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A marine deep seismic sounding survey over Winona Basin Thorleifson, Allan James 1978

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_ MARINE DEEP  SEISMIC SOUNDING SURVEY OVER WINONA BASIN  by ALLAN JAMES B. A*Sc. , U n i v e r s i t y  THORLEIFSON  o f B r i t i s h Columbia,  1976  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE  xn THE FACULTY (Department  OF GRADUATE STUDIES  o f G e o p h y s i c s and Astronomy)  We a c c e p t t h i s t h e s i s a s c o n f o r m i n g to  the required  THE UNIVERSITY OF July,  Allan  standard  BRITISH COLUMBIA 1978  James T h o r l e i f s o n ,  1978  In  presenting  this  an a d v a n c e d  degree  the  shall  Library  I f u r t h e r agree for  scholarly  by  his  of  this  written  thesis at  the U n i v e r s i t y  make that  permission  purposes  for  freely  may  is  financial  of  of  Columbia,  British  available  for  for extensive  be g r a n t e d  It  fulfilment  by  shall  copying  t h e Head o f  understood  gain  of  The U n i v e r s i t y  {-r<zc,pV*f$>\C<> of  British  2075 Wesbrook Place Vancouver, Canada V6T 1W5  Columbia  the  that  not  requirements  reference of  I  agree  and  or  this  tha  thesis or  publication  be a l l o w e d w i t h o u t  AbAYQl\Oft\y  f<  study.  my D e p a r t m e n t  copying  permission.  Department  Date  it  representatives. thesis  in p a r t i a l  my  i i  ABSTRACT  During the was  carried  basin  out  located  reversed  refraction  the  to  two  the  of the  basin  method.  The  for  2 km  o f s e d i m e n t s f o r one  the  other  data sets  although  profile  basin.  On  the  approximately to  3.5  slope  km/s on  below an  1 km  the  east  the  u p p e r 0.7  analyzed  km  in a  of the  to t h e  necessitated  the  using  profiles  seismic of  use  but  from  over  the  T ~X 2  with  on  Revere  energy  the  the  2  thicknesses .6 km f o r  and  data i n d i c a t e  were s h o t  sediments  two  delineate  shot  that  sides  The of  the  Ridge,  velocity  i n the  Over t h e  is strongly  range  2.5  continental scattered  sediments.  study using  the  a x i s of  synthetic  o f ray  t r a c i n g f o r the  75-2,2R and  they s a t i s f y  75-3,35. the  seismic  cross The  the  the  basin  basin  final  data  basin,  seismograms.  l a t e r a l inhomogeneities across  profiles,  non-unique  to  v o l c a n i c basement.  f l a n k of P a u l  previous  severe  are  profiles  p r o f i l e 75-1,IR, a l o n g  However, t h e  refraction  attempt  o v e r l i e s v o l c a n i c basement.  Refraction was  of  and  basin.  industry  profiles  Three  sub-critical  gave l a y e r v e l o c i t i e s and  penetrated  western  parallel  Several  were a n a l y z e d  petroleum  remaining r e f l e c t i o n  sedimentary  with penetration  i n an  sub-critical reflection  part  one  basin,  were a l s o s h o t of  survey  of Vancouver I s l a n d .  upper m a n t l e .  structure  sounding  a deep water  were s h o t ,  central  neither  end  axis of the  t o the  profiles  sedimentary The  a deep s e i s m i c  northern  profiles  the  ocean bottom  reflection  1975  o v e r Winona B a s i n ,  o f f the  perpendicular the  summer o f  very  models well  and  are c o n s i s t e n t with  75-1,1R, g r a v i t y data and  profile  views on p l a t e t e c t o n i c s .  They show deep c r u s t a l l a y e r s  d i p p i n g from both s i d e s of the basin towards the Evidence f o r subduction the Explorer and  center.  as well as l a t e r a l motion between  American p l a t e s has  t h a t o b l i q u e subduction  current  l e d to the  i s o c c u r r i n g at Hinona  conclusion Basin.  iv  TABLE OF CONTENTS Abstract  i i  T a b l e Of C o n t e n t s  iv  L i s t Of T a b l e s  v  L i s t Of F i g u r e s ......................................... v i Acknowledgements  ............v...........................viii  1. I n t r o d u c t i o n  1  1.1 S t r u c t u r e And T e c t o n i c s Of Winona B a s i n  ...........  3  1.2 P r o j e c t D e s c r i p t i o n ...............................  8  2. Data A c q u i s i t i o n And P r e l i m i n a r y A n a l y s i s ............ 10 2.1 Data A c q u i s i t i o n .................................. 10 2.2 P r e l i m i n a r y A n a l y s i s .............................. 11 3. S u b - c r i t i c a l R e f l e c t i o n Data ......................... 17 3.1 Methods Of A n a l y s i s ............................... 17 3.2 Bubble P u l s e D e c o n v o l u t i o n  19  3.3 A n a l y s i s Of R e s u l t s ............................... 21 4. R e f r a c t i o n A n a l y s i s  35  1.1 The R e f r a c t i o n Data S e t  35  4.2 Methods Of A n a l y s i s ............................... 42 4.3 D e s c r i p t i o n Of Ray T r a c i n g  Program  4.4 A p p l i c a t i o n Of Ray T r a c i n g 5. D i s c u s s i o n  ,.45 47  ........................................... 62  5.1 Sediments  62  5.2 Ray T r a c i n g Models ................................ 65 5.3 T e c t o n i c  S i g n i f i c a n c e Of Winona B a s i n  67  R e f e r e n c e s ....................... ........ •>......... ..... 69  V  L I S T OF CABLES I  II  Reflection interpretation results f o r profiles 75-2V and 75-3V. . . . . . . . . . . . . . . . . . . . . . . . . . . . .  ,23  Reflection interpretation results f o r profiles 75-2, 75-2R, 75-3 and 75-3S. . . . . . . . . . . . . . . . . . . . . . . . . 33  vi  L I S T OF FIG0RES 1.1  Location  map  f o r Sinona  Basin.  .................... 2  1.2  Location  map  f o r DSS p r o f i l e s ,  ..................., 4  1.3  Continuous seismic  profile  line  1.4  Continuous seismic  profile  l i n e 75-3.............. 8  2.1  The 6 s e i s m i c c h a n n e l s p l u s SIVB t i m e c o d e for  two s h o t s .  75-2. . . . . . . . . . . . . . .7  .......... . . . . » . . . . . . . . . . . . . ........ .13  3.1  Eecord s e c t i o n  for reflection  p r o f i l e 75-2V. ...... 22  3.2 3.3  Record s e c t i o n f o r r e f l e c t i o n Record s e c t i o n f o r r e f l e c t i o n shallow shots.  p r o f i l e 75-3V. ......25 p r o f i l e 75-2R, .. , 27  3.4  Record s e c t i o n  p r o f i l e 7 5-2B,  deep s h o t s .  for reflection  ............................ . . . . . . . . . . . . 28  3.5  Record s e c t i o n f o r r e f l e c t i o n  p r o f i l e 75-2.  29  3.6  Record s e c t i o n  for reflection  p r o f i l e 75-3. ....... 30  3.7  Record s e c t i o n  for reflection  p r o f i l e 75-3R.  4.1  Record s e c t i o n f o r r e f r a c t i o n  profile  4.2  Record s e c t i o n  for refraction  p r o f i l e 75-2K. ...... 37  4.3  Record s e c t i o n  for refraction  p r o f i l e 7 5 - 3 . .......38  4.4  Record s e c t i o n  f o r refraction  p r o f i l e 75-3R.  4.5 4.6  S t a r t i n g model f o r 75-3 r a y t r a c i n g . . . . . . . . . . . . . . . 48 Ray p l o t and t r a v e l t i m e c u r v e f o r 75-3 s t a r t i n g model. ..... ...........52  4.7  Ray p l o t and t r a v e l t i m e c u r v e f o r 75-3R s t a r t i n g model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ......53  4.8  Ray p l o t and t r a v e l t i m e c u r v e f o r 75-3 f i n a l model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55  4.9  Ray p l o t and t r a v e l model. . . . . . . . . . ,  4. 10  Ray p l o t and t r a v e l t i m e c u r v e f o r 75-2 f i n a l model. . . . » . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 9  ......31  75-2. ...... .36  ...... 39  t i m e c u r v e f o r 75-3R f i n a l ............56  vii  4.11  Say p l o t model.  and  travel  time c u r v e f o r 75-2B  final 60  v i i i  ACKNOWLEDGEMENTS For  h i s encouragement  phases o f t h i s R.M. C l o w e s . students their  I would  I would a l s o  Henry  Cheung,  like  like  helpfulness  thanks  with  also  t o thank  Andy J u r k e v i c s  my f e l l o w  and Barry  go t o K e n W h i t t a l l  t h e r a y t r a c i n g program.  provided  the opportunity  sections  i n Winona  I  f o rh i s enthusiastic  their  their  t o e x p r e s s my a p p r e c i a t i o n  a n d c r e w o f t h e C. F. A. V. ENDEAVOUR  Unit,  Pacific  assistance Funding  contract  f o rt h i s  and r e s e a r c h  Canada. Ltd.,  grant  Maritime  during  Canada, Department operating  Standard L t d .  reflection  LAYMORE a s w e l l a s t h e e x p l o s i v e s e x p e r t s Diving  record  to the  a n d C.F. A. V.  from  the Fleet  Command, E s g u i m a l t ,  B.C. f o r  the cruise.  project  was p r o v i d e d  agreements with  of Energy,  A 7 7 07  through  Research  were s u p p l i e d  Resources Ltd.  a  the Geological  Mines and Resources  of the National  A d d i t i o n a l funds  S h e l l Canada  Narod f o r  Basin.  would a l s o l i k e  officers  graduate  suggestions.  Chevron  t o examine  throughout a l l  t o t h a n k my s u p e r v i s o r D r .  s t i m u l a t i n g d i s c u s s i o n s and h e l p f u l  Special work  project  and open  research Survey of  and through  Council of  by M o b i l  O i l Canada  and Chevron Standard  Ltd.  1  U The  western Canadian  very complex area. ridges crust  continental  into  latter  the P a c i f i c  a n d J u a n de F u c a  plates  to the e a s t ;  p l a t e s i s t h e l a r g e N o r t h American  Explorer  and J u a n  Farallon plate,  plates  by t h e N o o t k a F r a c t u r e  North American  results  plate  the  oceanic  and  Hyndman, 1976).  Zone  motion  1977).  with  the E x p l o r e r  junction  plate  between  the  several  t h i s deep s e i s m i c  (Riddihough margin  motion from t h e c o n v e r g e n t  The change i n p o s i t i o n o f t h i s t r i p l e m i l l i o n years i s believed  the three junction  sounding  s u r v e y . ...  over  t o be a t l e a s t  f o r many o f t h e p r o m i n e n t f e a t u r e s  i n c l u d i n g Winona B a s i n ,  plates  and s u b d u c t i o n of  plates.  region,  transform  along the c o n t i n e n t a l  m o t i o n i s known as t h e t r i p l e  the  North of the  c o n v e r g e n c e o f the  of the oceanic crust  The p o i n t  and Hyndman,  and J u a n de F u c a  p l a t e beneath the c o n t i n e n t a l  responsible  The  o c c u r s between t h e P a c i f i c and  fault  partially  1.1).  (Riddihough  which s e p a r a t e s the t r a n s f o r m  past  of the  p l a t e , t h e western  a l o n g t h e Queen C h a r l o t t e  i n compression  smaller  a r e remnants o f t h e once  zone; south o f t h e Dellwood K n o l l s ,  North American  east  (see F i g .  communication,  lateral  plates  the oceanic  and a r e now s e p a r a t e d i n t o two s u b -  Hyndman, p e r s o n a l  Dellwood K n o l l s ,  crust  de F u c a p l a t e s  larger  fault  f a u l t s separates  spreading  p l a t e t o t h e west and t h e much  o f which h a s c o n t i n e n t a l  1976;  margin i s t e c t o n i c a l l y a  A series of active sea-floor  c o n n e c t e d by t r a n s f o r m  Explorer  part  , INTRODUCTION  of  the area o f i n t e r e s t f o r  Fig.  1. 1  3  ls.1 STRUCT0RE AND  TECTONICS OF  WINONA BASIN  Winona B a s i n i s a deep water s e d i m e n t a r y the  f o o t of the c o n t i n e n t a l s l o p e  Vancouver I s l a n d  R e v e r e R i d g e t o the (adjacent Dellwood  K n o l l s to the believed  i n t o two  obliquely A -160 eastern (1969)  smaller  being  the  is  Winona R i d g e , which  runs  basin.  a spreading  (Chevron S t a n d a r d  i n d i c a t e up  show t h e t y p i c a l with  sea  but  that the  of  Ltd., sediments.  which has  (personal  anomaly been  formed  communication,  o v e r l y i n g sediments  i s l o c a t e d near the  Explorer  p l a t e s and  of spreading  c o n t i n e n t a l margin. of t h e  floor crust  Couch  Petroleum  l i n e a r magnetic  Riddihough  b a s i n have a l m o s t c e r t a i n l y interaction  t o 4 km  the  could  a pattern.  Winona B a s i n A m e r i c a n , and  data)  center,  such  been i n t e r p r e t e d by  data  sugqested  obliterate  has  reflection  b a s i n d o e s not  patterns  the  which i s  age,  of sediments.,  patterns associated  the  by  the  basin i t s e l f ,  6 km  unpublished  has  of  basin  to  Calgary,  1978)  The  and  a to  due  seismic  at  southeast  Paul  Zone  mqal f r e e a i r g r a v i t y anomaly l o c a t e d o v e r  industry  The  t o the  basins  t i p of  I t i s bounded by  of P l i o c e n e - P l e i s t o c e n e  length  p o r t i o n of as  northern  located at  Brooks F r a c t u r e  northwest.  t o be  down the  & 1.2).  southwest, the  t o Brooks Peninsula)  generally divided  F i g s . 1.1  (see  o f f the  basin  area,  ridge readjustment In  a study  rates  of the  (197 7) and  Pacific,  main f e a t u r e s o f  been c o n t r o l l e d by  Riddihough  readjustment of spreading  the  j u n c t i o n of t h e  has  and  the  the  complex  subduction  magnetic shown  at  anomaly  that  d i r e c t i o n s over  the  past  4  Fig.  1.2  5  10  million  triple  years  has  r e s u l t e d i n a complicated  j u n c t i o n near t h e a r e a  a l s o been s u g g e s t e d  of Brooks P e n i n s u l a ,  by  Murray  and  evidence  of subduction  to the  southeast  Zone and  strike-slip  junction  i s now  migrated  northward  formation  have m i g r a t e d  the  i n the  would i m p l y  that the  temporarily  stuck  s i d e of the  However D a v i s  not  past  few  I t was  along  m. y.  coast  communication,  being  basin  being  considerably  in  the  Hyndman the  and  triple  which l i n e s  more than. 6 m.y.  t h i n and  Since  the northwest R i d d i h o u g h and  was  in  1978)  has  f o r m e d by  still  there  the  basin  (Tiffin  and  The  and  newly  definitely  1976), t h e  formed  warm, may  strike-slip  Chase et a l ,  the  oceanic have b e e n  i s subduction  to  to the  motion  to  1975;  n o r t h w e s t e r n edge o f  the  basin  personal  compression than  likely  e t a l , 1972;  Hyndman,  west.  which  slow convergence of  relatively  very  the  old.  (Chase,  the  least  shown, on  from  up  This  at  o l d e r than  i s t h a t Winona R i d g e ,  1 9 7 8 ) , was  of the  basin  measurements, t h a t b o t h s i d e s o f t h e  more s u s c e p t i b l e t o d e f o r m a t i o n  southeast  triple  Vancouver I s l a n d .  p o r t i o n of t h e  continental plates.  subduction.  that  Winona R i d g e ,  c o n s o l i d a t e d s e d i m e n t s have been d r e d g e d  crust,  I f the  resulting  1978)  o f f northern  eastern  Another p o s s i b i l i t y  and  Fracture  s u g g e s t e d by  (personal communication,  likely  oceanic  on  Brooks  t o the c o n t i n e n t a l p l a t e r e s u l t i n g  b a s i s of heat flow are  has  Queen C h a r l o t t e F a u l t , r a t h e r t h a n f o l l o w i n g  indentation  east  This  <1974) b a s e d of the  the  D e l l w o o d K n o l l s , i t must have  (personal communication,  j u n c t i o n may with  Tiffin  the n o r t h w e s t .  l o c a t e d at t h e  o f Winona B a s i n . ,  Riddihough  well  motion t o  over the  motion of  the  6  subducting the  p l a t e must be  basin.  of the  Riddihough  basin.  volcanic  this  (see  and  Figs.  order  uplift  to gain  s i g n i f i c a n c e of during  the  provide crust  summer o f  along  basin  two  across  profiles  profiles  with  (Clowes,  1977)  reflected  and  the  The  the  1975.  and  the the  long  shooting proceeded  ship  structural area.  center  minimizing  to shoot  of  Ridge.  survey  the  was  eastern  sediments, short  at the  carried  study  information  for  vertical  To near  penetration  to  to the  the  gain vertical the  marine s e i s m i c incidence  of e a c h  gas  the bubble  returned the  the  profile  cross  system  wide-angle  from  to  were s e t  the  at  ocean  shallow  b u b b l e would blow o u t pulse  problem.  i t s starting  entire profile  with  point the  out  profiles  i n t e r s e c t i o n s of  The  was  portion of  (see F i g . 1.2).  profile., near  of  mantle.  16 s h o t s  then  clear i f  Three reversed  of the  basin  (7 ra) i n hopes t h a t  surface,  the  Paul  the r e s u l t  o b j e c t i v e of the  r e f r a c t e d waves w i t h  first  just  southeast  from  i t i s not  a seismic  The  were run  upper  just  of  (C.S.P.*s) show  a f u r t h e r understanding  records  bottom t o t h e  southeast  PROJECT DJSCJRIPTIOS  i n f o r m a t i o n about the  incidence  but  Paul Revere  upper m a n t l e i n t h e one  further  1.4),  Winona B a s i n ,  were r u n ; and  being  profiles  plate or  along  detailed velocity  and  depths  1. 3 &  subducting  1.2 In  just  d i p p i n g b e n e a t h Winona B a s i n  i s actually a  deformation  (1977) shows i t a s  Continuous seismic  basement  Revere R i d g e  e i t h e r beneath, or  at  The and  shots  at  the  C.S. P.  LINE  75-2  i  Fig.  1.3  C o n t i n u o u s s e i s m i c p r o f i l e l i n e 75-2, p a r a l l e l to r e f r a c t i o n line 75-2E. S u p e r i m p o s e d above t h e s e c t i o n a r e p r o f i l e l i n e s s h o w i n g a p p r o x i m a t e s h o t l o c a t i o n s {numbers below the l i n e s ) and r e c e i v i n g s h i p l o c a t i o n s f o r t h e s e s h o t s (numbers above t h e lines) *  1  Fiq,  1.4  C o n t i n u o u s s e i s m i c p r o f i l e l i n e 75-3, p a r a l l e l to r e f r a c t i o n 75-3. F o r d e s c r i p t i o n of p r o f i l e l i n e s see F i q . 1. 3.  line  9  optimum d e p t h s  f o r maximum p e n e t r a t i o n o f s e i s m i c  these depths depending Profiles while t h i s 75-2R,  Lynch*s  thesis  75-3,  and  75—1  on t h e c h a r g e s i z e 75-1R  75-2V  and  work a r e an i n t e g r a l  references w i l l  (Shor, 1 9 6 3 ) .  were a n a l y z e d by L y n c h  p r e s e n t s an a n a l y s i s o f p r o f i l e s  75-3R,  energy,  75-3V,  75—2,  Since the results  part of this  be made t o h i s t h e s i s .  (1977)  s t u d y , many  of  10  2_.  DATA A C M I S J T I O N  2_! A detailed  principle Shor  to the two-ship  a t one  of the marine  ship,  deep  (1977).  refraction  one  ANALYSIS  ACQUISITION  i s g i v e n by C l o w e s  (1963) , i n which  freely  DATA  description  sounding system  AN D PRELIMINARY  seismic  It is similar in  technique described  the r e c o r d i n g  ship,  end o f t h e l i n e w h i l e t h e s e c o n d  by  drifts  ship  proceeds  along a predetermined course r e l e a s i n g e x p l o s i v e charges. our survey, s h i p On  p o s i t i o n s were d e t e r m i n e d by LORAN  the r e c e i v i n g  by s i x h y d r o p h o n e s cable.  The  filtered  suspended  to  manually  p l u s WtfVB t i m e code onto magnetic  100  recorder On  system  for guality  the  by b o t h a hydrophone  on d e c k .  were r e c o r d e d on  water  and  signals  written  14 b i t , m u l t i -  1977).  two  Five  data  6-channel  (D.W.H.)  behind the  signals plus  a 4 - c h a n n e l FM  tape  were r e c o r d e d  Brush c h a r t  being analyzed i n t h i s  wave  directly  These  WWVB s i g n a l s  onto a 2-channel h i g h speed  individual  were m o n i t o r e d on a  trailed  •HflVB t i m e code  profiles  Hz  (see C l o w e s ,  direct  located  hydrophone and  by  s i x analog  a t 312.5  m  control.  and a geophone  while the  The  a 610  hydrophone,  t a p e u s i n g an I.B»M. c o m p a t i b l e ,  the s h o o t i n g s h i p ,  detected  at the  shot.  were d i g i t i z e d  c h a n n e l s p l u s t h e W5JVB t i m e c o d e chart  s i g n a l s were d e t e c t e d  Hz and t h e n a m p l i f i e d  s e t f o r each  channel data a c g u i s i t i o n  A.  t o a d e p t h o f 45 m from  s i g n a l s were p r e - a m p l i f i e d  f r o m 0.8  amplifiers  ship, the seismic  For  thesis,  recorder..  was ship  the  transport directly F o r the  the c h a r g e s ranged i n  11  size  from  2.3  kg  t o 96  kg  of Geogel, a commercial  and  were s u s p e n d e d  The  shot-to-ship distances  focussed  on  the  at optimum  d e p t h s by  were measured  recorded  the  six seismic  digitally  processing  step  which a marker digitization,  arrival Lynch  in multiplexed  was  channel  of the  shot  (1977) was  errors  and  energy.  input  Shot O r i g i n  The  analog  comprise  determine the  one  records,  and  the  after  channels plus  data  since the  written  demultiplexing.  f o r missed data  on  for  unnecessary data  A c o m p u t e r program  tapes  data  time i n t e r v a l  both before  s i x seismic  and  data  by  The tape  onto  new  WWVB t i m e code  file. ,  Times  D.W.W. r e c o r d e d shot  origin  hydrophone c o u l d  at t h e  d e p t h s and  shooting  times.,  be t i m e d t o  2-channel chart recordings.  corrections  first  wrote the c o r r e c t e d , d e m u l t i p l e x e d  f o r a s i n g l e shot  various  out  used t o perform  magnetic tapes.  The  The  i d e n t i f i e d the  were used t o e d i t  WSVB t i m e c o d e were  form, t h e  demultiplexing.  program c h e c k e d t h e  at  rangefinder  ANALISIS  channels plus  s e v e r a l s e c o n d s were r e c o r d e d  the  a  Demultiplexing Since  2)  with  balloons.  balloons.  2j.2 PRELIWINAfiY  1)  large red  explosive,  The  arrival  b e t t e r than 5  Since  distances  s h i p was  the charges  f r o m the  were made f o r t h e t r a v e l  of the ms  to  signal  from  were  shooting  time,  used  at  the  detonated  ship,  g e n e r a l l y of  the  12  order  o f 100 ms, from  shot t o s h i p .  This introduced a further  15 t o 20 ms e r r o r .  3)  Shot-Receiver Distances The  shot t o r e c e i v e r  measuring velocity  d i s t a n c e s were d e t e r m i n e d by  t h e D.w.W. t r a v e l o f 1.49 km/s.  t i m e and assuming  a constant  To measure t h e a r r i v a l  time o f t h e  D.W.W., t h e s i x s e i s m i c and WWVB t i m e c h a n n e l s were w i t h a common  time  origin  a t a r a t e o f .126 s/cm  plotted  {.32 s / i n c h ) ,  e a c h c h a n n e l b e i n g n o r m a l i z e d t o a maximum a m p l i t u d e (see F i g . 2 . 1 ) . T h e e r r o r 10 - 20 ms and combined total  4)  error  with o r i g i n  Topographic  the eastern portion approximately  sediments. sub-bottom material, 2.0 km/s.  time e r r o r s t h i s  gives a  t h e C. S. P.  records (Figs.  1. 3 & 1.4),  f o r topography.  Since  of the b a s i n i s a t a u n i f o r m depth o f  2.0 km, a l l t r a v e l t i m e s were c o r r e c t e d t o t h i s  A velocity  sub-bottom  t h e D.W.W. a r r i v a l i s  Correction  was n e c e s s a r y t o make c o r r e c t i o n s  depth,  o f 1.9 cm  i n d i s t a n c e o f t h e o r d e r o f 50 - 80 m.  As c a n be seen f r o m it  i n measuring  water  o f 2.0 km/s was assumed f o r t h e i m m e d i a t e  material,  t h i s being representative  The e f f e c t  of this  correction  m a t e r i a l a b o v e 2.0 km d e p t h and a l l water  of shallow  i s to replace a l l  with  1. 49 km/s  below 2.0 km w i t h m a t e r i a l o f v e l o c i t y  The c o r r e c t i o n i s  <2.1)  13  n  O CM I I C\J I  D.VYW.  LO  •1 SEC-H  Fiq.  2.1  Two s e i s m o q r a m s t y p i c a l o f t h o s e used t o time the d i r e c t water wave (D.W.W. ) and the f i r s t refraction arrivals. The b u b b l e p u l s e s e q u e n c e i s c l e a r l y v i s i b l e f o l l o w i n g t h e D.W.W. a r r i v a l on t h e upper s e t o f t r a c e s . The l o w e r s e t o f t r a c e s shows t h e a r r i v a l oC r e f r a c t i o n energy at a d i s t a n c e o f 19 km.  14  where H i s t h e h e i g h t is  o f t h e topography  t h e water v e l o c i t y , V  t h e sub-bottom  5  angle  o f t h e r a y from  f r o m 2.0 km d e p t h ,  vertical.  This  V  w  v e l o c i t y and O t h e  c o r r e c t i o n , which i s  a p p r o x i m a t e l y 50 ms f o r 250 m o f t o p o g r a p h y , was a p p l i e d t o both t h e s h o o t i n g Since the  ship  and t h e r e c e i v i n g s h i p w a t e r  t h e two s i d e s o f t h e b a s i n a r e c l o s e  greatest  Revere  effect  o f the topographic  depths.  t o 2.0 km  depth,  c o r r e c t i o n i s over  Paul  R i d g e , Winona R i d g e and t h e c o n t i n e n t a l s h e l f and  slope.  The v e l o c i t i e s b e n e a t h t h e s e a r e a s a r e n o t w e l l known  so  the c o r r e c t i o n s could  that  1.0 km o f c o n t i n e n t a l s l o p e  be i n e r r o r .  material  F o r example, i f  i s corrected  a t 2.0 km/s  the  c o r r e c t i o n i s 170 ms, w h i l e  i f i t i s c o r r e c t e d a t 4.0  the  c o r r e c t i o n i s 4 20  must  analyzing  such  ms.  This  was no d e p t h  depth i n f o r m a t i o n ,  Hydrographic S e r v i c e water d e p t h s a l o n g  to the cruise.  a chart  was u s e d .  an e x t e n s i v e  from  this chart  which f o r LORAN  On t h i s  of  echo  In order to  chart  are plotted The  t h e d e p t h s below o u r i n ship  o f our survey  position,  i s of the order  1 km. It  was a l s o p o s s i b l e t o c a l c u l a t e t h e d e p t h s below t h e  shooting the  f o r workinq  s e r i e s of ship t r a c k s .  i s the u n c e r t a i n t y  A i n the region  equipment  p r o d u c e d by t h e C a n a d i a n  major s o u r c e o f e r r o r i n d e t e r m i n i n g ships  recording  on e i t h e r s h i p , a l t h o u g h a r e q u e s t  s o u n d e r s had been made p r i o r obtain  when  areas.,  During t h e survey t h e r e operating  be c o n s i d e r e d  km/s  s h i p by m e a s u r i n g  D.W.W. and t h e f i r s t  following  diagram:  the d i f f e r e n c e i n t r a v e l  water-bottom bounce.  time o f  Consider the  15  If  Ti  i s the a r r i v a l time of the  time of the water-bottom  D.W.S.  and  T2.  bounce then, assuming  the a r r i v a l near-vertical  incidence.  Tx=  (Z/(£) *** x  -<0/Vw  12.3)  where X i s the h o r i z o n t a l d i s t a n c e , d the shot depth, and Z the  water depth.  Let  AT  = T* - T| , then  2. = x/(VwAT • d <- y^FTc?) - - XV21  <2.4)  The u n c e r t a i n t y i n v o l v e d s h o u l d be no more than 30 m over a depth of 2000 m.  The r e s u l t s obtained using t h i s method agree  very w e l l with those taken from the bathymetric c h a r t so i t was  assumed that the r e c e i v i n g s h i p depths taken from the  c h a r t were r e a s o n a b l e as w e l l .  16  5) R e c o r d  Sections  In order  t o present  interpretation, profile.  the data  a r e c o r d s e c t i o n was p r o d u c e d  The program  used  t o compile  makes c o r r e c t i o n s f o r a m p l i f i e r spreading, The  for  (O'Brien,  spherical  large  f o r charge s i z e  reflection  gain, charge s i z e ,  is 8  (Cerveny  amplitudes..  isX  2  f o r head  and R a v i n d r a , Such a m p l i t u d e  g i v e s a record s e c t i o n with amplitudes arrivals  (Kanasewich,  1977).  f o r a weight o f W  wave a m p l i t u d e s  at  1971) a n d X f o r scaling  with  normalized  primarily  distance  such  that  The program  phase, f o u r pole B u t t e r w o r t h  1976) w h i c h was u s e d  noise.  spherical  (see Lynch,  a t a l l d i s t a n c e s c a n be s e e n c l e a r l y . ,  also contains a zero  frequency  f o r each  1960; M u l l e r e t a l , 1962) and t h e c o r r e c t i o n  spreading  distances  p o s s i b l e way f o r  the r e c o r d s e c t i o n s  and h y d r o p h o n e s e n s i t i v i t i e s  correctxon  pounds  i n the best  filter  t o reduce  high  17  3_.  SUB-CRITICAL REFLECTION  3_.1 METHODS OF All  reflection  profiles  " p i c k i n g " the a r r i v a l times horizons  DM A-  ANALYSIS  s e r e i n t e r p r e t e d by  vs. distance f o r a l l r e f l e c t i n g  which c o u l d be i d e n t i f i e d .  However, d i f f e r e n c e s i n  individual  profiles  made i t n e c e s s a r y  analysis.  Profiles  75—2V and 75—3? were s h o t  profiles CS.P.  records  Profiles not  (Telford  split-dip profiles  records that there  75—3V  for  were s h o t .  dipping  Method  For  plane,  travel  time  For these  profiles  from t h e  dip i n the layering.  from  hand, a r e  t h e C.S.P.  d i p i n the horizons reasons,  profiles  T -X 2  were a n a l y z e d  2  method  over  75-2V  and  for flat  using the eguation  layers.  1) T - X 2  multilayered  as s p l i t - d i p  on t h e o t h e r  using the standard  and t h e o t h e r  2  little  and i t i s o b v i o u s  is significant  were a n a l y z e d  layers  i s very  75-2, 75-2R, 75-3 and 75-3R  which t h e y  t o u s e two methods c f  e t a l , 1976) and i t i s o b v i o u s  that there  first  horizontal reflecting  horizons  in a  medium, t h e a p p r o x i m a t e r e l a t i o n s h i p between T and d i s t a n c e X i s  T ^ i ^ - T V  (3.1)  where V i s t h e a v e r a g e rms v e l o c i t y  down t o a h o r i z o n and  T  the  time.  side  2-way v e r t i c a l  incidence travel  The r i g h t - h a n d  0  18  of equation  3. 1 c o n s i s t s o f t h e  s e r i e s expansion  for T  vs X  a slope of  will  2  yield  For the case interval  velocity  (X) a b o u t  2  V*  J and  vertical  incidence travel  2)  are the  time layer  1  The dipping  to the  i s the angle  bed  Consider  rms  at t h e  intercept  Dix  of  T  2  T ., 2  (1955) g i v e s  the  (3.?)  velocity  and  2-way  t o the bottom o f the k * 1  i s then  between t r a v e l  T.Y  x  ©  an  given  time  h o r i z o n i s , (from T e l f o r d  T=  where  and  of  layer.  by  Approach  relationship plane  p o i n t X=0. . A p l o t  - ^sx:  average  t h i c k n e s s o f t h e k**  Dipping Layer  Taylor  T" - Tic-\  where \  The  2  terms of a  as  l;  K  1/V  two  the  of s e v e r a l layers,  x/ , / ^ T K  T  first  I  I + (X**  and  distance f o r a  e t a l , 19 76)  (3.4)  4hXS.*8i) 4K* /  o f d i p , and  h  i s the t h i c k n e s s normal  origin.  2 points, T  (  and  T  z  with c o r r e s p o n d i n g  X,  and  X , t  19  from  a travel  time  curve.  which c a s e T i =T =2h/V, 0  choose X , - 0 , i n  For convenience  the t r a v e l time i n t e r c e p t .  Therefore,  T,= o V a  (3. 5)  T l = To  V  Let  AT =T^-T%  1  then  l  (iToX^s.^e)^  -  AT"  In  this eguation  to f i n d from  V,  6  Xa  (3.7)  = O  a  AT  1  t h e r e a r e two unknowns, V and  must be known.  9.  F o r t u n a t e l y we c a n e s t i m a t e  t h e C.S.P. r e c o r d s , a l t h o u g h o n l y t h e f i r s t  l a y e r c a n be a n a l y z e d  w i t h a n y degree  A g a i n , t h e Dix f o r m u l a velocity,  In order  sub-bottcm  of accuracy.  can be used  which i n t u r n c a n be used  ©  to find  to find  the i n t e r v a l  the layer  thickness.  3.2  BOBBLE PULSE DECONVOLUTION  In m a r i n e s e i s m i c o p e r a t i o n s , t h e s o u r c e produced  by t h e e x p l o s i v e e n e r g y  impulses  caused  and  source  signature  i s a s e r i e s of  by t h e r e p e a t e d c y c l e o f gas b u b b l e  c o n t r a c t i o n ( s e e Kramer e t a l , 1 9 6 8 ) .  expansion  For s u b - c r i t i c a l  20  reflection profile  data  this  wavelet  attempt  deconvolution  was  filtering.  first  method was  level  parameter  technique  of  siqnature.  The  this  (Clayton, by  Hood e t a l  while  the  Minimum  b a s e d on  time  t r a c e with  the  data,  The  advantage  both  filters  limitations  zero  conventional  produced  Instead,  s h i p of  arrival  what was  from  T h i s method  (Wiggins,  was  used  depend on  was  not  source  S u c h a method  1977)  attempted  which i s have here  a l s o b e c a u s e f o r most a r r i v a l s  bubble o s c i l l a t i o n s d i d not  true  the water s u r f a c e .  give better results.  Deconvolution  and  no  t h e D.W.W. w h i c h i s  method w h i c h d o e s n o t  probably  moveout.  of  T h i s i s most l i k e l y b e c a u s e no  ghost  Entropy  source  i n t o a d e s i r e d shape  s e a r c h i n g e a c h t r a c e f o r s e g u e n c e s which  identical of  which c o n s i s t s  method.  by  s i g n a t u r e s would  a  o r i g i n a l bubble pulse s i g n a t u r e i s  receiving  A deconvolution  water-  a u t o c o r r e l a t i o n f u n c t i o n has a  a t the the  The  then c o n t a i n the a u t o c o r r e l a t e d  s i g n a t u r e s were a v a i l a b l e .  arrival  the  which c a u s e s p r o b l e m s when u s i n g t h e  improvement.  complicated  is  (1978), the  signature.  using  by  both  s e c o n d method was  be p r o c e s s e d  i s that the  When a p p l i e d t o t h e  the  The  l a g Wiener i n v e r s e f i l t e r .  Wiener s p i k i n g f i l t e r  source  1975) .  can  source  deconvolution  trace should  phase s p e c t r u m ,  noticeable  o f the  cross-correlating  procedure  mixed d e l a y ,  methods were t r i e d ,  knowledge  s i g n a t u r e which optimum  Two  divisional  devised  essentially  an  the  made t o r e d u c e t h e b u b b l e p u l s e p r o b l e m  requiring a detailed  by  seriously complicate  interpretation.  An  source  can  present a severe  problem.  because the For  21  later  arrivals  significantly noise  ratios  i t i s unlikely improve  bottom  75-2V  Profiles profiles.  and  The  the  ship,  receiving  75-3V  record  F i g . 3.1,  record  and  then  section  with  section  of the  the  water  shows r e l a t i v e l y  bottom  bounce  from  strong  arrival  the  explosion.,  The  bottom  The  long  A l l  flat  strong  i s shown  5 t o 30 H z .  The  i s virtually identical  with lying  little  o r no  sediments  d i p i n the  in this  a r r i v a l of energy  bubble created bounce a r r i v a l s are a l s o  bubble t r a i n  but because the b u b b l e  to  t h e C.S.P. r e c o r d ( F i g .  i n i t i a l explosive  oscillations  from  o f 75-2V  i s the  i m p u l s e . , The  the  the gas  record,  direction  bounce from  of  amplitudes.  the  corresponds to the bottom  oscillation  bubble  i n one  f i l t e r e d from  This agrees  first  while  eastern half  indicating  1.3)  The  first  western half  surfaces.  the basin.  freely  split-dip  i n the opposite direction.,  for the  f o r the eastern half,  which  shot as  m depth.  a l ltraces  f o r the  drifted  explosives,  reflecting  third  ms  be  j_ES0LTS  were b o t h  ship  were d e t o n a t e d a t 7 The  of  500  would  75-3V  receiving released  that  within  A N A L Y S I S OF  75-2V and  shooting ship  in  main a d v a n t a g e  to  arrival.  Profiles  shots  The  primary a r r i v a l s  i_J  1)  d e c o n v o l u t i o n would  m a t t e r s , because of the lower s i g n a l  encountered.  enhancement of  that  by  from  water  next  the  first  underwater the second  present, but at greatly  area  and  reduced  complicates the  p u l s e s h a v e an  identical  22  Fiq.  3.1  Record s e c t i o n f o r r e f l e c t i o n p r o f i l e 7 5 - 2 V , All t r a c e s have been f i l t e r e d f r o m 5 t o 30 h z . The water b o t t o m r e f l e c t i o n i s marked W and i s f o l l o w e d 2 5 0 ms l a t e r by the water b o t t o m r e f l e c t i o n of t h e f i r s t b a b b l e p u l s e . A, B, C, and D a r e c l e a r p r i m a r y r e f l e c t i o n s w h i l e E i s less certain. WW i s t h e f i r s t o r d e r m u l t i p l e o f Vi.  23  moveout t o t h e i n i t i a l  impulse  f o r a given r e f l e c t o r ,  possible to identify  a r r i v a l s from  different  Furthermore, s i n c e the time  moveouts.  Table I  "T—  ~i  I P r o f i l e | Layer | 1  1 | 75-2V  |  75-3V  t  -T  for profiles  T-  V e l o c i t y J T h i c k n e s s ! Depth (km) (km/s) 1 (km) |  w  1  1.*>9  1  2.00  |  2.00  1  A  I  1.62  I  • 33  |  2. 33  i  B  I  1.83  \  .39  |  2.72  I  c  I  2.04  1  .18  \  2.90  1  D  |  2.49  1  .30  |  3. 20  I  £  I  3.63  1  .77  |  3.97  |  H  |  1.49  1  2.05  I  2.05  1  A  I  1.67  1  .40  J  2. 45  1  B  1  2.03  1  .20  |  2.65  .i  -X.  successive  train,  results  bubble  i t i s nearly  _,. j .  always p o s s i b l e  interval  to the nearest a r r i v a l with  possible to distinguish A,  identical  arrivals i n this  clearly  B, C and D shown i n F i g . 3.1.  labeled  the f o u r  obscured  the time  moveout.  By  manner i t was  sub-bottom  A fifth  E, shows a d i s t i n c t l y d i f f e r e n t  partially  .j  t o determine the  o f an a r r i v a l a l o n g t h e t r a i n by m e a s u r i n g  a l l prominent  l i  pulse decreases along the  position  identifying  their  interval  1  i  between e a c h bubble  d e e p e r h o r i z o n s by  Reflection interpretation 75-2V and 75-3V.  t  i t is  reflectors  s e t of a r r i v a l s ,  moveout t h a n  D but i s  by b o t h n o i s e and b u b b l e o s c i l l a t i o n s  from  24  earlier ms  arrivals.  after  However, t h e  the event  marked, can  t h e s e c t i o n and  g i v e s credence  reflector.  T -X  in  Table I.  coherent for  2  Beyond  arrivals;  5.25  clearly  f o r these f i v e t o be  indicating earlier  visible;  250  this  l a y e r s are given  additional interval  that  sets  they are  arrivals.  But  at l e a s t  s the f i r s t i t obscures  one  deeper  most  a t a time  p o s s i b l e deep  of  enough  reflector,  s e t of water bottom any  of  velocities  w i t h s m a l l moveout, n o t c l e a r  which s u g g e s t s 5.4  p u l s e o f E,  t o the e x i s t e n c e o f  however t h e c a l c u l a t e d  s , t h e r e i s an e v e n t analysis,  bubble  be c o r r e l a t e d a c r o s s most c f  E t h e r e appear  r e v e r b e r a t i o n s from  fit a p p r o x i m a t e l y is  results  2  t h e s e a r e much t o o lew,  likely  for  The  first  multiples  crustal  reflections. The is  r e c o r d s e c t i o n f o r the  shown i n F i g , 3.2.  Only  and  B, a r e i d e n t i f i e d ,  3.5  s.  and  petroleum  There  industry  with  i t i s so  cannot  be  eastern  with  no  i s similar  The  below a b o u t  Since the CS.P,  in  the l a y e r i n g ,  of  the  profile  summarized  used  i n T a b l e I,  1.4)  s.  between W  and  that i t f o r the half  bubble  r e c o r d shows v e r y l i t t l e d i p  only the r e c o r d s e c t i o n  was  3.5  of the western by  after  deformed  oscillations  e x c e p t t h e a r r i v a l s a r e e v e n more o b s c u r e d oscillations.  (Fig.  record section  to t h a t  A  coherent energy  d a t a w h i c h show  by b u b b l e  identified.  75-3V  of primary a r r i v a l s ,  o f a primary a r r i v a l  badly obscured  o f 75—3V  h a l f of p r o f i l e  t h e C.S.P. r e c o r d  no c o n t i n u o u s l a y e r i n g  positively  half  sets  reflection  i s some p o s s i b i l i t y  A, b u t  two  with v i r t u a l l y  This i s consistent  sediments  western  i n the analysis.  f o r the The  western  results  are  half  Fiq.  3.2  Fecord  section  for  reflection  profile  75-3V.  26  2)  Profiles  75-2,  Profiles twice, at  first  45 m.  the  75-2B,  75-3  and  75-3R  7 5 - 2 , 7 5 - 2 R , 7 5 - 3 and 7 5 - 3 R with  shots  a t 7 m depth  and t h e n  None were s h o t a s s p l i t - d i p  r e c o r d s e c t i o n s f o r t h e two s h o t  reveals significant  much l e s s  shot  profiles  arrivals to  primary  records  arrivals  The b u b b l e  p e r i o d , which  f o r the s h a l l o w  pulse  that the shallow pulse  p e r i o d f o r t h e deeper  bubble a r r i v a l s i m p o s s i b l e  reverberatory nature.  the  o c e a n b o t t o m , b u t from c o m p a r i s o n that t h i s  shots.  o u t i n t i m e and were r e l a t i v e l y  p l a c i n g the shots  75-2,  pulse  i t was f o u n d  for  obvious  75-28  easy  shots i s  wave p e r i o d o f .06 - .10 s w h i c h made  individual  a much more  shots  Comparison of  were much b e t t e r b e c a u s e t h e b u b b l e  c l o s e t o t h e dominant identifying  with  d i f f e r e n c e s ( s e e F i g s , 3.3 S 3 . 4 ) . The  were more s p r e a d  identify.  shot  depths f o r p r o f i l e  f o r t h e deeper s h o t s than  For i d e n t i f y i n g  again  profiles.  major d i f f e r e n c e i s c a u s e d by t h e b u b b l e is  were e a c h  a t 4 5 m was t o d i r e c t  resulted.  and g a v e t h e  The i n i t i a l  reason  more e n e r g y  o f the r e c o r d s  into  i t i s not  The r e c o r d s e c t i o n s f o r p r o f i l e s  7 5 - 3 and 7 5 - 3 R a r e shown i n F i g s . 3 . 5 , 3 . 6 and 3.7  respectively. Another problem records  which c a u s e d some o b s c u r i n g  of the  was a r e s u l t o f t h e r e c o r d i n g p r o c e d u r e when  collecting  data  over  normal l a n d s e i s m i c  a dipping r e f l e c t o r . operations,  In contrast to  marine s e i s m i c  r e q u i r e s a s t a t i o n a r y a r r a y o f r e c e i v e r s and a sequence of shot direction considered  profiling varying  p o s i t i o n s . , I f the h o r i z o n i s d i p p i n g i n the  of the shooting ship the overall t o be a down-dip p r o f i l e  profile i s  and the a p p a r e n t  velocity  02  03  04  05  06  07  5-30 Hz Shots at 7m i  1.0  Csl  1.4  1.8  2.2  7 5 - 2 R  2.6  3.0  3.4  3.8  4.2  4.6  DISTANCE (KM)  r  Fig.  3,3  Record s e c t i o n f o r r e f l e c t i o n with s h o t s at 7 m depth.  profile  75—2R  {western  end o f l i n e )  1  17  18  19  20  5-20* Shots at *5m ^1.0  Fig.  1.4  3.4  1.8  / 2.2  2.6  3.0  C— I \  3.4  DISTANCE (KM)  Record s e c t i o n f o r r e f l e c t i o n depth.  3.8  profile  4.2  75-2R  4.6  5.G  with shots a t  45  m  J  0.4  Fig.  0.8  3.5  1.2  Record  1.6  section  2.0  for  2.4  2.8 DISTANCE  reflection  3.2  3.6  4.0  4.4  (KM)  profile  75-2  {eastern  end  of  line).  4.8  o LD"  in  UJ  -t  -00"  oo-  (M-  °  Fig,  5- 30 Hz Shots at 7m  .8  1.2  3.6  1.6  7 5 - 3  2.0  2.4  2.8 DISTRNCE  Record s e c t i o n f o r r e f l e c t i o n  (KM)  3.2  profile  3.6  75-3  4.0  {eastern end  4.4  4.8  of  line). o  ^.8  1.2  1.6  2.0  2.4  2.8  3.2  DISTANCE  Fig.  3.7  Record s e c t i o n line) .  for reflection  3.6  4.0  4.4  4.8  (KM)  profile  75-3R ( w e s t e r n end o f  32  across  the record s e c t i o n  horizontal. traveling  i s lower  than  i f the r e f l e c t o r i s  However, f o r a s i n g l e  shot  the energy i s  up-dip  t o t h e r e c e i v e r s and t h e a p p a r e n t  velocity  a c r o s s t h e a r r a y o f s i x hydrophones i s higher than horizontal  reflector.  smoothly continuous  This results  i n arrivals  from  t o some e x t e n t  75—3R,  floor,  which t h e C.S.P.  It  reflections This also  beyond  from  obscured As  as h a v i n g  of coherent  problem  75-2, 75-2R,  75-3  dipping sea  phases  reflection arrival  reflections,  data  very  times  mentioned  Examination  primary  four  profiles.  t h e C.S.P. r e c o r d s and  w h i c h show no c o h e r e n t for similar  petroleum  reflections  regions.  Any  i f p r e s e n t , have l o w a m p l i t u d e s  by n o i s e and b u b b l e  split-dip  o n l y one o r two  t h e r e c o r d s e c t i o n s of these  i s c o n s i s t e n t with  these  crustal  This  i n places.  was p o s s i b l e t o i d e n t i f y  industry  dip.  of p r o f i l e s  r e c o r d s show  and made t h e c o r r e l a t i n g  difficult  not  i n each  as a  e a c h o t h e r by a n amount  which depends on t h e d e g r e e o f r e f l e c t o r  and  not being  a c r o s s the s e c t i o n but appearing  s e r i e s o f s h o r t segments o f f s e t  occurred  fora  deeper and a r e  pulse reverberations.,  i n the previous section  and must be a n a l y z e d  these  p r o f i l e s are  using equation  3.1.  o f t h e C.S.P. r e c o r d s shows t h a t f o r p r o f i l e s  75—2R and 75—3R t h e basement d i p i s a b o u t 5° t o w a r d s t h e e a s t , whereas f o r p r o f i l e s Dsinq 0.8  t h e C.S.P.-determined v a l u e s q i v e s a sub-bottom  km t h i c k  thick  75-2 and 75—3, t h e d i p i s n o t c l e a r .  with  with  velocity  velocity  d i p s were t r i e d  with  2.45 km/s  2.42 km/s  f o r 75-2R  f o r 75—3R.  75-2 and 75-3.  layer  and 0.65  Several  km  different  V a r y i n g t h e d i p from  2°  33  Table  I I  Reflection interpretation results for profiles 75-2, 75-2B, 75-3 & 75-3B. Three sets of possible r e s u l t s a r e shown f o r t h e b o t t o m l a y e r o f e a c h p r o f i l e s i n c e t h e r e i s an a m b i g u i t y between v e l o c i t y , t h i c k n e s s and " d i p .  Profile  Layer  75-2R B B B — +  75-3R  B B B  7 5-2  — +  H A A A  75-3 A A A  Velocity (km/s)  •1.49 2.45 3. 01 3.47 3. 90 1. 49 2. 42 2.66 3. 09 3. 52 1. 49 2.42 2.85 3.24 1. 49 2. 51 2. 81 3. 11  Thickness (km)  Dip (deg)  1.80 .80 • 30 .35 .39 1.80 .65 .34 .40 .45 1.77 .59 .65 .71  0.0 5.0 3. 0 4.0 5.0 0. 0 5.0 3. 0 4.0 5. 0  1. 75 .78 .84 .91  0.0 2.0 4.0 6. 0 0.0 2.0 4.0 6.0  34  to  6° r e s u l t e d i n v e l o c i t i e s  3.24  km/s  with  varying  an a v e r a g e t h i c k n e s s o f 0.65 km  75-2 and v e l o c i t i e s  v a r y i n g from 2.51  an a v e r a g e t h i c k n e s s o f 0.85 km For  km/s t o for profile  km/s t o 3.11 km/s  with  f o r p r o f i l e 75—3,  p r o f i l e s 75-28 and 75-38 i t was p o s s i b l e t o a r r i v e  velocities  and t h i c k n e s s e s  range of p o s s i b l e d i p s . results  from 2.42  of t h i s  f o r a 2nd s u b - b o t t o m  These  values  layer for a  p l u s a summary  s e c t i o n a r e shown i n T a b l e I I .  of the  at  35  iii,  BEISACTION ANALYSIS  HaJ. THE BEFRACTION The  reduced  record  75-2R, 75-3 and 75-3R  DATA SET  sections f o r refraction profiles  a r e shown i n F i g s .  4.4, r e s p e c t i v e l y .  A reducing  u s e d and a l l t r a c e s  have been f i l t e r e d  set  of 6 traces  defective)  constant  v e l o c i t y o f 6.0  i s labeled  with  previously, water d e p t h  i t s shot  o f 2.0 km  2.0 km/s f o r t h e s u b - b o t t o m  sediments i n t h i s  and 2.0 km/s  area  For s h o t s over  continental  s h e l f and s l o p e ,  First  arrival  picks  arrival The  first  beyond  material.  of shallow  from t h e r e s u l t s o f waters o f the  2.0 km/s i s p r o b a b l y t o o l o w . for the last  were made from  triangles.  sections  breaks  km/s  few s h o t s o f  t h e same  p l o t s as the  ( F i g . 2.1) and a r e i n d i c a t e d on t h e r e c o r d  arrivals are indicated record  to a  a v e l o c i t y o f 1.49  i s representative  As  and 75-3R.  s e c t i o n s by s o l i d  four  ( F i g . 1.3 o r 1.4).  the shallower  was e n c o u n t e r e d  D.W.H. a r r i v a l s  Each  i s also  f o r t h e sub-bottom  as determined  C h a p t e r 3..  75-2R  has been  5 t o 20 h z .  number, w h i c h  using  The  profiles  km/s  t o p o g r a p h y has been c o r r e c t e d  t h e water l a y e r  problem  from  C.S. P. r e c o r d  for  This  4.1, 4.2, 4,3 and  {or l e s s i f p a r t i c u l a r c h a n n e l s were  shewn on t h e a p p r o p r i a t e described  75-2,  Weaker and more e m e r g e n t  by s l i g h t l y  show e x t e n d e d  b u t few c o h e r e n t  water bottom  multiple  t h e end o f t h e t r a c e s  smaller  triangles. A l l  wavetrains a f t e r the f i r s t  secondary a r r i v a l a t the shooting  branches.  ship  comes i n  f o r most s h o t s b u t i s v i s i b l e f o r  I I  Fig.  4.1  Reduced r e c o r d s e c t i o n f o r r e f r a c t i o n p r o f i l e 75-2. The s o l i d t r i a n g l e s i n d i c a t e f i r s t a r r i v a l p i c k s ; t h e s m a l l e r ones c o r r e s p o n d t o more emergent a r r i v a l s . A l l t r a c e s have been f i l t e r e d from 5 t o 20 hz. E a c h s e t o f s i x t r a c e s i s l a b e l e d w i t h i t s c o r r e s p o n d i n g s h o t number. Amplitude c o r r e c t i o n with distance i s r . 2  Fiq,  4,2  Reduced  record  section  for  refraction  p r o f i l e  75-2E.  "  Fig.  4.3  "  "  Seduced  .  .  record  - -  section  I  for refraction profile  75-3.  Fig.  4.4  Reduced  record  section  for  refraction  p r o f i l e  75-3R.  40  some o f t h e d i s t a n t profile  s h o t s , f o r example a t 5.6 s f o r s h o t  75-3B.  Close examination unusual  o f t h e r e c o r d s e c t i o n s r e v e a l s many  characteristics.  t h e C.S.P. r e c o r d s w h i l e (Fig.  corresponds  to a fault  visible  possibly  ridge  than  starting is  or  others are not.,  at the foot  on e i t h e r  higher  side.  are r e l a t i v e l y  on s h o t  51 and l a t e r  The  corresponds indication  time  precisely  either  at  48 km h a s a v e r y  could  side.  be p a r t l y  up d i p .  i s d i p p i n g towards for this  i n c r e a s e s a r e observed  ( F i g . 4.2) i s  advance c e n t e r e d  45.  with  m a t e r i a l beneath  high apparent  correction  of  6.0 km/s f o r t h e c o r r e c t i o n  of  10.1 km/s f o r t h e b r a n c h . must  a t shot  This  Winona Ridge and i s a c e r t a i n  arrival  velocity  due t o t h e low v e l o c i t y  velocity  branch,  o f 9.8 km/s which  The a m p l i t u d e s  The f u r t h e s t f i r s t  the l a r g e topographic  correction  arrival  f e a t u r e o f p r o f i l e 75—2R  of higher v e l o c i t y  cn  first  head waves t r a v e l i n g  although  Ridge  m a t e r i a l beneath t h e  i f the oceanic crust  uniform  Winona  shots.  most s t r i k i n g  t h e 0.25 s t r a v e l  up t h r o u g h  velocity  beneath the c o n t i n e n t a l c r u s t .  profile  s h o t s 38 a n d 39  & 0.1 s a d v a n c e a t  The f u r t h e s t  upper mantle  75-2  of the continental slope,  velocity  a t 44 km h a s an a p p a r e n t  would be e x p e c t e d  On p r o f i l e  a d v a n c e between  to rays t r a v e l i n g  indicates  c o n s i s t e n t with  This  a r e c o n s i s t e n t with  on C.S.P. r e c o r d 75-2.  s h o t 49 c o r r e s p o n d s and  Some o f t h e s e  4.1) a 0.2 s t r a v e l t i m e  clearly  54 o f  branch,  than  starting  o f 20 km/s.  This  o f 2.0 km/s u s e d f o r  necessary.  results  the r i d g e  Using  a  velocity  i n an a p p a r e n t  Since t h e proper  velocity  topographic  be between t h e s e two e x t r e m e s  there  41  is  a strong implication  arrivals  were c r i t i c a l l y  west s i n c e greater  and  end  45  On at  8.0  which  must be d i p p i n g  v e l o c i t y i s not l i k e l y  Amplitudes to about  beyond w h i c h  they  for this  30  km.  these  towards  t o be  profile  the  much  are  They i n c r e a s e  between  decrease s i g n i f i c a n t l y  to the  profile.  profile  s h o t 44  km/s.  u n i f o r m out km  of the  refracted  the r e f r a c t i n g  than  relatively 32  t h a t t h e i n t e r f a c e from  75—3  could  ( F i g . 4.3)  correspond  a slight  to e i t h e r  t r a v e l time  a fault  or a  advance  positive  i  horizontal  velocity  gradient.  is  s h o t s 49  and  seen on  The  furthest  apparent  first  variation,  12.0  km/s,  Amplitudes  consistent  for this  out  strong  fade g r a d u a l l y  and  T h i s wide v a r i a t i o n  t o 32 km.  over 58  km  75-3R  Winona R i d g e w i t h an  They  f o l l o w e d by  of the  the  velocity  km/s  f o r the  first  velocity  s travel  arrival  o f 20 km/s.  correction  of g r e a t e r than  10.0  westward d i p p i n g i n t e r f a c e . from to  32  30 km  t o 46 and  km and from  a t 59  48 t o 54  no  to very profile.  result  of  km/s,  r e s u l t s i n an  km.,  and  advance  starting  As f o r  at  profile small.  apparent  again i n d i c a t i n g  Large amplitudes are km,  time  branch  75—2R t h e t o p o g r a p h i c c o r r e c t i o n s a r e p r o b a b l y t o o U s i n g 6.0  eastward  show c o n s i d e r a b l e  t o t h e end  h a s a 0.15  a final  apparent  an  Ridge.  ( F i g . 4.4) and  w i t h an  has  then i n c r e a s e  be  Ridge  advance.  a t 52 km  t o 25 km  i n amplitudes could  f o c u s i n g o f r a y s by Winona Profile  profile  f r o m 22  detectable arrivals km  s t r a v e l time  branch, s t a r t i n g  w i t h weak a r r i v a l s  a t 43  i n f l u e n c e o f Winona  as a 0.2  arrival  velocity of  dipping crust.  50  The  a  observed  v e r y weak a r r i v a l s  from  23  42  In  general  direction  are c o n s i s t e n t with  particularly apparent For is and  the s e t s of p r o f i l e s  one a n o t h e r .  t r u e f o r the e f f e c t  velocities  s h o t i n t h e same This i s  o f Winona R i d g e and t h e  of the furthest  first  arrival  p r o f i l e s 7 5-2 and 75—3 an e a s t w a r d d i p p i n g indicated  deep  f o r t h e west s i d e o f t h e b a s i n w h i l e  75—3H a westward d i p p i n g  deep i n t e r f a c e  branches. interface  f o r 75—2R  i s indicated f o r  the east side of the basin.  H±2  METHODS OF ANALYSIS  S e v e r a l methods e x i s t data.  The b e s t  profiling of  First  constant dipping  arrival  case  refraction  depends on t h e o f t h e area  most a p p l i c a b l e t o t h i s t y p e o f  analysis,  s y n t h e t i c seismogram  and r a y t r a c i n g .  Arrival  This  to  Those t e c h n i g u e s  are f i r s t  computation  1}  one t o u s e f o r a s p e c i f i c  seismic  method and t h e p h y s i c a l c h a r a c t e r i s t i c s  interest.  study  f o r interpreting  Analysis  method assumes a model which c o n s i s t s o f d i s c r e t e , velocity  layers separated  interfaces.  the p r o f i l e  velocity,  by p l a n e ,  T h e r e c a n be no l a t e r a l  line.  Mota  (1954)  arbitrarily variations  derives equations  for dip,  d e p t h and t h i c k n e s s f o r t h e n - l a y e r c a s e .  p r o f i l e must be s h o t directions.  Since  i n both  the forward  only the a r r i v a l  refraction  energy  a r e used,  amplitudes  and s e c o n d a r y  times  The  and r e v e r s e of the f i r s t  much i n f o r m a t i o n , s u c h  arrivals,  normal  i s neglected.  as relative  43  2)  S y n t h e t i c Seismogram A method  which  Computation  makes use  data i s the computation  o f the e n t i r e  of synthetic  devised  t o d a t e , such  (1976),  impose t h e r e s t r i c t i o n  DRT  a starting  analysis. generate turn curve and  a ray  and  to generate  portion  was  (DRT)  profiles  by  75—1  and  Wiggins  homogeneity. first  For  arrival  distance  which i n  (P-X)  curve  time c u r v e , The  travel  r e a l data  and i f  and  the  to  a velocity-depth  process  times  repeated.  (1S77) t o a n a l y z e t h e s e t o f  75—1R t a k e n a l o n g t h e  o f Winona B a s i n , p a r a l l e l  eastern  t o the c o n t i n e n t a l  L a t e r a l v a r i a t i o n s along  small, j u s t i f y i n g the  by  i s used  with the  Lynch  Techniques  v e l o c i t y - d e p t h curve  curve i s a l t e r e d  used  F i g . 1.2).  likely  of l a t e r a l  a travel  a r e compared  t h e P—X  method  reversed  arrival  parameter vs.  amplitudes  This  theory"  a s e t of s y n t h e t i c seismograms.  necessary,  (see  first  of s e i s m i c  seismograms.  model i s o f t e n d e r i v e d f r o m  The  i s used  as " d i s c - r a y  suite  assumption  margin  these l i n e s  of  are  most  lateral  homogeneity.  3)  Ray  tracing  A method w h i c h i m p o s e s a minimum i n model ray  tracing.  arbitrarily  The  model c a n c o n s i s t  s h a p e d z o n e s o f any  Rays are s e n t out from and  " t r a c e d " through  i n t e r f a c e s and the  g r a d i e n t zones.  velocity  the o r i g i n  t h e model by  following The  t h e s u r f a c e i s computed  o f any  restrictions i s  number o f  or v e l o c i t y  at egual angular  and  increments  a p p l y i n g S n e l l * s Law  the a p p r o p r i a t e t r a j e c t o r y travel  gradient.  time  plotted  f o r each on  at  the  through  ray to r e t u r n t o  a time-distance curve  44  for  comparison  amplitudes spacing  with  the r e a l  c a n be o b t a i n e d  of a r r i v a l s  An i n d i c a t i o n  by o b s e r v i n g  on a r a y p l o t t h e  to increased  amplitudes.  Choosing a  model f o r a c o m p l e x a r e a c a n be v e r y  requires consideration of f i r s t  d i f f i c u l t and  arrival information,  geology,  r e g i o n a l t e c t o n i c s and any i n f o r m a t i o n  obtained  from o t h e r  Say in  h a s b e e n used  Gebrande  As record  (1976)  by C l e e e t a l (1974) a s an a i d  and G r u b b e  demonstrated  strong l a t e r a l  travel  studies of  4.1, c l o s e e x a m i n a t i o n w i t h t h e C.S.P.  v a r i a t i o n s i n both  time advance observed  i s i n d i c a t e d by p r o f i l e The h i g h  refraction  apparent  Winona  higher  side.  75-2 and p o s s i b l y b y  A major profile  v e l o c i t i e s a t t h e ends o f a l l f o u r  l i n e s i n d i c a t e i n t e r f a c e s deep w i t h i n t h e c r u s t  which d i p from  both  These o b s e r v a t i o n s  s i d e s o f t h e b a s i n towards immediately  an i n t e r p r e t a t i o n  technique  inhomogeneities  a laterally  s e t s of p r o f i l e s .  indicates significantly on e i t h e r  o f the  records  c o n s i s t e n t l y over  v e l o c i t i e s beneath the ridge than  lateral  and by M i l l e r  (1976) i n r e f r a c t i o n  i n Section  B i d g e on a l l p r o f i l e s  as  reflection-refraction  Northwest T e r r i t o r i e s ,  sections i n conjunction  75-3.  which c a n be  Europe.  indicates  fault  surface  methods.  of a detailed  near Y e l l o w k n i f e ,  Central  The  geophysical  the i n t e r p r e t a t i o n  study and  tracing  of r e l a t i v e  a t t h e s u r f a c e ; many a r r i v a l s i n a s h o r t  distance correspond starting  data.  r u l e o u t s y n t h e t i c seismograms f o r these  profiles.  a r e much t o o s e v e r e  homogeneous model.  the center.  The  t o attempt  Furthermore, f i r s t  t o use  arrival  45  a n a l y s i s cannot  be u s e d  appear  t o be  extend  over the e n t i r e  apparent  interfaces  velocities  means t h a t  the  identifiable This  a t the and  be  o b t a i n e d from  unreversed  first  refraction  line.  established  arrival The  results;  r e g u i r e s assumptions hypothetical  4.3  Having  plots.  The  several  layers  and  desired  no  two  existing  reguirements  lines.  This  easily  branches.  profiles  from C h a p t e r the s t a r t  a  inter-  starting  75-1 3  and  75-1B  and  of  each  provide  well  however, t h e f i r s t  arrival  analysis  concerning  not  high  n o t have  for selecting  a n a l y s i s of  apparent  which w i l l  velocities  of  give reasonable  layer  dips.  DESCRIPTION OF  r a y s and  type  of  with  tracing  produce  as the  travel  was  was  shaped  or l i n e a r  programs i n t h e  most  suitable  data i t  program t o p e r f o r m  needed  arbitrarily  a decision  PJOGMI-  o f the r e f r a c t i o n  program  velocity  TR1.CTJS  MI  t o o b t a i n a computer  of the  the  there  do  sources  method o f i n t e r p r e t a t i o n  any  results  first  do  arrival  refraction  d e c i d e d upon r a y  tracing  by  as the o n l y a c c e p t a b l e  reverse profiles  t h i c k n e s s e s and  necessary  first  Information  1977), r e f l e c t i o n  indicated  reverse lines  leaves ray t r a c i n g  model c a n  as  not p l a n e , or  ends o f t h e r e f r a c t i o n  corresponding  technique.  s e c t i o n s because  which a r e e i t h e r profile,  forward  pretation  (Lynch,  f o r the o v e r a l l  time one  d e p a r t m e n t met made t o d e v e l o p  the  actual  c u r v e s and which  polygonal  velocity  was  could  the  handle  boundaries  gradient. these  ray  Since  specific program  here.  46  The several layer  completed polygonal  program, w r i t t e n b y Ken W h i t t a l l , s h a p e d l a y e r s as i n p u t .  i s defined as being  varying l i n e a r l y  with  leave the o r i g i n  at egual  constant  along  The v e l o c i t y i n a i t s t o p boundary and  depth normal t o t h i s angular  boundary.  increments  over  r a n g e o f a n g l e s . , S i n c e a l l l a y e r s have n o n - z e r o gradients,  a l l ray paths  are circular  c e n t e r d e p e n d i n g on t h e v e l o c i t y  a r c s with  intersects  another  velocities  on e i t h e r s i d e o f t h e i n t e r f a c e ,  trajectory  i s computed  for  each c i r c u l a r  a  Rays specified  linear r a d i u s and  and g r a d i e n t .  boundary, S n e l l ' s  accepts  I f a ray  law i s a p p l i e d u s i n g t h e and a new  from t h e new g r a d i e n t .  segment i s c a l c u l a t e d  circular  The t r a v e l  time  and when t h e r a y i  e v e n t u a l l y r e t u r n s t o the s u r f a c e the t o t a l determined  by summation  distance.  Constant  by  specifying The  theory  velocity  a very  generation  and p l o t t e d  small  generated  waves i s n o t p r e d i c t e d by r a y  i n t e r s e c t s a boundary critical regular  reason,  artificially  intervals  traveling The  o f wave p r o p a g a t i o n pseudo head  by t h e r a y t r a c i n g  along  refracted  ray tracing  model w i t h  r a y paths  program  angle  rays are i d e n t i f i e d  of the  wave  at  t h e upward arrivals.  p r o d u c e s a computer  superimposed  full  I f a ray  rays a r e generated  t r u e head  vs. d i s t a n c e f o r a l l r a y a r r i v a l s . reflected  program.  t h e boundary t o s i m u l a t e  energy a s s o c i a t e d with  fora  wave a r r i v a l s a r e  w i t h i n some s p e c i f i e d  angle, c r i t i c a l l y  approximated  gradient.  o f head  For t h i s  against i t s a r r i v a l  layers are c l o s e l y  and r e q u i r e s t h e t h e o r y  description.  t r a v e l time i s  plot  and a p l o t o f t r a v e l On t h e t r a v e l  by c r o s s e s and head  time  o f the times plots,  waves by  47  X's.  The t r a v e l  time  plot  c a n be r e d u c e d  plotted  a t any s c a l e f o r e a s y  of  data.  real  travel and  Once a good  comparison  t o a n y v e l o c i t y and with  amplitudes. certainly travel  c a n be u s e d Fittinq first  a forward  as a q u a l i t a t i v e  a unique  time  measure o f profile  model; however,  i f the  a m p l i t u d e s c a n be s a t i s f i e d f o r b o t h  and r e v e r s e p r o f i l e , t h e n t h e r e l i a b i l i t y  model i s i m p r o v e d  arrival  on t h e t r a v e l  arrivals f o ra sinqle  does n o t q u a r a n t e e  t i m e s and r e l a t i v e  sections  f i t has been made t o f i r s t  times the c o n c e n t r a t i o n of a r r i v a l s  ray plots  record  of the  s i q n i f i c a n t l y , althouqh i t i s s t i l l not  unique.  i E £ L I £ A T I Q I O f RAY  In  order t o apply  interpretation first  generated compared  A ray plot  f o r both  relative  Profiles  technique t o the  profile, a starting  and t r a v e l  model  data.  until  amplitudes  must  time c u r v e a r e then  t h e f o r w a r d and r e v e r s e d i r e c t i o n s  with t h e r e a l  made t o t h e model the  the r a y t r a c i n g  of a r e f r a c t i o n  be c h o s e n .  TRACING  and  I f necessary, a l t e r a t i o n s a r e  the t r a v e l agree  time  f i t i s a c c e p t a b l e and  qualitatively.  7 5 - 3 and 7 5 - 3 R :  To i l l u s t r a t e t h e d e t a i l e d  application  technique the s e t o f reversed p r o f i l e s  of the r a y t r a c i n g  75—3 and 75—3R  w i l l be  used. a)  The S t a r t i n g  Model  The s t a r t i n q  model  for profile  75-3  i s shown i n F i q . 4 . 5 .  Fig.  4.5  S t a r t i n g model f o r r a y t r a c i n g of p r o f i l e 75-3. The i n s e t shows t h e f i n a l v e l o c i t y - d e p t h c u r v e s f o r r e f r a c t i o n p r o f i l e s 75-1 and 75-1R a r r i v e d a t by L y n c h (1977) u s i n g DRT s y n t h e t i c s e i s m o g r a m s . T h e s e were used a s a c o n s t r a i n t f o r t h e c r o s s - b a s i n p r o f i l e s . Numbers w i t h i n model b l o c k s a r e v e l o c i t i e s i n km/s.  r  j> '  ! •  ) \  0 0  '  49  The  results  reversed  of the f i r s t  p r o f i l e s 75-1  by  the v e r t i c a l l i n e  of  constraints  inset  that  t o p few  obtained few of  DRT  layers  24 km,  program, b a s e d on  for  the beginning of the  cannot  be d e t e r m i n e d  effectively  seismograms. o f the s e c t i o n  because  satisfy  n o t r e v e r s e d . , The  arrivals  virtually  be r e d u c e d ,  first  sub-bottom  results second dip  of Chapter layer  o f 11"  3.  i s consistent Choosing  i s consistent  4° to the e a s t  f o r the  km/s  with the  secondary  certain For  f o r the  reflection  of 4.3  w i t h p r o f i l e 75—1  second.  on  number o f  o f 2.5  a velocity  t o t h e west f o r t h e f i r s t  times  layers  o r d i p s o f the l a y e r s .  an a v e r a g e v e l o c i t y layer  The  are  however, by a p p l y i n g  r e s t r i c t i o n s t o the v e l o c i t i e s example, a s s u m i n g  velocities,  from t h e s e l a y e r s  and  can  was  segments a r e  r e v e r s e p r o f i l e 75-3B, i f p r e s e n t a t a l l ,  possibilities  1954)  the t r a v e l  the  impossible to i d e n t i f y .  A  velocity,  A unigue s e t o f  the p r o f i l e  km.,  (Mota,  with d i f f e r e n t  first  assumption  10 t o 20  plane l a y e r s  profile.  were  a n a l y s i s on t h e  the eguations f o r d i p ,  a l l o f which  The  amplitude  over the f i r s t  used t o g e n e r a t e s e t s o f l a y e r s  set  curves f o r these  and  arrival  thickness f o r several  dips,  crust.  of p r o f i l e 7.5-3, w i t h an  plane, continuous layers  t h i c k n e s s e s and  are represented  the e n t i r e  traveltime  synthetic  of the set of  provide the only  on t h e e a s t end  t r a v e l time branches  d e p t h and  and  1977)  velocity-depth  by p e r f o r m i n g a f i r s t  computer  (Lynch,  throughout  from a  using  interpretation  75-1S  through  final  p r o f i l e s determined  The  and  extend  shows L y n c h ' s  interpretation  arrival  but  sub-bottom  km/s  f o r the  reguires a interface  Conseguently, a v e l o c i t y  and of  50  3.5  km/s  was  chosen  as  because i t r e s u l t e d Applying layers  the  same p r o c e d u r e t o  east.  starting As  these  Since  a crust thickening  a l l profiles  represented  on  the  w i t h v e l o c i t y 3.0  layers.  Fig.  or  f o r the  as shown  towards the  and In the  the  be  the  not  of  i s no  shot  This  material  used t o c o m p l e t e  was  and  is  further the  The  made s i m p l y  by  intersecting  the  with s u b d u c t i o n  i s necessary to r e f l e c t The  Ridge.,  deeper l a y e r s .  sufficient  75—3R was  topography.  C.S.P.  west s i d e o f  continent  and  whether  f o r a s t a r t i n g model. directly  the  o v e r 75-3  (see  d i f f e r e n t than  differences in  for  profile  same s i t u a t i o n a l s o a p p l i e s  to  75-2R. summary, t h e  main c o n s t r a i n t s on  r e s u l t s of p r o f i l e  75-1,1R,  approximately reversed,  i i i ) the  v i e w s on  and  typical  towards  i n F i g . 4.5  i s consistent  i t will  be  i t s s t a r t i n g model i s s l i g h t l y  and  the  t h i s there  1.2),  length  with  block  profile  This  i)  not,  the  Since  75—3.  75-2  This  in  a t r a v e l time advance i s  which can  model, p a r t i c u l a r l y  them d i p p i n g  correct  Beyond  information  having  west.  7.5—1, a l l d i p p i n g  f o r s h o t s o v e r Winona  km/s.  of these l a y e r s  75—1  4.2,  s t a r t i n g model by  choice  to the  75-3.  i n Section  o b s e r v e d on  starting  to those of  l a y e r s were used f o r the  mentioned  layer  p r o f i l e 75—3R r e s u l t e d  this i s consistent  model f o r p r o f i l e  seismic  for this  i n both i n t e r f a c e s d i p p i n g  ( F i g . ..1.4) and  continent,  direct  s t a r t i n g value  with s i m i l a r v e l o c i t i e s  towards the record  the  plate tectonics oceanic  crust  to  v)  the models  i i ) the  profiles  C.S.P. r e c o r d s , the  crust  are  iv)  thickening  a thicker continental  being  crust  current from  (up  to  30  51  km) to  from Winona B a s i n t o the c o n t i n e n t a l s h e l f .  For any model  be a c c e p t a b l e i t must be c o n s i s t e n t with these  constraints. b) T e s t i n g the Models The ray p l o t s and  reduced t r a v e l time curves f o r ray  t r a c i n g on the s t a r t i n g models f o r p r o f i l e s 75—3 and 75-38 are shown i n F i g s . 4.6  and 4.7.  The c i r c l e s connected by  l i n e s on the t r a v e l time c u r v e s represent the f i r s t times taken from the r e c o r d s e c t i o n s  dashed  arrival  (Figs. 4. 3 6 4. 4).  For  p r o f i l e 75—3 the agreement i s q u i t e good over most o f the p r o f i l e , the a r r i v a l s being 250 l a t e from 50 km t o 60 km.  ms l a t e a t 24 km and 100  For 75-3R however, the agreement i s  good only at the s t a r t , which i s expected because in  which the i n i t i a l model was  remainder  of the p r o f i l e .  Winona Ridge 3.7  km/s,  (11 to 21 km)  ms  of the  way  c o n s t r u c t e d , and poor over the  To make the a r r i v a l s e a r l i e r the r i d g e v e l o c i t y  was  over  i n c r e a s e d to  more r e p r e s e n t a t i v e of c o n s o l i d a t e d sediments.  order t o g r e a t l y i n c r e a s e the apparent v e l o c i t y o f the  In  first  a r r i v a l s beyond 50 km some o f the l a y e r s a g a i n s t the east s i d e of  the b a s i n were made to d i p towards the west.  g r e a t l y improved  These changes  the 75-3R f i t without s e r i o u s l y a f f e c t i n g the  reasonably qood f i t of the 75-3  model.  Many subsequent  trials  were made, with the t r a v e l time f i t s g r a d u a l l y improving at each s t e p . c) The F i n a l Models E v e n t u a l l y , models were found which and 75-3R t r a v e l times very w e l l . superimposed,  These  are shown i n F i g s . 4.8  s a t i s f i e d both  75-3  models, with r a y s  and 4,9 along with t h e i r  52  ig.  4,6  Ray p l o t and t r a v e l time c u r v e f o r 7 5 - 3 startinq model. The d a s h e d l i n e on t h e t r a v e l t i m e c u r v e r e p r e s e n t s t h e f i r s t a r r i v a l p i c k s t a k e n from the record section. Numbers w i t h i n b l o c k s o u t l i n e d by h e a v y d a s h e d l i n e s a r e v e l o c i t i e s i n km/s.  53  Fig.  4.7  Bay p l o t and t r a v e l t i m e c u r v e f o r 75-3R s t a r t i n g model. The model i s i d e n t i c a l t o 7 5 - 3 e x c e p t f o r d i f f e r e n c e s a r i s i n g f r o m t h e two p r o f i l e s n o t being e x a c t l y c o i n c i d e n t . Note t h a t t h e model i s r e v e r s e d , west b e i n g on t h e l e f t , compared w i t h F i g . 4.6. Numbers w i t h i n b l o c k s o u t l i n e d by heavy dashed l i n e s a r e v e l o c i t i e s i n km/s.  54  respective  travel  from  a l l head  which  t h a t head as  first  profile next  arrivals, (beyond  48  ccme i n a l m o s t  of the  their  slight  the  simulating  3.5  branch of  interface  arrival  model t r a v e l  travel  from  This  velocity  velocities  very d i f f i c u l t  satisfactorily. corresponding  km/s  times  could  time f i t i s very  gradient  modeled  through  which  arrival  arrivals  from  a change i n  branches  agree  km very  the t h i c k n e s s o f t h e  a p p r o x i m a t e l y 3.7  km/s  As f o r p r o f i l e  This  f o r the r i d g e  75-3,  The  travel  upper  limit  ridge of  time curve f o r  f i t t o the observed  time a d v a n c e o v e r Winona R i d g e  modeled and t h e h i g h a p p a r e n t  have  velocity.  t h e model t r a v e l  75-3R shows a v e r y good  well  same r e s u l t s c o u l d  i m p l i e s an  by  would have made  36 t o 48  Winona R i d g e  of  produced  been o b t a i n e d by d e c r e a s i n g b o t h i t s velocity.  by  i n the v i c i n i t y  have been  of t h e s e l a y e r s  early  t o paths  was  but w o u l d have r e g u i r e d  to f i t the f i r s t The  km  The  arrivals.  be  profile  with the observed t r a v e l times.  profile  first  layers together,  same e f f e c t c o u l d  i n the sediments,  and  the  i s shown with  time c u r v e f o r  arrival travel  and t h e 4.3  a horizontal  d i p s and  layer  alcne  s i m u l t a n e o u s l y as  t h e bottom  found  each  plus a r r i v a l s  a d v a n c e o b s e r v e d a t 22  km/s  boundary.  fault  i t was  without i t .  The  placing  boundaries  i n t e r f a c e never appear  these a r r i v a l s  shows t h a t t h e f i r s t  good.  it  km)  the  were r e f r a c t e d ,  the f i n a l  marks s i n c e t h e f i r s t  Examination  the  a l t h o u g h on  For t h i s r e a s o n  satisfied  75-3  By i d e n t i f y i n g  t h e bottom  higher i n t e r f a c e  question  curves.  wave b r a n c h e s  waves from  arrivals.  a  time  velocity  of the f i n a l  first i s well branch  has  55  Fig.  4.8  Ray p l o t and t r a v e l t i m e c u r v e f o r 75—3 f i n a l model. T h e g u e s t i o n marks on t h e bottom interface i n d i c a t e t h a t the f i r s t a r r i v a l t r a v e l t i m e s c o u l d be s a t i s f i e d w i t h o u t a r r i v a l s f r o m t h i s b o u n d a r y ; t h e r e f o r e i t s p o s i t i o n i s not d e f i n i t e . Numbers w i t h i n b l o c k s o u t l i n e d by heavy d a s h e d l i n e s a r e v e l o c i t i e s i n km/s. V e r t i c a l e x a g g e r a t i o n i s 2X.  I  Fig,  4.9  56  Ray p l o t and t r a v e l t i m e c u r v e f o r 7 5 - 3 R f i n a l model. The model i s i d e n t i c a l t o 7 5 - 3 e x c e p t f o r d i f f e r e n c e s a r i s i n g from the two p r o f i l e s n o t being e x a c t l y c o i n c i d e n t . Note t h a t t h e model i s r e v e r s e d , west b e i n g on t h e l e f t , compared w i t h F i g . 4.8.  57  been a c h i e v e d by t h e at  the e a s t s i d e o f t h e The  single  head  arrivals.  This  arrivals  observed About  reflected stronger  and  t o 32  over t h i s  corresponds well  s later  on  arrival  34 t o 48  km  is  good.  areas of apparent  likely  and  on  of f a u l t s ,  to  with the o f 75-3  weak  first  o v e r t h e same  time curve a the  i s o b s e r v e d on  record  the  wave branch  from  observed  and t h e  large  large  observed there. in identifying  secondary  not p o s s i b l e  arrivals  to constrain  However, c l o s e c o m p a r i s o n time  arrivals  plots  since  on  the  f o r 75—3  and  c a n be  both s e t s o f  t h e models c o n s i s t  while  the  of  tentatively  the s e c t i o n s the amplitude  contradiction  material  km.  refracted  Of c o u r s e , i t i s n o t d i f f i c u l t  expected  velocity  zones  i s expected  75-38 t h e weak a r r i v a l s  some s e c o n d a r y  Elsewhere  t o 30  coincide to create  with the ray t r a v e l  identified.  T h i s i s t o be  20  a r e g e n e r a t e d by t h e  s e c t i o n s , i t was  75—3fi shows t h a t  generally  from  on the model t r a v e l t i m e c u r v e  models by t h e s e a r r i v a l s .  exception  which  for profile  of a r r i v a l s  sections  shows a  t h e model t r a v e l  branch  to the d i f f i c u l t y  on t h e r e c o r d  constant  section  a r e g e n e r a t e d by t h e head  concentration  record  arrival  with other r e f l e c t e d  a refracted  from  layers  7 5-3  i t s amplitude  on t h e r e c o r d  0.3  range  amplitudes  Due  branch  Similarly km  as t h e f i r s t  wave a r r i v a l ,  secondary  section. 22  westward d i p p i n g  basin.  wave b r a n c h  s m a l l i n comparison  range.  o f the  model t r a v e l t i m e c u r v e f o r p r o f i l e  B e i n g a p u r e head be  position  in actual fact,  to  agreement find  profiles., of blocks of with the  t h e b o u n d a r i e s between l a y e r s a r e most  of increased  velocity  gradient  with  very  smooth  58  lateral  variations.,  Profiles  75-2 and 7 5-2R:  The  modeling  was a l m o s t final and  identical  models, with  4.11 a l o n g  very w e l l with the l a s t  to that  their  for profiles  used  7 5-2 and 75-2B  f o r 75—3 and 75-3R.  o f 75-2R.  arrivals,  The t r a v e l  with  time  4.10  curves.  c u r v e s f o r p r o f i l e s 75—2 and 75-2B first  The  a r e shown i n F i g s .  r e s p e c t i v e t r a v e l time  the observed  branch  used  rays superimposed,  with  model t r a v e l t i m e  of  procedure  The  agree  the exception  advance  observed  between s h o t s 38 and 39 f o r 75-2 was g e n e r a t e d  by a 0.7  km  vertical  profiles  75-3  and  fault  i n the sediment  75-3B a v e l o c i t y  generated  o f 3.7 km/s  20 t o 32 km on 75-2R.  on t h e v e l o c i t y  a l s o have been u s e d . time c u r v e ,  arrival still  for  ( F i g . 4.1).  d i p s were n e c e s s a r y  high apparent  velocity  layer  the deepest  e v e n more would c e r t a i n l y  could  travel  interface first  However, t h i s i n t e r f a c e i s  layers  c o u l d be  satisfied  slightly.  on t h e e a s t s i d e  of the b a s i n  f o r 75-3 and 75-3B t o a c h i e v e  v e l o c i t y required f o r the l a s t  75-2H, which i s s t i l l  since the l a s t  i s an upper  c o r r e s p o n d i n g t o t h e weak  p r o f i l e s 75-2 and 75-2R t h a n  the very of  from  i t by v a r y i n g t h e o v e r l y i n g  Steeper  32 t o 40 km on  Again, t h i s  shown as q u e s t i o n a b l e s i n c e t h e d a t a  without  layer  Beyond 56 km on t h e 75-2 model  arrivals,  o f s h o t 54  a d v a n c e from  as a t h i n n e r , lower  head wave a r r i v a l s  a p p e a r as f i r s t  As w i t h  f o r t h e Winona R i d g e  the r e q u i r e d t r a v e l time  75—2 and from limit  layer.  not s a t i s f i e d . f i t the t r a v e l  two s h o t s o f t h e p r o f i l e  Steepeninq times  branch  the d i p  b e t t e r but  were i n a d v e r t e n t l y  Fig.  4. 10  1  Fig.  4.11  Bay p l o t and t r a v e l t i m e c u r v e f o r 75—2B f i n a l model. The model i s i d e n t i c a l t o 75-2 e x c e p t f o r d i f f e r e n c e s a r i s i n g from the two p r o f i l e s not being e x a c t l y c o i n c i d e n t . N o t e t h a t t h e model i s r e v e r s e d , west b e i n g on t h e l e f t , compared w i t h F i g . 4. 10.  60  61  shot over the c o n t i n e n t a l s h e l f the l a r g e t o p o g r a p h i c c o r r e c t i o n s r e q u i r e d say be i n e r r o r .  For t h i s reason and  a l s o because the d i p a l r e a d y seemed s t e e p , no f u r t h e r changes were made. The amplitudes on the r e c o r d s e c t i o n s f o r p r o f i l e s and  75—28  do not show as much v a r i a t i o n as f o r  75—3  and  75—2 75-3R.  However, the c o n c e n t r a t i o n s of a r r i v a l s on the model t r a v e l time curves do seem t o agree i n g e n e r a l with the observed results.  For p r o f i l e 7 5 - 2 t h e head wave branch from  12 t c 16  km corresponds to the weak f i r s t a r r i v a l s observed t h e r e and the l a r g e amplitudes past 4 4 km correspond t o the a r r i v a l of two head wave branches.  For p r o f i l e  model a r r i v a l s are very uniform out t o 3 0 km, concentrated and extended  from 3 0 t o 4 8 km,  with the observed amplitudes.  simultaneous  75-2B  the  becoming more  i n good aqreement  Beyond 4 8 km however, the  amplitudes are s m a l l e r than would be expected from the c o n c e n t r a t i o n of a r r i v a l s on the model t r a v e l time curve. T h i s i s p o s s i b l y an e f f e c t of the c o n t i n e n t a l s h e l f and s l o p e over which t h i s part of the p r o f i l e was The f i n a l  shot.  models presented are by no means unique.  combinations of l a y e r s could be found t r a v e l times e q u a l l y w e l l .  Other  which would s a t i s f y the  However, by r e s t r i c t i n q  layer  v e l o c i t i e s to accepted values f o r s i m i l a r c r u s t a l s e c t i o n s and by c o n s i d e r i n q only g e o l o g i c a l l y and  tectonically  s i t u a t i o n s , i t would be very d i f f i c u l t  feasible  t o a r r i v e a t other  s u i t a b l e models which would not have the same qross properties.  62  5_i  DISCUSSION  5,1  SEDIMENTS  Very r e c e n t l y t h e r e s u l t s reflection  profiles  Vancouver I s l a n d Ltd.  were s h o t  over  p a r t s o f Winona B a s i n ,  much d e e p e r p e n e t r a t i o n  than our r e f l e c t i o n  accurate  t o 4.5  determined  km  while  75-3  ( F i g . 1.4)  shows.  undisturbed,  Down t o 0.6  frcm  using  much t h i c k e r f l a t  length.  they  is little  lying  penetration  75-3,  2.7  0.9  s o f very  reflections  a r e o b v i o u s on t h e C h e v r o n s e c t i o n .  preliminary  and s i m p l e  NMO  at the foot  t o the t h i c k  Whereas o n l y  beyond  Towards t h e  sediments occur  record,  lying  a r e f o l d e d and deformed  corresponding  a r e i n d i c a t e d on t h e C.S.P.  with  s of sediments at  s t h e sediments appear f l a t  o f C. S. P. . r e c o r d 7 5 - 3 .  a  a l m o s t 2 s more t h a n o u r  and C.S.P. r e c o r d  the c o n t i n e n t a l slope  region  t h a n 2 km  b u t beyond t h i s  s on b o t h 7 5 - 3 V  ranged  were c o l l e c t e d  a n d shows 2.5  75-3V,  l o c a t i o n of p r o f i l e  continent of  i s more  p r o f i l e s c o i n c i d e s almost e x a c t l y  p o s s i b l y e x p l a i n i n g why t h e r e 0.6  a n d f a r more  from our p r o f i l e s  streamer o f l e s s  C.S.P. r e c o r d  and  and t h e s t a c k e d  However t h e v e l o c i t y -  the Chevron data  One o f t h e C h e v r o n  profile  profiles.  Standard  air-gun  because our s h o t - t o - r e c e i v e r d i s t a n c e s  multichannel  the  o f t h e 2400S& c o v e r a g e  Several  depth i n f o r m a t i o n  0.5  i n 197 2 o f f t h e west c o a s t o f  taken  r e c o r d s e c t i o n s show detail  set of  were made a v a i l a b l e t o us by C h e v r o n  of Calgary,  profiles  o f an e x t e n s i v e  sediment  s of sediments strong From a  a n a l y s i s . Chevron has i n t e r p r e t e d a  63  similar  section  as c o n s i s t i n g  velocity  material overlying  velocity  material.  These  2.5  fault  the  s of r e l a t i v e l y  to  0.5  reflection 0.5  km/s  75-3  t h e water  w h i c h shows o n l y bottom  o f 0.8  arrival.  km  Ridge,  i n very  basement  beneath 0 . 7  good  agreement  can e a s i l y  sediments to the east obscured  by a t l e a s t  1.3s  sharply  by a  reflection  Chevron  a velocity  section  of 2.6 -  sequence  shows t h e  s o f s e d i m e n t s on P a u l  Revere  dipping  of d e f o r m e d  Severe  profile  beneath  Ridge before  2.7  (see T a b l e  clearly  reflection  up  with  one c l e a r  section  with  To  sediments o v e r l y  The  f o r this  be f o l l o w e d  of Paul  average  average  This agrees well  To t h e s o u t h w e s t t h e C h e v r o n  v o l c a n i c basement  km/s  km/s  o f the c o n t i n e n t a l s l o p e .  a dip of 4°- 6° i n d i c a t i n g  and a t h i c k n e s s  II).  o f 3.7  undisturbed  sediments.  profile  s beyond  indicates  The  at the foot  2 s o f deformed  km  o f 2.05  km  sediments are truncated  deep v e r t i c a l east,  o f 1.3  75—3R.  1.5  s of  becoming  sediments beneath  Winona R i d g e . No  other Chevron  study although  two  the  10 km  basin,  one  and a n o t h e r 10 km first the  Chevron  east  of  profile,  were s h o t  To  with those i n  over the e a s t e r n  parallel  o f and p a r a l l e l  part  to p r o f i l e  to 75-2i  As  this of  75—2R with  the  up t o 3 s o f s e d i m e n t s a r e t r u n c a t e d  vertical  fault  the east  undisturbed  sediments.  i s coincident  s o u t h o f and  north  slope.  relatively  deformed  others  by a deep  continental  profile  a t the foot  of t h i s ,  This i s consistent  (Fig.  3 . 5 ) , which  shows cne c o r r e l a t a b l e  after  t h e bottom r e f l e c t i o n .  of t h e  approximately 0.5  sediments o v e r l y  up t o 2 s o f  with p r o f i l e reflection  As d i s c u s s e d  on  75—2 0.5  in section  s  3.3,  s  64  profile  75-2V  ( F i g , 3.1), o v e r t h e c e n t r a l  shews f o u r r e f l e c t o r s w e l l a s one  deeper  b e f o r e the bottom Chevron  data.  o f a b o u t 5.5 preliminary velocity  Deeper  3.6  km/s  multiple.  reflection  This corresponds well  km/s.  1.3  to  km/s  to p r o f i l e  Chevrons  relatively  undeformed  2 s of f o l d e d  25 km  shows 0.7  75-2JR. , The  thickness  s of sediments  data.  deformed  sediments  cf  E  and  overlying  agreement  d i p s beneath  with  2 s cf  o b s c u r e d by  up  the b u r i e d n o r t h e r n  clearly  4 km  by Couch  agreement  of winona  meters  Ridge  at  of the c o n t i n e n t a l  with the 4 - 6  (1969) on t h e b a s i s o f  compression.  slow convergence  show a s t e a d y  hundred  at t h e f o o t  f e a t u r e s of the s e c t i o n s ,  northeast-southwest either  profiles  i s in qualitative  Several  basement  t h i c k n e s s f r o m a few  R i d g e t o about  predicted  1.3  Ridge.  i n c r e a s e i n sediment  This  a  km  t o the northwest o f  s e d i m e n t s b e f o r e becoming  In summary, t h e C h e v r o n  slope.  2.5  of r e f l e c t o r  sediments which r e p r e s e n t  o f Winona  above,  s.  75-2  profile  P a u l Revere  traveltimes  data i n d i c a t e  v o l c a n i c basement a t P a u l S e v e r e R i d g e , i n good  extension  with the  w h i l e t h e s u g g e s t e d deep r e f l e c t o r i s  profile  reflection  as  just  2-way t r a v e l t i m e o f  The i n t e r v a l v e l o c i t y  Another Chevron  to  arrival  o f m a t e r i a l with average  This gives a  i n the s e c t i o n .  i s 3.63  km  basin,  (E) and one  of strong r e f l e c t i o n s  a n a l y s i s gave  a t r a v e l t i m e o f 5.3  parallel  s of the s e a - f l o o r  s a r e shown c l e a r l y , a n d , a s m e n t i o n e d  sediments.  ( T a b l e I) at  correlatable  A series  o f 2.05  s.  w i t h i n 1.3  p a r t o f the  km  gravity  particularly  the  a p p e a r t o be t h e r e s u l t This could  be c a u s e d  of the E x p l o r e r - A m e r i c a n p l a t e s  by or  of  65  subduction of The  vertical  similar of  the  to  the  Explorer  fault  that  at  the  plate foot  o b s e r v e d on  Queen C h a r l o t t e  o f the  the  American  continental  C.S.P. r e c o r d s o f f  Islands  i n d i c a t e a component o f  beneath  (Chase e t  lateral  plate.  slope  the  is  west  a l , 1975)  and  coast could  motion r e s u l t i n g from  oblique  subduction,  .!LL2  HAI  2 1 I C I N G MODELS  Before discussing  the  tectonic implications  tracing  models, a  resolution mentioned entire and is  be  words s h o u l d be  expected  previously,  crust  75-1E, of  to  few  the  are The  structure.  only  uncertainty 1 -  2 km.  depends on  The  various  from  i n the The  preconceived  possible.  few  layers  reguired and  The  to f i t the  i f the  cannot  first  the  t r a v e l t i m e s at the  much.  Of  course  the  notions  the of  ends of  models i s tectonic  primarily  the  s t a r t of to  be  to  are  models  the  assumed  t h i s assumption  75—1  layers  in their positions  v e l o c i t i e s f r o m 75-1,1H a r e  vary too  invalid  at  As  of these  d e e p e r l a y e r s h a v e been c h o s e n variations  the  refraction profiles  remainder of  and  ray  which a p p l y t o  positions  s a t i s f y t r a v e l times  the  layers.  constraints  those obtained  order of  n o n - u n i q u e and  the  for the  s a i d about  of  profiles they  would  i f s i g n i f i c a n t horizontal v e l o c i t y gradients  are  be  are  present. In  light  appears at thin.  of the  first  data r e c e n t l y  glance that  the  However a r e - e x a m i n a t i o n  acguired  from Chevron, i t  sediment l a y e r chosen i s of  profile  75-1,1R  results  too  66  shows t h a t  the  4.3  e x t e n d s f r o m 2.2 One  of  km/s  to  the Chevron  southeast  of  sedimentary  and  2.8  sediments t o  2.5  and  parallel  the  4.3  of  km/s  a lower  of  the  of  a higher  ridge  The  profile  75—1B and  entire profile of b u r i a l  3.7  km/s  to  the  shows d e f i n i t e  length  i t i s not km/s.  for  this  unreasonable  Thus b o t h the  the  s i d e of  very  high  75-2B and  velocity. not  75-3B.  the ray  no  on  required  the  i t could  c r u s t over  of the and  of  represent  crust  satisfy  still  satisfy  of be the  to  the  horizontal velocity  models i s under  the  a t r a n s i t i o n from However, i n  a negative  m o d e l , would be  the the  layers could  layers dipping  this distance.  to c o n t i n e n t a l t o the  of  to  t r a v e l time curves  both  slope  contrary  l a y e r s on  part  continental  from o c e a n i c  westward d i p p i n g  introducing  this  sediments  important  without  Since  indicative  surrounding  the  either side  gradients.  km/s,  velocity  most  configuration  possible  to c o n t i n e n t a l  probably  These a r e  to  possible  that the  3.7  Bidge  compression.  deepest point  t o 5 km  would be  than  than the  velocities The  have made  implies  greater  and  basin.  t i m e s , but  gradient,  This  models i s t h e  the  up  would  have r e s u l t e d from  apparent  by  been c h o s e n f o r Winona  layer thinner  most s t r i k i n g , of  has  v e l o c i t y sediment  east  east  to  5 km  l a y e r s c o n s t i t u t e s e d i m e n t s on  is likely  which c o u l d  travel  water b o t t o m a r r i v a l . ,  models.  use  shifted  the  (1977) i n t e r p r e t e d  runs approximately  t h i s depth  a l t h o u g h making t h i s  feature  Lynch  h a v e a v e l o c i t y o f 4.3  A v e l o c i t y of  and  beyond  profiles  At  for  the  s  l a y e r i n g over the  time i n t e r v a l .  tracing  layer that  oceanic going  horizontal  expected.  67  Regardless rapid  of the l a y e r  overall  The  f o r the  eastward  models s u g g e s t s dipping  layers  buckling of subduction  plate.  crust  i n response  that the  models  the g r a v i t y  be  the  gravity  a i r anomaly,  although  effects  satisfying  results  g e o p h y s i c a l and  the  of t h i s  c o u l d be  area to the Dellwood zone would  also  northwest.  As  have p e r f o r m e d  and  a  t o compare  with  much more work i s n e c e s s a r y  they  project,  migrated  Knolls,  American  be  have shown t h a t t h e  to  model i s  data.  WINONA BASIN  coupled  with  existing that  (ie.  from  (Figs.  Riddihough,  junction  has  has  1977; recently  the Brooks Peninsula  1.1  by R i d d i h o u g h p l a t e s has  oblique  I f , as  & 1.2)  the  t o make a c o r r e s p o n d i n g  p o i n t e d out  E x p l o r e r and  must  1978)  1974), t h e t r i p l e  have had  area.  check t h i s , Clowes  f o r model 7 5—3  some a u t h o r s  4 m,y.)  of  To  o c c u r r i n g a t Winona B a s i n .  by  Tiffin,  (within the past  the  the  westward  rate i n t h i s  g e o l o g i c a l i n f o r m a t i o n , suggest  been h y p o t h e s i z e d Murray and  west s i d e o f  Perhaps the  5a_3 TECTONIC SIGNIFICANCE OF  subduction  travel  to a recent i n i t i a t i o n  data.  calculation  remove unwanted end  The  the  a c c e p t a b l e , they  (personal communication,  of  f o r the s i d e s of  i n order to s a t i s f y  d i p p i n g c r u s t on  more  on t h e e a s t s i d e o f t h e b a s i n r e p r e s e n t a  with  preliminary  capable  with depth  or i n c r e a s e i n s u b d u c t i o n  consistent  the f r e e  center  a subducting  the  In o r d e r  Whittall  shewn, t h e r e must be a  increase in velocity  the b a s i n than times.  boundaries  subduction  shift  to  the  (1977) c o n v e r g e n c e  been o b l i q u e and  very  of  68  ( 1 . 9 t o 1 . 4 cm/yr) o v e r  slow  convergence without crust  subduction  i s presumably s t i l l  However i f t h e r e  3 m.y.;  t h e past  i s p o s s i b l e s i n c e t h e young  warm and p e r h a p s u n a b l e t o s u b d u c t .  has been a s l i g h t  component o f o b l i q u e .  subduction  a l o n g t h e Queen C h a r l o t t e t r a n s f o r m  subducting  p l a t e would  the  junction shift.  triple  consequently  already  fault, a  have been e s t a b l i s h e d p r i o r t o  The t h i c k s e d i m e n t - f i l l e d trough  with  evidence  like  s t r u c t u r e o f t h e d e e p e r c r u s t i n Winona B a s i n  evidence  of northeast-southwest  f o rsubduction.  east side of the basin in the at  compression  The westward d i p p i n q  could represent  and t h e bowl are strong  l a y e r s on t h e  b u c k l i n g o f the c r u s t  r e s p o n s e t o an i n c r e a s e i n t h e c o n v e r g e n c e r a t e f o l l o w i n g triple  junction s h i f t .  Similarly  t h e f o o t of the c o n t i n e n t a l slope  component o f l a t e r a l  motion.  explained  subduction  by o b l i q u e  t h e deep  vertical  fault  indicates a definite  These o b s e r v a t i o n s  are best  between t h e E x p l o r e r and  American p l a t e s . In of  order  Winona B a s i n  geophysical t h i s study and  t o q e t a complete  of the s t r u c t u r e  and t h e t e c t o n i c f o r c e s o p e r a t i n q  information place  understandinq  i s needed.  important  any c o n c l u s i o n s  on i t , more  However t h e r e s u l t s o f  c o n s t r a i n t s on t h e o v e r a l l  made must be c o n s i s t e n t w i t h  them.  picture  69  Cerveny, V. and fi. Bavindra 1971. Theory of S e i s m i c Head Haves. U n i v e r s i t y of Toronto P r e s s , 312 pp. Chase, R. L., D. L. T i f f i n and J.W. Hurray 1975. The western Canadian c o n t i n e n t a l margin. Canadian S o c i e t y of Petroleum G e o l o g i s t s , Memoir 4, pp. 701-721. C l a y t o n , R.W. 1975. The d e c o n v c l u t i o n of t e l e s e i s m i c r e c o r d i n g s . U n p u b l i s h e d M.Sc. t h e s i s . U n i v e r s i t y of B r i t i s h Columbia. . C l e e , T.E., K,G. Barr and M.J. Berry 1974. F i n e s t r u c t u r e c f the c r u s t near Y e l l o w k n i f e . Can. J . E a r t h S c i . 11, pp. 1534- 1549. Clowes, R.M. 1977. A marine deep s e i s m i c sounding system. J . E a r t h S c i . 14, pp. 1276-1285.  Can.  Couch, R.W. 1969. G r a v i t y and s t r u c t u r e s of the c r u s t and subc r u s t i n the n o r t h e a s t P a c i f i c ocean west of Washington and B r i t i s h Columbia. Unpublished PhD. t h e s i s , Oregon S t a t e U n i v e r s i t y , C o r v a l l i s , OR. D i x , C.H. 1955. S e i s m i c v e l o c i t i e s from s u r f a c e measurements. Geophysics 20, pp. 68-86. Grubbe, K. 1976. S e i s m i c - r e f r a c t i o n measurements along two c r o s s i n g p r o f i l e s i n n o r t h e r n Germany and t h e i r i n t e r p r e t a t i o n by a r a y - t r a c i n g method. I n : E x p l o s i o n Seismology i n C e n t r a l Europe. P. G i e s e , C. P r o d e h l and A. S t e i n ( e d s . ) , S p r i n g e r - V e r l a g , B e r l i n 1976, pp. 268282. Kanasewich, E. R. 1976. Time S e r i e s A n a l y s i s i n G e o p h y s i c s , 2nd ed., U n i v e r s i t y of A l b e r t a P r e s s , 352 pp. Kramer, F.S., R.A. P e t e r s o n and W.C. energy s o u r c e s handbook, 1968. C o r p o r a t i o n , 57 pp.  Walter 1968. S e i s m i c United G e o p h y s i c a l  Lynch, S. 1977. The c r u s t a l s t r u c t u r e o f Sinona B a s i n as determined by deep s e i s m i c sounding. Unpublished M.Sc. t h e s i s , U n i v e r s i t y o f B r i t i s h Columbia. M i l l e r , H. and H. Gebrande 1976. C r u s t a l s t r u c t u r e i n s o u t h e a s t e r n B a v a r i a d e r i v e d from s e i s m i c - r e f r a c t i o n measurements by r a y - t r a c i n g methods. I n : E x p l o s i o n Seismology i n C e n t r a l Europe. P. G i e s e , C. P r o d e h l and A. S t e i n ( e d s . ) , S p r i n g e r - V e r l a g , B e r l i n 1976, pp. 3393 46. Mota, L. 1954.  Determination  of d i p s and  depths o f g e o l o g i c a l  70  l a y e r s by t h e s e i s m i c r e f r a c t i o n method. pp. 242-254.. fluller,  Geophysics  19,  St., A. S t e i n and B. V e i s 1962. S e i s m i c s c a l i n g l a s s f o r e x p l o s i o n s on a l a k e b o t t o m . Z e i t . f . Geophys. .28, pp. 258-280.  Murray, J.W. and D.L. T i f f i n 1974. P a t t e r n s o f d e f o r m a t i o n , s e d i m e n t a t i o n and t e c t o n i s m , s o u t h w e s t e r n Canadian c o n t i n e n t a l m a r g i n . Ann. de l a Soc. G e o l , de B e l g i q u e 97, pp. 169-183. O ' B r i e n , P.N.S. 1960. J.R. a s t r . Soc.  S e i s m i c energy 3, pp. 29-44.  from e x p l o s i o n s . Geophys.  R i d d i h o u g h , R.P. and R.D. Hyndman 1976. C a n a d a » s a c t i v e w e s t e r n m a r g i n - t h e c a s e f o r s u b d u c t i o n , G e o s c i , Canada 3, pp. 269-278. R i d d i h o u g h , R.P. 1977. A model f o r r e c e n t p l a t e i n t e r a c t i o n s o f f C a n a d a » s west c o a s t . Can. J . E a r t h S c i . J 4 , pp. 384396. , Shor,  G.G,, J r . 1963. R e f r a c t i o n and r e f l e c t i o n t e c h n i g u e s and p r o c e d u r e . I n : The Sea. M.N. H i l l ( e d . ) , I n t e r s c i e n c e , New Y o r k , pp. 20-38,  T e l f o r d , W.M., L.P. G e l d a r t , B.E. S h e r i f f and D.A. Keys A p p l i e d G e o p h y s i c s . Cambridge U n i v e r s i t y P r e s s , C a m b r i d g e , 860 pp.  1S76.  T i f f i n , D.L., B.E.B. Cameron and J.W. Murray 1 9 7 2 . „ T e c t o n i c s and d e p o s i t i o n a l h i s t o r y o f t h e c o n t i n e n t a l m a r g i n o f f V a n c o u v e r I s l a n d , B r i t i s h C o l u m b i a . Can. J . E a r t h S c i . 9, pp. 280-296. T i f f i n , D.L. and D. ,5eeman 1975. B a t h y m e t r i c map o f t h e c o n t i n e n t a l m a r g i n of w e s t e r n Canada. Open f i l e map, G e o l . S u r v . o f Canada. W i g g i n s , B.A. 1976. Geophys. J.R.  Body wave a m p l i t u d e a s t r . S o c . ^6, pp.  calculations II. 1-10.  W i g g i n s , R.A. 1977. Minimum e n t r o p y d e c o n v o l u t i o n . P r o c e e d i n g s o f t h e I n t e r n a t i o n a l Symposium on Computer A i d e d S e i s m i c A n a l y s i s and D i s c r i m i n a t i o n , J u n e 9 & 10, 1977, F a l m o u t h , Mass., IEEE Computer S o c i e t y , pp. 7-14. Wood, L.C., R.C. R e i s e r , S. T r e i t e l and P.L. R i l e y 1978. d e b u b b l i n g of marine s o u r c e s i g n a t u r e s . G e o p h y s i c s pp. 715-729.  The 43.,  

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