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The structure of the earth's crust in the vicinity of Vancouver Island as ascertained by seismic and… White, William Robert Hugh 1962

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THE STRUCTURE OP THE EARTH'S CRUST IN THE VICINITY OP VANCOUVER ISLAND AS ASCERTAINED BY SEISMIC AND GRAVITY OBSERVATIONS  By WILLIAM ROBERT HUGH WHITE M.A., The University of Saskatchewan, 195^  A THESIS SUBMITTED IN PARTIAL FULFILMENT OP THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OP PHILOSOPHY  i n the Department of Physics  We accept t h i s thesis as conforming to the required standard  THE UNIVERSITY OF BRITISH COLUMBIA October, 1962  In p r e s e n t i n g the  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  requirements f o r an advanced degree a t t h e  B r i t i s h Columbia, I agree t h a t the a v a i l a b l e f o r reference  and  study.  University  of  L i b r a r y s h a l l make i t f r e e l y I f u r t h e r agree t h a t  permission  f o r e x t e n s i v e copying o f t h i s t h e s i s f o r s c h o l a r l y purposes may g r a n t e d by  the  Head o f my  Department o r by h i s  be  representatives.  It i s understood t h a t copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l gain  Department  s h a l l not  of  The U n i v e r s i t y o f B r i t i s h Vancouver 8, Canada. Date  be a l l o w e d w i t h o u t my  Columbia,  written  permission.  The University of B r i t i s h Columbia FACULTY OF GRADUATE STUDIES  PROGRAMME OF THE FINAL ORAL EXAMINATION  PUBLICATIONS  FOR THE DEGREE OF 1.  2.  3.  DOCTOR OF PHILOSOPHY  Milne, W.G. and W.R.H. White. A seismic i n v e s t i g a t i o n of mine "bumps" i n the Ciiewsnest Pass Coal F i e l d . The Canadian Mining and M e t a l l u r g i c a l B u l l e t i n , 51, No. 559, 1958.  of WILLIAM ROBERT HUGH WHITE  Milne, W.G. and W.R.H. White. A seismic survey i n the v i c i n i t y of Vancouver Island, B r i t i s h Columbia. Publications of the Dominion Observatory, XXIV, No. 7, 1960.  Scrimger, J.A., W.G. Milne and W.R.H. White, Sub-bottom v e l o c i t i e s i n the Esquimalt Lagoon, B r i t i s h Columbia. Report 61-4, P a c i f i c Naval Laboratory, Esquimalt, B. C , 1961.  B.A. Hons., University of Saskatchewan, 1951 M.A., | !  University of Saskatchewan,  MONDAY, JULY 23, 1962, at 2:30  1954 P.M  0  IN ROOM 303, PHYSICS BUILDING  COMMITTEE IN CHARGE Chairman:  F.H. SOWARD  J„A„ JACOBS V . J o OKULITCH G.L. PICKARD  R„D RUSSELL W.F. SLAWSON R„W. STEWART 0  W_H  C  WHITE  External Examiner: J H„ HODGSON Dominion Observatory, Ottawa 0  GRADUATE STUDIES Field of Study: THE STRUCTURE OF THE EARTH'S CRUST IN THE VICINITY OF VANCOUVER ISLAND FROM SEISMIC AND GRAVITY OBSERVATIONS  ABSTRACT A seismic explosion programme has been carried out in the Vancouver Island-Strait of Georgia area of Western Canada. The programme included a relatively intensive survey in the Strait of Georgia between Campbell River and the south end of Texada Island, as well as a number of longer range refraction lines extending from Kelsey Bay along the coast as far south as northern California, and east through the mountains to a distance of 700 km. Gravity readings were obtained at intervals of about ten km. along the east coast of Vancouver Island as well as for a number of east-west traverses. Readings were also obtained for a few locations on the British Columbia mainland. Except for a marked positive trend in the Victoria area the regional value of the Bouguer anomaly for the Vancouver Island area is nearly zero. The average structure for the area, derived from the seismic refraction observations consists of a layer of volcanic and granitic strata less than five km. in thickness, and an intermediate layer 46 km. thick with a constant velocity for compressional waves of 6.66 km/sec. A velocity of about 7.7 km/sec. for the mantle has been observed along unreversed refraction lines, both along the coast and east through the mountains. Interpretation of the refraction observations has been based mainly on f i r s t arrival phases. The observed regional gravity anomaly is compatible with the crustal model obtained from the seismic results.  The Crustal Structure of the Earth in Western Canada  Advanced Geophysics  J.A. Jacobs  Nuclear Physics  J.B. Warren  Introduction to Dynamic Oceanography  G.L. Pickard  Advanced Dynamic Oceanography .... R.W.  Stewart  Related Studies: Geodesy Electronic Instrumentation Computational Methods  H.R.  Bell  F.K. Bowers C. Froese  - i ABSTRACT A seismic explosion program has been carried out in the Vancouver IslandStrait of Georgia area of Western Canada. The program included a relativelyintensive survey in the Strait of Georgia between Campbell River and the south end of Texada Island, as well as a number of longer range refraction lines extending from Kelsey Bay along the coast as far south as northern California, and east through the mountains to a distance of 700 km.  Gravity readings were  obtained at intervals of about ten km. along the east coast of Vancouver Island as well as for a number of east-west traverses. Readings were also obtained for a few locations on the British Columbia mainland.  Except for a marked  positive trend in the Victoria area, the regional value of the Bouguer anomaly for the Vancouver Island area i s nearly zero. The average structure for the area, derived from the seismic refraction observations consists of a layer of volcanic and granitic strata less than five km. in thickness, and an intermediate layer with a constant velocity for compressional waves of 6.66 km/sec, 46 km. thick.  A velocity of about 7.7 km/sec.  for the mantle has been observed along unreversed refraction lines, both along the coast and east through the mountains.  Interpretation of the refraction  observations has been based mainly on f i r s t arrival phases. The observed regional gravity anomaly i s compatible with the crustal model obtained from the seismic results.  - iv ACKNOWLEDGMENT The author is especially indebted to Dr. J. C. Savage who suggested an outline for the program discussed in this thesis, assisted in the f i e l d operations and supervised the research throughout. The author would also like to express his indebtedness to Mr. W. G. Milne of the Dominion Observatory staff who assisted in a number of ways and in particular organized the seismic f i e l d program of May I 9 6 0 .  Thanks are also  due to Dr. P. L. Willmore, formerly of the Dominion Observatory staff, who kindly supplied data for the Ripple Rock explosion. Mr. Malcolm Bancroft of the Dominion Observatory operated two f i e l d stations during the May I960 program. The Gravity Division of the Dominion Observatory, in cooperation with this program, carried out observations for a network of base stations in the survey area.  This part of the survey was carried out by Mr. A. K. Goodacre. Arrangements for ship time were made by Mr. E. P. Fleischer, Coordinator  of Research Ships at the Pacific Naval Laboratory, and Mr. J . S. Williams, Supervisor of Auxiliary Vessels.  Thanks are due to Captain J. E. Francois of  C. N.A.V. Laymore and Captain D. M. McFarlane of C.N.A.V. Whitethroat and the crews of these two vessels for handling the explosives and carrying out the navigation.  The explosives for the November 1961 program were handled by LCDR  D. B. Perrin, R.C.N., of the Underwater Weapons, Fleet School, H.M.C.S. Naden, and LT D. B. Hope, R.C.N., of the R.C.N. Diving Establishment (West). A number of the members of the staffs of the Dominion Astrophysical Observatory and the Institute of Earth Sciences at the University of British Columbia assisted in the f i e l d programs. These include B. Caner, R. Hemmings, C. Hemmings, H. Draper, C. Sansbury, J. White, R. Bennett, H. McNaughton, L. Manslhna, C. Westphal, B. Whittles and W. McReynolds. Mr. H. Draper and Miss T. Thompson assisted in the preparation of the thesis.  - V -  A number of submarine gravity readings have been used by permission of Professors Worzel and Ewing of the Lamont Geological Observatory, and one gravity reading by permission of Dr. G. D. Garland of the Department of Physics, University of Alberta. The research in this thesis has been supported by the Dominion Observatory and through grants made to the University of British Columbia by the National Research Council.  - ii TABLE OF CONTENTS Page 1.  INTRODUCTION  1  1.1  Definition of the Earth's Crust  1  1.2  Methods of Investigation of Structure i n the Earth's Crust  2  1.2.1  Seismic reflection and refraction surveys 1.2.2. Analysis of surface waves 1.2.3 Gravity measurements 1.2.4 Other methods  2  Some General Aspects of Crustal Structure Studies  6  1.3  1.3.1 1.3.2 1.3.3 1.3.4 1.3.5  History of pioneer refraction studies Continental and oceanic crustal structure The reality of the intermediate layer Velocities of compressional waves in the mantle Investigations of the earth's continental crustal structure  4  4 6  6 8 10 10 J  11  2.  RESULTS OF SOME SURVEYS IN WESTERN NORTH AMERICA  13  3.  THE SEISMIC PROGRAM  23  3.1  General Description  23  3.2  Instrumentation and Recording Techniques  25  3.3  Reduction of Data  28  3.4  Accuracy of Measurements  29  3.5  Additional Data  32  3.6  Seismograms  33  Amplitudes of Recorded Energy  33  Shorter Range Refraction Data for the I960 and 1961 Programs  35  3.8.1 3.8.2 3.8.3 3.8.4 3.8.5 3.8.6 3.8.7 3.8.8  36 43 44 44 44 46 47 47  .3.7 3.8  Profile I Profile II Profile III Prpfile IV Profile V Profile VI Profile VII Profiles VIII to XI  - iii 3.9  Ripple Rock Data  51  3.9.1  51  Profile XII  3.10 Longer Range Data 3.10.1  53  3.11 Methods of Interpretation  56  3.12 Interpretation and Discussion of the Refraction Data  59  3.12.1 3.12.2 3.12.3 3.12.4 3.12.5 3.12.6  4.  Profiles XIV to XVII  53  Results for the Strait of Georgia area Results for the Kelsey Bay and Johnstone Strait area Estimated thickness of granitic and volcanic strata Results of the longer range data Ripple Rock-east data Lg waves  61 64 66 68 72 72  THE GRAVITY SURVEY  73  4.1  General Description of Program  73  4.2  Instrumentation  73  4.3  Procedures Used in the Survey  74  4.4  Elevations and Positions  76  4.5  Reductions  77  4.6  Density Measurements  79  4.7  Precision of Measurements  79  4.8  Discussion of Data  81  5.  DISCUSSION AND COMPARISON OF REFRACTION AND GRAVITY RESULTS  86  6.  COMPARISON WITH OTHER SURVEYS  89  7.  CONCLUSION  91  8.  BIBLIOGRAPHY  95  I  1.  1.1  INTRODUCTION  D e f i n i t i o n of the E a r t h ' s Crust  Various d e f i n i t i o n s of the e a r t h ' s  c r u s t have been s u g g e s t e d .  example, t h r e e c o n c e p t s have been c i t e d by Ewing and P r e s s (1956) (1)  For as  The d i v i s i o n o f t h e o u t e r p a r t o f t h e e a r t h i n t o two z o n e s , t h e  follows: outer  s h e l l b e i n g d e s i g n a t e d t h e l i t h o s p h e r e , c h a r a c t e r i z e d by t h e f a c t t h a t m a t e r i a l possesses s u f f i c i e n t features  shear s t r e n g t h t o s u p p o r t t h e t o p o g r a p h i c  i n e v i d e n c e at t h e s u r f a c e o f the e a r t h .  T h i s i s u n d e r l a i n by t h e  a s t h e n o s p h e r e , i n which t h e shear f o r c e s a r e n e g l i g i b l e and o n l y forces are important.  the  hydrostatic  When s u r f a c e e l e v a t i o n s o r d e p r e s s i o n s have  lateral  dimensions s e v e r a l t i m e s t h e t h i c k n e s s o f t h e l i t h o s p h e r e , t h e y are s u p p o r t e d m a i n l y by t h e h y d r o s t a t i c (2)  forces of the asthenosphere.  The d i v i s i o n on t h e b a s i s o f t h e maximum depth at which earthquakes  observed.  are  The m a t e r i a l above such a boundary must be s u f f i c i e n t l y r i g i d t o  a l l o w s h e a r s t r a i n energy t o b u i l d up t o v a l u e s r e q u i r e d t o account f o r o b s e r v e d magnitude o f e a r t h q u a k e s .  The q u e s t i o n o f t h e time s c a l e  which s t r a i n energy i s m a i n t a i n e d must be c o n s i d e r e d .  Earthquakes  the  over are known  t o have depths o f f o c u s i n c e r t a i n areas i n excess o f 700 k i l o m e t r e s . (3) The d i v i s i o n between t h e c r u s t and t h e mantle as marked by the Mohorovic'ie' d i s c o n t i n u i t y .  T h i s d i s c o n t i n u i t y i n the v e l o c i t y of  wave p r o p a g a t i o n and t h e i n f e r r e d d i s c o n t i n u i t y i n t h e p h y s i c a l  elastic  constants  - 2o f t h e m a t e r i a l , have been d e t e c t e d by r e f r a c t i o n and r e f l e c t i o n  seismology.  I t may be c o n s i d e r e d t o be t h e deepest abrupt 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 upper 100  km. o f t h e e a r t h .  The m a t e r i a l below t h e d i s c o n t i n u i t y i s  referred  t o as t h e mantle and i s c h a r a c t e r i z e d by a v e l o c i t y o f about 8 km/sec. compressions! e l a s t i c  waves.  t o be remarkably c o n s i s t e n t and o c e a n i c a r e a s .  It  Measurements o f t h i s v e l o c i t y have been found i n most p a r t s o f t h e e a r t h b o t h f o r  has t h u s become customary t o a s s o c i a t e  continental  t h e Mohorovifcic  d i s c o n t i n u i t y w i t h t h e upper boundary o f t h e l a y e r h a v i n g t h i s Recently v e l o c i t i e s  for  velocity.  l e s s than 8 km/sec. have been o b s e r v e d f o r some a r e a s .  These areas must be c o n s i d e r e d anomalous i n view o f t h e l a r g e amount o f  data  p r e s e n t l y e x i s t i n g which c o n s i s t e n t l y  The  i n d i c a t e t h e 8 km/sec. v e l o c i t y .  i d e n t i f i c a t i o n o f t h e s e lower v e l o c i t y measurements w i t h t h e m a n t l e , appears t o be i n d i c a t e d .  C o n f i r m a t i o n by mapping from normal areas  however, into  anomalous ones i s r e q u i r e d . The l a s t  o f t h e s e d e f i n i t i o n s i s adopted i n t h i s t h e s i s ,  p r i m a r y method o f measurement i s  1.2  1.2.1  Methods o f I n v e s t i g a t i o n  essentially  since  a measure o f v e l o c i t y  the structure.  o f S t r u c t u r e i n the E a r t h ' s Crust  S e i s m i c r e f l e c t i o n and r e f r a c t i o n  surveys:  The most d i r e c t approach t o t h e d e t e r m i n a t i o n o f s t r u c t u r e i n t h e e a r t h ' s c r u s t i s t h a t o f r e f l e c t i o n and r e f r a c t i o n s e i s m o l o g y . waves propagated from a d i s t u r b a n c e w i t h i n t h e c r u s t o r a t t h e s u r f a c e a r e r e c o r d e d w i t h seismograph i n s t r u m e n t s h a v i n g t h e  Elastic earth's  appropriate  frequency r e s p o n s e , l o c a t e d a t s u i t a b l e i n t e r v a l s o f d i s t a n c e from t h e disturbance.  T r a v e l t i m e s a r e measured f o r the v a r i o u s a r r i v a l s o f wave  energy and t i m e - d i s t a n c e p l o t s made from t h e reduced d a t a . a t i o n o f t h e s e p l o t s an attempt  i s made t o a s s o c i a t e  By an examin-  the various a r r i v a l s  - 3 of energy with c e r t a i n ray paths of the propagation. When t h i s has been done, s t a t i s t i c a l methods may be applied to groups of p l o t t e d points to obtain quantitative information as to the structure, including the depth of the v e l o c i t y d i s c o n t i n u i t i e s and the v e l o c i t i e s of propagation of the e l a s t i c waves i n the proposed layers.  In most investigations interpretation has been  based on r e f r a c t i o n rather than r e f l e c t i o n data. the  safest approach, since f o r most r e f r a c t i o n data, the interpretation may  be based on the f i r s t a r r i v a l s of seismic energy. the  In the past t h i s has been  case w i l l be pointed out l a t e r .  That t h i s i s not always  More recently the use of spreads of  detectors, and more d e t a i l e d observations, have made possible the c o r r e l a t i o n of secondary a r r i v a l s from one detector to adjacent ones.  This has greatly  enhanced the value of r e s u l t s from r e f l e c t i o n observations as w e l l as those of secondary r e f r a c t i o n a r r i v a l s from intermediate l a y e r s .  The existence of  r e f r a c t i o n and r e f l e c t i o n a r r i v a l s , and the geometry of ray paths w i l l be discussed i n a l a t e r section. A v a r i e t y of energy sources has been used, including earthquakes, rockbursts commonly found i n mining areas, and man-made explosions.  Quarry  b l a s t s , underwater explosions and underground nuclear t e s t explosions have been used. the  The magnitude of some explosions has made i t possible to record  radiated seismic energy well beyond the distances at which refracted rays  from the mantle become f i r s t a r r i v a l s on the seismograms.  An alternative to  t h i s procedure i s to use smaller explosions, observed i n greater d e t a i l at shorter ranges, and to base the interpretations p a r t l y on secondary a r r i v a l s of refracted and r e f l e c t e d energy. Explosions used as energy sources are to be preferred to earthquakes and other n a t u r a l l y occurring seismic phenomena, i n that l o c a t i o n , depth of focus and o r i g i n time may a l l be accurately measured quantities.  In addition,  - 4t h e p r e c i s e time o f o c c u r r e n c e  o f t h e d i s t u r b a n c e may be p r o v i d e d t h r o u g h  a p p r o p r i a t e communication systems so t h a t temporary s t a t i o n s may be s e t up f o r a minimum o f t i m e and h i g h t i m e r e s o l u t i o n may be o b t a i n e d by r u n n i n g r e c o r d e r s a t h i g h speed f o r s h o r t p e r i o d s o f t i m e . F o r the g r e a t e r p a r t o f t h e i n v e s t i g a t i o n r e p o r t e d i n t h i s t h e s i s underwater e x p l o s i o n s were u s e d as energy s o u r c e s .  N a v a l depth charges  which had become o b s o l e t e f o r m i l i t a r y purposes were p o s i t i o n e d from a s h i p as s i n g l e u n i t s and i n packages o f t e n when g r e a t e r ranges o f r e c o r d i n g s were required.  The e x p l o s i v e was d e t o n a t e d  a t s u f f i c i e n t depth t h a t , i n almost  a l l c a s e s , t h e water c o v e r c o n f i n e d t h e e x p l o s i o n s and ensured  efficient  t r a n s f o r m a t i o n o f t h e energy r e l e a s e d by t h e e x p l o s i o n i n t o s e i s m i c energy.  1.2.2  A n a l y s i s o f s u r f a c e waves: A c o n s i d e r a t i o n o f s o l u t i o n s o f t h e wave e q u a t i o n s  which a p p l y f o r an  e l a s t i c medium i n d i c a t e s t h e p o s s i b i l i t y o f t h e p r o p a g a t i o n The  o f s u r f a c e waves.  a p p l i c a t i o n o f v a r i o u s boundary c o n d i t i o n s r e s u l t s i n t h e p r e d i c t i o n o f  s u r f a c e waves o f v a r i o u s t y p e s .  R a y l e i g h waves may be propagated a t a f r e e  s u r f a c e i n a homogeneous medium, i n which case t h e r e i s no wave d i s p e r s i o n . F o r a medium w i t h v e l o c i t y s t r u c t u r e , R a y l e i g h waves a r e d i s p e r s e d . v e l o c i t y s t r u c t u r e may p r o v i d e channels p r e d i c t e d by t h e t h e o r y .  f o r g u i d e d waves o f v a r i o u s  The d i s p e r s i o n observed  The types  f o r such waves may be  examined from seismograms o f earthquakes and u s e d t o i n f e r t h e dimensions and p h y s i c a l c o n s t a n t s o f t h e s t r u c t u r e .  E x t e n s i v e use o f t h i s method has  been made by many i n v e s t i g a t o r s . B r i e f l y , t h e t e c h n i q u e  i s t o compute d i s -  p e r s i o n c u r v e s o f group and phase v e l o c i t y f o r v a r i o u s c r u s t a l models and t h e n t o choose t h e one which b e s t f i t s t h e observed  data.  R a y l e i g h waves  and Love waves have been used t o i n v e s t i g a t e t h e c o n t i n e n t a l and o c e a n i c  - 5crtfstal' structure. Other surface waves known as Lgand Rg have been observed r  for"purely continental paths/' 'Variousi types of structure in the crust and upper mantle have been proposed 'to account for the efficient propagation of v  these" waves. A -summary* is' given by"'Byerly'(1956).n  The present ihvestigati bn ihc-Iuded a' study of Rayleigh waves recorded 4  l  r  at the permanent s4i8mograph stations at 'Victoria, Alberni and Horseshoe Bay :  from earthquakes in the south-east Pacific.""Because of inadequate informc  ation related to the constants" of the "instruments, "an attempt to make corrections for the phase' shift's "iii "the RetTerogerieous recording systems was not successful. The' lack of'succTess was evidenced by the very large scatter  1  in "the phase 'velocity-wave'period" plots,;' The analysis is not, reported in 1  this thesis. It is. hoped that with the instrumentation now in operation, a further'attempt may bem'atie'to"6btain dispersion curves. o*  1.2.3 Gravity measurements:  '.lie  P-^"ibi\ity  The analysis "of gravity  t»&v  *  of  •• -  .  ,^ ,  ,-•  -does riot le'a'd to a unique crustal structure. 1  L  However, i f a relation between * the -veloc*ity 6'f ^propagation of seismic waves 5  d  J  and the density of the material.through 'which they propagate can be established, "a quantitative'comparison may be made between the structure determined by the direct-measurements of refraction and reflection seismology and the observed gravity values; SucJr a-relation has been found by Nafe and Drake (1958) by a statistical treatment of labora1iory and^ iii situ measurements of w  rl  e  the physical properties'of various media. -A curve showing their results is found iri Talwani'(1959)t^ 'It'"is of "course necessary to set up a standard for comparison.  This has been done'by Press'(I960) and Woollard (1959). Press  has chosen a'crustal'structure for Africa^ which'agrees with the observed seismic 'dataj as 'being typical of a 'cdntinenta! crust and as one which gives €  1  e  - 6 a Bouguer g r a v i t y anomaly o f -30 mgals.  On some b a s i s such as t h i s , one may  p r o c e e d t o compare s t r u c t u r e s determined by s e i s m i c  r e f r a c t i o n and r e f l e c t i o n  s t u d i e s w i t h t h e o b s e r v e d r e g i o n a l g r a v i t y anomalies. I f a theory  o f i s o s t a s y i s assumed, t h e c r u s t a l s t r u c t u r e must p r o v i d e f o r  compensation f o r t h e v a r i a t i o n i n l o a d imposed by v a r i a t i o n s i n t h e r e g i o n a l l a n d e l e v a t i o n ' o r ocean d e p t h . the Pratt-Hayford  Two systems o f compensation have been proposed,  i s o s t a t i c system i n v o l v i n g compensation by v a r i a t i o n s i n  d e n s i t y , o r t h e A i r y - H e i s k a n e n system i n v o l v i n g v a r i a t i o n s i n t h i c k n e s s o f the lower d e n s i t y c r u s t a l m a t e r i a l s . In t h e present  study, g r a v i t y readings  have been o b t a i n e d  f o r t h e south  p a r t o f t h e B r i t i s h Columbia c o a s t , as w e l l as a l o n g t h e e a s t c o a s t l i n e o f Vancouver I s l a n d and f o r some east-west p r o f i l e s on t h e I s l a n d . ings obtained  1.2.4  by o t h e r  A few r e a d -  i n v e s t i g a t o r s a r e i n c l u d e d and g r a t e f u l l y acknowledged.  O t h e r methods: The  s t r u c t u r e o f t h e earth's  c r u s t must a l s o be expected t o be r e f l e c t e d  i n such measurements as heat f l o w and v a r i a t i o n s i n t h e e a r t h ' s  magnetic  field.  1.3  1.3.1  Some G e n e r a l A s p e c t s o f C r u s t a l S t r u c t u r e  H i s t o r y o f the pioneer  Studies  refraction studies:  A b r i e f summary o f t h e e a r l y r e f r a c t i o n s t u d i e s , and t h e n o t a t i o n which has  developed, i s p r e s e n t e d h e r e .  Jeffreys The  (1959), B y e r l y (1956), first  Comprehensive accounts a r e g i v e n by  and more r e c e n t l y by S t e i n h a r t and Meyer  evidence f o r s t r u c t u r e i n t h e earth's  Mohorovic'ic' i n 1909.  Seismograms o b t a i n e d  (1961).  c r u s t was o b s e r v e d by  f o r an earthquake which  occurred  i n C r o a t i a on O c t o b e r 8 o f t h a t y e a r i n d i c a t e d two c o m p r e s s i o n a l wave  arrivals and two transverse wave arrivals.  The P and S waves had been pre-  viously identified on seismograms of earthquakes. Mohorovicic" concluded that the earthquake had occurred i n the upper layer of the crust and that the two P waves represented propagation paths through the upper (granitic) layer and by refraction through a deeper layer of higher velocity. The former arrival was named P and the latter P. The corresponding transverse waves were named S and S. P and S have also been referred to as Pg and Sg, and P^ and S^.  It was found that near the focus of the earthquake only P  and S appeared, that at greater distances P and S were observed and became f i r s t arrivals on the seismograms. P  n  and S « n  P and S are frequently referred to as  The velocity discontinuity between the two layers has been named  the MohoroviSic discontinuity. Gutenberg observed similar features i n records of earthquakes occurring in Europe i n 1911 and 1913.  Recorder Source Upper (granitic) layer Conrad discontinuity  Intermediate (basaltic) layer  P n  Figure 1.  Mohorovic'iS discontinuity Mantle  Notation for Refracted Rays i n a Two Layer Crustal Model.  - 8Conrad in 1 9 2 5 and Jeffreys in 1 9 2 6 found evidence for arrivals from an intermediate layer.  These arrivals were characterized by velocities inter-  mediate between the crustal and subcrustal values.  They have been referred  to as P and S and also as ?^ and Sg, and the layer as the 'basaltic layer'„ The various propagation paths and associated notation are shown i n Figure 1 .  A similar diagram may be drawn for S waves.  Frequently the number subscripts are used with parameters referring to the sequence of layers beginning with the most s u r f i c i a l (i.e., P , P., n  1.3.2  Continental and oceanic crustal structure: Following the pioneer observations of refraction arrivals from earth-  quakes, data have been obtained at many locations of the earth from earthquakes, other naturally occurring seismic disturbances and especially from explosions.  Data have been obtained for both continental and oceanic areas.  The results related to continental crustal structures have been summarized in detail by Byerly  ( 1 9 5 6 )  to the date of publication. Some of the more  recent results w i l l be summarized in a section to follow.  Oceanic data have  been obtained mainly by workers from Columbia, Cambridge and California. A basic difference exists between continental and oceanic crustal structure. Because of the large variations in the proposed structure for the various continental areas, i t i s d i f f i c u l t to describe a typical continental crust. Most models indicate a crustal thickness of  3 0 - 4 0  km. in low lying continental  areas, consisting of the granitic and intermediate layers, the lower boundaries of these layers being referred to as the Conrad and Mohorovic"i6 discontinuities.  Some doubt has been cast on the existence of the  intermediate  layer and as a result, single layered models have been proposed with or without  -  an increase in velocity with depth.  9  -  The thickness of the continental crust  has been found to increase for mountainous and plateau areas. of up to 70 km. have been found to f i t the observed data. structures have been proposed with more than two layers. granitic layer vary from about 5.6  to 6.4  to 8.4  to 7.2  km/sec. have been observed.  consistent values were between 7.8  and 8.3  Velocities for the  km/sec. Mantle Until recently the most  km/sec. The oceanic crustal  structure consists typically of 5 km. of water, about 0.5 sediments and about 1.0  A number of crustal  km/sec. for compressional waves and  those fbr the intermediate layer from about 6.6 velocities of 7.5  Thicknesses  km. of unconsolidated  km. of volcanic pr .granitic material. Underlying  these layers i s the intermediate layer with a typical compressional wave velocity of 6.8 less to 6 km.  km/sec. This layer varies i n thickness from about 4 km. or The granitic layer as i t exists for the continental structure  appears to be missing in oceanic areas.  A typical crustal section for an  oceanic area i s shown in Figure 2. Thickness (km.)  5  Water  0.5  Unconsolidated sediments  1  Volcanic or granitic layer  5  Intermediate layer  Mantle Figure 2.  A Crustal Section Typical of Oceanic Areas.  1.3.3  The reality of the intermediate layer: The evidence for the intermediate layer from seismic data i s in the form  of refracted arrivals.  As w i l l be shown later, the refracted arrival from an  intermediate layer may not become a f i r s t arrival for any recording distance from the focus of a disturbance.  Thus many of the refraction data for the  intermediate layer have been based on secondary arrival phases. The reading of such a phase is often made d i f f i c u l t because of the background signal from an earlier arrival.  It i s also f e l t that i f the Conrad discontinuity exists,  seismic reflections should be observed at suitable recording distances i n addition to the refracted arrivals. These d i f f i c u l t i e s have led many workers to doubt the existence of the intermediate layer and to favor a single layered crust. This view i s expressed, for example, by Press and Ewing (1956).  Byerly (1956, page 146)  suggests that the established existence of the layer for oceanic areas also points to i t s existence as a separate entity in continental structures. More recent investigations with improved techniques and more detailed observations tend to confirm the existence of this layer in many continental areas.  Its thickness, as well as the depth of the Conrad discontinuity varies  over rather wide ranges from area to area.  Richards and Walker (1959),  reporting on a survey on the Alberta plains, interpret a strong arrival at a distance of about 120 km. as being a reflection from the Conrad discontinuity. Hales and Sachs (1959) have identified secondary arrivals as  refractions  from an intermediate layer in the eastern Transvaal area of South Africa. A number of other investigators also include the intermediate layer in their proposed crustal models. 1.3.4 Velocities of compressional waves in the mantle: It has already been pointed out that most refraction studies have given  - 11  -  compressional wave velocities of 7 . 8 to 8 . 3 km/sec. in the mantle.  Recently  somewhat lower velocities have been found for both continental and oceanic areas.  These velocities (e.g., 7.5-7.6 km/sec.) are higher than would be  expected for the intermediate layer, and there i s a strong suggestion that they are associated with the mantle, since in some areas at least, a higher velocity i s not observed.  Menard  (1961)  has pointed out a correlation between  the lower velocity and the proximity of the refraction line to the crest of the East Pacific Rise. Velocities of about  7.6  km/sec. have been found by Press  eastern California and by Berg et a l Province.  (I960)  (I960)  for  for the Eastern Basin and Range  Both, however, report arrivals indicating a velocity of about  8 . 0 km/sec. i n a deeper layer.  The upper boundary of the deeper layer i s  placed at about 50 km. for California and 7 0 km. for the Basin and Range Province.  1.3.5  Investigations of the earth's continental crustal structure: No attempt i s made in this section to summarize in detail the results  of a l l of the crustal determinations which have been made. Rather, the results of a few surveys are briefly reviewed for various parts of this continent and other continental areas of the earth.  In a section to follow,  results of surveys carried out i n areas adjkcent to the area of the present investigation w i l l be described in slightly greater detail. Hodgson  (1953)  has used rockbursts as sources of energy in the Canadian  Shield area of eastern Canada. He has found a single layered crust with velocities of  6.2  km/sec. above the MohoroviXic and  The thickness of the crust i s  36  km.  Katz  (1955)  model for the New York and Pennsylvania area.  7.9-8.2  km/sec. below i t .  has found a similar crustal  - 12 -  W i l l m o r e (1949) r e c o r d e d t h e H e l i g o l a n d e x p l o s i o n t o a d i s t a n c e o f 1000  km.  i n Europe and from h i s a n a l y s i s o f t h e d a t a , proposed a c r u s t a l s t r u c t u r e c o n s i s t i n g o f a s i n g l e l a y e r e d c r u s t , having  a t h i c k n e s s o f 28 km.  His struc-  t u r e does not i n c l u d e an i n t e r m e d i a t e l a y e r which has been proposed by some o t h e r workers i n Europe. Hales  and Sachs (1959), u s i n g e a r t h tremors o f t h e W i t w a t e r s r a n d have  c a r r i e d out a s t u d y i n t h e e a s t e r n T r a n s v a a l .  They have found two  s t r u c t u r e s f o r t h e p l a t e a u a r e a , i n c l u d i n g an i n t e r m e d i a t e l a y e r . conclude  that early arrivals of P  possible They a l s o  on r e c o r d s o b t a i n e d i n t h e c o a s t a l a r e a n  o f lower e l e v a t i o n i n d i c a t e d a decrease  i n the thickness o f the c r u s t .  Weizman e t a l (1957) have r e p o r t e d on surveys  c a r r i e d out from 1949-1955  i n v a r i o u s areas o f R u s s i a , u s i n g methods i n which t h e waves p r o p a g a t i n g e x p l o s i o n s were o b s e r v e d a r r i v a l s was  possible.  are presented.  i n g r e a t d e t a i l , and continuous  areas t o more t h a n 70 km. The  t o 40  km.  areas  i n 'platform*  i n t h e h i g h mountain a r e a o f N o r t h e r n  Pamir.  c r u s t a l s t r u c t u r e s proposed i n t h e s e i n v e s t i g a t i o n s a r e summarized  in Figure  Depth i n km.  correlation of  In t h e i r r e p o r t , c r u s t a l models f o r t h e v a r i o u s  C r u s t a l t h i c k n e s s e s ranged from 30  from  3. (Hodgson) Canadian S h i e l d Area o f E . Canada  0 r  20  (Willmore) Europe  4.4 Granitic 6.2 km/sec,  (Hales and Sachs) South A f r i c a  (Weizman) Northern Pamir Mts. Russia  km/sec.  Granitic 6.0 km/sec.  Granitic 6.03 'km/sec, Intermediate 6.71 km/sec.  Granitic  40 60  80 F i g u r e 3.  20  40 Mantle 7.9 t o 8.2 km/sec.  Mantle 8.2 km/sec. s  Mantle 7.96 km/sec.  Intermediate  Mantle C r u s t a l S e c t i o n s f o r V a r i o u s C o n t i n e n t a l Areas o f t h e E a r t h .  60  80  - 13 The greater thicknesses of the continental crust in mountainous areas is in agreement with the idea of isostasy in that the load due to the elevated surface features is compensated for by the extension of the lower density crustal material to greater depths. Woollard (1959) has assembled in one diagram the measured crustal structures for a section extending from west to east beginning at the eastern Pacific and ending at the Indian Ocean and including the continents of North America, Europe and Africa. he shows also the observed Bouguer gravity anomaly values.  For comparison  This work has  been cited also by Heiskanen and Vening Meinesz (1959) as strong evidence for the Airy-Heiskanen system of isostatic equilibrium. It may be remarked that the more recent refraction measurements in the Eastern Basin and Range Province (Berg, I960) and the Colorado Plateau (Pakiser et a l , 1962) have indicated somewhat greater crustal thicknesses than those shown in Woollard's diagram.  2.  RESULTS OF SOME SURVEYS IN WESTERN NORTH AMERICA  Most of the explosions of the present study were located in the Strait of Georgia and Johnstone Strait between Vancouver Island and the British Columbia mainland, along a line paralleling the structure of the physiographic features. The recording stations were also distributed along the coast.  The area inves-  tigated l i e s at a distance of about 200 km. east of the edge of the continental shelf, and may be expected to be transitional between oceanic and continental types of structure. In addition, data have been obtained from the Ripple Rock explosion in the Strait of Georgia as recorded at stations east through the mountains,, as far as Banff, Alberta. In this section, the results of a number of surveys carried out in the western part of this continent are summarized.  Two refraction lines at sea,  - l i -  near the coast of British Columbia, are also included. As a part of a program in which refraction and reflection studies were carried out in various parts of the United States, Tatel, Adams and Tuve (1953) and Tatel and Tuve (1955) have conducted a survey in the State of Washington. Explosions detonated in Puget Sound were recorded along lines in various directions from the explosion area, to a maximum distance of 290 km. Although a relatively large number of observations at what was considered to be the correct range of distance was obtained, no secondary arrivals which could be interpreted as critical reflections from the Mohorovicic discontinuity were detected. This led the authors to suggest that the discontinuity does not exist in this area, or that i t exists in a broken and irregular form. The early arrival of a group of observations recorded at a range of distances of 110 to 140 km. to the west of the explosion area led the authors to postulate that the arrivals are refracted arrivals from the Mohorovic'ie' discontinuity and that the crust becomes thinner as the edge of the continent is approached. A crustal thickness of 19 km. is calculated using a velocity of 8.1 km/sec. for P . ' n Neumann (1957) has analysed the data from a large number of seismograms obtained for earthquakes occurring in the State of Washington and in southwestern British Columbia. Arrival times of compressional waves at stations of the University of Washington network have been used as well as those for the western network of the Dominion Observatory.  A best f i t for the data  has been obtained by adopting a structure with three layers having velocities for compressional waves of 5.8, 6.4 and 7.0 km/sec. With one exception, arrivals from earthquakes along the coast as far distant as southern California do not indicate propagation with a velocity reaching 8 km/sec. These  -  15  -  . observations have led Neumann to propose a very great depth to the Mohorovic'ic  .discontinuity, or non-existence of the discontinuity in the area of his  observations. A reversed profile program was carried out by A. R. Milne  (I960)  using the  standard techniques of seismic refraction and reflection investigations at sea. The area of this survey is located between 135* and 136* W. longitude and at ..about 48" 20* N. latitude, the profiles running approximately east-west. Crustal sections obtained by treating the profiles independently show a typical  oceanic crustal layer having a velocity of about 6.8 km/sec. underlain by material with typical mantle velocity slightly in excess of 8 km/sec. The basaltic layer is from 2.6 to 4.0 km. i n thickness and i s overlaid by 1.0 to 1.5 km. of sediment and material having a velocity typical of volcanics or  granitics. The structure found for the east end "of the profile i s shown in Figure 4.  Thickness (km.)  Velocity (km/sec.)  Water  Figure 4.  Structure for North Pacific Ocean Basin  (A.  R. Milne, I 9 6 0 ) .  - 16 Among numerous other p r o f i l e s , Shor (1962) has obtained a reversed p r o f i l e running approximately east-west at the north end of the Queen Charlotte Islands i n Dixon Entrance.  This p r o f i l e i s on the continental shelf.  V e l o c i t i e s of  about 6.8 km/sec, t y p i c a l o f the oceanic structure, have been well established by the reversed p r o f i l e s , with r e l a t i v e l y long segments on the travel-time curves representing f i r s t a r r i v a l s .  Based on secondary a r r i v a l s , which he  interprets as possible refractions from the Mohorovic'ic discontinuity, a thickness of 26 km* has been suggested f o r the crust.  The structure f o r t h i s  work i s shown i n Figure 5. Thickness (km.)  Velocity (km/sec.)  .34-.45.36-. 9 6 ^ 1.73—^  !>2.42 ^-3.05  -Water  3.98-4.46  5.78  19.8  6.81  8.49  Figure 5.  Structure f o r Dixon Entrance (Shor, 1962).  The r e s u l t s of e a r l i e r surveys c a r r i e d out by Milne and White (I960) i n the Vancouver Island area were based on records obtained f o r explosions detonated  - 17 in the Strait of Georgia and a second profile of explosions in the Strait of J-uaxu-de Fuca and for some distance along the west coast of Vancouver Island. These events were recorded at the permanent seismograph stations at Victoria, Alberni and Horseshoe Bay.  The results obtained by plotting the time distance  data for a l l stations for each series of explosions indicated crustal velocities of 6 . 0 and 6.4. km/sec. for the two profiles.  In addition to these  longer range profiles, some very short range refraction work was done to measure velocities at the surface for the various geological structures outcropping in the Vancouver Island area. Richards and Walker (1959) have carried out a seismic investigation of the crust on the plains of Alberta. The profile is 130 km. in length and parallels the Rocky Mountains. It is located approximately 100 km. to the east of the foothills.  Fifteen prospecting units, using spreads up to 4 km.  in length were stationed at intervals along the profile.  The seismic energy  was supplied by explosions at each end of the profile consisting of about 400 kilograms of conventional seismic explosive. The interpretation is based on first arrivals in granitic or limestone strata having a velocity of about 6 km/sec. A slight increase in the apparent velocity across the spreads at greater distances appears to indicate an increase of velocity with depth in the granitic layer. Two discontinuities are proposed by interpretation of secondary arrivals.  A decrease in apparent  velocity across the spreads with an increase in range indicates that the arrivals are due to reflections. Real velocities of about 7.2 km/sec. and 8.2 km/sec. have been obtained for the intermediate layer and the mantle, respectively. Depths to the Conrad and Mohorovic'iS discontinuities are 29 km* and 43 km. These results are summarized in Figure 6.  - 18 Thickness (km.)  2 1  Velocity (km/sec.) 3.6 6.1  26  6.2  Conrad discontinuity 7.2 Mohorovic'ic' discontinuity  8.2  Figure 6. Structure for Alberta Plains (Richards and Walker, 1959).  The underground atomic tests conducted at the Nevada Test Site and more recently near Carlsbad, New Mexico, have provided energy sources for a number of refraction studies. Somewhat varying structures have been proposed for areas in Utah, Nevada, Colorado and California. by Berg  (I960)  These have been summarized  to the date of his publication.  From a composite plot of arrivals for a l l events (including two nuclear explosions and other large quarry explosions) Berg has arrived at the velocity model shown i n Figure 7 (page 19). Press (I960) has discussed a crustal model for California from the point of view of the three methods outlined earlier.  The refraction results are for  records obtained from high explosive events at Corona and Victorville, and a number of nuclear events at the Nevada Test Site. i s as shown in Figure 8 (page 19).  The velocity model obtained  -1?  Thickness (km.) 9 16  47  -  Velocity (km/sec.). 5.73 6.33  7.59  7.97  Figure 7.  Average Structure for Area Surrounding Explosions in Northern Utah (Berg, I 9 6 0 ) . Thickness (km.) 1  Velocity (km/sec). sedimentary  23  6.11  26  7.66  8.11  Figure 8.  Structure for California-Nevada Region (Press, I 9 6 0 ) .  - 20 More recent results obtained by the United States Geological Survey (Pakiser, et a l , 1962) indicate the following approximate structure f o r a p r o f i l e north-south  running  in Colorado, about 100 miles east of Denver (Figure 9). Thickness (km.)  Velocity (km/sec.)  30  20  7 8  Figure 9.  Structure f o r Colorado (Pakiser, 1962). Thickness (km.)  Velocity (km/sec.)  4.0  4.6  19.4  6.2  25.2  7.0  8.2  Figure 10.  Structure f o r the Area North of Carlsbad, New Mexico (Pakiser,  1962).  - 21 -  A refraction line over a distance range of 70 to 430 km. in a direction north from the underground nuclear explosion Gnome, near Carlsbad, New Mexico, was observed by the United States Geological Survey and the Colorado School of Mines.  The structure shown in Figure 10 (page 20), based on these data  has been proposed by Pakiser (1962). Healey (1962) has found the structure shown in Figure 11 for a refraction line from Santa Monica to San Francisco. Santa Monica —x 35  km.  Camp Roberts  6.1 km/sec.  8.0 km/sec. Figure 11.  25 km.  8.2 km/sec.  Structure along the Coast from Santa Monica to San Francisco.  By using reverse  San Francisco  data  the data obtained  for this refraction line and additional  from the Salinas explosion, Berg (1962) has proposed the struc-  ture shown i n Figure 12 for the coastal area south of San Francisco. Salinas  San Francisco 13.5 km.  7.8 to 8.0 km/sec. Figure 12.  Structure for Coastal Area, Salinas to San Francisco (Berg, 1962)  - 22 Healy (1962) has obtained observations for the area east from San Francisco to Fallon and Eureka, Nevada. The structure shown in Figure 13 has been obtained. San Francisco  Central Valley  Sierra Nevada  Fallon  36 km.  28 km.  25 km.  Figure 13.  Eureka 36 km.  Structure East from San Francisco (Healy, 1962).  Herrin (1962) has mapped P  FI  velocities for the United States and has  drawn velocity contours, which show a low value of about 7.6 km/sec. in the area of the Nevada Test Site, with values increasing radially to about 8 km/sec. Healy (1962) reports the following significant variations in the velocity of P for the western United States: n Colorado  8.2 km/sec.  New Mexico  8.0 to 8.2 km/sec.  Korthern Basin and Range Province  7.7 km/sec.  Santa Monica-Lake Meade  7.8 km/sec.  Santa Monica-San Francisco  8.0 to 8.2 km/sec.  Sierra Nevada Mountains  7.8 km/sec.  A brief summary such as the preceding serves to indicate the complexity of the earth'8 crustal structure in the coastal, mountain, and plateau areas of western North America. The need for detailed mapping rather than interpolation of structure*between widely spaced areas of known structure is indicated.  - 23 3.  3.1  THE SEISMIC PROGRAM  General Description I n May  I960 and June 1961,  s e i s m i c e x p l o s i o n programs were c a r r i e d out i n  t h e S t r a i t o f G e o r g i a and Johnstone S t r a i t areas o f B r i t i s h Columbia.  The  s b i s m i c energy was s u p p l i e d by 1 3 5 - k i l o g r a m depth c h a r g e s d e t o n a t e d a t a depth o f a p p r o x i m a t e l y 150 metres below t h e s u r f a c e . a r e r e f e r r e d t o as s h o t s 1 t o 31. r e f e r r e d t o as s h o t s A t o T.  The events o f t h e June 1961  I n November 1961  out t o o b t a i n supplementary d a t a .  The events o f t h e I960 program program a r e  a f u r t h e r program was  carried  Two 1350-kilogram charges were d e t o n a t e d  e l e c t r i c a l l y and r e c o r d e d a t d i s t a n c e s up t o 510 km.  from t h e s h o t p o i n t s .  a d d i t i o n t e n 1 3 5 - k i l o g r a m charges were d e t o n a t e d s e p a r a t e l y .  In  The events o f  t h i s program a r e r e f e r r e d t o as s h o t s A (Nov.), B (Nov.) and 1 (Nov„) t o 10  (Nov.)  F o r t h e I960 program seismograms were o b t a i n e d a t t h e permanent seismograph s t a t i o n s a t V i c t o r i a , A l b e r n i and Horseshoe Bay. s t a t i o n s were a l s o o p e r a t e d a t S c a n l o n Dam, and K e l s e y Bay.  Temporary  recording  Hornby I s l a n d , U c l u e l e t , B l o e d e l  F o r t h e 1961 programs seismograms were o b t a i n e d a t t h e perm-  anent s t a t i o n s a t V i c t o r i a and A l b e r n i .  Temporary s t a t i o n s were o p e r a t e d a t  Campbell R i v e r f o r t h e June program and a t E l k F a l l s and K e l s e y Bay f b r t h e November program. Some o f t h e e x p l o s i o n s were a l s o r e c o r d e d a t Longmire, a s t a t i o n o p e r a t e d by t h e U n i v e r s i t y o f Washington, and a t t h e Dominion O b s e r v a t o r y s t a t i o n a t Penticton. The l o c a t i o n s o f t h e s t a t i o n s and shot- p o i n t s f o r t h e s h o r t e r range r e f r a c t i o n work a r e shown i n F i g u r e 14 and f o r t h e l o n g e r range work i n F i g u r e 15.  The c o o r d i n a t e s a r e l i s t e d i n T a b l e 1 (page 2 4 ) .  LOCATIONS OF SHOT POINTS AND RECORDING STATIONS FOR SHORTER RANGE REFRACTION PROFILES FIGURE  14  125°  I20»  LOCATIONS OF SHOT POINTS AND RECORDING STATIONS FOR LONGER RANGE PROFILES FIGURE  15  - 24 Table 1.  Locations of Explosions and Recording Stations  Station or Shot  Latitude 0  Victoria Alberni Horseshoe Bay Penticton Ucluelet (1)  Patricia Bay Seattle Longmire Mineral 1  2 3 4 5 6 7 8 9 10 11  12 13 14 15 16 17 18 19 20 21 22 23 24  9  48 31.16  49 49 49 49 49 (2) Kelsey Bay 50 Scanlon Dam 49 Bloedel 50 Hornby Island 49 Campbell River 50 Elk Falls 50 Kelsey Bay (Nov.) 50  Longitude  16.23 22.65 19 02.9  04.8  23.82  47.80 06.03 37.60 03.37 01.89  21.38  48 38.94  47 39.3 46 45.0 40 20.8 49 37.60 49 35.61 49 33.02 49 32.45 49 34.35 49 36.35 49 41.00 49 43.88 50 33.55 50 30.20 50 28.20 50 23.74 50 20.22 50 1 0 . 7 1 49 57.30 49 53.61 49 48.92 49 43.95 49 38.10 49 33.27 49 32.20 49 27.72 49 25.75 49 23.82  i  123 24.91 124 49.30 123 119 125 125 125 124 125 124 125 125 125  Station or Shot  16.55 37 35.90  e  25 26  27 28  57.55  29 30 31  18.60  A  27.90 24.62  21.82 55.28 123 28.83 122 18.5 121 48.6  B C D E F G H I  34.09  K L M N  27.26  24.47  121 36.1 124 27.26 124 29.70 124 32.85  124 124 124 124 124 126 126 126 125 125 125 125 125 124 124 124 124 124 124 124 124  31.62 28.65 17.65  18.21  51.35 31.27 10.40  53.00  25.70 21.62 05.55 00.40  54.08 47.05 40.90 35.70 32.10 38.47 31.10 23.55  Latitude  J  0  P Q R S T A B 1 2 3 4 5 6  (Nov.) (Nov.) (Nov.) (Nov.) (Nov.) (Nov.) (Nov.) (Nov.)  7 (Nbv.)  8 (Nov.) 9 (Nov.) 10 (Nov.) Ripple Rock  49 49 49 49 49 48 48 49 49 49 49 49 49  Longitude 0  21.95  18.12  11.40  08.00  00.00  56.70 53.40 48ol5  46.07 44.17  42o37 40.53 38.61 49 36.79 49 35.60 49 33.91 49 32.33 49 31.45 49 32.97 49 34.64 49 36.21 49 37.87 49 39.38 49 41.38 49 43.19 49 45.12 49 46.95 50 24.47 48 18.6 50 23.6 50 23.4 50 22.2 50 22.4 50 22.5 49 59.6 49 53.8 49 51.1 49 48.9 50 21.4 50 07.9  t  124 16.80 124 02o40 123 41o30 123 35.90 123 30.50  123 23.15  123 124 124 124 124 124  15.55 51.92 48o23 44.50 40.55 36.65  124 124 124 124 124  29.02 24.58  124 124 124 124 124 124 124 125 123 125  26.71 30.79  124 33 d i 20.31 16.40 14.04  124 18.04 124 22.34  34.75 38.60 42.46 46.26  50.32 58.27  37.7 54.6  125 50.0  125 125 125 125 125 124 124 125 125  46.4 42.3  37.3  12.1 02.7 58.8  55.3  55.3  21.2  - 25 -  3.2  Instrumentation and Recording Techniques The instrumentation for the May I960 program i s summarized in Table 2„  Table 2. Summary of Instrumental Constants Station  Seismometer Recorder  Seismometer Galvanometer Paper Period Speed Period (sec) (mm/sec 5 (sec.)  Victoria  Benioff  Benioff  1.0  0.2  lo0  Alberni  Willmore  Sprengnether  1.0  0.03  lo0  Horseshoe Bay  Willmore  Sprengnether  1.0  0.25  1.87  Penticton  Benioff  Benioff  1.0  0.20  1.0  Ucluelet (1)  Willmore  Willmore  1.0  0.25  0.89  Ueluelet (2)  Willmore  Willmore  1.0  0.25  0.89  Kelsey Bay  Willmore  Willmore  1*0  0.25  0.89  Scanion Dam  Willmore Electro-tech and Geophbne ER-101 Spread  0.222  0,005  60.0  Bloedel  Willmore  Heiland  1.0  0.025  9.4  Century Hornby Island Willmore and Geophone Spread  1.0  0.06  90.0  The recording equipment at Victoria consisted of 3 component instruments. The other stations were equipped with vertical components only. The stations at Scanlon Dam, Bloedel and Hornby Island a l l had spreads of detectors. The June 1961 explosions were recorded at the permanent stations at Victoria and Alberni with the same instrumentation as indicated above, and also at the temporary station at Campbell River near the John Hart Dam,  This station  was equipped with a spread of geophones recording into an Electro-Technical ER-101 recorder mounted i n a truck.  The spread was approximately 1050 metres  in length with geophones at intervals of 15Q metres.  The geophones had a  - 26 ~  natural frequency of 4.5 cycles per second and were c r i t i c a l l y damped. Included also were two low impedance Willmore seismometers having a natural frequency of 1 cycle per second.  The outputs were amplified with conven-  tional amplifiers and recorded on channels 1-10 of the photographic paper recorder.  A l l geophones were located near bedrock.  75 mm/sec. was used.  A paper speed of about  For the November 1961 program this equipment was  operated at Kelsey Bay.  The site did not permit the use of the whole spread  of geophones. The layout for this operation i s further described in a later section.  In addition to this station and the permanent stations at Victoria,  Alberni and Penticton, two other temporary stations were operated at Elk Falls Forestry Lookout and at Patricia Bay.  At Elk Falls a short spread of  three Willmore seismometers spaced at 75-metre intervals recorded through low frequency amplifiers into a Century recorder.  The paper speed used was 26.5  mm/sec. The Pacific Naval Laboratory of the Defence Research Board at Esquimalt operated the instrument at Patricia Bay, which recorded the output of a hydrophone.  Well recorded events were obtained at this station from  the smaller explosions up to a distance of 195 km. Time control at the recording sites was provided by Times chronometers in practically a l l cases. These chronometers are operated by a synchronous motor which i s driven by an electronically generated power supply. The frequency of the power i s controlled by a tuned vibrator.  At Scanlon Dam a  Mercer self-winding chronometer was used and at Hornby Island a Dent Chronometer with 2-second contacts was used.  WWV and Dominion Observatory time  signals were used to correct and rate the chronometers.  The accuracy of time  control at the temporary stations and on the ship was sufficient to detect a delay of a few hundredths of a second in the Dominion Observatory time signal as broadcast from the Canadian Broadcasting Corporation station in Vancouver,  -  27  -  The delay Is presumably Introduced by the transmission system between Eastern Canada and the West Coast. Shot times were recorded on the ship on a two-channel tape recorder.  A  Times chronometer was used to trigger pulses at intervals of one second and these were recorded on one channel of the tape recorder. WWV  signals were  recorded with the chronometer pulses almost continuously throughout the program. The shock from the explosion was detected by a microphone fastened to the hull of the ship and recorded on the other channel of the tape recorder. I960  For the  program communication was carried on between the ship and those record-  ing stations which used high paper speeds by means of a 60-watt shortwave transmitter on the ship. Communication provided the recorder operator with a warning signal at an appropriate time for switching on the recording equipment. In some cases the communication was intermittent. For the June program, in addition to the shortwave equipment, communication was  1961  maintained  between the ship and the Elk Falls Forestry Lookout near Campbell River through British Columbia Telephone lines. into the telephone circuit.  The ship's radio was used to patch  The communication link to the seismic truck was  completed by V.H.F. Pye transmitter-receiver s e t s located at the truck site and at the Forestry Lookout. The telephone lines were again used, supplemented by an f-m telephone unit on the ship for the November 1961  program to  provide communication from the ship to the stations at Elk Falls and Kelsey Bay.  From a l l areas in which the ship operated i t was possible to work into  one of the telephone company's coastal stations with the f-m unit installed on the ship.  Telephones were installed conveniently in the Elk Falls Lookout  and the seismic truck at Kelsey Bay and connected to local lines of the Campbell River exchange. The only d i f f i c u l t y with transmission was  experienced  1  in the case of shot B (Nov.) as a result of elevated topography between the ship and the telephone land station.  - 28 -  Shot times f o r the two l a r g e e x p l o s i o n s o f t h e November 1961 were o b t a i n e d i n two ways. was  The shock wave d e t e c t e d by the h u l l  program  microphone  r e c o r d e d w i t h time r e f e r e n c e s i g n a l s on a two-channel t a p e r e c o r d e r .  a g a t i n g c i r c u i t was t r i g g e r e d by t h e c u r r e n t i n t h e d e t o n a t o r c i r c u i t . t u r n e d on an a u d i o t o n e f o r a d u r a t i o n o f about one second, and t h i s t r a n s m i t t e d from t h e l a u n c h from which t h e charge was where i t was  Also This  was  detonated t o the ship,  r e c o r d e d by a second t a p e r e c o r d e r w i t h t h e same time r e f e r e n c e  signals. Some o f t h e t a p e r e c o r d i n g s were p l a y e d back i n t o a two-channel b r u s h r e c o r d e r and o t h e r s , i n t o t h e C e n t u r y p h o t o g r a p h i c paper r e c o r d e r .  Examples  o f t h e v i s u a l r e c o r d s o f shot times a r e shown i n F i g u r e 19. N a v i g a t i o n was  c a r r i e d out by members o f t h e crews o f t h e CNAV Laymore  and CNAV Whitethroat„  B e a r i n g s were t a k e n a t t h e time t h e charges were  dropped i n t o t h e water, on l a n d marks which were w e l l d e f i n e d on t h e Hydrog r a p h i c Survey C h a r t s . direction.  The s h i p ' s gyro was u s e d t o o b t a i n t h e r e f e r e n c e  F o r s h o t s 1 t o 6 o f t h e I960 program  shot p o s i t i o n s was  some e x t r a c o n t r o l on t h e  o b t a i n e d by r e a d i n g s t a k e n w i t h l a n d - b a s e d t h e o d o l i t e s on  L a s q u e t i and Hornby I s l a n d s .  3.3  Reduction o f Data O r i g i n t i m e s ( d e f i n e d as t h e times o f a r r i v a l o f t h e shock waves a t t h e  bottom below t h e charges) were o b t a i n e d by making a p p r o p r i a t e c o r r e c t i o n s t o t h e r e c o r d e d shock i n s t a n t s . charges was  The p r e s s u r e d e t o n a t i n g mechanism on t h e depth  s e t t o t r i g g e r at a depth o f about 150 metres.  depth s e t t i n g v a r i e d from t h i s f i g u r e , but a r e c o r d was setting.  I n some c a s e s t h e  kept o f t h e a c t u a l  A r a t e o f descent o f t h e depth charge i n t h e water was  be 3 m/sec.  An average speed o f t h e s h i p was  under each charge was  calculated.  assumed t o  The depth o f water  o b t a i n e d w i t h s u f f i c i e n t a c c u r a c y from t h e soundings  - 29 -  marked on a Hydrographic Survey chart, published by the Federal Department of Mines and Technical Surveys„  For the explosions of the I960 program, depth  > •soundings were supplied by the ship's echo sounder.  The two large charges of  the November 1961 program, shots A (Nov.) and B (Nov,), were detonated on the bottom in water depths of 194 m. and 102 m., respectively. The data for obtaining the origin times of the explosions are shown in Tables 3 (page 30) and 4 (page 3 1 ) . Coordinates were obtained for the temporary stations by plotting the positions on 1:50,000 scale topographic maps. Distances between shot points and stations were calculated using the Richter short distance formula (1943). Arrivals of seismic energy were read for a l l stations at which events were recorded. Drift curves were drawn up for each chronometer based on the recorded standard radio signals.  Corrections to each arrival time were then  made by referring to the d r i f t curves. In many cases i t was possible to record WWV  signals on the ship and at the land-based stations, through the  recording time of the explosion. 3.4  Accuracy of Measurements The error in the origin time i s due to the following uncertainties in  the data used in i t s calculation. (l) The correction to the time of arrival of the shock at the ship for travel time in the water i s calculated on the basis of an average velocity of the shock or sound wave in the water.  The sinking rate of the depth charge was  taken as 3 m/sec. with a depth setting of 150 metres, approximately, giving a delay of about 50 seconds.  A figure of 364 m. (1200 ft.) with an uncertainty  of about 30 m. (100 ft.) was taken as the distance from the ship to the charge at the time of detonation.  The effect of the uncertainties i n distance and  velocity i s shown by the relation  - 30 -  T a b l e 3.  R e d u c t i o n o f Recorded Shock Time a t t h e S h i p t o 0-Time f o r E x p l o s i o n s o f t h e June 1961 Program  Recorded Shock I n s t a n t  Correction f o r F i l t e r i n Playback C i r c u i t (+.05 s e c . ) and T r a v e l Time t o S h i p (-.25 s e c . )  Depth o f Water  Correction f o r T.T. t o Bottom  (metres)  (sec.)  0-Time (Pacific Daylight Time)  A  1105  09.97  1105  09.77  218  + .05  1105  09.82  B  1121  59.20  1121  59.00  290  .10  1121  59.10  C  1138  25.00  1138  24.80  326  .12  1138  24.92  D  1152  40.18  1152  39.98  302  .10  1152  40.08  E  1209  39.67  1209  39.47  326  .12  1209  39.59  F  1227  06.92  1227  06.72  272  .08  1227  06.80  G  1245  38.40  1245  38.20  290  .10  1245  38.30  H  1303  08.18  1303  02.98  326  .12  1308  03.10  I  1320  31.69  1320  31.49  218  .05  1320  31.54  J  1337  13.29  1337  13.09  326  .12  1337  13.21  K  1415  41.42  1415  41.22  326  .12  1415  41.34  L  1429  51.66  1429  51.46  326  .12  1429  51.58  M  1444  21.09  1444  20.89  290  .10  1444  20.99  N  Not  0  1514  40.86  1514  40.66  290  .10  1514  40.76  P  1528  04.14  1528  03.94  272  .08  1528  04.02  Q  1544  31.24  1544  31.07  272  .08  1544  31.12  R  1556-  01.68  1556  01.48  326  .12  1556  01.60  S  1611  04.74  1611  04.54  326  .12  1611  04.66  T  1625  38.94  1625  38.74  208  .04  1625  38.78  recorded  - 31 -  T a b l e 4.  R e d u c t i o n o f Recorded Shock Time a t t h e S h i p t o 0-Time f o r E x p l o s i o n s o f t h e November 1961 Program  Shot  Corrected D e t o n a t i o n Time  1 (Nov.)  No shot t i m e .  2 (Nov.)  0750  16.19  3 (Nov.)  0810  01.56  4 (Nov.)  0830  5 (Nov.)  Correction for Travel Time t o Ship (sec.)  Correction for Travel Time t o Bottom (sec.)  -  + 0  0-Time ( P a c i f i c S t a n d a r d Time)  .17  .15  0750  16.04  .01  .18  0810  01.39  04.61  .03  .18  0830  04.46  0850  06.80  .07  .18  0850  06.69  6 (Nov.)  1329  29.80  .12  1329  29.68  7 (Nov.)  1410  07.78  .02  .24  1410  07.56  8 (Nov.)  1429  59.65  .04  .24  1429  59.45  9 (Nov.)  1450  09.86  .23  1450  09.63  10 (Nov.)  1510  11.89  .17  1510  11.75  A (Nov.)*  1729  59.30  1729  59.30  B  1044  59.68  1044  59.68  (Nov.)*  0  0  0 .03  *These t i m e s were o b t a i n e d from t h e p u l s e t r i g g e r e d by t h e d e t o n a t i n g c i r c u i t .  t t o be about  D  v  .02 s e c . where a v a r i a t i o n o f one p e r c e n t i s a l l o w e d i n sound  wave v e l o c i t y . (2)  The depth o f water as i n t e r p o l a t e d from t h e H y d r o g r a p h i c Survey C h a r t s  has an u n c e r t a i n t y o f 45 m.,  i n t r o d u c i n g an u n c e r t a i n t y o f 0.03 s e c . i n t o  t h e c o r r e c t i o n ! f o r t r a v e l time from charge t o bottom. Q t o M, t h e u n c e r t a i n t y may be as l a r g e as 90 m.  I n t h e case o f s h o t s  - 32 •  These and o t h e r u n c e r t a i n t i e s a r e summarized i n T a b l e 5.  T a b l e 5.  U n c e r t a i n t i e s i n Time D i s t a n c e Data  C o r r . due to t r a v e l time i n water (sec.)  Corr. f o r filter delay  Measurement errors on brush recording  Combined correction to absolute time  Reading errors on seismograms  Position  Probable error  H i g h Paper Speed Recorders with Spreads  .05  .01  .05  .04  .05  .02  .10  Continuous Oper&ting Recorders  .05  .01  .05  .10  .2  .02  .23  P o s i t i o n s were o b t a i n e d from b e a r i n g s t a k e n on l a n d markers.  T h i s work  was done by e x p e r i e n c e d n a v i g a t o r s who had become accustomed t o a c c u r a t e p o s i t i o n i n g i n l a y i n g mines. gyroscope.  The b e a r i n g s were t a k e n r e l a t i v e t o t h e s h i p ' s  Assuming an a c c u r a c y o f b e t t e r than 0.5° i n t h e b e a r i n g s , i t i s  r e a s o n a b l e t h a t t h e u n c e r t a i n t y i n p o s i t i o n i s l i m i t e d t o 0.1  3.5  km.  A d d i t i o n a l Data  On A p r i l 5, 1958, about 1.5 k i l o t o n s o f h i g h e x p l o s i v e were  detonated  i n a s i n g l e e x p l o s i o n i n Seymour Narrows i n o r d e r t o remove t h e t w i n p i n n a c l e s o f R i p p l e Rock which had i n t h e p a s t p r e s e n t e d a n a v i g a t i o n a l h a z a r d .  The  Dominion O b s e r v a t o r y o p e r a t e d a number o f seismograph s t a t i o n s t e m p o r a r i l y at p o i n t s t r a v e r s i n g t h e mountains as f a r e a s t as B a n f f .  O t h e r s t a t i o n s were  o p e r a t e d i n t h e C a l g a r y a r e a by a number o f o i l p r o s p e c t i n g crews, was o p e r a t e d by t h e U n i v e r s i t y o f A l b e r t a , n e a r Edmonton.  A station  The e x p l o s i o n was  - 33 also recorded by permanent seismograph stations at Spokane, Longmire, Seattle and Mineral.  Data from the recordings of this explosion have also been  included in this thesis. Data from a few explosions recorded during depth charge programs carried out in October of 1957 and July of 1959 are also included.  The 1957 program  has been described in a previous publication (Milne and White, I960). 3.6  Seismograms Examples of seismograms are shown in Figures 16 to 19.  pretation has been based on f i r s t arrival phases.  Most of the inter-  A few secondary arrivals  which were considered to be well defined were used on the long range timedistance plots. 3.7  Amplitudes of Recorded Energy A quantitative analysis of the relation of trace amplitudes and maximum  distances of recording to size of explosion, amplitude of background noise, depth of detonation and other factors has not been made in this thesis. However, a brief discussion is in order.  The observations w i l l be useful in  setting up future programs. The two large charges, A (Nov.) and B (Nov.), were detonated in order to obtain records for distances beyond the point where P arrival.  r  becomes a f i r s t  The greatest distance for which these charges were recorded was  from shot A (Nov.) to Longmire (510. km.).  The amplitude of the disturbance  on this seismogram i s small, but differs in frequency from the background noise so that the onset may be recognized with some certainty. Stations along the coast record a rather high level of microseismic background during the winter months. This reduces the signal to noise ratio for these stations. The frequency of the recorded seismic energy i s , however, considerably higher  a SHOT A(NOV.), 1350 Kilograms Explosive, Water Depth - 194 Metres Recorded at Alberni, A = I5III Kilometres  b SHOT B(NOV.), 1350 Kilograms Explosive, Water Depth - 102 Metres Recorded at Alberni, A = 138-89 Kilometres  SB c SHOT B(NOV.)  1  SEISMOGRAMS OBTAINED AT PERMANENT FIGURE  16  STATIONS  a SHOT B(NOV.), Recorded at Longmire, A = 220-97 Kilometres  b SHOT A (NOV.), Recorded at Victoria, A = 280-1 Kilometres  c  RIPPLE ROCK, 1-5 Kilotons Explosive Recorded at Mineral, A= 1126-6 Kilometres SEISMOGRAMS  OBTAINED FIGURE  17  AT PERMANENT  STATIONS  HIM IQCWS\VWA/^S'V^ I  WW***  SHOT C, 135 Kilograms Explosive  Recorded at Campbell River, A= 59-64 Kilometres Geophone Spacing 145 Metres  ^AA//  *'* •  '  v"  /  >\  yv  .'  ,  J  V^V.-'H.'V\/."..'-S^.-*.  ;  _\,  - _v  , »,  1  f  —— : ——  . • •' •' *^^' ^• .S A ,  ill  1  I  li  SHOT B(NOV.), Recorded at Elk Falls, A = 229-50 Kilometres Geophone Spacing 75 Metres SEISMOGRAMS OBTAINED AT TEMPORARY  FIGURE  18  STATIONS  e,»*T" i{toO (km  ''•  V ' —— i  -  1  SHOT 7 (NOV.), 135 Kilograms Explosive, Recorded at Elk Falls, A= 26-29 Kilometres S E I S M O G R A M OBTAINED AT E L K F A L L S  = SHOT A (NOV.)  SHOT B(NOV.)  VISUAL RECORDS OF DETONATION TIME  FIGURE 19  FOR SHOTS A(NOV) AND B(NOV.)  -  34  -  t h a n t h e frequency: c h a r a c t e r i s t i c o f t h e m i c r o s e i s m i c background,,  This i s  b e s t i l l u s t r a t e d i n t h e V i c t o r i a r e c o r d of s h o t A shown i n F i g u r e 17bo p r e c i s e time o f a r r i v a l may  A  be r e a d f o r t h e i n i t i a l d i s t u r b a n c e on the  seismograme It  i s o f i n t e r e s t t h a t a f i r s t a r r i v a l a t V i c t o r i a has been r e c o r d e d f o r  s h o t 12,.a  shot w h i c h was  V i c t o r i a as shot A  d e t o n a t e d a t a p p r o x i m a t e l y the same d i s t a n c e f r o m  (Nov.) but c o n t a i n e d o n l y 1/10  t h e amount of e x p l o s i v e  e  That t h e f i r s t a r r i v a l o f energy has been chosen i s i n d i c a t e d by t h e good correspondence for  o f t h e t r a v e l times of t h e s e two  s h o t 12 i s v e r y s m a l l .  (Nov.) as r e c o r d e d a t V i c t o r i a .  (Nov.) and B  be made by  I t w i l l be  f o r s h o t A i s somewat g r e a t e r .  d i s t a n c e from t h e r e c o r d i n g s t a t i o n .  km.)  A comparison of t h e a m p l i -  (Nov.) a t A l b e r n i may  e x p l o s i v e used i n each case i s the same and  noted  The w e i g h t o f  shot A i s a t a s l i g h t l y g r e a t e r  I t s h o u l d be n o t e d t h a t s h o t A  detonated, on t h e bottom i n 194 nu o f w a t e r as compared w i t h 102 m. O t h e r f a c t o r s w h i c h may  has  The d i s t a n c e i s s l i g h t l y g r e a t e r (315  r e f e r r i n g t o t h e seismograms shown i n F i g u r e 16a and b . t h a t t h e o v e r a l l amplitude  amplitude  (Nov.), as compared w i t h t h a t f o r shot  t h a n f o r t h e s h o t A - V i c t o r i a d i s t a n c e (280 km.). tudes recorded f o r shots A  The  A considerably higher s i g n a l to noise r a t i o  been o b t a i n e d a t P e n t i c t o n f o r shot B A  explosions*  was  f o r shot  B.  account f o r t h e v a r i a t i o n s a r e t h e bottom c o n d i t i o n s ,  and t h e . p o s s i b l e e x i s t e n c e o f major d i s c o n t i n u i t i e s i n t h e s u r f i c i a l s t r u c t u r e in  the propagation path of the seismic - An. e x a m i n a t i o n  energy.  of t h e seismograms f o r o t h e r events i n d i c a t e s an  a m p l i t u d e v a r i a t i o n f r o m one  explosion t o another.  overall  A c u r s o r y a n a l y s i s would  i n d i c a t e t h a t t h e v a r i a t i o n i s observed a t a l l s t a t i o n s and i s a r e s u l t of t h e c o n d i t i o n s l o c a l t o t h e charge and n p t , f o r example, a f u n c t i o n of t h e azimuth  of the recording s t a t i o n .  - 35 The necessity for finding a 'quiet' site was emphasized by an examination of the seismograms obtained at Kelsey Bay during the November 1 9 6 1 program. The spread of detectors was laid out on the alluvium at the mouth of the Salmon River.  This uncompacted sediment had the effect of transmitting with  rather large amplitude the ground noise set up by logging operations and electrical generating equipment, even though the site was located at what was expected to have been a safe distance from the latter.  Local industrial noise  of this nature gives rise to background of about the same frequency as the seismic energy radiating from the explosions and makes precise reading of onsets d i f f i c u l t . It would appear that the 1350 kilogram charges supply sufficient seismic energy to carry out refraction studies to distances where  is a first  arrival, provided that quiet sites are chosen for recording. This condition is better realized for stations located some distance away from the coast. 3.8  Shorter Range Refraction Data for the I960 and 1961 Programs In this section the layout of the shot points and recording stations i s  described for each profile of the shorter range data.  In the two following  sections a similar description i s given for the Ripple Rock data and for the longer range data obtained for the I960 and 1961 programs. The time-distance data are presented i n tabular form together with the plots, and the least squares equations for the various segments of the time-distance relations. General features of the travel time curves are observed.  Discussion and  interpretation of the time-distance plots in the form of suggested structures follow in section 3.12. The data are presented for the various profiles which are suggested by the geographical locations of the shot points and stations.  In most cases  the profiles consist of a spread of shots recorded at a single station.  In  - 36  -  a few cases the profile i s for a single shot recorded on a spread of stations. The observations have been treated in a manner conventional in seismic refraction studies. Time-distance plots have been made and the various segments treated by least squares to obtain equations of the form * " 0 T  where t  +  v  = travel time i n sec.  TQ = zero distance intercept on the time axis in sec. A  = distance between shot and recording station in km.  v  = velocity in km/sec. for the refracting layer being considered.  The following assumptions are inherent in this approach. (1) The layers of the crustal model are assumed to have plane, but not necessarily horizontal, boundaries. (2) The velocity in each layer i s constant. (3) The travel time i s not affected by the interchange of a shot and a recording station. 3.8.1  Profile I: The time-distance data used for profile I, and consisting of events  recorded at Campbell River and at Elk Falls Forestry Lookout, are shown i n Tables 6 (page 37) and 7 (page 3 8 ) .  Since these two stations differ only  slightly in location and are underlain by the same geological structure i t was considered justifiable to plot the data for both on the same plot.  The loca-  tions for shots 6 (Nov.) to 10 (Nov.) were laid out to f i l l in the short range part of the profile. The data are plotted in Figure 20. The arrival times for this profile with the exception of shot 6 appear to f a l l along two lines with a discontinuity between shots P and F.  This suggests a discontinuity in the structure  such as a buried fault with vertical displacement, the high side underlying  - 37 -  shot P.  Events C and R show late arrivals which may be due to deep deposits  of low velocity sediment below the shot positions,  A shot located between  6 (Nov.) and 7 (Nov.) had to be omitted due to local marine t r a f f i c .  Shot  6 (Nov.) shows some evidence of a layer having a typical granitic velocity underlying the lower velocity Cretaceous sediments.  Table 6.  Time-Distance Data for Elk Falls used for Profile I  Shot  t (sec.)  A (km.)  6 (Nov.)  2.32  12.41  7 (Nov.)  4.80  26.29  8 (Nov.)  5.96  34.01  9 (Nov..)  7.00  39.88  10 (Nov.)  8.32  49.78  It may be noted that these shots were detonated in two stages, during the June 1961 program. Ten were dropped while the ship made a traverse in one direction and the second group of ten were dropped at staggered positions on the return traverse.  This procedure gave the ship's crew more time for  handling the explosives and i t i s also felt that errors in timing and position would tend to cancel out since each event i s independent of the errors i n the immediately preceding and following event. A summary of the time-distance relations, in the form of least squares equations i s given i n Table 8 (page 39) for each profile. are probable errors.  The uncertainties  - 38 Table 7.  Time-Distance Data for Campbell River used for Profile I  Shot  t (sec.)  A (km.)  A  8.02  48.10  B  8.85  53.95  C  9.85  59.64  D*  Not recorded  65.45  £  11.37  71.25  F  12.54  76.79  G  13.22  82.76  H  14.02  88.38  I  14.91  94.37  J  15.60  99.91  K  16.20  103.21  L  15.31  97.64  M  14.48  91.60  N*  A  Not recorded  85.63  0  12.84  79.87  P  11.74  74.36  Q  10.89  68.43  R  10.31  62.71  S  9.18  56.90  T  8.44  50.97  *The recorder was not operating at the correct time due to a breakdown i n the communications at the time of shot D and i n the case of shot N, to an unusual delay i n the detonating mechanism associated with the depth charge.  T a b l e 8.  L e a s t Squares E q u a t i o n s R e p r e s e n t i n g Time-Distance R e l a t i o n s  Profile  I  (Campbell  Uncompacted Sediments  Cretaceous Sediments  Granitic or Volcanic  Intermediate  t -  River, E l k Falls)  Mantle  1.10 ± .21 +  A  6.94 ±  .20  t - 1.48 ± .15 t 7.04 ± .07 A  II  (Alberni)  t -  .70 ±  .10 A  t  6.85 ±  .08  t - 2.51 ± 9 7.19 ± 3  t - .38 ±  .56  3.61 ± .16  III (Elk Falls)  t - .47 ±  1.28  6.66 ±  1.20  Table 8 (Continued) Uncompacted Sediments  Profile  Cretaceous Sediments  IV (Kelsey Bay)  V (Kelsey Bay)  VI (Hornby Island)  Intermediate  Granitic or Volcanic t  t *  - •«°*J?jB  A  1.56 ± .02  t - .05 ± .04 + 3.81 ± .16  t  A  VII (Scanlon Dam)  t = .56 ± .86 + 6.45 ± 1.00  VIII (Victoria)  t = 1.55 ± .49 + 6.76 ± .13  A  A  +  t - .92 ± 3.11 +  IX (Ucluelet)  A  3.62 ± .24  t = 1.13 ± .60 + 6.63 ± .30 A  Mantle  Table 8 (Continued)  Profile  Uncompacted Sediments  Cretaceous Sediments  Granitic o r Volcanic  Mantle  Intermediate  X ( K e l s e y Bay I960)  t -  .03 ±  .49 A 7.22 ± .23  XI (Horseshoe Bay)  t - 1.69 ± .41 6.74 ± .16  +  t - 4.81 ± .64  XII (Ripple Rock)  7.66 ±  +  .07  t - 6.0 ± 1.3 7.78 ±  .15  t - 2.00 ± 2.49 3.64 ±  XIII (Ripple Rock)  t = .11 ± .53  XIV  t = .97 ± .28  i  +  .06  t = 7.93 ± .19  A  6.60 ±  .10  6.71 ± .06  7.75 ±  t  -"  0 9  *  5  +  .02  7.97  Table 8 (Continued)  Profile  XV  Uncompacted Sediments  Cretaceous Sediments  Granitic or Volcanic  Intermediate  t--A_ 6.66  Mantle  t - 7.17 ± 2.5 +  XVI  t - .22 ± .08 A 6.64 ± .03 +  XVII  t - 1.31 ± .26 +  Composite Plot  6.66 ± .09  t = 0.63 ± .30 6.66 ± .07  7.68 ± .35  - 43 3.8.2 Profile H i The tine-distance observations for profile II, consisting of events recorded at Alberni, are l i s t e d i n Table 9. It will be noted by reference to Figure Ik that although the Alberni station is not collinear with the line of shots, this profile i s an approximation to a reversal of profile I. The p r o f i l e includes the Ripple Rock event which has also been included in the least squares reduction of the data.  I t also includes shots A (Nov.) and  1 (Nov.) to 5 (Nov.) which are shown i n the distance range 135 to 151 km. on the plot of the data shown i n Figure 21. These arrivals are late and have been treated separately to obtain a least squares segment of the travel time curves. Table 9. Time-Distance Data for Profile II as Recorded at Alberni Shot  *  3 20 21 P Q D C S B T a 16 15 R.R. A (Nov.) 1 (Nov.) 2 (Nov.) 3 (Nov.) 4 (Nov.) 5 (Nov.) 6 (Nov.) 7 (Nov.) 8 (Nov.) 9 (Nov.) 10 (Nov.)  t (sec.) 5.7 6.3 6.1 5.9 7.4 7.8 7.9 8.2 8.4 8.6 9.1 9.4 11.0 12.1 15.7 23.75 22.61 22.52 22.49 21.59 21.19 13.29 11.17 10.40 9.32 8.40  A  (km.)  35.25 36.94 35.61 35.19 46.37 48.38 49.59 52.11 53.68 55.33 56.96 59.18 70.57 78.61 103.10 151.11 147.40 144.21 140.14 138.13 135.65 84.95 71.54 65.34 59.07 53.00  - 44 -  3.8.3. P r o f i l e I I I :  The t i m e - d i s t a n c e d a t a f o r p r o f i l e I I I a r e shown i n T a b l e 10.  The d a t a  a r e f o r s h o t s A (Nov.) and 2 (Nov.) t o 5 (Nov.) as r e c o r d e d a t E l k F a l l s .  T a b l e 10.  Time-Distance  D a t a f o r P r o f i l e I I I , E v e n t s Recorded a t E l k F a l l s  Shot  t  (sec.)  A  (km.)  A (Nov.)  9.74  60.03  1 (Nov.)  8.74  56.24  2 (Nov.)  8.19  52.26  3  (Nov.)  7.69  47.68  4  (Nov.)  7.21  45.16  5 (Nov.)  6.91  42.42  The p l o t  f o r t h i s p r o f i l e i s shown i n F i g u r e 22.  The p o i n t s do not  f i t the l i n e obtained f o r p r o f i l e I.  3.8.4  P r o f i l e IV:  The d a t a f o r p r o f i l e IV were o b t a i n e d from r e c o r d s a t K e l s e y Bay o f s h o t s 2 (Nov.) t o 5 (Nov.).  The p l o t  w i t h two p o i n t s on each segment. shown i n T a b l e 11.  i s i n t e r p r e t e d as h a v i n g two segments  I t i s shown i n F i g u r e 23 and t h e d a t a a r e  A t h i r d low v e l o c i t y  segment t a k e n from p r o f i l e V has  been drawn i n .  3 . 8 . 5 - . P r o f i l e V:  P r o f i l e V ( F i g u r e 24) i s f o r a s h o r t range r e f r a c t i o n o b t a i n e d by d e t o n a t i n g a s m a l l charge  shot.  I t was  (about one k i l p g r a m ) o f n i t r o n e i n  - 45 -  s e v e r a l metres o f water a t one end o f t h e geophone l i n e and r e c o r d i n g t h e e x p l o s i o n a c r o s s t h e s p r e a d o f geophones l a i d o u t a t K e l s e y Bay (Nov.). purpose o f t h e p r o f i l e i s t o o b t a i n t h e v e l o c i t y  i n t h e sediments a t t h e  mouth o f t h e Salmon R i v e r , on which t h i s  was l o c a t e d .  station  The  The o b s e r v e d  dataware shown i n T a b l e 12.  T a b l e 11.  D a t a f o r P r o f i l e IV, E v e n t s Recorded a t K e l s e y Bay  Shot  t  (sec.)  x  A (km.)  2 (Nov.)  1.45  3 (Nov.)  2.07  10.64  4 (Nov.)  2.91  15.44  5 (Nov.)  3.80  21.42  T a b l e 12.  7.29  D a t a f o r P r o f i l e V, f o r E x p l o s i o n Detonated n e a r t h e South End o f t h e Geophone L i n e a t K e l s e y Bay  A (km.)  Geophone Number  t (sec.)  1  .000  0.000  2  .033  0.061  4  .138  0.247  5  .206  0.348  6  .282  0.456  7  .344  0.556  8  .400  0.625  - 46 Onsets f o r s h o t s A (Nov.) (A =* 6.70  km.)  and 2 (Nov.) r e c o r d e d a t s t a t i o n  K e l s e y Bay were sharp and v e l o c i t i e s were r e a d a c r o s s t h e geophone s p r e a d f o r t h e s e two e v e n t s . shot 2 (Nov.). a  result  valley.  The apparent  v e l o c i t y o f 5.85  km/sec. was o b t a i n e d f o r  v e l o c i t y o b t a i n e d f o r shot A was v e r y h i g h , as  o f r e f r a c t i o n from t h e h i g h v e l o c i t y r o c k o f t h e w a l l o f t h e r i v e r The geophone l i n e was o r i e n t a t e d  with the v a l l e y  3.8.6  An apparent  30*  a t an a n g l e o f a p p r o x i m a t e l y  wall.  P r o f i l e VI: The t i m e - d i s t a n c e d a t a f o r p r o f i l e V I , events r e c o r d e d on Hornby  a r e shown i n T a b l e 13.  Hornby i s t h e o n l y s t a t i o n l o c a t e d  Island,  on t h e C r e t a c e o u s  sediments.  T a b l e 13.  Time-Distance  D a t a f o r P r o f i l e V I , Events. Recorded a t Hornby  t  Shot  A (km.)  (sec.)  3  1.1  4.42  4  0.6  2.60  15  9.8  59.12  19  2.9  13.09  20  0.7  2.75  21  1.2  4.76  The p l o t t e d p o i n t s shown i n F i g u r e 25 intersecting  a t a d i s t a n c e o f 8.03  km.  Island  appear t o f a l l a l o n g two l i n e s  The low v e l o c i t y segment i s a s s o c i -  a t e d w i t h t h e low v e l o c i t y sediments o f C r e t a c e o u s  age.  T h e r e i s no i n d i -  c a t i o n o f a l a y e r h a v i n g v e l o c i t y o f t h e o r d e r o f 6 km/sec. above t h e l a y e r h a v i n g v e l o c i t y 6.67  km/sec.  The d a t a , however, a r e s p a r s e , and t h e r e f r a c -  t i o n from a t h i n i l a y e r between t h e C r e t a c e o u s  l a y e r and t h e h i g h e r v e l o c i t y  - 47 layer would not become a first arrival. 3.8.7  Profile VII: The data for records from Scanlon Dam used in the plot for profile VII  are shown in Table 14.  Table 14.  Time-Distance Data for Profile VII, Events Recorded at Scanlon Dam t (sec.)  Shot  A (km.)  17  7.2  42.61  18  6.0  34.92  19  5.8  32.29  20  5.5  33.89  A plot of the data is shown in Figure 26.  The uncertainties are very  large. 3.8.8  Profiles VIII to XI: The data obtained at a number of stations at greater distances from shots  detonated in the same area are also presented. The data for Victoria, Patricia Bay, Ucluelet, Kelsey Bay and Horseshoe Bay are shown in Tables 15 to 19. The time-distance data for Patricia Bay for events 6 (Nov.) to 10 (Nov.) are combined with those for Victoria for profile VIII (Figure 27) and were also used in the least squares determination. Included also, but not used in the least squares, are Ripple Rock, shot 12 and shot A (Nov.), observed travel times to Victoria.  Patricia Bay and Victoria are located on the same  geological formation. The points plotted as solid circles are probably a l l located on Cretaceous sediments.  The open circles at the short distances  - 48 represent charges detonated on the Fraser River deposits.  Ripple Rock, 12,  and A (Nov.) plotted at distances greater than 230 km. were a l l detonated close to a competent basement rock. Table 15. Time-Distance Data for Profile VIII, Events Recorded at Victoria  Shot  t (P) (sec.)  t (Lg) (sec).  3 4 20  22.4 22.7 22.7 24.4 25.3 25.4 25.8 26.1 26.3 27.1 27.8 31.5 28.6 29.4 30.8 32.52  40.5  P Q D C  S B T A 15 1 7  *  1-18J 1-19  6 (Nov.) 7 (Nov.) 8 (Nov.) 9 (Nov.) 10 (Nov.).  39.7  47.8 50.4 51.6 56.1  30.44 28.76 27.78 27.72  A (km.) 141.43 141.49 143.85 152.39 158.06 160.91 166.39 169.07 171.88 174.71 177.63 201.03 181.66 186.36 196.58 209.22 194.04 187.00 181.14 171.42  *  Shots from the explosion program of October 2 5 , 1957 (Milne, I 9 6 0 ) .  Table 1 6 . Time-Distance Data for Profile VIII, Events Recorded at Patricia Bay  Shot  6 (Nov.) 7 (Nov.) 8 (Nov.) 9 (Nov.) 10 (Nov.)  t (P) (sec.)  t (Lg) (sec.)  30.5 28.5 27.5 26.3 25.1  54.5 47.8 48.3 45.4  A (km.) 194.97 179.56 172.66 166.79 157.05  - 49 Lg arrivals are included of events for which they were well defined. The data for both Ucluelet stations are plotted together for profile IX (Figure 28).  The location of the shotpoiht^ tend to l i e along a line at  right angles to the line joining them to the stations. The Ripple Rock arrival time is plotted with the Horseshoe Bay data on profile XI (Figure 30) but is not included in the least squares.  Table 17. Time-Distance Data for Profile LX, Events Recorded at Ucluelet Stations (1) and (2) Shot  t (sec).  20 (2) 19 (2) 18 (2) 20 (1) k (1) 19 (1) 3 (1) 21 (1) 18 (1) 17 (1) 16 (1) 15 (1)  13.5 14.0 14.5 15.0 15.7 15.5 15.8 15.2  A  82.33 83.93 87.78 92.15 92.82 93.23 94.04 94.50 96.33  15.8  16.1 16.6 17.6  (km.)  99.14  103.30 107.27  The number in brackets identifies the two Ucluelet stations.  Table 18.  Time-Distance Data for Profile X, Events Recorded at Kelsey Bay Shot  t (sec.)  15 16 17 18 19 20 21 13 14  10,93 11.71 14.08 15.73 17.65 18.54 19.10 4.50 7.37  A  (km;) .  79.06 88.16 99.56 111.97 124.77 135.49 139.99 38.36 49.11  - 51  however t h a t t h i s chronometer had across the spread o f shots t i m e i n t e r c e p t has Figure  -  a low d r i f t  i s reasonably  l i t t l e meaning.  The  r a t e and  t h a t the  r e l i a b l e but t h a t the time-distance  velocity zero  distance  p l o t i s shown i n  29.  Correction (+ sec.)  i 12 May F i g u r e 31.  3.9  3.9.1  i i i i i 18 24 06 P a c i f i c S t a n d a r d Time 18, I960 May 19  D r i f t Curve f o r Horseshoe Bay  i  i _  12  Chronometer  R i p p l e Rock Data  P r o f i l e XII:and P r o f i l e X I I I :  Temporary s t a t i o n s were o p e r a t e d  eastward from t h e R i p p l e Rock e x p l o s i o n ,  t h r o u g h t h e Rocky Mountains a t t h e g e o g r a p h i c a l l o c a t i o n s l i s t e d i n T a b l e (p)3ge 52). W i l l m o r e who  The  d a t a from t h i s program were k i n d l y s u p p l i e d by Dr. P.  20  L.  c o o r d i n a t e d t h e s e i s m i c program o f the Dominion O b s e r v a t o r y a t  t h e t i m e o f t h e R i p p l e Rock e x p l o s i o n .  - 52 -  Table 20. Time-Distance Data for Profile XII for Ripple Rock Recorded at Stations East through the Mountains  Station  t (P) (sec.)  t (Lg) (sec.)  A (km.)  Cache Creek Deadman Cherry Creek Knutsford Chase Revelstoke Glacier McMurdo Banff Edmonton  43.8 47.4 50.0 53.3 56.7 71.8 79.6 85.3 96.3  86.5 87.7 96.1 102.2 111.6 143.2 155.6 172.1  296.1 336.8] 345.8! 362.0 401.2 514.8 568.3 617.8 702.3 896.3  252.2  No adjustment has been made to travel times for height above sea level. Events A (Nov.) and B (Nov.) as recorded at Penticton are shown on this plot, but not included in the least squares calculation (Figure 32). Data for stations along the Coast south-east from Ripple Rock are shown in Table 21.  Table 21.  The data are plotted in Figure 33 for profile XIII.  Time-Distance Data for Profile XIII, Ripple Rock Recorded at Stations South East along the Coast  Station  t (sec.)  Alberni Horseshoe Bay Victoria Brother's Island Bellingham Seattle Longmire Mineral  15.69 26.29 34.5 35.9 39.8 54 67.0 153.1  A (km.) 103.10 171.52 228.1 235.5 259.6 358.2 458.5 1126.6  -  53/-  The Ripple Rock explosion was recorded at two of the University of Washington stations at Seattle and Longmire, and at Bellingham, Washington. It was also recorded at a University of California station at Mineral. data from these stations are included in the plot.  The  The seismogram of this  event recorded at Mineral i s shown in Figure 17c.  3.10  Longer Range Data The data presented in this section are for the larger explosions which  were capable of recording at distances for which P  R  becomes a f i r s t arrival,  i.e., Shot A (Nov.), B (Nov.), Ripple Rock and Constance Bank. 3.10.1  Profiles XIV to XVII s  The data for the longer range profiles are shown in Table 22 (Page 54). The plots for the four profiles are shown in Figure 34. The data for profile XIII are also replotted.  In plotting these observations i t has been assumed  that the interchange of a shot point and a station does not affect the travel time between them. Profile XIV i s obtained from records of Shot A (Nov.) along a spread of stations running southeast on Vancouver Island and the Puget Sound area as far south as Longmire near Mt. Rainier. A well-defined secondary P arrival has been read for Victoria. events recorded at Longmire.  Data for profile XV are for  The line representing the relation t = •gygg  has been drawn i n by assuming zero intercept, and using the time-distance data for shot B (Nov.) recorded at Longmire as a point on the line.  The  open circles plotted on this profile represent data for shots 28, 29, 30 and 31 detonated on the low velocity sediments associated with the depositions of the Fraser River, They have not been included in the least squares analysis. Their position on the plot suggests that i f properly corrected for sediments, they would l i e on the t = 7~rr line rather than the t = 7.17 + TTTa line.  - 54 -  Table 22. Time-Distance Data for Longer Range Profiles Profile XIV Shot A - Recorded at Stations Southeast to Longmire Station  t (sec.)  A  Elk Falls Alberni Victoria Longmire  9.74 23.75 42.56 73.4  151.11 280.1 509/7  (km.) 60.03  '44.6  Profile XV Events Recorded at Longmire Shot  t (sec).  A  B (Nov.)  33.2 45.3 46.0 41.7 40.8 67.0 73.4  220.97 296 280.4 271.1 261.5 458.5 509.7  28 29 30 31  Ripple Rock A (Nov.)  35.9  (km.)  Profile XVI Shot B (Nov.) Recorded at Stations Northwest to Elk Falls; Ripple Rock and Shot A Recorded at Victoria Station or Shot  t (sec.)  A  Victoria Patricia Bay Alberni Elk Falls Ripple Rock A (Nov.)  4.34 6.2 21.34 34.52 34.5 42.56  28.29 39.13 138.89 229.50 228.1 280.1  (km.)  - 55 -  T a b l e 22.  Time-Distance  Data f o r Longer Range P r o f i l e s  (Cont.).  P r o f i l e XVII Constance Bank Recorded a t S t a t i o n s Northwest t o R i p p l e Rock Camp  Station  t  Victoria  4.18 24.28  19.93 150.86  38.31  247.3  R i j i p l e Rock , Camp •-  (sec.)  A (km.)  P r o f i l e XVI r e p r e s e n t s t h e d a t a from s t a t i o n s r e c o r d i n g Shot B t o t h e north-west as f a r as E l k F a l l s , as w e l l as events A (Nov.) and R i p p l e Rock recorded at V i c t o r i a .  The assumption r e g a r d i n g i n t e r c h a n g e o f shot p o i n t  and r e c o r d i n g s i t e has a l s o been made here i n p l o t t i n g t h e d a t a f o r both spreads o f shots and spreads o f r e c o r d i n g s t a t i o n s .  The j u s t i f i c a t i o n f o r  p l o t t i n g d a t a f o r V i c t o r i a and shot B on t h e same p l o t i s i n d i c a t e d by t h e good correspondence  between t r a v e l times o f R i p p l e Rock t o V i c t o r i a and Shot B  to E l k F a l l s . P r o f i l e X V I I r e p r e s e n t s d a t a o b t a i n e d from t h e e x p l o s i o n a t Constance Bank as r e c o r d e d a t s t a t i o n s t o t h e north-west as f a r as R i p p l e Rock Camp. The good correspondence  o f the v e l o c i t i e s obtained f o r t h i s p r o f i l e with that  o f p r o f i l e XVI i s i n d i c a t e d .  The d i s c r e p a n c y i n t h e i n t e r c e p t s i n d i c a t e s  e i t h e r t h e presence o f low v e l o c i t y m a t e r i a l below t h e Constance Bank e x p l o s i o n o r t h e p o s s i b i l i t y o f an e r r o r i n t h e shot i n s t a n t o b t a i n e d .  The l a t t e r  possi-  ' b i l i t y i s f a v o u r e d , s i n c e no g e o l o g i c a l evidence f o r a l o w v e l o c i t y l a y e r o f t h e r e q u i r e d depth i s i n d i c a t e d .  The shot i n s t a n t was o b t a i n e d by r e c o r d i n g  t h e e x p l o s i o n w i t h a hydrophone a t a l o c a t i o n s e v e r a l k i l o m e t r e s from t h e P.  s h o t p o i n t and c o r r e c t i n g f o r t h e d e l a y on t h e b a s i s o f an assumed v e l o c i t y .  i — i — i — i — i — i — i — i  0  10  20  30  i  40  i  i  50  i  60  i  i  70  i  i  i  80  A (KILOMETRES) PROFILE  I  EVENTS RECORDED AT CAMPBELL RIVER AND ELK FALLS FIGURE  20  i  90  i  r  1  r  "i  i  i  i  r  i  1  1  r  RIPPLE ROCK  6 85  I  80  I  120  A (KILOMETRES)  PROFILE H  EVENTS RECORDED AT ALBERNI  FIGURE 21 )  I  I  140  I  160  FIGURE  22  6  8  10  12  14  16  18  A (KILOMETRES)  PROFILE TST  EVENTS RECORDED AT KELSEY BAY (NOV.)  FIGURE 23  20  01  02  0-3  0-4  0-5  0-6  A (KILOMETRES)  PROFILE TT  TIME - DISTANCE PLOT FOR ALLUVIUM AT MOUTH OF SALMON RIVER  FIGURE  24  0  10  20  30  40  50  A (KILOMETRES)  PROFILE 1ZI  EVENTS RECORDED AT HORNBY ISLAND FIGURE  25  10  20  30 A  P R O F I L E HR  EVENTS  40  (KILOMETRES)  R E C O R D E D AT S C A N L O N DAM  FIGURE 26  r  FIGURE  27  0  20  40  60  80  100  A (KILOMETRES) PROFILE IX  EVENTS  RECORDED AT UCLUELET (I) and (2) FIGURE  28  120  0  20  40  PROFILE  60  3!  80 A (KILOMETRES)  EVENTS  FIGURE  100  RECORDED AT KELSEY BAY  29  120  140  PROFILE X I  EVENTS  RECORDED AT H O R S E S H O E BAY  FIGURE 30  A (KILOMETRES)  PROFILE 2 H  RIPPLE ROCK RECORDED AT STATIONS EAST THROUGH MOUNTAINS FIGURE  32  0  200  4 0 0  600  800  A  PROFILE 33E  1000  1200  (KILOMETRES)  RIPPLE ROCK RECORDED AT STATIONS SOUTH-EAST ALONG THE COAST  FIGURE  33  1400  80  1  1  1  1  1  1  1  1  1  1  70  60  \ . t-7 17 + V^r \. 7 68  -  50 029  ,  40  _  PROFILE  \  PROFILE SET."* \ \  t= 22 + d b 6 64  20  -  PROFILE T  =  9  7  m/  X*  211-^^ +  This line has only one Observation on it ond Zero Intercept has been assumed  m  30  /^-PROFILE  ""' s T PROFILE  =4^  N  t- ir + 6 60  / ^/  6 T ^ /  SH -  13  \ \  10  \ 0  i  SHOT A (NOV)  C)  RIPPLE ROCK ELK FALLS  1  ALBERNI  1  100  1  N !  1  I  HORSESHOE  1  VICTORIA BELLINGHAM SHOT B(NOV) CONSTANCE BANK  200  1  1  SEATTLE  1  300  A (KILOMETRES) PROFILES  FIGURE  ZHt, XTJ.Xg.  34  1  400 Y7T.XZII  \  \  \ l  LONGMIRE  1  1  500  FIGURE  45  - 56 3oil  Methods of Interpretation The existence of a refraction arrival has been treated theoretically by  a number of authors. Ewing, Jardetzky and Press  (1957) have shown that for  an explosive source and a receiver on the lower velocity side of a discontinuity between two semi-infinite media in contact there i s an arrival at a time, t = —  + (z + h) cos  V 2  V  6c  l  The parameters of this equation are illustrated in Figure 35*  Source Receiver  Figure 35.  Refraction from an Interface between Two Semi-Infinite Media where v < v . 1  2  where v^ and  are the compressions! wave velocities in the two media and l v^ <^ V g , and where 6c i s the c r i t i c a l angle such that sin 0c • — . This i s v  2 the expected time of arrival for a wave propagated over the path shown by the broken line. This result has been obtained for the case of two liquids in contact. The theory of refracted arrivals has also been developed by these authors for the case of two solid media in contact. The interpretation of the refraction data obtained in the present investigation i s based on the following geometrical considerations, where the seismic energy radiating from the explosive source i s considered in terms of ray paths.  - 57 The diagram of Figure 36 represents the case of refraction from a layer of velocity v^, underlying a layer of velocity v^, where v^<  v^.  The layers  are separated by a plane interface, dipping from the horizontal at an angle 4 • A and B are shot point and recording station, and are interchangeable. and  are taken as the thicknesses of the layer below points A and B.  H 0c  is the c r i t i c a l angle defined by the relation, • -1 v, 0c = sin _1 o  V  Figure 36.  2  Refracted Rays in a Dipping Structure with Plane Interface.  The following relation gives the travel time from A to B or B to A ,  t =  (H + H^) cos 0c  A cos'  fi  V  l  V  For the case of horizontal layering where t =  2 H  c o s  6 c v  l  + ±  V  <f>  (3-U-l)  2  = 0 the relation reduces to  2  (3-11-2)  Conventional procedure is to plot the observed data and obtain by a least squares analysis an equation relating t and A in the same form as 3-11-2:  58 for the direct and refracted arrivals, v^ being the inverse of the slope of the line obtained for refracted arrivals. Thus from the value of T  q  obtained from the observational data, together  with the velocities v^ and V g , the depth of the layer i s determined. The following relation may also be shown to hold  22  H where A ^ i  s  2  V  2  V  2  + V  V  l  (3-11-3)  l  the distance at which the direct and refracted arrivals arrive  at the same time. For the general case of a structure with m - 1 layers i t may be shown that m-1  t  m  =  2  r H -. n n=l  cos 0c T—^ +  A n  (3-11-4)  v m  th where t i s the travel time of the ray refracted in the m layer H i s the m n thickness of the n*"* layer, 0 i s the c r i t i c a l angle, such that, ' ' nm n sin 0 — nm v m 1  th th v , v are the velocities in the n and m layers respectively, n m This follows directly i f the geometry of Figure 37 i s considered, and i t i s shown that 0 = 0c n nm v  V  l 2  V  m  n =  L/e  1  J7  n =  2  V  n  n  n = m  Figure 37. Geometry of Refracted Ray in Multi-Layered Structure.  -  59  -  The f i r s t arrival data for the multi-layered case may be expected to define m segments of the travel time curve.  This assumption w i l l hold only  for the cases where an appropriate relation exists between the thicknesses and velocities of the layersj e.g., in a case where m = 2, f i r s t arrivals of energy refracted in the second layer w i l l only be observed when the thickness of this layer exceeds a certain value. Leet (1938) has shown the criterion (after Maillet and Bazerque) for determining the minimum thickness for refracted f i r s t arrivals from an intermediate layer.  A quantity Y i s  formed, cos  ^ where sin 9^  =  sin 0 ^  sin  - cos  cos  +  sin(G^2 -  6^3)  (1 - sin 0^)  sin $23°  Then the line for V 2 passes above the point b-^> ^3  h  2  h  l  ^  ^  The geometrical relationships have been derived for the case of dipping interfaces (Leet - pages 138, 139), and these w i l l be cited in interpreting the observed data. 3.12  Interpretation and Discussion of the Refraction Data The Strait of Georgia i s underlain by sediments of Cretaceous age through-  out most of the area where explosions have been located. These sediments consist mainly of sandstones, shales and loosely cemented conglomerate.  The  formation outcrops on Vancouver Island and on the adjacent islands in the Strait of Georgia.  Small patches of the outcrop are also found on Lasqueti  and Texada Islands. Hornby Island was the only recording station located on the G retaceous formation.  The other stations in the Vancouver Island area were located on  outcrops of granitic or volcanic rock, with the exception of Kelsey Bay (Nov.)  - 60  -  This station was located on the alluvium at the mouth of the Salmon River. The shot points A (Nov.), B (Nov.), 1 to 5 (Nov.), Ripple Rock and Constance Bank were also located on volcanic or granitic type formations. Short range refraction surveys indicate the average velocity of compressions! waves for the Cretaceous formation to be 4.05 km/sec.  Velocities  of 5.4 to 6.0 km/sec. have been found for the more competent volcanic and granitic formations (Milne and White, I 9 6 0 ) . Figure 38 shows the form of model which represents the results of the refraction data, along a line from Kelsey Bay to Victoria.  Kelqey B.  Ripple Rock Campbell R. Hornby Cretaceous Sediments  Shot 31 v - 4 km/sec.  Victoria  Granitic and Volcanic Strata  v = 5.4 - 6.0 km/sec.  Intermediate  v = 6.7 - 7.0 km/sec.  Mantle  v = 7.7 km/sec.  Figure 38. Form of Model Representing Structure from Kelsey Bay to Victoria. Reference to Table 8 (page 39) indicates that most of the refraction arrivals are from the intermediate layer with apparent velocities varying from 6.6 to 7.0 km/sec, except for two extreme values of 6.5 and 7.2 km/sec.  - 61 -  for  p r o f i l e s V I I and X.  t u r e o f t h e Cretaceous P  n  is a first  The  d a t a f o r the s h o r t e r ranges which g i v e t h e s t r u c -  and v o l c a n i c l a y e r s a r e r a t h e r s p a r s e .  a r r i v a l have been o b t a i n e d by e x t e n d i n g t h e l i n e o f o b s e r v a t i o n s  i n t o t h e c o a s t a l a r e a o f t h e S t a t e o f Washington and N o r t h e r n  3.12.1  Data f o r which  Results f o r the S t r a i t of Georgia  California.  Area:  P r o f i l e VI i s an u n r e v e r s e d p r o f i l e f o r which t h e r e c o r d i n g s t a t i o n (Hornby I s l a n d ) and t h e s i x shot p o i n t s a r e l o c a t e d on t h e C r e t a c e o u s ation.  The t h i c k n e s s o f t h e C r e t a c e o u s  s t r u c t u r e , found from t h e d a t a o f  F i g u r e 25 by t h e r e l a t i o n e x p r e s s e d i n e q u a t i o n (3-11-3), i s 2.33  ±  The u n c e r t a i n t y has been found by p l a c i n g r e a s o n a b l e l i m i t s on t h e  .43  t r a v e l time measurements f o r s h o t s 15 and 19 and u s i n g t h e p r o b a b l e  of  t h e low v e l o c i t y segment o f t h e t r a v e l time c u r v e d e r i v e d from  first of  km.  accuracy  of  squares.  form-  errors  least  On t h e b a s i s o f t h e few d a t a o b t a i n e d , t h e r e i s no evidence f o r  a r r i v a l energy  e v i d e n c e may  from a l a y e r with a v e l o c i t y n e a r 6 km/sec.  The l a c k  be due t o t h e absence of events a t t h e a p p r o p r i a t e range o f  d i s t a n c e s , o r i t may  be due t o t h e f a c t t h a t t h e second  s u f f i c i e n t l y t h i c k to g i v e r i s e t o f i r s t  arrivals.  l a y e r i s not  I f we  assume t h e f o l l o w i n g  numerical values, v ^ = 3.81  km/sec.  = 6.00  km/sec.  v^ *" 6.70  km/sec.  and r e f e r t o L e e t ' s curve t o o b t a i n a v a l u e f o r Y, we o b t a i n a minimum r a t i o for  hg:h^ If  o f about  0.3.  a segment r e p r e s e n t i n g t h e 6 km/sec. l a y e r i s proposed  give the greatest thickness of the l a y e r , with f i r s t =  7.16  km.  and ending a t  = 13.09 H  x  H_  km.  a r r i v a l s s t a r t i n g at  we o b t a i n t h e  - 1.69  km.  = 1.06  km.  which would  results  - 62 -  The t o t a l t h i c k n e s s o f t h e two l a y e r s i s 2.75 km. as compared w i t h km. o b t a i n e d by assuming a s i n g l e l a y e r c a s e .  I t appears  2.33  t h a t f u r t h e r observ-  a t i o n s i n t h e range'below 13.09 km. c o u l d r e v e a l t h e presence o f a 6 km/sec. layer.  The maximum t h i c k n e s s o f t h i s l a y e r i s 1.06 km.  The a l t e r n a t i v e  s t r u c t u r e s f o r t h e a r e a o f p r o f i l e VI a r e shown i n F i g u r e 39.  Thickness (km.)  2.33  V'el. (km/sec.)  Cretaceous  Intermediate  F i g u r e 39.  Thickness (km.)  Vel. (km/sec.)  1.69  Cretaceous  1.06  Granitic or V o l c a n i c  3.81  3.81  6.67  Intermediate  6.67  A l t e r n a t i v e S t r u c t u r e o f Upper C r u s t a l L a y e r s f o r P r o f i l e V I .  The o n l y d i r e c t e v i d e n c e f o r a 6 km/sec„ basement l a y e r o v e r l y i n g t h e 6.8 km/sec. m a t e r i a l i s shown by p r o f i l e I . below t h e v e l o c i t y c u r v e 1.10 + ^ o ^ ° 0  t h e p l o t f o r event 6 (Nov.) f a l l s  The c a l c u l a t e d t r a v e l time f o r t h e  d i r e c t r a y , propagated n e a r t h e s u r f a c e , f i r s t and t h e n i n t h e v o l c a n i c s t r a t a n e a r E l k F a l l s ,  i n the Cretaceous i s 3.10 s e c .  The p r o p a g a t i o n  p a t h i n c l u d e s 10.41 km. o f C r e t a c e o u s and 2.00 km. o f v o l c a n i c s . t r a v e l time i s somewhat l e s s (2.32 s e c ) .  sediments  The o b s e r v e d  I t i s thus i n d i c a t e d t h a t the p l o t  f o r 6 (Nov.) i s on a v e l o c i t y l i n e c h a r a c t e r i s t i c o f t h e higher, v e l o c i t y v o l c a n i c rock.  U n f o r t u n a t e l y a shot planned f o r t h e i n t e r v a l between 6 (Nov.)  and 7 (Nov.) had t o be c a n c e l l e d . The d i s c o n t i n u i t y between t h e t i m e - d i s t a n c e segments f o r p r o f i l e  I  - 63  -  between events P and F i s indicative of a fault structure with the vertical displacement being up on the side represented by shot P.  The same effect may  be the result of a rapid increase in the thickness of the Cretaceous strata along the profile between these two events.  In either case the displacement  of about 0.3 sec. in the time-distance curve requires a change of path length of the seismic energy in the Cretaceous strata of about 1 km. The variation i n travel time for shots C and R i s in excess of the expected errors of measurement and probably indicates thicker deposits of Cretaceous or lower velocity materials under these events. The apparent velocities obtained for the two segments of this profile agree, within the uncertainties as shown in Table 8. The same series of shots, as far along profile I as shot P has been If  plottia for Alberni in Figure 21. The data for Ripple Rock and some other explosions are also included. The apparent velocity obtained i s 6.85 km/sec. Profile II forms an approximate reversal to profile I. The calculation of the angle of dip of interfaces which would give rise to the apparent velocities of profiles I and II i s somewhat complicated by the fact that two layers may be overlying the intermediate velocity layer. The geometry and related formulas for a three-layer case, where the two interfaces are dipping in opposite directions, are given by Leet (1938). The ambiguity in the interpretation of the data presented here i s that no apparent velocity has been obtained for a layer between the low velocity .Cretaceous sediments and the layer having velocity near 7 km/sec. If a constant thickness of Cretaceous sediments underlying the area of the explosions used for profile II and the shorter range segment of profile I i s assumed, an angle of dip may be computed for the interface between a 6 km/sec. layer and the 6.9 km/sec. layer.  - 64 We have that sin (6,2 where v, Vg v  +  *2)  and sin (8^  =  - ^)  -  ^-  ° 6 km/sec. =6.85 km/sec.  2+ " ^*^  f  k m  /  s e c  *  where 0^ * sin" _1 and <f>^ = angle of dip. V  2  The angle of dip obtained i s 42 minutes toward the Elk Falls end of the spread of shots.  The real velocity v  2  i s found to be 6.90 km/sec.  If profile VIII for the data obtained at Victoria i s used as a reversal of profile I, the dip is 1 degree 19 minutes and a real velocity of 6.86 km/sec. i s obtained.  These results are subject to the uncertainties associated  with the apparent velocities obtained for the profiles. 3.12.2 Results for the Kelsey Bay and Johnstone Strait Areas: The data for the area near Kelsey Bay and east in Johnstone Strait are presented i n Figures 23 and 24 for profile IV and V.  From a consideration  of profile IV, a calculated thickness for the alluvium and volcanic strata may be obtained.  The equations for the three segments are; t1  A  1.58 A  t„ - 0.10 + 2 5.41 *3  " °'  6 0  +  6T70  The average thicknesses obtained are 0.08 km. (280 ft.) of alluvium and 1.5 km. of volcanic strata.  The thickness of the alluvium may be greater  under the recording station since rather high velocity t i d a l currents occur in the area of the shot points. That this i s the case i s indicated in that  - 65 for the data of profile V, no break i s observed in the arrival times within a distance of 0.6 km.  The data for this profile were obtained for a short  range refraction measurement on the alluvium at the site of the recording truck. If shot A is plotted on profile IV (A = 6.70 km.), sec.  i t i s late by  0.23  This indicates a variation in the thickness of river deposited s i l t of  about 0.4 km. (1320 ft.) i f a velocity of 1.58 km/sec. is used.  An error in  position for shot A could also account for the late arrival at the Kelsey Bay recorder.  In any case i t is to be noted that the origin time and location  for shot A are well controlled for use on the longer range profiles. An examination of the time-distance plots for profiles II and III reveals a change in structure between the Ripple Rock - Campbell River area and the Kelsey Bay - Johnstone Strait area.  The arrivals of shots A (Nov.)  and 1 (Nov.) to 5 (Nov.), shown in the upper right hand corner of Figure 21, are late by about 1 sec. relative to the velocity line fitting the other data at shorter ranges.  This requires a somewhat greater depth to the upper bound-  ary of the intermediate layer than that obtained from the results of profile IV.  The existence of a layer of 5.6 km/sec. strata about 10 km. in thickness  in the Kelsey Bay area would explain the delay of the arrivals for profile I I . Direct arrivals (propagated in the 5.6 km/sec. strata) at Elk Falls from the ---same group of shots, however, would be later for such a structure than the observations shown in Figure 22 would indicate. An increase in the velocity of the upper layer to about 6 km/sec. and aih increase in the thickness of the layer at Kelsey Bay could explain the results from both profiles II and III. The proposed structure i s shown in Figure 40 (page 66). A low velocity plug in the Kelsey Bay - Alberni path of propagation but not in the Kelsey Bay - Elk Falls path  would also introduce  - 66 the observed  effect.  The  e a r l y a r r i v a l s on p r o f i l e I I I would then be due  t h e absence o f t h e C r e t a c e o u s  Elk  f o r m a t i o n i n t h e K e l s e y Bay  Falls  6.2  Bay  v = 6 km/sec. 1 0  Figure 4 0 .  area.  Kelsey  km.  to  + (?) km.  Proposed S t r u c t u r e f o r E l k F a l l s - K e l s e y Bay  P r o f i l e X f o r s h o t s r e c o r d e d a t K e l s e y Bay  during the  Area.  I 9 6 0  program do  not c o n t r i b u t e t o a d e f i n i t i o n o f t h e s t r u c t u r e " i n t h e K e l s e y Bay  area since  t h e t i m e - i n t e r c e p t i s not known w i t h any degree o f p r e c i s i o n and most o f t h e events; r e c o r d e d a r e south o f Campbell R i v e r . o b t a i n e d may  be a t t r i b u t e d t o a s m a l l d r i f t  program proceeded. program was t h e observed  3 . 1 2 . 3  The h i g h apparent  velocity  r a t e f o r t h e chronometer as t h e  A d r i f t o f 0 . 2 s e c . i n t h e f o u r hours d u r i n g which t h e  i n p r o g r e s s would r a i s e t h e apparent  v e l o c i t y from 7 . 0 km/sec. t o  value ( 7 . 2 km/sec).  E s t i m a t e d t h i c k n e s s o f g r a n i t i c and v o l c a n i c s t r a t a :  The l e a s t squares  e q u a t i o n s o b t a i n e d f o r p r o f i l e s V I I , V I I I , IX, X  X I t e n d t o s t r e n g t h e n t h e evidence  and  f o r the intermediate l a y e r with a r e l a t i v e l y  t h i n l a y e r o f s u r f i c i a l s t r a t a above i t .  I f t h e second s t r u c t u r e f o r p r o f i l e  VI i s assumed and used as a s t a r t i n g p o i n t , t h e t h i c k n e s s o f t h e s t r a t a above t h e i n t e r m e d i a t e l a y e r may  be computed f o r t h e v a r i o u s o t h e r p r o f i l e s .  has been done f o r p r o f i l e s I , I I , V I I I and X I .  The  v e l o c i t y i n the  l a y e r a t A l b e r n i and E l k F a l l s - Campbell R i v e r has been t a k e n as 5.6  This  surficial km/sec,  - 67 an average v e l o c i t y f o r v o l c a n i c s t r a t a as o b t a i n e d by M i l n e and White The v e l o c i t y a t V i c t o r i a and Horseshoe Bay has been t a k e n as 6.0 where t h e s u r f a c e bedrock i s g r a n i t i c i n c h a r a c t e r . t i o n s , t h e a n g l e o f d i p has been n e g l e c t e d . s t a t i o n i s shown i n F i g u r e Thickness (km.)  Velocity  [ta&feec.) 13.1  14.8  The  (I960).  km/sec.  I n making t h e s e  calcula-  r e s u l t i n g s t r u c t u r e a t each  41.  Thick.  Vel.  Thick.  6.2 Granitic  rranitic Strata  6.0  Vel.  Thickness  2.3  Velocity  5.6  Volcanic  Volcanic5.6  6.9  S.9  6.8  6.8 Horseshoe  F i g u r e 41.  Bay  Campbell R. Elk Falls  Victoria - Pat Bay  Estimated Thicknesses o f Volcanic or G r a n i t i c  Alberni  Layer.  I t must be p o i n t e d out t h a t t h e t h i c k n e s s e s o f t h e s e s u r f i c i a l  strata  depend on t h e v e l o c i t i e s chosen f o r them and a l s o on t h e s t r u c t u r a l column o b t a i n e d f o r p r o f i l e VI f o r t h e shot p o i n t a r e a . s m a l l number o f o b s e r v a t i o n s .  The l a t t e r i s based on  An i n c r e a s e i n t h i c k n e s s o f t h e  a  surficial  l a y e r s i n t h e shot p o i n t a r e a o f t h e S t r a i t o f G e o r g i a would r e s u l t i n a decrease  i n t h e computed t h i c k n e s s e s o f t h e s t r a t a above t h e  layer f o r the structures i n Figure  intermediate  41.  I t has been assumed t h a t t h e v e l o c i t y a c r o s s t h e s p r e a d o f s h o t s i s a l s o t h e v e l o c i t y between t h e spread o f shots and t h e r e c o r d i n g s t a t i o n s . t h i s i s a good a p p r o x i m a t i o n l o n g e r range p r o f i l e s .  That  i s shown by t h e v e l o c i t i e s o b t a i n e d f o r t h e  - 68  3.12.4  -  Results of the longer range data:  These data are plotted in Figures 32, 33 and 34.  The interpretation i s  based mainly on f i r s t arrival P phases. Two well-defined secondary phases (designated by solid squares on the time-distance plots) are used.  The events  designated by open circles are not included in the least squares analysis. The least squares equation for the f i r s t segment of the data for profile XIII  along the coast south-east from the Ripple Rock explosion indicates a  velocity of 6.60 km/sec. with a small intercept. This i s in agreement with the velocities obtained for the shorter range profiles. line represents the arrivals to a distance of 340 km.  This lower velocity The higher velocity  segment of this profile is based on the data from three recording stations: Seattle, Longmire and Mineral.  The velocity of 7.75 km/sec. obtained i s con-  siderably lower than the value 8 km/sec. commonly found for the upper mantle. No records of the explosion are available in the distance interval between Longmire and Mineral.  The f i r s t detectable arrival of energy for the Mineral  record does not indicate refraction from a higher velocity layer. The arrival read i s well defined and time control is good. an earlier weak arrival i s not detected.  It i s of course possible that  The seismogram for this reading i s  shown in Figure 17. The structure determined from this unreversed profile is shown in Figure 42 (page 6 9 ) . The thin layer inferred by the small intercept has been omitted. The five profiles of Figure 34 display a l l of the longer range data along the coast for explosions A (Nov.), B (Nov.), Ripple Rock and Constance Bank. Four smaller explosions (28 to 31) as recorded at Longmire are also included. in 3.10.  Two reversed profiles are obtained by treating the data as explained Profiles XIV and XV represent a reversal for the area between  - 69 -  shot A (Nov..) and Longmire.  These may  be compared w i t h p r o f i l e X I I I which  i s f o r e s s e n t i a l l y t h e same a r e a , except t h a t i t i s extended t o M i n e r a l  and  the north-west end o f t h e p r o f i l e i s d i s p l a c e d from t h e l o c a t i o n o f shot  A,  a d i s t a n c e o f about 50 km.  t o t h e R i p p l e Rock l o c a t i o n .  XVII form r e v e r s a l s f o r both X I I I and XIV a r r i v a l s shown f o r p r o f i l e s XVI  P r o f i l e s XVI  and  from V i c t o r i a and shot B.  The  and XVII do not appear t o i n d i c a t e a t r a n s -  i t i o n t o a h i g h e r v e l o c i t y segment i n t h e d i s t a n c e r e p r e s e n t e d (280  km.)  T h i s r e s u l t i s not i n c o n s i s t e n t w i t h t h e t h r e e o t h e r l o n g e r range p r o f i l e s f o r which t h e c r o s s - o v e r d i s t a n c e i s a t a p p r o x i m a t e l y  R i p p l e Rock  F i g u r e 42.  The  350  Mineral  _—:  v = 6.60  km/sec.  v = 7.75  km/sec.  48.1  km.  C r u s t a l S t r u c t u r e Determined from D a t a f o r P r o f i l e X I I I .  d a t a r e p r e s e n t e d by t h e open c i r c l e p l o t s f o r events 28 t o 31  not used i n t h e l e a s t squares a r r i v a l s r e a d a t Longmire. low v e l o c i t y segment may  s o l u t i o n f o r p r o f i l e XV.  The  events  were  are f o r  The l a t e n e s s of t h e i r a r r i v a l w i t h r e s p e c t t o the  be t h e r e s u l t o f t h e low v e l o c i t y sediments under-  l y i n g the area o f these explosions.  The  apparent  v e l o c i t y r e p r e s e n t e d i s an  i n d i c a t i o n t h a t the a r r i v a l s r e a d are p o s s i b l y secondary a t e d w i t h t h e h i g h e r v e l o c i t y segment. squares would lower t h e v e l o c i t y (7.68 The  km.  ones, and are  Including these a r r i v a l s i n the  associleast  km/sec.) s l i g h t l y .  a r r i v a l time f o r t h e underground n u c l e a r e x p l o s i o n B l a n c a , a t t h e  - 70 Nevada Test Site, as recorded at Victoria (A = 1390 km.) XIII.  i s shown on profile  It i s late by approximately 3 seconds relative to the velocity line  t = 7.93  y ^«  +  It i s possible tihat the f i r s t arrival of seismic energy  has not been detected.  In this regard Romney (1959) has observed that P  decreased to very small amplitudes at distances beyond about 1000 km. and i s not often detected as the f i r s t arrival, a later phase being picked. examination of Romney's published seismograms indicates that the P  n  An arrival  is missing at a distance of about 1200 km. and i s again present at about 1400 km. ; The: distance from Ripple Rock to Mineral i s 1127 km. and from Blanca to Victoria i s 1390  km.  The crustal model computed for the reversed profiles XIV and XV is shown in Figure 43.  Shot A (Nov.) T3.2  Longmire  km. 5.6 km/sec.  v = 6.68 km/sec. 51.1  39.9  km.  km.  v = 7.83 km/sec.  Figure 43.  Crustal Structure Determined from the Reversed Profiles XIV and XV.  - 71 It i s to be observed that the high velocity segment of profile XIV i s based on only two observations, one of which i s a secondary arrival.  The  velocity pbtained i s considerably higher than that for profile XIII.  Also,  the low velocity segment of Profile XV has been obtained from one observation and the assumption of zero time intercept. The form of the 5.6 km/sec, layer i s subject to the validity of this assumption.  The average thickness  of the 6.7 km/sec. layer i s consistent with the results obtained from the other profiles of Figures 42 and 44. The dipping lower boundary of the 6.7 km/sec. layer i s a result of the high apparent velocity (7.97 km/sec.) observed for profile XIV. The crustal model computed from the results for the composite plot of long range data (Figure 45) i s shown in Figure 44•  3.26 km.  49.1 km.  v = 5.6 km/sec.  v = 6.66 km/sec,  v = 7.76 km/sec.  Figure 44.  Crustal Structure Determined from Composite Plot of Longer Range Data.  A velocity of 5.6 km/sec. has,been chosen to represent the average velocity of the thin surface layer. This i s based on measurements made on basement type rocks in the Vancouver Island area.  - 72 3.12  Ripple Rock - east data: The data for Ripple Rock recorded at stations east through the mountains,  as plotted in,Figure 32, indicate an apparent velocity of 7.66  km/sec.  observations have been obtained for distances smaller than about 300  No  km.  There i s thus no f i r s t arrival data for arrivals in the upper crustal layers. A depth of 30.6  km. to the upper interface of the layer having velocity  7.66  km/sec. is obtained i f the velocity for the short range data along the coast is used.  No arrivals may be interpreted as refractions from an 8.2 km/sec.  layer, i f the 7.66  km/sec. velocity i s accepted as a real velocity.  The  absence of refracted arrivals from an 8.2 km/sec. layer may be due to the fact that Banff is hot at a great enough distance from the explosion to make the refraction a f i r s t arrival. the 7.66  This infers a thickness exceeding 55 km. for  km/sec. layer. A second possibility i s that sufficient energy i s  not refracted through the 7.66  km/sec. layer to record as a refraction from  an 8 km/sec. layer. The third and most preferred possibility i s that the 7.66 km/sec. layer i s the mantle.  3.12.61: Lg waves: A later arriving phase (designated by the solid triangles on the time-: distance plots') has been read in cases where i t is well defined. arrivals have been plotted for profiles II, VIII, XI and XII. tance time intercept i s small for each profile.  These  The zero dis-  Except for profile XI, the  velocity i s about 3.6 km/sec.,, even for the longer ranges represented in profile XII.  The arrivals have been identified as the Lg phase which i s  propagated over paths of continental crustal structure. The velocity is higher than 3.5 km/sec. reported by other investigators for this wave. (Ewing, Jardetzky and Press, 1957,  page 219)  - 73 4.  4.1  THE GRAVITY SURVEY  General Description of Program The gravity survey was carried out i n two parts. A network of base  stations was f i r s t established at the locations shown in Figure 46. This part of the survey was carried out during the month of December I960 by Mr. A. K. Goodacre of the Gravity Division of the Dominion Observatory.  The  author accompanied Mr. Goodacre for the greater part of this operation and readings were taken on two Worden gr^vimeters.  This procedure afforded an  opportunity to check the performance of the gravimeters and to arrive at a revised calibration constant for the Worden No. 35 instrument used on the subsequent part of the survey.  The base station readings wefce referred to  absolute datum by including the pendulum station at the University of British Columbia in the network. During the month of January 1961, detailed readings at intervals of about 10 km. were obtained.  These readings were taken on Vancouver Island  along the highway between Victoria and Kelsey Bay.  Some observations were  also obtained along two east-west lines from Victoria to River Jordan and from Duncan to the west end of Cowichan Lake.  In May 1961 the observations  were extended from Port Alberni to Ucluelet and Tofino. These gravity stations are shown in Figure 46.  4.2  Instrumentation The observations used in establishing the base network of stations were  obtained with a Worden No. 546 gravimeter with a temperature control unit incorporated. division.  This instrument had a calibration constant of .39881 mgal/  It had recently been calibrated by taking readings for a  BOUGUER  ANOMALIES FOR GRAVITY STATIONS IN VANCOUVER ISLAND AREA  FIGURE  46  - 74 network of stations in Eastern Canada for which gravity values had been established.  During a part of the program for the establishing of a base network,  readings were also taken on the Worden No. 35.  It thus seemed advisable to  calibrate the Worden No. 35 against the Worden No. 546.  A calibration constant  pf 0.416 mgal/div. was obtained for the Worden No. 35« Elevations for almost a l l of the base stations w0re obtained from geodetic •bench marks.  A pair of altimeters was used to obtain elevations for most of  the detail stations.  4.3  Procedures Used in the Survey A 'base looping' procedure was followed in obtaining readings for the  base network.  In extending the observation of the gravity value from station  A to station B, readings were taken in the order A B A B.  It was then poss-  ible to construct two drift curves for the instrument and to obtain an estimate of the uncertainty of the readings. In almost a l l cases the uncertainty was not greater than 0.1 scale divisions.  If a deviation from the mean of more  than ±0.2 occurred, the loop was repeated. The distance between base stations varied from 40 to 70 km. and i t was thus possible to keep the delay between readings to within about an hour. operation was three to four hours. 35 i s shown in Figure 47 (page 75).  The total time for carrying out one looping A typical drift curve for the Worden No. Loops were done by aircraft from the  Vancouver International Airport to the Victoria International Airport, from Vancouver to Powell River, from Campbell River to Vancouver and from Vancouver to Comox. Also loops were made from the airports to stations which were part of the land based survey.  It was then possible to form circuits from the  data for most of the base station network.  Closure errors are shown in Figure  48 (page 75) together with the reading differences between the stations of  - 75 the base network.  667.0  546.0  545.0 (Scale div.)  Comox 1700  1800  1900  2000  2100  Pacific Standard Time (Dec.10, I960) Figure 47.  Typical Drift Curve for the Worden No. 35.  Figure 48.  Closure Errors in Base Looping Circuits.  2200  - 76 -  Detail readings were made with the Worden No. 35 gravimeter at intervals of about 10 km. between base stations.  The base stations were included in  the observations and drift curves were drawn up for the instrument by converting back from the gravity differences at base stations to instrumental reading differences using the calibration constant k ° 0.416 mgal/div.  There was no  check on irregularities in the instrumental drift between the base stations. In the case of the observations made from Victoria to River Jordan, Duncan to Gowichan Lake, and Port Alberni to Ucluelet and Tofino, readings were taken at the base station at the beginning and end of the observations. were drawn from these readings.  Drift curves  For the Port Alberni to Ucluelet and Tofino  profile, observations were taken at six intermediate points both on the outward traverse and the return traverse. An average drift curve was drawn from these observations.  4.4  Elevations and Positions Elevations for most of the base network stations were obtained from  bench marks placed by the Geodetic Survey of Canada. For stations located at a i r terminals, elevations were obtained from the Department of Transport. The elevation for the pendulum station at the University of British Columbia walg obtained by correcting the elevation given by the University Department of Buildings and Grounds to geodetic datum, a correction of -93 feet. Elevations for almost a l l of the detail stations were obtained by the use of a pair of altimeters.  Drift curves were drawn for the altimeters by  taking readings at a l l bench marks. Temperature and humidity observations were made at each station and corrections applied to altimeter readings. Coordinates were obtained from 1:50,000 scale maps compiled by the Surveys and Mapping Branch of the Department of Mines and Technical Surveys.  - 77  4.5  -  Reductions The gravity values obtained at each station were used to obtain the  values of the Bouguer anomaly. This was done by applying the Bouguer correction to the observed gravity values and comparing the reduced value with the gravity value given by the International Gravity Formula:  y=  Y f i  ( l + B sin  2  i> + e s i n  2  20)  (4-5-1)  where *y_ = 978.0490, the gravity value at the equator, i n gal. . B - 0.0052884 e =  .0000059  ^ i s the latitude. The Bouguer reduction consists of a correction to the observed value for the effect of elevation above sea level of the observing station and the effect of the attraction of the mass of material between the observing point and sea level on the vertical component of gravity.  If the mass i s considered  to be in the form of an infinite plate, i t s effect may be shown to be z = 2rrkoh  (4-5-2)  where z = vertical component of the attraction k = gravitational constant P = density of material h = thickness of plate (i.e., elevation above sea level) The second part of the correction i s that due to the effect of height of the observing point above sea level, the free a i r correction.  It i s given  by the following relation g where  f  = 2 ^ h o  ( l - n | | - + ....) o  = average gravity value in gal R  = average radius of curvature of the earth i n cm.  (4-5-3)  - 78 -  Making the substitution R •gT where M = mass of the earth  k  in (4-5-1) i t follows that the Bouguer correction has the form »_ = 2 - h (1 - - — )  %  I where p  m  ^Pm  • mean density of the earth and the second term of the free air  correction has been neglected. P = 2.67  (4-5-4)  3  gm/cm. and p  m  For the reduction made on this data the values  = 5.576  3  gm/cm. were used.  The correction then takes  the form g_ = 0.1978 h  where h i s in m.  D  No terrain correction has been made to the data.  In this connection i t may  be noted that no extreme variations i n topography occur near most of the stations, and i t i s assumed that the correction i s small. correction i n many cases i s only a few mgal. survey i s 310 m.  The total Bouguer  The greatest elevation of the  The data and reductions are presented i n Tables 23 and 24.  Table 23.  Locations and Gravity Data for Stations of the Base Network  Station  B.M. No.  *  Vancouver 776J Victoria 762J Duncan Nanaimo 772J Qualicum 809J Port Alberni Comox 83U Campbell River 843J3 Kelsey Bay Vancouver Airport Victoria Airport Powell R. Airport  Latitude  Longitude  49 48 48 49 49  16.0 123 25.2 123 46.7 .123 10,'p 123 21.0 124  49 49  14.5 40.5  15.0 22.1 42.4 56.1  26.8  124  48.3  124  56.2  125  15.0  126  57.5  123  10.0  48 38.8  123  25.3  49  124  30  50 02.0 50 49  22.8 10.7  49  Elev- Observed ation Gravity (m.)  Bouguer Corr.  Calculated Gravity  Bouguer Anom-  aly (mgal.)  87 10.1 15.0 16.3 52.4  980.9370 980.9756 980.9700 980.9974 980.9961  +17.4 2.0 2.8 3.2 10.3  981.0132 980.9375 980.9696 981.0044 981.0202  38.1 36.3  981.0037 981.0278  5.6 7.2  981.0110 981.0498  - 1.7 -14.8  2.7  981.0782  .6  981.0815  -  2.7  5  981.1030  .9  981.1124  -  8.5  3  980.9316  .6  981.0057  -73.5  16.5  980.9463  3.3  980.9578  -  119.5  981.0063  23.6  981.0623  -32.5  U.B.C. Physics Bldg. - Pendulum Station.  -58.8  +40.1 + 3.2 - 3.8 -13.8  8.2  - 79 -  Table 24.  Locations and Gravity Data for the Detail Stations (Cont.)  Station Name  Latitude  Campbell River) Kelsey Bay 1  Elev- Observed ation Gravity (m.)  Bxtguer Corr.  Calculated Gravity  Bouguer Anomaly  + 1.1 + 4.7 - 4.8 - 2.8 - 4.5 -13.2  Kelsey Bay Leg 50 50 50 50 50  2 3 4 5 6  Longitude  50  07.2 10.0 15.3 18.3 23.8 16.5  125 125 125 125 125 125  23.1 27.9 39.7 53.8 57.6 49.8  24 264 112 20 9 66  981.0852 981.0457 981.0739 981.0986 981.1074 981.0765  4.7 52.2 22.1 4.0 1.8 13.0  981.0888 981.0932 981.1008 981.1054 981.1137 981.1027  123 124 124 124 123  49.8 03.0 10.6 22 29.5  140 175 175 181 192  980.9517 980.95$! 980.9517 980.9536 980.9509  27.7 34.6 34.5 35.8 38.0  980.9696 980.9739 980.9732 980.9788 980.9819  125 125  31.9 56.3  0 0  980.9846 981.0037  0 0  980.9847 981.0028  (Duncan) Lake Cowichan Lake Cowichan 1 2 3 4 5  48 48 48 48 48  46.6 49.7 49.2 53 55.0  + 9.8  +11.8 +13.0 +10.6 + 7.0  (Port Alberni) Tofino Leg Ucluelet Tofino  48 49  56.8 09.1  - 0.1 + 0.9  Note - The station i n parenthesis i s the base station at the beginning of each leg. The one following i s the station at the end of the leg.  4.6  Density Measurements Representative rock samples were collected for various locations in the  area of the survey.  Density measurements were made on these samples. The  density measurements and locations are given i n Table 25 (page 80) and are also shown i n Figure 49.  4.7  Precision of Measurements A s t a t i s t i c a l treatment has not been applied to these data in order to  obtain precision indices.  However, some estimate of the accuracy of the  »  - 80 Table 25. Data for Collected Rock Samples Longitude  Latitude  Sample No.  e  Density (gm/cm.3)  1  50  2  05  125  19  2.94  49  19  124  16  2.64  3  49  07  123  55  2.62  4  48  52  123  42  2.66  5  48  39  123  34  2.74  6(  48  33  123  3.02  48  26.0 21.9 21.8 23.8 24.2  32,  123  29.7  2.81  123  44.1  2.77  123  48.2  2.76  123  52.6  2.97  123  59.2  2.72  1 8  48 48 48 48  A.It i s thought that sample 10 may not be representative. measurements may be made. Two standards of precision exist in the survey and these are discussed separately. (1) The base network; The d r i f t curves for the Worden No. 5 4 6 indicated an uncertainty of mgal in the measured difference in the value of gravity between any two stations. It may be assumed that the calibration constant k =  .39881  i s determined to within ±  .0001.  The great-  est reading difference for this instrument occurs between Vancouver (g or  980.9370)  416.24  and Kelsey Bay (g -  scale divisions.  981.1030),  a difference of 1 6 6 . 0 mgal  This leads to an uncertainty of ±  the extreme range of measurement.  .04  mgal for  The probable error in the observed  gravity for any station is thus ± 0 . 0 6 mgal. The closure errors i n the two large circuits are shown in Figure 4 8 to be 0 . 2 and 0 . 7 scale divisions or 0 . 0 7 and 0 . 2 8 mgal. The Bouguer reduction i s dependent on the value assumed for p . It would appear that the conventional value area.  P = 2 . 6 7 i s somewhat low for this  - 61 (2) The d e t a i l  stations:  The s t a n d a r d o f a c c u r a c y f o r t h e d e t a i l s t a t i o n s i s n o t w e l l The  c a l i b r a t i o n c o n s t a n t f o r t h e Worden No. 35  determined.  used i n t h i s p a r t o f t h e  survey  has been o b t a i n e d by comparing r e a d i n g d i f f e r e n c e s o b t a i n e d w i t h i t f o r some o f t h e base network s t a t i o n s and t a k i n g an average v a l u e f o r t h e c o n v e r s i o n factor. of ±  I t may  .002.  be assumed t h a t the v a l u e o f 0.416  f o r k has an u n c e r t a i n t y  T h i s would l e a d t o an u n c e r t a i n t y o f about ± 1 mgal o v e r the  range o f about 160 mgal u s e d i n t h i s  survey.  I t has been p o i n t e d out t h a t except i n a few c a s e s , t h e r e i s no  check  on i r r e g u l a r i t i e s i n t h e d r i f t between base s t a t i o n s , o r between base s t a t i o n r e a d i n g s t a k e n a t t h e b e g i n n i n g and end o f a l e g .  I n some i n s t a n c e s , r e a d -  i n g s were t a k e n a t s t a t i o n s on t h e r e t u r n t r i p f o r a p a r t i c u l a r l e g . examination  o f t h e d r i f t c u r v e s would i n d i c a t e t h a t e r r o r s a r i s i n g  An  from  i r r e g u l a r i t i e s i n d r i f t a r e p r o b a b l y l e s s than 2 s c a l e d i v i s i o n s , o r about 1 mgal.  for t h e d stall s t a t i o n s by the use of two  E l e v a t i o n s were o b t a i n e d altimeters.  The u n c e r t a i n t y  of t h e s e elevations i s p r o b a b l y not g r e a t e r  than i 2 m e t r e s .  T h i s would g i v e r i s e to an «xi: e r t a i n t y i n the Bouguer  c o r r e c t i o n of 0.4  mgal.  4.8  D i s c u s s i o n of D a t a  The base network s t a t i o n s and t h e d e t a i l s t a t i o n s a r e shown i n F i g u r e 46, with t h e Bouguer anomalies An attempt  f o r each  station.  t o draw i s o a n o m a l i e s has not been made s i n c e most of  the  o b s e r v a t i o n s were made along a l i n e p a r a l l e l i n g t h e l i n e o f t h e r e f r a c t i o n p r o f i l e s i n the S t r a i t of G e o r g i a . lines.  Some of t h e s t a t i o n s l i e a l o n g  east-west  However, t o draw i s o a n o m a l i e s would r e q u i r e g r e a t e r d e t a i l i n t h e  - 82  -  observations. The g e n e r a l f e a t u r e s o f t h e d a t a are as f o l l o w s : (1) The anomaly v a l u e s a l o n g t h e e a s t c o a s t l i n e o f Vancouver I s l a n d (except f o r the V i c t o r i a area) as f a r n o r t h as K e l s e y Bay  do not d i f f e r much from  zero.  l e g s from Duncan t o Lake  The  same may  Cowichan and  a l s o be s a i d of t h e east-west  from Qualicum west t o P o r t A l b e r n i , Uc£Luelet and T o f i n o .  (2) The n e g a t i v e anomaly a t Vancouver and t h e s m a l l e r n e g a t i v e anomaly a t P o w e l l R i v e r are t o be expected mountainous a r e a , i f i s o s t a t i c  due t o the e f f e c t o f compensation i n t h e equilibrium exists.  G a r l a n d and Tanner (1957)  have c a r r i e d out a g r a v i t y survey a c r o s s t h e southern Canadian and have found  an i s o s t a t i c anomaly o f - 4.2  Vancouver ( B r o c k t o n  mgal  Cordillera  f o r a measurement made a t  Point).  (3) T h e r e i s an a r e a o f p o s i t i v e Bouguer anomaly i n t h e V i c t o r i a a r e a which becomes even more p o s i t i v e t o t h e west along the s h o r e l i n e o f the S t r a i t Juan de F u c a toward R i v e r J o r d a n .  T h i s v a l u e o f from +40  mgal t o +70  i s i n c o n t r a s t w i t h an anomaly o f n e a r l y zero a t P o r t Angeles  of  mgal  on t h e o p p o s i t e  s i d e o f the S t r a i t , and w i t h t h e l a r g e n e g a t i v e anomaly i n t h e Puget Sound area. I n F i g u r e 50 a s e c t i o n o f t h e topography i s ishown f o r a l i n e taken approximately  49° 30'  l a t i t u d e and t r a v e r s i n g the a r e a f o r which g r a v i t y  v a l u e s are b e i n g d i s c u s s e d . found t o be 0.48 of Georgia,  km.  at  The  average e l e v a t i o n along t h i s s e c t i o n i s  S i n c e t h e f e a t u r e s o f low e l e v a t i o n , such as t h e  Strait  i n c l u d e d by t h i s s e c t i o n have dimensions s m a l l e r t h a n , o r of t h e  same o r d e r as, the minimum dimension  f o r l o c a l compensation as g i v e n  Heiskanen and Vening Meinesz (1958) (about 100 km.), compensation e x i s t s on a r e g i o n a l b a s i s ,  Woollard  we may  by  assume t h a t t h e  (1959) c i t e s the g e n e r a l  r u l e t h a t l o c a l compensation e x i s t s o n l y f o r f e a t u r e s which have h o r i z o n t a l  Kilometres  VANCOUVER  ISLAND  STRAIT of GEORGIA  COAST  RANGE  Kilometres  CROSS SECTION OF ELEVATIONS TAKEN NEAR ^> = 49°3o' FROM X=I22°35' TO X=I26°34'  FIGURE  50  Kilometres  - 83 dimensions exceeding three times the regional crustal thickness. The expected Bouguer anomaly i s - 5 5 mgal. the Vancouver station.  This i s close to the observed value obtained for  The difference between this value and the average  value obtained for the profile along the east side of Vancouver Island appears as an isostatic anomaly of about 50 mgal and requires an explanation. Sufficient gravity data are available to draw a gravity profile along a line from Vancouver across the Strait of Georgia, Vancouver Island and out for some distance into the Pacific Ocean.  The submarine observations are  taken from measurements made by Worzel and Ewing ( 1 9 5 2 ) .  The profile of  Bouguer anomalies i s shown in Figure 51 together with the elevations. The elevations taken for the continental part of the profile are along the same line as that shown in Figure 50.  This profile is displaced about 20" north  in latitude from the gravity profile.  This was done in order to obtain a  more representative figure for the average elevation of the land mass for the region, since a cross section taken through Vancouver would follow the Fraser River Lowlands.  The oceanic part of the profile is taken along a line west  from the Tofino gravity station at latitude 49° 1 0 ' .  The gravity reading  included to the east of Vancouver is for Agassiz and is taken from the results of a survey carried out by Garland and Tanner ( 1 9 5 7 ) ,  The data obtained from  this source as well as those from the submarine measurements are shown in Table 26 (page 84),  The Bouguer anomaly reductions do not include terrain  corrections for any of the data given. The deviation of the observed anomaly from the calculated curve for the Vancouver,Island area is about +50 mgal. For the oceanic observations the deviation is about - 2 0 mgal.  FIGURE 51  - 84 -  T a b l e 26.  Station No.  A d d i t i o n a l Data O b t a i n e d from O t h e r O b s e r v e r s  X  i  Observer  0  Observed Gravity (gal)  Elevation  20  (m.)  Reduced Gravity (gal)  Gravity from International Gravity Formula (gal)  Bouguer Anomaly (mgal)  Garland and Tanner  49  14.1  121  46  980.8977  7  Worzel and Ewing  49  12  127  33  980.963  -1906  981.092  981.007  85  8  Worzel and Ewing  50  05  130  26  981.086  -2282  981.241  981.086  155  120  Worzel and Ewing  49  43  132  21  981.034  -3216  981.252  981.033  197  157  109.7  The Bouguer r e d u c t i o n as g i v e n by Heiskanen and Vening Meinesz (1958) f o r submarine o b s e r v a t i o n s r e q u i r e s t h a t t h e f o l l o w i n g c o r r e c t i o n be made to t h e observed v a l u e o f g r a v i t y :  -.3086 (1 - r — ) d + .3086 (£ 4  where  P  fm  4  ^ ) t w  Pm  1  + 2 w cos 6 v  = d e n s i t y o f r o c k w i t h which t h e water i s r e p l a c e d and i s t a k e n as  2.67 gm/cm.-  3  3 i  = d e n s i t y o f water, 1.027 gm/cm. d  = depth o f o b s e r v a t i o n i n m.  pm  = mean d e n s i t y o f t h e e a r t h i n gm/cm.  V  = depth o f water i n m.  v  1  = east-west v e l o c i t y i n m/sec„  The f i n a l term i s t h e C o r i o l i s  f o r c e which r e s u l t s from t h e east-west  - 85 -  component of the submarine velocity and has not been considered. In computing the Bouguer anomaly values for the three submarine readings, the observed gravity values as received in a communication from Worzel and Ewing were corrected only for the second term of the above expression. The effect due to the depth at which the observations were taken was neglected. If a depth of 50 m. is taken as the maximum expected depth of the submarine, the error in correction is limited to about 10 mgal. The small variations in the gravity anomaly values observed along the profiles shown in Figure 46 may be easily accounted for by the variations in depth and density of the s u r f i c i a l geological structures. As may be noted from Table 25 and Figure 49 there is a considerable density contrast between the sedimentary rocks of the Cretaceous strata and the volcanic rocks and basic and granitic intrusives underlying the gravity stations at the north and south ends of the profile.  A variation of 20 mgal may be due to a layer  3 of thickness 2.4 km. i f a density contrast of 0.2 gm/cm. is assumed. The rather large positive anomaly at the south end of Vancouver Island requires further observations to define the boundaries.  It may be assumed  from the high horizontal gradient in the gravity values that the anomaly is the result of a high density body at rather small depth.  If the highest  value of the gradient indicated in the observations i s considered, i.e., 3.9 mgal/km. taken between station Sooke-2 and Duncan-2, an estimate of the maximum depth to the upper surface of the anomaly producing body may be made. A formula given by Bancroft  where  dQ  (I960) has been  used:  = maximum possible depth to the top of the anomaly producing body in km.  Ag. = total anomaly in mgal U  = maximum horizontal gradient in mgal/km.  - 86 Using the value 3.9 mgal/km. as the gradient and the total anomaly of 60 mgal, one obtains an estimate of 4.9 km. as the maximum possible depth to the top of the anomaly producing body.  5.  DISCUSSION AND COMPARISON OF REFRACTION AND GRAVITY RESULTS  The locations of the Alberni and Victoria stations relative to the spread of shots in the Strait of Georgia are convenient for testing the variation of velocity with depth in the 6.8 km/sec. layer. Observations from a layer with velocity tincreasing with depth would result in a non linear time-distance curve.  The equation representing the  time-distance relation for a layer in which the velocity increases linearly with depth for the case of zero time intercept has the form:  t = 1  where a = — v o  aA  + bA  (5-1)  3  2  and b = - ^  24v 3  o  g is the velocity gradient with depth and V is the surface velocity. For q  the case of constant velocity which has been assumed in the analysis of this study, the constant b reduces to zero and the relation to the linear form t = aA  (5-2)  From (5-1) we have that | | = a + 3bA  2  (5-3)  dt ^  is the reciprocal of the apparent velocity for any value of A. An increase  in apparent velocity i s indicated for an increase in A. A continuous increase in velocity with depth of another form yields a similar result.  The observ-  ational results for Alberni and Victoria as shown for profiles II and VIII  -  g  l  v  e  dtl dK\ A * 70 km,  87  -  , dt\  ~  l  dA \ A * 170  ^  km.  Reference to Table 8 indicates values for these slopes as follows: dt I dA | Alberni  1  D  6.85  ±  .08  and dtl HAJ Victoria  1  =  6.76  ±  .13  Since the observed velocities are about equal, an increase in velocity with depth i s not indicated.  The small variation in the apparent velocities  may be due to slightly different orientation of the profiles in an area with dipping structures. If the gravitational f i e l d associated with an infinite plate of density and thickness h i s considered, we have from Poisson's equation that, ^7 S* = 0  (5-4)  2  for a l l space external to the plate, where <f> i s the potential. Since 0 is presumably not a function of x and y  vre have  s  and 0  =  AZ  +  B.  The gravitational force is  |4  - - A  The gravitational force due to an infinite plate is thus a constant, in the space above the plate, and not a function of the z coordinate. The vertical attraction due to an infinite plate may be shown to be: z' - 2tTkph 8  (5-5)  (Heiskanen and Vening Meinesz, 1958) where k is the gravitational constant.  - 88  This of the  r e s u l t f a c i l i t a t e s the  expected d i f f e r e n c e s  -  comparison of v a r i o u s c r u s t a l models i n terms  i n r e g i o n a l g r a v i t y anomalies.  process to i n s e r t a l a y e r of p o s i t i v e or negative density,  I t i s a simple ±Aip, and  thickness  h i n t o a model at any  c r u s t a l depth and  to c a l c u l a t e i t s e f f e c t on t h e  v a l u e , as l o n g as the  assumption o f the  i n f i n i t e p l a t e i s c o n s i d e r e d t o be  good  a  one. P r e s s (I960) has  for  gravity  assumed a r e g i o n a l Bouguer g r a v i t y anomaly of -30  an average c o n t i n e n t a l  A f r i c a which i s c o n s i s t e n t Sachs (1959) and  the  crust.  He  with the  surface  adopts  the  explosion  wave d i s p e r s i o n  mgal  c r u s t a l model f o r South  d a t a as determined by H a l e s data (Figure  and  52).  Thickness (km.)  Density (gm/cnr )  Velocity (km/sec.)  21  2.78  6.03  15  3.00  6.71  3.37 F i g u r e 52.  Structure  Found by H a l e s and  Sachs (1959) f o r South A f r i c a .  I f t h i s model i s adopted as a s t a n d a r d f o r a comparison w i t h t h e o f t h i s study, a depth f o r the  6.7  km/sec. l a y e r may  measured r e g i o n a l Bouguer g r a v i t y anomaly. e f f e c t o f the has  surface  been n e g l e c t e d .  sub-crustal  material  be  v a r i a t i o n i n the  between t h e two  from t h e  I n making t h i s c a l c u l a t i o n  l a y e r s o f lower v e l o c i t y , f o r the A l s o the  calculated  models has  the  a r e a o f t h i s study,  v e l o c i t i e s obtained f o r not  structure  been taken i n t o  the  account.  - 89 3  The value of p = 3,00 gm/cm. for the 6.7 km/sec„ material has been obtained, as in the analysis by Press, from the empirical relation arrived at by Nafe and Drake (1958) between the velocity of compressional waves and density. The value of the depth obtained i s 46 km., a result which compares well with the seismic models obtained i n 3.12.4. It may be stated that the expected effect on the gravity values is very sensitive to the values of density chosen for the different layers of the structure.  For example, a change in the assumed value of p for the 21 km. 3  layer of 0.10 gm/cm. would lead to a difference in the expected gravity value of 100 mgal. } 6. COMPARISON WITH OTHER SURVEYS A brief comparison of the results of the present investigation with other investigations in adjacent areas i s presented. The plot of the data given by Tat el > . Adams and Tuve (1953) for the Puget Sound area has been examined. The plot represents the data obtained from about 60 observations from 17 explosions in Puget Sound. The refraction lines run i n directions radial from the explosion area.  The data of Figure 10 of  the above publication have been plotted separately for each of these lines, in order to compare the results with those of the present study.  The refrac-  tion lines to the south-east and east of the explosion area extend to distances of about 280 and 290 km. respectively.  The velocity line t = ^  obtained  for the composite plot of the data of the present study with a somewhat larger time intercept f i t s the data for these profiles reasonably well.  The refrac-  tion profile to the southwest of the explosion area indicates an apparent velocity of about 6 km/sec. The early arrivals recorded on a line to the west of the explosion area, have been interpreted by the authors as refractions  - 90  -  from the Mohorovicic discontinuity which they suggest is rising toward the surface as the continental margin is approached. The spread of recording stations (110 - 140 km.) apparent velocity.  i s rather short for the determination of a reliable  However, the plot of the points does appear to f a l l along  a line parallel to t = ^ ^ . A quotation from Steinhart and Meyer (1961) refers in part to the work of Tatel and Tuve in the Puget Sound area. crust gives 24 km. to the M.  "An interpretation with a uniform  Woollard finds a much greater depth indicated  to the west but there are not enough data to compute a structure. (National report for Canada 1959-60, Contributions  Hodgson  Dominion Observatory,  Ottawa) gives about 32 km. for this area." The following values are also given in the same reference: Velocity in the crust:  6.0 increasing to 7.0 km/sec. at base of crust  Mean crustal velocity:  6.5 km/sec.  Mantle velocity:  8.0 km/sec  Depth to mantle:  30 km.  The reversed profile observed by Shor on the continental shelf in Dixon Entrance at the north end of the Queen Charlotte Islands has indicated an intermediate velocity for this area.  The equations for the segments of the  velocity lines with a conversion of units are approximately,  * - 1 M + sAo t =, where A is in km.  2.17 •  These results are quite comparable with those of the short  range profiles of the present investigation. Shor has interpreted some secondary arrivals as possible refractions from the mantle and has calculated a depth of 26 km. for the crust in the area of  - 91 -  the profile,,  The present investigation requires a thicker intermediate layer  for the Vancouver Island area. The reversed profiles observed by A. R. Milne (I960) seaward from the edge of the continental shelf indicate velocity segments on the time-distance plots of 6.78 km/sec. and 6.76 km/sec, typical for the intermediate layer which forms the greater part of the oceanic crustal section. The thickness of the layer varies from about 2.5 km. at the west end of the profile to 4.0 km. at the east end.  The present investigation requires a thickening of the  intermediate layer at the continental margin. A previous investigation by Milne and White (I960) based on a smaller number of observations in the Vancouver Island area gave a somewhat lower velocity for the upper crustal strata.  It was pointed out that the Alberni  recording of the Ripple Rock explosion showed an earlier than expected arrival.  This observation is accounted for by the present interpretation.  The observations of Neumann (1957) include the use of a 7.0 km/sec. velocity in epicentral determinations of local earthquakes, the proposal of a very thick crustal section in the Seattle area, and the failure to observe velocities approaching 8 km/sec. from earthquakes along the coast as far as southern California.  These are in general agreement with the conclusions  from the present investigation.  7.  CONCLUSIONS  The crustal model which has been derived from a composite plot of a l l longer range refraction data, is shown in Figure 44 (page 71) and consists of the following structure (Table 27, page 9 2 ) . Structures determined by treating the data in the form of refraction profiles do not differ much from this model.  Considerable variations in the  - 92 -  t h i c k n e s s and c h a r a c t e r o f t h e g r a n i t i c and v o l c a n i c l a y e r a r e d e t e c t e d i n t h e r e s u l t s o f t h e s h o r t e r range r e f r a c t i o n  T a b l e 27.  0  Average C r u s t a l S t r u c t u r e f o r C o a s t a l A r e a  Layer  P-wave V e l o c i t y (km/sec.)  V o l c a n i c and granitic strata  The  data  5,6 - 6.0  Intermediate  6.66  Mantle  7.76  Thickness (km.)  3.26 45.8  s h o r t e r range p r o f i l e s o b t a i n e d from c l o s e l y spaced e x p l o s i o n s i n  the S t r a i t o f Georgia  a r e a extending  e a s t , show s t r o n g e v i d e n c e  from Campbell R i v e r about 100 km.  south-  f o r t h e i n t e r m e d i a t e l a y e r a t s h a l l o w depth.  a t i o n s o f 0.7 km/sec. have been found across this,spread of explosions.  i n t h e observed  Except  apparent  Vari-  velocities  f o r two p r o f i l e s , the v a r i a t i o n i s  l i m i t e d t o 0.3 km/sec.  The v a r i a t i o n s may be e x p l a i n e d i n p a r t by t h e e x i s t e n c e  of dipping i n t e r f a c e s .  T h i s e x p l o s i o n a r e a i s u n d e r l a i n by a l a y e r o f  Cretaceous  sediments having  There i s some evidence  a compressional  wave v e l o c i t y o f about 4 km/sec.  f o r major f a u l t i n g i n t h e a r e a .  An anomalous s i t u a t i o n e x i s t s n o r t h o f Campbell R i v e r which g i v e s t o e a r l i e r than expected  rise  a r r i v a l s from t h e e x p l o s i o n s A (Nov.) and 1-5 (Nov.)  a t E l k F a l l s and l a t e r a r r i v a l s a t A l b e r n i ,  A l t e r n a t i v e s t r u c t u r e s have been  proposed t o e x p l a i n t h e s e d a t a c o n s i s t i n g o f a c o n s i d e r a b l e t h i c k e n i n g o f the upper c r u s t a l l a y e r toward K e l s e y Bay o r t h e e x i s t e n c e o f a low v e l o c i t y p l u g i n t h e path o f p r o p a g a t i o n Strait  between A l b e r n i and the K e l s e y Bay - Johnstone  area.  Except  f o r t h e case o f one p r o f i l e , where t h e v e l o c i t y segment i s based  - 93 -  on only two observations, a velocity approaching 8 km/sec„ has not been observed.  The velocity obtained for observations along the coast from the  Ripple Rock explosion as far distant as 1127 km. is 7.7 km/sec. This velocity has also been found for a profile in the reverse direction, of events recorded at Longmire from a distance of 510 km. at Mineral (A = 1127 km.)  The possibility that the arrival read  i s a secondary arrival has been considered.  tunately an opportunity to follow the character of P  R  Unfor-  with increasing distance  is not afforded in the data because of the gap in the observations between Longmire and Mineral.  An apparent velocity of 7.66 km/sec. has been obtained  for observations along a line east from Ripple Rock for the distance range 300-700  km.  It is suggested that these velocities are associated with the  mantle. Since no observations have been made east from Ripple Rock in the distance range 0-300 km. the data are not complete enough to determine a structure. It is to be noted, however, that the time intercept of 4.8 seconds is somewhat smaller than for the P Y  n  curve for the coast data.  A test for a possible increase in velocity with depth in the intermediate (6.7 km/sec.). layer has shown that no appreciable increase exists. The results of the gravity observations, taken between Victoria and Kelsey Bay on Vancouver Island, show that except for the south end of the Island, the regional Bouguer gravity anomaly is about zero. positive anomaly of about 40-65 mgal  A pronounced  has been found in the Victoria area  and west along the south shore of Vancouver Island. The horizontal gradients observed indicate that the positive anomaly is due to dense material near the surface.  An east-west gravity profile starting on the British Columbia  mainland, traversing Vancouver Island and extending out into the Pacific Ocean indicates a positive isostatic anomaly of about 50 mgal in the Vancouver  - 94 -  Island area.  The isostatic anomaly at Vancouver and seaward from Vancouver  Island appears to be small. If the seismic model, requiring an intermediate layer at shallow depth is accepted, then in order to explain the observed regional gravity results, this layer must be assumed to be relatively thick.  The positive effect on  the gravity value resulting from the existence of the intermediate density material near the surface must be compensated for by extending the intermediate densities to greater depths.  Some of the higher density material of  the mantle must be replaced with intermediate density material. The density of the intermediate layer has been inferred from the compressional wave velocity.  A comparison of the structure of the Vancouver Island area with  that of a standard model gives a thickness of 46 km, for the intermediate layer based on the gravity observations.  i  -  8.  B a n c r o f t , A.M. I960. R e s e a r c h , 65, No.  95  -  BIBLIOGRAPHY  G r a v i t y anomalies o v e r a b u r i e d s t e p . 5, pp. 1630-1631.  J . Geophys.  Berg,Eduard, Bob H a m i l t o n , and A l a n Ryall„ 1962. T r a v e l t i m e s and c r u s t a l s t r u c t u r e from b l a s t n e a r S a l i n a s , C a l i f o r n i a . Program, meetings of the S e i s m o l o g i c a l S o c i e t y o f America, A p r i l 16-18, 1962. B e r g , Joseph W., J r . , Kenneth L. Cook, H a r r y D. Narans, J r . , and W i l l i a m M. Dolan. I960. S e i s m i c i n v e s t i g a t i o n s o f c r u s t a l s t r u c t u r e i n t h e e a s t e r n p a r t o f t h e B a s i n and Range P r o v i n c e . B u l l . Seism. Soc. Am. 50, No. 4, pp. 511-535. B i r c h , F. 1958. I n t e r p r e t a t i o n of the seismic s t r u c t u r e of the c r u s t i n t h e l i g h t of e x p e r i m e n t a l s t u d i e s o f wave v e l o c i t i e s i n r o c k s . Cont r i b u t i o n s i n G e o p h y s i c s , 1, Pergamon P r e s s , New York, pp. 158-170. Byerly, Perry. evidence.  1956. S u b c o n t i n e n t a l s t r u c t u r e i n the l i g h t o f s e i s m o l o g i c a l Advances i n G e o p h y s i c s , V o l . 3, Academic P r e s s I n c . , New York.  Ewing, M a u r i c e , and F r a n k P r e s s . 1956. Handbuch Der P h y s i k , V e r l a g , B e r l i n , G o t t i n g e n , H e i d e l b e r g , pp, 246, 248.  47,  Springer-  G a r l a n d , G.D., and J . G. Tanner. 1957. I n v e s t i g a t i o n s o f g r a v i t y and i s o s t a s y i n the s o u t h e r n Canadian C o r d i l l e r a . P u b l i c a t i o n s o f the Dominion O b s e r v a t o r y , 19, No. 5, pp. 169-208. Gutenberg, B. Science,  1950. S t r u c t u r e o f the 111, pp. 29-30.  earth's  c r u s t i n the  continents.  H a l e s , A. L., and I . S. Sachs. 1959. E v i d e n c e f o r an i n t e r m e d i a t e l a y e r from c r u s t a l s t r u c t u r e s t u d i e s i n E a s t e r n T r a n s v a a l . Geophysical J o u r n a l , 2, pp. 1 5 - 3 3 . H e a l y , J . H., S. W. Stewart, and W. H. Jackson, 1962. Crustal studies i n t h e western C o r d i l l e r a . Program, meetings o f t h e S e i s m o l o g i c a l S o c i e t y of America, A p r i l 16-18, 1962. Heiskanen, W. A., and P. A. Vening M e i n e s z . field. McGraw H i l l , New Y o r k , pp. 138,  1958. The e a r t h and 1 5 9 , 208-209.  i t s gravity  H e r r i n , Eugene. 1962, I n t e r p r e t a t i o n and adjustment o f r e g i o n a l t r a v e l times and phase v e l o c i t i e s u s i n g h i g h speed computers. Program, meeti n g s o f t h e S e i s m o l o g i c a l S o c i e t y o f America, A p r i l 16-18, 1962. Hodgson, J . .H. 1953. A s e i s m i c s u r v e y i n t h e Canadian S h i e l d , Is Refraction s t u d i e s based on r o c k b u r s t s a t K i r k l a n d Lake, O n t a r i o . Publications of t h e Dominion O b s e r v a t o r y , 16, No. 5, pp. 111-163. J e f f r e y s , S i r Harold. 1959. P r e s s , Chapter I I I .  The  Earth  (Fourth E d i t i o n ) .  Cambridge U n i v e r s i t y  96 -  Katz, Samuel. 1955. Seismic study of crustal structure i n Pennsylvania and New York, Bull. Seism. Soc. Am., 45, No. 4, pp. 303-325. Leet, L. D. 1938. Practical seismology and seismic prospecting, Century-Crofts, Inc., New York, Chapter 4. Menard, H, W.  1961.  1961, pp. 52-61,  The East Pacific Rise.  Appleton-  Scientific American, December  Milne, A. R. I960. A reversed seismic refraction profile, North Pacific Ocean Basin. Report 60-10, Pacific Naval Laboratory, Esquimalt, B. C. Milne, W. G., and W.R.H. White, I960, A seismic survey in the vicinity of Vancouver Island, British Columbia, Publications of the Dominion Observatory, 24, No, pp. 145-154.  7,  Nafe, J. E., and C. L. Drake. 1958. Physical properties of crustal materials as related to compressional wave velocities. Abstr, in Geophysics, 23, p. 403. Neumann, Frank. 1957. Crustal structure i n the Puget Sound area. Publications du Bureau Central Seismologique International, Serie A, Travaux Scientifiques, Fascicule 20 (Memoires presentes a l a Conference de Toronto 1957). Pakiser, L. C , J. H. Healy, W. H. Jackson, and S, W. Stewart (U. S. Geological Survey, Denver, Colorado), 1962, Seismic refraction exploration of the continental crust in eastern Colorado and the Basin and Range Province. Abstract, J. Geophys, Research, 67, No. 4. Press, Frank, and Maurice Ewing. 1952. Two slow surface waves across North America, Bull. Seism, Soc, Am., 43, pp. 219-228. Press, Frank, I960, Crustal structure in the California-Nevada region, J. Geophys. Research, 65, No, 3, pp. 1039-1051. Richards, T. C , and D. J. Walker. 1959, Measurement of the thickness of the earth's crust i n the Alberta Plains of Western Canada. Geophysics,  24, No. 2, pp. 262-284.  Richter, C. F. 1943.  33, pp. 243-250.  Calculation of small distances.  Bull. Seism. Soc. Am.,  Romney, Carl. 1959. Amplitudes of seismic body waves from underground nuclear explosions. J, Geophys, Research, 64, pp. 1489-1498. Shor, George G., Jr„ 1962. Seismic refraction studies off the coast of Alaska, Bull, Seism, Soc. Am,, 52, No. 1, pp. 37-57. Steinhart, J. S., and R. P. Meyer. 1961, Explosion studies of continental structures. Publication 622, Carnegie Institute of Washington, D,C. Talwani, Manik, George H. Sutton, and J. Lamar Worzel. 1959. A crustal section across the Puerto Rico Trench, J, Geophys, Research, 64, No. 10,  pp. 1545-1555.  - 97 Tatel, Ho E., L. H. Adams, and Mo A. Tuve. 1953= Studies of the earth's crusto Proceedings of the American Philosophical Society, 97, No. 6,  pp. 658-669.  Tatel, H. E., and Tuve, M. A. 1955. Seismic exploration of a continental crust. Geol. Soc. Am. Special Pubis. 62, pp. 35-50. Weizman, P. S., Kosminskaja, I. P., Risnichenko, J. V. 1957. New deep seismic sounding data on the structure of the earth's crust and on mountain roots. 11th General Assembly of I.U.G.G. at Toronto. Willmore, P. L. 1949. Seismic experiments on the north German explosions, 1946 to 1947. Phil. Trans. Roy. S o c , A 242, pp. 123-151. Woollard, G. P. 1959. Crustal structure from gravity measurements. J . Geophys. Research, 64, No. 10, pp. 1521-1544. Worzel, J. L., and Maurice Ewing. 1952. Gravity measurements at sea, 1948 and 1949. Transactions, American Geophysical Union, 33, No. 3, pp.  453-460.  

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