UBC Theses and Dissertations

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UBC Theses and Dissertations

A refraction survey across the Canadian cordillera Forsyth, David A.G. 1973-12-31

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A REFRACTION SURVEY ACROSS THE CANADIAN CORDILLERA BY DAVID A.G. FORSYTH B.Sc.  Queen's U n i v e r s i t y  a t K i n g s t o n , 1968  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE  REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE  i n t h e Department of GEOPHYSICS AND ASTRONOMY  We a c c e p t t h i s t h e s i s as conforming required  THE  to the  standard.  UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1973  "In presenting  t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements  for 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 Library 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 study.  and  I further agree that permission f o r extensive copying of  t h i s thesis f o r s c h o l a r l y purposes may Department or by h i s representative.  be granted by the Head of  my  I t i s understood that copying  or p u b l i c a t i o n of t h i s thesis f o r f i n a n c i a l gain s h a l l not be allowed without my written permission". •  Department of Geophysics and The  University  Date June  1973  Astronomy  of B r i t i s h Columbia  ii i  ABSTRACT  Record s e c t i o n s from p a r t i a l l y r e v e r s e d r e f r a c t i o n n o r t h e r n B r i t i s h Columbia show t h a t  the a m p l i t u d e s o f upper  a r r i v a l s v a r y smoothly w i t h d i s t a n c e . a m p l i t u d e s i s n o t smooth. amplification effects  lines i n mantle  The p a t t e r n o f c r u s t a l a r r i v a l  N o r m a l i z a t i o n o f t h e seismograms  t o remove t h e  caused by s h o t s i z e and i n s t r u m e n t response show t h e  o f r e c o r d i n g s i t e s on P Models  n  amplitudes are minimal.  d e r i v e d from r a y t h e o r y i n d i c a t e  from about 40 km i n t h e Omineca C r y s t a l l i n e B e l t I n s u l a r Trough.  The average P^ v e l o c i t y  crustal velocity  i s 6.4 km/s.  models a r e g r e a t l y  a c r u s t which  thins  t o about 25 km i n t h e  i s 8.06 km/s.  The average  The secondary energy would  i n d i c a t e the  simplified.  A t i m e - t e r m p r o f i l e between t h e Omineca C r y s t a l l i n e B e l t and the  C o a s t Mountains  characterized  s u g g e s t s a M o h o r o v i c i c t r a n s i t i o n which i s  by two s i g n i f i c a n t t o p o g r a p h i c w a v e l e n g t h s .  (200 km) wavelength  correlates  elements o f Wheeler  e t a l . (1972).  have t e c t o n i c  significance.  roughly with the C o r d i l l e r a n The l a r g e r  The s h o r t e r structural  (800 km) wavelength  may  iii TABLE OF CONTENTS Page v  LIST OF TABLES  vi  LIST OF FIGURES  vii  ACKNOWLEDGEMENTS CHAPTER 1  1  INTRODUCTION  1-1  Background  1  1-2  P r e v i o u s work i n t h e a r e a  1  1- 3  Present  2  CHAPTER 2  study  5  SYSTEMS  2- 1  Components and c o n f i g u r a t i o n s  5  2- 2  Calibration  7  CHAPTER 3  9  RECORD SECTIONS  3- 1  Systems r e s p o n s e s and shot  3-2  Formation  3- 3  Normalization  CHAPTER 4  correction factors  of record sections  9 10 10 18  INTERPRETATION  4- 1  G e n e r a l p r i n c i p l e s and method  18  4-2  L e a s t square  19  4-3  T r a v e l - t i m e curves  20  4-4  F i t t i n g t r a v e l - t i m e curves  22  4-5  Record  s e c t i o n from Greenbush  29  4-6  Record  s e c t i o n from B i r d  33  4-7  The anomaly a t 820 km d i s t a n c e  34  4-8  Time-term study  39  4-9  D i s c u s s i o n o f time-term  4-10 L i m i t a t i o n s  analysis  Lake  study  41 42  CHAPTER 5  SUMMARY  5-1  T r a v e l - t i m e models  5-2  Time-term models  5-3  Conclusions  J  APPENDIX A REFERENCES  ,  V LIST OF  TABLES Page  TABLE 1 la  BC 69 Shot d a t a  12  BC 70 Shot d a t a  13  vi LIST OF FIGURES FIGURE  Page  1  The l o c a t i o n map  2  The c h a r t  3  The response c u r v e s  4  The system f a c t o r s  5  The r e c o r d respect  5a  6  7  recording  systems  6 8 11  s e c t i o n from Greenbush n o r m a l i z e d w i t h 15  s e c t i o n from Greenbush n o r m a l i z e d w i t h  to distance  The r e c o r d respect  o f the f i e l d  to distance  The r e c o r d respect  4  squared  16  s e c t i o n from B i r d Lake n o r m a l i z e d  to distance  with  squared  The t r a v e l - t i m e model  17  (model 1) f o r t h e Greenbush  section  23  7a  The v e l o c i t y - d e p t h  s t r u c t u r e f o r model 1  8  The t r a v e l - t i m e model  24  (model 2) f o r t h e Greenbush  section  25  8a  The v e l o c i t y - d e p t h  s t r u c t u r e f o r model 2  26  9  The t r a v e l - t i m e model f o r the B i r d Lake s e c t i o n  27  9a  The v e l o c i t y - d e p t h  28  s t r u c t u r e f o r the B i r d Lake model  10  The f i t o f model 2 t o t h e d a t a o f F i g . 5a  32  11  The f i t o f the B i r d Lake model t o t h e d a t a  35  12  Part  37  13  The p l o t o f the time-terms c o n v e r t e d t o depth  43  14  The c o r r e l a t i o n o f the time-term s u r f a c e  47  o f t h e Greenbush s e c t i o n expanded  with g e o l o g y  ACKNOWLEDGEMENTS With sincere appreciation, the writer wishes to extend thanks to Dr. M.J. Berry for his patient advice and many helpful comments during both the f i e l d work and the interpretation at the Seismology Division, Earth Physics Branch, and to Dr. R.M. E l l i s for supervision during the writer's residence at the University of British Columbia and c r i t i c a l reading of the manuscript. In addition, the author i s indebted to Mr. M.N. Bone of the Seismology Division and Mr. R.D. Meldrum of the Department of Geophysics, University of British Columbia, for assistance in understanding the recording systems and to Dr. R. Stacey of the Earth Physics Branch for discussion of the f i n a l results. Data for this project was collected by teams from the Earth Physics Branch and the University of British Columbia and analysed in Ottawa. Basic computer programs for data handling and interpretation pre-existed at the Seismology Division and were modified by the author to cope with the present study. Mrs. Lois Turgeon who typed the thesis was of great help during the f i n a l preparation of the work. This research has been sponsored by the Seismology Division, Earth Physics Branch, Department of Energy, Mines and Resources, the University of British Columbia, and the National Research Council (Grant A-2617).  1 CHAPTER 1  INTRODUCTION 1-L  Background With increasing i n t e r e s t i n plate tectonics, the emphasis on  large scale r e f r a c t i o n studies has declined i n favour of surface-wave and array analyses to study the lithosphere-asthenosphere r e l a t i o n s h i p . However, r e f r a c t i o n and r e f l e c t i o n l i n e s used to determine the v e l o c i t y depth structure and i t s l a t e r a l behaviour may yet provide a means of r e l a t i n g surface features to l i t h o s p h e r i c plates i n some d e t a i l . p a r t i c u l a r , the study of t h i s signature i n the neighbourhood  In  of plate  boundaries may prove most i n t e r e s t i n g . The geology of the Canadian C o r d i l l e r a n region, both onshore and offshore, would strongly favour i t s candidacy as a region where plates are and have been active (Souther, 1972; Monger, 1972; Berry et a l . , Johnson and Couch, 1970; Stacey, 1972).  1971;  When subdivided into geological  provinces, the region includes the Rocky Mountain Trench, the zone containing the Cassiar, Omineca and Columbia Mountains, the  Intermontane  Belt, the Coast Mountains and the Insular Belt (Monger, 1972).  The present  r e f r a c t i o n study covers the area from Greenbush Lake i n the Columbia Mountains 1-2.  to Bird Lake on Graham Island i n the Insular Belt (Fig. 1).  Previous work i n the area From a seismic viewpoint, the p i c t u r e of the Canadian C o r d i l l e r a  has been incomplete as a r e s u l t of l i m i t e d surveys and rather incoherent data.  The i n t e r p r e t a t i o n i s rendered a d d i t i o n a l l y d i f f i c u l t due to the  complexity of at l e a s t the upper crust as evidenced by the surface geology.  2  Work a d j a c e n t t o t h e a r e a p r e s e n t l y under s t u d y has been o u t l i n e d by Shor  (1962), by White  e t a l . (1968) and by Jacoby  (1970).  I n the a r e a  o f the I n s u l a r B e l t northwest o f P r i n c e R u p e r t , Shor suggested t h a t the M occurred  a t a depth o f 26 km.  concentrated along p r o f i l e s  I n s o u t h e r n B r i t i s h Columbia, s t u d i e s were  from Puntchesakut Lake and from near  t o Osoyoos and from Greenbush  Lake  a b l e d p a r t i a l r e v e r s a l o f the l i n e was  to Hope.  Barkerville  Mine b l a s t s a t M e r r i t t  from Puntchesakut Lake.  en-  The Hope l i n e  unreversed. White  e t a l . proposed a l t e r n a t i v e i n t e r p r e t a t i o n s f o r t h e i r d a t a .  Both models were c h a r a c t e r i z e d by minor topography on the M d i s c o n t i n u i t y a t a depth o f from 28 t o 30 k i l o m e t e r s . l a t e r a l change i n upper mantle v e l o c i t y Lake t o 7.8 km/s 8.0  i n the s o u t h .  T h e i r f i r s t model i n d i c a t e d a from 8.1 km/s  beneath W i l l i a m s  The a l t e r n a t e model d e p i c t e d a v e l o c i t y o f  km/s. D u r i n g 1967,  s h o t s d e t o n a t e d a t the s o u t h end o f  I s l a n d were r e c o r d e d a l o n g t h e l i n e used i n 1966, sites.  Vancouver  r e - o c c u p y i n g many o l d  T h i s d a t a , plagued w i t h c o n s i d e r a b l e n o i s e , gave an apparent  v e l o c i t y o f 8.43  km/s,  p e r t i n e n t t o the upper mantle e a s t o f M e r r i t t .  A  p r e l i m i n a r y time-term a n a l y s i s i n d i c a t e d a s o u t h w e s t e r l y d i p p i n g M d i s c o n t i n u i t y w i t h an upper mantle v e l o c i t y o f 8.27 1-3.  km/s  (Jacoby, 1970).  Present study D u r i n g the summers o f 1969  and 1970,  r e f r a c t i o n p r o f i l e s were  r e c o r d e d by E a r t h P h y s i c s Branch and U n i v e r s i t y o f B r i t i s h Columbia teams a c r o s s the t h r e e i n t e r i o r g e o l o g i c a l p r o v i n c e s . s h o t s d e t o n a t e d a t Greenbush  In August  (UBC)  1969,  Lake were r e c o r d e d by teams from the E a r t h  3  P h y s i c s Branch a l o n g roads r u n n i n g from southwest I s l a n d , from L i t t l e W i l l i a m s Lake  o f R e v e l s t o k e t o Vancouver  F o r t t o McLeod Lake, from Nazko t o B a r k e r v i l l e and from  t o B e l l a C o o l a ( F i g . 1 ) . A m o n i t o r s t a t i o n was l o c a t e d a t  Lumby t o r e c o r d a l l s h o t s .  Teams from UBC r e c o r d e d a l o n g roads from  P r i n c e George t o P r i n c e Rupert. D u r i n g J u l y o f 1970 an attempt was made by t h e same teams t o r e v e r s e t h e P r i n c e R u p e r t - P r i n c e George l i n e C o o l a - W i l l i a m s Lake l i n e  from R i p l e y Bay.  from B i r d Lake and t h e B e l l a  The N a z k o - B a r k e r v i l l e l i n e was  r e v e r s e d from R i p l e y Bay and an upper c r u s t a l v e l o c i t y was o b t a i n e d a l o n g t h i s same l i n e  from a shot i n Puntchesakut  Lake.  The p r e s e n t s t u d y i n c l u d e s d a t a from L i t t l e u s i n g t h e Greenbush  shot p o i n t and from P r i n c e Rupert  u s i n g t h e B i r d Lake  shot p o i n t .  stations.  Fort  t o P r i n c e Rupert  t o P r i n c e George  F i g . 1 shows t h e l o c a t i o n o f s h o t s and  Fig. 1 The  location  map.  T r i a n g l e s show s i t e s r e corded from B i r d Lake; d o t s show s i t e s r e c o r d e d from Greenbush Lake. (Cordilleran structural elements a f t e r Wheeler, et a l . , 1972)  49°  _L_——'48°  5  CHAPTER 2  SYSTEMS  2-1.  Components and The f i e l d  configurations  r e c o r d i n g systems a r e o u t l i n e d i n F i g . 2.  The  Earth  P h y s i c s Branch systems had two v a r i a t i o n s w i t h i n the arrangement shown. On one o f the systems, E l e c t r o - T e c h SPA-10 a m p l i f i e r s were used i n s t e a d o f the  AS-330 models.  The E l e c t r o - T e c h a m p l i f i e r s were m o d i f i e d t o produce  the  same o v e r a l l g a i n as t h e AS-330 models and the d i f f e r e n c e  responses i s n e g l i g i b l e data.  between  f o r the frequencies encountered i n the present  The o t h e r system employed  E l e c t r o - T e c h EV-17  seismometers w i t h a  n a t u r a l p e r i o d o f one second, Texas Instrument a m p l i f i e r s and an Ampex tape r e c o r d e r .  The shape o f the v e l o c i t y  system i s n o t s i g n i f i c a n t l y d i f f e r e n t  s e n s i t i v i t y curve f o r t h i s  from the c u r v e f o r the UBC  systems  (0.8 Hz-12.5 Hz) shown i n F i g . 3. All  d a t a p r e s e n t e d h e r e i n from the 1969  u s i n g a three-component  field  seismometer c o n f i g u r a t i o n .  teams c o n t i n u e d w i t h the three-component  trip  were r e c o r d e d  In 1970, t h e UBC  arrangement, w h i l e the E a r t h  P h y s i c s Branch teams used s i x v e r t i c a l seismometers s e t a t a s p a c i n g o f 500 meters. or was  The a r r a y n o m i n a l l y p r o v i d e d the o p t i o n o f s t a c k i n g  d e t e r m i n i n g a phase v e l o c i t y t o more c o n f i d e n t l y i d e n t i f y a r r i v a l s . found, however, t h a t the l a c k o f freedom i n c h o o s i n g l o c a t i o n s  spreads compared  to p i c k i n g i s o l a t e d o r bedrock seismometer s i t e s  t h r e e components  r e s u l t e d i n s i g n i f i c a n t l y n o i s i e r seismograms  if  records  any, advantage was  gained i n d i s c e r n i n g P  first  arrivals.  and  It  f o r the f o r the little,  UNIVERSITY OF BRITISH COLUMBIA BC 69-70 GEOTECH AMPLIFIERS 3 COMPONENT MODEL AS-330 ARRANGEMENT-AT 0 db ATTENUATION WILLMORE MK II SEISMOMETERS. LOW LEVEL 0/P GIVES 70 db GAIN NATURAL FREQUENCY HIGH LEVEL 0/P GIVES 70 db PLUS SET AT 1 Hz. SEPARATION (18,24,30 or 30,36,42 db) - 3 db POINTS OF FILTER SET AT .01,5Hz or .8,12.5Hz  O O  GEOTECH FM TAPE RECORDER -SIX DATA CHANNELS - 1 CHRONOMETER OR RADIO TIME CHANNEL -RECORDS AT I5/I60 IN/SEC -SET UP FOR FULL MODULATION AT t 1 VOLT RMS  O-  CHRONOMETER OR WWV/WWVB RECEIVER  EARTH PHYSICS BRANCH * BC 69 3 COMPONENT ARRANGEMENT: WILLMORE MK H SEISMOMETERS. NATURAL FREQUENCY SET AT 1 Hz.  O O O  PRECISION INSTRUMENT FM TAPE RECORDER -SIX DATA CHANNELS - 1 CHRONOMETER CHANNEL •1 RADIO TIME SIGNAL EDGE TRACK -RECORDS AT I5/I6 IN/SEC -SET UP FOR FULL MODULATION AT t 5 VOLTS  GEOTECH AMPLIFIERS MODEL AS-330 -AT 0 db ATTENUATION LOW LEVEL 0/P GIVES 92 db GAI HIGH LEVEL 0/P GIVES 92 db PLUS -SEPARATION (18,24,30 db) -3db POINTS OF FILTER SET AT 0.1,17.0 Hz CHRONOMETER  EARTH PHYSICS BRANCH  BC 70  o SPREAD ARRANGMENT: 6 VERTICAL WILLMORE MK n SEISMOMETERS. SET AT 500 METER SPACING. NATURAL FREQUENCY SET AT 1 Hz  WWV/WWVB RECEIVER  GEOTECH AMPLIFIERS  a  MODEL AS-330 -HIGH LEVEL 0/P ONLY  o -  USED (92 db GAIN AT 0 db ATTENUATION)  o -  -3 db POINTS OF FILTER SET AT 0.1, 17.0Hz  PRECISION INSTRUMENT FM TAPE RECOROER -SIX DATA CHANNELS - 1 CHRONOMETER CHANNEL •1 RADIO TIME SIGNAL EDGE TRACK -RECOROS AT I5/I6 IN/SEC -SET UP FOR FULL MODULATION AT± 5 VOLTS  a o Fig.  2  CHRONOMETER WWV/WWVB RECEIVER  The c h a r t o f the r e c o r d i n g systems used i n t h e f i e l d . * V a r i a t i o n s on t h i s system a r e d e s c r i b e d i n the t e x t .  7  2-2.  Calibration The U n i v e r s i t y o f B r i t i s h Columbia systems were c a l i b r a t e d  b e f o r e each f i e l d K o l l a r and R u s s e l l  season u s i n g a Maxwell b r i d g e i n the manner d e s c r i b e d by (1966).  I n the f i e l d ,  t h e systems were checked b e f o r e  each s h o t by m o n i t o r i n g the response t o an a c c e l e r a t i o n s t e p g i v e n t o the seismometer mass.  K tests  ( B a n c r o f t and Basham, 1967) were a l s o performed  b e f o r e each s h o t . E a r t h P h y s i c s Branch systems were checked i n Ottawa b e f o r e each f i e l d s e a s o n , w h i l e a m p l i f i e r r e s p o n s e , K t e s t and system response checks were made b e f o r e each shot i n t h e f i e l d .  T y p i c a l band-pass  curves f o r the v a r i o u s systems a r e shown i n F i g . 3.  response  Fig. 3  The response  curves o f the systems shown i n F i g . 2.  9  CHAPTER 3  RECORD SECTIONS  3-1.  System response As t h e f i e l d  was, deemed w o r t h w h i l e to  and shot c o r r e c t i o n  factors  u n i t s were c o n t i n u a l l y checked  and c o r r e c t e d , i t  t o c o n s i d e r a l l t h e system f a c t o r s from  seismometer  computer and use t h e s e , a l o n g w i t h shot f a c t o r s , i n c o n s t r u c t i n g r e c o r d  sections.  The r e c o r d s from any one shot p o i n t might then be e x p e c t e d t o  show some c o h e r e n t  energy  p a t t e r n - n o t w i t h s t a n d i n g the e f f e c t s o f  seismometer s i t e - a l t h o u g h r e c o r d s e c t i o n s would d i f f e r b y r e l a t i v e p o i n t response. was used  The system f a c t o r s a r e show i n F i g . 4.  t o account  f o r the s e i s m o m e t e r - a m p l i f i e r  shot  A s i n g l e number  response,  s i n c e the  r e c o r d e d P coda f r e q u e n c i e s a r e n a t u r a l l y r e s t r i c t e d t o t h e passband 3 t o 6 Hz.  The response  o f t h e systems i s e s s e n t i a l l y f l a t  i n this  from range.  No r e c o r d s from t h e system w i t h a 0.01-5 Hz f i l t e r a r e i n c l u d e d i n the present  study. The  Greenbush Lake s h o t s were r e l a t i v e l y r a t e d u s i n g  from h e l i c o r d e r r e c o r d s o b t a i n e d a t a s t a t i o n n e a r Lumby.  The l a r g e s t  amplitude was g i v e n t h e r a t i n g 1.0 and weaker s h o t s were then by a number which would make a l l s h o t s e f f e c t i v e l y same form o f r a t i n g was used  t h a t o f u s i n g nominal  Table l a .  t h e same s i z e .  This  T h i s mode o f shot r a t i n g was p r e f e r r e d  charge weight,  t r u e s e i s m i c energy y i e l d .  normalized  f o r t h e 1970 B i r d Lake s h o t s u s i n g h e l i c o r d e r  r e c o r d s from a s t a t i o n a t S a n d s p i t . to  amplitudes  as i t g i v e s a b e t t e r e s t i m a t e o f  The shot f a c t o r s a r e shown i n T a b l e 1 and  10  3-2.  Formation The  o f Record  analog f i e l d  Sections d a t a were d i g i t i z e d and a r r a n g e d on permanent  d a t a tapes i n a manner which f a c i l i t a t e d the f o r m a t i o n o f r e c o r d s e c t i o n s . All  programs, except  to work on a CDC  3100  o p e r a t e s on a DDP used  124  the d i g i t i z i n g program, were f i r s t w r i t t e n  computer.  The  program used t o d i g i t i z e the d a t a  computer a t the E a r t h P h y s i c s Branch.  The  t o form permanent d a t a tapes were o r i g i n a l l y w r i t t e n by K.G.  and M.J.  seismograms was  on a CDC  6400 computer.  w r i t t e n by M.J.  Berry.  The program used  to f i l t e r  of M.J.  B e r r y and  the w r i t e r .  The  l i m i t a t i o n s o f the  and d i g i t i z i n g systems e n a b l e d a d i g i t i z i n g  1-7/8  Records d i g i t i z e d a t 200  f i l t e r e d t o remove 60 Hz energy s e r i e s were decimated  t o 100  significantly  Record  The  or  Data d i g i t i z e d a t  sample p e r second lowest N y q u i s t  time 50  standard  frequency i s  g r e a t e r than the passband of r e c o r d e d  energy.  s e c t i o n s were then c o n s t r u c t e d i n a reduced t r a v e l - t i m e  For 3-component systems, a v e r t i c a l  v e r t i c a l seismometers were d e p l o y e d , 3-3.  200  the r e s u l t a n t  form w i t h the system and shot f a c t o r s a p p l i e d t o the i n d i v i d u a l series.  second  samples per second were then  samples p e r second.  upon f o r m a t i o n o f permanent d a t a t a p e s .  playback  tapes r e c o r d e d a t  (from power l i n e s ) and  samples p e r second were expanded to the 100  thus 25 Hz,  The  i n c h e s per second were d i g i t i z e d a t e i t h e r 100  samples p e r second.  the  r a t e o f 50 samples p e r  tapes r e c o r d e d at 15/160 i n c h e s p e r second. o r 15/16  the  The program which c o n s t r u c t s the  r e c o r d s e c t i o n s has e v o l v e d at the E a r t h P h y s i c s Branch through efforts  Barr  B e r r y and were m o d i f i e d by the w r i t e r t o cope w i t h the p r e s e n t  d a t a and o p e r a t e  for  programs  t r a c e was  a " b e s t t r a c e " was  used.  time  Where s i x  chosen.  Normalization To d i s t i n g u i s h  i n d i v i d u a l seismograms, i t was  necessary  to  UNIVERSITY  OF  BRITISH  - B C 69.70  COLUM8IA  M K II  AS-330 MAX. 0/P - ± 2.82 VOLTS  3 COMPONENT  EFFECTIVE K = 1.11 V/CM/SEC <fi.HVOLTSV-  CM " SEC.  -\.CM/SECJT  LOW LEVEL GAIN-70 db-ATTENUATION HIGH LEVEL GAIN = (70 db + SEPARATION -ATTENUATION) ~xf  V O L T A G E GAIN (CORRESPONDING; — I.TO db S E T T I N G  < (2.50 V.")—  < f 2 0 4 7 UNITS"!-  - \ . 2 . 8 2 V./*  -\  5 VOLTSj-r  .  DIGITAL UNITS  EARTH PHYSICS BRANCH -BC69  MK  n  V  3 COMPONENT EFFECTIVE K =I.20V/CM/SEC. - B C 70  PLAYBACK R I. DIGITIZER TAPE DECK RECORDER MAX. 0/P 2.5 V MAX. 0/P ± 5 V. SSSA MAX. NUMBER IS ± 2 0 4 7 CORRESPONDING TO ± 5 V.  AS-330  LOW L E V E L GAIN = 92 db-ATTENUATION HIGH L E V E L GAIN= (112 db-ATTENUATION)  6 VERTICAL EFFECTIVE K = 0.57V/CM/SEC. <fl.20(0 57)vV—  CM ' SEC.  A  - B C 70  CM/SEC. J+ •  MK  n  TEXAS INSTRUMENT AMPLIFIER  - X f1.13 VOLTS Si — \ CM/SEC.- > :. J-r-«  4  5  VOLTS,  DIGITAL UNITS  MAX. 0/P = ± 1.35 VOLTS EFFECTIVE K = 1.13 V/CM/SEC.  Fig.  \  AMPEX PRE AMP  6 VERTICAL  CM SEC  X<2047 UNITS  *xf VOLTAGE GAIN (CORRESPONDING — LTO db SETTING  GAIN TO RECORDER (107.5 db-ATTENUATION) -Xf  VOLTAGE GAIN ^ (CORRESPONDING) L TO db SETTING J-r-«  »-X/2.50 V. V ( > U - 3 5 V.JT  •Xf"2047 UNITS") — — I. 5  VOLTSJ-r-  The system f a c t o r s f o r c o n v e r s i o n o f ground v e l o c i t y t o d i g i t a l u n i t s and t o ground v e l o c i t y . Note e f f e c t i v e K a p p l i e s t o passband c e n t r e o n l y .  "DIGITAL UNITS  back  TABLE 1 BC 69 SHOT DATA Greenbush Lake - Shot P o i n t Coordinates  Number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20  5 0 ° 46.90' N 118° 20.66' W Date  Aug.5, '69 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24  Water Depth  60 m  Charge  Time GMT  Type  07:05:00.15 07:05:00.14 07:05:00.18 07:05:00.23 07:05:00.23 07:05:00.18 07:05:00.20  17,100 17,100 12,027 13,293 13,293 12,660 13,293  l b s TNT l b s TNT l b s MINOL l b s MINOL l b s MINOL l b s MINOL l b s MINOL  Pattern Point Pattern Pattern Pattern Pattern Pattern  07:05:00.19 07:05:00.17 07:05:00.17 07:05:00.17 07:05:00.16 07:05:00.17 07:05:00.15 07:05:00.19 07:05:00.17 07:05:00.17 07:05:00.16  12,660 13,293 13,293 13,293 13,293 13,293 13,293 13,293 12,660 13,293 17,100  l b s MINOL l b s MINOL l b s MINOL l b s MINOL l b s MINOL l b s MINOL l b s MINOL l b s MINOL l b s MINOL l b s MINOL l b s TNT  Pattern Pattern Pattern Pattern Pattern Pattern Pattern Pattern Pattern Pattern Pattern  i'  ,Amplitude a t Lumby  -  Shot Factor  2 1 6 11/2" 6 2 2  4.75 9.5 1.55 6.3 1.55 4.75 4.75  2 2 8 9 8 8 9 9 8 7 9  3.8 4.75 1.19 1.0 1.12 1.12 1.05 1.0 1.12 1.27 1.0  1/2  1/2 1/2 1/2 1/2 1/2 1/2 1/2  TABLE l a  BC 70  B i r d Lake - Shot  Point  Coordinates  Number 1 2 3 4  SHOT DATA  53° 35.83' N 132° 23.92' W  Date J u l y 25, 1970 26 27 28  Water depth  Time GMT 07:05:00.00 07:05:00.01 07:04:59.90 07:05*00.00  24 m  Charge 2,000 l b s 6,000 l b s 7,000 l b s 8,000 l b s  NITRONE NITRONE NITRONE NITRONE  SM SM SM SM  Type  Amplitude at S a n d s p i t  Shot Factor  Pattern Pattern Pattern Pattern  4.7 11.0 10.3 14,2  3.0 1.3 1.4 1.0  14  n o r m a l i z e t r a c e amplitudes i n t h e f o l l o w i n g manner. discern f i r s t energy.  a r r i v a l s , i t was n e c e s s a r y  Hence, t h e s e c t i o n s  20 o r 25 seconds.  to e l i m i n a t e  i n order to  t h e l a r g e r shear  a r e t e r m i n a t e d a t a reduced t r a v e l time o f  Next, t h e t r a c e c o n t a i n i n g  wavelet - normally a record  Firstly,  i n the v i c i n i t y  the l a r g e s t  of crossover  amplitude - was chosen, i t s  l a r g e s t peak-to-peak a m p l i t u d e s e t t o 1 i n c h and t h e r e s t o f t h e t r a c e s in  t h e s e c t i o n were n o r m a l i z e d r e l a t i v e t o t h i s t r a c e .  p o s s i b l y d i s t i n g u i s h the f i r s t  a r r i v a l beyond c r o s s o v e r  headwave o r a bodywave and t o make d i s t a n t r e c o r d s additional m u l t i p l i c a t i v e factor of distance  F i n a l l y , to as e i t h e r a  more l e g i b l e , an  or d i s t a n c e  squared was  applied. The  r e s u l t s as shown i n F i g s . 5 and 6 i n d i c a t e t h a t by removing  most o f t h e s h o t and system a m p l i t u d e e f f e c t s , a r a t h e r smoothly energy p a t t e r n may be o b t a i n e d .  C l o s e l y spaced, a d j a c e n t r e c o r d s  d i f f e r by a f a c t o r o f two i n a m p l i t u d e .  I n t h e case o f t r a c e s  81802 ( F i g . 5) which have been a r b i t r a r i l y of an i n c o r r e c t l y l o g g e d a m p l i f i e r s e t t i n g . considered It  varying rarely  40702 and  reduced, the p o s s i b i l i t y  T r a c e s 81102 and 81202 a r e  t o be l o c a t e d on s i t e s s u i t a b l y "tuned" t o 3-4 Hz energy.  i s notable that only  exists  2 out o f 51 s i t e s respond i n t h i s manner.  15  T-X/7=0  •20  ,«  RV  .WW,  30  -  T^^^VN^  o  CD CD  I  -CD 60402 S0SO2  ^3 BMOJ 11 SOI _  B3.0J  .  ,  . 1000 Fig.  5  The r e c o r d s e c t i o n r e c o r d e d a t s i t e s from L i t t l e F o r t t o P r i n c e Rupert from s h o t s i n Greenbush Lake. The seismograms have been m u l t i p l i e d by d i s t a n c e and the f o l l o w i n g o p e r a t i o n s performed: - - n o r m a l i z e d w i t h r e s p e c t t o t r a c e 40402 - - t r a c e 40702 reduced by 4 - - t r a c e s 81102, 81202, 81802 reduced by 2 — b a n d p a s s f i l t e r e d from 2 t o 9 Hz. _ — t i c s on x - a x i s are a t 40 km. i n t e r v a l s  16  T-X/7=0  20  30  •0 ci  CD  o I ----- ^ N A V v ^ W - * " " -V^Wl'v -,  " ,  CD I  B0702 . ^ ^ / S ^ ^ J ^ ^ ^ ^ v ^ ^ - ^ ^ ^ ^ v ^ ^ |faaeo2 _ w BO90Z . „_ SIO03 -w-VV e n oz 8' 20?  CD  yyj^.  VAV^'ITVS^/VN^W^-^VVV'^W^^  ^J,"?.!  .~»  t  1000 F i g . 5a  The r e c o r d s e c t i o n from Greenbush as shown i n F i g . 5 except t h a t a f a c t o r o f d i s t a n c e squared has been a p p l i e d . Note t h e r a t h e r u n i f o r m l e v e l o f t h e amplitude o f t h e f i r s t a r r i v a l out t o a t l e a s t t r a c e 82302. T i c s on x - a x i s a r e a t 40 km. i n t e r v a l s .  • 30  T  "  X / 7  -  0  25  CD C) —] -  o ? 0  co-.  "3-D [\3 '~D 03  rn o  Fig.  6  The r e c o r d s e c t i o n r e c o r d e d from P r i n c e Rupert t o P r i n c e George from s h o t s i n B i r d Lake. A f a c t o r o f d i s t a n c e squared has been a p p l i e d a l o n g w i t h t h e f o l l o w i n g operations: - - n o r m a l i z e d w i t h r e s p e c t t o t r a c e 86102 (2nd t r a c e ) — b a n d p a s s f i l t e r e d from 3 t o 9 Hz — t i c s on x - a x i s a r e a t 20 km. i n t e r v a l s  18  _  CHAPTER 4  INTERPRETATION  4-1.  G e n e r a l p r i n c i p l e s and method In  this thesis f i r s t  a v e l o c i t y g r e a t e r than 7.5  a r r i v a l s beyond about  km/s  150 km,  w i l l be d e s i g n a t e d as P . n  c h a r a c t e r i z e d by a f r e q u e n c y o f about  4 Hz,  t r a v e l l i n g with T h i s event i s  a somewhat emergent  arrival  and an amplitude which i s g e n e r a l l y l e s s than any o t h e r w e l l - c o r r e l a t e d event on t h e r e c o r d s e c t i o n s .  I t i s i n t e r p r e t e d as the a r r i v a l  behaves k i n e m a t i c a l l y l i k e an event which i s c r i t i c a l l y  that  r e f r a c t e d a t the  Mohorovicic  (M) t r a n s i t i o n and w h i c h t r a v e l s w i t h the v e l o c i t y o f the  immediately  u n d e r l y i n g medium.  The l a r g e r amplitude secondary a r r i v a l i s i n t e r p r e t e d as the reflection  from the M.  I t i s assumed t h a t t h i s r e f l e c t i o n i s preceded  a s m a l l e r amplitude, d i r e c t l y r e f r a c t e d a r r i v a l .  The  former  behaves as a  wave c o n t i n u o u s l y r e f r a c t e d w i t h i n the c r u s t above the M and appears first  a r r i v a l from Greenbush o n l y out t o about W i t h i n 340 km  first  arrivals  t r a v e l l i n g w i t h P^ v e l o c i t i e s may  upon i n the next  of  from Greenbush,  i n d i c a t e energy  from  T h i s w i l l be e l a b o r a t e d  section.  a r r i v a l s were p i c k e d from analogue  record sections.  weighted  r e c o r d s w i t h the a i d  L e a s t square v e l o c i t i e s were determined  using a  a n a l y s i s and p r e l i m i n a r y models were computed u s i n g p l a n e  t r a v e l - t i m e e q u a t i o n s (Jakosky, In  as a  km.  o f Greenbush and beyond 800 km  s h a l l o w e r and deeper h o r i z o n s r e s p e c t i v e l y .  First  175  by  layer  1961).  o r d e r to r e f i n e the p r e l i m i n a r y models, t r a v e l - t i m e  curves  19  based on r a y t h e o r y f o r t h e case o f h o r i z o n t a l gradient  (Bullen, 1 9 6 5 ; Officer,  perturbed u n t i l  first  layers with a v e l o c i t y  1 9 5 8 ) were c a l c u l a t e d .  G r a d i e n t s were  and secondary a r r i v a l branches were o b t a i n e d which  s u b j e c t i v e l y b e s t f i t t h e observed a m p l i t u d e d i s t r i b u t i o n . The program which uses a weighted l e a s t - s q u a r e a n a l y s i s t o der i v e v e l o c i t i e s from t r a v e l - t i m e s was o r i g i n a l l y w r i t t e n by K.G. B a r r , modif i e d by M.J.  B e r r y and adapted f o r the CDC  6400  by the w r i t e r .  M u e l l e r wrote t h e o r i g i n a l v e r s i o n o f the t r a v e l - t i m e program Appendix A.  T h i s program has been a l t e r e d by M.J.  D r . Gerhard outlined i n  B e r r y and t h e w r i t e r t o  c a l c u l a t e and p l o t t h e p-A graph and o p e r a t e on the CDC  6400.  U s i n g t h e r e s u l t s o f t h e t r a v e l - t i m e a n a l y s i s as b a s i c models for  t h e e a s t e r l y and w e s t e r l y ends o f the a r e a , a time-term approach was  t h e n used t o s t u d y s t r u c t u r e i n t h e i n t e r v a l between t h e shot p o i n t s . program which computes t h e t i m e - t e r m p r o f i l e was w r i t t e n by M.J. adapted f o r t h e 4-2.  6400  L e a s t square First  The  B e r r y and  by t h e w r i t e r . analysis  a r r i v a l d a t a observed westward from Greenbush  Lake t o  P r i n c e Rupert - e x c l u s i v e o f d a t a from s i t e s 4 0 1 and 4 0 2 where t h e f i r s t arrival ±0.01 Bird  i s c o n s i d e r e d a c r u s t a l phase  km/s.  First  - indicate a P  8.03  ±0.03  km/s.  L o c a l c o n s i s t e n t v a r i a t i o n s from "normal" P From s i t e s 4 0 3 t o 4 0 8 i n c l u s i v e , f o r example,  a velocity of clusive  7.61  ±0.06  km/s.  indicate a velocity of  over the r e l a t i v e l y short (Fig.  1 ) , first  velocity of 8.09  a r r i v a l s p i c k e d on r e c o r d s observed eastward from  Lake i n d i c a t e a v e l o c i t y o f  noted.  N  First 8.41  r  v e l o c i t y were the data i n d i c a t e  a r r i v a l s at s i t e s  ±0.07  km/s.  408  to  417  F a r t h e r t o the n o r t h ,  ( 5 0 km) l i n e from B a r k e r v i l l e t o Quesnel  a r r i v a l s from Greenbush  in-  indicate a P  velocity of  20  10.04  ±.3 km/s, whereas from Q u e s n e l t o Nazko the observed P^ v e l o c i t y i s  8.16 ±.4 km/s.  T h i s l i n e as r e v e r s e d  from R i p l e y Bay i n d i c a t e s v e l o c i t i e s  o f 6.94 ±.3 km/s and 8.12 ±.1 km/s o v e r t h e same r e s p e c t i v e s e c t i o n s . S i n c e t h e a r e a from s i t e 401 t o s i t e 417 i s u n r e v e r s e d , problem o f s e p a r a t i n g t h e e f f e c t s o f v e r t i c a l  the  (or l a t e r a l ) v e l o c i t y  t i o n from M topography cannot be r e s o l v e d a t p r e s e n t .  varia-  Assigning the l o c a l  v e l o c i t y change from 401 t o 417 t o d i p would r e q u i r e an M " s u r f a c e " which d i p s down a t about 4 degrees t o t h e a r e a immediately e a s t o f s i t e 408 and r i s e s with River  t h e same s l o p e t o t h e r e g i o n immediately e a s t o f t h e F r a s e r  ( F i g . 1 ) . The apparent v e l o c i t i e s  immediately e a s t o f Quesnel a l s o  r e q u i r e an M s u r f a c e which r i s e s a t about 10 degrees towards the F r a s e r River. From F i g . 1 i t i s n o t e d t h a t , r a t h e r than a p r o f i l e , a r e a c t u a l l y " f a n s h o t " over r a t h e r s m a l l a n g l e s .  these  The f a n a n g l e s  data  subten-  ded  a t Greenbush and B i r d Lake a r e about 20 and 24 degrees r e s p e c t i v e l y .  The  r e s u l t a n t model t h e r e f o r e must r e p r e s e n t  covered. and  The a r e a l d i s t r i b u t i o n o f r e c o r d i n g s i t e s ,  the d i s t a n c e  range  the r e v e r s e d v e l o c i t i e s would i n d i c a t e t h a t 8.06 km/s i s c l o s e t o a  t r u e upper mantle v e l o c i t y 4-3.  an average f o r t h e a r e a  Travel-time  f o r t h e r e g i o n under  study.  curves  A program, based on r a y t h e o r y which uses as i n p u t s i m p l y 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 , was used t o c o n s t r u c t t r a v e l - t i m e curves f o r comparison w i t h  t h e r e c o r d s e c t i o n s (Appendix A ) .  t o t h e wave s o l u t i o n , t h e use o f r a y t h e o r y  As an a p p r o x i m a t i o n  i s v a l i d provided  i n v e l o c i t y g r a d i e n t over a wavelength i s s m a l l compared w i t h where c i s t h e v e l o c i t y and X  i s t h e s u r f a c e wavelength  t h e change c/X  Q  (Officer,  , 1958).  o F o r a s u r f a c e wavelength o f 1 km, any change i n v e l o c i t y g r a d i e n t  at M  21  surface depths must then be small compared with a gradient of about 8 s In fact, the derived models shown in Fig. 7 and Fig. 8 violate this restriction in the neighbourhood of each boundary. These areas are precisely those which determine the nature of the cusps in the travel-time curves. From published wave solutions, i t i s instructive to note some of the limitations of ray theory which should be considered when interpreting real data.  For the case of spherical, longitudinal waves in a plane-  layered structure, Cerveny (1961) has shown that the amplitude maximum is displaced beyond the ray theory c r i t i c a l point by a distance dependent mainly upon the index of refraction and the frequency of the incident wave. This distance interval increases and the shape of the amplitude maximum broadens as the refractive index approaches unity and the incident wave frequency decreases. Therefore, the amplitude maximum would be expected to appear on a record section as a region of large amplitudes displaced from the ray-theoretical c r i t i c a l point.  Comparison of the  present models with curves given by Cerveny (1966) and Fuchs (1970, p 536) suggests that the true amplitude maximum may be displaced some 40 to 50 km beyond the ray-theoretical c r i t i c a l point. Ray theory predicts that the reflected and head wave travel-time branches meet at the c r i t i c a l point.  The wave solutions indicate that the  travel-time branches, in fact, intersect beyond the ray-theoretical c r i t i c a l point in the zone of interference of the reflected and head waves. However, the difference in time between travel times predicted by ray theory and those computed using wave solutions - for a model characterized by velocities and frequencies not grossly different from  22  those i n the p r e s e n t s t u d y - appears r e a l data  (Cerveny,  too s m a l l to be d e t e c t a b l e i n the  1966).  I t would t h e r e f o r e appear t h a t t r a v e l - t i m e curves might good e s t i m a t e s o f a r r i v a l  times a l o n g b o t h  branches t o w i t h i n about 40 km  the r e f l e c t e d and  of c r i t i c a l .  be  refracted  Used as such, t h e y would  g i v e o n l y a g e n e r a l i n d i c a t i o n o f where the l a r g e s t a m p l i t u d e s  might  be  expected. 4-4.  F i t t i n g t r a v e l - t i m e curves I t was  t o be e x p e c t e d ,  i n a d d i t i o n t o the t h e o r e t i c a l  limitations,  t h a t i n such a g e o l o g i c a l l y complex a r e a a s i m p l e , c o n t i n u o u s t r a v e l - t i m e c u r v e c o u l d not e x p l a i n a l l the apparent therefore necessary and  t o d e c i d e which energy  correlations.  Primary  well-developed  c o n s i d e r a t i o n was  p o i n t d i s p l a c e d from the amplitude p l a c i n g the cusps  a r r i v a l s and  Since ray theory p r e d i c t s a maximum, the attempt  c o i n c i d e n t w i t h the l a r g e s t energy  R a t h e r , because the r e f l e c t e d ' branch  was  limitations.  g i v e n to the f i r s t  r e f l e c t e d branches.  It  c o r r e l a t i o n s must be e x p l a i n e d  t o produce a model which would have acknowledged '  at  energy  was  not  on t h e  the  critical directed section.  i s r e l a t i v e l y w e l l d e f i n e d , the  models were a d j u s t e d so as to g i v e a good f i t t o the c u r v a t u r e o f t h i s branch.  The  c u r v a t u r e , o f c o u r s e , i s r a t h e r s e n s i t i v e t o the  g r a d i e n t s i n the r e g i o n o f the  transition.  U s i n g the models suggested by the l e a s t square with discontinuous v e l o c i t y branches was  velocity  velocities  i n c r e a s e s - the c u r v a t u r e o f the  a poor f i t to the observed  data.  -  reflected  In o r d e r t o produce a  b e t t e r f i t , the g r a d i e n t s shown i n F i g . 7a, F i g . 8a and F i g . 9a were required.  The  g r a d i e n t s a l s o b r i n g the cusp B w i t h i n about 50 km  of  23  1000  ig.  7  The t r a v e l - t i m e (model 1) f o r the Greenbush r e c o r d s e c t i o n . Note the g r a d i e n t n e c e s s a r y t o produce the c u r v a t u r e o f t h e r e f l e c t e d A-B b r a n c h . T i c s on x - a x i s a r e a t 20 km. i n t e r v a l  24  BC69 LTFJ-PR -RPT .  Fig.  7a  The  velocity-depth  s t r u c t u r e f o r Model  1.  25  Fig.  8  The t r a v e l - t i m e model 2 f o r the Greenbush Lake s e c t i o n . Minor t r i p l i c a t i o n s , C-D and E-F, have been added t o e x p l a i n s i g n i f i c a n t secondary energy. T i c s on x - a x i s are a t 20 km. i n t e r v a l s .  26  BC69 LTFT.-PR . RPT .  i  0  —  •  —  —  •  —  •  —  •  —  2  10B 4 Fig.  8a  _i  1—  KM/S The v e l o c i t y - d e p t h  1  10  s t r u c t u r e f o r Model 2.  27  Fig. 9  The t r a v e l - t i m e model f o r the B i r d Lake r e c o r d T i c s on x - a x i s a r e a t 20 km. i n t e r v a l s .  section.  28  Fig. 9a The velocity-depth structure for the Bird Lake Model.  29  what may 4-5.  be t h e l a r g e s t energy on t h e s e c t i o n ( F i g . 10) ,  Record s e c t i o n from Greenbush F i t t i n g t r a v e l - t i m e c u r v e s t o d a t a w i t h i n 360 km o f Greenbush  was d i f f i c u l t due t o t h e p o s s i b i l i t y o f an i n t e r m e d i a t e l a y e r . l a y e r i s s u g g e s t e d by t h e f i r s t a r r i v a l s on t r a c e s 403 t o 407, w i t h a v e l o c i t y o f 7.6 of  km/s.  This inclusive,  However, t h e q u a s i - c o r r e l a t a b l e n a t u r e  t h e s e c o n d a r y e n e r g y which must be used t o d e f i n e o r suggest i t s  t r a v e l - t i m e t r i p l i c a t i o n l e a v e s some doubt as t o t h e r e a l i t y o f t h i s layer.  Assuming  t h e f i r s t a r r i v a l on t r a c e s 403 t o 407 d e f i n e s a r e -  f r a c t e d t r a v e l - t i m e b r a n c h , t h e n t h e l a r g e r secondary energy immediately following P  n  on t r a c e s 411 t o 417, i n c l u s i v e , would make t h e  b r a n c h e s t o t h e cusp a t C ( F i g . 8 and F i g . 10) appear p l a u s i b l e . energy i n t h i s r e g i o n i s c o n f u s e d by i t s p r o x i m i t y t o P C r i t i c a l energy a l o n g t h e r e f l e c t e d b r a n c h , CD,  n  The  arrivals.  i s s u g g e s t e d by t h e  l a r g e r secondary energy on t r a c e s 407 and 408, but a g a i n t h e r e f l e c t e d b r a n c h i s not c l e a r .  The f i r s t p a r t o f t h e r e f r a c t e d b r a n c h f r o m the  cusp a t D ( F i g . 9) i s perhaps i n d i c a t e d by secondary e v e n t s (on r e c o r d s 402 and 404) which o c c u r between t h e more w i d e l y s e p a r a t e d t r a v e l - t i m e b r a n c h e s p r o d u c i n g t h e cusp a t B.  The a l t e r n a t i v e i n t e r p r e t a t i o n , i n  terms o f an i n t e r f a c e w i t h a r e a s o n a b l e d i p and t h e f a c t t h a t t h i s p a r t o f t h e s e c t i o n i s u n r e v e r s e d , l e a v e s t h e q u e s t i o n open.  Synthetic  seis-  mograms c o n s t r u c t e d f o r the d i f f e r e n t models might p r o v i d e some o t h e r insight. I t i s p e r t i n e n t t o n o t e here a s t u d y , done on d a t a a p p l i c a b l e t o the  a r e a b e g i n n i n g 100 km e a s t o f Greenbush Lake a l o n g l a t i t u d e 50°30',  by Chandra and Cumming (1972). Greenbush shot p o i n t .  P a r t o f t h e d a t a was r e c o r d e d from t h e  The r e s u l t s show a 7.2 km/s  l a y e r a t a depth o f  30  about  30 km which p i n c h e s out towards the T r e n c h .  I n c l u s i o n o f an  i n t e r m e d i a t e l a y e r i n the p r e s e n t model would thus be c o m p a t i b l e w i t h d a t a t o the e a s t . from one and one  However, s i n c e the two  areas a r e some 300 km  distant  a n o t h e r , s e p a r a t e d b o t h by a zone o f major g e o l o g i c a l c o m p l e x i t y o f the l a r g e s t Bouguer anomalies  p r o b a b l y not v a l i d  i n the C o r d i l l e r a , i t i s  t o r e l a t e the a r e a s i n any  detail.  From F i g . 10, d a t a on the r e c o r d s e c t i o n from Greenbush are relatively 360  s p a r s e i n the r e g i o n about  t o 480 km  400 km.  However, the d a t a  suggest t h a t , w h i l e the amplitude o f the P  n o r m a l l y , the amplitude creases abruptly.  o f energy a l o n g the A-B  shows p r e c i s e l y the same amplitude  a r r i v a l s behaves  t r a v e l - t i m e branch i n -  I t i s t o be noted t h a t a t 450 km  seismogram ( i n a d d i t i o n t o t r a c e 431  r  from  there e x i s t s  another  shown) r e c o r d e d a t s i t e 801 which  character.  G e o l o g i c a l l y , the phenom-  enon c o i n c i d e s w i t h the f a u l t t r e n d i n g s o u t h e a s t from McLeod Lake t o the a r e a immediately  e a s t o f P r i n c e George ( F i g . 1 ) .  Upper P a l e o z o i c r o c k s immediately v o l c a n i c s t o the west. related  t o the e a s t from  c o n t i n u e s to the depths o f the M n  802  t o 812  immediately  Triassic-Jurassic  this surface d i v i s i o n  becomes more u n c e r t a i n w i t h the o c c u r r e n c e o f s i -  r a t h e r abrupt appearance  B r i t i s h Columbia  separates  transition.  n i f i c a n t l y more n o i s e i n the s i g n a l passband  regional effect.  fault  Assuming the b e h a v i o u r o f the c r u s t a l phase i s  to the s u r f a c e geology, i t suggests t h a t  Picking P  The  from s i t e s  802  The  and d i s a p p e a r a n c e o f the n o i s e would suggest a  S u p e r p o s i t i o n o f the s i t e map  on the g e o l o g i c a l map  r e v e a l s t h a t , e x c e p t i n g s i t e s 808  and 809,  l i e on T e r t i a r y v o l c a n i c f l o w s and p y r o c l a s t i c s . p r i o r t o 802  t o 812.  a l l sites Sites  l i e on T r i a s s i c and J u r a s s i c v o l c a n i c s  and  of from  31  pyroclastics, sites 808 and 809 occur on Jurassic granites and sites immediately following 812 f a l l on Jurassic volcanics and pyroclastics. It appears that over this central region sites located on the tertiary sediments are characterized by a higher level of noise in the signal passband.  The amplitude character of the f i r s t one or two seconds of  energy i n this range may be undergoing some interesting changes; however, the noise effectively precludes an interpretation. Beginning at about 660 km (site 814), the character of the f i r s t second of energy has changed notably. The f a i r l y consistent 4 to 5 Hz P phase which characterized records to at least site 809 i s now n more emergent i n character and i s followed at about 0.8 seconds by a larger amplitude energy band with a similar velocity arid frequency. This phase i s well developed from sites 818 to 823.  Here again, the nature of  the f i r s t energy begins to change and by sites 825 a very emergent cycle ' of 3 Hz energy i s immediately followed by significantly larger amplitude 4 to 5 Hz energy. An enlarged picture of this part of the section i s shown i n Fig. 12.  At subsequent sites, the f i r s t amplitudes decay and the  f i r s t arrival may have a velocity of about 8.4 km/s at the end of the section. A problem of phase identification arises here. Is the amplitude variation i n the range about 820 km produced by a triplication of the P^ travel-time curve or i s the larger secondary arrival the upper mantle P phase, seen i n earthquakes studies? f i r s t arrival at about 820 km.  The P phase would then become the  For the present study, i t i s assumed  that the effect i s not due to a local surface effect. planations to be considered are: *  Alternative ex-  32  Fig.  10  The  f i t o f Model 2 t o the d a t a o f F i g . 5a.  T i c s on x - a x i s a r e a t 40 km.  intervals.  33 (a)  structural focusing by M topography of either P  by Barr  (1971),  or  P  r  energy as described  energy as described by Mereu  Both  (1969).  effects could produce a triplication of the travel-time curve. (b)  a triplication of the travel-time curve due to a discontinuous velocity increase in the upper mantle.  These alternatives w i l l be discussed after examining data from Bird Lake , 4-6.  Record section from Bird Lake The f i r s t arrival branches on the section recorded from Bird Lake are  less clearly defined than on i t s reversed counterpart (Fig. 6 ) .  This i s  in part due to fewer stations and a correspondingly larger station spacing but also to the use of spreads.  Since these arrays could feasibly be set  up only along linear segments of roadway, there was l i t t l e choice of seismometer site and hence background noise was relatively severe.  Sites  8 4 0 to 8 4 9 f a l l on tertiary sediments and, like the reversed sites in the same area, are characterized by considerable background noise. Those recordings west of 8 4 9 where three components could be set on bedrock ( 8 5 0 , 8 5 6 , 8 5 7 , 8 5 9 ) show relatively minor background noise.  It is noted  that where an array was used, a l l six seismograms were used in determining a f i r s t arrival time.  Thus, the confidence of many of the picks is not  as poor as the noise level on the chosen records might suggest. On the Bird Lake section i t i s noted that, due to a thinner crustal section near the coast ( P intercept of 5 . 5 seconds) than near L i t t l e Fort, the larger c r i t i c a l energy is effectively missing.  From the coast  eastwards, the main phases are identified as P ^ and a larger coherent second arrival with a velocity of 6 . 4 km/s.  The amplitude of the secondary  phase drops abruptly at about 2 9 0 km (from trace 8 5 7 to 8 5 6 ) .  It is  34 succeeded t h e r e by a s l o w e r , more emergent phase. l a r g e r amplitude phase  The c e s s a t i o n o f t h e  c o i n c i d e s a p p r o x i m a t e l y w i t h t h e w e s t e r n edge o f  the H a z e l t o n mountains.  The s u r f a c e topography changes  c h a r a c t e r i z e d by seven thousand f o o t peaks  i n t h e west t o the B u l k l e y  ranges i n t h e e a s t where peaks r e a c h n i n e thousand  feet.  The q u e s t i o n o f phase i d e n t i f i c a t i o n a r i s e s a g a i n . phase, w i t h a v e l o c i t y  from an a r e a  I s the secondary  l e s s than 6.4 k m / s e c , s i m p l y t h e mantle  reflection  which has been p e r t u r b e d by l a t e r a l s t r u c t u r e i n the r e g i o n about 857, o r i s i t a s e p a r a t e a r r i v a l ?  Because  site  of the d i s t i n c t p o s s i b i l i t y o f  l a t e r a l s t r u c t u r e e f f e c t s and because  the t r a v e l - t i m e branches a r e so  c l o s e a t t h i s d i s t a n c e , t h e cusp at A  ('Fig..  distances.  9) i s not c a r r i e d  out t o g r e a t e r  The e f f e c t o f t h i s i s o n l y t o make t h e f i n a l g r a d i e n t above  the M (lower l e f t  graph i n F i g . 11) s l i g h t l y  less.  The v e l o c i t y - d e p t h s t r u c t u r e f o r t h i s s e c t i o n i s shown i n F i g . 9a. The  first  a r r i v a l s show no w e l l - d e v e l o p e d v a r i a t i o n s i n v e l o c i t y and  d e v i a t i o n s from t h e l e a s t square v e l o c i t y i n d i c a t i v e o f topography.  (8.03 km/s) a r e p l a u s i b l y  Thus, t h e p r e s e n c e o f a s i g n i f i c a n t  inter-  mediate l a y e r i s n o t s u g g e s t e d and M v e l o c i t y i s reached a t a depth o f 30 km. 4-7.  The anomaly a t 820 km I n an attempt  distance  t o d e c i d e which o f t h e s u g g e s t e d e x p l a n a t i o n s o f the  phenomena a t 820 km i s more p l a u s i b l e , the f o l l o w i n g p o i n t s a r e pertinent: (a)  Comparison  o f F i g . 5 and 5a shows t h a t  squared f a c t o r appears t o r e s t o r e P  n  the a p p l i c a t i o n o f a d i s t a n c e  amplitude t o an a p p r o x i m a t e l y  c o n s t a n t l e v e l i n the d i s t a n c e range from 0 t o .800 km.  The  35  BC70  Fig.  PR . R P T . - P R . G E O . ( 1-3/9-12 )  1 1 . The f i t o f t h e B i r d Lake model t o t h e record section. T i c s on x - a x i s a r e a t 2 0 km. i n t e r v a l s .  36 suggestion i s that to this distance the f i r s t arrival travels as a head wave, probably connected with the M boundary. Beyond 800 km the amplitude of the f i r s t distinct arrival increases uniformly to a maximum at about 815 km and subsequently decays (Fig. 12).  The  length of the section precludes determination of the exact nature of the decay. However, this pattern is very similar to that calculated by Mereu (1969) using ray theory to analyse the focusing effect of M topography on P energy. Topographically, the phenomena occurs in the region characterized by the highest mountains in the coast range at this latitude.  Culbert  (1971) points out that a line between Douglas Channel and Nass River (at a distance of approximately 810 km on the record section) marks a prominent scarp line in the summit envelope. Geologically, ''the phenomenon occurs at the eastern edge of the Coast Plutonic Complex in the region of the Skeena Arch (Wheeler, 1970; Monger et a l . , 1972). The magnitude of the velocity discontinuity required to produce a travel-time triplication which might account for the amplitude anomaly is approximately 0.2 km/s  at a depth of 95 km.  However,  this raises the question of upper mantle homogeneity. Assuming this model i s true, the inference i s that no similar velocity discontinuity occurs between the M and 95 km, since no effect similar to that at 820 km distance is observed on the record section. Construction of synthetic seismograms for this model would aid in deciphering data from below the M.  37  890  Fig.  12  P a r t o f t h e Greenbush r e c o r d s e c t i o n expanded t o show t h e amplitude i n c r e a s e n e a r 820 km. T i c s on x - a x i s a r e a t 4 km. i n t e r v a l s .  38 The r e v e r s e d d a t a from B i r d Lake i s r a t h e r i n c o n c l u s i v e w i t h r e s p e c t to e i t h e r a l t e r n a t i v e . t o 630 km  S i n c e the seismograms are o n l y o b t a i n e d out  from B i r d Lake, e f f e c t s from a depth o f 95 km  a r e not  recorded. I f t h e a m p l i t u d e b e h a v i o u r a t 820 km on the Greenbush is  section  due t o t h e f o c u s i n g o f P energy by M topography, a s i m i l a r  suitably offset  t o about  effect,  the a r e a o f t r a c e 852 on the r e v e r s e d s e c -  t i o n , might t h e n be e x p e c t e d .  U n f o r t u n a t e l y , the l a c k o f r e v e r s e d  d a t a i n t h i s r e g i o n l e a v e s t h e q u e s t i o n open. the apparent l a c k o f f o c u s s e d energy may  P a r t o f the r e a s o n f o r  be because  the d i s t a n c e  range from B i r d Lake i s t o o s m a l l t o w i t n e s s s i g n i f i c a n t P energy r e t u r n from below the M. However, t h e r e a r e two may  f e a t u r e s o f the B i r d Lake s e c t i o n  i n d i c a t e t h a t the lower c r u s t and perhaps the M t r a n s i t i o n i s  different  i n t h e r e g i o n o f the H a z e l t o n Mountains.  d e s c r i b i n g the s e i s m i c a r r i v a l s from B i r d Lake t h a t the phase 290 km.  I t was  r e f l e c t e d from the M t r a n s i t i o n dropped a b r u p t l y a t about T h i s c o u l d i n d i c a t e a major d i s c o n t i n u i t y e x t e n d i n g t o  summit e n v e l o p e r e p o r t e d by C u l b e r t  (1971).  s t r o n g secondary w a v e l e t on t r a c e s 856  t o 851  i n the  The o t h e r f e a t u r e i s a ( F i g . 11) s i m i l a r t o  t h e l a r g e amplitude secondary a r r i v a l on r e c o r d s 816 t o 822 Greenbush  ( F i g . 10).  f r e q u e n c y o f 4-5 n  noted i n  the amplitude o f  the M i n the a r e a r o u g h l y c o i n c i d e n t w i t h the s c a r p l i n e  P  which  from  Both w a v e l e t s a r e c h a r a c t e r i z e d by a  Hz, an amplitude a p p r o x i m a t e l y f o u r times t h a t o f  and a v e l o c i t y o f 7.7 km/s.  r e l a t e d , i t suggests that  Assuming  the two w a v e l e t s a r e  the M t r a n s i t i o n beneath  the Skeena A r c h  39 has the rather unique property of producing this reverberation. 4-8.  Time-term study In an effort to see what structure i s implicit in interpreting  arrivals as indicating depths to the M and whether the reversed traveltimes are compatible, a time-term study was undertaken.  Although the  experiment was not designed to exploit this type of study, the small d i f ference between reversed, least-square velocities and the fact that at least one site had observed both shot points while many sites from each survey l i e within a few kilometers of each other are points in favour of its application.  The rather limited areal distribution of sites means  that the resulting model may only be valid in a, zone trending west-northwest, coincident with the general line of recordings. To determine the general topography on the M between L i t t l e Fort and Prince Rupert, a simplified version of the "Delay-Time-Function" method outlined by Morris (1972) was used.  Conventionally, the refractor sur-  face i s determined by calculating a time term for every s i t e .  The delay-  time method assumes that the time-term surface may be represented by a simple mathematical function of position.  In doing so, the procedure  eliminates the necessity of determining the arbitrary constant which may be subtracted from a l l shot time-terms and added to the recording site time-terms (Scheidegger and Willmore, 1957). In addition, the degree of smoothing of the data can be controlled somewhat by specifying the form of the function to be f i t t e d .  In the present study the time-term surface  is described as a function of only one parameter - the distance from an arbitrary reference point.  40 Since the s i t e s are r e s t r i c t e d to a zone trending to the northwest, the present data were used to construct a time-term p r o f i l e , rather than a surface, along a l i n e coincident with an approximate of the zone of s i t e s .  centre-line  Polynomials i n x - the distance from the mid-point  of the l i n e - were f i t t e d to the data to determine the p r o f i l e . In d e t a i l , a l i n e centered near the midst of the survey area with an azimuth coincident with the trend of the s i t e s was chosen. Secondly, the distances from the mid-point of t h i s l i n e to the s i t e s were projected onto the l i n e and o f f s e t a distance equal to the step-out of the c r i t i c a l ray i n the appropriate d i r e c t i o n .  The set of points determined  by the o f f s e t distances and the travel-times was then least-square f i t t e d by polynomials of increasing order.  I n i t i a l l y , a P^ v e l o c i t y , an  average c r u s t a l v e l o c i t y and a single shot (recording) s i t e time term were input to determine the o f f s e t distances.  Subsequent  f i t s used  previous estimates of time•terms and P^ v e l o c i t i e s to compute new o f f s e t distances and hence new p r o f i l e s .  An average c r u s t a l v e l o c i t y of 6.4 km/s  determined from the travel-time models was used throughout. Herein the t r a v e l time from shot  i  to s t a t i o n  j  i s given by  the usual formula T  «  =  J  —  + t  i  + t  j  where A.. i s the h o r i z o n t a l shot-station distance, ij shot and s t a t i o n time terms and constant f o r the p r o f i l e zone. distance  x  V  t . are j  I t i s assumed that the time-term  n  fc  and  i s the r e f r a c t o r v e l o c i t y , assumed  can be adequately described by k t(x) = E a, x k=0  t. i  ,  t ( x ) where  t at  and n i s the degree of the p o l y n o m i a l .  Then the t r a v e l - t i m e i s g i v e n  by A  T  H  k  n  k  n  « " r \% V i f 1  Jo  + k  +  +  r -^  M  n  m-1  1 3  m-1  k=0  squares,  , K  .  1  A p p l y i n g the p r i n c i p l e o f l e a s t M  ^  0  1  ,  the q u a n t i t y A,,  J  V  1 J  (where M i s the number o f o b s e r v a t i o n s ) I s minimized. yields  the normal e q u a t i o n s which may  a^ and  V.  4-9.  D i s c u s s i o n of time-term  This operation  be s o l v e d f o r the  coefficients  study  To examine the e f f e c t o f a r b i t r a r i l y c h o o s i n g the l i n e o f profile,  s o l u t i o n s were o b t a i n e d  f o r the p o s i t i o n  54° l a t i t u d e , 125°  l o n g i t u d e (azimuth  longitude  of 282°) and  293°).  (azimuth  The  of the l i n e e n t e r e d  o f 3 1 0 ° ) ; 54° l a t i t u d e ,  53.5° l a t i t u d e , 124°  g e n e r a l topography of the time-term  each s o l u t i o n .  s t a t i s t i c a l F t e s t was I t was  found  polynomial nomial  that a polynomial  p r o f i l e changed i n s i g f i t numerically.  including  gave a markedly improved RMS  Unexpectedly, residual.  of  terms t o the  Subsequent s o l u t i o n s w i t h  improvement.  i s s t i l l n o t an i n t e r p o l a t i v e  (azimuth  used to t e s t t h e s i g n i f i c a n c e  f o u r t h o r d e r gave a s i g n i f i c a n t f i t . o r d e r terms showed l i t t l e  125°  longitude  n i f i c a n t l y ; however, the l a s t mentioned l i n e gave the b e s t The  at  higher  the e l e v e n t h  order  Since t h i s p o l y -  f i t , the s u g g e s t i o n i s t h a t  the  i n c l u s i o n o f the e l e v e n t h o r d e r term, which i n t r o d u c e s a wavelength o f  42 about 200  km,  is significant.  A p l o t o f the  t h a t the smoothing e f f e c t o f the simplification. the  In f a c t , the  not matched u n t i l the e l e v e n t h n o t a b l e i s the w i t h the p o i n t s i z e d by The  f a c t that  a shorter  (=200 km)  from Greenbush.  The  Perhaps most compatible  i s thus  character-  s i g n i f i c a n t wavelengths.  s t r u c t u r e has  an amplitude o f about 10  from  wavelength which i s  profile  a wavelength o f about 800  w h i l e the s h o r t e r  over-  from B i r d Lake are  reversed  shows  residuals  order polynomial i s f i t t e d .  l a t e r a l elements w i t h at l e a s t two  about 5 km,  4-10.  d e v i a t i o n o f the  the p r o f i l e p o i n t s  b r o a d e r s t r u c t u r e has  and  f o u r t h o r d e r p o l y n o m i a l i s an  consistent  fourth order p r o f i l e depicts  time-terms ( F i g . 13)  km  and  an amplitude  a wavelength o f about 200  km  km.  Limitations E l e v a t i o n c o r r e c t i o n s were not  doubtful feet.  t h a t any  survey p o i n t s  R e c o r d i n g s i t e s were not  r o u t e s which f o l l o w the  a p p l i e d t o the d a t a s i n c e i t i s  d i f f e r i n e l e v a t i o n by more than 2500 g r e a t l y d i s t a n t from the main a c c e s s  r i v e r v a l l e y s i n regions  where c o r r e c t i o n s might  be s i g n i f i c a n t . The  v a l i d i t y o f the  h a v i o u r o f the  residuals.  time-term method i s i n d i c a t e d by  In p r e l i m i n a r y  o f 62  c o u l d be  to them i n the 49  are  r e j e c t e d on  least-square  the b a s i s  analysis.  Of  be-  p r o f i l e s o l u t i o n s , d a t a were  re-examined i f r e s i d u a l s were anomalously l a r g e . out  the  Only four  observations  of large u n c e r t a i n t i e s the r e m a i n i n g 58  shown to have a t o t a l e s t i m a t e d u n c e r t a i n t y  assigned  observations,  o f l e s s than  0.2  is in reality  a  seconds. The  p r o f i l e shown i n F i g . 13  and  F i g . 14  p r o j e c t i o n of the s t r u c t u r e on e i t h e r s i d e o f a l i n e c e n t e r e d at 53.5°  of  Fig.  13  The p l o t o f time-terms c o n v e r t e d t o depth. The o r i g i n o f d i s t a n c e s c a l e i s a t 33.5°N l a t i t u d e , 124°E l o n g i t u d e . The p r o f i l e i s a l o n g a l i n e w i t h an azimuth o f 293°. Note the d e v i a t i o n s from the f o u r t h o r d e r p o l y n o m i a l are c o n s i s t e n t i n o u t l i n i n g a wavelength o f about 200 km.  44 l a t i t u d e , 124° l o n g i t u d e w i t h an azimuth  o f 293°.  Inherent i n t h i s  p r o j e c t i o n a r e two t y p e s o f d i s t o r t i o n .  The f i r s t  i s the unknown e f f e c t  o f f o r c i n g onto t h e p r o f i l e s t r u c t u r e which  i s l a t e r a l l y displaced.  How-  e v e r , s i n c e 80 p e r c e n t o f the s t a t i o n s a r e w i t h i n 60 km o f the l i n e  of  p r o f i l e and the r e v e r s e d d a t a a r e c o n s i s t e n t , the p r o f i l e i s p r o b a b l y a good average p i c t u r e .  The second type o f d i s t o r t i o n i s a s h o r t e n i n g  (<10%) o f t h e p r o f i l e i n t r o d u c e d by p r o j e c t i n g the i n t e r - s t a t i o n onto the l i n e w i t h an azimuth o f 293°.  The e f f e c t  i n t e r - s i t e d i s t a n c e by an amount dependent  distance  s h r i n k s the t r u e  upon the c o s i n e o f the a n g l e  between the l i n e o f s i t e s and the p r o f i l e l i n e .  Examination of t h i s  e f f e c t i n d e t a i l shows t h a t s i g n i f i c a n t s h o r t e n i n g o c c u r s l o c a l l y i n the a r e a s where t h e p r o f i l e  c r o s s e s the l i n e o f s i t e s .  In t h e s e a r e a s the  apparent d i p between l o c a l p a i r s o f p r o f i l e p o i n t s may  be  considerably  i n e r r o r ; however, the g e n e r a l shape o f the p r o f i l e remains t h e same. The incorrect.  tendency t o make the topography more d r a m a t i c may I f t h e M topography  elements, which latitude  f o l l o w s the s t r i k e o f the  not  be  structural  i s about North 30° West i n the a r e a between 52° and  56°  ( F i g . 1 ) , then the p r o f i l e l i n e s t r i k i n g North 67° West c r o s s e s  the s t r u c t u r e o b l i q u e l y .  T h i s would r e s u l t  w i t h lower d i p s than a s e c t i o n normal Thus, any e f f e c t which curate p i c t u r e .  i n an apparent  t o the s t r u c t u r a l  enhances the topography may  cross-section  elements.  p r e s e n t a more a c -  The s i g n i f i c a n c e o f the time-term p r o f i l e w i l l  o u t l i n e d i n Chapter  5.  be  45 CHAPTER 5  SUMMARY  5-1.  T r a v e l - t i m e models Acknowledging  some o f the b a s i c shortcomings o f r a y t h e o r y w i t h  r e s p e c t t o p u b l i s h e d wave s o l u t i o n s , the models shown i n F i g . 7a, F i g . 8a and F i g . 9a have been d e r i v e d f o r the end r e g i o n s o f the s u r v e y .  The  e a r t h ' s c u r v a t u r e has been taken i n t o account i n the manner d e s c r i b e d by Mereu (1969).  A l t e r n a t i v e i n t e r p r e t a t i o n s are p r e s e n t e d f o r the d a t a  w i t h i n 360 km o f Greenbush. l a y e r was  Lake.  The v e l o c i t y o f f i r s t  from B i r d Lake s h o t s and the v e l o c i t y  Nazko (J.A. M a i r , p e r s o n a l communication) this  f o r the s u r f a c e  chosen on the b a s i s o f r e c o r d i n g s made from Nazko t o B a r k e r v i l l e  o f a s h o t i n Punchesakut pit  The v e l o c i t y o f 5.6 km/s  a r r i v a l s at  l o g of a w e l l d r i l l e d  Sands-  near  are i n c l o s e agreement w i t h  figure. In the s i m p l e r model ( F i g . 7 a ) , the v e l o c i t y between 3.5 km  24 km  i s n e a r 6.2  km/s.  At t h i s depth a g r a d i e n t i s i n t r o d u c e d  produces the cusp a t 550 km and 36 km, produce 110 km 8.0  ( s i m i l a r to cusp A i n F i g , 8 ) .  and  which  Between 24  km  the v e l o c i t y g r a d i e n t i s v a r i e d i n the manner shown t o  the n e c e s s a r y c u r v a t u r e t o the r e f l e c t e d branch and the cusp a t ( s i m i l a r t o cusp B i n F i g . 8 ) .  The upper mantle v e l o c i t y i s  km/s. The a l t e r n a t e model from Greenbush  ( F i g . 8a) i n t r o d u c e s a l a y e r  a t the base o f the c r u s t w i t h a v e l o c i t y o f 7.5 km/s  and a v e l o c i t y  c o n t i n u i t y at 95 km i n an e f f o r t t o e x p l a i n v a r i a t i o n s i n P^ and s i g n i f i c a n t energy immediately f o l l o w i n g the P  arrivals.  dis-  velocity These  46 same o b s e r v a t i o n s may  a l s o be e x p l a i n e d i n terms o f topography on the M.  On t h e b a s i s o f the p r e s e n t s t u d y , the two e f f e c t s cannot be s e p a r a t e d . The B i r d Lake model i s shown i n F i g . 9a. surface layer velocity, and 20.5 km,  U s i n g 5.6 km/s  the r e q u i r e d t h i c k n e s s i s 4.5 km.  the v e l o c i t y  i s n e a r 6.4  km/s.  as a  Between 4.5  From 20.5 km  t o 29.5 km,  km the  v e l o c i t y g r a d i e n t s shown i n F i g . 9a a r e r e q u i r e d t o produce the t r a v e l time curve o f F i g . 11. 5-2.  The upper mantle v e l o c i t y  i s 8.0  km/s.  Time-term model Assuming  the v e l o c i t y o f the r e f r a c t i n g h o r i z o n and the c r u s t a l  v e l o c i t y w i t h i n the c r i t i c a l l y  r e f r a c t e d ray cone remain n e a r l y  constant  and the s l o p e and c u r v a t u r e o f the r e f r a c t i n g s u r f a c e are not t o o g r e a t ( B e r r y and West, 1966), the time-term method may  be a p p l i e d .  l e n g t h o f energy o b s e r v e d i n the p r e s e n t d a t a would l i m i t resolution  to about 2 km.  The wave-  vertical  The s t a t i o n s p a c i n g would r e s t r i c t  lateral  s t r u c t u r e r e s o l u t i o n to f e a t u r e s w i t h a wavelength g r e a t e r than about 20 km.  Consistent w i t h these l i m i t a t i o n s ,  the p r e s e n t s t u d y has  an M t r a n s i t i o n c h a r a c t e r i z e d by two prominent wavelengths  delineated  ( F i g . 14 ) .  The l a r g e r f e a t u r e w i t h an apparent wavelength o f about 800 km and an a m p l i t u d e o f about 5 km i s d e p i c t e d by the f o u r t h o r d e r p o l y n o m i a l . the s u r f a c e , t h i s f e a t u r e has as c o r r e l a t i v e s metamorphic  terrains  (Monger  the d i s t r i b u t i o n o f  and H u t c h i s o n , 1971)  and, not  the p h y s i o g r a p h i c r e g i o n s o f the Canadian C o r d i l l e r a . metamorphic  On  surprisingly,  The low grade  t e r r a i n o v e r l i e s the t h i n n e r c e n t r a l c r u s t a l s e c t i o n .  The  " h a l f w a v e l e n g t h " p o i n t has r e c e n t l y been d e p i c t e d as the boundary between the P a c i f i c and Columbian Orogens  (Wheeler and G a b r i e l s e ,  1972).  I t i s a l s o n o t e d t h a t the 800 km wavelength i s s i m i l a r to the dimension  47  Fig.  14  The c o r r e l a t i o n o f the time-term s t r u c t u r e w i t h Geology ( C o r d i l l e r a model a f t e r Wheeler and G a b r i e l s e , 1972). The t r i a n g l e s a r e d a t a from B i r d Lake, the d o t s a r e d a t a from Greenbush.  48 o f topography at the l i t h o s p h e r e  i n t e r f a c e as d e p i c t e d  t e c t o n i c model f o r the e v o l u t i o n  o f the  Mid-Cretaceous  (100  MY)  to O l i g o c e n e  West, then the presently  the  (25 MY),  ibid.).  e n v i s a g e d p l a t e models (Isacks  continental lithospheric flexure quite reasonable. explained  by  The  Assuming t h a t  s t r u c t u r e at 40° km.  the  and wavelengths f o r  ( W a l c o t t , 1970), t h i s number appears  l o n g wavelength s t r u c t u r e , o f c o u r s e , may  also  be  smaller  f e a t u r e w i t h an apparent wavelength o f about 200  km  i s o u t l i n e d by  to the  fourth order polynomial.  The  feature  s t r u c t u r e elements and  their subdivisions,  topographic expression  on  consistent  suggests t h a t  the  the M t r a n s i t i o n .  the  area presently  under s t u d y .  This  i s already  1971)  two  at l e a s t 50 km  and  the  f a c t that  deep beneath Vancouver I s l a n d  M  considerably  suggested by  o f aeromagnetic Z component r e s i d u a l s regions  Cordilleran  I f t h i s i s so, then the  different pattern between the  the  residuals  t e c t o n i c elements, have a  t r a n s i t i o n beneath s o u t h e r n B r i t i s h Columbia s h o u l d d i f f e r  5-3.  to  In terms o f  e t a l . , 1968)  amplitude o f about 10 km,  be  the  t r u e s t r u c t u r e onto a  about 640  an  from the  c,  a regional variation i n velocity.  The and  s t r i k e of the  t r u e wavelength would be  plate  Canadian C o r d i l l e r a ( p a r t  p r o f i l e s e c t i o n i s a c t u a l l y a p r o j e c t i o n o f the v e r t i c a l p l a n e which c r o s s e s  i n the  the  (Haines e t a l . ,  the M d i s c o n t i n u i t y  may  (Tseng, 1968).  Conclusions On  the b a s i s o f the p r e s e n t study and  the p r e s e n c e of t y p i c a l o c e a n i c c r u s t i s not Graham I s l a n d  (Queen C h a r l o t t e  Graham I s l a n d  ( F i g . 14),  c o a s t a l mountains and  fault?).  the work o f Shor (1962),  s u g g e s t e d u n t i l west  From a depth of 26 km  of  beneath  the M t r a n s i t i o n deepens s l i g h t l y beneath  the  t h e n r i s e s beneath the H a z e l t o n Mountains i n  the  49  r e g i o n o f the Skeena A r c h  ( F i g . 14).  peaks which r e a c h 9000 f e e t .  The  to the e a s t o f these mountains.  T h i s a r e a i s c h a r a c t e r i z e d by  c r u s t a l s e c t i o n then t h i c k e n s The  immediately  major aeromagnetic anomaly a l s o  o c c u r s j u s t t o the e a s t o f the major e l e v a t i o n s at t h i s l a t i t u d e . records  from Greenbush which a r e c h a r a c t e r i z e d by  amplitude f i r s t and  The  M t r a n s i t i o n then r i s e s  Columbian Orogens ( F i g . 1 ) .  s e c t i o n i s unreversed profile.  with  the e x c e p t i o n  T h i s s h o r t p r o f i l e agrees w i t h  suggesting  anomalously l a r g e  energy c o u l d o r i g i n a t e from t h i s r e g i o n between 126°30'  127°30' l o n g i t u d e .  the P a c i f i c and  The  an M s t r u c t u r e which r i s e s  to the a r e a  separating  South o f P r i n c e George  the  o f the Nazko to B a r k e r v i l l e the u n r e v e r s e d  data i n  from the e a s t to the v i c i n i t y  of  the F r a s e r R i v e r . The  present  White e t a l . (1968). 03 o f the 1968  study The  i s i n good agreement w i t h apparent v e l o c i t i e s  study, r e i n f o r c e the p r e s e n t  topography f o r the a r e a e a s t o f the  the r e s u l t s  f o r p r o f i l e s 01,  of  02  and  i n t e r p r e t a t i o n i n terms o f M  Fraser.  For the P r i n c e Rupert a r e a , the model o b t a i n e d by Johnson e t a l . (1972) f o r t h e c o a s t c r u s t a l s e c t i o n i s r a t h e r more complex than present  d a t a would i n d i c a t e .  The  two  s t u d i e s concur i n s u g g e s t i n g  average c r u s t a l v e l o c i t y o f a p p r o x i m a t e l y evidence  f o r a s t r o n g r e f r a c t e d event w i t h  t h e P r i n c e Rupert a r e a i s m i s s i n g  the  6.4  km/s;  however, the  a v e l o c i t y o f 6.7  i n the p r e s e n t  an  data.  km/s  in  T h i s may,  of  c o u r s e , be e x p l a i n e d i n terms o f the d i f f e r e n t azimuths o f  recording  l i n e s and M topography, o r the c o n s i d e r a b l y g r e a t e r s t a t i o n s p a c i n g i n the study by Johnson e t a l .  However, i t i s o b s e r v e d t h a t the major  geomagnetic anomaly which extends northwest from R i p l e y Bay  terminates  in  50 the area of Prince Rupert.  This may  suggest a rather d i f f e r e n t c r u s t a l  section i n d e t a i l f o r the area approximately mid-way between Ripley Bay and Prince Rupert. The analysis of gravity data over the general area of the survey has not been completed.  However, a preliminary i n t e r p r e t a t i o n for the  area between L i t t l e Fort and the Fraser River i s i n good agreement with the present suggestion  of M topography (R. Stacey, personal communication).  In addition to the s t r u c t u r a l pattern described above, the following general points are considered pertinent. (A)  Careful consideration of systems' responses and the monitoring  of a l l  shots at one l o c a t i o n enables construction of record sections which show a coherent energy pattern.  The main e f f e c t s of poor seismometer  s i t e s are, apparently, an a m p l i f i c a t i o n of background noise, but only rare a m p l i f i c a t i o n of the e n t i r e record by more than a factor of about (B)  two.  The passband of the recorded seismic energy i s n a t u r a l l y r e s t r i c t e d to the range from about 3 to 6 Hz. the energy i s not r e a d i l y explained.  The monochromatic nature of Other puzzling features of  the record sections which remain unexplained 1.  the regional l e v e l s of background noise,  2.  the anomalous v a r i a t i o n i n amplitude of the c r u s t a l phase with respect to P  (C)  are:  r  energy.  The B i r d Lake and Greenbush Lake shotpoints, although greatly separated and with d i f f e r e n t water depths, apparently i n response.  differ  little  51 (D)  P  n  i s w e l l recorded  Lake t o a t l e a s t  as a f i r s t  a r r i v a l northwest from Greenbush  800 km, and e a s t from B i r d Lake t o 610 km.  t o r a t i o n o f P ^ energy t o an a p p r o x i m a t e l y  constant  Res-  l e v e l by the  a p p l i c a t i o n o f a d i s t a n c e squared f a c t o r suggests t h a t t h e wave t r a v e l s as a head wave connected w i t h  t h e M t r a n s i t i o n and, t e n t a -  t i v e l y , t h a t upper mantle g r a d i e n t s a r e e x t r e m e l y s m a l l over r e g i o n o f the C o r d i l l e r a (E)  (Hill,  this  1971a).  The r a t h e r complex energy p a t t e r n s which f o l l o w t h e major events (P  and c r u s t a l r e f l e c t i o n s ) suggest t h e models d e r i v e d h e r e i n a r e  s i m p l i f i e d v e r s i o n s o f the t r u e p i c t u r e . seismograms f o r comparison w i t h of the data recorded and  from W i l l i a m s  Construction of synthetic  the r e c o r d s e c t i o n s and e x a m i n a t i o n  a l o n g l i n e s from L i t t l e F o r t t o McLeod Lake  Lake t o B e l l a C o o l a may h e l p t o d i s t i n g u i s h be-  <•tween t h e e f f e c t s o f topography and v e l o c i t y depth s t r u c t u r e .  52 Appendix A Travel-Time  Curves  The distance and t r a v e l time along a ray can be expressed i n terms of the angle of i n c l i n a t i o n of the ray and depth  z by the  equations x = f Z tan0dz o where  s  and  S  i s length along the ray and v Using the ray parameter r  i s velocity.  p =  dz cos  e  pc'(z)  o  0 = —, , > l sin0d0 = pc'(z) 6 c ( z ) i s the gradient.  I  1  rr~\ (cos0 - cos0} pc'(z) o  o  1  The t r a v e l time i s then t  =r  *§  e  6 c'(z)sin0 o  i  r,  1 =  and since  -cPJzJ  1 c  r  ,  6/2 .  tan —072 . tan o  1  { L O G  tan 0/2' = sin©/  }  \ yl - cosvJ a  sin0 ° l^o70  l-cos0 • lHB~ oi o  g  W  V  1  =  = fz ^ z o c COS0  z sin ,  x = I °  where  t = / — o c  -cPJzJ  l  o  g  {  (  _  1-COS0  Q  vT '0 o  }  x  i^o70  q  O ) }  using a l i n e a r gradient within each layer such that c'(z)  z (v _ ±  1  - V^/h^  53 where  h^  i s the t h i c k n e s s  o f the i  th  Then f o r t h e l a y e r i n which  layer. the r a y bottoms  P - i/v  and the time and d i s t a n c e  t  .  (  V..-V, . _ ^ l r  are given  l  o  g  by  l-cos6. _ ,  {  i + 1  }  A program based on the above e q u a t i o n s was travel-time  graphs.  The program  used t o produce  e f f e c t i v e l y sums the time and  distance  c o n t r i b u t i o n from a s p e c i f i e d number o f r a y s p e r l a y e r , f o r as many l a y e r s as d e s i r e d , t o produce a t r a v e l - t i m e p l o t . o r i g i n a l form, was  w r i t t e n by Dr. Gerhard  Mueller.  The program, i n i t s  54 REFERENCES  1.  B a n c r o f t , A.M.,  and Basham, P.W.  2.  B a r r , K.G.  1971  B e r r y , M.J.  C r u s t a l r e f r a c t i o n experiment.  and West, G.F.  the f i r s t  76,  B e r r y , H.J.,  1966.  A time-term i n t e r p r e t a t i o n o f  a r r i v a l d a t a o f the 1963  Jacoby, W.R.,  Yellowknife  Lake S u p e r i o r experiment.  166-180.  N i b l e t t , E.R.,  and S t a c e y , R.A.  A review o f g e o p h y s i c a l s t u d i e s i n the Canadian  Can. J. Earth Sci. 8_, 5.  B u l l e n , K.E.  1965.  Cerveny,  V.  1961.  waves around  1971.  Cordillera.  788-801.  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