5.18 uses the c r ack m o d e l parameters (5.13), a n d assigns a s y m b o l for the constant (B = 1.1 x I O - 9 ) . T o est imate the effects of b r emss t r ah lung w e need express ions for the r a d i a t i o n f ie lds i n the t ime d o m a i n , w h i c h neglec t ing c o n d u c t i o n losses are ( L o r a i n 2 a n d C o r s o n , 1988) N o t e that c ( w h i c h is c o m p l e x i n the f requency d o m a i n ) is f requency d e p e n d e n t for f requencies w h e r e c o n d u c t i o n cur ren ts p r e d o m i n a t e . T h u s , at l o w f requencies (<100 k H z ) the magne t i c f i e l d s igna ture w i l l be d i s to r t ed a n d w i l l not resemble the o r i g i n a l pu l se ( a l though i t w i l l be a pulse) . H o w e v e r , the electric f ie lds are p r o p o r t i o n a l to the acce le ra t ion , b a r r i n g f requency dependen t a t t enua t ion d u e to c o n d u c t i v e losses ( w h i c h are not s ign i f ican t u n t i l 1 M H z for t y p i c a l u n d e r g r o u n d c o n d i t i o n s a n d dis tances i n m y f i e l d t r ia l s ) a n d o ther d i s p e r s i v e effects ( F u l l e r a n d W a i t e , 1976). I n t e g r a t i n g the m a g n i t u d e s q u a r e d of the electric f i e ld i n 5.19 ove r a l l poss ib le d i p o l e or ienta t ions g ives the f o l l o w i n g express ion for the expected m a g n i t u d e of the electric f i e ld H Psint? (5.19) 1*1 ~ HP (5.20) 6nr S u b s t i t u t i n g equa t ions 5.16, 5.18, a n d ad jus t ing for the l i m i t e d b a n d w i d t h o f the E M sensors (e.g. see 5.11) g ives a n expres s ion that can be u s e d to ca lcu la te the expec ted m a g n i t u d e of the electric f i e ld f rom a gas d ischarge be tween the crack surfaces, Chapter 5: A Physical Model for RPE 127 1*1- 2r \ mel (5.20) as m e a s u r e d b y l o w - b a n d w i d t h detectors. S u b s t i t u t i n g i n the n u m b e r s f r o m 5.13 a n d 5.14 a n d the f o l l o w i n g va lues in to equa t ion 5.20 g ives an expected electric f i e ld m a g n i t u d e of 4 m V / m . T h i s v a l u e is e q u a l i n m a g n i t u d e to m a n y la rge s igna l s f r o m R P E seen i n m y f i e l d t r ia l s (about 5 m V / m typ.) . T o ob ta in a b a l l p a r k v a l u e for the magne t ic f i e ld f r o m equa t i on 5.19 I w i l l use the v e l o c i t y of l i gh t at 10 k H z for m y t y p i c a l u n d e r g r o u n d c o n d i t i o n s (about 3000 k m / s ), w h i c h g ives a p r e d i c t e d magne t ic f i e ld a m p l i t u d e o f about 1.3 nT . A g a i n , th is n u m b e r is e q u a l to the o b s e r v e d m a g n i t u d e s of la rge s igna l s . H e n c e , th i s m o d e l of excess r a d i a t i o n f r o m the o p e n i n g of cracks p r o v i d e s a means to e x p l a i n the la rge E M fields f r o m R P E . m = 1 x 10 k g Z = 50 e = 1 . 6 x l O _ 1 9 C /J = l . l x l 0 " 9 C H A P T E R 6 T H E S I S C O N C L U S I O N S M a n y aspects of the R P E p h e n o m e n o n d e s c r i b e d b y S o b o l e v et a l . (1982) h a v e been c o n f i r m e d i n m y f i e ld t r ia ls . These i n c l u d e : the p u l s e - l i k e na ture of the E M s igna ture , the non- l inea r r e l a t ionsh ip be tween the se i smic force a n d E M pu l se a m p l i t u d e s , p o o r to fair r e p r o d u c t i o n character is t ics , o r e b o d y "exhaust ion", a n d the assoc ia t ion of R P E w i t h s u l p h i d e m i n e r a l s . Tests w i t h e x p l o s i v e sources i n u n d e r g r o u n d f i e l d t r i a l s h a v e s h o w n that a p p r o x i m a t e l y 100-500 k P a of s e i smic stress needs to be a p p l i e d to the s u l p h i d e o r e b o d y i n o r d e r to p r o d u c e R P E e m i s s i o n s . F u r t h e r m o r e , i n c r e a s i n g the se i smic force does not increase the E M s igna l a m p l i t u d e . Instead, greater force tends to increase the n u m b e r of E M pulses . M a x i m u m elec t romagnet ic f i e l d a m p l i t u d e s of R P E pulses are t y p i c a l l y 5 m V / m a n d 2 n T w h e n measured w i t h a 30 k H z b a n d w i d t h . W i t h a r e c o r d i n g b a n d w i d t h greater t han 1 M H z , these pu l ses m a y reach a p p r o x i m a t e l y 100 m V / m a n d 10 n T i n a m p l i t u d e . T h e f requency content of the E M p u l s e is o rde r s of m a g n i t u d e greater than the se i smic inpu t , a n d has been obse rved to range f r o m 1 k H z to 3 M H z . In s u m m a r y , the charac te r i s t i cs of the E M p u l s e s p r o d u c e d i n m y f i e l d exper iments are exactly as desc r ibed by the R u s s i a n scientists (Sobolev et a l . , 1982). In a d d i t i o n to c o n f i r m i n g m a n y R P E character is t ics , I have f o u n d t w o n e w proper t i es . O n e of these d i s c o v e r i e s is that the d i r e c t i o n w i t h w h i c h the s e i s m i c w a v e i m p i n g e s u p o n the o r e b o d y can s t r o n g l y in f luences the a m o u n t of E M ac t i v i t y . T y p i c a l l y , the o rebody ceases to p r o d u c e a s ignif icant E M response after severa l shots are f i r ed f r o m the same loca t ion . H o w e v e r , the o r e b o d y responds a n e w i f the sho tpo in t is m o v e d to a n e w area , w h e r e the d i r e c t i o n to the o r e b o d y is d i f f e ren t , u n t i l t h i s l o c a t i o n is a l so "exhausted" . In a d d i t i o n , it w a s f o u n d that i f n o shots are f i r ed for a p e r i o d of severa l hour s then the o rebody tends to recover; this type of recovery has been m e n t i o n e d to us b y the R u s s i a n s (persona l c o m m u n i c a t i o n to R. D . R u s s e l l a n d M . M a x w e l l i n 1983 a n d 128 Chapter 6: Thesis Conclusions 129 1991). T h e other d i s c o v e r y is that the r e l a t i v e l y p o o r r e p l i c a t i o n of shot r eco rds i s m a i n l y i n the a b i l i t y to r ep roduce the largest pulses . Spec t rograms f r o m the L y n x M i n e s h o w that the pe r iods of pu l se ac t iv i ty can be r e p r o d u c e d a lmos t exact ly , bu t the large a m p l i t u d e events w e r e not r e p r o d u c e d w e l l . H e n c e , the r e p r o d u c t i o n of shot da ta is s ign i f i can t ly better i f the largest s ignals are g i v e n less emphas i s . M y s p e c u l a t i o n about the o r i g i n of R P E is that the se i smic w a v e p r o d u c e s t r iboe lec t r ic charge s epa ra t i on d u e to m i c r o - c r a c k i n g , a n d that the E M f ie lds are p r o d u c e d f r o m a r a p i d r e c o m b i n a t i o n of these charges. S u l p h i d e s are able to p r o v i d e large a m o u n t s of t r iboelec t r ic charge because t e l lu r i c currents a n d the e lec t ronic c o n d u c t i o n m e c h a n i s m w i t h i n s u l p h i d e m i n e r a l s causes o p p o s i t e l y cha rged e lec t r ica l l ayers to f o r m o n f l aws a n d joints w i t h i n the rock assemblage. Tens i le stresses of the o rde r of 5 M P a are needed to o p e n cracks w i t h large surface charge densi t ies (10 m C m " 2 ) , a n d this tensi le stress is p r o v i d e d b y a c o m b i n a t i o n of stress f r o m the s e i s m i c w a v e a n d p r e - e x i s t i n g stat ic stresses. T h e se i smic w a v e f r o m a 0.5 k g e x p l o s i v e canno t p r o v i d e 5 M P a of stress b e y o n d 10 m , thus , mos t of the stress is s u p p l i e d f r o m p re -ex i s t i ng static stresses for targets b e y o n d this dis tance. F a i l u r e of the ma te r i a l u n d e r the a d d e d tensi le l o a d f r o m the se i smic w a v e p r o d u c e s a crack, w h i c h p roduces a n electr ic d i p o l e v i a t r iboe lec t r ic charge separa t ion . In m y p r o p o s e d m o d e l of R P E , r a p i d r e c o m b i n a t i o n of the t r iboelect r ic charges f r o m a gas d i scharge i n the crack gap causes a large E M pulse . The large m a g n i t u d e of the E M f i e l d is a t t r i b u t e d to a n i m b a l a n c e b e t w e e n the E M f i e ld s of a n e l e c t r o n u n d e r acce lera t ion a n d the dece le ra t ion process of b r e m s s t r a h l u n g r ad i a t i on . B r e m s s t r a h l u n g processes a lso e x p l a i n the subs tan t ia l a m o u n t s of l i gh t a n d X - r a y s seen b y the R u s s i a n s (Sobolev et a l . , 1982; Sobo lev et a l , 1984). M y m o d e l of R P E exp la ins the p r o d u c t i o n of an E M p u l s e f r o m the se i smic w a v e a n d the non- l inear , t h r e sho ld r e l a t i onsh ip be tween the se i smic force a n d the E M response. It also m o d e l s m a n y other aspects of R P E s u c h as the d o m i n a n c e of a s ing le p o l a r i t y i n the E M response , the poor - to - fa i r r e p r o d u c t i o n Chapter 6: Thesis Conclusions 130 charac te r i s t i cs of repeat expe r imen t s , o r e b o d y "exhaus t ion" a n d " recovery" , a n d the co inc idence of u l t r a s o u n d co inc iden t w i t h the E M emiss ions (Sobolev et a l . , 1982a). T h e Russ i ans have p r o v i d e d ve ry l i t t le i n f o r m a t i o n about the m e t h o d s u s e d to acqu i r e a n d in te rpre t R P E s igna l s . M a t e r i a l abou t u s i n g R P E for e x p l o r a t i o n i s p a r t i c u l a r l y sparse. T h i s thesis p r o v i d e s a subs tan t ia l c o n t r i b u t i o n i n this area b y f u l l y d e s c r i b i n g a n u m b e r of m e t h o d s for a c q u i r i n g a n d a n a l y z i n g R P E d a t a . I ssues a b o u t the i n t e r p r e t a t i o n a n d p r e s e n t a t i o n o f s i g n a l s that h a v e a n u n k n o w n cause a n d fa i r r e p r o d u c t i o n character is t ics have been e x a m i n e d , a n d the in te rpre ta t ion a l g o r i t h m s take these issues in to cons ide ra t ion . T h e m e t h o d s of t o m o g r a p h i c r econs t ruc t ion , s ta t i s t ica l ana lys i s of a r r i v a l da ta , a n d spec t rograph ic ana lys i s are n e w m e t h o d s of t rea t ing R P E data . These techniques p r o v i d e a s o l i d f o u n d a t i o n for the t rea tment of R P E da ta for e x p l o r a t i o n purposes . A n u m b e r of s ign i f ican t advances were m a d e i n the area of i n s t r u m e n t a t i o n a n d f i e ld t e c h n i q u e o v e r the m e t h o d s a n d i n s t r u m e n t s u s e d i n ea r l i e r tests. H i g h v e l o c i t y exp los ives , s u c h as pentol i te , have p r o v e n to be the best se i smic source for u n d e r g r o u n d w o r k . T h i s is d u e to the m a t c h i n g of e x p l o s i v e i m p e d a n c e w i t h that of u n d e r g r o u n d rock. A charge s ize of a p p r o x i m a t e l y 0.5 k g is able to i n d u c e R P E m o r e than 100 m f r o m the sho tpo in t . These exp los ives w o r k mos t effect ively w h e n u s e d i n a shor t bo reho le , w i t h a f iber opt ic blast sens ing c i rcu i t to p r o v i d e a t i m i n g s igna l . T h e f iber op t ic t r igger has v i r t u a l l y e l i m i n a t e d the E M interference f r o m the blast. D i g i t a l r e co rd ing of the E M signals w i t h a 5 M H z b a n d w i d t h p r o v i d e d the best results i n m y f i e l d t r ia ls . H i g h b a n d w i d t h r e c o r d i n g y i e l d s better d i s c r i m i n a t i o n b e t w e e n s i g n a l a n d noise , a n d a l l o w s the use of spec t rog ram ana lys i s . T h e use of a 12 b i t d i g i t i z e r or better is r e c o m m e n d e d , a l t hough , an 8 bi t d ig i t i z e r suff iced i n m y h i g h b a n d w i d t h w o r k . V a r i o u s E M sensor con f igu ra t i ons w e r e tested, a n d b o t h e lec t r ic a n d m a g n e t i c f i e l d sensors were u s e d w i t h g o o d results. T h e pa ra l l e l plate d i p o l e w i t h a l o w - n o i s e F E T pre-a m p l i f i e r de l i ve r s the best c o m b i n a t i o n of b a n d w i d t h , s ignal - to-noise , a n d p o r t a b i l i t y of Chapter 6: Thesis Conclusions 131 a l l the tested conf igura t ions , a n d this is m y r e c o m m e n d e d sensor. T h e f i e l d t r ia ls at the S u l l i v a n , M o b r u n a n d L y n x M i n e s p r o v i d e a m p l e d e m o n s t r a t i o n of the po t en t i a l a p p l i c a t i o n of R P E for u n d e r g r o u n d e x p l o r a t i o n for m a s s i v e s u l p h i d e s . A t the M o b r u n M i n e the boundar ie s of the 1100 lens were located, a n d the sp l i t s t ructure of the lens was revea led i n the g r o u p i n g of a r r i v a l t imes of E M s igna l s . S i g n a l a r r i v a l t imes f r o m l e v e l 14 i n the L y n x M i n e a n d f r o m the S u l l i v a n M i n e w e r e s h o w n to be consis tent w i t h the l oca t ion a n d shape of the nearby o r e b o d y mass , a n d the b o u n d a r i e s of the ore-zones c o u l d be loca ted f r o m this data w i t h some geo log i ca l cons t ra in t s f r o m d r i l l d a t a . H o w e v e r , the m o s t c o n v i n c i n g d e m o n s t r a t i o n o f t he p r a c t i c a l i m p l e m e n t a t i o n of R P E was the p r o d u c t i o n of a t o m o g r a p h i c i m a g e of the o r e b o d y f r o m data a c q u i r e d o n l e v e l 10 i n the L y n x M i n e . Based u p o n these f i e ld t r ia ls , I expect that w i t h a 0.5 to 1 k g e x p l o s i v e charge the R P E m e t h o d is capab le o f r e l i a b l y de t ec t ing mass ive s u l p h i d e orebodies 100 to 150 meters f r o m the shotpoin t . T h e d i s c o u r a g i n g resul ts f r o m the t w o surface t r ia ls i n Q u e e n s l a n d , A u s t r a l i a , i nd ica te that the m e t h o d needs i m p r o v e m e n t before it can be a p p l i e d o n the surface. H i g h -b a n d w i d t h data w o u l d a l l o w better d i s c r i m i n a t i o n be tween spher ics a n d R P E pulses , a n d p r o b a b l y w o u l d rect ify mos t of the a c q u i s i t i o n p r o b l e m s . H o w e v e r , the p o o r s e i smic c o u p l i n g i n w e a t h e r e d rock m a y l i m i t the a p p l i c a t i o n of R P E i n e x p l o r a t i o n f r o m the surface to v e r y s h a l l o w targets, o r to borehole w o r k . T h e p r i n c i p a l conc lu s ions that can be d r a w n f r o m m y thesis is that the h i g h f requency se i smoe lec t r i c p h e n o m e n o n of R P E d e s c r i b e d b y S o b o l e v , D e m i n , M a y b u k , a n d L o s exists, a n d that it can be u s e d to explore for s u l p h i d e minera l s . R E F E R E N C E S A b u e l m a ' a t t i , M . T., 1995, Rad io f r equency interference b y d e m o d u l a t i o n m e c h a n i s m s present i n b i p o l a r opera t iona l ampl i f i e r s : IEEE Trans, on EM Compability, 37, 306-310. A u l d , B . A . , 1973, Acoustic Fields and Waves in Solids, Vol I: J o h n W i l e y a n d Sons . B a r l o w , R. J., 1989, Statistics: John W i l e y a n d Sons. B a u m , C . E . , 1986, E lec t romagne t i c sensors a n d measurement techniques: T h o m p s o n , J .E. a n d Lues sen , L . H . , Eds . , Fast Electrical and Optical Measurements, Vol I, N A T O A S I series E , 71-137. 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A P P E N D I X A S E I S M I C S O U R C E C H A R A C T E R I S T I C S A.1 Explosive to Rock Coupling T o u n d e r s t a n d w h y pento l i te w o r k s so w e l l w e need to c o n s i d e r h o w h i g h e x p l o s i v e s w o r k , a n d h o w they c o u p l e to the rock . F i r s t l y , h i g h e x p l o s i v e s c a n o n l y be r e l i a b l y de tona ted b y means of a s h o c k or a h i g h pressure i m p u l s e (Dav ie s , 1987; T o u r , 1992). E l e v a t i n g the tempera ture of h i g h explos ives does cause t hem to b u r n , but the process is the m u c h s l o w e r process of def lagra t ion . W h e n a h i g h exp lo s ive detonates the c h e m i c a l r eac t ion w i t h i n the e x p l o s i v e proceeds a l o n g a p r o p a g a t i n g s h o c k w a v e t h r o u g h the e x p l o s i v e m a t e r i a l ( D a v i s , 1987). W i t h d e f l a g r a t i o n o r r a p i d b u r n i n g the d i f f u s i v e p rocess o f h e a t i n g the reactants to i g n i t i o n t e m p e r a t u r e p r o p a g a t e s the c h e m i c a l r eac t ion , w h i c h is a m u c h s l o w e r process t han d e t o n a t i o n . A c o m b i n a t i o n o f h i g h pressure a n d tempera ture is needed to sus ta in the shock w a v e a n d the r a p i d c h e m i c a l react ions of de tona t i on w i t h i n it. The speed of the de tona t ion shock w a v e is c a l l ed the ve loc i ty of de tona t ion . P ressure f r o m the c h e m i c a l react ions w i t h i n the h i g h e x p l o s i v e are d i s t r i b u t e d i n t w o w a y s , shock a n d heave. H e a v e is the p o r t i o n of energy that is d i s t r i b u t e d t h r o u g h o u t the spent c h e m i c a l reactants, a n d shock is the par t that a c c o m p a n i e s the d e t o n a t i o n front. Pen to l i t e e x p l o s i v e s have v e r y h i g h d e t o n a t i o n p ressures a n d a lo t o f s h o c k energy. In s m a l l scale se i smic app l i ca t i ons it is the shock that con t r ibu tes m o s t to the steep wavef ron t of the se i smic d i s tu rbance (pr iva te c o m m u n i c a t i o n w i t h C . I . L . s e i s m i c e x p l o s i v e represen ta t ives ) . H e n c e , the use of d y n a m i t e a n d o the r h i g h v e l o c i t y exp los ives i n se i smic w o r k . T h i s c l a i m is s u p p o r t e d b y m y u n d e r g r o u n d tests because no difference i n the peak se i smic a m p l i t u d e , as m e a s u r e d b y a remote geophone , w a s f o u n d be tween t a m p e d (confined) a n d u n t a m p e d explos ives . If the heave p o r t i o n of the 138 Appendix A : Seismic Source Characteristics 139 energy w e r e to p r o v i d e a subs tan t ia l boost then t a m p i n g the e x p l o s i v e w o u l d increase the a m p l i t u d e of the se i smic w a v e . H e a v e is m o r e i m p o r t a n t i n m i n i n g a n d q u a r r y i n g because the r e l a t i ve ly s l o w e x p a n s i o n of the gases comple tes the b r e a k - u p of r o c k a n d m o v e s the rock mass. In a d d i t i o n to h a v i n g a greater p o r t i o n of energy i n the shock front, pento l i te exp los ives c o u p l e m o r e e f f i c ien t ly to u n d e r g r o u n d r o c k t h a n d o the l o w d e t o n a t i o n v e l o c i t y exp los ives . F r o m basic acoust ic p r i n c i p a l s w e k n o w that the m a x i m u m energy transfer across a n interface occurs w h e n the acoust ic impedance ' s of the t w o mate r ia l s are equa l ( Z o e p p i t z equat ions , Sher i f f a n d G e l d a r t , 1985). A c o u s t i c i m p e d a n c e Z is d e f i n e d as a p r o d u c t of the d e n s i t y a n d the v e l o c i t y of acous t ic p r o p a g a t i o n (I w i l l o n l y cons ide r acoust ic o r P -waves here as these are the d o m i n a n t w a v e s i n u n d e r g r o u n d explos ions) . Z = Pvp (A.1) C o u p l i n g of the energy f r o m exp los ive shock front to the rock m e d i u m f o l l o w s a s i m i l a r ru le . E x p e r i m e n t a l s tud ies b y N i c h o l l s (1962) s h o w that the c o n d i t i o n for m a x i m a l t ransfer of ene rgy f r o m e x p l o s i v e is that the p r o d u c t of the v e l o c i t y of d e t o n a t i o n ( V . O . D . ) a n d dens i ty of the exp lo s ive equa l the acoust ic i m p e d a n c e of the s u r r o u n d i n g m e d i u m . ZexP; = VD x p e x p ; (A.2) a n d for m a x i m u m energy transfer. Z ™ * = Z e x p / (A.3) It is d i f f icu l t for exp los ives i n h a r d rock to satisfy A . 3 because the acoust ic i m p e d a n c e of h a r d rock is h i g h a n d the densi t ies of mos t exp los ives are qu i t e l o w (1100-1700 k g / m ^ ) . F o r example , cons ide r m y t y p i c a l u n d e r g r o u n d s i tua t ion : Pro* = 2 7 0 0 k g n T 3 and vp = 5500 m s " 1 => Z = 14.8 x 1 0 6 k g n T 2 s _ 1 Appendix A : Seismic Source Characteristics 140 P exP/= 1700 k g m - 3 and V f l = 7500 m s - 1 => Z = 1 2 . 7 x l 0 6 k g m " 2 s _ 1 T h i s is a f a i r l y g o o d ma tch , bu t the Z of pen to l i t e (used i n the e x a m p l e above) is the u p p e r b o u n d for c o m m o n l y ava i l ab le exp los ives , mos t f a l l w e l l short . F o r ins tance, the m o r e c o m m o n l y a v a i l a b l e b l a s t i n g agents have p rope r t i e s that f a l l w e l l shor t for o u r u n d e r g r o u n d c o n d i t i o n s : Pexp/ = !300 k g m ~ 3 and VD = 2500 m s"1 => Z = 3.2 x 1 0 6 k g m - 2 s"1 In s u m m a r y , pen to l i t e a n d other v e r y h i g h v e l o c i t y exp los ives c o u p l e w e l l because the exp los ive i m p e d a n c e matches c lose ly that of the rock genera l ly f o u n d u n d e r g r o u n d . A.2 Source Energy Calculations M a t c h e d i m p e d a n c e does not guarantee that a l l the energy c ros s ing the interface w i l l be seen i n the far f i e l d . If the i m p e d a n c e ' s of the r o c k a n d e x p l o s i v e are r e a s o n a b l y m a t c h e d t h e n the p ressures i n the host r o c k near the e x p l o s i v e w i l l be s i m i l a r to pressures w i t h i n the d e t o n a t i o n front , w h i c h are a p p r o x i m a t e l y 10 G P a for pen to l i t e . A n e m p i r i c a l l y d e r i v e d e x p r e s s i o n for the p e a k d e t o n a t i o n p r e s s u r e for v a r i o u s explos ives is g i v e n b y T o u r (1992) w h e r e D is the specif ic dens i ty of the explos ive . T y p i c a l va lues of Pd (1-10 G P a ) exceed the elastic l i m i t for mos t rocks (300-600 M P a t y p i c a l c o m p r e s s i v e s trengths for grani tes , basalts a n d andesites, Jaeger a n d C o o k , 1971; C a r m i c h a e l , 1989), a n d as a resul t m u c h of the energy diss ipates i n the f o r m of cracks a n d heat. N o t e that the v e r y competen t r o c k t y p i c a l l y f o u n d u n d e r g r o u n d w i l l behave e l a s t i c a l l y at m u c h h i g h e r stresses, thus , t r ans f e r r i ng a greater a m o u n t of energy f r o m the e x p l o s i v e . N i c h o l l s et a l . (1962) o b s e r v e d a n ene rgy t ransfer ef f ic iency of a p p r o x i m a t e l y 5% (best) f r o m c h e m i c a l to se i smic energy i n fa i r ly compe ten t rock ( N i c h o l l s c l a i m s that h i g h e r eff ic iencies , u p to 450 D 1 + 0 . 8 D (A.4) Appendix A : Seismic Source Characteristics 141 20%, can be ob ta ined , but I w i l l use the l o w e r f igures ob ta ined b y h i s exper iment ) . If i t is a s s u m e d that the shock w a v e is r e spons ib le for mos t o f the se i smic d i s tu rbance then w e c a n calcula te the po ten t ia l m e c h a n i c a l energy w i t h dW = VdP (A.5) ( F i n n , 1993). N o t e that the peak se i smic a m p l i t u d e w i l l d e p e n d u p o n the geomet ry of the e x p l o s i v e / h o s t r o c k contact , ra ther t h a n the mass of e x p l o s i v e (or to t a l c h e m i c a l energy). T h e re la t ive ly s l o w gaseous expans ion phase (heave) conta ins m o r e energy, but ach ieves a s m a l l e r p e a k a m p l i t u d e because of the m u c h greater d u r a t i o n o f f o r c i n g . E x p l o s i v e charges u s e d i n m y f i e ld t r ials are c y l i n d r i c a l i n shape to m a t c h the bo reho le i n w h i c h they are p l aced . Therefore, a d d i n g charges (or mass) w i l l l i n e a r l y increase the surface area p resen ted to the rock , a n d se i smic a m p l i t u d e s w i l l be p r o p o r t i o n a l to the square root of the charge mass. H o w e v e r , a sphe r i ca l geomet ry , s u c h as that m a d e b y a s m a l l exp lo s ive for a la rger charge, w i l l resul t i n a cube root s c a l i n g o f a m p l i t u d e w i t h charge mass ( N i c h o l l s , 1962). N o t e that for la rge scale se i smic w o r k the h i g h f requency p o r t i o n of the s e i s m i c d i s t u r b a n c e is a t t enua ted , a n d the s h o c k p r o p e r t i e s o f the exp lo s ive cont r ibu te l i t t le to the response. E q u a t i o n A . 5 c a n be u s e d to ca lcula te the a m o u n t of energy p u t in to the se i smic pu lse . T h e shock pressure increase is g i v e n by equa t ion A . 4 , a n d is abou t 10 G P a for pentol i te . F o r V , the v o l u m e of compres sed gases i n the shock front, I w i l l use the outer s h e l l of the exp lo s ive that ac tua l ly touches the borehole w a l l . It is this p o r t i o n of the exp los ive that p r o v i d e s the i m p u l s i v e force u p o n the host r o c k to p r o d u c e the se i smic i m p u l s e . A n o rde r of m a g n i t u d e f igure for the w i d t h of the d e t o n a t i o n front (1) is about 1 c m (Davies , 1983). T y p i c a l d i m e n s i o n s of o u r charges are 10 -15 c m (h) i n l eng th a n d 5 c m i n d i a m e t e r (d) (225 to 450 g charge) . I w i l l a s sume that the energy coup le s c o m p l e t e l y across the b o u n d a r y a n d then losses 95% of the e n e r g y i n heat a n d m e c h a n i c a l de fo rma t ion , g i v i n g a to ta l efficiency ((3) of 5%. Therefore, A . 5 becomes: Appendix A : Seismic Source Characteristics 142 E, = B Kdlh Pd (A.6) Substituting my estimates of the parameters in equation A.6 gives an expected seismic energy of approximately 100 kj from a potential energy of 2 MT. Telford (1986) gives a lib (450 g) explosive 4 MJ of potential energy, which is in fair agreement with my estimate. To estimate the stress/strain in the far field I will treat the seismic disturbance as a pulse of P-wave energy of duration At, and assume that the stress field is radially symmetric about the shotpoint (i.e. a homogeneous medium). Firstly, consider the energy flux, or Poynting vector (S), at some distance r from the shotpoint (Auld, 1973) S = - v \ T (A.7) where v and T are the particle velocity and stress respectively (for P-waves T can be represented as a vector, but it is actually a tensor). The acoustic wave and the energy are propagating radially outward. For convenience, I will progress further with scalar notation with the understanding that T and v are co-linear and radially directed. Integrating A.7 over time and a closed surface enclosing the shotpoint gives, Es = \drjdtS ( A 8 ) s At the total energy of the source. If equation A.8 is integrated over the surface of a sphere centered upon the shotpoint, and over the pulse duration At, then Es = Anr2MS (A.9) Substitution of a simple relationship between the average stress (T) and average particle velocity (v), T = Zv (A.10) into A.7 to expresses the source energy (A.9) in terms of radial particle velocity at some Appendix A : Seismic Source Characteristics 143 dis tance, r, f r o m the sho tpo in t Et = 4nr2AtZv2 ( A . l l ) E q u a t i o n A . 10 is d e r i v e d f r o m the elastic s t r e s s / s t r a in cons t i t u t ive equa t ions (Hooke ' s eqn, Sher i f f a n d G e l d a r t , 1985), a n d f r o m n o t i n g that energy is exchanged f r o m kine t ic to po ten t i a l energy i n the se i smic w a v e osci l la t ions . F r o m A . l l w e can d e v e l o p t w o usefu l exp re s s ions : a n e q u a t i o n for R M S r a d i a l p a r t i c l e v e l o c i t y at a d i s t a n c e r as the d is turbance passes b y , 1 E v = - * — (A.12) r \AntstZ a n d an express ion for the R M S stress T = - Z E s (A.13) r v 4 ^ A r W i t h A . 1 3 a n d A . 1 2 I n o w have the means to estimate se i smic f i e ld a m p l i t u d e s . E. = 100 k J Z = 1 4 . 8 x l 0 6 k g m " 2 s"1 A f = 3 ms r = 5 0 m A.3 Source Parameter Measurements (A.14) T = 125 k P a v = 0 . 8 4 x l 0 " 2 m s 1 A n u n d e r s t a n d i n g of w h a t is m e a s u r e d w i t h a g e o p h o n e i n s i d e the m i n e is n e e d e d before a c o m p a r i s o n can be m a d e w i t h m y theore t i ca l es t imates of p a r t i c l e v e l o c i t y (A.14) . F i r s t l y , I use a fa i r ly s t anda rd type of ve r t i ca l (spike) geophone , s i m i l a r to those u s e d i n s ing le c o m p o n e n t exp lo ra t i on su rveys o n the surface. T h e geophones have a 10 H z resonance a n d are c r i t i ca l ly d a m p e d . In the m i n e they are genera l ly p l a c e d in to the f l oo r of the t u n n e l , as th is is the o n l y p l ace that a s p i k e c a n be d r i v e n i n . T h i s o r i en ta t ion is not i dea l because the exp lo s ive is genera l ly loca ted at the s ame e l eva t ion as the geophone ; therefore, the P - w a v e par t ic le m o t i o n is r a d i a l , w h i c h is the d i r e c t i o n Appendix A : Seismic Source Characteristics \4A that the geophone is least sensitive (supposedly non-sensitive). Nonetheless, record clear signals of appreciable amplitude are recorded. Some simple elastic theory can explain this, and give an estimate of the radial particle velocity from the geophone signal. z B a l a n c e d F o r c e s Figure A . l Vertical ground motion due to a radial P-wave. The geophone is mounted on a stress free boundary. Radial forces on the rock beneath the geophone causes the material to bulge upward. Understanding the role of the tunnel is necessary to solve the problem. Consider the forces on a cube of rock beneath the seismometer. One, axis of the cube is being forced, another axis has stress free boundaries (the geophone axis), and the last axis has balancing forces and no particle displacement because of the adjacent rock (Figure A .2). This model treats the seismic wave as a plane wave incident on the first axis (wavefront curvature is not significant tens of meters away from the source). The role of the tunnel is to provide a free surface, which the geophone rides on. Note the similarity of this model to the definition of Poisson's ratio, CT, the amount of strain in a direction perpendicular to an applied stress. Using the same approach we can obtain a modified Appendix A : Seismic Source Characteristics 145 Poisson ' s ra t io , & ' , (Jaeger a n d C o o k , 1971). E q u a t i o n A . l 5 can be u s e d to relate the ve r t i ca l m o v e m e n t of the geophone to the r a d i a l s t r e s s / s t r a in s o n the m e d i u m . F o r m o s t h a r d - r o c k types (e.g.. grani tes) < T ~ 0 . 2 , a n d O " ' ~ 0 . 2 5 . There fo re , If I h a v e a m e a s u r e m e n t of v e r t i c a l p a r t i c l e v e l o c i t y ( in the tunne l ) , V, then the r a d i a l par t ic le ve loc i ty , v , is W i t h e q u a t i o n A . 16 m e a s u r e d pa r t i c l e ve loc i t i e s f r o m m y u n d e r g r o u n d tests c a n be c o m p a r e d to the estimates g i v e n i n A .14 . T h e first e x a m p l e comes f r o m the L y n x M i n e , B . C . , w h e r e I m e a s u r e d a peak pa r t i c l e ve loc i ty of v' = 2.7 x 10~ 3 m s"1 (an 80 m V s igna l w i t h a 30 V s / m geophone) o n a ve r t i ca l geophone 75-80 meters f r o m a 0.5 k g shot. U s i n g equa t ion A . 1 5 , a n d n o r m a l i z i n g the a m p l i t u d e for a d i s t a n c e o f 50 mete rs f r o m the sho t w i t h vlrl=v2r2(so that a c o m p a r i s o n w i t h A . 1 4 can be made) , a par t ic le ve loc i ty of v ( r = 50 m ) = 1.6 x 10" 2 m s"1 is o b t a i n e d . T h i s i s w i t h i n a f ac to r of t w o o f m y t h e o r e t i c a l e s t i m a t e o f v ( r = 50 m ) = 0.84 x 10" 2 m s"1. N o t e that the theore t ica l est imate is an average o r R M S va lue a n d that the measurement is a peak va lue . H e n c e , the t w o f igures are comparab le . T h e peak stress (at 50 m) is a p p r o x i m a t e l y 250 k P a (us ing A . 1 0 a n d Z = 1.5 x 10 7 ) . A s i m i l a r e x a m p l e f r o m the M o b r u n M i n e , b u t w i t h a 0.22 k g charge , g ives a v e r t i c a l par t ic le v e l o c i t y of v' = 1 . 3 x l 0 - 3 m s"1 at a d i s tance of 90 meters. O n c e the a m p l i t u d e s have been n o r m a l i z e d for charge s ize (0.5 kg) a n d dis tance (50 m ) , peak par t ic le v e l o c i t y a n d stress at 50 meters are v = 1.3 x 1 0 - 2 m s _ 1 and 200 k P a respect ively . These f i e l d m e a s u r e m e n t s agree w i t h the expe r i ence o f o thers . D a t a f r o m N i c h o l l s (1962) g ives a par t ic le ve loc i ty of 0.013 m / s at 50 m for 0.5 k g , a n d a ca lcu la ted ( f rom h is s u p p l i e d Z ) peak stress of 140 k P a . U s i n g an e m p i r i c a l express ion d e v e l o p e d b y the U . S . v = — or (A.16) Appendix A : Seismic Source Characteristics 146 Bureau of Mines (Sheriff and Geldart, 1985), I calculate that approximately 240 kPa of stress, and a particle velocity of 0.014 m/s at a distance of 50 meters from a 0.5 kg charge can be expected. In summary, a 0.5 kg explosive charge should be able to deliver P-wave pressures of about 200 kPa at a distance of 50 m if the explosive is well coupled to the rock. A P P E N D I X B INSTRUMENTATION DETAILS B.l The Fiber Optic Time-Break T h e i d e a b e h i n d the f iber op t i c t ime-break is that the in tense l i g h t e m i t t e d b y h i g h exp los ives can be u s e d to ob ta in an accurate t ime break. In pract ice , a s m a l l l eng th of u n s h e a t h e d f iber is a t tached to the de tonator or exp los ive . S o m e o f the l i g h t f r o m the e x p l o s i o n is co l l ec ted b y the fiber op t i c s t rand , a n d is t r ansmi t t ed t h r o u g h the l eng th of the f iber op t i c cable to a n o p t i c a l rece iver , w h i c h i n t u r n t ranslates the l i g h t in to a n electr ical s i gna l (Kep ic et a l . , i n press). O p t i c a l cable m a d e b y H e w l e t t - P a c k a r d , the H F B R 501 ser ies , w a s u s e d as the l i g h t c o n d u i t . T h e f iber op t i c cable is a n o p t i c a l l y c lear p las t i c f iber , 1 m m i n d i ame te r , shea thed b y P V C , w i t h the comple t e f iber h a v i n g a to ta l d i a m e t e r o f 2.2 m m . P las t i c c o n s t r u c t i o n a l l o w s the cable to be v e r y l igh t (5 g per meter) , f l ex ib le (it m a y be bent d o w n to a 3 c m radius) , a n d it costs o n l y $1 per meter. L o w cost is of conce rn because the e n d of the fiber l i n k is des t royed b y the exp los ion . I t y p i c a l l y lose less than 1 m w i t h 0.5 k g charges , bu t some t imes a greater a m o u n t is lost d u e to f l y - r o c k d a m a g e . S t a r t i n g lengths of the cable are be tween 40 a n d 100 meters i n l eng th . L e n g t h s greater t han 70 meters are not r e c o m m e n d e d b y H e w l e t t - P a c k a r d because of a t t enua t ion of the l i g h t s i g n a l , bu t I have u s e d 100 me te r l eng ths w i t h no p r o b l e m s (the e x p l o s i o n is v e r y p o w e r f u l l igh t source). A c r i m p - o n connector is p l aced o n the cable at the r e c e i v i n g end , but the other end is u s e d bare. T h e o p t i c a l receiver , a H e w l e t t - P a c k a r d H F B R - 2 5 2 2 , has a 3 m i c r o s e c o n d response t ime , a n d can detect a blast w i t h as m u c h as 100 m of f iber opt ic cable (longest l eng th tested). T h e rece ive r p r o d u c e s a d i g i t a l s i g n a l , w h i c h is l o w (0 V ) i f the l i g h t exceeds a s m a l l t h r e sho ld a n d h i g h (5 V ) o therwise , a n d can be in terfaced w i t h T T L c o m p a t i b l e c i rcu i t s . 147 Appendix B: Instrumentation Details 148 This signal may be used directly, but to produce a consistent signal a pulse-generator is used to produce a 300 ms pulse; otherwise, the fiber optic receiver may produce a pulse so short in duration that it is not detected by the acquisition electronics. A n attenuator placed in series with the output of the pulse generator reduces signal levels on the cable connecting the optical receiver box to the recording unit, thus, reducing cable cross-talk. The explosive end of the fiber optic cable is prepared by stripping approximately 3 to 5 cm of PVC cladding, and then taping the bare fiber to the detonator or inserting it directly into the explosive. If the end of the fiber optic cable is not stripped then there wil l be no signal. After use, the burnt end of the cable can be cut off and prepared for another shot. Unlike glass fiber optics, no special care is needed to put a connector on a cable; therefore, cable repair or construction is easily done in the field. The fiber optic circuit is resistant to dirt and mud, but the connection between the receiver and optical cable should be kept fairly clean. Testing the integrity of the connection and electronics between salvoes is advisable, as we have on occasion forgotten to reconnect the power or signal cable to the optical receiver electronics after moving the equipment to new area. In addition, flyrock from the explosion can ruin the cable with little or no visible signs of damage. A camera flash unit with a manual test button provides a reliable simulation of the explosion and is very portable. A side-by-side comparison of the fiber optic and wire-break methods on a detonator showed that the fiber optic trigger occurs first, and the wire-break signal arrives 10-20 Lts later. This demonstrates the great accuracy of the fiber optic time-break because the wire-break is known to be a very accurate method of determining time of detonation (Burrows, 1936). B.2 Electromagnetic Noise from Various Triggering Methods As mentioned previously, the large transients at time zero from blasting box currents are intolerable for seismoelectric research. A n example of this type of interference is Appendix B: Instrumentation Details 149 shown in Figure B. l , which shows the response of three grounded dipoles (labeled Dipole 1, Dipole 2 and HF antenna) in an underground seismoelectric experiment to the detonation of a 50 ms delay electrical detonator in a 225 g pentolite booster. A 50 ms delay was chosen so that the initiation interference would be finished before the blast. The record shows a large transient (some of the signals are clipped because of the high gain on the recording amplifiers) when the detonator is initiated, and approximately 50 ms later the explosion occurs, producing further signals: blast EM and possibly some other seismoelectric signals. Al l reasonable measures were taken to reduce the transient amplitude and duration, such as keeping signal cables and detonator wires separate and the use of a blasting box that quickly disconnects once the detonator is initiated. 80 uV/m 600uV/m 1 mV/m 30 45 Time (ms) Figure B.l Electric field from detonator initiation and blast. In this example a 50 ms delayed blasting cap is used to start the explosion of a 0 22 ke charge. & Appendix B: Instrumentation Details 150 In F i g u r e B . l it is apparent that the blast is a c c o m p a n i e d b y cop ious amoun t s o f e lectr ical s ignals . T h i s type of seismoelectr ic s i g n a l has a lso been seen b y O 'Kee fe a n d T h i e l (1992) i n q u a r r y blasts, a n d Endres (1982) i n s m a l l exp los ions i n a n u n d e r g r o u n d m i n e . E a r l y blas t - re la ted E M has a l l bu t d i s appea red f r o m o u r records s ince the i n t r o d u c t i o n of the fiber opt ic t ime break w i t h safety fuse detonators . Blast associa ted E M is thought to be d u e to the electr i f icat ion of rocks d u r i n g fracture near the shotpoin t . It appears that the w i r e s connec ted to the electr ical de tona tor re-radiate m u c h of the e lec t r ica l no i se f r o m the b las t area , a n d c o m m u n i c a t e these s i g n a l s to sensors nea r to the w i r e s . T h e r e d u c t i o n of the blast E M a m p l i t u d e i s i m p o r t a n t as i t often obscures ea r ly s igna ls f r o m the target. Electric Cap + Wire-Break Electric Cap + Fiber-Optic Fuse Cap + Wire-Break 20mV/m 20mV/m 1 mV/m Fuse Cap + Fiber-Optic + 2m Wire 1 mV/m Fuse Cap + Fiber-Optic 1 mV/m -0.6 -0.3 0.0 0.3 0.6 Time (ms) F i g u r e B.2 E lec t r i ca l noise verses t ime-break m e t h o d . Resu l t s are f r o m a s m a l l -scale test w i t h a detonator. Appendix B: Instrumentation Details 151 At a site near Vancouver, Canada we performed a small scale test of the E M noise produced by the fiber optic method and compared it to the wire-break method. In addition, electrical and safety fuse detonators were compared. The explosive charge used in the tests consisted of a detonator plus a small pentolite booster (about 8 g). Charges were placed into water filled holes 2 cm in diameter and 30-40 cm deep; the holes were drilled into a large, partially buried boulder. Results from the tests, displayed in Figure B.2, demonstrate that a wire-break with only 0.2 mA sense current (traces A and B) creates considerable interference, and that the fiber optic with a safety fuse detonator (E) produces the least amount of noise. The fiber optic and electrical (250 ms delay) detonator combination (C) produced a significant transient, confirming our belief that long wires leading into the explosive increase the amount of blast related noise. A 2 m length of shorted twin-strand wire placed with a safety fuse and fiber optic assembly (D) produced a barely measurable transient, but demonstrates that it is best to keep conductors away from the blast area if the lowest possible amount of E M interference is desired. The excellent electrical isolation of the fiber optic method also provides another benefit, it can be safely used in areas where it would be hazardous to use electric/seismic detonators. B.3 O p t i m i z i n g S o l e n o i d a l M a g n e t i c Sensors There are three basic electrical arrangements for using a coil (Figure B.3). The first is the induction coil arrangement: the coil is connected to a high input impedance amplifier. A disadvantage of the induction coil is that the voltage is proportional to the derivative of the magnetic field, so higher frequencies dominate the response. This behavior can be remedied with numerical post-processing of the data, but it requires low noise (especially at the lower frequencies) and demands a large dynamic range from the electronics and digitizer to be successful. Alternatively, an analog integrator may be used before digitization. However, the greatest drawback of this scheme is the self-resonance of the coil (the parallel combination of capacitance and inductance) because it Appendix B: Instrumentation Details 152 creates a v e r y u n d e s i r a b l e f r equency r e sponse , a n d d y n a m i c r a n g e p r o b l e m s i f u n d e r d a m p e d . x l Induction Coil Damped Resonator r - A / W W Current to Voltage Converter F i g u r e B.3 Prac t ica l magnet ic sensor topologies u s i n g a c o i l . T h e o the r t w o a r r a n g e m e n t s i n F i g u r e B.3 are s i m i l a r ; b o t h p r o v i d e a v o l t a g e p r o p o r t i o n a l to the magne t ic f i e ld over a l i m i t e d b a n d of frequencies. In the d a m p e d resonator d e s i g n a shunt resistor is a d d e d to the c o i l to p r o d u c e a flat response be tween the frequencies de t e rmined b y the R / L ( lower l i m i t ) a n d 1 / R C (upper l i m i t ) poles . N o t o n l y does the res is tor set the b a n d w i d t h , bu t i t p r o v i d e s the role of cur ren t - to-vol tage c o n v e r s i o n . T h e t rans-conductance (or cur ren t - to -vo l tage conver te r ) a m p l i f i e r d e s i g n uses the feedback a rch i tec ture of a n o p e r a t i o n a l a m p l i f i e r ( O p - A m p ) to conver t the Appendix B: Instrumentation Details 153 cu r r en t at the nega t ive t e r m i n a l to a n e q u i v a l e n t vo l t age . In t heo ry , the pa ra s i t i c capaci tance p l ays no part i n this t o p o l o g y because the po ten t ia l difference across the c o i l is m a i n t a i n e d , b y nega t ive feedback, to be zero . In pract ice , the capaci tance w i l l cause s tab i l i ty p r o b l e m s w i t h the a m p l i f i e r (i.e. h i g h f requency osc i l la t ions) a n d c o m p e n s a t i n g capaci tance is a d d e d across the feedback resistor , thus f o r m i n g a p o l e that l o w e r s the f requency response of the sys tem. T h e t rans-conductance a m p l i f i e r has a r e l a t i ve ly flat response d o w n to the R / L po le f o r m e d by the series resistance o f the c o i l itself. T h i s can be a n advan tage s ince it m a y ex tend the b a n d w i d t h d o w n to 1-10 H z ( M a c i n t y r e , 1980; L a b s o n et a., 1985; H a u s e r , 1990). H o w e v e r , it is a severe d i s a d v a n t a g e i f there i s s u b s t a n t i a l p o w e r - l i n e no i se because the a m p l i f i e r w i l l c l i p the s i g n a l s i f there is insuff ic ient d y n a m i c range. I chose to i m p l e m e n t the d a m p e d resonator de s ign because it is fa i r ly robust , a n d i t has a na tu r a l h igh-pass response that can be d e s i g n e d to attenuate p o w e r - l i n e frequencies (60 H z a n d s u b - k i l o h e r t z h a r m o n i c s ) . In a d d i t i o n , I i n h e r i t e d s o m e g o o d d e s i g n s f r o m ear l ier w o r k b y B . N a r o d to b u i l d u p o n . N o t e that i f the same v a l u e of res is tor is u s e d for the d a m p e d resonator a n d the t rans -conduc tance a m p l i f i e r t hen the t w o sys t ems h a v e the same f u n d a m e n t a l l i m i t s o n s igna l - to -no i se ( a s s u m i n g a perfect no i se less ampl i f i e r ) . T h e choice be tween the t w o des igns is based p r i m a r i l y u p o n the d e s i r e d l o w frequency response. The band-pass p o r t i o n of the transfer func t ion is ob ta ined b y subs t i t u t ing CO = 2nf0 i n to equa t ion 2.4 (see C h a p t e r 2), —^—L = NA— (B l) T h i s is e q u i v a l e n t to t r ea t ing the res is tor as the o n l y r e t u r n p a t h o f m a g n e t i c f i e l d i n d u c e d current p r o d u c e d by the c o i l . In genera l , a w e l l d e s i g n e d t ransducer p roduces the largest poss ib le vo l tage for a g i v e n va lue of magne t i c f i e l d . F r o m e q u a t i o n B.24 it can be seen that b y m a x i m i z i n g the effective area of the c o i l w h i l s t m i n i m i z i n g the c o i l Appendix B: Instrumentation Details 154 i n d u c t a n c e p r o d u c e s the largest s i g n a l . In a d d i t i o n , the s h u n t res is tance s h o u l d be m a x i m i z e d . H o w e v e r , these s i m p l e gu ide l ines do not p r o d u c e use fu l sensors because of trade-offs i n sensor b a n d w i d t h ( in ad jus t ing L or R ) , a n d i n the s ignal - to-noise rat io. A f t e r c o m p l e t i n g the S u l l i v a n a n d M o b r u n M i n e exper iments i n 1991 I n o t i c e d that the noise s p e c t r u m of m y best magnet ic sensor ( U B C I) appea red to be G a u s s i a n o r w h i t e i n na tu re . F u r t h e r i n v e s t i g a t i o n r e v e a l e d that th is n o i s e c o u l d be a t t r i b u t e d to the d a m p i n g resis tor a n d p r e a m p l i f i e r w i t h i n the sensor. T h i s pa r t i cu l a r sensor , d e s i g n e d a n d b u i l t b y B . N a r o d , offers v e r y g o o d pe r fo rmance , bu t s ign i f i can t i m p r o v e m e n t s c o u l d be m a d e i n this area. N e w sensors for b road -band measurement (1 k H z to 3 M H z ) w e r e n e e d e d ( in 1991), so these sensors w e r e o p t i m i z e d for l o w n o i s e as w e l l as b a n d w i d t h . L u k o s c h u s (1979) a n d M a c i n t y r e (1980) have presented analyses o n o p t i m i z i n g i n d u c t i o n c o i l magnetometers , bu t thei r t reatments are too specif ic . M a c i n t y r e ' s (1980) is t i e d to a p a r t i c u l a r c o i l geomet ry , a n d L u k o s c h u s (1979) is c o n c e r n e d w i t h o p t i m a l w e i g h t (his sensor was to be u s e d i n space). N e i t h e r g ive any genera l gu ide l ines . In the f o l l o w i n g , I w i l l g i v e a t rea tment that leads to s o m e f a i r l y i n t u i t i v e c o n c l u s i o n s abou t s o l e n o i d a l sensors . A s o l e n o i d is a l o n g c y l i n d r i c a l c o i l , a geome t ry u s e d i n m a n y g e o p h y s i c a l i n s t r u m e n t s because it 's p rope r t i e s are w e l l de f ined , a n d has a c o n v e n i e n t shape for borehole a n d a i rborne w o r k . The sel f - inductance of a s o l e n o i d is a p p r o x i m a t e d b y ( L o r a i n a n d C o r s o n , 1988), w h e r e 1 is the l eng th , N the n u m b e r of turns , A the cross-sec t iona l area, a n d fl is the magne t i c p e r m e a b i l i t y . N o t e that this a p p r o x i m a t i o n is accurate i f the l eng th is m u c h greater than the d iameter . S u b s t i t u t i n g equa t ion B .2 in to B . l g ives the pass-band response for a s o l e n o i d L = fi N2A (B.2) V = IRB RH UN or (B.3) Appendix B: Instrumentation Details 155 w h e r e B = flH. E q u a t i o n B.3 is u n d e r c o n s t r a i n e d because the l o w f requency corner , coL = 2nfL, of o u r d e s i g n has not been des igna t ed . F o r a g i v e n c o i l a n d l o w e n d frequency the r equ i r ed v a l u e of R is (eqn. 2.6, see C h a p t e r 2) R = COLL (B.4) S u b s t i t u t i n g equa t i on B.4 a n d B.2 in to B.3 expresses the response o f the s o l e n o i d g i v e n d e s i g n constra ints o n the l o w frequency per formance of the magnetometer . V = coLflNAH (B.5) E q u a t i o n B.5 descr ibes the magne tomete r b e h a v i o r w e l l , bu t the r ea l ly i m p o r t a n t f igure of m e r i t i n o p t i m i z i n g the sensor is the s igna l - t o -no i se r a t io ( S / N ) , o r e q u i v a l e n t magne t ic f i e ld noise . It is m o r e impor t an t to have l o w noise t han h i g h t r ansducer ga in ; the la t te r c a n c o m p e n s a t e d b y g o o d p r e a m p l i f i e r d e s i g n . T h e m i n i m u m n o i s e is d e t e r m i n e d b y the Johnson noise of the d a m p i n g resistance (Ott, 1988). V2n=4kTRM (B.6) J o h n s o n no i se has a G a u s s i a n o r w h i t e p o w e r s p e c t r u m , a n d i t is p r o p o r t i o n a l to the a b s o l u t e t e m p e r a t u r e (T) , r e s i s t ance (R) , a n d b a n d w i d t h (Af ) . T h e cons tan t , k (& = 1.38 x 1 0 2 3 J K " 1 ) , is B o l t z m a n n ' s constant . S u b s t i t u t i o n of e q u a t i o n B.4 i n to B.6 a n d equa t ing to the square of B.5 g ives the sensor no ise l e v e l i n terms of an equ iva len t magne t i c f i e ld • 2 AkTM H n = 77 (B.7) E q u a t i o n B . 7 i s the k e y to o p t i m i z i n g a s o l e n o i d a l t r a n s d u c e r . T h e i m p o r t a n t i n f o r m a t i o n that i t conta ins is that the l o w e n d of the magne tome te r pa s s -band s h o u l d be set as h i g h as the i n t e n d e d a p p l i c a t i o n w i l l a l l o w , a n d that it is the effective v o l u m e of the c o i l that is impor tan t ; not, as i nd i ca t ed i n eqn. B .5 , the effective area. T h i s is an easy concep t to c o m p r e h e n d : a l a rge r c o i l v o l u m e encompasses a greater a m o u n t of Appendix B: Instrumentation Details 156 magne t i c energy (the v o l u m e t r i c energy dens i ty is p r o p o r t i o n a l to flH2 ) for a g i v e n noise p o w e r f r o m the resistor . N o i s e f r o m the p r e a m p l i f i e r o r buffer connec ted to the c o i l w i l l a d d some noise, bu t it is genera l ly not a p r o b l e m w i t h the w i d e se lec t ion of h i g h q u a l i t y ampl i f i e r s ava i lab le . B.4 UBC Magnetic Sensor Design T h e f irst s tep i n d e s i g n i n g the sensor is d e f i n i n g the p a s s - b a n d e n d p o i n t s . F o r m y a p p l i c a t i o n this was set to a p p r o x i m a t e l y 1 k H z to 5 M H z . N e x t , a ferrite r o d that has the greatest v o l u m e a n d p e r m e a b i l i t y that budge t a n d s ize constra ints w i l l a l l o w is selected. Fe r r i t e r o d s 510 m m l o n g a n d 17 m m i n d i ame te r , t y p e C N - 2 0 m a d e b y C e r a m i c M a g n e t i c s , w e r e u s e d i n the U B C I V a n d U B C V magne t ic sensors. C N - 2 0 ferrite has a b u l k p e r m e a b i l i t y 800 t imes that o f free space o v e r a f r e q u e n c y r a n g e o f D C to a p p r o x i m a t e l y 4 M H z . B e y o n d this f requency the pe rmeab i l i t y of the ferrite falls r a p i d l y a n d losses w i t h i n the ma te r i a l increase. A l l of the v a r i o u s ferrites a n d magne t i c a l l oys share a p p r o x i m a t e l y the same a s y m p t o t i c b e h a v i o r at h i g h f requencies . V e r y h i g h p e r m e a b i l i t y mater ia l s reach this a sympto te at l o w e r frequencies, l i m i t i n g the i r ab i l i t y to g ive a flat pass -band over the des i r ed range. Therefore, i t is the u p p e r f requency b o u n d o n the sensor that de te rmines the ma te r i a l u s e d for the core of the c o i l ; as a resul t , the u p p e r b o u n d inf luences the noise per formance . F o r example , i f m y requ i remen t s w e r e o n l y for f requencies b e l o w 100 k H z then u s i n g M N - 6 0 ferrite w o u l d be advan tageous because i t has a re la t ive p e r m e a b i l i t y of 7000 (be low 100 k H z ) , a n d i t w o u l d reduce the noise l eve l by a factor of 3 over C N - 2 0 . O n c e the o p e r a t i n g f requency range has been d e t e r m i n e d then it is mat te r o f w i n d i n g the w i r e onto the ferrite core. M y requi rement for a range of 1 k H z to 5 M H z leads to a resonant f requency of 71 k H z for the c o i l (see equat ions B.23). A n a r rangement of three pies (or bobb ins ) w i t h about 500 turns each gave the r e q u i r e d resonant f requency. The three pies were sp read over 200 m m i n the center of the ferrite r o d a n d w i r e d i n series. A c c o r d i n g to equa t ion B.30 a l o w e r noise l e v e l can be o b t a i n e d i f the f u l l l eng th of the Appendix B: Instrumentation Details 157 r o d w e r e u sed , but this does not w o r k i n pract ice because the effective p e r m e a b i l i t y is i n f l uenced b y the aspect ra t io of the r o d ( L o r a i n a n d C o r s o n , 1988) a n d the a m o u n t o f r o d b e y o n d the c o i l ; the effect of the ferrite is to focus the f lux t h r o u g h the center of the c o i l , a n d w o r k s mos t effectively near the center of the r o d . T h e r e are n u m e r o u s sub t le t i e s i n c o i l d e s i g n for m i n i m u m s t r ay c a p a c i t a n c e a n d r e d u c i n g other resonances, bu t I w i l l o n l y m e n t i o n a coup l e of i m p o r t a n t po in t s . O n e is that the inne rmos t l ayer of w i n d i n g s o n the p ie can exhib i t s econdary resonances, w h i c h s h o u l d be t u n e d o r d a m p e d out w i t h either a capaci tor or resistor (Labson et a l . , 1985). A 3.3 k f i d a m p i n g resistor across each layer of w i n d i n g s o n the p ie (4 per p i e o f #30 A W G wi re ) s u b d u e d a secondary resonance at about 1.6 M H z . A shun t resistor across the c o i l a s s e m b l y adjus ts the e f fec t ive r e s i s t ance seen b y the c o i l , a n d the p a s s - b a n d characteris t ics of the sensor. A sp l i t s h i e l d to reduce capac i t ive or electric f ie ld p i c k - u p is p l a c e d o n the c o i l sensors . T h e sp l i t s h i e l d is a c o n d u c t i v e sheet w r a p p e d a r o u n d the sensor, bu t it is sp l i t so that the s h i e l d does not m a k e a shor ted w i n d i n g a r o u n d the r o d . I have f o u n d that a s h i e l d is necessary , desp i t e the a d d i t i o n a l pa ras i t i c capac i tance , because i t r educes p o s i t i v e feedback p r o b l e m s encoun te red w h e n the sensor is p l a c e d near ra i l s a n d o ther l a rge conduc tors . C a l i b r a t i n g b r o a d - b a n d m a g n e t i c sensors w i t h test co i l s is p r o b l e m a t i c . A m p l i t u d e ca l ib ra t ion is r e l a t ive ly easy: at the resonance f requency, o r at f requency w e l l w i t h i n the pass -band , check w i t h a k n o w n cur ren t g o i n g in to H e l m h o l t z co i l s o r a p re -ca l ib ra t ed s o l e n o i d (the a p p r o a c h I used) . P r o b l e m s occur i f the c a l i b r a t i o n c o i l has a resonance near the test f requency, or i f the ferrite r o d alters the test co i l ' s character is t ics . T e s t i n g the f requency response of a b road -band sensor is s t r a igh t fo rward u s i n g a cur ren t source , a n d a lmos t i m p o s s i b l e u s i n g test co i l s . T h e m a g n e t i c f i e l d c a n be r ep resen ted b y a cur ren t source across the c o i l ; hence, a cur ren t source across the c o i l m a y be u s e d to s imu la t e the effects of a n external magnet ic f ie ld . Appendix B: Instrumentation Details 158 A s i m p l e cur ren t source c a n be m a d e f r o m a func t ion generator ( A C vo l t age source) i n series w i t h a large (say 100 k Q ) resistor. N o t e that this a r rangement requi res s o m e care as the paras i t ic capaci tance across the resistor (< 1 pF) w i l l l i m i t the b a n d w i d t h of s u c h a n a r rangement (<5 M H z i n this case). In a d d i t i o n , the s u p p l i e d cur ren t is s h o u l d be r e l a t i v e l y i n d e p e n d e n t of l o a d , so the res is tor m u s t be at least a n o r d e r of m a g n i t u d e greater t han the i m p e d a n c e o f the t r ansduce r e lement (the c o i l / r e s i s t o r a r rangement . A l t e r n a t i v e l y , a b r i d g e c i rcu i t a r rangement m a y be u s e d (Russe l l a n d W a tanabe, 1980). N o t e that the f requency response tests s h o u l d be d o n e w i t h the sensor p r e - amp l i f i e r a n d s h i e l d in s t a l l ed as these w i l l in f luence the self-resonance of the c o i l . T i m e d o m a i n p e r f o r m a n c e w a s tes ted w i t h a s p a r k d i s c h a r g e source . T h e s p a r k d i s cha rge is c rea ted w i t h a p i ezoe lec t r i c d r i v e n gas- l ighter . W i t h each s p a r k a la rge current f l ows for a v e r y shor t d u r a t i o n , thus genera t ing a n i m p u l s i v e f i e ld . Tests w i t h a 100 M S / s d i g i t a l osc i l loscope a n d a h i g h qua l i t y lOx probe s h o w e d that the current f l ows for less than 10 ns; a near perfect i m p u l s e source. F i g u r e 2.4 (see C h a p t e r 2) s h o w s the t ime d o m a i n response a n d F F T of U B C TV, w h i c h is de s igned to have a 1.5 k H z to 3 M H z pass-band. B.5 Long Wire Antenna: Theory of Operation T h e l o n g w i r e antenna can be treated as a s u m m i n g charge a m p l i f i e r w i t h each segment of w i r e ac t ing as a loca l capaci t ive p i c k - u p . R . D . R u s s e l l has f o r m a l i z e d this a p p r o a c h b y t r ea t ing the w i r e as a c o n t i n u o u s c o n d u i t of cu r ren t c a p a c i t i v e l y c o u p l e d f r o m the g r o u n d (F igure B.4). T h e cur ren t f l o w i n g t h r o u g h the feedback capac i to r cancels the cur ren t that ends at the s u m m i n g junc t i on d u e to the w i r e . . _ dV2 „dV0 i = -C2—± = C1—± (B.8) dt dt T h e current f l o w i n g in to Cx is d u e to the capac i t ive c o u p l i n g of the po ten t i a l difference u n d e r the w i r e , w h e r e each segment of w i r e dx contr ibutes Appendix B: Instrumentation Details 159 di = C'(x)dx—(V(x)-V0) (B.9) ikiilh -I F i g u r e B.4 Equ iva l en t c i rcui t of the l o n g w i r e a n d charge ampl i f i e r . T h e capaci tance per un i t l eng th is C, a n d to s i m p l i f y the p r o b l e m I w i l l c o n s i d e r i t constant , w h i c h is a fa i r ly g o o d a p p r o x i m a t i o n i n n o r m a l use. In tegra t ing B.9 over the l eng th of the w i r e g ive the total current .dV(jc) Jo dt dx-C'l „dV0 dt (B.10) E q u a t i n g B.8 a n d B.10 a n d in tegra t ing w i t h respect to t ime (x a n d t are i ndependen t of o rde r of in tegrat ion) gives (C0 + CW)V0 =9*.? I Jo V(x)dx ( B . l l ) The total capacitance of the w i r e isCw =C'l. N o t e that r i gh t -hand s ide of B . l l is the first m o m e n t of potent ia l u n d e r the wi re . If w e denote this average po ten t ia l (w i th respect to g round) as V then equat ion B . l l becomes C„ c +c ^ ^o V ( B . l 2) Appendix B: Instrumentation Details 160 Subs t i tu t ing B.12 in to B.8 expresses the ou tpu t of the charge amp l i f i e r for a g i v e n pat tern of po ten t i a l unde rnea th the sens ing w i r e V 2 = ^ ^ ' ^ , V V=ifV(x)<fc (B.13) / Jo -C C — — 1 C2(Cj + C^ ; ) E q u a t i o n B.13 is n o r m a l l y s i m p l i f i e d b y the use o f a large v a l u e (about 10 n F ) for the series i n p u t capaci tance C1. In a d d i t i o n , i f the po ten t i a l g rad ien t is r e l a t i ve ly constant u n d e r the w i r e then V can s i m p l i f i e d a n d B.13 can be expressed i n terms of average f i e ld s t rength. B.6 Noise Characteristics of the Parallel Plate Dipole T h e h i g h i m p e d a n c e a n d r e l a t i ve ly s m a l l effective he igh t of the an tenna m a y cause s ignal- to-noise p rob lems . T h e Johnson noise is h i g h at the l o w frequency end of the of the spec t rum, a n d it is often c o m p o u n d e d b y the current no ise of the ampl i f i e r . F i g u r e B.5 s h o w s the equiva len t c i rcu i t of the pa ra l l e l plate d i p o l e a n d the noise sources. c = c 8+q F i g u r e B.5 Pa ra l l e l d i p o l e antenna c i rcu i t a n d in t r in s i c noise sources. T h e J o h n s o n no i se is m o d e l e d as a cur ren t source to a i d the ana lys i s (rather t h a n it 's u s u a l vol tage source representation). Spectra l dens i ty of the noise (e, i n V / VHz) is e? = e2a+(Z + il)Z2 (B.14) where Z is the impedance of the pa ra l l e l c o m b i n a t i o n of R a n d C . Appendix B: Instrumentation Details 161 R2 l + 0)2R2C2 where C = C: + C„ (B.15) The to ta l vo l tage noise is f o u n d b y in tegra t ing B . l 4 over the pass -band , fj to f 2 • + i2\df ( B . l 6) VN j f e ( at e.Q2 ^ + k[l + (02R2C2J[ R w h e r e CO — Ini. E q u a t i o n B . l 6 can be s i m p l i f i e d to y;=<(s2 - f,)+(4*77?+i i aR 2) f 0 tan ^ - t a n (B.17) w h e r e f 0 = (2KRC) 1 , the h igh-pass corner frequency. Severa l observa t ions can be m a d e f r o m B . l 7 . F i r s t l y , better noise per formance is ob ta ined i f the h igh-pass is as set as l o w as poss ib le . N o t e that the a m p l i f i e r cur ren t noise ia m u s t be v e r y l o w for th is d e s i g n to w o r k w e l l . Subs t i t u t i ng the v a r i o u s parameters of m y sys t em in to equat ions 2.9 (see C h a p t e r 2) a n d B.17 g ives the h igh-pass f requency a n d R M S noise referred to the i npu t . R = 100 M Q C a = 1 2 p F , C a = 8 p F e a = 8 n V V H z , ia = 10 f A ^ H z A f = 1 k H z to 5 M H z f o = 80 H z V n = 1 8 ^ V R M S T h e no i se l e v e l of th is an tenna compare s f a v o r a b l y w i t h o ther sensor c o n f i g u r a t i o n s , w h i c h h a v e l o w e r i n t r i n s i c source impedances . T h i s c a n be a t t r i b u t e d m o s t l y to the p e r f o r m a n c e of the h i g h b a n d w i d t h p r e - a m p l i f i e r . M o s t h i g h - s p e e d d e v i c e s are d e s i g n e d w i t h l o w impedences i n m i n d , a n d t y p i c a l l y exhib i t a large a m o u n t of cur rent noise . Therefore , h i g h speed F E T dev ices are n o r m a l l y u s e d for th is t ype of an tenna (Casey a n d Bansa l , 1991, a n d M a t s u i , 1991) because of thei r l o w current noise . Appendix B: Instrumentation Details B.7 T h e Effects of R e d u c i n g S i g n a l B a n d w i d t h 162 F r o m Parseva l ' s t h e o r e m w e have the equ iva l ence of energy i n the f requency d o m a i n a n d t ime d o m a i n where f(t) is the t ime d o m a i n desc r ip t i on of a pu lse , a n d F(co) is the F o u r i e r t r ans fo rm of f(t). The in tegra ls i n B . l 8 m a y be a p p r o x i m a t e d b y in t eg ra t ing ove r the d u r a t i o n of the p u l s e a n d o v e r the b a n d w i d t h , as these are the t imes a n d f r equenc ies w i t h s igni f icant a m o u n t s of energy. B o t h f(t) a n d F(co) c an be r ep laced w i t h m e a n or R M S a m p l i t u d e s / „ a n d F 0 , ove r the pu l se d u r a t i o n a n d b a n d w i d t h respec t ive ly , to g i v e an equ iva len t express ion of Parseval ' s theorem for a s i m p l e pu l se T h e r e l a t i o n s h i p b e t w e e n the d u r a t i o n of a p u l s e A t a n d i t ' s b a n d w i d t h A f i s (Bracewel l , 1965) A t A f > — or AtA<y = l ( B 2 0 ; An Subs t i tu t ion of equa t ion B.20 in to B . l 9 g ives E q u a t i o n B.21 s h o w s that the he igh t of a s i m p l e p u l s e i s e q u a l to s p e c t r a l d e n s i t y m u l t i p l i e d b y the b a n d w i d t h . F o r s i m p l e , b r o a d - b a n d pu lses F 0 is r e l a t i v e l y cons tan t over the frequencies s p a n n e d b y A f . Therefore, l ow-pass f i l t e r ing a pu l se (to Af j ) b e l o w it's na tu r a l b a n d w i d t h ( A f 0 ) l ower s the a m p l i t u d e b y ( B . l 8) / 2 A t = F 2 A < y (B.19) f0 =F0A(O (B.21) fi = Af fo where Af, < Af Af, o (B.22) o W h i t e o r G a u s s i a n noise behaves d i f ferent ly , desp i te h a v i n g a s i m i l a r p o w e r s p e c t r u m Appendix B: Instrumentation Details 163 (Ott, 1988). If g0 is the R M S va lue of the noise then the n e w R M S v a l u e of no ise is C o m p a r i s o n of B.22 a n d B.23 s h o w s that the s igna l - to -no ise ra t io / / g is l o w e r e d b y r e d u c i n g b a n d w i d t h . In s u m m a r y the b a n d w i d t h of a pu l se s h o u l d p rese rved i f poss ib le , or o the rwise m a x i m i z e d , w h e n the d o m i n a n t noise source has a w h i t e p o w e r spec t rum. B.8 A M Demodulation in Operational Amplifiers S u t u a n d W h a l e n (1983, 1985), a n d C h e n a n d W h a l e n (1981) desc r ibe th is p r o b l e m a t i c interference a n d p r o v i d e S P I C E c i rcu i t m o d e l s to emula te the effects, bu t d o not e x p l a i n w h y i t h a p p e n s . F r o m m y tes t ing it appears to be m o s t l y r e l a t ed to the s lew-ra te per formance of the ampl i f i e r . T h e ou tpu t stage of most ampl i f i e r s are class C i n de s ign , a n d con t a in a P N P a n d N P N transis tor to pu t out the negat ive a n d p o s i t i v e po r t i ons of the o u t g o i n g w a v e f o r m r e s p e c t i v e l y . N P N t rans i s to r s are faster t h a n t h e i r P N P coun te rpa r t s . T h i s is u s u a l l y ref lec ted i n the p e r f o r m a n c e of the p o s i t i v e s lew-ra te verses the nega t ive : the nega t ive is nea r ly a l w a y s s l o w e r . Therefore , a fast c h a n g i n g s i g n a l s u c h as an A M rad io s i gna l tends to p u s h the negat ive ha l f cyc le e lectronics to it 's l i m i t b e f o r e the p o s i t i v e . T h i s a s y m m e t r y causes r e c t i f i c a t i o n , h e n c e , A M d e m o d u l a t i o n . A n ea r l y d e s i g n of the T - b o x p r e - a m p l i f i e r p r o v e d to be s u s c e p t i b l e to A M r a d i o t r ansmis s ions . Pa s s ive f i l t e r i ng w a s i n t r o d u c e d to the f ron t -end of the a m p l i f i e r to compensa t e for this b e h a v i o r . T h e pas s ive f i l ter is a s i m p l e R C l o w - p a s s w i t h the res is tor i n series w i t h the i n p u t . A t the sugges t i on of R . D . R u s s e l l , a n L C l o w - p a s s a r rangement w a s a v o i d e d because i n d u c t o r s w i l l often i n t r o d u c e m o r e no i se t han they e l imina te (v ia i n d u c t i v e c o u p l i n g ) . T h e resistance was selected to be f a i r l y large , about 500 kI2, so that the frequency response of the a m p l i f i e r w o u l d be r e l a t i ve ly i n d e p e n d e n t of the source impedance . (B.23) Appendix B: Instrumentation Details 164 A side-effect of the f i l ter is the extra Johnson noise i n t r o d u c e d b y this resis tor . T h i s no i se c o n t r i b u t i o n can be neg lec ted i n m a n y s i tua t ions , desp i t e a d d i n g a n extra 90 n V / V H z . T o p u t this i n perspec t ive , c o n s i d e r a 3 mete r d i p o l e w i t h a T - b o x pre -ampl i f i e r . O v e r a 50 k H z (effective) b a n d - w i d t h the noise amoun t s to a 160 p V peak- to-peak s igna l , a n d is capable of m a s k i n g a 50 p V / m s igna l . S ince the expected a m p l i t u d e s o f R P E s igna l s are i n the range of 1 to 10 m V / m th is a m o u n t o f no i se i s no t too bothersome. H o w e v e r , it can obscure s m a l l s ignals a n d de t a i l o n the larger ones. 100 0 500 1000 1500 2000 2500 Input Voltage (mV) F i g u r e B .6 R F I d e m o d u l a t i o n effects i n four c o m m o n O p - A m p s . T h e amp l i f i e r s w e r e tested i n an n o n - i n v e r t i n g c o n f i g u r a t i o n (ga in of 6) w i t h a test f requency of 800 k H z . N o t e the d e m o d u l a t e d vo l t age is the refered to i n p u t vol tage (i.e. adjusted for gain). T o a v o i d A M d e m o d u l a t i o n a n a m p l i f i e r s h o u l d be c h o s e n for h i g h s l e w - r a t e per formance , or a f u l l p o w e r b a n d w i d t h of a p p r o x i m a t e l y 1 M H z or better (the t w o are essent ia l ly the same specification). E x a m p l e s of g o o d a n d b a d ampl i f i e r s can be f o u n d i n f igure B.6 where the LF411 (barely) a n d O P - 4 2 have sui table per formance , but the OP-27 , Appendix B: Instrumentation Details 165 LF-441 d o not. B o t h the O P - 2 7 a n d the LF-441 were u s e d i n o u r i n i t i a l des igns of the T-b o x pre -ampl i f ie r s , bu t w e h a d lea rned our lesson by the t ime these ampl i f i e r s were u s e d i n surface w o r k for R P E . N o w w e use the O P - 4 2 part a n d pass ive R C filters to guarantee pe r fo rmance . B.9 D e m o d u l a t o r s a n d Spec t r a l D e c o m p o s i t i o n T h e a c q u i s i t i o n b a n d w i d t h o f mos t of m y f i e ld t r ials have been l i m i t e d b y the speed of the ana log- to d i g i t a l c o n v e r s i o n speed , a n d the n u m b e r of r e c o r d e d channe l s . A t it 's h ighest rate the R C E d i g i t i z e r is capable of s a m p l i n g one channe l at 500 k H z b a n d w i d t h (see sec t ion 2.9 for d i g i t i z e r details); a n o rde r of m a g n i t u d e b e l o w the u p p e r e n d reached b y Sobolev ' s g r o u p . O n e channe l is not g o o d r e d u n d a n c y . A l s o , I f o u n d that I needed eight channels to u n d e r s t a n d (and feel conf ident about) w h a t w a s g o i n g on . Therefore , eight channels were u s e d i n general , g i v i n g a usable b a n d w i d t h of 62 k H z . Sobo lev et a l . have l o n g m a i n t a i n e d that there is a lo t of s i g n a l b e y o n d 100 k H z , a n d that m u c h of this energy is con t a ined w i t h i n spec t ra l peaks (Sobolev et a l . , 1986) i n the H F f requency range ( 500 k H z to 5 M H z ) . In fact, D e m i n a n d M a y b u k w e r e u s i n g sensors w i t h a 100 k H z to 5 M H z pass-band (personal c o m m u n i c a t i o n , 1992). S o m e R P E s igna ls w i l l be o s c i l l a t o r y because of the h i g h Q of the spec t r a l peaks . T h e p u r p o s e o f the d e m o d u l a t o r is to extract the a m p l i t u d e enve lope of these osc i l l a to ry s igna ls so that the presence of these s ignals w i l l be k n o w n . B r o a d - b a n d pulses w i l l be pas sed w i t h m u c h r e d u c e d a m p l i t u d e s , l i k e the d i rec t m e a s u r e m e n t s , bu t o s c i l l a t o r y s i g n a l s f r o m the d e m o d u l a t o r w i l l be a n o m a l o u s l y l a r g e because o f the d o w n w a r d f r e q u e n c y t r a n s f o r m a t i o n . T h e i m p l e m e n t a t i o n of the d e m o d u l a t o r has v a r i e d , but a l l m y des igns (see a p p e n d i x ) are e s sen t i a l ly f u l l - w a v e rect i f iers . T h i s t ype of i m p l e m e n t a t i o n c a n be d o m e w i t h pass ive d iodes o n l y , bu t because of f o r w a r d c o n d u c t i o n p r o b l e m s w i t h s m a l l s igna ls i n d i o d e s (i.e. a pass th resho ld) I u s e d an O p - A m p a n d a nega t ive feedback (or a trans-Appendix B: Instrumentation Details 166 conductance) ar rangement i n later designs. B ia sed d i o d e c i rcui ts w i l l w o r k w e l l f r o m H z to G H z ; the feedback ar rangement is l inear u n t i l severa l M e g a h e r t z , a n d suffices for m y purposes . S p e c t r a l d e c o m p o s i t i o n is a p a r t i a l s o l u t i o n to the p r o b l e m of f i n d i n g the spec t r a l charac ter i s t ics of R P E s igna l s w i t h o u t h a v i n g to pu rchase a v e r y e x p e n s i v e d i g i t i z e r . T h i s s o l u t i o n comes at the expense of u s i n g a large n u m b e r of channels . T h e m e t h o d is based u p o n a n u m b e r of bandpass f i l ters , w i t h each fi l ter f o l l o w e d b y a d e m o d u l a t o r . E a c h b a n d p a s s f i l t e r / d e m o d u l a t o r c o m b i n a t i o n is g i v e n a c h a n n e l . E i g h t c h a n n e l s a l l o w s u p to eight reg ions w i t h i n the s p e c t r u m to be m o n i t o r e d . I b u i l t s u c h a sy s t em w i t h seven pass bands 20-50 k H z , 50-100 k H z , 100-200 k H z , 200-500 k H z , 0.5-1 M H z , 1-2 M H z a n d 2-5 M H z . T h e sy s t em was u s e d i n o n l y one exper imen t , near the C e n t u r y depos i t i n A u s t r a l i a (see C h a p t e r 4 for expe r imen t de ta i l ) . U n f o r t u n a t e l y , this w a s a p o o r e x p e r i m e n t to d e m o n s t r a t e the c a p a b i l i t i e s of the t e c h n i q u e as w e c o u l d not c o n v i n c i n g l y p r o d u c e a n y R P E signals . A d e m o d u l a t o r a l l o w s the m o n i t o r i n g o f a c t i v i t y i n f r e q u e n c y b a n d s n o r m a l l y inaccess ib le to the d i g i t i z i n g sys tem, a n d yet re ta in a reasonable n u m b e r o f channe ls to w o r k w i t h . I f o u n d the d e m o d u l a t o r to be p a r t i c u l a r l y use fu l w i t h the R C E d i g i t i z e r , a n d I feel that the spectra l d e c o m p o s i t i o n m e t h o d can be ve ry usefu l i f a spare l o w speed d i g i t i z e r (such as the R C E board) is avai lable . A P P E N D I X C E L E C T R I C A L C I R C U I T S C H E M A T I C S 167 Appendix C: Electrical Circuit Schemattics Transmitter Circuit 39K-Sig In 1K ' +5V .10K 1/2 LM393 2N2222 47 LL 2 -a 3 HFBR-1522 Power Supply may be up to 7 V. Suggested source 6V lantern battery 4.7 uF and 10 nF Decoupling capacitors used. Receiver Circuit +5V 47 nF Bypass capacitor used Sig Out More than 250 mV is needed to give a logic low, and anything less gives a logic high ouput. The design with 60 m of cable will work up to 400 kHz, and with 1:3 (low:high) duty cycle to 1.4 Mhz. Glitches as short as 200 nS are captured. Falling edge is slower than rising edge. Geophysical Instrumentation Group Title : Fiber Optic Trigger Relay Drawn By : AK 13 Dec 1991 Ver 1.0 F i g u r e C l F iber opt ic c i rcui t schematics. Appendix C: Electrical Circuit Schemattics > E X > CD i t m = oc to cc ca ^ o >« m O z Q ro c CD o o •o c o CD o ro o CO < a. O A A / W V VWVSA-T3 CO O O CD C O CM cn i re CD 13 6 8 Q . O O C L O CD C9 ~ 10 I 2 « < > o g " = « r T J O CD c Q-- £ o re T J C CM re CD : CM T J _ X I re co c 2 w CD ro o re 5 L U •5 -S » CO co > c r E * - CD o m "> 2 ~ .—* m _ CO v S re co . re TO CD CD O C L CM co Z E o O XT CM — — E CD 2 ro _ '« '8 CO o CO t CO 3 CO _ >% ~ •° o T J E co ro ~° S" CD • i : CD < o CO c D ) 0 ro co 2 CD O >» E i5 | — co co O CO > » CO " ° 1 -g u ro CD E £.<2 CD sa o J3 E T J O CD O s -0 — 1 o T J 3 £ ° -CD 5 o E .<2 CD CO CO CD CO CO x: >. 5 -° T J CD CD J i • " . C M -to o CO o T J ° T J .E, C T J CO CO ^ CD CO J= F i g u r e C.2 U B C V magnet ic sensor c i rcu i t schemat ic . Appendix C: Electrical Circuit Schematics •° o TJ f - A A A A A , — 1 | 81 * « 8 "S.=i= ^ S ™^ to g> CM £ T : rj CD < C L > C L S .£ O — Q) •11 TJ 3 ra ,-X I C CD (0 £ S .»> £ ra o> ^ CO (0 Q D_ > X TJ 0) co 3 — C L cz CD -r-5= <B t -CO M T J C O ? O CO ~ TJ CO CD cn "5 CD - ° £ .« <" « CD ™ °> ~ CO OJ CO o, C L CD CD — 2 >. S3 S CO - 1 °-d o — — o X . £ s °> <  a . CO CO co C L x: E x: CO _o 3 M S- C L => CO o o CD U -x: C L — OJ co .2 <D O •— <D CU Q-"> g TJ CO CO o o s ; 1 1 ! S-o: CJ =J o C L -<= O „ OT OT "OT CO • cj CC O CD CO > . CO CO I . X ) OJ ' , „ CM !2 <o CO -<= Q O < " TJ J Z d o to ra -m CN • . J <o . 2 < ° Q_ So 8 o CD E cc F E "S O -3> m = to ro , O ® o ^ e E i = TJ X to ^ LU 5 u. . CD £ 5 * O T J £ CD 5 to < CO* C N .2 | ^ . » £ o •* o 2 a § ra CD CO ra C L ^ CO u_ CO g x: c o °- = 2 co to 2 £ — » - . _ CO « u to a CD X- C L TJ E CD CO TJ E o B sz O L L CD x: F i g u r e C .3 H B W pre -ampl i f i e r c i r cu i t schemat ic . Appendix C: Electrical Circuit Schemattics Passive Demodulator (7) + 0.485 V Ideally the voltages at 1 and 2 should be 0.6 V. The original circuit came from Horowitz and Hill and used 1K resistors on the bias arrangement. So the threshold for any signal is about 110 mV before (2) - 0.485 V any output will occur. This neccessitates a high bandwidth preamp to be put before the input from the sensor, usually a AM502. This circuit consists of two biased rectifiers to form an absolute value output from the input. The outputs are to be connected to a differential amplifier or buffer. The signal must have an amplitude greater than 110 mV to give output. Geophysical Instrumentation Group Title : Demodulator Drawn By : AK APR 11 1991 Ver : 0.1 F i g u r e C.4 D e m o d u l a t o r c i r cu i t schematic . Appendix C: Electrical Circuit Schemattics Four Channel 60 Hz Notch Filter + E Pin 1 XLR Input Set R1=180 K R2=15.0 K for 60 Hz Pin 5 Four notch filter sections One for each channel Bypass Switch Pin 1 XLR Output Pin 5 - E + V Pin 2 . GND Pin3 . - v P in4 Supply Pins Shared on Inputs and Outputs 1uF 4= lOOuhT -4~ 1 l l F 1uF X 100uH X 1uF Pin 2 + v Pin3 GND Pin4 - V Note that the filters have a 200 K series resistance. The filter box should not be directly connected to the RC electronics card since that has only a 20 K input impedance. So connect this to the TEK AM 502 or a high input impedance device. Also the output and input supply lines are decoupled by the above set of LC low pass filters. Geophysical Instrumentation Group Title : 60 Hz Notch Filter Drawn By : AK MAR 20 1991 Ver : 0.0 F i g u r e C.5 N o t c h filter c i rcu i t schematic . Appendix C: Electrical Circuit Schemattics 173 Replacement PreAmplifier for LWA and Dipole Antenna BANANA JACK INPUT P 499K -AA/VvV 100N 1M BANANA JACK INPUT N 499K 100N 1 M XLR CONNECTOR SIGNAL PIN r E1 — — GND E2 r v 2 1 3 5 4 The choice of the 499K resistor is based on requirement that the lo-pass formed with the 10 pF capacitor have a corner at approx 30 kHz and that this corner change little with various source impedences. If the noise is a problem then replace the resistor and capacitor with 47K and 100pF respectively. Nominal Gain is 30 HP Corner for Dipole configuration is 2 Hz and for Long Wire Antenna is 160 Hz LP Corner for both configurations is 33 kHz with 12 dB/Oct rolloff until 1 Mhz Power Supply rails are decoupled by a 100 nH inductor and a 10 uF tantalum capacitor Geophysical Instrumentation Group Title : The Best Preamplifier Drawn By : AK Ver : 1.3 Jan 1992 F i g u r e C . 6 T - B o x p re -ampl i f i e r c i rcu i t schemat ic : 

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