UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

A marine deep seismic sounding survey in the region of Explorer Ridge 1976

You don't seem to have a PDF reader installed, try download the pdf

Item Metadata

Download

Media
UBC_1976_A6_7 M34.pdf [ 7.36MB ]
Metadata
JSON: 1.0052376.json
JSON-LD: 1.0052376+ld.json
RDF/XML (Pretty): 1.0052376.xml
RDF/JSON: 1.0052376+rdf.json
Turtle: 1.0052376+rdf-turtle.txt
N-Triples: 1.0052376+rdf-ntriples.txt
Citation
1.0052376.ris

Full Text

A MARINE DEEP SEISMIC SOUNDING SURVEY IN THE REGION OF EXPLORER RIDGE by STEVEN JEROME MALECEK B.A., S o u t h w e s t M i n n e s o t a S t a t e C o l l e g e , 1973 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n t h e D e p a r t m e n t o f GEOPHYSICS AND ASTRONOMY We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA J a n u a r y , 19 76 In p re sent ing t h i s t he s i s in p a r t i a l f u l f i l m e n t o f the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I f u r t h e r agree tha t permiss ion fo r ex tens i ve copying of t h i s t he s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r ep re sen ta t i ve s . It i s understood that copying or p u b l i c a t i o n of t h i s t he s i s f o r f i n a n c i a l ga in s h a l l not be a l lowed without my w r i t t e n permi s s ion . Department of Ci € O ^ K ^ \CS> L A~S f^O 0 OYt\y The U n i v e r s i t y of B r i t i s h Columbia 20 75 Wesbrook P l a c e Vancouver, Canada V6T 1W5 Date YQ WlkCK r / A 79 7(o ABSTRACT During July 1974, two reversed deep seismic sounding (DSS) p r o f i l e s extending about 75 km were recorded i n the Explorer Ridge region of the northeastern P a c i f i c , one p a r a l l e l and the other perpendicular to the ridge. A two-ship operation was used to record near- v e r t i c a l incidence to wide-angle r e f l e c t e d waves and refracted waves with penetration from the ocean bottom to the upper mantle. Signals from s i x i n d i v i d u a l hydrophones suspended at 45 m depth from a 600 m cable t r a i l e d behind the receiving ship were recorded in d i g i t a l form. The shooting ship detonated charges ranging from 2.3 kg to 280 kg and recorded the d i r e c t a r r i v a l plus the WWVB time code. Processing of the data recorded at distances beyond 4 km included demultiplexing, stacking, and f i l t e r i n g . Before the data were presented i n record section form, traveltime corrections were made for topography and shot distance, and amplitude corrections were made for amplifier gain, charge s i z e , and spherical spreading. The i n t e r p r e t a t i o n procedure consisted of two steps. A homogeneous, layered velocity-depth model was i n i t i a l l y constructed from f i r s t a r r i v a l traveltime data. The p-A curve corresponding to this model was then altered u n t i l an amplitude f i t was obtained using synthetic i i seismograms. Weichert-Herglotz integration of the resultant p-A curve produced the f i n a l velocity-depth model. This traveltime and amplitude i n t e r p r e t a t i o n required the introduction of v e l o c i t y gradients into the model. The p r o f i l e run across the ridge showed no anomalous behaviour as the ridge was crossed; the p r o f i l e on the Juan de Fuca plate, p a r a l l e l i n g the ridge, exhibited traveltime branch o f f s e t s and delays. These have been interpreted as due to f a u l t i n g with a. v e r t i c a l component of o f f s e t of about 5 km. The reversed upper mantle v e l o c i t i e s are 7.8 and 7.3 km/s in directions perpendicular and p a r a l l e l to the ridge. Anisotropy i s proposed to explain these d i f f e r e n t v e l o c i t i e s . Compared with c r u s t a l sections from other ridge areas, the data require a thick "layer 3" (up to 7 km) near the ridge crest. The t o t a l depth to the base of the oceanic crust varies between 10 and 12 km except in the faulted region. The results of this study favor the hypothesis that Explorer Ridge i s presently an inactive spreading center. i i i TABLE OF CONTENTS Page ABSTRACT i TABLE OF CONTENTS i i i LIST OF TABLES iv LIST OF FIGURES v ACKNOWLEDGEMENTS vii 1. INTRODUCTION 1.1 Tectonic significance of Explorer Ridge 1 1.2 Previous studies of Explorer Ridge 5 1.3 Purpose and scope of the project 5 2. DATA ACQUISITION 2.1 The two-ship operation 7 2.2 The shooting ship 8 2.3 The receiving ship 18 3. DATA PROCESSING 3.1 Field tapes 21 3.2 Shot-receiver distances 22 3.3 Traveltime corrections 23 3.4 Amplitude corrections 25 3.5 Seismic record sections 28 4. INTERPRETATION 4.1 Review of methods 40 4.2 Traveltime interpretation 41 4.3 Traveltime and amplitude interpretation 52 5. DISCUSSION OF RESULTS 5.1 Determining a petrologic model 81 5.2 Oceanic crustal structure in the 84 region of Explorer Ridge CONCLUSION 9 7 REFERENCES 101 APPENDIX 105 i v L I S T OF TABLES T a b l e , Page I Summary o f l e a s t s q u a r e s a n a l y s i s o f t r a v e l t i m e d a t a . 51 I I S e i s m i c and p e t r o l o g i c models o f t h e o c e a n i c c r u s t . 82 i L I S T OF FIGURES F i g u r e 1.1 L o c a t i o n o f deep s e i s m i c s o u n d i n g p r o f i l e s n e a r E x p l o r e r Ridge. 2.1 T w o - s h i p deep s e i s m i c s o u n d i n g system. 2. 2A The s h o o t i n g and r e c e i v i n g s h i p s . 2.2B S h o o t i n g p r o c e d u r e s w i t h g e l a t i n e x p l o s i v e s . 2.2C S e t t i n g o u t t h e r a f t u s e d f o r t h e l a r g e c h a r g e s ( p r o j e c t i l e s ) . 2.2D P r o j e c t i l e s b e i n g p l a c e d o v e r b o a r d . 2.3 B l o c k d i a g r a m o f s h o o t i n g s h i p i n s t r u m e n t a t i o n . 2.4 B l o c k d i a g r a m o f r e c e i v i n g s h i p i n s t r u m e n t a t i o n . 3.1 R e c o r d s e c t i o n o f 6 - c h a n n e l d a t a f r o m p r o f i l e 74-2. 3.2 R e c o r d s e c t i o n o f d a t a f r o m p r o f i l e 74-2 a f t e r s t a c k i n g . 3.3 P r o c e s s e d r e c o r d s e c t i o n f o r p r o f i l e 74-1. 3.4 P r o c e s s e d r e c o r d s e c t i o n f o r p r o f i l e 74-1R. 3.5 P r o c e s s e d r e c o r d s e c t i o n f o r p r o f i l e 74-2. 3.6 P r o c e s s e d r e c o r d s e c t i o n f o r p r o f i l e 74-2R. 4.1 Examples o f s e i s m o g r a m s u s e d f o r t r a v e l t i m e p i c k s . 4.2 L e a s t s q u a r e s f i t t e d t r a v e l t i m e p l o t s f o r p r o f i l e s 74-1 and 74-1R. 4.3 L e a s t s q u a r e s f i t t e d t r a v e l t i m e p l o t s f o r p r o f i l e s 74-2 and 74-2R. VI F i g u r e Page 4.4 V e l o c i t y - d e p t h c u r v e s d e t e r m i n e d f r o m l e a s t s q u a r e s a n a l y s i s . 53 4.5 P-A c u r v e and s y n t h e t i c s e i s m o g r a m s w i t h s u p e r i m p o s e d t r a v e l t i m e c u r v e f o r homogeneous l a y e r e d v e l o c i t y - d e p t h m o d e l . 58 4.6 P r o c e d u r e u s e d i n a d j u s t i n g t h e p-A c u r v e f o r an a m p l i t u d e f i t . 63 4.7 The p-A c u r v e f o r a homogeneous l a y e r e d m odel and t h e m o d i f i e d v e r s i o n d e t e r m i n e d f o r a t r a v e l t i m e and a m p l i t u d e f i t . 64 4.8 D a t a r e c o r d s e c t i o n , s y n t h e t i c s e i s m o g r a m s and t r a v e l t i m e c u r v e f o r p r o f i l e 74-2. 66 4.9 Same as F i g . 4.8 f o r p r o f i l e 74-2R. 71 4.10 Same as F i g . 4.8 f o r u p - f a u l t r e g i o n s o f p r o f i l e s 74-1 and 74-1R. 73 4.11 V e l o c i t y - d e p t h c u r v e s d e t e r m i n e d f r o m t h e t r a v e l t i m e and a m p l i t u d e f i t . 76 4.12 R e c o r d s e c t i o n f o r p r o f i l e 74-2 s h o w i n g t r a v e l t i m e f i t f o r m u l t i p l e s . 79 v i i ACKNOWLEDGMENTS I w o u l d l i k e t o f i r s t o f a l l e x p r e s s my a p p r e c i a t i o n to: Dr. R.M. Clowes f o r h i s guidance and encouraging support throughout the p r o j e c t . I am a l s o g r a t e f u l t o Dr. R.A. W i g g i n s f o r t h e u s e o f h i s c o m p u t e r r o u t i n e (HRGLTZ) and h i s h e l p f u l s u g g e s t i o n s r e g a r d i n g t h e i n t e r p r e t a t i o n p r o c e d u r e . Due t o t h e n a t u r e o f t h e p r o j e c t c o n s i d e r a b l e a s s i s t a n c e was r e q u i r e d d u r i n g t h e f i e l d o p e r a t i o n s . The t i m e and a s s i s t a n c e g i v e n by R. C l a y t o n and E. W a d d i n g t o n on t h e s h o o t i n g s h i p and J . D a v i e s and B. N a r o d on t h e r e c e i v i n g s h i p i s g r e a t l y a p p r e c i a t e d . I n a d d i t i o n , I w o u l d l i k e t o t h a n k E. W a d d i n g t o n f o r t h e use o f s l i d e s t a k e n d u r i n g t h e c r u i s e . I am a l s o i n d e b t e d t o C. West and R.D. Meldrum f o r t h e i r g e n e r o u s e f f o r t s c o n c e r n e d w i t h t h e i n s t r u m e n t a t i o n and r e c o r d i n g s y s t e m s f o r t h e p r o j e c t . I am p l e a s e d t o a c k n o w l e d g e t h e use o f t h e s h i p s CFAV ENDEAVOUR and CFAV LAYMORE and t h e c o o p e r a t i o n and a s s i s t a n c e g i v e n by t h e i r o f f i c e r s and c r e w s . The F l e e t D i v i n g U n i t , P a c i f i c M a r i t i m e Command, E s q u i m a l t p r o v i d e d e x p l o s i v e e x p e r t s and t h e i r a s s i s t a n c e i s g r a t e f u l l y a c k n o w l e d g e d . The s e i s m o l o g y d i v i s i o n o f t h e E a r t h P h y s i c s B r a n c h p r o v i d e d t h e a n t i - s u b m a r i n e p r o j e c t i l e s . D u r i n g p a r t o f t h i s work, t h e a u t h o r was s u p p o r t e d by a G r a d u a t e R e s e a r c h F e l l o w s h i p f r o m t h e U n i v e r s i t y o f B r i t i s h Columbia. Fi n a n c i a l support for the project was provided by National Research Council equipment grant E3235 and operating grant A7707. Additional funds were contributed by Mobil O i l Canada Limited and Shell Canada Limited. 1. INTRODUCTION 1.1 T e c t o n i c S i g n i f i c a n c e o f E x p l o r e r R i d g e E x p l o r e r R i d g e i s a b r o a d n o r t h e a s t - t r e n d i n g t o p o g r a p h i c f e a t u r e i n t h e n o r t h e a s t P a c i f i c l o c a t e d b e tween 49° and 51° N o r t h l a t i t u d e and 129° and 131° West l o n g i t u d e . I t i s c o n s i d e r e d p a r t o f a r i d g e - t r a n s f o r m f a u l t s y s t e m w h i c h marks t h e w e s t e r n b o u n d a r y o f t h e J u a n de F u c a p l a t e ( F i g . 1 . 1 ) . A t w a t e r (1970) has c o n c l u d e d t h a t t h e J u a n de F u c a p l a t e i s a remnant o f t h e F a r a l l o n p l a t e ( M c K e n z i e and Morgan, 1969) w h i c h was u n d e r t h r u s t i n g t h e A m e r i c a n p l a t e b u t b r o k e up d u r i n g m i d - T e r t i a r y t i m e . B e c a u s e o f t h e complex, i n t e r a c t i o n among t h e P a c i f i c , J u a n de F u c a , and A m e r i c a n p l a t e s f o l l o w i n g t h e b r e a k u p , t h e p r e s e n t r o l e o f E x p l o r e r R i d g e i n t h e t e c t o n i c a c t i v i t y i s n o t c l e a r l y u n d e r s t o o d . Is i t , i n f a c t , an a c t i v e s p r e a d i n g c e n t e r , o f f s e t by t r a n s f o r m f a u l t s f r o m o t h e r l i k e l y s p r e a d i n g c e n t e r s as t h e i n s e t i n F i g . 1.1 s u g g e s t s ? W h i l e J u a n de F u c a R i d g e has g e n e r a l l y b e e n a c c e p t e d as an a c t i v e s p r e a d i n g c e n t e r on t h e b a s i s o f f l a n k i n g m a g n e t i c l i n e a t i o n s ( V i n e and W i l s o n , 1965; V i n e , 1966) , t h e d i s t r i b u t i o n o f e a r t h q u a k e e p i c e n t e r s ( T o b i n and S y k e s , 1 9 6 8 ) , and numerous g e o p h y s i c a l and g e o l o g i c a l s t u d i e s n e a r t h e r i d g e ( D e h l i n g e r e t a l , 1 9 7 0 ) , no s u c h c o n c l u s i o n c a n be r e a c h e d f o r E x p l o r e r R i d g e . 2 Figure 1.1 Location of deep seismic sounding p r o f i l e s near Explorer Ridge. S o l i d c i r c l e s show d r i f t track of the receiving ship; heavy s o l i d l i n e s show track of shooting ship. Regions of normal magnetic p o l a r i t y i n the area of the survey are s t r i p p l e d . Bathymetric contours i n meters (from T i f f i n and Seeman, 1975). The inset shows the major tectonic features i n the northeastern P a c i f i c and outlines the map area given i n d e t a i l .  4 B e r t r a n d (1972) has p r e s e n t e d e v i d e n c e i n d i c a t i n g t h a t t h e D e l l w o o d K n o l l s l y i n g n o r t h w e s t o f E x p l o r e r R i d g e a r e p a r t o f a p r e s e n t l y a c t i v e s p r e a d i n g segment, He p r o p o s e s '.. t h a t t h e e a s t e r n arm o f E x p l o r e r R i d g e (as d e l i n e a t e d by t h e b i f u r c a t e d c e n t r a l m a g n e t i c anomaly p a t t e r n i n F i g . 1.1) c e a s e d b e i n g a c t i v e when t h e D e l l w o o d segment came i n t o e x i s t e n c e (<1 m.y. a g o ) . The p o s s i b i l i t y t h a t E x p l o r e r R i d g e i s no l o n g e r a c t i v e seems t o t i e i n w e l l w i t h t h e e a r t h q u a k e d a t a f o r J u a n de F u c a p l a t e . The e p i c e n t e r s g i v e n by T o b i n and Sykes (1968) p l u s t h o s e c o m p i l e d by B a r r and Chase (1974) s u g g e s t t h a t t h e Sovanco F r a c t u r e Zone i s p r e s e n t l y n o t an a c t i v e t r a n s f o r m f a u l t . R a t h e r , t h e Queen C h a r l o t t e F a u l t p r e s e n t l y may e x t e n d t o t h e n o r t h e r n t i p o f J u a n de F u c a R i d g e i m p l y i n g t h a t E x p l o r e r R i d g e no l o n g e r f u n c t i o n s as an a c t i v e s p r e a d i n g c e n t e r ( B a r r and C h a s e , 1 9 7 4 ) . Such a change c o u l d be due t o t h e r e o r i e n t a t i o n o f t h e J u a n de F u c a R i d g e w h i c h b e g a n a b o u t 7.5 m.y. ago a c c o r d i n g t o m a g n e t i c anomaly p a t t e r n s ( V i n e , 1966; Menard and A t w a t e r , 1968) . E x p l o r e r R i d g e i s t h e n an o f f s e t segment o f t h e J u a n de F u c a R i d g e (McManus e t a l , 19 72) and t h e D e l l w o o d K n o l l s a r e d e s c e n d a n t s o f t h e E x p l o r e r segment ( B e r t r a n d , 1972) . From t h e i r s t u d y o f o c e a n r i d g e s and f r a c t u r e zones o f f C a l i f o r n i a , M e nard and A t w a t e r (1969) c o n c l u d e d t h a t ". . . an e n t i r e s e c t i o n o f a r i d g e may become i n a c t i v e i f i t i s t o o f a r f r o m t h e g e n e r a l l i n e o f a r i d g e c r e s t . " I f E x p l o r e r R i d g e i s now " t o o f a r " f r o m J u a n de F u c a R i d g e , i t may i n d e e d be i n a c t i v e . A c c o r d i n g t o t h a t r e a s o n i n g t h e 5 D e l l w o o d K n o l l s s h o u l d a l s o be i n a c t i v e . However, B e r t r a n d (1972) a l s o c o n s i d e r s t h e d i f f u s e n a t u r e o f t h e Queen C h a r l o t t e F a u l t Zone as an a l t e r n a t i v e e x p l a n a t i o n f o r t h e a c t i v i t y i n t h e D e l l w o o d a r e a . 1.2 P r e v i o u s S t u d i e s o f E x p l o r e r R i d g e The b a t h y m e t r i c , m a g n e t i c , and e a r t h q u a k e d a t a p r e s e n t l y a v a i l a b l e have u n d o u b t e d l y b e e n v a l u a b l e f o r i n f e r r i n g t h e p o s s i b l e s t a t u s o f E x p l o r e r R i d g e i n t e r m s o f t h e r e g i o n a l t e c t o n i c s . However, few i n v e s t i g a t i o n s have been a t t e m p t e d t o d e t e r m i n e t h e c r u s t a l / u p p e r m a n t l e s t r u c t u r e n e a r t h e r i d g e and t h e n t e s t t h e c o m p a t i b i l i t y o f t h i s s t r u c t u r e w i t h t h e t e c t o n i c h y p o t h e s e s . E x c e p t f o r a l i m i t e d i n t e r p r e t a t i o n o f g r a v i t y d a t a f r o m t h e a r e a ( S t a c e y , 1 9 7 3 ) , and t h e r e s u l t s o f an i n p r o g r e s s d e t a i l e d s t u d y w h i c h i n c l u d e s c o n t i n u o u s s e i s m i c p r o f i l i n g and d r e d g e s a m p l e s (A.G. T h o m l i n s o n , p e r s . comm., 1 9 7 5 ) , l i t t l e i n f o r m a t i o n a b o u t t h e c r u s t a l s t r u c t u r e n e a r E x p l o r e r R i d g e i s known t o e x i s t . 1.3 P u r p o s e and Scope o f t h e P r o j e c t In o r d e r t o o b t a i n d a t a f o r a d e t a i l e d s t u d y o f t h e c r u s t a l s t r u c t u r e n e a r E x p l o r e r R i d g e , a deep s e i s m i c s o u n d i n g (DSS) e x p e r i m e n t was c a r r i e d o u t d u r i n g J u l y , 19 74. U s i n g a t w o - s h i p o p e r a t i o n , two r e v e r s e d p r o f i l e s a b o u t 75 km l o n g were r e c o r d e d - one p a r a l l e l (on t h e J u a n de F u c a p l a t e ) , a n d t h e o t h e r p e r p e n d i c u l a r , t o t h e r i d g e . A l t h o u g h s e i s m i c a r r i v a l s 6 ranging from n e a r - v e r t i c a l i n c i d e n c e r e f l e c t i o n s to wide-angle r e f l e c t i o n s and r e f r a c t i o n s were r e c o r d e d , o n l y data o b t a i n e d beyond the c r i t i c a l d i s t a n c e f o r the f i r s t c r u s t a l l a y e r (about 4 km) were completely processed and i n t e r p r e t e d . With these data, the purpose o f t h i s t h e s i s p r o j e c t became t w o f o l d : (1) to o b t a i n a v e l o c i t y versus depth model f o r each p r o f i l e through an i n t e r p r e t a t i o n based on s e i s m i c ray theory; (2)to c o n s i d e r the g e o l o g i c a l and t e c t o n i c i m p l i c a t i o n s of these models. The major emphasis o f t h i s t h e s i s i s on the f i r s t t o p i c . The i n t e r p r e t a t i o n u t i l i z e s both t r a v e l t i m e and amplitude i n f o r m a t i o n , although a p r e l i m i n a r y i n t e r p r e t a t i o n was made on the b a s i s o f f i r s t a r r i v a l times al o n e . C o n s i d e r a b l e a t t e n t i o n was t h e r e f o r e g i v e n to p r o c e s s i n g the data i n a manner which would f a c i l i t a t e t h i s type o f i n t e r p r e t a t i o n . Although determining what the v e l o c i t y - d e p t h models mean i n terms of the geology and t e c t o n i c s i s probably the most rewarding p a r t o f such a study, i t i s a l s o the most ambiguous. The chapter e n t i t l e d " D i s c u s s i o n o f R e s u l t s " attempts to c o n s t r u c t a g e o l o g i c a l model f o r the c r u s t i n the r e g i o n covered by the p r o f i l e l i n e s . I t was s a t i s f y i n g to note t h a t s e v e r a l c o n s i s t e n c i e s between the g e o l o g i c a l i n t e r p r e t a t i o n and present ideas r e g a r d i n g p l a t e t e c t o n i c s i n the r e g i o n d i d appear. 7 2. DATA ACQUISITION 2.1 The Two-Ship O p e r a t i o n The t w o - s h i p o p e r a t i o n u s e d t o r e c o r d t h e f o u r DSS p r o f i l e s marked on F i g . 1.1 i s s i m i l a r , i n p r i n c i p l e , t o t h e t e c h n i q u e d e s c r i b e d by S h o r ( 1 9 6 3 ) . A s t a n d a r d p r o c e d u r e i n v o l v e s u s i n g one o f t h e s h i p s f o r s h o o t i n g and t h e o t h e r f o r r e c e i v i n g . " D u r i n g t h i s p r o j e c t CFAV LAYMORE s e r v e d as the s h o o t i n g s h i p w h i l e CFAV ENDEAVOUR s e r v e d as t h e r e c e i v i n g s h i p . D u r i n g a p r o f i l e r u n ENDEAVOUR w o u l d d r i f t f r e e l y f r o m t h e s t a r t i n g p o i n t o f t h e p r o f i l e ( i n d i c a t e d by d o t s i n F i g . 1.1) w h i l e t r a i l i n g a ma i n c a b l e w h i c h s u p p o r t e d s i x i n d i v i d u a l h y d r o p h o n e s y s t e m s . M e a n w h i l e LAYMORE w o u l d p r o c e e d a l o n g a p r e d e t e r m i n e d c o u r s e ( s o l i d t r a c k l i n e s i n F i g . 1.1) as c h a r g e s were d e t o n a t e d a t s e l e c t e d d i s t a n c e s . Shot s p a c i n g was l e s s t h a n 0.5 km f o r t h e s m a l l e s t c h a r g e s (2.3 kg) where s u b - c r i t i c a l r e f l e c t i o n s were a n t i c i p a t e d , b e t w e e n 1 and 4 km f o r i n t e r m e d i a t e c h a r g e s (4.5-45.4 k g ) , and a r o u n d 5 km f o r t h e l a r g e s t c h a r g e s (94 and 282 kg) u s e d b e y o n d 45 km. S h i p - t o - s h i p d i s t a n c e s were d e t e r m i n e d by r a d a r o u t t o a b o u t 22 km. Beyond t h a t , L o r a n f i x e s were r e q u i r e d t o d e t e r m i n e l a t i t u d e and l o n g i t u d e f r o m w h i c h d i s t a n c e s c o u l d be c a l c u l a t e d . To r e v e r s e t h e p r o f i l e , ENDEAVOUR w o u l d s t e a m t o t h e f i n a l p o s i t i o n o f LAYMORE where s h o o t i n g w o u l d b e g i n i n t h e 8 opposite d i r e c t i o n . Two such reversed p r o f i l e s , one p a r a l l e l and the other p e r p e n d i c u l a r to E x p l o r e r Ridge were planned and c a r r i e d out. However as F i g . 1.1 shows, p r o f i l e s 74-1 and IR are not reversed w e l l . P r o f i l e 74-1 was terminated at a di s t a n c e of about 48 km due to sea c o n d i t i o n s which prevented safe detonation of l a r g e charges. N a v i g a t i o n of LAYMORE r e l a t i v e to ENDEAVOUR and the l a r g e n o r t h w e s t e r l y d r i f t of the l a t t e r s h i p f o r p r o f i l e 74-1R caused the bow i n the shooting t r a c k . In c o n t r a s t , p r o f i l e s 74-2 and 2R are w e l l reversed. The upper p a r t of F i g 2.1 s c h e m a t i c a l l y shows the o p e r a t i o n a l program. D e t a i l s of the equipment and procedures are given i n the f o l l o w i n g s e c t i o n s . 2.2 The Shooting Ship SHOOTING PROCEDURES Two types of e x p l o s i v e s served as se i s m i c energy sources during a p r o f i l e run. For s h o t - r e c e i v e r distances I l e s s than 45 km, 2.3 kg (5 lb ) c a r t r i d g e s of Geogel , a g e l a t i n e x p l o s i v e , were used to co n s t r u c t charge s i z e s up to 45.4 kg (100 l b ) . At longer d i s t a n c e s , surplus Mark IV H.E. anti-submarine p r o j e c t i l e s were used. These contained 94 kg (207 l b ) of Minol high e x p l o s i v e . Detonating three p r o j e c t i l e s simultaneously provided the maximum charge s i z e I Trademark of CIL explosives. 9 Figure 2.1 Top - The two-ship deep seismic sounding system with ray paths ind icated for the d i r e c t water wave and some r e f l e c t e d and re f rac ted waves. Bottom - A de ta i l ed view of one of the s ix hydrophone systems connected to the main cable.  11 u s e d , 282 kg (621 l b ) . B e c a u s e t h e e x p l o s i v e s u s e d were so d i s s i m i l a r i n t y p e and s i z e , s e p a r a t e s h o o t i n g t e c h n i q u e s h a d t o be e m p l o y e d . S h o o t i n g p r o c e d u r e s u s i n g G e o g e l were r e l a t i v e l y s i m p l e and a r e e s s e n t i a l l y t h e same as t h o s e o u t l i n e d by Sho r (19 6 3 ) . C h a r g e s were p r e p a r e d f o r d e t o n a t i o n w i t h a t i m e d - f u s e / S e i s m o c a p 2 a s s e m b l y and P r i m a c o r d 3 . A f t e r t h e c h a r g e was f a s t e n e d t o a 45 m l i n e w h i c h had one o r more b a l l o o n s t i e d t o t h e end o f i t , t h e .fuse was l i t and t h e c h a r g e was d r o p p e d o v e r b o a r d . A f o u r m i n u t e f u s e a l l o w e d t i m e f o r t h e c h a r g e t o s i n k t o t h e optimum depth 1* (45 m) . I t a l s o gave LAYMORE t i m e t o t r a v e l a s a f e d i s t a n c e a h e a d o f t h e s h o t . A f t e r t h e s h o t h a d d e t o n a t e d , t h e r a n g e and b e a r i n g o f t h e b a l l o o n s were e s t i m a t e d and r e c o r d e d . B e c a u s e t h e w e i g h t o f t h e p r o j e c t i l e s (170 kg i n c l u d i n g c a s i n g ) made them u n w i e l d y , a s p e c i a l p r o c e d u r e was u s e d t o d e t o n a t e them s a f e l y . The t i m e d - f u s e / S e i s m o c a p a s s e m b l y was c o n n e c t e d t o t h e P r i m a c o r d and t h e f u s e i g n i t e d 2 T r a d e m a r k o f C I L e x p l o s i v e s . 3 T r a d e m a r k o f E n s i g n - B i c k f o r d Co., S i m s b u r y , Conn. "* The c r i t e r i a f o r optimum d e p t h i s t h a t , f o r a g i v e n c h a r g e s i z e , t h e d e t o n a t i o n d e p t h s h o u l d e q u a l a q u a r t e r o f the w a v e l e n g t h f o r t h e b u b b l e p u l s e f r e q u e n c y ( S h o r , 1 9 6 3 ) . S i n c e c h a r g e s i z e and d e t o n a t i o n d e p t h c an be r e l a t e d t o b u b b l e p u l s e f r e q u e n c y by t h e R a y l e i g h - W i l l i s f o r m u l a (Kramer e t a l , 1 9 6 8 ) , a c u r v e r e l a t i n g optimum c h a r g e d e p t h o r f r e q u e n c y t o c h a r g e w e i g h t c a n be o b t a i n e d . Such a c u r v e i s g i v e n by R a i t t (1952) and was u s e d t o d e t e r m i n e t h e 45m d e p t h f o r t h e G e o g e l c h a r g e s (601 d y n a m i t e ) . T h i s d e p t h r e p r e s e n t s an a v e r a g e f o r t h e r a n g e o f G e o g e l c h a r g e s i z e s , and s i n c e t h e c u r v e was c a l i b r a t e d f o r TNT, t h e t r u e optimum d e p t h may d i f f e r f r o m t h i s v a l u e by up t o 15 m f o r a p a r t i c u l a r s h o t . 12 when t h e p r o j e c t i l e s were a s a f e d i s t a n c e f r o m t h e s h i p , r a t h e r t h a n on b o a r d . To do t h i s , a p r o j e c t i l e o r a g r o u p o f t h r e e mounted on a p a l l e t were s u s p e n d e d f r o m a r a f t a t a d e p t h o f 10 7 m 5 w h i l e LAYMORE moved a b o u t 2 km away. U s i n g a m o t o r i z e d r u b b e r b o a t , t h e C a n a d i a n F o r c e s d e m o l i t i o n team a p p r o a c h e d t h e r a f t , made t h e c o n n e c t i o n s and r a d i o e d b a c k t o t h e s h i p when t h e f u s e was i g n i t e d . The ra n g e and b e a r i n g o f th e s h o t were d e t e r m i n e d by t h e s h i p ' s r a d a r . A v i e w o f t h e s h o o t i n g o p e r a t i o n s t h a t have b e e n d e s c r i b e d above i s p r o v i d e d by t h e p h o t o g r a p h s i n F i g . 2 . 2 . INSTRUMENTATION F i g . 2 . 3 shows t h e i n s t r u m e n t a r r a n g e m e n t u s e d on t h e s h o o t i n g s h i p t o r e c o r d t h e d i r e c t a r r i v a l f r o m t h e s h o t and t h e WWVB t i m e code. The d i r e c t a r r i v a l was d e t e c t e d by a hy d r o p h o n e t r a i l e d i m m e d i a t e l y b e h i n d t h e s h i p and a geophone p l a c e d on t h e d e c k . T h e s e s i g n a l s , t o g e t h e r w i t h t h e t i m e code were r e c o r d e d on FM t a p e f o r s u b s e q u e n t p l a y b a c k . A two- c h a n n e l c h a r t r e c o r d e r m o n i t o r e d t h e h y d r o p h o n e and WWVB o u t p u t . In a d d i t i o n t o t h e s e i n s t r u m e n t s , a f a t h o m e t e r was u s e d t o o b t a i n a c o n t i n u o u s r e c o r d o f t h e w a t e r d e p t h d u r i n g t h e p r o f i l e . 5 A g a i n , t h i s d e p t h (107 m) was c o n s i d e r e d optimum f o r t h e c h a r g e s i z e s i n v o l v e d . An e x p e c t e d c o n s e q u e n c e o f t h e i n c r e a s e d d e t o n a t i o n d e p t h i s t h e i n c r e a s e i n b u b b l e p u l s e f r e q u e n c y . The change i n f r e q u e n c y c o r r e s p o n d i n g t o t h e change f r o m t h e G e o g e l t o t h e p r o j e c t i l e s c a n be o b s e r v e d on t h e s e i s m i c r e c o r d s e c t i o n s and w i l l be p o i n t e d o u t l a t e r . 13 CFAV LAYMORE CFAV ENDEAVOUR F i g u r e 2.2A The s h o o t i n g s h i p (CFAV LAYMORE) and r e c e i v i n g s h i p (CFAV ENDEAVOUR) u s e d f o r t h e DSS p r o j e c t . 14 •Figure 2.2B Shooting procedures with the g e l a t i n explosives (Geogel). Top - A 11.2 kg (25 lb) charge is prepared for detonation. Bottom - a larger charge, 22.5 kg (50 lb) is ready to be dropped overboard with the t i l t b o a r d . 15 . F i g u r e 2.2C Top - P l a c i n g o v e r b o a r d t h e r a f t w h i c h s u p p o r t s t h e p r o j e c t i l e s . B o t t o m - T o w i n g t h e r a f t away f r o m t h e s h i p as d e t o n a t i n g c o r d i s p a i d o u t f r o m t h e z o d i a c . F i g u r e 2 . 2 D A s i n g l e a n t i - s u b m a r i n e p r o j e c t i l e ( l e f t ) a n d a p a l l e t o f t h r e e ( r i g h t ) b e i n g l i f t e d o v e r b o a r d . ANTENNA V D e v e l c o t i m e code r e c e i v e r WWVB G o u l d - C l e v i t e CH-24 h y d r o p h o n e HYDROPHONE B u r r - B r o w n l O x g a i n a m p l i f i e r GEOPHONE S h i p - d e c k geophone 4v B r u s h 220 2 - c h a n n e l c h a r t r e c o r d e r CHART H e w l e t t - P a c k a r d 3960 4 - c h a n n e l FM t a p e t r a n s p o r t F i g u r e 2.3 B l o c k d i a g r a m o f s h o o t i n g s h i p i n s t r u m e n t a t i o n . 18 2.3 The R e c e i v i n g S h i p RECEIVING PROCEDURES Compared t o s h o o t i n g , t h e r e c e i v i n g o p e r a t i o n r e q u i r e d f e w e r manual t a s k s t o r u n a p r o f i l e . B e f o r e s h o o t i n g b e g a n , t h e 610 m m a i n c a b l e was s t r e a m e d f r o m t h e s t e r n and t h e s i x h y d r o - phone s y s t e m s were c o n n e c t e d t o t a k e o u t s s p a c e d 91 m a p a r t i n a manner much as F i g . 2.1 ( l o w e r ) shows. The p r o c e d u r e a l s o i n v o l v e p e r f o r m i n g a t e s t f o r c o n t i n u i t y .between t h e h y d r o p h o n e and r e c o r d i n g e q u i p m e n t a f t e r e a c h s y s t e m h a d b e e n p l a c e d o v e r b o a r d . D u r i n g a p r o f i l e r u n t h e main c a b l e was g e n e r a l l y l e f t t o t r a i l f r e e l y b e h i n d t h e s h i p . Shock c o r d e x t e n d i n g f r o m t h e main c a b l e t o t h e b a t t e r y box, and a f l e x i b l e 15 m c a b l e w h i c h l e d t o t h e h y d r o p h o n e s y s t e m s , p r o v i d e d m e c h a n i c a l damping t o m i n i m i z e t h e e f f e c t s o f s u r f a c e waves. A t some o f t h e g r e a t e r d i s t a n c e s however, where h i g h s i g n a l - t o - n o i s e r a t i o s were more d i f f i c u l t t o a c h i e v e , a d d i t i o n a l methods were e m p l o y e d t o m i n i m i z e t h e s e e f f e c t s . F i r s t o f a l l , m a n e u v e r i n g ENDEAVOUR t o p l a c e t h e main c a b l e i n a "U" c o n f i g u r a t i o n i m p r o v e d t h e d a t a as t h e t e n s i o n on t h e c a b l e was r e d u c e d . In c o n j u n c t i o n w i t h t h i s , a t e n s i o n i n g / s l a c k e n i n g p r o c e d u r e was a l s o c a r r i e d o u t a t l a r g e r d i s t a n c e s . By m a i n t a i n i n g t e n s i o n on t h e c a b l e w i t h a w i n d l a s s , and t h e n r a p i d l y r e l e a s i n g i t j u s t b e f o r e t h e s h o t , t h e h y d r o p h o n e s were s l o w l y d r i f t i n g downward when t h e s e i s m i c a r r i v a l s were d e t e c t e d A l t h o u g h s u c h t e c h n i q u e s i n v o l v e d an e x t r a e f f o r t f o r e a c h s h o t , t h e improvement i n t h e q u a l i t y o f t h e d a t a was s i g n i f i c a n t as 19 n o i s e l e v e l s were o b s e r v e d t o d e c r e a s e by f a c t o r s o f a t l e a s t two o r t h r e e . INSTRUMENTATION The r e c o r d i n g s y s t e m u s e d on t h e r e c e i v i n g s h i p i s shown i n F i g . 2.4. P r e s s u r e waves i n c i d e n t on p i e z o e l e c t r i c c r y s t a l s p r o d u c e a s i g n a l w h i c h i s p r e a m p l i f i e d (20 db) and t r a n s m i t t e d t o t h e s e i s m i c a m p l i f i e r s on b o a r d . I n t h e l a t t e r , t h e s i g n a l i s b a n d p a s s f i l t e r e d between 0.8 and 100.0 h z , t h e n a m p l i f i e d w i t h t h e g a i n m a n u a l l y s e t f o r e a c h s h o t . The o u t p u t s f r o m t h e s i x s e i s m i c a m p l i f i e r s p l u s t h e WWVB t i m e code a r e r e c o r d e d on m a g n e t i c t a p e w i t h an I B M - c o m p a t i b l e , 14 b i t , m u l t i c h a n n e l d i g i t a l a c q u i s i t i o n s y s t e m . As a b a c k u p , t h e d a t a a l s o were r e c o r d e d i n a n a l o g f o r m on a s e v e n c h a n n e l FM ta p e r e c o r d e r . I f t h e d i g i t a l s y s t e m ha d f a i l e d , t h e FM t a p e s c o u l d have been d i g i t i z e d a t a l a t e r t i m e . F i v e d a t a c h a n n e l s and t h e WWVB s i g n a l a r e m o n i t o r e d on a s i x - c h a n n e l c h a r t r e c o r d e r . T h i s e n a b l e d q u a l i t y c o n t r o l o f th e s e i s m i c d a t a s i n c e r e q u i r e d c h a n g e s i n c h a r g e s i z e c o u l d be r e l a y e d t o t h e s h o o t i n g s h i p v i a two-way r a d i o . A f a t h o m e t e r on t h e r e c e i v i n g s h i p p r o v i d e d a r e c o r d o f t h e s e a f l o o r t o p o g r a p h y as t h e s h i p d r i f t e d . ANTENNA V S i x G e o t e c h AS-330 a m p l i f i e r s ( b a n d p a s s f i l t e r s : 0.8 - 100 hz) S i x G o u l d - C l e v i t e h y d r o p h o n e s y s t e m s i n c l u d i n g CH-2A h y d r o p h o n e and CE-25L p r e a m p l i f i e r (20db g a i n , 24 v DC) S A E M I P S L M I I F C I E R S F o u r G e l c e l 6v b a t t e r i e s I 1 1 I I H e w l e t t - P a c k a r d 7 - c h a n n e l | FM TAPE FM t a p e transport WWVB A/D JJJLi D e v e l c o t i m e code r e c e i v e r BUFFER FOR- MATTER Kennedy 8208 d u a l b u f f e r f o r m a t t e r CHART A n a l o g i c AN5800 1 6 - c h a n n e l , 14 b i t a n a l o g / d i g i t a l c o n v e r t e r B r u s h 260 6 - c h a n n e l c h a r t r e c o r d e r DIGITAL TAPE Kennedy 8108 9-track, synchronous d i g i t a l tape transport, 25 ips with read a f t e r w r i t e F i g u r e 2.4 B l o c k d i a g r a m o f r e c e i v i n g s h i p i n s t r u m e n t a t i o n . 2 1 3. DATA PROCESSING A description i s given here of the procedure followed to process the seismic data and present them i n record section form. By making appropriate traveltime and amplitude corrections the f i n a l record section can be r e l i a b l y interpreted using synthetic seismograms. 5 . 1 F i e l d Tapes Following the cruise, the magnetic tapes recorded on board the receiving ship were checked to ensure that the d i g i t a l system had functioned properly. However the data were i n an impractical form to access for routine processing. The multichannel data had been multiplexed which meant that reconstructing the data for each channel would be time-consuming from the computing standpoint. Also, the d i g i t a l system writes the data i n an unblocked format (each l o g i c a l record i s one block) which i s i n e f f i c i e n t for tape input/output (I/O) operations. In addition, the amount of data recorded was excessive. Normally d i g i t i z a t i o n lasted for about a minute for each shot i n order to acquire a complete time code. However, useful seismic data may have been recorded during less than h a l f of this i n t e r v a l . F i n a l l y , the recording density was 800 bytes per inch (BPI) rather than the more standard 22 1600 BPI. This meant the amount of tape used was inherent ly la rger than necessary. For the above reasons, and others as w e l l , the raw data were ed i ted , demult iplexed, and wr i t ten on new tapes i n a blocked format at the higher recording density. The IBM 370/68 computer which operates under the Michigan Terminal System (MTS) at the Un iver s i t y of B r i t i s h Columbia was used for these and a l l fu r ther d i g i t a l processing operations. 3.2 Shot-Receiver Distances Shot - rece iver distances for each shot were ca l cu la ted using d i rec t water wave (DWW) travelt imes and a constant for the DWW v e l o c i t y . A seven-channel computer p lo t of d i g i t a l data was used to determine DWW a r r i v a l times r e l a t i v e to UT with in 0.01s. The o r i g i n time of the shot r e l a t i v e to UT was obtained from an FM playback of the single hydrophone and WWVB s ignals onto a two-channel chart recorder. Although the shot could be timed to bet ter than 0.005s on the chart record, an add i t iona l error of up to 0.015s was introduced when the cor rec t ion for the estimated shot - to - sh ip distance was considered. A DWW v e l o c i t y of 1.48 km/s (Minkley et a l , 1 9 70) was assumed to convert travelt imes to d istances. Although th is value does change with depth and season, the v a r i a t i o n is less than one per cent. Taking into account the error associated with DWW travelt imes and the DWW v e l o c i t y , the error 23 i n d i s t a n c e c a l c u l a t i o n s was l e s s t h a n t h r e e p e r c e n t . 3.3 T r a v e l t i m e C o r r e c t i o n s E r r o r s i n s e i s m i c t r a v e l t i m e s a r e p r i m a r i l y due t o i n a c c u r a t e o r i g i n t i m e s and t o p o g r a p h y . C o r r e c t i o n s f o r t h e s e e r r o r s a r e d i s c u s s e d b e l o w . ORIGIN TIMES The d i s t a n c e b e t w e e n t h e s h o t and t h e h y d r o p h o n e b e h i n d t h e s h o o t i n g s h i p i n t r o d u c e s a t i m e d e l a y b e f o r e t h e s h o t i s r e c o r d e d . As a r e s u l t , t h e o b s e r v e d s e i s m i c t r a v e l - t i m e s w i l l be l e s s t h a n t h e y a c t u a l l y a r e . The c o r r e c t i o n f o r t h e d e l a y t i m e i s c a l c u l a t e d f r o m t h e e s t i m a t e d s h o t - h y d r o p h o n e d i s t a n c e and DWW v e l o c i t y o f 1.48 km/s. F o r s m a l l c h a r g e s u s i n g G e o g e l , t h e s h o t d i s t a n c e s were e s t i m a t e d a c c o r d i n g t o t h e l o c a t i o n o f t h e s u r f a c e b a l l o o n s a f t e r t h e s h o t . D i s t a n c e e s t i m a t e s r a n g e d f r o m 90 t o 275 m, o r i n terms o f o r i g i n t i m e c o r r e c t i o n s , f r o m 0.061s t o 0.186s. F o r t h e l a r g e c h a r g e s where r a d a r was u s e d , d i s t a n c e s r a n g e d f o r m 0.5 t o 1.8 km; c o r r e s p o n d i n g t i m e c o r r e c t i o n s were between 0.337 and 1.216s. I t i s a l s o i m p o r t a n t t o c o n s i d e r t h e e r r o r a s s o c i a t e d w i t h t h e o r i g i n t i m e c o r r e c t i o n s , p a r t i c u l a r l y s i n c e some o f t h e d i s t a n c e e s t i m a t e s were v e r y s u b j e c t i v e . A l t h o u g h t h e d i s t a n c e t o t h e b a l l o o n s was n e v e r m e a s u r e d and compared w i t h e s t i m a t e s , an a t t e m p t was made t o c a l c u l a t e i t . Some t i m e l y p h o t o g r a p h s p r o v i d e d s l i d e s s h o w i n g t h e b a l l o o n s i m m e d i a t e l y a f t e r t h e s h o t . From t h e s l i d e s , t h e s i z e o f t h e b a l l o o n s ' 24 image p r o d u c e d by t h e camera l e n s c o u l d be d e t e r m i n e d . S i n c e t h e o b j e c t s i z e a nd image d i s t a n c e ( f o c a l l e n g t h o f t h e l e n s ) were known, t h e o b j e c t d i s t a n c e c o u l d be c a l c u l a t e d u s i n g g e o m e t r i c a l o p t i c s . The r e s u l t s compare f a v o r a b l y w i t h t h e e s t i m a t e s ; t h e d i f f e r e n c e was l e s s t h a n 10 m i n e a c h o f t h e t h r e e s h o t s c o n s i d e r e d . B a s e d on t h e a c c u r a c y o f t h e measurements u s e d i n t h e d i s t a n c e c a l c u l a t i o n , a maximum e r r o r o f s e v e n p e r c e n t was a s s i g n e d t o t h e d i s t a n c e e s t i m a t e s . A s s u m i n g a maximum one p e r c e n t e r r o r i n t h e DWW v e l o c i t y , t h e e r r o r i n t h e o r i g i n t i m e c o r r e c t i o n f o r t h e s m a l l c h a r g e s i s 8 p e r c e n t , o r up t o 0.015s. The e r r o r i n d i s t a n c e s d e t e r m i n e d by r a d a r was much s m a l l e r , a r o u n d two p e r c e n t o r l e s s . T h e r e f o r e , a maximum e r r o r o f 0.015s i s a l s o c o n s i d e r e d a p p r o p r i a t e f o r t h e l a r g e . c h a r g e s . TOPOGRAPHY T r a v e l t i m e s a l s o were c o r r e c t e d f o r s e a f l o o r t o p o g r a p h y as r e c o r d e d on b o t h t h e s h o o t i n g and r e c e i v i n g s h i p s . A w a t e r d e p t h datum o f 2.40 km was c h o s e n b e c a u s e i t m i n i m i z e d t h e m a g n i t u d e o f t h e c o r r e c t i o n s . The b a t h y m e t r y r e c o r d s f r o m t h e r e c e i v i n g s h i p showed l i t t l e v a r i a t i o n t h r o u g h o u t any p r o f i l e e x c e p t 74-1R where c o n s i d e r a b l e d r i f t o c c u r r e d . R e c o r d e d w a t e r d e p t h s on t h e s h o o t i n g s h i p r a n g e d f r o m 2000 m t o 2600 m i n th e r i d g e a r e a s b u t were g e n e r a l l y w i t h i n 75 m o f t h e datum l e v e l on t h e f l a n k s . To c a l c u l a t e t h e t o p o g r a p h i c c o r r e c t i o n a t e i t h e r t h e s h o t o r r e c e i v e r l o c a t i o n , t h e f o l l o w i n g e x p r e s s i o n was u s e d : 25 At = (D - d r ) ( V * " Vw) V* vw where D i s t h e datum d e p t h , d r i s t h e r e c o r d e d d e p t h , i s t h e v e l o c i t y o f t h e u p p e r m o s t l a y e r , and V w i s t h e DWW v e l o c i t y . F o r a l l c a l c u l a t i o n s , , v a l u e s o f 3.0 km/s and 1.48 km/s were assumed f o r V~£ and V w , r e s p e c t i v e l y . The t r a v e l t i m e must be i n c r e a s e d ( A t > 0) t o compensate f o r t h e h i l l s a nd d e c r e a s e d ( A t . <0) t o c o m p e n s a t e f o r t h e v a l l e y s i n t h e t o p o g r a p h y . The n e t t o p o g r a p h i c c o r r e c t i o n f o r e a c h s h o t i s s i m p l y t h e a l g e b r a i c sum o f t h e s h o t and r e c e i v e r c o r r e c t i o n s . The m a g n i t u d e o f t h e n e t c o r r e c t i o n was g e n e r a l l y l e s s t h a n 0.035s e x c e p t i n a r e a s s h o w i n g l a r g e t o p o g r a p h i c r e l i e f . H e r e , c o r r e c t i o n s up t o 0.220s were c a l c u l a t e d . The e r r o r i n At was d i f f i c u l t t o e v a l u a t e s i n c e t h e a c c u r a c y o f v a l u e s f o r and d r a r e n o t w e l l d e t e r m i n e d . However, an e r r o r e s t i m a t e o f 0.25 km/s i n b a s e d on t h e r a n g e i n v e l o c i t i e s t h a t has b e e n a s s o c i a t e d w i t h t h e s e d i m e n t a r y / b a s e m e n t l a y e r , and an e r r o r o f 0.005 km i n d r a s s o c i a t e d w i t h r e a d i n g t h e b a t h y m e t r i c r e c o r d , y i e l d s a t o t a l e r r o r o f 16% i n A t . T h i s amounts t o 0.005s o r l e s s f o r l o c a t i o n s h a v i n g s m a l l t o p o g r a p h i c c o r r e c t i o n s . 3.4 A m p l i t u d e C o r r e c t i o n s I n o r d e r t o f a c i l i t a t e an a m p l i t u d e i n t e r p r e t a t i o n o f t h e d a t a , c o r r e c t i o n s were made f o r v a r i a t i o n s i n a m p l i f i e r s e t t i n g s , c h a r g e s i z e , and s h o t - r e c e i v e r d i s t a n c e . A s i m p l e c o m p u t e r r o u t i n e was u s e d t o c a l c u l a t e t h e s e c o r r e c t i o n s f o r 26 t h e s i x c h a n n e l s o f e v e r y s h o t and a l s o r e t u r n a n o r m a l i z e d s e t o f c o r r e c t i o n f a c t o r s f o r t h e e n t i r e p r o f i l e . AMPLIFIERS M a n u a l g a i n c o n t r o l on t h e s e i s m i c a m p l i f i e r s a l l o w e d n e a r s h o t s t o be a t t e n u a t e d and d i s t a n t s h o t s t o be a m p l i f i e d r e l a t i v e t o t h e f i r s t s t a g e o f a m p l i f i c a t i o n o f 70 db. I n o r d e r t o d e v e l o p an e x p r e s s i o n f o r t h e n e c e s s a r y c o r r e c t i o n f a c t o r s , t h e f o l l o w i n g d e f i n i t i o n i s f i r s t c o n s i d e r e d : V o l t a g e g a i n (db) = 20 l o g i o V2 V i where V i and V 2 a r e two v o l t a g e l e v e l s o f t h e s i g n a l . S i n c e a b s o l u t e v o l t a g e v a l u e s a r e n o t r e l e v a n t h e r e , t h e e x p r e s s i o n c a n be s i m p l i f i e d by a s s u m i n g an i n s t a n t a n e o u s v a l u e o f l v f o r t h e s i g n a l a f t e r b e i n g i n i t i a l l y a m p l i f i e d by 70 db. I f t h e s i g n a l has b e e n a m p l i f i e d by an a d d i t i o n a l G db so t h a t G = 20 l o g i o V2, t h e n m u l t i p l y i n g t h e r e c o r d e d s i g n a l by I/V2 w i l l b r i n g i t b a c k t o i t s o r i g i n a l v a l u e , i . e . l v . The - G/ c o r r e c t i o n f a c t o r , I/V2, d e n o t e d as C i i s t h e n , C i = I/V2 = 10 ' Note t h a t i f t h e i n p u t s i g n a l has b e e n a t t e n u a t e d (G <0) r a t h e r t h a n a m p l i f i e d , t h e e x p o n e n t i s p o s i t i v e . M u l t i p l y i n g by C i w i l l t h e n i n c r e a s e t h e a m p l i t u d e o f t h e r e c o r d e d s i g n a l . CHARGE SIZE B a s e d on s t u d i e s o f u n d e r w a t e r e x p l o s i o n s , O ' B r i e n (1960) has shown t h a t f i r s t a r r i v a l s e i s m i c a m p l i t u d e s a r e p r o p o r t i o n a l 2 t o W"3" where W i s t h e w e i g h t o f t h e c h a r g e . L a t e r e x p e r i m e n t a l work by M u l l e r e t a l (1962) a l s o p l a c e d t h e v a l u e o f t h e e x p o n e n t c l o s e t o two t h i r d s . The a m p l i t u d e c o r r e c t i o n f o r 2 c h a r g e s i z e , d e n o t e d as C 2 , i s C 2 = 1/W3 where W i s g i v e n i n l b s . T h i s c o r r e c t i o n was a p p l i e d t o a l l s h o t s i n e a c h p r o f i l e . However, s i n c e two t y p e s o f e x p l o s i v e s were u s e d and were d e t o n a t e d a t d i f f e r e n t d e p t h s , t h e y a r e n o t e q u i v a l e n t i n terms o f e n e r g y y i e l d p e r pound. No a t t e m p t was made t o c o r r e c t f o r t h i s . The d i f f e r e n c e may t h e r e f o r e c a u s e some p e c u l i a r a m p l i t u d e (and f r e q u e n c y ) c h a n g e s t o a p p e a r on t h e r e c o r d s e c t i o n . SPHERICAL SPREADING A c o r r e c t i o n must a l s o be made f o r t h e d e c r e a s e i n a m p l i t u d e as s e i s m i c e n e r g y s p r e a d s away f r o m t h e s h o t . C e r v e n y and R a v i n d r a (1971, p. 147) show t h a t f o r l a r g e e p i c e n t r a l d i s t a n c e s ( r ) , t h e h e a d wave a m p l i t u d e s d e c r e a s e a p p r o x i m a t e l y as 1 / r 2 w i t h i n c r e a s i n g r . F o r s m a l l r , n e a r t h e c r i t i c a l d i s t a n c e , t h e r a t e may be c l o s e r t o 1 / r 3 o r 1/r 1*. Wide - a n g l e r e f l e c t i o n s , on t h e o t h e r hand, have a m p l i t u d e s w h i c h f a l l o f f as 1/r w i t h i n c r e a s i n g r . ( B r a i l e and S m i t h , 1 9 7 5 ) . A l t h o u g h i t was n o t c o m p l e t e l y s u i t a b l e , t h e c o r r e c t i o n f a c t o r C 3 = r 2 was a p p l i e d t o a l l d a t a . THE NORMALIZED CORRECTION FACTOR The a m p l i t u d e c o r r e c t i o n (C) a p p l i e d t o a g i v e n d a t a c h a n n e l c a n now be w r i t t e n as C = £1 where C"= Ci • C 2 • C 3 = 1 0 " G / 2 ° • W^ • r 2 max and C i s t h e maximum p r o d u c t c a l c u l a t e d . C i s u s e d max c max t o n o r m a l i z e v a l u e s o f C t o l i e b e t w e e n z e r o and one. 28 3.5 S e i s m i c R e c o r d S e c t i o n s The raw d a t a f r o m e a c h p r o f i l e were i n i t i a l l y p r e s e n t e d i n r e c o r d s e c t i o n f o r m by p l o t t i n g a l l s i x c h a n n e l s o f d e m u l t i p l e x e d d a t a f o r e a c h s h o t . C o r r e c t i o n s were made f o r traveltimes but not amplitudes. For p l o t t i n g , traveltimes were reduced by x/6s where x i s . t h e s h o t - r e c e i v e r d i s t a n c e . F i g . 3.1 shows t h e raw d a t a f r o m p r o f i l e 74-2 b e g i n n i n g a t a d i s t a n c e o f a b o u t 4.5 km. F a r more d a t a i s p r e s e n t e d h e r e t h a n w i l l be i n t e r p r e t e d l a t e r . However, t h e i n t e n t was t o show l a t e r p h a s e s s u c h as b o t t o m r e f l e c t i o n s and m u l t i p l e s , and a l s o t o p r e s e n t a b e t t e r v i e w o f t h e o v e r a l l q u a l i t y o f t h e d a t a . STACKING The n e x t s t e p i n v o l v e d s t a c k i n g t h e s i x s e i s m o g r a m s t o f o r m a s i n g l e t r a c e f o r e a c h s h o t . The p r i m a r y r e a s o n f o r s t a c k i n g was t o enhance t h e q u a l i t y o f t h e d a t a . A t t h e same t i m e i t r e d u c e d t h e amount o f d a t a t o be h a n d l e d and, t h e r e f o r e c u t c o m p u t i n g and p l o t t i n g c o s t s . In g e n e r a l , t o s t a c k m u l t i c h a n n e l d a t a , t h e summation i s p e r f o r m e d a l o n g some p r e d e t e r m i n e d l a g t r a j e c t o r y . As t h e t i t l e i m p l i e s , t h e l a g t r a j e c t o r y i s d e f i n e d by t h e l a g , o r e q u i v a l e n t l y , t h e moveout o f a s i g n a l f r o m one c h a n n e l t o t h e n e x t due t o r e c e i v e r s p a c i n g . F o r r e f l e c t i o n a r r i v a l s , t h e t r a j e c t o r y i s h y p e r b o l i c . I f , however, we assume t h e t r a j e c t o r y i s a s t r a i g h t l i n e f o r r e f r a c t i o n a r r i v a l s , t h e 29 F i g u r e 3.1 R e c o r d s e c t i o n o f s i x c h a n n e l d a t a f r o m p r o f i l e 74-2 r e c o r d e d b e y o n d t h e c r i t i c a l d i s t a n c e f o r t h e f i r s t c r u s t a l l a y e r . T r a v e l t i m e s have b e e n c o r r e c t e d f o r t o p o g r a p h y and s h o t d i s t a n c e , b u t no a m p l i t u d e c o r r e c t i o n s h a v e b e e n a p p l i e d .  31 s t a c k i n g v e l o c i t y w i l l be t h e v e l o c i t y o f t h e c r i t i c a l l y r e f r a c t i n g l a y e r . More s o p h i s t i c a t e d t e c h n i q u e s o f d e t e r m i n i n g t h e optimum s t a c k i n g v e l o c i t y employ c o h e r e n c y ' m e a s u r e s w h i c h t e s t a r a n g e o f v e l o c i t i e s . C o h e r e n c y m e a s u r e s were n o t a p p l i e d h owever, s i n c e t h e number o f d a t a c h a n n e l s was c o n s i d e r e d i n a d e q u a t e f o r r e l i a b l e r e s u l t s . A l s o , t h e s p a t i a l d i g i t i z a t i o n was s u c h t h a t t h e v e l o c i t y r e s o l u t i o n w o u l d be no b e t t e r t h a n 16 km/s. T h e r e f o r e , r e f r a c t i o n v e l o c i t i e s d e t e r m i n e d f r o m t h e i n t e r p r e t a t i o n o f f i r s t a r r i v a l t r a v e l t i m e d a t a were used' i n s t e a d . F o r a p a r t i c u l a r s h o t , t h e optimum s t a c k i n g v e l o c i t y f o r t h e f i r s t a r r i v a l was u s e d t o o b t a i n t h e e n t i r e s t a c k e d t r a c e . T h i s c h o i c e was made o n l y a f t e r t h e e f f e c t s o f u s i n g d i f f e r e n t v e l o c i t i e s t o o b t a i n l a t e r segments o f t h e t r a c e were shown t o be i n s i g n i f i c a n t . The d a t a f r o m p r o f i l e 74-2 a f t e r a m p l i t u d e c o r r e c t i o n s and s t a c k i n g were a p p l i e d a r e shown i n F i g . 3.2. Note t h a t most o f t h e l a r g e a m p l i t u d e b o t t o m r e f l e c t i o n s and t h e i r m u l t i p l e s have been o m i t t e d . The improvement i n t h e d a t a i s q u i t e s t r i k i n g , p a r t i c u l a r l y f o r t h o s e s h o t s r e c o r d e d b e y o n d 55 km where t h e s i g n a l / n o i s e r a t i o was v e r y low. FILTERING A l t h o u g h t h e a m p l i f i e r f i l t e r s p r o v i d e d f i l t e r i n g b e t w e e n 0.8 and 100.0 h z , some h i g h f r e q u e n c y n o i s e r e m a i n e d s u p e r i m p o s e d on t h e s t a c k e d t r a c e s . A l s o , t h e e f f e c t o f t h e low f r e q u e n c y s w e l l o f the sea was s t i l l a p p a r e n t on some t r a c e s . T h e r e f o r e , t h e s t a c k e d d a t a f o r e a c h p r o f i l e were b a n d p a s s f i l t e r e d b e tween 2.0 and 30.0 hz w i t h a z e r o p h a s e s h i f t , 32 Figure 3.2 Record section of data from profile 74-2 after stacking to form a single trace for each shot. Both traveltime and amplitude corrections have been applied. 1 PROFILE 74-2 34 fourth order, Butterworth f i l t e r . The mathematical background for Butterworth f i l t e r s i s given by Kanasewich (1973) . The frequency bandwidth was determined by measuring the period of prominent a r r i v a l s on the stacked traces; most values ranged between 8 and 12 hz. Such an approach was adequate since the purpose of bandpass f i l t e r i n g was simply to improve the general appearance of the data. In f a c t , the i n t e r p r e t a t i o n could have been accomplished equally well without any f i l t e r i n g . A f t e r applying the amplitude corrections, stacking, and f i l t e r i n g , the processed version of the data f o r each p r o f i l e was written on another tape. No data beyond 9.0s reduced traveltime were stored however. The processed data for the four p r o f i l e s are presented i n Figs. 3.3 through 3.6. 35 Figure 3.3 Record section of data from p r o f i l e 74-1 a f t e r processing which included stacking, bandpass f i l t e r i n g (Butterworth, 2.0 to 30.0 hz) and applying traveltime and amplitude corrections. PROFILE 74-1 41 1 lo '. ii 20 25 To S HT. 5 3> DISTANCE (KM) ON PROFILE 74-IR f <• 20 25 IT IT DISTANCE (KM) Figure 3.4 Processed record section for p r o f i l e 74-1R.  PROFILE 74-2R L U CO c to, X I T o T o is jJT" as" J J so 3s DISTANCE (KM) Figure 3.6 Processed record sect ion for p r o f i l e 74-2R 65 10 40 4. INTERPRETATION 4.1 Review o f Methods The i n t e r p r e t a t i o n o f m a r i n e r e f r a c t i o n d a t a i n t h e p a s t has g e n e r a l l y r e l i e d on t h e s l o p e - i n t e r c e p t method t o c a l c u l a t e v e l o c i t i e s and d e p t h s f r o m a p l o t o f t r a v e l t i m e v e r s u s d i s t a n c e . The method was o r i g i n a l l y p r o p o s e d f o r a p p l i c a t i o n t o m a r i n e d a t a by D e G o y l e r ( 1 9 3 2 ) . S i n c e t h e n , i t has b e e n w i d e l y u s e d t o i n t e r p r e t l a r g e q u a n t i t i e s o f d a t a f r o m deep o c e a n a r e a s . W h i l e t h e method i s s i m p l e t o a p p l y , two r e s t r i c t i v e a s s u m p t i o n s a r e made - t h a t e a c h l a y e r i i b o u nded above and be l o w by p l a n e s and has a c o n s t a n t v e l o c i t y w h i c h i s h i g h e r t h a n t h e one i n t h e l a y e r above i t . I n r e a l i t y , o f c o u r s e , t h e s e c o n d i t i o n s a r e n o t p e r f e c t l y s a t i s f i e d , and t h e r e s u l t i n g v e l o c i t y - d e p t h model may be h i g h l y a r t i f i c i a l . A more c o m p l e t e i n t e r p r e t a t i o n o f t h e d a t a c a n be made by u t i l i z i n g b o t h t r a v e l t i m e s and a m p l i t u d e s . T h i s method o f i n t e r p r e t i n g m a r i n e r e f r a c t i o n d a t a h as b e e n d e m o n s t r a t e d b y H e l m b e r g e r (1968) , and H e l m b e r g e r and M o r r i s (1969, 1970) t h r o u g h t h e use o f s y n t h e t i c s e i s m o g r a m s . A s i m i l a r method i s u s e d i n t h i s work. I t c o n s i s t s o f o b t a i n i n g f i r s t a s i m p l e l a y e r e d model u s i n g t h e s l o p e - i n t e r c e p t method on f i r s t a r r i v a l t r a v e l t i m e s ; t h e n u s i n g t h i s model as a s t a r t i n g p o i n t , c h a n g es a r e made t o i t u n t i l t h e s y n t h e t i c 4 1 seismograms match the a m p l i t u d e c h a r a c t e r i s t i c s o f t h e d a t a w i t h o u t d i s t u r b i n g t h e o r i g i n a l t r a v e l t i m e f i t . The d e t a i l s of t h i s approach a r e g i v e n below. 4 . 2 T r a v e l t i m e I n t e r p r e t a t i o n TRAVELTIME PLOTS The t r a v e l t i m e p l o t s used t o d e r i v e the i n i t i a l v e l o c i t y - d e p t h models were c o n s t r u c t e d from f i r s t a r r i v a l p i c k s made on computer p l o t t e d seismograms. F i g . 4.1 shows f o u r s e t s o f such seismograms from p r o f i l e 74-1R t h a t were p l o t t e d at a p p r o x i m a t e l y 100 mm/s. Each s e t i s c h a r a c t e r i s t i c o f t h e d a t a r e c o r d e d on o t h e r p r o f i l e s a t d i s t a n c e s s i m i l a r t o t h o s e i n d i c a t e d . The f i r s t r e f r a c t e d a r r i v a l was r e a d i l y i d e n t i f i e d on n e a r l y a l l r e c o r d s . However, l o c a t i n g i t s f i r s t b r e a k was sometimes r a t h e r s u b j e c t i v e e s p e c i a l l y a t l o n g e r d i s t a n c e s where the o n s e t was more emergent and the background n o i s e l e v e l was h i g h e r . A l s o , a t a d i s t a n c e o f about 12 km the d i r e c t wave and the s e i s m i c a r r i v a l i n t e r f e r e d w i t h one a n o t h e r c a u s i n g the o n s e t to be d i s t o r t e d f o r a few s h o t s . The maximum e r r o r i n t i m i n g the f i r s t a r r i v a l p i c k s was d e t e r m i n e d t o be 0.015s. T h e r e f o r e , t o g e t h e r w i t h the e r r o r a s s o c i a t e d w i t h the o r i g i n time and t o p o g r a p h i c c o r r e c t i o n s , the e s t i m a t e d t r a v e l t i m e e r r o r was l e s s t h a n 0.035s. F i g s . 4.2 and 4.3 are r e d u c e d t r a v e l t i m e p l o t s o f the f i r s t a r r i v a l d a t a from the f o u r p r o f i l e s ; the l e a s t s q u a r e s f i t t e d t r a v e l t i m e b ranches a l s o are shown. In t h e l o w e r 42 F i g u r e 4.1 Computer p l o t t e d seismograms from f o u r shots at d i f f e r e n t d i s t a n c e s along p r o f i l e 74-1R. One hydrophone system was i n o p e r a t i v e f o r three o f the f o u r shots shown. 43 44 Figure 4.2 Reduced t r a v e l t i m e data (symbols) and l e a s t squares f i t s ( l i n e s ) f o r p r o f i l e s 74-1 and 74-1R. The inv e r s e slopes of the l i n e s are v e l o c i t i e s i n km/s; i n t e r c e p t s are i n sec. The upper part i s an expanded s c a l e to show the f i r s t 20 km of each p r o f i l e . Symbols enclosed i n parentheses were assigned a r e l a t i v e weight of 0.5. DISTANCE (KM)  47 p a r t o f e a c h d i a g r a m a s i n g l e d a t a p o i n t i s p l o t t e d f o r e a c h s h o t a l t h o u g h a l l t h e t r a c e s were g e n e r a l l y u s e d t o make t h e p i c k . F o r t h e t r a v e l t i m e b r a n c h e s a t t h e b e g i n n i n g o f e a c h p r o f i l e , t h e number o f p o i n t s a v a i l a b l e f o r a l e a s t s q u a r e s a n a l y s i s was c o n s i d e r e d i n a d e q u a t e . The u p p e r p a r t o f e a c h d i a g r a m t h e r e f o r e shows, on an e x p a n d e d s c a l e , t r a v e l t i m e s d e t e r m i n e d f r o m e a c h s e i s m i c t r a c e r e c o r d e d o v e r t h e f i r s t 20 km. S i n c e t h e f i r s t a r r i v a l d a t a o v e r t h i s p o r t i o n o f t h e p r o f i l e s a r e n o t r e v e r s e d , t h e u p p e r p l o t s b o t h have t h e same o r i g i n . A l e a s t s q u a r e s l i n e a r f i t was made and a / v e l o c i t y - d e p t h model computed f r o m s l o p e s and i n t e r c e p t s . In a d d i t i o n t o t h e d a t a p o i n t s , i n p u t t o t h e c o mputer p r o g r a m i n c l u d e d a w e i g h t i n g v a l u e b e t w e e n z e r o and one f o r e a c h p o i n t a c c o r d i n g t o i t s e s t i m a t e d r e l i a b i l i t y . D e c i d i n g on w h i c h p o i n t s t o a s s i g n t o e a c h t r a v e l t i m e b r a n c h was a m a t t e r o f j u d g e m e n t . In some c a s e s , d a t a p o i n t s n e a r a b r a n c h c r o s s o v e r p o s i t i o n were a s s i g n e d t o e a c h b r a n c h f o r t h e l e a s t s q u a r e s a n a l y s i s . As F i g s 4.2 and 4.3 i n d i c a t e , p r o f i l e s 74-2 and 2R d i s p l a y n o r m a l t r a v e l t i m e c u r v e s ( l a y e r v e l o c i t i e s i n c r e a s e w i t h d e p t h ) w h i l e p r o f i l e s 74-1 and IR show anomalous b e h a v i o r . The a n o m a l i e s a r e t o o l a r g e t o a t t r i b u t e t o t i m i n g e r r o r s and so must be c o n s i d e r e d a r e a l f e a t u r e t o be e x p l a i n e d . L e t us examine i n more d e t a i l t h e t r a v e l t i m e c u r v e s f o r p r o f i l e s 74-1 and IR t h a t a r e shown i n F i g . 4.2. F o l l o w i n g t h e f i r s t b r a n c h t h e r e a r e i n t e r m e d i a t e b r a n c h e s w i t h a p p a r e n t v e l o c i t i e s o f 5.43 km/s (74-1) and 6.41 km/s ( 7 4 - 1 R ) . T h e s e p e r s i s t as f i r s t a r r i v a l s f o r a s h o r t d i s t a n c e a f t e r w h i c h a 48 b r a n c h w i t h a h i g h e r v e l o c i t y (7.30 km/s and 7.12 km/s f o r 74-1 and IR, r e s p e c t i v e l y ) i s o b s e r v e d . However t h i s b r a n c h e x t e n d s o n l y f o r a few km on 74-1 and f o r a b o u t 10 km on 74-1R. A t g r e a t e r d i s t a n c e s t h e f i r s t a r r i v a l s l i n e up on b r a n c h e s w i t h s i m i l a r v e l o c i t i e s , 7.37 km/s f o r 74-1 and 7.24 km/s f o r 74-1R b u t s h o w i n g a c l e a r t i m e d e l a y . The s i m i l a r i t y o f t h e s e a p p a r e n t v e l o c i t i e s w h i c h a r e p a r t i a l l y r e v e r s e d s u g g e s t s / t h a t d i p on t h e r e f r a c t i n g h o r i z o n i s n e g l i g i b l e . In a d d i t i o n t h e r e e x i s t s an i n t e r m e d i a t e b r a n c h on 74-1 w h i c h has an a p p a r e n t v e l o c i t y o f 5.80 km/s, a v a l u e n e a r t h e a v e r a g e f o r t h e two i n t e r m e d i a t e b r a n c h e s a t s h o r t e r d i s t a n c e s . How a r e s u c h o b s e r v a t i o n s e x p l a i n e d i n a r e a s o n a b l e manner? A low v e l o c i t y zone w i l l i n t r o d u c e a t i m e d e l a y and o f f s e t i n t o t h e f i r s t a r r i v a l t r a v e l t i m e c u r v e . But i f t h i s were t h e c a s e , d a t a f r o m 74-2 s h o u l d e x h i b i t a s i m i l a r e f f e c t . They do n o t . A l s o a low v e l o c i t y l a y e r does n o t e x p l a i n t h e i n t e r m e d i a t e 5.80 km/s b r a n c h . FAULTING An a l t e r n a t i v e e x p l a n a t i o n f o r t h e o b s e r v a t i o n s i s a f a u l t w i t h a m a j o r component o f v e r t i c a l s h i f t e i t h e r due t o downthrow i n t h e d i r e c t i o n o f t h e t r a v e r s e o r e q u i v a l e n t l y , u p l i f t i n t h e r e g i o n n e a r t h e s t a r t o f t h e p r o f i l e . I f t h e h o r i z o n t a l l a y e r s a r e t e r m i n a t e d by a f a u l t - l i k e s t r u c t u r e w i t h a v e r t i c a l o f f s e t i n t h e o r d e r o f a few k i l o m e t e r s , t h e s e i s m i c waves w i l l be d i f f r a c t e d and s c a t t e r e d . S u f f i c i e n t e n e r g y may t h e n be i n c i d e n t upon t h e d o w n - f a u l t e d b l o c k t o c r e a t e a n o t h e r h e a d wave. F o r p r o f i l e 74"IR t h e f i r s t a r r i v a l d a t a a t d i s t a n c e s o f 20 and 22 km ( F i g 4.2 u p p e r ) c l e a r l y show a t r e n d 49 c h a r a c t e r i s t i c of d i f f r a c t i o n s that would occur near a f a u l t . In addition, the amplitude of the seismic trace at 22 km (shot 29, F i g . 3.4) i s inconsistent with the amplitudes of adjacent records and may be a t t r i b u t e d to a focusing e f f e c t associated with the f a u l t . No such evidence i s apparent for p r o f i l e 74-1. For this p r o f i l e however, the o f f s e t may be such that f o r a short distance f i r s t a r r i v a l s are from an unfaulted layer with apparent v e l o c i t y 5.80 km/s before these waves are overtaken by a r r i v a l s from the material with v e l o c i t y 7.40 km/s. VELOCITY-DEPTH MODELS Except for the faulted region of p r o f i l e 74-1 and IR, the velocity-depth structures for the four p r o f i l e s can be obtained using two f a m i l i a r expressions from r e f r a c t i o n seismology; namely, 1 fdt] , T . " v 1 , ( V n 2 - V i 2 } h x? ~ A— and T-; = ) 2zi ,T—TT—'—J— V n [ d x j n x n j = x J v j V n where (dt/dx) n and T^ n are the slope and intercept, respectively of the traveltime branch for the nth layer. V n i s the apparent v e l o c i t y of the n — layer and z n _ i i s the thickness of the layer above i t . Table I summarizes the results of the computations for each profile. Using the v e l o c i t i e s and intercepts, and assuming homogeneous layers, f a u l t throws could be calculated according to the method outlined i n the Appendix. The f a u l t throws for p r o f i l e s 74-1 and IR are 3.9 and 4.7 km, respectively. The data given i n Table 1 for each p r o f i l e and the calculated f a u l t structures for p r o f i l e s 74-1 and IR are presented graphically 50 Summary of least squares analysis of traveltime data. Traveltime branches are numbered from near to far distances. One standard deviation in the determination of the slopes has been converted to an equivalent standard deviation in v e l o c i t y . The calculated standard deviation in the intercept i s given. Maximum residuals enclosed by parantheses are for points assigned a weighting factor of 0.5. Profile Figure Reference Traveltime Branch No. P t s . Velocity (km/s) Intercept (s) Layer Thickness (km) Maximum Residual (s) 74-1 74-IR 74-2 74-2R 3-upper 3-lower 3-upper n II 3- lower 4- upper 4-lower 4-upper n 4-lower n 1 2 3 4 5 1 2 3 4 1 2 3 4 1 2 3 4 1.48 (assumed) 2.68 21 4.10 ± .06 3.38 ± .02 1.71 24 5.43 ± .04 3.70 ± .01 .78 21 7.30 ± .11 4.10 ± .02 5 , 5.80 ± .10 3.78 ± .06 9 7.40 ± .12 4.67 ± .09 1.48 2.44 21 4.20 ± .06 3.08 ± .02 .97 34 6.41 ± .05 3.50 ± .01 .92 21 7.12 ± .10 3.67 ± .03 10 7.24 ± .05 4.12 ± .07 1.48 2.62 18 4.06 ± .08 3.30 ± .02 1.55 54 5.33 ± .03 3.61 ± .01 2.14 7 6.55 ± .13 4.24 ± .08 3.44 7 7.83 ± .24 5.03 ± .25 1.48 2.52 14 4.17 ± .10 3.19 ± .03 1.09 57 5.17 ± .03 3.42 ± .01 1.40 14 6.59 ± .09 3.88 ± .05 6.36 6 7.87 i .07 5.06 i .08 .013 .015 .025 .023 (.118) .018 .011 .040 .083 .008 .030 .059 (.304) .014 .037 .153 .036 (TABLE I) tn 52 i n F i g . 4.4. T h e s e m o d e l s w i l l be m o d i f i e d i n t h e c o u r s e o f t h e a m p l i t u d e a n a l y s i s d i s c u s s e d i n t h e n e x t s e c t i o n . 4.3 T r a v e l t i m e and A m p l i t u d e I n t e r p r e t a t i o n COMPUTING SYNTHETIC SEISMOGRAMS S y n t h e t i c s e i s m o g r a m s p r o v e d v a l u a b l e f o r d e t e r m i n i n g a v e l o c i t y - d e p t h m odel r e q u i r e d by t h e t r a v e l t i m e and a m p l i t u d e c h a r a c t e r i s t i c s o f t h e d a t a . T h e s e were c a l c u l a t e d a c c o r d i n g t o t h e r e c e n t l y d e v e l o p e d d i s c r a y t h e o r y (DRT) o f W i g g i n s ( 1 9 7 6 ) , w i t h a c o m p u t e r r o u t i n e (HRGLTZ) w r i t t e n by R.A. W i g g i n s . T r a v e l t i m e s and a m p l i t u d e s o f t h e s y n t h e t i c s e i s m o g r a m s a r e c a l c u l a t e d f r o m a s p e c i f i e d p-A c u r v e ( d e s c r i b e d b e l o w ) . The p r o g r a m a l s o p e r f o r m s a W e i c h e r t - H e r g l o t z i n t e g r a t i o n o f t h e p -A c u r v e t o d e t e r m i n e a v e l o c i t y - d e p t h m o d e l . T h e r e f o r e , c h a n g e s a p p l i e d t o t h e p-A c u r v e w h i l e f i t t i n g s y n t h e t i c s e i s m o g r a m s t o t h e d a t a a l s o i n f l u e n c e d t h e shape o f t h e v e l o c i t y v e r s u s d e p t h c u r v e . To g e n e r a t e s e i s m o g r a m s w h i c h m a t c h e d t h e d a t a i t was n e c e s s a r y t o s p e c i f y t h e c h a r a c t e r i s t i c s o f t h e d a t a i n terms o f t h e p -A c u r v e . The p -A c u r v e r e f e r s t o a p l o t o f t h e r a y p a r a m e t e r (p) a g a i n s t e p i c e n t r a l d i s t a n c e (A). The r a y p a r a m e t e r i s g i v e n by t h e d i s t a n c e d e r i v a t i v e o f t h e t r a v e l t i m e , dt / d A , o r by p=r s i n i / v , where r i s t h e d i s t a n c e f r o m t h e c e n t e r o f t h e E a r t h , i i s t h e a n g l e b e t w e e n t h e r a y and t h e r a d i u s v e c t o r , and v i s t h e v e l o c i t y a t r . An i n i t i a l p-A c u r v e c a n t h e r e f o r e be c o n s t r u c t e d f r o m t h e s l o p e s o f t h e 53 F i g u r e 4.45 V e l o c i t y - d e p t h c u r v e s d e t e r m i n e d f r o m f i r s t a r r i v a l d a t a f o r e a c h o f t h e f o u r p r o f i l e s . Datum w a t e r d e p t h i s 2.4 km. Dashed l i n e s show t h e c a l c u l a t e d w a t e r d e p t h d e t e r m i n e d f r o m t h e i n t e r c e p t o f t h e f i r s t b r a n c h . The d o t t e d c u r v e s show t h e d o w n - f a u l t e d s t r u c t u r e s as d i s c u s s e d i n t h e t e x t . 54 V (km/s) V (km/s) 0 2 4 6 8 0 2 4 0 i 1—|—i 1 1 r 1 1 1 0 I 1—I—i 1 r E S3 ^ 6 10L 74-1 8 h 10 «- 6 8 -1 1 1 74-1R 0 2 4 6 8 0 2 4 6 8 0 I 1—r—i 1 1 1 1 1 1 0 I 1—I—i 1 1 r 1 1 1 6 CS3 10 12 L 74-2 10 12 L 74-2R 55 t r a v e l t i m e c u r v e s , b u t s i n c e t h e t r a v e l t i m e c u r v e s i n F i g s . 4.2 and 4.3 show o n l y r e f r a c t i o n b r a n c h e s , t h e p o r t i o n o f t h e p-A c u r v e c o r r e s p o n d i n g t o t h e r e f l e c t i o n b r a n c h e s w o u l d have t o be e x t r a p o l a t e d . A l t h o u g h s u c h an a p p r o a c h may have b e e n a c c e p t a b l e , a more c o n v e n i e n t method o f o b t a i n i n g an i n i t i a l p-A c u r v e was u s e d . A c o m p u t e r r o u t i n e (MDLPLT, a l s o by R.A. W i g g i n s ) a p p r o x i m a t e d a c o n t i n u o u s A ( p ) 6 c u r v e f r o m a s p e c i f i e d v e l o c i t y - d e p t h c u r v e a c c o r d i n g t o t h e e q u a t i o n s g i v e n i n B u l l e n (1965, p. 1 1 2 ) . The v e l o c i t y - d e p t h m o d e l s d e t e r m i n e d f r o m t h e l e a s t s q u a r e s a n a l y s i s f o r t h e f o u r p r o f i l e s were t h e r e f o r e u s e d t o o b t a i n t h e i n i t i a l p-A c u r v e i n e a c h c a s e . F o r p r o f i l e s 74-1 and IR, o n l y t h e u p - f a u l t s t r u c t u r e s c o u l d be s p e c i f i e d s i n c e t h e p r o g r a m s a r e n o t d e s i g n e d t o h a n d l e d i s c o n t i n u i t i e s s u c h as f a u l t s . The c o n t i n u o u s p-A c u r v e s were t h e n " d i g i t i z e d " f o r s u b s e q u e n t use i n HRGLTZ. The s a m p l i n g i n t e r v a l v a r i e d s i n c e t h o s e p a r t s o f t h e c u r v e s h o w i n g c o n s i d e r a b l e c u r v a t u r e r e q u i r e d more p o i n t s f o r t h e i n t e r p o l a t i o n i n HRGLTZ. A f t e r t h e i n i t i a l p-A c u r v e was c o n s t r u c t e d , a t r i a l - a n d - e r r o r p r o c e d u r e was u s e d t o m o d i f y i t so t h a t t h e s y n t h e t i c r e c o r d s e c t i o n m a t c h e d t h e d a t a . S i n c e b o t h t r a v e l t i m e s and a m p l i t u d e s were c o n s t r a i n t s i n m a k i n g t h e f i t , a t r a d e - o f f b e t w e e n t h e two was u s u a l l y n e c e s s a r y . As a r e s u l t , t h e t r a v e l t i m e c u r v e s f i t t e d w i t h HRGLTZ were n o t 6 The p-A c u r v e i s r e f e r r e d t o h e r e w i t h A as a f u n c t i o n o f p a l t h o u g h i t i s u s u a l l y p l o t t e d as p ( A ) , w i t h p b e i n g t h e o r d i n a t e . S i n c e p ( A ) may be m u l t i v a l u e d (as i t was f o r t h e s e d a t a ) , A(p) c o r r e c t l y e x p r e s s e s t h e one t o one r e l a t i o n s h i p b e t w e e n p and A i n t h e a b s e n c e o f low v e l o c i t y z o n e s . 56 i d e n t i c a l t o t h o s e o b t a i n e d f r o m t h e l e a s t s q u a r e s method. To s e r v e as a g u i d e f o r m a k i n g t r a v e l t i m e and a m p l i t u d e a d j u s t m e n t s w i t h t h e p-A c u r v e , t h e f o l l o w i n g r e l a t i o n s were b o r n e i n m i n d : (1) The t r a v e l t i m e , T ( p , A ) , c a n be w r i t t e n as T ( P > A ) = PA+ p A ( p ) d p P = pA + T ( p ) where p = R/v(R) i s t h e maximum p v a l u e t h a t c a n be o b t i n e d IUclX and R i s t h e r a d i u s o f t h e E a r t h ( W i g g i n s and M a d r i d , 1974) . I t was t h e r e f o r e u s e f u l t o t h i n k o f t h e t r a v e l t i m e i n terms o f t h e a r e a u n d e r t h e A(p) c u r v e , and a " c o u n t i n g t h e s q u a r e s " t e c h n i q u e was o f t e n u s e d t o a d j u s t t h e t r a v e l t i m e as d e s i r e d . (2) The a m p l i t u d e , A ( p , A ) , i s o f t e n e x p r e s s e d as A(P,A) = F ( p , A ) |dp/dA| 2 The f a c t o r F ( p , A ) i s u s u a l l y s l o w l y v a r y i n g compared t o dp/dA ( B u l l e n , 1 9 6 5 ) ; h e n c e , t h e a m p l i t u d e may be c o n s i d e r e d i p r o p o r t i o n a l t o |dp/dA| 2. A c c o r d i n g t o t h a t , a p v s . A p l o t s h o u l d be s t e e p where l a r g e a m p l i t u d e s a r e n e e d e d and n e a r l y f l a t f o r s m a l l a m p l i t u d e s . I n p r a c t i c e , more a m p l i t u d e c o n t r o l was o b t a i n e d by c a r e f u l l y p o s i t i o n i n g t h e A r a n g e s where t r a v e l t i m e t r i p l i c a t i o n o c c u r r e d s i n c e t h e c o n s t r u c t i v e i n t e r f e r e n c e o f t h e r e f r a c t e d and r e f l e c t e d p h a s e s w o u l d c a u s e a l a r g e a m p l i t u d e b u i l d u p . T h i s w i l l be i l l u s t r a t e d more s p e c i f i c a l l y w i t h t h e a c t u a l d a t a . A l t h o u g h t h e p r o c e d u r e was l a r g e l y by t r i a l and e r r o r , t h e t i m e r e q u i r e d t o o b t a i n a r e a s o n a b l e f i t was g r e a t l y 57 r e d u c e d t h r o u g h t h e u s e o f t h e Adage g r a p h i c s t e r m i n a l a t t h e UBC C o m p u t i n g C e n t e r . W i t h t h e d a t a and s y n t h e t i c r e c o r d s e c t i o n s on d i s k f i l e , t h e g r a p h i c s t e r m i n a l a l l o w e d b o t h o f them t o be shown on CRT d i s p l a y i m m e d i a t e l y a f t e r a d j u s t i n g t h e p-A c u r v e and r u n n i n g HRGLTZ. T h i s e l i m i n a t e d t h e n e e d f o r e x p e n s i v e c o m p u t e r p l o t s w h i c h h a d a l o w e r t u r n a r o u n d r a t e . APPLICATION OF HRGLTZ TO THE DATA The manner i n w h i c h t h e a p p r o p r i a t e p - A c u r v e s were co n - s t r u c t e d f o r t h e d a t a f r o m t h e f o u r p r o f i l e s i s now d i s c u s s e d . The s y n t h e t i c s e c t i o n s computed f r o m t h e s e c u r v e s s h o u l d r e s e m b l e t h o s e i n F i g s . 3.3 t h r o u g h 3.6, a t l e a s t f o r t h e i n i t i a l 0.5s o r so o f d a t a . P r o b a b l e r e a s o n s f o r some o f t h e d i f f i c u l t i e s e n c o u n t e r e d d u r i n g t h e f i t t i n g p r o c e d u r e a r e a l s o c o n s i d e r e d . As p r e v i o u s l y m e n t i o n e d , HRGLTZ c o u l d n o t be u s e d i n t h e n o r m a l way f o r a l l o f p r o f i l e s 74-1 and 74-1R; p r o f i l e s 74-2 and 74-2R a r e t h e r e f o r e d i s c u s s e d f i r s t . PROFILES 74-2 AND 74-2R The l o w e r p a r t o f F i g . 4.5 shows t h e p-A c u r v e f o r p r o f i l e 74-2 b a s e d on t h e s i m p l e l a y e r e d v e l o c i t y - d e p t h model shown i n F i g . 4.4. S y n t h e t i c s e i s m o g r a m s computed f r o m t h i s c u r v e (0.46 - p - 0.12) a t 4 km i n t e r v a l s a r e p l o t t e d w i t h t h e s u p e r i m p o s e d t r a v e l t i m e c u r v e s on t h e u p p e r p a r t o f t h e d i a g r a m . S i n c e t h e d a t a had b e e n c o r r e c t e d f o r s p h e r i c a l s p r e a d i n g , a l l s y n t h e t i c a m p l i t u d e s have b e e n m u l t i p l i e d by t h e r 2 f a c t o r d i s c u s s e d i n S e c . 3.4. On a l l f u t u r e p l o t s 58 Figure 4.5 Bottom - The p-A curve corresponding to the homogeneous layered ve loc i ty -depth model for p r o f i l e 74-2 (See p ig . 4.4). Top - Synthetic seismograms and travel t ime curve computed from the p-A curve. Inset shows the source wavelet used for convolut ion. 3 to X irj 3.0 0,1 _ U 1 SEiOKD IT —I so "7T 20 IT 35 IT DISTANCE (KM) 4 DI5 TRNCE (KM) in to 60 o f t h i s t y p e , t h i s c o r r e c t i o n w i l l be assumed. A l s o , as an a t t e m p t t o a c h i e v e a more r e a l i s t i c w a v e form, t h e s o u r c e w a v e l e t shown i n t h e i n s e t o f F i g . 4.5 was c o n v o l v e d w i t h t h e s p i k e d s e i s m o g r a m computed by DRT a t e a c h s p e c i f i e d d i s t a n c e ( s e e W i g g i n s , 1 9 7 6 ) . T h i s w a v e l e t was u s e d f o r a l l r e c o r d s e c t i o n s and was o b t a i n e d by s i m p l y e x t r a c t i n g t h e f i r s t a r r i v a l f r o m s h o t 25 o f p r o f i l e 74-2R ( F i g . 3 . 6 ) . A c o m p a r i s o n o f t h e s y n t h e t i c r e c o r d s e c t i o n ( F i g . 4.5) and t h e d a t a r e c o r d s e c t i o n f o r p r o f i l e 74-2 ( F i g 3.5) r e v e a l s some d i s t i n c t a m p l i t u d e d i f f e r e n c e s . P a r t i c u l a r l y n o t i c e a b l e a r e t h e l a r g e a m p l i t u d e s e c o n d a r y a r r i v a l s w h i c h a p p e a r c o n s i s t e n t l y on a l l s y n t h e t i c s e i s m o g r a m s o u t t o t h e g r e a t e s t d i s t a n c e s , b u t no s u c h l a r g e a m p l i t u d e s a p p e a r on t h e d a t a b e y o n d 45 km. The l a r g e s y n t h e t i c a m p l i t u d e s l i e on t h e r e f l e c t i o n b r a n c h e s o f t h e t r a v e l t i m e c u r v e , and s i n c e r e f l e c t i o n s a t t e n u a t e as 1 / r , t h e r 2 c o r r e c t i o n f a c t o r e x a g g e r a t e s t h e s e a m p l i t u d e s . However, s i n c e t h e same c o r r e c t i o n was a p p l i e d t o t h e d a t a , t h e d e c r e a s e i n a m p l i t u d e b e g i n n i n g w i t h s h o t 35 (44 km) was c o n s i d e r e d i n d i c a t i v e o f some r e a l p r o p e r t y o f t h e v e l o c i t y - d e p t h s t r u c t u r e . A t t h e n e a r e r d i s t a n c e s t h e a b r u p t i n c r e a s e i n a m p l i t u d e s on t h e d a t a a t s h o t 31 (27 km) i s a l s o o f p a r t i c u l a r i n t e r e s t s i n c e i t i s n o t c h a r a c t e r i s t i c o f t h e s y n t h e t i c s e c t i o n . T h ese d i f f e r e n c e s i m p l y t h a t a s i m p l e m odel c o n s i s t i n g o f homogeneous l a y e r s i s n o t a d e q u a t e t o e x p l a i n t h e o b s e r v e d a m p l i t u d e f e a t u r e s . To o b t a i n a more r e a l i s t i c m o d e l , t h e 61 p-A curve was progress ive ly a l te red u n t i l the large amplitudes were approximately confined to the observed range. At the same time attempts were made to preserve the o r i g i n a l travelt ime f i t s . In order to decide on appropriate changes for the p-A curve, more a t tent ion should be given to the inf luence of th i s curve on amplitudes. The prev ious ly stated r e l a t i o n , 1 A(p,A) a j dp/dA| 2, suggests a simple way of c o n t r o l l i n g amplitudes with the p-Acurve. However, synthet ic seismogram amplitudes were found - to be much more dependent upon the pos i t ions of c r i t i c a l points on the travel t ime curve (the distance at which a head wave f i r s t ex i s t s ) than on the slope of the p-A curve alone. This observation is consistent with the geometric ray theory p red i c t i on that the maximum amplitude for r e f l e c t e d a r r i v a l s occurs at the c r i t i c a l po int . To see the re l a t i onsh ip between the p-A and travelt ime curves consider the p lots in F i g . 4.5. C r i t i c a l points on the travelt ime curve are located at approximately 2.0, 4.5, 10.5, and 17.0 km for r e f r a c t i o n v e l o c i t i e s of 4.06, 4.33, 6.55, and 7.83 km/s, re spec t i ve l y . These distances also locate the corners of a ser ies of lens shaped curves comprising the p-A curve. The bottom part of the lens corresponds to r e f r a c t i o n a r r i v a l s , with the highest v e l o c i t i e s being associated with the lowest p values. The upper part of the lens corresponds to r e f l e c t e d a r r i v a l s ranging from near c r i t i c a l to wide angle. 62 F o r homogeneous l a y e r s t h e r e f l e c t i o n and r e f r a c t i o n b r a n c h e s a r e a s y m p t o t i c a l t h o u g h F i g . 4.5 does n o t c l e a r l y i l l u s t r a t e t h i s f a c t . By a l l o w i n g t h e s e c u r v e s t o become t a n g e n t t o e a c h o t h e r a t some f i n i t e d i s t a n c e however, t h e l a r g e r e f l e c t i o n a m p l i t u d e s w h i c h s t a r t a t t h e c r i t i c a l p o i n t c a n be t e r m i n a t e d where d e s i r e d . L a r g e a m p l i t u d e s w i l l t h e n be c o n f i n e d , f o r t h e most p a r t , t o t h e r e g i o n b etween c u s p s on t h e t r a v e l t i m e c u r v e , o r e q u i v a l e n t l y , t h e zone o f t r a v e l t i m e t r i p l i c a t i o n . F i g . 4.6 i l l u s t r a t e s how t h i s c r i t e r i a f o r f i t t i n g a m p l i t u d e s was u s e d t o a d j u s t t h e p -A c u r v e f o r e a c h l a y e r . The p r o c e s s i n v o l v e d m o v i n g t h e e n d p o i n t s (A v a l u e s ) o f t h e t r a v e l t i m e t r i p l i c a t i o n a c c o r d i n g t o where l a r g e a m p l i t u d e s .appeared on t h e d a t a . F o r p r o f i l e 74-2 however, t h e l a s t two t r i p l i c a t i o n s a r e b a r e l y d i s c e r n i b l e . R e c a l l i n g t h a t t h e t r a v e l t i m e i s e q u a l t o t h e a r e a u n d e r t h e A(p) c u r v e , i t i s c l e a r t h a t s u c h c h a n g e s w o u l d a l t e r t h e t r a v e l t i m e u n l e s s b o t h e n d p o i n t s had b e e n moved i n o p p o s i t e d i r e c t i o n s by t h e c o r r e c t amount. C o m p e n s a t i o n was t h e r e f o r e made by e i t h e r i n c r e a s i n g o r d e c r e a s i n g t h e t h i c k n e s s o f t h e l e n s s h a p e d c u r v e s f o r t h e l a y e r s c o n c e r n e d a l t h o u g h c h a n g i n g t h e s l o p e d i d have an a d d i t i o n a l e f f e c t on a m p l i t u d e s . F i g . 4.7 shows t h e p-A c u r v e b e f o r e ( d a s h e d l i n e ) and a f t e r ( s o l i d l i n e ) b e i n g a d j u s t e d f o r a t r a v e l t i m e and a m p l i t u d e f i t . The change i n t h e p o s i t i o n o f t h e c r i t i c a l p o i n t was g r e a t e s t f o r t h e l a s t two r e f r a c t o r s (p = 0.152 and p = 0.127). To compensate f o r t h e l a r g e i n c r e a s e i n sample points p-A curve constructed from a homogeneous layered model Corresponding (reduced) travelt ime curve large amplitudes | t originally here _ large amplitudes 1 Modif ied vers ion of the p-A curve for des ired amplitude f i t with compensation for the increase in t rave l t ime Corresponding (reduced) t rave l t ime curve Figure 4.6 The procedure used in adjust ing the p-A curve to contro l the amplitudes of synthetic seismograms. Figure. 4.7 The p-A curve for the homogeneous layered model of p r o f i l e 74-2 (dashed l ine ) superimposed on the curve adjusted for a travelt ime and amplitude f i t ( so l i d l i n e ) . The p range shown represents the approximate range over which synthet ic seismograms were computed (0.125- - p ^ 0.46). I I la ii Si 5 3i ii 35 5 15 5 e£ S To H 3> DISTANCE (KM) 65 t r a v e l t i m e a c c o m p a n y i n g t h i s c h a n g e , t h e r e s p e c t i v e " l e n s e s " were t h i c k e n e d as much as p o s s i b l e . As a f u r t h e r a t t e m p t t o l o w e r t h e t r a v e l t i m e t h e r e f r a c t i o n v e l o c i t y o f an o v e r l y i n g l a y e r was i n c r e a s e d s l i g h t l y , o r as shown i n t h e d i a g r a m , t h e p v a l u e was d e c r e a s e d by a c o n s t a n t amount. The s y n t h e t i c s e i s m o g r a m s computed f r o m t h e r e v i s e d p-A c u r v e and a t r u n c a t e d v e r s i o n o f t h e r e c o r d s e c t i o n f o r p r o f i l e 74-2 a r e shown i n F i g . 4.8. B o t h a r e p l o t t e d on t h e same s c a l e and have i d e n t i c a l t r a v e l t i m e c u r v e s s u p e r i m p o s e d . The p r o b l e m s d i s c u s s e d w i t h t h e c o n s t r u c t i o n o f t h e p-A c u r v e a r e r e a d i l y a p p a r e n t i n F i g . 4.8. The l a r g e s t a m p l i t u d e s on t h e s y n t h e t i c s e c t i o n b e g i n a r o u n d 24 km r a t h e r t h a n 27 km as o b s e r v e d on t h e d a t a . A l s o , t h e c a l c u l a t e d t r a v e l t i m e a l o n g t h e r e f r a c t i o n b r a n c h e s b e y o n d 24 km a r e t o o l o n g w h i l e t h o s e b e f o r e 24 km a r e g e n e r a l l y t o o s h o r t t o f i t t h e f i r s t a r r i v a l s . T h e s e d i f f i c u l t i e s i l l u s t r a t e t h e t r a v e l t i m e - a m p l i t u d e t r a d e - o f f t h a t h a d t o be made t o o b t a i n a r e a s o n a b l e f i t . To be s u r e , e x p e r i m e n t a l e r r o r s a c c o u n t f o r some o f t h e d i f f i c u l t i e s w i t h f i t t i n g . The r e s u l t s o f a t h e o r e t i c a l s t u d y o f r e f l e c t i o n and h e a d wave a m p l i t u d e s by C e r v e n y (1966) may, however, p r o v i d e a f u r t h e r e x p l a n a t i o n . He showed t h a t t h e a m p l i t u d e s o f r e f l e c t e d a r r i v a l s n e a r t h e c r i t i c a l p o i n t do n o t r e a c h a maximum u n t i l some d i s t a n c e b e y o n d t h e c r i t i c a l p o i n t where t h e r e f l e c t e d wave and h e a d wave i n t e r f e r e . T h i s d i s t a n c e i s b e t w e e n 10 and 50 km d e p e n d i n g on t h e f r e q u e n c y o f t h e r e f l e c t e d wave; i t i s l a r g e r f o r low f r e q u e n c i e s 66 4.8 Top - Synthetic seismograms and the travel t ime curve computed from the modif ied p-A curve shown in F i g . 4.7. Bottom - Data from p r o f i l e 74-2 (approximately 1.8s of each t r a ce ) , with the travelt ime curve of the upper diagram superimposed. O N 68 and s m a l l e r f o r h i g h f r e q u e n c i e s . T h r e e o f t h e s e i s m o g r a m s w h i c h have l a r g e a m p l i t u d e s ( s h o t s 31, 32, 33) were u s e d t o d e t e r m i n e t h e 6.55 km/s b r a n c h f r o m t h e f i r s t a r r i v a l d a t a ( F i g . 4 . 3 ) . The c r i t i c a l d i s t a n c e f o r t h i s b r a n c h i s c e r t a i n l y l e s s t h a n t h e c r o s s o v e r p o i n t (17 km) w i t h t h e 5.17 km/s b r a n c h . The d i s t a n c e b etween t h e c r i t i c a l p o i n t and t h e l a r g e a m p l i t u d e s i s t h e n a t l e a s t 10 km. However, s i n c e t h e c r i t i c a l p o i n t h a d t o be p l a c e d a t 24 km t o o b t a i n l a r g e a m p l i t u d e s t h e r e , i t s p o s i t i o n i s n o t c o r r e c t a c c o r d i n g t o t h e o r i g i n a l t r a v e l t i m e f i t . I n o t h e r w o r d s , u s i n g t h e g e o m e t r i c r a y t h e o r y c r i t e r i a t o g e n e r a t e l a r g e a m p l i t u d e s has d i s p l a c e d t h e c r i t i c a l p o i n t and i n c r e a s e d t h e t r a v e l t i m e by more t h a n what c o u l d be c o m p e n s a t e d . A s i m i l a r s i t u a t i o n e x i s t s w i t h t h e l e a s t s q u a r e s d e t e r m i n e d 7.83 km/s b r a n c h . A c c o r d i n g t o i t s p o i n t o f i n t e r s e c t i o n w i t h t h e 6.55 km/s b r a n c h , t h e c r i t i c a l d i s t a n c e i s l e s s t h a n 31 km. A g a i n , t h e c r i t i c a l p o i n t was p l a c e d a t a g r e a t e r d i s t a n c e (34 km) t o p r o d u c e a m p l i t u d e s s i m i l a r t o t h o s e on t h e d a t a . In t h i s c a s e t h o u g h , t h e t r a v e l t i m e i n c r e a s e w i l l a l s o i n c l u d e t h e i n c r e a s e on t h e p r e v i o u s (6.55 km/s) b r a n c h . A l t h o u g h t h e above d i s c u s s i o n has p o i n t e d o u t t h e d i f f i c u l t i e s e x p e r i e n c e d i n u s i n g t h e c r i t i c a l p o i n t c r i t e r i a t o f i t l a r g e a m p l i t u d e s , i t s h o u l d be remembered t h a t c r o s s o v e r p o i n t s (and t h e r e f o r e c r i t i c a l p o i n t s ) were n o t w e l l d e t e r m i n e d on t h e d a t a . C r o s s o v e r p o i n t s i n t h e l e a s t 69 s q u a r e s a n a l y s i s were q u i t e d e p e n d e n t upon w h i c h p o i n t s were i n c l u d e d on a p a r t i c u l a r b r a n c h . T h i s i s e s p e c i a l l y t r u e a t l a r g e r d i s t a n c e s where f e w e r d a t a p o i n t s were a v a i l a b l e and a s s i g n i n g two o r t h r e e d a t a p o i n t s d i f f e r e n t l y c o u l d s h i f t t h e c r o s s o v e r p o i n t by 5 km o r more. The a m p l i t u d e f i t a l o n g t h e l a s t t r a v e l t i m e b r a n c h i s good e x c e p t f o r t h e l a s t two s h o t s where t h e d a t a s u d d e n l y a p p e a r more a t t e n u a t e d . F o r t h e s e s h o t s however, t h e l a r g e s t charge s i z e was u s e d ( t h r e e p r o j e c t i l e s ) , and s i n c e t h e e n e r g y y i e l d o f t h e p r o j e c t i l e s was n o t w e l l known, t h e c h a r g e s i z e c o r r e c t i o n f a c t o r may have b e e n e x c e s s i v e . A t t h e n e a r d i s t a n c e s t h o u g h (<25 km), t h e f i r s t a r r i v a l a m p l i t u d e s a r e r e a s o n a b l y w e l l f i t . The p r o c e d u r e f o r d e t e r m i n i n g t h e p-A c u r v e f o r p r o f i l e 74-2R was t h e same as f o r p r o f i l e 74-2 d e s c r i b e d a b o v e . The r e s u l t a n t c u r v e was, however, s i g n i f i c a n t l y d i f f e r e n t . The main r e a s o n f o r t h e d i f f e r e n c e was t h e c o n t r a s t i n g n a t u r e o f t h e t r a v e l t i m e c u r v e s as shown by F i g . 4.3. A c o m p a r i s o n o f t h e r e c o r d s e c t i o n s f o r p r o f i l e s 74-2 and 74-2R ( F i g s . 3.5 and 3.6) shows t h a t t h e a m p l i t u d e c h a r a c t e r i s t i c s o f t h e s e p r o f i l e s a r e q u i t e s i m i l a r . L a r g e a m p l i t u d e s were a g a i n o b s e r v e d o v e r a l i m i t e d r e g i o n f o r p r o f i l e 74-2R, b u t i n t h i s c a s e , t h e i r o n s e t was more g r a d u a l . T h a t i s , i n c r e a s e d a m p l i t u d e s were o b s e r v e d as e a r l y as 21 km, b u t t h e maximum d i d n ' t a p p e a r u n t i l a b o u t 30 km. As w i t h p r o f i l e 74-2, t h e r e was a d e f i n i t e f a l l - o f f 70 i n a m p l i t u d e s b e y o n d a c e r t a i n d i s t a n c e , i n t h i s c a s e , 43 km (shot 3 5 ) . A l t h o u g h o n l y l a r g e p r o j e c t i l e s were u s e d a f t e r shot 35 (as t h e h i g h e r f r e q u e n c y i n s h o t 36 m i g h t s u g g e s t ) , t h e d e c r e a s e i n a m p l i t u d e was c o n s i d e r e d s i g n i f i c a n t i n t e r m s o f c r u s t a l s t r u c t u r e . F i g . 4.9 shows t h e d a t a and s y n t h e t i c s e i s m o g r a m s f o r p r o f i l e 74-2R w i t h t h e t r a v e l t i m e c u r v e s u p e r i m p o s e d on e a c h . Many o f t h e same p r o b l e m s d i s c u s s e d w i t h p r o f i l e 74-2 were e n c o u n t e r e d a g a i n . The l a r g e a m p l i t u d e s e x t e n d i n g b e y o n 50 km on t h e s y n t h e t i c s e c t i o n were p a r t i c u l a r l y d i f f i c u l t t o r e d u c e i n s i z e . F o r t h i s p r o f i l e a l l l a r g e a m p l i t u d e s were g e n e r a t e d o v e r t h e r a n g e o f one t r i p l i c a t i o n b e g i n n i n g a t t h e c r i t i c a l p o i n t o f t h e l a s t r e f r a c t i o n b r a n c h . T h e r e f o r e , m o v i n g t h e l a r g e a m p l i t u d e s f u r t h e r t o t h e l e f t w o u l d have meant d e c r e a s i n g t r a v e l t i m e s on t h e l a s t r e f r a c t i o n b r a n c h more. The s i t u a t i o n i s a c t u a l l y o p p o s i t e t o what i t was f o r p r o f i l e 74-2 i n t h a t t h e c r i t i c a l p o i n t has b e e n moved t o t h e l e f t by t o o much t o be a d e q u a t e l y c o m p e n s a t e d . A g a i n , h owever, i t i s i m p o r t a n t t o r e a l i z e t h e r a n g e i n p o s s i b l e p o s i t i o n s f o r t h e c r o s s o v e r p o i n t s t h a t w o u l d t e n d t o " s o f t e n " t h e s e d i f f i c u l t i e s . F o r t h i s p r o f i l e as w e l l , f i r s t a r r i v a l a m p l i t u d e s f o r t h e s m a l l e r d i s t a n c e s ( < 25 km) have b e e n f i t w e l l . PROFILES 74-1 AND 74-1R B e c a u s e o f t h e o f f s e t and d e l a y o f t h e 7.3 km/s b r a n c on p r o f i l e s 74-1 and 74-1R, a c o n t i n u o u s p-A c u r v e c o u l d n o t DISTANCE (KM) Figure 4.9 Same as F ig . 4.8 for p r o f i l e 74-2R. 72 be c o n s t r u c t e d f o r e i t h e r p r o f i l e . A l t h o u g h HRGLTZ c a n be u s e d t o o b t a i n s y n t h e t i c s e i s m o g r a m s f r o m a d i s c o n t i n u o u s p-A c u r v e ( t h e same p v a l u e f o r two d i f f e r e n t A ) , t h e p h y s i c a l i n t e r p r e t a t i o n o f t h e d i s c o n t i n u i t y i s t h e n a low v e l o c i t y z o n e . F o r a l a t e r a l d i s c o n t i n u i t y s u c h as a f a u l t , HRGLTZ c a n n o t be u s e d t o p r o d u c e m e a n i n g f u l s e i s m o g r a m s b e y o n d t h e f a u l t . T h u s , f o r p r o f i l e s 74-1 and 74-1R w h i c h show e v i d e n c e f o r f a u l t i n g on t h e Moho, s y n t h e t i c s e i s m o g r a m s were computed f o r t h e u p - f a u l t s t r u c t u r e s o n l y (A < 25 km) F i g . 4.10 shows t h e d a t a f r o m t h e two p r o f i l e s o v e r t h e u p - f a u l t r e g i o n w i t h t h e a s s o c i a t e d s y n t h e t i c s e c t i o n s and t r a v e l t i m e c u r v e s d i s p l a y e d i n t h e i r u s u a l manner. As w i t h p r o f i l e s 74-2 and 74-2R, t h e w e l l d e f i n e d f i r s t a r r i v a l s and u n i f o r m a m p l i t u d e s a t n e a r d i s t a n c e s a l l o w e d a g ood t r a v e l t i m e and a m p l i t u d e f i t t o be made. A l t h o u g h s y n t h e t i c s e i s m o g r a m s were n o t computed f o r t h e l a t t e r p a r t o f e i t h e r p r o f i l e , i t i s i n t e r e s t i n g t o n o t e t h a t t h e l a r g e a m p l i t u d e s c h a r a c t e r i s t i c o f p r o f i l e s 7 4 _ 2 and 74-2R a l s o a p p e a r on p r o f i l e 74-1R w i t h i n a p p r o x i m a t e l y t h e same d i s t a n c e i n t e r v a l (30-50 km) ( s e e F i g . 3 . 4 ) . The d a t a f r o m p r o f i l e 74-1, on t h e o t h e r hand, show no s u c h p r o m i n e n t f e a t u r e ( s e e F i g . 3 . 4 ) . The l a r g e a m p l i t u d e s on s h o t 37 a r e r e g a r d e d as a n o m a l o u s , p r o b a b l y c a u s e d by an e r r o r i n t h e a m p l i t u d e c o r r e c t i o n f a c t o r . D e s p i t e t h e l i m i t a t i o n s on t h e a m p l i t u d e a n a l y s i s f o r t h e s e p r o f i l e s , HRGLTZ c o u l d be u s e d f o r f i t t i n g F i g u r e 4.10 Same as F i g . 4.8 f o r u p - f a u l t r e g i o n s o f p r o f i l e s 74-1 ( l e f t ) and 74-1R ( r i g h t ) . 74 t r a v e l t i m e s beyond t h e f a u l t . The t h r o w o f t h e f a u l t was t h e n d e t e r m i n e d f r o m t h e d i f f e r e n c e b e t w e e n t h e u p - f a u l t a n d d o w n - f a u l t v e l o c i t y - d e p t h s t r u c t u r e s . The method u s e d f o l l o w s t h e one d e v e l o p e d by B e n n e t t (1973). S i n c e HRGLTZ computes h o r i z o n t a l l a y e r s o n l y , t h e f a u l t t h r o w must be t h o u g h t o f i n t e r m s o f an i n c r e a s e i n l a y e r t h i c k n e s s . F o r a n o r m a l f a u l t , s i m p l e g e o m e t r i c r a y t h e o r y shows t h a t t h e t i m e d e l a y due t o t h e f a u l t w i l l be l e s s t h a n t h e d e l a y due t o i n c r e a s i n g t h e l a y e r t h i c k n e s s b y t h e amount o f t h e f a u l t t h r o w . T h e r e f o r e , t o c a l c u l a t e t h e d o w n - f a u l t s t r u c t u r e w i t h HRGLTZ, a t i m e c o r r e c t i o n AT e x p r e s s i n g t h i s d i f f e r e n c e must be o b t a i n e d . The d e t a i l s on c a l c u l a t i n g AT, a s s u m i n g a f a u l t on t h e Moho o n l y , a r e g i v e n i n t h e A p p e n d i x . F o r p r o f i l e s 74-1 and 74-1R t h e AT v a l u e s were 0.261s and 0.235s, r e s p e c t i v e l y . The d o w n - f a u l t p-A c u r v e was t h e n c o n s t r u c t e d f r o m t h e u p - f a u l t v e r s i o n t h a t h a d b e e n d e t e r m i n e d u s i n g s y n t h e t i c s e i s m o g r a m s . F o r a f a u l t e d Moho, t h e a r e a u n d e r t h e A ( p ) c u r v e was i n c r e a s e d a c c o r d i n g t o t h e above s t a t e d AT v a l u e s w i t h i n t h e p r a n g e a p p l i c a b l e t o Moho r e f l e c t i o n s and h e a d waves. S i n c e t h e r e were no a m p l i t u d e c o n s t r a i n t s i m p o s e d w h i l e m a k i n g t h e c h a n g e , t h e s h a p e o f t h e l o w e s t p a r t o f t h e p-A c u r v e was c o m p l e t e l y a r b i t r a r y . However, u n l i k e t h e t r a v e l t i m e i n t e g r a l , t h e d e p t h i n t e g r a l i s w e i g h t e d . T h u s , some r e a s o n a b l e shape was m a i n t a i n e d f o r t h e f i n a l l e n s o f t h e p-A c u r v e . 75 FINAL VELOCITY-DEPTH MODELS The f i n a l v e l o c i t y - d e p t h m o d e l s o b t a i n e d a f t e r f i t t i n g a r r i v a l t r a v e l t i m e s and a m p l i t u d e s o f t h e d a t a f o r t h e f o u r p r o f i l e s a r e p r e s e n t e d g r a p h i c a l l y i n F i g . 4.11. T h e s e s h o u l d be compared w i t h t h e models o b t a i n e d f r o m t h e t r a v e l t i m e i n t e r - p r e t a t i o n a l o n e ( F i g . 4 . 4 ) . The most o b v i o u s change i n e a c h c a s e i n v o l v e s g o i n g f r o m a c r u s t a l model composed o f homogeneous l a y e r s t o one where p o s i t i v e v e l o c i t y g r a d i e n t s e x i s t w i t h i n t h e l a y e r s . The change i s most d r a s t i c f o r p r o f i l e 74-2 where t h e v e l o c i t y d i s c o n t i n u i t y i n t h e l o w e r c r u s t has b e e n n e a r l y e l i m - i n a t e d . The g r a d i e n t s shown f o r t h e d o w n - f a u l t models o f 74-1 and 74-1R ( d a s h e d l i n e s ) a r e , however, n o t s i g n i f i c a n t s i n c e no a m p l i t u d e c o n t r o l was m a i n t a i n e d w h i l e a d j u s t i n g t h e p-A c u r v e . B e s i d e s b e i n g i n t u i t i v e l y more r e a l i s t i c , t h e v e l o c i t y g r a d i e n t s a l s o e x p l a i n some o f t h e a m p l i t u d e f e a t u r e s on t h e d a t a . C e r v e n y and R a v i n d r a (1971) d e s c r i b e a wave c a l l e d an i n t e r f e r e n c e h e a d wave w h i c h i s c a u s e d by a p o s i t i v e v e l o c i t y g r a d i e n t b e l o w a p l a n e i n t e r f a c e . A l t h o u g h t h e k i n e m a t i c s o f t h e s e waves a r e s i m i l a r t o t h o s e o f p u r e h e a d waves, i . e . t h e t r a v e l t i m e s a r e n e a r l y t h e same, t h e a m p l i t u d e and s p e c t r a l c h a r a c t e r i s t i c s a r e v e r y d i f f e r e n t . E v e n f o r s m a l l g r a d i e n t s (0.01 km/s/km) i n t e r f e r e n c e a m p l i t u d e s may be one o r two o r d e r s o f m a g n i t u d e l a r g e r t h a n t h e a m p l i t u d e s o f p u r e h e a d waves ( C e r v e n y , 1 9 6 6 ) . The models i n F i g . 4.11 show t h a t a g r a d i e n t o f a b o u t 0.30 km/s/km e x i s t s , i n t h e l o w e r c r u s t f o r p r o f i l e 74-2 and a b o u t 0.10 km/s/km f o r p r o f i l e 74-2R. I n l i g h t o f t h e p r e c e d i n g d i s c u s s i o n , t h e s e g r a d i e n t s seem t o be a l i k e l y e x p l a n a t i o n f o r t h e l a r g e 76 VELOCITY (KM/SEC) ? 1 * o m 5« VELOCITY IKM/SEC) ? 5_ F i g u r e 4.11 V e l o c i t y - d e p t h curves determined f o r each of the f o u r p r o f i l e s by f i t t i n g both t r a v e l t i m e s and amplitudes with s y n t h e t i c seismograms. Dashed l i n e s r e p r e s e n t down-fault s t r u c t u r e s . 77 a m p l i t u d e s a p p e a r i n g a t a p p r o x i m a t e l y 30 km on t h e r e c o r d s e c t i o n s . The v e l o c i t y g r a d i e n t s may a l s o a c c o u n t f o r t h e d e c r e a s e i n a m p l i t u d e s b e y o n d 50 km. A t t h e s e d i s t a n c e s a g r a d i e n t may be p r o d u c i n g a shadow zone f o r Moho r e f l e c t i o n s (C'erveny and R a v i n d r a , 1 9 7 1 ) . As a r e s u l t , t h e i n t e r f e r e n c e h e a d wave w i l l no l o n g e r e x i s t . In t h e c o u r s e o f a d j u s t i n g t h e p-A c u r v e t o f i t a m p l i t u d e s t h e l a y e r t h i c k n e s s e s were a l t e r e d f r o m t h e i r l e a s t s q u a r e s v a l u e s . The d i f f e r e n c e s a r e g e n e r a l l y l e s s t h a n 0.20 km, e x c e p t f o r p r o f i l e 74-2 where t h e t h i c k n e s s o f t h e s e c o n d c r u s t a l l a y e r was i n c r e a s e d by more t h a n 2 km. The t o t a l c r u s t a l t h i c k n e s s f o r p r o f i l e 74-2 ( i n c l u d i n g 2.40 km o f w a t e r ) was t h e n 11.8 km, w h i c h a g r e e s f a v o r a b l y w i t h t h e 11.2 km f o r p r o f i l e 74-2R. F o r p r o f i l e s 74-1 and IR, a d o w n - f a u l t c r u s t a l d e p t h o f 9.9 km was computed. T h i s i n c r e a s e s t h e r e s p e c t i v e f a u l t t h r o w v a l u e s t o 4.5 and 5.1 km f r o m t h e e a r l i e r v a l u e s o f 3.9 and 4.7 o b t a i n e d f r o m t h e t r a v e l t i m e c a l c u l a t i o n . A more c o m p l e t e d i s c u s s i o n o f t h e s e m o d e l s i s g i v e n i n C h a p t e r 5 where t h e i r g e o l o g i c a l s i g n i f i c a n c e i s c o n s i d e r e d . LATER DATA A l t h o u g h t h e t r a v e l t i m e and a m p l i t u d e a n a l y s i s o f t h i s s t u d y was p r i m a r i l y c o n c e r n e d w i t h t h e f i r s t 1.0s o f d a t a , i t was i n t e r e s t i n g t o o b s e r v e t h a t t h e l a r g e a m p l i t u d e s c h a r a c t e r i s t i c o f f i r s t a r r i v a l s b etween 30 and 50 km c o n s i s t e n t l y r e a p p e a r e d a t a l a t e r t i m e on t h e r e c o r d s e c t i o n s . 78 The d a t a f r o m p r o f i l e 74-2 a r e t a k e n as an example o f t h i s and a r e a g a i n p r e s e n t e d i n r e c o r d s e c t i o n f o r m ( F i g . 4 . 1 2 ) . The o n s e t o f t h e l a r g e a m p l i t u d e s i n i t i a l l y o c c u r s a t a p p r o x i m a t e l y 3.5s r e d u c e d t r a v e l t i m e . A l t h o u g h t h e a m p l i t u d e s a r e somewhat a t t e n u a t e d , t h e w a v e f o r m i s much t h e same f o r a r r i v a l s a t a p p r o x i m a t e l y 7.0s. A s i m p l e e x p l a n a t i o n f o r t h i s o b s e r v a t i o n i s t h a t t h e l a t e r a r r i v a l s r e p r e s e n t m u l t i p l e p h a s e s , i . e . due t o e n e r g y w h i c h has r e f l e c t e d once f r o m t h e w a t e r s u r f a c e b e f o r e p e n e t r a t i n g t h e s u b s t r a t u m . To c h e c k t h i s p o s s i b i l i t y , t h e two-way n o r m a l i n c i d e n c e t r a v e l t i m e o f 3.24s ( a s s u m i n g 2.4 km datum an d 1.48 km/s w a t e r v e l o c i t y ) was a d d e d t o r e l e v a n t s e c t i o n s o f t h e s u p e r i m p o s e d t r a v e l t i m e c u r v e o b t a i n e d w i t h t h e HRGLTZ r o u t i n e . As F i g . 4.12 shows, t h i s p r o c e d u r e r e s u l t e d i n b r a n c h e s w h i c h f i t t h e t r a v e l t i m e s o f t h e l a r g e a m p l i t u d e s e c o n d a r y a r r i v a l s r e a s o n a b l y w e l l . A t g r e a t e r d i s t a n c e s s u c h a f i t i s n o t c l e a r l y e v i d e n t . A p r o m i n e n t f e a t u r e o f t h e data b e y o n d 45 km ( s h o t 35) i s t h e p r e s e n c e o f s t r o n g a r r i v a l s w h i c h a p p e a r t o be m u l t i p l e s o f s e c o n d a r y p h a s e s . A more d e t a i l e d i n t e r p r e t a t i o n i n c l u d i n g t h e s e l a t e r s e c o n d a r y p h a s e s may r e v e a l t h e i r s i g n i f i c a n c e . 79 F i g u r e 4.12 R e c o r d s e c t i o n o f d a t a f r o m p r o f i l e 74-2 w i t h t h e s u p e r i m p o s e d t r a v e l t i m e c u r v e d e t e r m i n e d w i t h HRGLTZ ( l o w e r c u r v e ) and a s e t o f r e f r a c t i o n b r a n c h e s d e l a y e d by t h e n o r m a l i n c i d e n c e two- way t r a v e l t i m e ( 3 . 2 4 s ) , a s s u m i n g a w a t e r d e p t h o f 2.4 km ( u p p e r c u r v e ) . The r e s u l t a n t f i t i m p l i e s t h a t l a t e r a r r i v a l s a r e m u l t i p l e s . PROFILE 74-2 U J vt 10« X I To 2 0 2 5 30 as 4a 4 s s o 5 2> 5 la DISTANCE (KM) oo o 81 5. DISCUSSION OF RESULTS 5.1 D e t e r m i n i n g a P e t r o l o g i c M o d e l To d i s c u s s more c o m p l e t e l y t h e models o b t a i n e d f r o m t h e t r a v e l t i m e and a m p l i t u d e a n a l y s i s , a p e t r o l o g i c m odel c o n s i s t e n t w i t h t h e s e r e s u l t s i s f i r s t s o u g h t . A s a t i s f a c t o r y p e t r o l o g i c model must, however, t a k e i n t o a c c o u n t n o t o n l y s e i s m i c s t r u c t u r e , b u t a l s o t h e l i t h o l o g i e s o f s e a f l o o r d r e d g e s a m p l e s and t h e r e s u l t s o f s e i s m i c v e l o c i t y measurements on t h e s e s a m p l e s ( C h r i s t e n s e n and S a l i s b u r y , 1975) . S i n c e o p h i o l i t e s a r e r e g a r d e d as a l l o c h t h o n o u s s l a b s o f o c e a n i c c r u s t t h a t h a v e b e e n e m p l a c e d on t h e c o n t i n e n t a l m a r g i n s (Coleman and I r w i n , 1 9 7 4 ) , t h e i m p o r t a n c e o f r e l a t i n g t h e o p h i o l i t e s t o a p e t r o l o g i c m odel must a l s o be r e c o g n i z e d . S i n c e no d r e d g e sample a n a l y s i s a c c o m p a n i e d t h e DSS s t u d y , i t i s w o r t h w h i l e c o n s i d e r i n g o t h e r s e i s m i c r e s u l t s and t h e p e t r o l o g i c c o r r e l a t i o n s t h a t have been made w i t h them. The a n a l y s i s o f r e f r a c t i o n d a t a by R a i t t (1963) has i m p l i e d a r e l a t i v e l y s i m p l e s e i s m i c s t r u c t u r e f o r " n o r m a l " o c e a n i c c r u s t . V e l o c i t y and t h i c k n e s s v a l u e s o f R a i t t ' s model f o r t h e c r u s t b e l o w t h e u s u a l l y t h i n ( < 1 km) s e d i m e n t a r y l a y e r a r e g i v e n i n T a b l e I I . More r e c e n t l y , d e t a i l e d s o n o b u o y r e f r a c t i o n s t u d i e s have r e f i n e d t h i s model somewhat. P r o f i l e s c o n d u c t e d by T a l w a n i e t a l , (1971) o v e r t h e R e y k j a n e s R i d g e have i n d i c a t e d t h a t l a y e r 2 i s composed o f TABLE II* Layer Raitt's Seismic Model Velocity Thickness (km/s) (km) 2 (basement) 5.07 ± 0.63 1.71 ± 0.75 3 (oceanic) 6.69 ± 0.26 4.86 ± 1.42 Mantle 8.13 ± 0.24 Revised Model Based on Sonobuoy Data Velocity Thickness Layer (km/s) (km) 1 (sediments) 1.7-2.0 2A 2B 3A 3B Mantle 0.5 2.5 - 3.8 0.5 - 1.5 4.0 - 6.0 0.5 - 1.5 6.5 - 6.8 2.0 - 3.0 7.0 - 7.7 2.0 - 5.0 8.1 Petrologic Model Layer Lithology 1 unconsolidated sediments 2A 2B 3A 3B (fractured) basalt flows and pillows metabasalt, diabase gabbro (cumulate) gabbro and peridotite Mantle peridotite * Velocity-depth models f o r the oceanic crust based on early r e f r a c t i o n results (after R a i t t , 1963) and the re s u l t s of more recent sonobuoy data (after Peterson et a l , 1974). The pe t r o l o g i c model correlated with the seismic layers is s i m i l a r to the model given by Peterson et a l (1974 ). 83 two v e l o c i t y l a y e r s . S t u d i e s by S u t t o n e t a l (1971) h a v e r e v e a l e d t h e p r e s e n c e o f a h i g h v e l o c i t y b a s a l c r u s t a l l a y e r i n t h e P a c i f i c B a s i n m e a n i n g t h a t l a y e r 3 s h o u l d a l s o be s u b d i v i d e d . A l t h o u g h t h e q u a n t i t y o f so n o b u o y d a t a i s l i m i t e d , t h e f a i r l y c o n s i s t e n t o c c u r r e n c e o f t h e s e v e l o c i t y s u b d i v i s i o n s has l e d t o a m o d i f i c a t i o n o f R a i t t ' s m o d e l . The n o m e n c l a t u r e 2A and 2B, and 3A and 3B has s u b s e q u e n t l y b e e n i n t r o d u c e d t o s p e c i f y t h e s u b d i v i s i o n s . A s e i s m i c c r u s t a l model b a s e d on so n o b u o y d a t a has b e e n c o n s t r u c t e d by P e t e r s o n e t a l , (1974) and i s a l s o shown i n T a b l e I I . Owing t o t h e comp l e x n a t u r e o f t h e g e o l o g i c p r o c e s s e s r e s p o n s i b l e f o r t h e e v o l u t i o n o f t h e o c e a n i c c r u s t , no s i n g l e p e t r o l o g i c model has b e e n f o u n d w h i c h c o n s i s t e n t l y a c c o u n t s f o r t h e g e o p h y s i c a l and g e o l o g i c a l d a t a o b t a i n e d i n a l l s i t u a t i o n s . V a r i o u s m o d els h a v e b e e n p r o p o s e d among s e v e r a l r e s e a r c h e r s on t h e b a s i s o f s e i s m i c r e s u l t s t o g e t h e r w i t h d a t a c o l l e c t e d f r o m d r e d g e s a m p l e s ( e . g . C h r i s t e n s e n , 1970; Cann, 1970) o r f r o m t h e o p h i o l i t e c o m p l e x e s ( e . g . P e t e r s o n e t a l , 1974; Mqores and J a c k s o n , 1 9 7 4 ) . No a t t e m p t w i l l be made h e r e t o r e v i e w i n d e t a i l t h e p r o p o s e d m o d e l s , o r p o i n t o u t t h e i r d i f f e r e n c e s . But f o r t h e p u r p o s e o f d i s c u s s i n g t h e v e l o c i t y d e p t h c u r v e s i n F i g . 4.11, a p e t r o l o g i c model s i m i l a r t o t h e one d e s c r i b e d by P e t e r s o n e t a l , (1974) w i l l be assumed. The model i s p r e s e n t e d i n T a b l e I I and i s i n t e n d e d t o be q u i t e g e n e r a l . The l i t h o l o g i e s i n c l u d e a b r o a d r a n g e o f r o c k t y p e s , e.g. p e r i d o t i t e , and no d e p t h has b e e n s p e c i f i e d a l t h o u g h a 84 s c a l e i s i m p l i e d by t h e i n d i c a t e d c o r r e l a t i o n s w i t h t h e s e i s m i c l a y e r s . 5.2 The O c e a n i c C r u s t i n t h e R e g i o n o f E x p l o r e r R i d g e The v e l o c i t y - d e p t h models d e r i v e d f r o m t h e s e i s m i c d a t a a r e c o m b i n e d w i t h t h e p e t r o l o g i c model o f S e c . 5.1 and t h e t e c t o n i c s i n v o l v i n g J u a n de F u c a p l a t e , t o p r e s e n t a more c o m p l e t e d e s c r i p t i o n o f t h e c r u s t i n t h e r e g i o n o f E x p l o r e r R i d g e . A l t h o u g h t h e d i s c u s s i o n i s o r g a n i z e d a c c o r d i n g t o t h e l a y e r s e q u e n c e commonly a s s o c i a t e d w i t h t h e o c e a n i c c r u s t , i t s h o u l d be e m p h a s i z e d t h a t t h e m o d els o f F i g . 4.11 do n o t s u g g e s t homogeneous l a y e r s . The v e l o c i t y g r a d i e n t s were i n t r o d u c e d by f i t t i n g b o t h t r a v e l t i m e s and a m p l i t u d e s d u r i n g t h e i n t e r p r e t a t i o n p r o c e d u r e . T h e r e f o r e , t h e t e r m " l a y e r " u s e d i n o t h e r m o d e l s w i l l now c o r r e s p o n d t o t h e d e p t h i n t e r v a l where t h e v e l o c i t y g r a d i e n t i s c o n t i n u o u s . UPPER CRUSTAL STRUCTURE A l l f o u r v e l o c i t y - d e p t h models i n F i g . 4.11 show t h a t t h e i m m e d i a t e s u b - b o t t o m m a t e r i a l has a v e r y h i g h v e l o c i t y g r a d i e n t - t h e v e l o c i t y i n c r e a s e s f r o m 2.5 t o 4.2 km/s o v e r a 0.5 km i n t e r v a l . S i n c e v e l o c i t i e s o v e r 3 km/s a r e n o t c h a r a c t e r i s t i c o f s e d i m e n t s , t h e s e d i m e n t a r y c o v e r i s c o n s i d e r e d m i n i m a l . T h a t t h e r e a r e l i t t l e s e d i m e n t s i s a l s o c o n f i r m e d by t h e c a l c u l a t e d w a t e r d e p t h s . As t h e d i a g r a m s show (s e e F i g . 4.4 o r F i g . 4 . 1 1 ) , t h e s e r a n g e f r o m 85 2.4 t o 2.6 km and compare f a v o r a b l y w i t h t h e datum o f 2.4 km. I n a d d i t i o n , t h e l a c k o f p e n e t r a t i o n on c o n t i n u o u s s e i s m i c p r o f i l i n g i n t h e r e g i o n i n d i c a t e s t h a t t h e a c o u s t i c b asement ( b e l o w t h e s e d i m e n t s ) i s t h e p r e d o m i n a n t s u b - b o t t o m f e a t u r e w i t h some i s o l a t e d a r e a s c o n t a i n i n g a t h i n v e n e e r ( < 250 m) o f s e d i m e n t s (A.G. T o m l i n s o n , p e r s . comm., 1 9 7 5 ) . From 0.5 t o 1.0 km s u b - b o t t o m , t h e v e l o c i t y has a n e a r l y c o n s t a n t v a l u e o f 4.2 km/s. The g r a d i e n t i n c r e a s e s c o n s i d e r a b l y o v e r t h e n e x t 0.4 km, as t h e v e l o c i t y ( d e p e n d i n g on t h e p r o f i l e ) r e a c h e s 5.2 - 6.3 km/s a t a b o u t 1.4 km. The 1.4 km o f c r u s t d i s c u s s e d so f a r c a n be a s s o c i a t e d w i t h l a y e r 2A, t h e u p p e r p a r t o f t h e b a s a l t i c l a y e r . F o r most o f t h i s d e p t h , t h e r a n g e i n o b s e r v e d v e l o c i t i e s i s n e a r l y t h e same as t h e 2.5-3.8 km/s r a n g e o b t a i n e d f r o m t h e a n a l y s i s o f b a s a l t d r e d g e s a m p l e s ( P e t e r s o n e t a l , 1 9 7 4 ) . The low s e i s m i c v e l o c i t i e s and l a r g e g r a d i e n t s w i t h i n t h e u p p e r 0.5 km may be due t o c r a c k s and p o r e s p a c e s i n t h e p i l l o w b a s a l t s as s u g g e s t e d by Matthews e t a l ( 1 9 7 1 ) . As t h e y p o i n t o u t , v e l o c i t i e s a r e s t r o n g l y i n f l u e n c e d by c a v i t i e s i n r o c k s when c o n f i n i n g p r e s s u r e s a r e l e s s t h a n 1 k b a r . T h e r e f o r e , a t d e p t h s where t h e h y d r o s t a t i c p r e s s u r e i s g r e a t e r t h a n 1 k b a r , t h e v e l o c i t y w i l l show a s m a l l e r change w i t h i n c r e a s i n g d e p t h . T h i s may e x p l a i n t h e n e a r l y c o n s t a n t v e l o c i t y (4.2 km/s) o b s e r v e d o v e r t h e n e x t 0.5 km. A l s o , e x t e n s i v e f r a c t u r i n g may have t a k e n p l a c e o n l y i n t h e up p e r m o s t p a r t o f t h e c r u s t t o a c c o u n t f o r t h e l i m i t e d r a n g e o f t h e l o w e s t v e l o c i t i e s . S u p p o r t f o r t h e s e i d e a s may be c l a i m e d 86 f r o m d e d g i n g r e s u l t s i n an a r e a n e a r t h e p r o f i l e l i n e s o f t h i s s t u d y . F o r ex a m p l e , v e s i c u l a r p i l l o w l a v a s h a v e b e e n d r e d g e d f r o m t h e n o r t h e r n end o f E x p l o r e r R i d g e n e a r P a u l R e v e r e Ridge, ( S r i v a s t a v a e t a l , 1 9 7 1 ) . A t t h e s o u t h e r n end, on t h e So v a n c o f r a c t u r e z o n e , p i l l o w s and p i l l o w f r a g m e n t s c o m p r i s e many o f t h e b a s a l t s a m p l e s o b t a i n e d t h e r e ( B a r r and C h a s e , 1 9 7 4 ) . F o r p r o f i l e s 74-2 and 2R a v e l o c i t y o f a b o u t 5.4 km/s i s n o t e d a t a d e p t h o f 1.4 km, w h i l e f o r 74-1 and IR t h e v e l o c i t y i s c l o s e r t o 6.0 km/s a t a p p r o x i m a t e l y t h e same d e p t h . W i t h t h e e x c e p t i o n o f 74-2 and t h e d o w n - f a u l t s t r u c t u r e s o f 74-1 and IR, t h e s e v a l u e s change o n l y s l i g h t l y o v e r t h e n e x t 1.0 km i n t e r v a l . T h e s e v e l o c i t i e s a r e w i t h i n t h e r a n g e o f l a b o r a t o r y m e a s u r e d v e l o c i t i e s (4.0 - 6.0 km/s) ( P e t e r s o n e t a l , 1974) f o r m e t a b a s a l t s w h i c h a r e p r o p o s e d t o c o n s t i t u t e l a y e r 2B. I n t h i s c a s e no d r e d g e s a m p l i n g r e s u l t s were a v a i l a b l e t o s u p p o r t t h e p r e s e n c e o f s u c h r o c k s . However, t h e y o u t h o f t h e p i l l o w l a v a s and t h e h i g h h e a t f l o w v a l u e s o b t a i n e d i n t h e v i c i n i t y o f E x p l o r e r R i d g e ( L i s t e r , 1972) i m p l y t h a t h i g h t h e r m a l g r a d i e n t s a r e p r e s e n t and c o u l d p r o d u c e s u c h l o w - g r a d e metamorphism i n t h e b a s a l t i c l a y e r . T h e r e f o r e , i f t h i s 1.0 km l a y e r i s assumed t o c o r r e s p o n d t o l a y e r 2B, t h e t o t a l t h i c k n e s s o f t h e l a y e r 2 s y s t e m i s a r o u n d 2.5 km. Such a t h i c k n e s s i s g r e a t e r t h a n t h e m e d i a n v a l u e o f 1.4 km f o r t h e P a c i f i c Ocean b u t n o t u n i q u e ( c f . S h o r and R a i t t , 19 69). The f a c t t h a t s u c h r e s u l t s a r e o b t a i n e d i n t h e v i c i n i t y o f young c r u s t i s i n t e r e s t i n g and 87 w i l l be commented on l a t e r . THE LOWER CRUST AND MOHOROVICIC DISCONTINUITY The m o d e ls o f F i g . 4.11 i m p l y t h a t t h e v e l o c i t y - d e p t h s t r u c t u r e i s much more v a r i e d i n t h e l o w e r p a r t o f t h e c r u s t t h a n i t i s i n t h e u p p e r p a r t . F o r p r o f i l e 74-2R v e l o c i t i e s c h a r a c t e r i s t i c o f t h e l a y e r 3 s y s t e m o f P e t e r s o n e t a l , (1974) a r e f o u n d o v e r a 6.6 km i n t e r v a l . F o r 74-2, where g r a d i e n t s a r e more p r o n o u n c e d t h r o u g h o u t t h e l o w e r c r u s t , i t i s d i f f i c u l t t o d e c i d e on a l a y e r 3 e q u i v a l e n t . I f t h e s m a l l d i s c o n t i n u i t y i n t h e g r a d i e n t a t a p p r o x i m a t e l y 8 km i s i n t e r p r e t e d as a l a y e r 2 - l a y e r 3 b o u n d a r y , t h e l a y e r 3 t h i c k n e s s w o u l d be a r o u n d 3.7 km. T h i s w o u l d mean a d i f f e r e n c e o f a l m o s t 3 km between t h i c k n e s s v a l u e s o b t a i n e d f r o m two p r o f i l e s , b o t h o f w h i c h c r o s s t h e r i d g e . H e n c e , i t i s p r o b a b l y more a p p r o p r i a t e t o s i m p l y c o n s i d e r t h e g r a d i e n t c o n t i n u o u s , as i t i s f o r 74-2R, and assume t h a t t h e 1.0 km t h i c k l a y e r 2B i n t e r p r e t e d on o t h e r p r o f i l e s i s n o t d i s c e r n i b l e . I f t h e u p p e r 1 km o f t h e g r a d i e n t i s c o n s i d e r e d p a r t o f l a y e r 2, a t h i c k n e s s o f a b o u t 6.9 km c o u l d be t a k e n as t h e a v e r a g e v a l u e f o r t h e l a y e r 3 s y s t e m b e l o w t h e r i d g e . The e s s e n t i a l d i f f e r e n c e b e t w e e n t h e v e l o c i t y - d e p t h m o d e ls o f p r o f i l e s 74-2 and 74-2R w o u l d t h e n be t h e shape o f t h e g r a d i e n t i n t h e l o w e r c r u s t . F o r 74-2R t h e v e l o c i t y c h a n g e s by o n l y 1 km/s o v e r a 5.5 km i n t e r v a l and t h e n i n c r e a s e s t o 7.8 km/s w i t h i n 1 km. W i t h 74-2, on t h e o t h e r hand, t h e v e l o c i t y i n c r e a s e s s t e a d i l y f r o m 6.0 t o 7.8 km/s o v e r t h e e n t i r e d e p t h r a n g e a s s o c i a t e d w i t h l a y e r 3. 88 S i n c e p r o f i l e s 74-2 and 2R a r e r e v e r s e d and e x t e n d to n e a r l y 80 km, t h e 7.8 km/s o b t a i n e d a t a d e p t h o f 11.8 km f o r 74-2 and 11.2 km f o r 74-2R i s t a k e n as t h e medium v e l o c i t y o f t h e u p p e r m o s t m a n t l e i n t h e d i r e c t i o n p e r p e n d i c u l a r t o t h e r i d g e . An a v e r a g e o f 9.1 km i s t h e r e f o r e o b t a i n e d f o r t h e t o t a l s u b - b o t t o m c r u s t a l t h i c k n e s s n e a r t h e r i d g e c r e s t . S u ch a v a l u e i s i n c o n s i s t e n t w i t h t h e r e s u l t s o f s u r v e y s n e a r o t h e r a x i a l z o n e s i n w h i c h a t h i n n e r c r u s t o f t h e o r d e r o f 4.4 km s u b - b o t t o m i s d e t e r m i n e d ( L e P i c h o n e t a l , 1965; Keen and T r a m o n t i n i , 1970; P o e h l s , 1 9 7 4 ) . However, i n a s t u d y o f t h e P a c i f i c b a s i n n o r t h w e s t o f E x p l o r e r R i d g e , S r i v a s t a v a e t a l , (1971) have i n t e r p r e t e d s e i s m i c r e f r a c t i o n d a t a t o show a c r u s t a l s e c t i o n w i t h a l a y e r 3 t h i c k n e s s o f 5.5 km and a d e p t h b e l o w s e a l e v e l t o t h e Moho o f a b o u t 11 km. T h e s e v a l u e s a r e s i m i l a r t o t h o s e f o r t h e a v e r a g e P a c i f i c b a s i n c r u s t ( S h o r and R a i t t , 1969) and a l s o a g r e e r e a s o n a b l y w e l l w i t h t h e d e p t h s ( n o t v e l o c i t i e s ) f r o m p r o f i l e s 74-2 and 2R. The s i m i l a r i t y o f t h e c r u s t a l d e p t h f r o m t h i s s t u d y and t h a t o f S r i v a s t a v a e t a l , (1971) may be r e l e v a n t t o some o f t h e i d e a s r e g a r d i n g p l a t e t e c t o n i c s i n t h e n o r t h e a s t P a c i f i c . McManus e t alp (1972) p r o p o s e t h a t t h e n o r t h w e s t - s a l i e n t b l o c k o f t h e J u a n de F u c a p l a t e ( r e g i o n b e t w e e n J u a n de F u c a and E x p l o r e r r i d g e s ) i s b e i n g s e v e r e d f r o m t h e r e m a i n d e r o f * t h e p l a t e due t o t h e r e o r i e n t a t i o n o f E x p l o r e r R i d g e w h i c h b e g a n a b o u t 7.5 m.y. ago. The Queen C h a r l o t t e F a u l t may now e x t e n d s o u t h t o i n t e r s e c t t h e n o r t h e r n end o f J u a n de F u c a R i d g e (McManus e t a l , 19 72; B a r r and C h a s e , 19 74) 89 m e a n i n g t h a t E x p l o r e r R i d g e i s no l o n g e r an a c t i v e s p r e a d i n g c e n t e r . I f s u c h a s i t u a t i o n e x i s t s , t h e r e g i o n o f t h e r i d g e w o u l d be e x p e c t e d t o c o o l s i n c e t h a t t i m e e n a b l i n g a more u s u a l n o r t h e a s t e r n P a c i f i c c r u s t t o e v o l v e i n t h e v i c i n i t y o f t h e ( c u r r e n t l y i n a c t i v e ) r i d g e c r e s t . One q u e s t i o n a b l e a s p e c t o f t h i s s u g g e s t i o n i s , however, w h e t h e r a p e r i o d o f l e s s t h a n 1 m.y. s i n c e t h e most r e c e n t s p r e a d i n g t o o k p l a c e i s s u f f i c i e n t f o r t h e c r u s t t o e v o l v e t o t h i s e x t e n t . Due t o t h e complex g e o l o g y and t e c t o n i c s a s s o c i a t e d w i t h a s p r e a d i n g c e n t e r , t h i s i s n o t r e a d i l y a n s w e r a b l e . F o r p r o f i l e s 74-1 and IR t h e l o w e r v e l o c i t y - d e p t h s t r u c t u r e shown i n F i g . 4.11 i s more c o m p l i c a t e d t h a n i t i s f o r 74-2 and 2R. B o t h t h e u p - f a u l t ( s o l i d c u r v e s ) and down- f a u l t ( d a s h e d c u r v e s ) s t r u c t u r e s a r e shown, and t h e y i n d i c a t e f a u l t t h r o w s on t h e 7.3 km/s r e f r a c t o r o f 4.9 and 5.1 km f o r 74-1 and IR, r e s p e c t i v e l y . A l t h o u g h t h e p r o f i l e s were r e v e r s e d and t h e medium v e l o c i t y f o r t h e d e e p e s t p e n e t r a t i o n o f t h e r a y s was w e l l d e t e r m i n e d o u t t o 80 km, i t was q u e s t i o n a b l e as t o w h e t h e r an u p p e r m a n t l e v e l o c i t y h a d b e e n o b s e r v e d . I n o t h e r w o r d s , do t h e m o d els r e p r e s e n t f a u l t s on t h e Moho? The i n t e r p r e t a t i o n o f t h i s v e l o c i t y was o f f u r t h e r i m p o r t a n c e s i n c e a v e l o c i t y o f 7.3 km/s must be r e g a r d e d as a n o m a l o u s l y low f o r t h e u p p e r m a n t l e . I t i s , however, w i t h i n t h e r a n g e o f v e l o c i t i e s d e t e r m i n e d i n o t h e r r e f r a c t i o n s u r v e y s n e a r r i d g e c r e s t s ( L e P i c h o n e t a l , 1965; Keen and T r a m o n t i n i , 19 7 0 ) . 90 One way o f r e s o l v i n g t h e q u e s t i o n was t o assume t h a t 7.3 km/s was a v e l o c i t y o f t h e l o w e r c r u s t and c a l c u l a t e t h e d e p t h t o Moho f o r a 7.8 km/s u p p e r m a n t l e i n w h i c h h e a d waves a r e r e c o r d e d as f i r s t a r r i v a l s a t a d i s t a n c e o f 80 km and b e y o n d . U s i n g t h e v e l o c i t y and l a y e r t h i c k n e s s v a l u e s f r o m t h e t r a v e l t i m e model ( T a b l e I ) , a computed Moho d e p t h o f 18.7 km was o b t a i n e d . T h i s v a l u e i s w e l l o u t s i d e t h e 10-12 km r a n g e n o r m a l l y a s s o c i a t e d w i t h t h e o c e a n i c c r u s t and was t h e r e f o r e c o n s i d e r e d u n r e a l i s t i c . R e g a r d i n g 7.3 km/s as an u p p e r m a n t l e v e l o c i t y i m p l i e s t h a t t h e c r u s t i s e x t r e m e l y t h i n on t h e u p - f a u l t s t r u c t u r e , a b o u t 2.6 km s u b - b o t t o m , o r 5 km t o t a l . A l s o , i n t h e u p - f a u l t s t r u c t u r e , l a y e r 3 v e l o c i t i e s a p p e a r t o be m i s s i n g o r s l o w , e.g. t h e v e l o c i t i e s a r o u n d 5.8 and 6.2 km/s f o r p r o f i l e s 74-1 a n d IR, r e s p e c t i v e l y . The t h i c k l a y e r 2 s y s t e m n o t e d i n t h e p r e v i o u s d i s c u s s i o n , and t h e low l a y e r 3 v e l o c i t i e s o b s e r v e d h e r e may be due t o a t h e r m a l d e p r e s s i o n o f l a y e r 3 v e l o c i t i e s as s u g g e s t e d b y C h r i s t e n s e n and S a l i s b u r y ( 1 9 7 5 ) . T hey c o n c l u d e t h a t a b n o r m a l l a y e r 2 t h i c k n e s s , a t h i n o r a b s e n t l a y e r 3, and t h e p r e s e n c e o f a b n o r m a l m a n t l e v e l o c i t i e s a r e i n t e r r e l a t e d phenomena a t many y o u n g s i t e s . A n o t h e r p o s s i b i l i t y i s t h a t e x t e n s i v e f r a c t u r i n g , p e r h a p s r e l a t e d t o f a u l t i n g , has o c c u r r e d i n l a y e r 3 and d e c r e a s e d t h e s e i s m i c v e l o c i t i e s . As p o i n t e d o u t e a r l i e r , t h e shape o f t h e g r a d i e n t w h i c h e x t e n d s t o t h e Moho on t h e d o w n - f a u l t s t r u c t u r e s o f p r o f i l e s 74-1 and IR i s n o t n e c e s s a r i l y r e a l i s t i c , s i n c e 91 b o t h t r a v e l t i m e s and a m p l i t u d e s c o u l d n o t be u s e d t o c o n s t r a i n t h i s p o r t i o n o f t h e m o d e l . T h e r e f o r e , i t i s d i f f i c u l t t o s a y w h e t h e r v e l o c i t i e s c h a r a c t e r i s t i c o f l a y e r 3 a r e p r e s e n t o v e r any a p p r e c i a b l e d e p t h e x t e n t i n t h e down- f a u l t e d r e g i o n . B a s e d on t h e l o c a t i o n o f p r o f i l e l i n e s ( F i g . 1.1), t h e l o w e r v e l o c i t y - d e p t h s t r u c t u r e i n t h e down- f a u l t e d r e g i o n o f 74-1R s h o u l d be s i m i l a r t o t h e s t r u c t u r e d e t e r m i n e d f r o m 74-2. However, a Moho d e p t h o f 11.8 km was o b t a i n e d f r o m 74-2 w h i l e t h e d o w n - f a u l t d e p t h f r o m 74-1R was o n l y 9.9 km. P a r t o f t h e d i f f e r e n c e may be due t o t h e f a c t t h a t 74-2 r e a c h e d t h e deep c r u s t a l s t r u c t u r e c l o s e r t o t h e r i d g e t h a n 74-1R d i d . The e r r o r s a s s o c i a t e d w i t h t h e v e l o c i t i e s u s e d i n c o m p u t i n g d e l a y t i m e s f o r m o d e l l i n g t h e f a u l t may f u r t h e r a c c o u n t f o r t h i s d i f f e r e n c e . The f a u l t s p r o p o s e d i n t h e r e g i o n o f p r o f i l e s 74-1 and 74-1R r e p r e s e n t a s i g n i f i c a n t r e s u l t o f t h e d a t a i n t e r p r e t a t i o n . Some a t t e n t i o n i s now g i v e n t o t h e g e o l o g i c and t e c t o n i c e v i d e n c e w h i c h s u p p o r t a f a u l t m o d e l . E v i d e n c e b a s e d on t h e s e i s m i c o b s e r v a t i o n s was m e n t i o n e d i n S e c . 4.2. A c c o r d i n g t o t h e f i r s t a r r i v a l d a t a d i s c u s s e d i n Se c . 4.2, t h e p r o p o s e d f a u l t s s h o u l d be p r e s e n t i n t h e r e g i o n n e a r t h e s t a r t o f e a c h p r o f i l e ( < 25 km). F o r p r o f i l e 74-1R t h e l o c a t i o n c o i n c i d e s w i t h b o t h a s t e e p t o p o g r a p h i c g r a d i e n t and t h e edge o f a p e c u l i a r j u t i n t h e m a g n e t i c anomaly p a t t e r n ( F i g . 1 . 1 ) . To t h e n o r t h , on t h e u p - f a u l t s i d e , t h e anomaly i s o f n o r m a l p o l a r i t y and a s s o c i a t e d w i t h t h e B r u h n e s e p o c h ; t o t h e s o u t h , t h e anomaly i s o f r e v e r s e d 92 polarity and presumably represents older rocks. In addition, a CSP profile over the topographic high shows evidence of high angle faulting, with negligible vertical offset, from the bottom to the maximum penetration depth of 0.6s two-way time, in approximately the same location as the proposed fault (R iL. Chase, pers. comm., 1975). No such data is present in the immediate vicinity of the starting point of profile 74-1 to support a fault interpretation. However, the existence of faults for both profiles would be consistent with present ideas regarding the regional tectonics. According to Atwater (1970), the motion of the Juan de Fuca plate relative to the American plate is 2.5 cm/yr toward the northeast, and thus implies subduction. Paul Revere Ridge trends northwest along the northern end of Explorer Ridge and may even be within the subduction zone. Evidence for this is shown by basalts which dip beneath the ridge and sediments which thicken away from the ridge toward the base of the continental slope where they are folded and faulted (Srivastava et al, 1971). If this zone is considered broad enough to include the steep topography which profile 74-IR crossed, a sub due tion-related fault may be possible. For profile 74-1 it is noted from Fig. 1.1 that the initial part of the profile is near the northern extent of the Sovanco transform fault zone which joins Juan de Fuca Ridge to Explorer Ridge. As previously mentioned, a reorientation of Explorer Ridge began about 7.5 m.y. ago. The transform faults of the Sovanco fracture zone then 93 represent "leaky" transform f a u l t s (Menard and Atwater, 1969) due to t h e i r clockwise r o t a t i o n (McManus et a l , 1972). Thus, a component of v e r t i c a l o f f s e t associated with t h i s process i s not unreasonable. Another r e s u l t which deserves further attention i s the difference i n mantle v e l o c i t i e s p a r a l l e l (7.3 km/s) and perpendicular (7.8 km/s) to the ridge crest. The existence of seismic anisotropy i n the uppermost mantle i s well established (Christensen and Salisbury, 1975) and i s the most a t t r a c t i v e explanation for these r e s u l t s . In experiments designed to measure the magnitude of anisotropic e f f e c t s the general conclusion has been that the ve l o c i t y i s a maximum in a d i r e c t i o n approximately perpendicular to the ridge and a minimum p a r a l l e l to i t (Raitt et a l , 1969; Keen and Barrett, 1971). The work of Keen and Barrett i s p a r t i c u l a r l y relevant to this study as i t was undertaken i n a region farther to the northwest on the P a c i f i c p l a t e . While the o v e r a l l v e l o c i t i e s were higher than those determined here, the magnitude of the anisotropic e f f e c t was about 8%. Assuming the v e l o c i t i e s determined along the two DSS p r o f i l e s i n thi s study represent maximum and minimum values, the anisotropic e f f e c t i s l%y which compares favorably with the above percentage. This suggests that the anisotropy due to c r y s t a l realignment as a resu l t of spreading or flow of " s o f t " material (Hess, 1964), is "frozen" and completed at or near the ridge. Because the material i s younger and warmer i n this region the magnitudes of the v e l o c i t i e s are 94 l e s s . As t h e r o c k s s p r e a d f u r t h e r , t h e y c o o l , become more c o m p a c t e d and as a r e s u l t h a v e a h i g h e r o v e r a l l medium v e l o c i t y . B u t t h e a n i s o t r o p y i s m a i n t a i n e d as t i m e p r o g r e s s e s . The i m p l i c a t i o n s o f t h e s e i s m i c m o d els on t h e p e t r o l o g i c s t r u c t u r e o f t h e l o w e r c r u s t / u p p e r m a n t l e i s now c o n s i d e r e d . B e c a u s e o f t h e l a t e r a l d i s c o n t i n u i t i e s i n t r o d u c e d by t h e f a u l t s o f p r o f i l e s 74-1 and IR, most a t t e n t i o n i s g i v e n to t h e models o f 74-2 and 2R. The s e i s m i c m o d e l s i m p l y t h a t a t h i c k (6.9 km), w e l l d e v e l o p e d l a y e r 3 u n i t e x i s t s b e l o w t h e r i d g e c r e s t . A model f o r t h e e v o l u t i o n o f o c e a n i c c r u s t p r e s e n t e d by Cann (1970, 1974) may be r e l e v a n t f o r e x p l a i n i n g t h i s . Cann d e s c r i b e s t h e f o r m a t i o n o f t h e l o w e r c r u s t i n terms o f t h e c r y s t a l l i z a t i o n o f an a x i a l magma chamber. The p r o c e s s a c t u a l l y i n v o l v e s two components. A c u m u l a t e l a y e r i s f o r m e d a t t h e b a s e o f t h e chamber due t o f r a c t i o n a l c r y s t a l l i z a t i o n o f t h e magma. A t t h e t o p o f t h e chamber t h e magma i s c o o l e d by h e a t c o n d u c t i o n t h r o u g h t h e o v e r l y i n g c r u s t t o f o r m an u p p e r u n i t o f i s o t r o p i c g a b b r o . The l i t h o l o g i c a l s e q u e n c e i s t h e r e f o r e much t h e same as t h e one p r e s e n t e d i n S e c . 5.1. T h a t i s , t h e l a y e r 3 s y s t e m w i l l c o n s i s t , f r o m t o p t o b o t t o m , o f u n a l t e r e d g a b b r o f o l l o w e d by a l a y e r o f c u m u l a t e g a b b r o and f i n a l l y t h e Moho w h i c h i s t h e b o u n d a r y b e t w e e n g a b b r o and p e r i d o t i t e . The b o u n d a r y between t h e u n a l t e r e d g a b b r o and t h e c u m u l a t e s i s t h o u g h t t o c o r r e s p o n d t o t h e l a y e r 3A-3B s e i s m i c b o u n d a r y ( P e t e r s o n e t a l , 1974) w h i c h has b e e n d e t e c t e d by. S u t t o n e t a l ( 1 9 7 1 ) . 95 One explanation for the thick layer 3 unit below Explorer Ridge i s , therefore, that i t represents such a magma chamber which has cooled since the ridge ceased being an active spreading center. Based on the magnetic anomalies (Brunhes), spreading has occurred i n the Explorer area within the l a s t 0.7 m.y. This would imply that a rapid cooling process has taken place i n order for the near normal crust to develop at a spreading s i t e . Such a cooling rate may be reasonable, however, since the lack of sediments and increased permeability due to fr a c t u r i n g within the crust (an expected consequence of the i n t e r n a l deformation of Juan de Fuca plate suggested by McManus et a l , [1972]) would improve the e f f i c i e n c y of a mechanism such as hydrothermal convection ( L i s t e r , 1972) in cooling an a x i a l magma chamber. A f a i r l y rapid cooling process i s also consistent with the fact that no basal layer (layer 3B) was detected on the seismic data. The velocity-depth models of p r o f i l e s 74-2 and 2R show a nearly constant gradient throughout the lower crust implying that no s i g n i f i c a n t cumulate layer has developed. Or, equivalently, the rapid freezing of the magma chamber has led to a layer 3 composed almost e n t i r e l y of i s o t r o p i c gabbro. The sharp increase i n the gradient within the f i n a l 1 km of the model for 74-2R may perhaps be due to the presence of a thin cumulate layer. The fact that mantle anisotropy was observed in this study adds support to a p e r i d o t i t e composition for the upper mantle. Olivine i s the chief mineral constituent of p e r i d o t i t e s and the s t a t i s t i c a l o r i e n t a t i o n of o l i v i n e c r y s t a l s has been regarded as the cause for the widely observed anisotropy i n the uppermost mantle (Hess, 1964). 97 6. CONCLUSION The s i g n i f i c a n c e of u t i l i z i n g both traveltimes and amplitudes i n the i n t e r p r e t a t i o n of marine r e f r a c t i o n data has been c l e a r l y i l l u s t r a t e d . The most notable outcome of th i s approach i s the introduction of gradients into the velocity-depth models. V e l o c i t y gradients are expected since they r e f l e c t v a r i a t i o n s with depth of density, thermal conditions, and petrology. In general, homogeneous layered models are inadequate for providing a detailed d e s c r i p t i o n of the oceanic crust. ' Results from the r e f r a c t i o n data along the reversed DSS p r o f i l e perpendicular to Explorer Ridge indicate a t o t a l c r u s t a l thickness of about 11.5 km near the ridge crest. This was an unexpected r e s u l t and i s a t y p i c a l of other a x i a l zones. The velocity-depth structure of the 1.4 km sub-bottom material i s s i m i l a r on either side of the ridge, but very d i f f e r e n t v e l o c i t y gradients appear i n the lower crust (see Fig . 4.11). The v e l o c i t y of the uppermost mantle i s 7.8 km/s in a d i r e c t i o n perpendicular to the ridge. A s i m i l a r reversed DSS p r o f i l e recorded p a r a l l e l to Explorer ridge and 60 km east of i t showed some unusual results at either end. V e l o c i t i e s of 7.3 km/s were determined at depths of about 5 km, only 2.6 km sub-bottom. On an o f f s e t and delayed branch at greater distances, s i m i l a r v e l o c i t i e s 98 were c a l c u l a t e d . A f a u l t zone w i t h a b o u t 5 km o f thr o w i s s u g g e s t e d t o e x p l a i n t h e r e s u l t s Csee F i g . 4 . 1 1 ) . A d d i t i o n a l e v i d e n c e b a s e d on t o p o g r a p h y and t h e m a g n e t i c a n o m a l y p a t t e r n i s u s e d t o s u p p o r t t h i s h y p o t h e s i s f o r t h e n o r t h e r n end o f t h e p r o f i l e . A l t h o u g h t h e r e i s l i t t l e s u p p o r t i n g e v i d e n c e o f t h i s t y p e f o r t h e p r o p o s e d s i m i l a r f e a t u r e a t t h e s o u t h e r n end, f a u l t i n g w o u l d be c o n s i s t e n t w i t h t h e t e c t o n i c a c t i v i t y t h a t has b e e n s u g g e s t e d f o r t h e a r e a . A s s u m i n g t h a t t h e 7.3 km/s m a t e r i a l i s r e p r e s e n t a t i v e o f t h e u p p e r m a n t l e , an a s s u m p t i o n n e c e s s i t a t e d by d a t a t o a d i s t a n c e o f 80 km, t h e t o t a l c r u s t a l t h i c k n e s s a t e i t h e r e nd o f t h e p r o f i l e p a r a l l e l i n g E x p l o r e r R i d g e i s a b o u t 5.0 km w h i l e on t h e deep s i d e o f t h e f a u l t i t i s 9.9 km. The l a t t e r f i g u r e compares o n l y m o d e r a t e l y w e l l w i t h t h e 11.8 km c r u s t a l t h i c k n e s s d e t e r m i n e d f o r t h e same r e g i o n f r o m d a t a a l o n g p r o f i l e 74-2, p e r p e n d i c u l a r t o t h e r i d g e . D i f f e r e n c e s a r e e x p e c t e d and w o u l d be due a t l e a s t p a r t i a l l y t o t h e d i f f e r e n t v e l o c i t i e s w h i c h were u s e d i n m o d e l l i n g t h e f a u l t and t h e c u m u l a t i v e e r r o r s w h i c h a c c r u e i n t h e c a l c u l a t i o n s . V a r i a t i o n due t o a n i s o t r o p y i s s u g g e s t e d t o e x p l a i n t h e d i f f e r e n c e i n t h e u p p e r m a n t l e v e l o c i t i e s d e t e r m i n e d p a r a l l e l and p e r p e n d i c u l a r t o t h e r i d g e . I f t h e c a l c u l a t e d v a l u e s o f 7.3 and 7.8 km/s r e p r e s e n t t h e minimum and maximum, r e s p e c t i v e l y , t h e v a r i a t i o n due t o a n i s o t r o p y i s a b o u t 1%. T h i s compares w e l l w i t h t h e 8% a n i s o t r o p i c e f f e c t d e t e r m i n e d by Keen and B a r r e t t (1971) i n a s p e c i a l s t u d y c o n d u c t e d t o t h e n o r t h w e s t . The a v e r a g e v e l o c i t y o f 7.58 km/s f o r t h e 99 u p p e r m o s t m a n t l e i n . a r e g i o n n e a r a r i d g e c r e s t compares w e l l w i t h t h e r a n g e o f v e l o c i t i e s d e t e r m i n e d f o r c r e s t a l a r e a s by o t h e r r e s e a r c h e r s C e«g- L e P i c h o n e t a l , 1965). The r e s u l t s o f t h i s s t u d y f a v o r t h e h y p o t h e s i s t h a t E x p l o r e r R i d g e i s p r e s e n t l y an i n a c t i v e s p r e a d i n g c e n t e r . The t h i c k c r u s t d e t e r m i n e d n e a r t h e r i d g e w o u l d be a l i k e l y c o n s e q u e n c e o f E x p l o r e r R i d g e b e i n g r o t a t e d t o o f a r f r o m t h e a x i s o f J u a n de F u c a R i d g e and t h u s r e n d e r i n g i t i n a c t i v e . The p r o p o s e d f a u l t s a r e c o n s i s t e n t w i t h t h e e f f e c t s o f r o t a t i o n and a l s o t h e i n t e r n a l d e f o r m a t i o n o c c u r r i n g w i t h i n J u a n de F u c a p l a t e i f an e x t e n s i o n o f t h e Queen C h a r l o t t e F a u l t i s s e v e r i n g t h e n o r t h w e s t p o r t i o n f r o m t h e r e m a i n d e r o f t h e p l a t e . The f a c t t h a t a n i s o t r o p y and m a g n e t i c l i n e a t i o n s a r e o b s e r v e d i m p l i e s t h a t s e a f l o o r s p r e a d i n g has o c c u r r e d i n t h e a r e a and E x p l o r e r R i d g e was a t one t i m e a c t i v e . W h i l e a number o f i m p o r t a n t c o n c l u s i o n s have b e e n made, i t i s w o r t h n o t i n g t h a t an abundance o f a d d i t i o n a l i n f o r m a t i o n i s c o n t a i n e d i n t h e e n t i r e s u i t e o f s e i s m i c d a t a . A d d i t i o n a l p r o c e s s i n g and t h e i n t e r p r e t a t i o n o f s e c o n d a r y p r i m a r y wave and t r a n s f o r m e d s h e a r wave a r r i v a l s may r e f i n e t h e p r e s e n t m o d e l s . A l s o , i n terms o f t h e i n t e r p r e t a t i o n method, a l e a s t s q u a r e s a p p r o a c h t o f i t t i n g t r a v e l t i m e s and a m p l i t u d e s w o u l d r e s t r i c t t h e models more t h a n t h e y c o u l d be i n t h i s s t u d y . F i n a l l y , no a t t e n t i o n has b e e n g i v e n t o t h e n e a r v e r t i c a l i n c i d e n c e r e f l e c t i o n d a t a r e c o r d e d on t h e s e p r o f i l e s ; however, t h e i n t e r p r e t a t i o n o f t h e s e d a t a w o u l d 100 be invaluable f o r c o r r e l a t i o n with the r e f r a c t i o n r e s u l t s e s p e c i a l l y i f Moho r e f l e c t i o n s were observed. 101 REFERENCES A t w a t e r , T. 1970. I m p l i c a t i o n s o f p l a t e t e c t o n i c s f o r t h e C e n o z o i c t e c t o n i c e v o l u t i o n o f w e s t e r n N o r t h A m e r i c a . G e o l . S o c . Am. B u l l . 8_1, pp. 3513-36. B a r r , S.M. and C h a s e , R.L. 1974. G e o l o g y o f t h e n o r t h e r n e n d o f J u a n de F u c a R i d g e and s e a - f l o o r s p r e a d i n g . Can. J . E a r t h S c i . 11_, pp. 1384-1406. B e n n e t t , G.T. 1973. A s e i s m i c r e f r a c t i o n s u r v e y a l o n g t h e s o u t h e r n Rocky M o u n t a i n T r e n c h . U n p u b l . M.Sc. t h e s i s , 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 . B e r t r a n d , W.G. 1972. A g e o l o g i c a l r e c o n n a i s s a n c e o f t h e D e l l w o o d seamount a r e a , n o r t h e a s t P a c i f i c O c e an, and i t s r e l a t i o n s h i p t o p l a t e t e c t o n i c s . U n p u b l . M.Sc. t h e s i s , 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 . B r a i l e , L.W. and S m i t h , R.B. 1975. G u i d e t o t h e i n t e r p r e t a t i o n o f c r u s t a l r e f r a c t i o n p r o f i l e s . G e ophys. J.R. a s t r . Soc. 4_0, pp. 145-176. B u l l e n , K.E. 1963. An I n t r o d u c t i o n t o t h e T h e o r y o f S e i s m o l o g y , 3 r d e d . , C a m b r i d g e U n i v e r s i t y P r e s s , New Y o r k , 381 pp. Cann, J.R. 1970. New model f o r t h e s t r u c t u r e o f t h e o c e a n c r u s t , N a t u r e 226, pp. 928-30. . 1974. A model f o r o c e a n i c c r u s t a l s t r u c t u r e d e v e l o p e d . Geophys. J.R. a s t r . S o c . 39, pp. 169-87. C e r v e n y , V., 1966. On d y n a mic p r o p e r t i e s o f r e f l e c t e d and h e a d waves i n t h e n - l a y e r e d E a r t h ' s c r u s t . G eophys. J.R. a s t r . S o c . 11_, pp. 139-47. . and R a v i n d r a , R. 19 71. T h e o r y o f S e i s m i c Head Waves, U n i v e r s i t y o f T o r o n t o P r e s s , 312 pp. C h r i s t e n s e n , N . I . 1970. C o m p o s i t i o n and e v o l u t i o n o f t h e o c e a n i c c r u s t . Mar. G e o l . 8_, pp. 139-154. . and S a l i s b u r y , M.H. 1975 . S t r u c t u r e and c o n s t i t u t i o n o f t h e l o w e r o c e a n i c c r u s t . Rev. G eophys. Space P h y s . 1_3, pp. 57-86. 102 Coleman, R.G. and I r w i n , W.P., 1974. O p h i o l i t e s and a n c i e n t c o n t i n e n t a l m a r g i n s . I n : The G e o l o g y o f C o n t i n e n t a l M a r g i n s , C A . B u r k and C L . Drake (Ed.) S p r i n g e r - V e r l a g , New Y o r k , pp. 921-31. D e G o l y e r , E. 1932. The a p p l i c a t i o n o f s e i s m i c methods t o s u b m a r i n e g e o l o g y . Tran. Am. Geophys.. Un. 1_3, pp. 37-40. D e h l i n g e r , P., C o u c h , R.W., McManus, D.A. and G e m p e r l e , M. 1970. N o r t h e a s t P a c i f i c s t r u c t u r e . I n : The S e a , P a r t I I , A.E. M a x w e l l (Ed.) W i l e y - I n t e r s c i e n c e , New Y o r k , pp. 133-89. D o b r i n , M.B. 1960. G e o p h y s i c a l P r o s p e c t i n g 2nd e d . , M c G r a w - H i l l , New Y o r k , 446 pp. H e l m b e r g e r , D.V. 1968. The c r u s t - m a n t l e t r a n s i t i o n i n t h e B e r i n g S ea. B u l l . S e i s m o l . S o c. Am. 5_8_, pp. 179-214. . and M o r r i s , G.B. 1969. A t r a v e l t i m e and a m p l i t u d e i n t e r p r e t a t i o n o f a m a r i n e r e f r a c t i o n p r o f i l e : p r i m a r y waves. J . Geophys. Res. 7_4, No. 2, pp. 483-94. . and M o r r i s , G.B. 1970. A t r a v e l t i m e and a m p l i t u d e i n t e r p r e t a t i o n o f a marine r e f r a c t i o n p r o f i l e : t r a n s f o r m e d s h e a r waves. B u l l . S e i s m o l . S o c. Am. 60, No. 2, pp. 593-600. H e s s , H. 1964. S e i s m i c a n i s o t r o p y o f t h e u p p e r m o s t m a n t l e u n d e r t h e o c e a n s . N a t u r e 203, pp. 629-31. K a n a s e w i c h , E.R. 1973. Time Sequence A n a l y s i s i n G e o p h y s i c s , U n i v e r s i t y o f A l b e r t a P r e s s , 352 pp. Keen, C.E. and B a r r e t t , D.L. 1971. A measurement o f s e i s m i c a n i s o t r o p y i n t h e n o r t h e a s t P a c i f i c . Can. J . E a r t h S c i . 8, pp. 1056-64. . and T r a m o n t i n i , C. 1970. A s e i s m i c r e f r a c t i o n s u r v e y on t h e M i d - A t l a n t i c R i d g e . Geophys. J.R. a s t r . S o c . 20_, pp. 473-91. Kramer, F.S., P e t e r s o n , R.A. and W a l t e r , W.C (Ed.) 1968. S e i s m i c E n e r g y s o u r c e s 1968 Handbook, U n i t e d G e o p h y s i c a l C o r p o r a t i o n , 57 pp. L e P i c h o n , X., H o u t z , R.E., D r a k e , C L . and N a f e , J . E . 1965. C r u s t a l s t r u c t u r e o f t h e m i d - o c e a n r i d g e s , 1, S e i s m i c r e f r a c t i o n m e asurements. J . Geophys. Res. 70_, pp. 319-40. L i s t e r , C.R.B. 1972. On t h e t h e r m a l b a l a n c e o f a m i d - o c e a n r i d g e . G eophys. J.R. a s t r . S o c . 26_, pp. 515-35. 103 M a t t h e w s , D.H., L o r t , J . , V e r t u e , J . , P o s t e r , C.K. and G a s s , I.G. 1971. S e i s m i c v e l o c i t i e s a t t h e C y p r u s o u t c r o p . N a t u r e P h y s . S c i . 321, pp. 200-01 M c K e n z i e , D.P. and Morg a n , W.J. 1969. E v o l u t i o n o f t r i p l e j u n c t i o n s . N a t u r e 224, pp. 125-33. McManus, D.A., Holmes, M.L., C a r s o n , B., and B a r r , S.M. 1972. L a t e Q u a t e r n a r y t e c t o n i c s , n o r t h e r n end o f J u a n de F u c a R i d g e ( n o r t h e a s t P a c i f i c ) . M a r i n e G e o l . 12, pp. 141-64. Menard, H.W. and A t w a t e r , T. 1968. Changes i n d i r e c t i o n o f s e a - f l o o r s p r e a d i n g . N a t u r e 219, pp. 463-67. . and A t w a t e r , T. 1969. O r i g i n o f f r a c t u r e zone t o p o g r a p h y . N a t u r e 222, pp. 1037-40. M i n k l e y , B.G., T r i p e , R.L.K. and H e a l e y , D.A. 1970. O c e a n o g r a p h i c o b s e r v a t i o n s a t Ocean S t a t i o n P (50°N, 1 4 5 ° W ) 27 J u n e t o 25 September 1969. F i s h e r i e s R e s e a r c h B o a r d Canada, T e c h . R e p o r t 184 222 pp. M o o r e s , E.M. and J a c k s o n , E.D. 1974. O p h i o l i t e s and o c e a n i c c r u s t . N a t u r e 250, pp. 136-38. M u l l e r , S t . , S t e i n A. and V e i s , R. 1962. S e i s m i c s c a l i n g laws f o r e x p l o s i o n s on a l a k e b o t t o m . Z e i t . f . Geophys. 28, pp. 258-80. O ' B r i e n , P.N.S. 1960. S e i s m i c e n e r g y f r o m e x p l o s i o n s . Geophys. J.R. a s t r . S o c . 3_, pp. 29-44. P e t e r s o n , J . J . , F o x, P . J . and S c h r e i b e r , E. 1974. N e w f o u n d l a n d o p h i o l i t e s and t h e g e o l o g y o f t h e o c e a n i c l a y e r . N a t u r e 24 7, pp. 194-96. P o e h l s , K. 1974. S e i s m i c r e f r a c t i o n on t h e m i d - A t l a n t i c R i d g e a t 37°N. J . Geophys. Res. 79_, pp. 3370- 73. R a i t t , R.W. 1952. O c e a n o g r a p h i c i n s t r u m e n t a t i o n . N a t . Res. C o u n c i l Pub. 509, p. 70. . 1963. The c r u s t a l r o c k s . I n : The S e a , M.N. H i l l , (Ed.) I n t e r s c i e n c e , New Y o r k , pp. 85-102. ., S h o r , G.G., J r . , F r a n c i s , T.J.G. and M o r r i s , G.B. — — 1969. A n i s o t r o p y o f t h e P a c i f i c u p p e r m a n t l e . J . Geophys. Res. 7_4, pp. 3095-3109. S h o r , G.G., J r . 1963. R e f r a c t i o n and r e f l e c t i o n t e c h n i q u e s and p r o c e d u r e . I n : The S e a , M.N. H i l l (Ed.) I n t e r s c i e n c e , New Y o r k , pp. 20-38. 104 S h o r , G.G., J r . and R a i t t , R.W. 1969. E x p l o s i o n s e i s m i c r e f r a c t i o n s t u d i e s o f t h e c r u s t and u p p e r m a n t l e i n t h e P a c i f i c and I n d i a n O c e a n s . I n : The E a r t h ' s C r u s t and Upper M a n t l e , P . J . H a r t (Bd.) A.G.U. Mono. 13, pp. 225-30. S r i v a s t a v a , S.P., B a r r e t t , D.L., Keen, C.E., M a n c h e s t e r , K.S., S h i h , K.G., T i f f i n , D.L., C h a s e , R.L., T h o m l i n s o n , A.G., D a v i s , E . E . and L i s t e r , C.R.B. 1971. P r e l i m i n a r y a n a l y s i s o f g e o p h y s i c a l measurements n o r t h o f J u a n de F u c a r i d g e . Can. J . E a r t h S c i . 8̂ , pp. 1265-81. S t a c e y , R.A. 1973. G r a v i t y a n o m a l i e s , c r u s t a l s t r u c t u r e , and p l a t e t e c t o n i c s , i n t h e C a n a d i a n C o r d i l l e r a . Can. J . E a r t h S c i . l f J , pp. 615-28. S u t t o n , G.H., M a y n a r d , G.L. and H u s s o n g , D.M. 1971. W i d e s p r e a d o c c u r r e n c e o f a h i g h - v e l o c i t y b a s a l . l a y e r i n t h e P a c i f i c c r u s t f o u n d w i t h r e p e t i t i v e s o u r c e s and s o n o b u o y s . I n : The S t r u c t u r e and P h y s i c a l P r o p e r t i e s o f t h e E a r t h ' s C r u s t , J.G. H e a c o c k (Ed.) A.G.U. Mono. 14_, pp. 193-209. T a l w a n i , M., W i n d i s c h , C.C. and L a n g s e t h , M.G., J r . 1971. R e y k j a n e s R i d g e C r e s t : A d e t a i l e d g e o p h y s i c a l s t u d y . J . G e ophys. Res. 7(3, pp. 473-517. T i f f i n , D.L. and Seeman, D. 19 75. B a t h y m e t r i c map o f t h e c o n t i n e n t a l m a r g i n o f W e s t e r n Canada. Open f i l e map, G e o l . S u r v . Canada. T o b i n , D.G. and S y k e s , L.R. 1968. S e i s m i c i t y and t e c t o n i c s o f t h e n o r t h e a s t P a c i f i c Ocean. J . Geophys. Res. 72_, pp. 3821-46. V i n e , F . J . 1966. S p r e a d i n g o f t h e o c e a n f l o o r : New e v i d e n c e . S c i e n c e 154, pp. 1405-1415. _. and W i l s o n J . T . 1965. M a g n e t i c a n o m a l i e s o v e r a y o u n g o c e a n i c r i d g e o f f V a n c o u v e r I s l a n d . S c i e n c e 150, pp. 485-89. W i g g i n s , R.A. 1976. Body wave a m p l i t u d e c a l c u l a t i o n s I I . Geophys. J.R. a s t r . Soc. ( s u b m i t t e d f o r p u b l i c a t i o n ) . . and M a d r i d , J.A. 1974. Body wave a m p l i t u d e c a l c u l a t i o n s . Geophys. J.R. a s t r . S o c . 37, pp. 423-33. 105 APPENDIX FAULT THROW CALCULATIONS The assumed ray paths for two rays incident on a faulted Moho are shown in the diagram below. One ray i s c r i t i c a l l y r efracted along the up-fault interface and produce a head wave a r r i v a l at a distance d. The other, s l i g h t l y steeper ray i s refracted into the lower medium and i s then c r i t i c a l l y r efracted along the down-fault interface to produce a head wave a r r i v a l at a greater.distance d^- 106 In order to consider the traveltime delay due to the fault the t r a v e l t i m e s must be compared at the same r e c e i v e r d i s t a n c e . Let the d i s t a n c e be d'for the l i m i t i n g case. Therefore, the head wave along the u p - f a u l t i n t e r f a c e must have the q u a n t i t y Aa/Vi, added to i t s t r a v e l t i m e . In terms of the geometry and n o t a t i o n presented i n the diagram, the t r a v e l t i m e d i f f e r e n c e at d^ i s AT = ( a 2 + h 2 ) * 2 - a v 3 cos e h tan 9 Vu Eq. 1 where a = d - 2zi tan \i - 2zz tan i 2 - 2z 3 tan x3. Using S n e l l ' s law the tangents can be expressed i n terms of the l a y e r v e l o c i t i e s using s i n i i = V i / V h , s i n 12 = V 2 / V 4 , e t c . Thus, the d i s t a n c e a can be expressed as 2z 2 z 2V 2 2z3.V3 Eq. 2 /Vn' - V i ' /V* - V 2 /V* - V 3 Note that i f the assumption a 2 >> h 2 i s made, so that ( a z + h 2 ) ' 5 = a, AT becomes, AT = h tan 9 V3 COS 6 Vi, o r , by using the f a c t that 9 = s i n 1 ( V 3 / V O , t h i s can be w r i t t e n as Eq. AT v 3 3 11 v3 v, Eq. 4 i s the same as the expression given by Dobrin (1960, p. 81). Since AT can be measured from the data, and V 3 and Vn are known, the f a u l t throw (h) can be e a s i l y determined. Eq. 4 107 To solve Eq. 1 for h without this assumption, the quantity ( a 2 + h 2 ) ^ i s f i r s t expanded by the binomial theorem. By r e t a i n i n g two terms, the approximation ( a 2 + h 2)* 5 = a + h 2/2a i s obtained. Substituting this r e s u l t into Eq. 1 gives, a f t e r simplyfying, r 1 tan 6 A T =2a"V, h 2 + h V 3 cos 6 V i Using Eqs. 3 and 4, t h i s can be written i n a standard quadratic form as, 1 h 2 + h - AT = 0 2avlf V 3 Solving for h and retaining the p o s i t i v e root, the f a u l t throw can be expressed as h = - a cot 9 + / C a cot ^~^VV^ Eq. 5 where a i s given by Eq. 2 and cot 9 = / y z To calculate the f a u l t throw for p r o f i l e s 74-1 and 74-1R, the following numerical values obtained from the traveltime analysis were used: P r o f i l e 74-1 Vi = 1.48 km/s - z, = 2.40 km V 2 = 4.10 km/s z 2 = 1.80 km V 3 = 5.80 km/s z 3 = 0.90 km V„ = 7.35 km/s d = 12.5 km AT = 4.67 - 4.10 = 0.57s 108 Erom Eq. 2 the distance a i s found to be 6.78 km. Using t h i s i n Eq. 5 and making the remaining s u b s t i t u t i o n s , the f a u l t throw becomes h = 3.92 km. P r o f i l e 74-1R Vi = 1.48 km/s z i = 2.40 km V 2 = 4.20 km/s z 2 = 1.00 km V 3 = 6.40 km/s z 3 = 0.90 km V* = 7.24 km/s d = 20 km AT = 4.12 - 3.67 = 0.45s From Eqs. 2 and 5, the calculated f a u l t throw i s h = 4.69 km. If Eq. 4 had been used instead to calculate h, the f a u l t throw values would be 5.38 km for p r o f i l e 74-1 and 6.16 km for p r o f i l e 74-1R. To cal c u l a t e the f a u l t throw using HRGLTZ, a quantity AT must be found which expresses the difference between the time delay due to the f a u l t (AT) and the delay due to increasing the layer thickness by the amount of the f a u l t throw. For a h o r i z o n t a l l y layered model based on the up-fault structure the intercept for the Moho r e f r a c t i o n branch i s 109 For a model based on the down-fault structure the intercept i s " 2 Z i v3 v„ Therefore, by increasing the layer thickness by an amount h = z' 3 - z 3, the traveltime i s increased by the amount T u ' - Tu = 2h - 2 h (V^ 2 - V 3 2 ) % The difference AT = ( T i ^ - T i J - AT then expresses the amount of time that must be added to the down-fault intercept i n the data to produce the down-fault structure with HRGLTZ. Substituting for T ^ - T^ and then for h from Eq. 5, the correction AT becomes A T, _ 2 cos 9 AT = 77 - a cot 9 + "'(a cot 9 ) 2 + 2aVuAT -AT Eq. 6 Note that i f the expression for h obtained by solving Eq. 4 were used, the r e s u l t would be much simpler, namely AT = AT. Although v e l o c i t y gradients were introduced by HRGLTZ, the layer v e l o c i t i e s stated above were taken as v a l i d averages for c a l c u l a t i n g angles. Substituting the numerical data y i e l d s AT values of 0.261s and 0.235s for p r o f i l e s 74-1 and 74-1R, respectively. These values represent the amount that the area under the A(pj curve was increased for HRGLTZ and corresponds to depth increases, i . e . f a u l t throws, of 4.5 and 5.1 km for the two p r o f i l e s .

Cite

Citation Scheme:

    

Usage Statistics

Country Views Downloads
France 4 0
United States 3 1
City Views Downloads
Unknown 4 1
Redmond 2 1
Ashburn 1 0

{[{ mDataHeader[type] }]} {[{ month[type] }]} {[{ tData[type] }]}

Share

Share to:

Comment

Related Items