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The crustal structure of Winona Basin as determined by deep seismic sounding Lynch, Steven 1977

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THE CRUSTAL STRUCTURE OF WI NONA BASIN DETERMINED BY DEEP SEISMIC  AS  SOUNDING  by  STEVEN LYNCH B.Sc.,  University  A THESIS SUBMITTED  of Guelph,  1975  IN PARTIAL FULFILLMENT OF  THE REQUIREMENTS  FOR THE DEGREE OF  MASTER OF SCIENCE  in  THE FACULTY OF GRADUATE STUDIES (Department  We  of  accept to  Geophysics  this  thesis  the required  THE UNIVERSITY  as  Astronomy)  conforming  standard  OF B R I T I S H COLUMBIA  June,  ©  and  Steven  1977  Lynch,  1977  In p r e s e n t i n g t h i s  thesis  an advanced degree at  agree  fulfilment  of  the  requirements  the U n i v e r s i t y of B r i t i s h Columbia, I agree  the L i b r a r y s h a l l make it I further  in p a r t i a l  freely  available  for  for  that  reference and study.  t h a t p e r m i s s i o n f o r e x t e n s i v e copying o f t h i s  thesis  f o r s c h o l a r l y purposes may be granted by the Head of my Department o r by h i s r e p r e s e n t a t i v e s . of this  thesis for  It  financial  i s understood that copying o r p u b l i c a t i o n gain s h a l l not be allowed without my  written permission.  Depa rtment The  U n i v e r s i t y o f B r i t i s h Columbia  2075 wesbrook P l a c e Vancouver, Canada V6T 1W5  Date  UfZ  ^1  /?77  i  ABSTRACT  During profiles  Auqust  1975, t h r e e  reversed  deep s e i s m i c  were r u n o v e r W i n o n a B a s i n , a deep w a t e r  basin  located  west o f t h e n o r t h e r n  This  thesis  r e p o r t s on t h e a n a l y s i s o f t h e d a t a  profiles  t h a n 90 km a l o n g  The  seismic  multi-channel The  Vancouver  acquisition  sub-critical  gain,  Immediately are  f o r reversed  to  distances  of  size  prior  data  recorded  on  and t h e r e f r a c t i o n  f o r spherical and  varying  to compilation  bandpass  and  a  system.  reflection  corrected  charge  Island.  the a x i s of the b a s i n .  hydrophones, a m p l i f i e d  digital  are amplitude  sedimentary  s i g n a l s from e x p l o s i v e charges a r e detected  six individual  data  of  75-1 and 75-1R, w h i c h w e r e r e c o r d e d  greater  by  end  sounding  filtered  spreadinq,  hydrophone  into  record  to  improve  data  amplifier sensitivity.  sections, their  the  general  appearance. T h r e e methods o f a n a l y s i s w e r e u s e d t o depth ray  information parameter,  comparison shows  that  essentially is  T -X 2  2  and  the  the  ray  parameter  t h e same r e s u l t ,  basin  T -X 2  2  methods.  from t h e t h r e e  and  T -X 2  2  data; the  methods 2  t h e data  below t h e s e c o n d  75-1H d a t a  A  techniques  whereas t h e s t r i p p e d T - X  The a n a l y s i s o f p r o f i l e  the  velocity-  reflection  stripped  the r e s u l t s obtained  o f no use i n a n a l y s i n g  layer. of  of  from t h e s u b - c r i t i c a l  obtain  yield 2  method  sub-bottom  a t t h e n o r t h w e s t end  gave a s e d i m e n t a r y s t r u c t u r e d i v i d e d i n t o  three  ii  prominent h o r i z o n s , with v e l o c i t i e s  ranging from  km/s.  The t o t a l  depth to t h e b a s a l t i c l a y e r  be 1.8  km. T h i s  thickness  is  suggested by other authors assumptions presence  southeast,  no  from the a n a l y s i s  of  less  information  first  arrival  analysis  provided  final  amplitude  was  the  These  model  first  s t e p i n the layered  i n t e r p r e t i v e guide and a  starting  of f i r s t  Based that  sub-sediment Average km/s, km/s, crustal of 7.8  layers,  km; 5.26  3.76  km.  section  having and  oceanic c r u s t a l  and  the  the seismograms,  the  seismogram  the r e f r a c t i o n  km/s, The  is  thus  values, layers  significant  thicknesses 2.75  sections  data  indicate  velocity  km/s,  sub-sediment  2a,  from 75-1 layers 2b,  3a,  4.13  thickness  12 km. An unreversed  the  gradients.  f o r the l a y e r s  km; 6.28  total  km/s was i n t e r p r e t e d  velocity  arrivals  order  u n d e r l y i n g Winona Basin i s s e p a r a t e d i n t o four  velocities  1.6  of  In  data.  on such an a n a l y s i s ,  the c r u s t  and secondary  made use of s y n t h e t i c  comparison with the r e a l  based  iso-velocity  characteristics  interpretation  the  could be obtained  f o r the c a l c u l a t i o n of s y n t h e t i c seismograms.  relative  the  times  an i n i t i a l  to u t i l i z e t r a v e l t i m e s  for  Due to  75-1.  of the r e f r a c t i o n d a t a .  models place  travel  that  h o r i z o n s in the  C a l c u l a t i o n of a preliminary velocity-depth on  to  s t u d i e s and  profiles.  dips on the r e f l e c t i n g  profile  2.1  than  on the b a s i s of g r a v i t y  velocity-depth  to  was determined  considerably  concerning continuous seismic  of s i g n i f i c a n t  1.7  results.  mantle  are;  4.28  km;  7.04  of  velocity  On the b a s i s of  have been i d e n t i f i e d 3b.  this  with  iii  The plate the  thick crust  interactions occurring  data  from  Explorer  and  conclusion 3  to  North  i s postulated  this Juan  that  ii my,  by  work w i t h de  Fuca  Winona B a s i n the  slow  America-Explorer  i n the  to  has  the  region.  previous Ridges  be  result A  studies  has  northward  plate triple  progression point.  complex  comparison i n the  l e d to the  been c r e a t e d  of  of  region  of  speculative  within of the  the  last  Pacific-  iv  TABLE OF CONTENTS ABSTRACT TABLE OF CONTENTS L I S T OF TABLES L I S T OF FIGURES ACKNOWLEDGEMENTS Pa qe la. I N T R O D U C T I O N  1. 1 A r e a o f S t u d y 1.2 T e c t o n i c S i g n i f i c a n c e o f Winona B a s i n 1.3 D a t a A c q u i s i s t i o n a n d P r o j e c t Description  1 13 16  li. . PRELIMINARY ANALYSIS 2.1 2.2 2.3 2.4  Demultiplexing O r i g i n Times Shot-receiver distances Record S e c t i o n s  20 .....21 24 29  Is. S U B z C R I T I C A L REFLECTION DATA 3.1 M e t h o d s o f A n a l y s i s 3.2 A n a l y s i s o f R e s u l t s i i .REFRACTION 4.1 4.2 4.3 '4.4 4.5  DATA  First Arrival Interpretation S y n t h e t i c Seismograms A p p l i c a t i o n t o Data Record S e c t i o n s Comparison o f S y n t h e t i c s w i t h Real  5. INTERPRETATION  Data  60 71 ...74 IS 88  ANE> DISCUSSION  5.1 V e l o c i t y - D e p t h 5.2 D i s c u s s i o n REFERENCES  34 47  Models  105 110 119  V  L I S T OF TABLES Table  Paqe  2.1  Shot-receiver distance errors  3.1  Comparison of r a y parameter, T - X s t r i p p e d T - X methods o f reflection analysis 2  2  3.2  .....28 z  and  2  52  V e l o c i t y v s d e p t h model f o r r e f l e c t i o n p r o f i l e 75-1R  53  3.3  T - X s l o p e s and i n t e r c e p t s p r o f i l e 75-1  57  4.1  L e a s t s q u a r e s v e l o c i t i e s and i n t e r c e p t s for r e f r a c t i o n f i r s t a r r i v a l a n a l y s i s  .....67  V e l o c i t y v s d e p t h model f r o m r e f r a c t i o n f i r s t arrival analysis  67  4.2  2  2  for reflection  vi  List  of Figures  Fiq  '  1.1  L o c a t i o n map f o r Winona B a s i n  .„  1.2  Bathymetric  1.3  Continuous seismic p r o f i l e plus i n t e r p r e t a t i o n  l i n e 75-1  1.4  Continuous seismic p r o f i l e plus i n t e r p r e t a t i o n  l i n e 75-2  1.5  A i r qun p r o f i l e  4  map o f Winona B a s i n  over  .6 ~8- t\ potjj^T  deep w a t e r  I 4r l*> J**«*  portion  o f Winona b a s i n 2.1  Four t y p i c a l  3.1  Processed profile  .11  seismograms from p r o f i l e  record section f o r 75-1R  3.3  Two t y p i c a l s e c t i o n s u s e d t o t i m e reflection arrivals T v s x a n d T - X * p l o t s f o r 75-1R r e f l e c t i o n data Same as 3.4 f o r p r o f i l e 7 5 - 1  4.1 4.2 4.3 4.4  ......26  ...36  Same as a b o v e f o r p r o f i l e  3.5  75-1  reflection  3.2  3.4  Paqe  7 5-1  ......37  ,  39  2  ..50 51  T vs X p l o t 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 f i r s t a r r i v a l a n a l y s i s  63  Expanded T v s X p l o t showing t h e f i r s t two p h a s e s o f F i g 4.2 o n l y  65  V e l o c i t y vs depth curve first arrival analysis  derived  from ...70  Processed record 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 75-1  77  4.5  Same a s 4.4 f o r p r o f i l e  78  4.6  Stacked record s e c t i o n f o r p r o f i l e 75-1  80  P r o f i l e 75-1R r e c o r d showing only channel  82  4.7 4.8  Expanded p l o t o f l a s t  7 5-1 R  section t h r e e data 40 km o f p r o f i l e  7 5 - 1 ....87  vii  4.9 4.10 4.11  P-A curve generated l a y e r e d model  from  iso-velocity .90  S y n t h e t i c seismograms f o r i s o - v e l o c i t y l a y e r e d model Final data  synthetic  s e i s m o g r a m s and t h e  for p r o f i l e 75-1  4.12  Same as a b o v e f o r p r o f i l e 7 5-1R  4.13 4.14  Final Final  4.15  Comparison of f i n a l 7 5 - 1 and 7 5 - 1 R  5.1  p-A c u r v e f o r p r o f i l e 7 5 - 1 p-& c u r v e f o r p r o f i l e 7 5 - 1 R  .92  real 94 95 97 98  p-A c u r v e s f o r  F i n a l v e l o c i t y vs depth models f o r p r o f i l e s 7 5 - 1 and 7 5 - 1 R  99  107  viii  ACKNOWLEDGEMENTS For  h i s encouragement, patience  entirety  of  this  appreciation Cumming  project,  I  and  wish  support  to  express  t o D r . R.M. C l o w e s . I am a l s o  who p r o v i d e d  me w i t h  great  during  my d e e p e s t  indebted  assistance  the  to  during  W.B.  t h e more  t e c h n i c a l a s p e c t s o f t h e work. This  project  has  q u a l i t y of t h e data, Bo and  Chandra, C h r i s George  shooting  g r e a t l y by t h e e x c e l l e n t  due i n no s m a l l  way t o t h e e f f o r t s o f Dr.  West and B i l l  Spence,  Patrick  officers  and  I  Shore and  Rob  d i l i g e n c e , enthusiasm  wish  crew  LAYMORE f o r t h e i r  to  of  express  the  assistance  by t h e e x p l o s i v e s  Pacific  Cumming on t h e r e c e i v i n g Clayton  ship,  on  the  and d e d i c a t i o n a r e  acknowledged.  Further,  given  aided  ship. Their  grat'efully  been  my  appreciation  C. F. A.V. during  experts  t o the  ENDEAVOUR a n d C F . A.V.  t h e c r u i s e . The  from t h e  Fleet  assistance  Diving  Unit,  M a r i t i m e Command i s a l s o g r a t e f u l l y a c k n o w l e d g e d . The  a n t i - s u b m a r i n e p r o j e c t i l e s were Division of t h e Earth Funding  for this  Research C o u n c i l  project  by M o b i l  by  the  Seismology  Branch. was s u p p l i e d  o f Canada, o p e r a t i n g  f u n d s were s u p p l i e d Resources  Physics  supplied  Oil  by t h e N a t i o n a l  g r a n t A 7707.  Canada  Additional  L t d . , Shell  Canada  L t d . a n d C h e v r o n S t a n d a r d . The D e p a r t m e n t o f E n e r g y  Mines and R e s o u r c e s  ( G e o l o g i c a l S u r v e y o f Canada) p r o v i d e d t h e  contract  the  assist  to acquire  i n i t s analysis.  data  and  a  research  agreement  to  1 Jt INTRODUCTION The say of  tectonic  structure  west o f V a n c o u v e r  the l e a s t , extremely complicated. interaction  American, plate  This  between two manor p l a t e s ,  and two m i n o r p l a t e s ,  ( F i g 1.1).  These  region  plates  ancient  plate,  the  Farallon  according  to  Atwater  (1970)  has  been  American  plate.  study  the  is  plates. 131°  location  I t i s thought  Basin, the  Pacific, how  of  t o be somewhere n e a r  North  this American  i t i s related  much o f t h e i m p e t u s survey  triple  point  for  the  to  the region  plate  marine  the this  of 51° N  i s questionable. is  plates.  t o t h e complex  which by  Winona  located  and i s s u r r o u n d e d  and E x p l o r e r  Fuca  between t h e t h r e e  the area of i n t e r e s t f o r t h i s study,  region  and  consumed  point  position  de  plate,  Of s p e c i a l i n t e r e s t  of the t r i p l e  W; h o w e v e r , i t s a b s o l u t e  and N o r t h  a r e t h e remnant o f a  and  North  i s t h e area  and J u a n  much l a r g e r  overriding  i s , to  the P a c i f i c  the Explorer  smaller  Island  near by t h e  What i t s r o l e i s ,  tectonics,  provided  seismic  sounding  deep  which .was c a r r i e d o u t i n 1975.  1.1 A r e a o f S t u d y  •  The  name  sedimentary margin  be and  basin  off  Srivastava  Winona  Basin  located  the  at  northwest  was  first  the  foot  t i p of  of  uplift,  zone,  an a r e a  whereas t h e n o r t h e r n  the  Vancouver  e t a l ( 1 9 7 1 ) . The s o u t h e r n m o s t  a t t h e Brooks f r a c t u r e  applied  continental Island,  by  boundary i s t a k e n t o  of intense  boundary  t o t h e deep  i s taken  fracturinq t o b e some  2  150 km  to the northwest  bounded the  on  the  west by P a u l  of  the  the  basin.  of  the  area  The o n l y  basin  Winona R i d g e . Revere the  basin.  with  Ridge  a -160 ragal f r e e  depth  (1973),  is  who  approximately minor c o a l  consistent  suggests 4  km  of d e f i n i t e  On t h e b a s i s crustal  from  Winona the km  Basin  basin  at  obliguely  the i n t e r i o r  by  named  to  Paul  sediments of of the  Couch  basin  2000  of  Winona  the  being basin.  work by T i f f i n  basin  i s  m.  (1969) has  between  portion  later  raudstone,  et al  underlain  by  s a n d s t o n e , c o n g l o m e r a t e and  Pliocene-Pleistocene  of h i s g r a v i t y d a t a ,  an o b l i g u e  interior  He i n t e r p r e t e d t h i s a s  "the  134° W t o 5 2 ° N,  the  low-relief structure  in this  and s u b - c r u s t a l c r o s s  4 8 ° N,  within  area  with  that of  f e a t u r e s of  depth o f a p p r o x i m a t e l y the  by  topographic  a i r anomaly l o c a t e d  t o 4 t o 6 km o f s e d i m e n t s  is  bathymetric  by t h e s t r a t i f i e d  a mean w a t e r over  is  somewhat  and t h e c o n t i n e n t a l s h e l f .  This  a  trends  this feature,  survey  basin  a  located  a broad  and i s u n d e r l a i n  gravity  revealed  is  feature  Other than  lying  A  due  This  F i g 1.2  t h e major  major f e a t u r e  itself  Ridge  flat  detailing  K n o l l s . The  R e v e r e R i d g e and on t h e e a s t  edge o f t h e c o n t i n e n t a l s l o p e .  map  is  at t h e Dellwood  age".  Couch  (1969)  s e c t i o n along  126°  W.  This  angle  at r o u g h l y  and shows an a p p r o x i m a t e d e p t h  generated  a line  section  running crosses  t h e m i d p o i n t of  t o the mantle  of  15  sub-bottom. The  probably given  by  basin  i s thought  t o be a r e l a t i v e l y  of Pliocene-Pleistocene Tiffin  et  al  ( 1972)  age. T h i s on  the  young  proposal basi-:  structure, has of  been  limited  3  J L I.  Fig  Location  o f Winona  the  Pacific,  and  Juan  de  North Fuca  the  rectangle  1.2.  PA  P,  American JF  P,  line  shows  boundary plates  i s shown  Pacific  de  and  P,  plate.  the  between  The  respect  Explorer  area  AM  within  P,  North  Explorer  plate;  The  assumed  Explorer/Juan  the North  to  e n l a r g e d on F i q  plate; EX  Fuca  with  American,  plates.  plate;  Juan  Basin  American  hatchured convergent de plate.  Fuca  4  132  130  128  126  Location  of  1975  sounding  profiles  circles  show  receiving heavy  ship  the basin  study.  Bathymetric Tiffin  the the  Profiles  along  (from  drift  during show  ship.  deep  seismic  i n Winona B a s i n .  the  lines  shooting  marine  track  of the  profile track  75-1  contours  run;  of  and  are the subject  a n d Seemen,  Open  in  1975).  the 75-lfi  of  this  meters  7  formaniferal The the  evidence.  sediments i n the  northwest,  as  basin are  shown  r e a r c o v e r ) ; however, they the  southeast,  1.5).  with  by  undeformed  1.3  (pocket  and  are severely  the  Continuous seismic  Figs  relatively  faults  profile  f o l d e d and  trending  basement d i p p i n g e a s t w a r d  Ridge.  dips  identified  then  f u r t h e r on  beneath  Winona  possibility  taking  place  i n the  (1977),  however,have r e c e n t l y p o i n t e d  off  ridge  currently  majority  of  (see  Fig 1.4),  Paul-Revere  and  can  not  region  be  (Tobin  and  in  On  Sykes 1968). M i l n e out  basis  progress  i t  are  located  earthguakes  Ridge/Revere-Dellwood  the  fracture  have  i s presently  that  zone  of  appears l i k e l y  and  the  not  is  a  earthquakes  this  on  et a l  there  e p i c e n t r a l l o c a t i o n s f o r the  coast.  research  in  (Fig  from  that a c t i v e deformation  basin  in  Canada's west  faulted  the" p r o f i l e .  suggested the  bias  the  northwest  E p i c e n t r a l l o c a t i o n s f o r earthquakes i n the  systematic  on  (C.S.P.) l i n e 75-2  shows t h e o c e a n i c It  1.4  in  in  work  and  that  the  Explorer the  basin  itself. The  area  maqnetic of . the  of  survey evidence  Winona by  Basin  R a f f and  for  the  was  Mason  c o n c e p t . I n Winona B a s i n  none  the  spreading relatively west  magnetic  centre,  the  of  the  the  however, the  l i n e a t i o n s normally  magnetic s t r u c t u r e of  smooth. There a r e  i n the  of  oriqinal  (1961) w h i c h p r o v i d e d  introduction  spreading of  part  sea-floor  a n o m a l i e s show  associated the  basin  normal magnetic l i n e a t i o n s  region of Explorer  much  Ridge; however, they  with a being to  the  terminate  A. C o n t i n u o u s s e i s m i c p r o f i l e parallel  to  Penetration  i s  DSS  profile  achieved  only  maximum o f 4 s two-way t r a v e l B.  Interpretation  courtesy  of  75-1. to  a  time.  o f C.S.P. l i n e 7 5 - 1 ,  Hopkins  continuity  along  interpreted  reflecting  in  l i n e 75-1,  the  p o c k e t on t h e b a c k  (1976). profile horizons cover).  Note  the  of  the  (located  9  Fig. JL 4  A.  Continuous  paralleling relatively and  seismic DSS  profile  undeformed  the increase  profile  line  75-2.  upper  in fold  75-2  Note  the  sediments,  amplitude  with  depth. B. I n t e r p r e t a t i o n o f C.S.P. also right  from  Hopkins  corner,  interpreted in  pocket  line  (1 9 7 6 ) . On  basement  the  • has  as d i p p i n g t o t h e e a s t  on b a c k c o v e r ) .  75-2, lower been (also  An  a i r gun  seismic  profile  water  portion  base  of the c o n t i n e n t a l  thick  folded  sequence, arrows  Taken  o f Winona B a s i n  and  indicates  from  of  Area  Hurray  the  showing  a  sedimentary between  the  reflection  deep  beyond  slope  faulted  f=fault„  position  o v e r the  the  approximate profile  and T i f f i n  75-1.  (1974).  11  12  formaniferal The  evidence.  sediments i n the  the  northwest,  rear  cover);  the  southeast,  1.5).  as  shown  by  however, they  are  with  the  Continuous seismic  shows t h e Ridge.  oceanic  It  identified  basin  then  f u r t h e r on  Fiqs  profile  undeformed  1.3  (pocket  and  beneath  1.4  f o l d e d and  trending  (C.S.P.)  dipping  the  relatively  severely  faults  basement dips  are  eastward  Winona  taking  the  place  off  bias  basin  in  C a n a d a ' s west  (Tobin  ridge  currently  majority  of  in  On  and  Sykes out  the  basis  progress  i t  are  located  earthquakes  Ridge/Revere-Dellwood  (see  Fiq 1.4),  Paul-Severe  and  can  not  region is  be  fracture  zone  presently et a l  that  is  of  there  this  on  the  not  a  earthguakes  appears l i k e l y  and  have  1968). M i l n e  e p i c e n t r a l l o c a t i o n s f o r the  coast.  research  in  (Fig  from  that a c t i v e deformation  ( 1 9 7 7 ) , however, have r e c e n t l y p o i n t e d systematic  faulted  profile.  possibility i n the  the  75-2  E p i c e n t r a l l o c a t i o n s f o r e a r t h q u a k e s i n the suggested  on  northwest  line  in  in  work  and  that  the  Explorer the  basin  itself. The  area  magnetic of  1  the  of  survey evidence  spreading  concept.  none  the  of  spreading relatively west  Winona by  Raff  for In  the  region  of  was  Mason  part (196 1)  introduction  Winona B a s i n  of  of  normal  Explorer  magnetic  Ridge;  original  provided  the  however, t h e  magnetic s t r u c t u r e of are  the  which  l i n e a t i o n s normally  smooth. There  i n the  and  the  magnetic  centre,  Basin  sea-floor  a n o m a l i e s show  associated the  much  basin  with  being  l i n e a t i o n s to  however, t h e y  a  the  terminate  13  at  P a u l R e v e r e R i d g e and  J.s-2 The North  do n o t  T e c t on i c S i g n i f i c a n c e o f Winona  position  American  of the t r i p l e and  and  Bertrand  other as  location  hand has  a  Early  boundary  of  the  suggested  discrete  plates  has  work  of  triple  Pacific-  a source  Srivastava  Basin,  p o i n t . Chase  al  Dellwood  marks  the  (1975) on  p o i n t does not  rather i t exists  of  et  that the  Winona  t h a t the t r i p l e  p o i n t but  the  been  by  itself.  Basin  between  (1972) h a v e s u g g e s t e d  K n o l l s , the n o r t h e r n present  point  Explorer  c o n j e c t u r e f o r some t i m e . (1971)  penetrate the basin  the  exist  as a b r o a d  area  of  deformation.  the to  Basing  their  shelf  s e d i m e n t s n o r t h of B r o o k s f r a c t u r e zone as  the  c o n c l u s i o n s on the. l a c k  extreme d e f o r m a t i o n  of the f r a c t u r e zone, that  the  until  approximately  off  triple  northern  subduction,  p o i n t was  f r a c t u r e z o n e , and  strike-slip  however,  in  4 -my-,- r e s u l t i n g  B a s i n by  t h a t the  t h e moving t r i p l e  Riddihough  a  From  have  south  proposed  triple  i n the  zone  c o n t i n e n t a l marqin  two  different  to  the south  motion to the  regimes: of  Brooks  n o r t h . They  p o i n t has  formation  l e s s than  suggestion  (1977).  opposed  have  migrated of  the  north Winona'  p o i n t . T h i s would r e q u i r e the c r u s t  u n d e r l y i n g t h e b a s i n t o be Such  into  deformation  suggested, the l a s t  (1974)  This d i v i d e s the  Island  and  Tiffin  of  s t a b l e at the Brooks f r a c t u r e  4 mya.  uplift  deformation  of the s h e l f s e d i m e n t s t o the  M u r r a y and  Vancouver  of  is a  4 my  supported  'detailed  old. by  recent  re-examination  work  of  of  the  14  existing  magnetic  Explorer-Juan  de  anomaly  patterns,  show t h a t  near the  B r o o k s f r a c t u r e zone u n t i l  the  triple  s t a r t e d a slow northward  recently  Pacific-North  by  the  Winona B a s i n  slow  northward  zone.  To  the  the  south, If  indeed  position  the  P-A-E  of  Queen  the  This  requires  plate,  that  of 51°  N.  of  the  point. Pacific-^  transform  the  fault  location  is  less well  Knolls  do  form  the  o f 5 1°  the  separated  be  created  N o r t h o f 5 1°  p o i n t , then south  Winona B a s i n  or a t l e a s t t h a t i t  been  p o s i t i o n of t h e  its  triple  American p l a t e s are  idea  stable  progression  Dellwood  P a c i f i c and  has  Charlotte  however,  established.  His  whereupon  (P-A-E) p l a t e t r i p l e  America p l a t e boundary south by  my.  the  migration.  American-Explorer  boundary i s d e f i n e d  10  past  a b o u t 2 t o 3 mya  Another matter of debate i s the North  calculated  p o i n t remained 'remarkably  I t i s p o s s i b l e then that very  has  Fuca p l a t e motions f o r the  results  it  he  be  by  Winona  part of  independent  of  the  Basin. Explorer  the  American  plate. Barr plate  and  Chase  boundary  edge o f J u a n de the. of  region  the  lies  along  Fuca Ridge w i t h the  o f 51°  N,  131°  W.  Explorer  through  Winona  R i d q e and bottom  communication,  Basin.  1977)  the  Pacific-American  connecting  the  northern  Queen C h a r l o t t e  fault  was  the  This and  s u g g e s t e d on  requires that  the  This would e s s e n t i a l l y  thus render i t  seismometer  that  a line  a v a i l a b l e earthquake data  extend  ocean  now  (1974) s u g g e s t e d  inactive.  results  (G.C.  have e s t a b l i s h e d t h e  However, Roqers,  presence of  in  basis fault isolate recent  personal seismic  15  activity the  on t h e n o r t h e r n  high  et a l in  heat  flow values  (1971),  the  recent  of  It  Explorer  re-exaraination  with  believed i s  the  recent  then  make  that  earthquake  extent.  data  The  (1972) this,  a  f o r t h e area  biases i n the  most o f t h e e a r t h q u a k e s a r e now fracture  zone  system.  t h a t t h e Queen C h a r l o t t e F a u l t e x t e n d s Ridge.  r e l o c a t i o n o f the earthquake data  fracture  plus  the conclusion of  In addition to  t o t h e J u a n de F u c a  zone  does n o t c l a r i f y  Pacific  Ridge,  the  t h e i d e a o f Chase  Dellwood  great  of  result  unlikely  supports  This,  i n t h e r e g i o n by S r i v a s t a v a  t o f o l l o w the Eevere-Dellwood  The  Ridge.  h a s shown s e r i o u s s y s t e m a t i c  t h r o u g h Winona B a s i n  this  detected  of the ridge unacceptable.  ( M i l n e e t a l 1977) data  of Explorer  a n d t h e f r e s h b a s a l t d r e d g e d by B e r t r a n d  region  inactivity  branch  et i s  al  a transform  the position basin  (1975)  could  of  that fault.  Winona  the  the  to  any  s e c t i o n of o l d  part o f t h e North  p l a t e o r a r e c e n t l y formed a d d i t i o n to  Revere-  Unfortunately Basin  be an i s o l a t e d  p l a t e m a t e r i a l , an i n t e q r a l  f o rt h e area  American  Explorer-Juan  de  Fuca p l a t e . In  any e v e n t ,  Winona B a s i n additional was  of  geological  and  qeophysical  p u r p o s e i n mind t h a t  was r u n o v e r t h e a r e a  1975.  t o completely  understand the r o l e  p l a y s i n the complex t e c t o n i c s  with t h i s  survey  i n order  a  data  deep  o f Winona B a s i n  of  this  reqion,  are required. I t seismic durinq  soundinq t h e summer  16  li.2  Project  and  structural  beneath  deep  seismic  sounding  refraction  be  further  method  ocean bottom shallow  (Clowes,  i n the  mantle.  sedimentary  profiles  profile  and  the  profiles  with p r o f i l e s  placing  the  structure  75-1  of  and  at t h i s  structure  reflections,  more e n e r g y  was  t h e s h o t s a t 7 m.  profiles  were r u n u s i n g  the  were r u n  over  The  deep  the r e c o r d i n q and  interpreter  of each  75-1R. I t was  To  s h o r t sub7  complete of  hoped  t h e y would delineate obtain  source  start to  of trace  m DSS  the cross> that  by  blow  out  the  deep  sub-basement  n e e d e d t h a n c o u l d be o b t a i n e d  As a r e s u l t ,  the  delineating  the shots at  intersection  possibly  the  the  of  of the b a s i n ,  s h a l l o w depth  and  placing  enabling  three  reflections  profiles.with  points  sedimentary  comprised  end  1.2.  As a method  minimise the bubble p u l s e problem.  profiles  Fig  angle  were r e c o r d e d p a r a l l e l t o t h e s t a r t  shots  this  w i t h p e n e t r a t i o n b e i n q a c h i e v e d from  t o the upper  at  To  1977), e n a b l e s  wide  c r i t i c a l incidence reflection  and  i n order that i t s role  b e i n g shown on  reflections,  arrivals,  upper  (DSS)  crust  detailed  and  understood.  sounding  locations  sub-critical  depth  Description  p r o j e c t i s to supply  Basin,  seismic  the b a s i n , t h e i r  the  Profile  i n f o r m a t i o n f o r the  Winona  t e c t o n i c s may  reversed  of  and  main o b j e c t i v e o f t h i s  velocity  local  Acgu i s i t i o n  Description :  The  mantle  Data  by  s u b - c r i t i c a l incidence  at each the  45  m  DSS  depth. profile,  transition  These thus from  17  reflection  to r e f r a c t i o n  Continuous length  o f each  seismic  standard  by H o p k i n s  Acquisition  sea drift  freely  vessel,  shooting Malecek  brief  is  one  acquisition  similar  in  for  ship,  in  the  principle  A  300  subsequently  explosives.  procedures  t h i s case  t o t h e two s h i p Operations  while  the  Detailed  the  methods  at  second  along a predetermined descriptions  and d a t a a c q u i s i t i o n s y s t e m  of  seismic  C.F.A.V. E n d e a v o u r , t o  (1976) and C l o w e s ( 1 9 7 7 ) . F o r t h i s  description  deep  (1963).  and a c t a s t h e r e c o r d i n g s h i p  the  lines.  These p r o f i l e s  C.F.A.V. L a y m o r e , p r o c e e d s  releasing  by  paper.  along the  with the recordings being  t e c h n i q u e d e s c r i b e d by S h o r  reguire  run  (1976).  data  project  refraction  also  :  The method o f sounding  were  as t h e s o u r c e  electrostatic  were a n a l y s e d  Data  profiles  of the three reversed r e f r a c t i o n  cu i n a i r gun was u s e d on  arrivals.  of  the  are provided  reason  and system  path  only  a  w i l l be q i v e n  here. Two t y p e s o f s h o o t i n g p r o c e d u r e s profiles.  Geogel,  a  commercial  explosive source f o r the r e f l e c t i o n distance ranged  of  i n size  suspended balloons.  in  70  km  explosive, profiles  the r e f r a c t i o n  used  during  was u s e d and  profiles.  out The  to  the  water  assembly  a  charges  2.3 kg (5 l b ) t o 96 kg (200 l b s ) a n d  was  the  as t h e  from  Detonation  fuse/Seismocap  on  were  were  by t w i n e a t t a c h e d t o l a r g e r e d p a r t y accomplished and  Primacord.  by  use  of  a  timed  For these s h o t s , the  18  shot-to-ship distance  was m e a s u r e d b y  focussed  balloons.  on t h e p a r t y  use of a  Beyond 70 km t h e e x p l o s i v e s o u r c e IV  H.E. a n t i - s u b m a r i n e  projectiles  rangefinder  consisted of three  per shot.  Each  Hark  projectile  contained  t h e e q u i v a l e n t o f 94 k g o f M i n o l  resultinq  i n an e q u i v i l e n t charqe s i z e  to  68 k q c a s t i n g s t h e p r o j e c t i l e s w e r e f a r t o o h e a v y t o  be  their  s u s p e n d e d by t h e p a r t y  were  suspended  from  ballo.bns.  a  raft  Detonation  was s i m i l a r  explosives  with the exception  p l a s t i c e x p l o s i v e . As a detonated ship.  As  a  determined The  result,  fixes  an  alternative,  they  used  f o r t h e commercial  t h e bombs were p r i m e d  precaution,  with  t h e bombs  were  i n e x c e s s o f 1 km f r o m t h e s h o o t i n q distance  h a d t o be  radar. d i s t a n c e s were d e t e r m i n e d by r a d a r o u t  of approximately  h a d t o be  that  safety  by t h e s h i p ' s  to a d i s t a n c e  o f 282 kq p e r s h o t . Due  As  the shot-to-ship  ship-to-ship  explosive,  made o f empty 45 q a l l o n drums.  to that  at distances  hiqh  used  22 km.  t o determine  Beyond  this,  LORAN  the ship's  A  relative  positions. The shooting behind The  direct ship  water by  wave  means  (D.W.W.)  was d e t e c t e d  o f a hydrophone t r a i l e d  at  immediately  t h e s h i p , a n d by a g e o p h o n e l o c a t e d o n t h e s h i p ' s  two s i g n a l s were r e c o r d e d  t i m e c o d e on a 4 c h a n n e l WWVB s i g n a l s recorder  with  deck.  t h e WWVB  FM t a p e t r a n s p o r t . The h y d r o p h o n e a n d  were r e c o r d e d  played  simultaneously  the  directly  on a 2 - c h a n n e l B r u s h  a t a s p e e d o f 125 mm/s. T h e s e r e c o r d i n q s  u s e d t o t i m e t h e D.W.W. a n d t h e d a t a  recorded  chart were  on t a p e was u s e d  19  as a b a c k u p . The  receiving ship trailed  a 610 m c a b l e f r o m  which,  at  i n t e r v a l s o f 91 n , 6 i n d i v i d u a l h y d r o p h o n e s y s t e m s a n d b a t t e r y boxes  were  de-coupled by  s u s p e n d e d t o a d e p t h o f 45 m. The b a t t e r y b o x was f r o m t h e main c a b l e and h e n c e s u r f a c e  _ shock  cord.  In order  t o provide  damping, t h e hydrophone and  a  15  m  wave  action  a d d i t i o n a l mechanical  cable  leading  b a t t e r y b o x w e r e made n e u t r a l l y b o u y a n t by a t t a c h i n g  to  the  flotation  material. The  signal  amplified  output  from  by 20 db a n d t r a n s m i t t e d t o a m p l i f i e r s i n t h e s h i p ' s  l a b o r a t o r y . They a r e f i l t e r e d and  the hydrophone element i s p r e -  then  amplified  by  using  l i m i t s o f 0.0  individual  fixed  manually s e t f o r each s h o t .  The s i x a n a l o g  time  on  code  freguency using  are digitized  o f 312.5 h z a n d t h e n an  I.B.M.  acquisition channels channel  system  plus  compatible, (Clowes  t h e WWVB  chart-recorder  board  1977).  time  amplifiers,  signals  plus  WWVB  the r e c e i v i n g s h i p a t a  written 14  gain  t o 100 hz  onto  magnetic  tape  b i t , multi-channel  data  Five  code  of  the s i x data  are monitored  on a s i x -  t o e n s u r e good g u a l i t y c o n t r o l  of  the  nor  the  data. Unfortunately, shooting  ship  although  this  the sea f l o o r  neither  had o p e r a t i o n a l was s u p p o s e d  the recording depth  ship  sounding  eguipment,  t o be a v a i l a b l e . Thus no r e c o r d o f  t o p o g r a p h y was o b t a i n e d  during  the c r u i s e .  20  2 PRELIMINARY ANALYSIS  ZsA D e m u l t i p l e x i n c j As d i s c u s s e d i n S e c t i o n 1.4, used  in  digital  this field  digitize  study tapes.  the  sampling freguency  were r e c o r d e d  is  signals  hz.  e l i m i n a t e any  aliasing  filter  hz  (100  board  is  .0032  hz was  the s  and  T h i s high sampling  rate  a  12db/octave  rolloff)  set  on  two  used  to  ship.  Thus t h e  the  Nyguist  was  problems a s s o c i a t e d w i t h  with  data  i n m u l t i p l e x e d form  on  of t h e d a t a  156  channel  A s a m p l i n g r a t e o f 312.5  analog  interval  the seven  chosen  the  to  high-cut  of the  analog  amplifiers. F a r more d a t a w e r e s t o r e d on actually and  needed.  edited  the  For t h i s r e a s o n  simultaneously  analysis.  Approximately  the f i r s t  arrival  as  the data  a  for  the  reflection  immediately  after  the  end  c h o s e n t o be wave and A  the  immediately  super-critical  data  step  was  water  bottom  cutoff  for  the r e f r a c t i o n  bottom were  demultiplexing  of  the data.  During the process of  the d i g i t i z e r  occaisionally  from  "lost"  be  was water  reflections.  problems  signals  data  of the d i r e c t  of  w r i t i n g the analog  The  to  second  water  before  chosen  the  the a r r i v a l  the  r e f r a c t i o n data.  of  after  was  to  were k e p t  number  and  than  were d e m u l t i p l e x e d  t h r e e seconds of data  f o r b o t h r e f l e c t i o n and  whereas  tapes  preliminary  termination  reflection  field  encountered  during  digitizing  the hydrophones onto data.  Clowes  the  (1977)  tape, has  21  described  how  acguisition board  such  s y s t e m . A l s o on  operators  failed  t e r m i n a t i o n of the several  errors  data  can  occur  a number  to  At  of  write  the  in  time  of the  this  demultiplexing  p r o g r a m t h a t would i d e n t i f y  errors  available  was  considerable would This  effort  handle  was  in  the  of  study and  writing  was  As a  use.  field  distance  technique  traveltime the o r i g i n  e x p l o s i v e charges are by  a  know t h e known  timed  exact  distance  the  the  calculations time of the  determination  seismic  is  impulse,  arrival  i s of primary used,  result, that  field  data.  the  time  O r i g i n Times  I n a l l s e i s m i c work a c c u r a t e  certain  a  of  in the  source  of  that associated  detonated  our  case  signal  seismic  error with  the at  i m p o r t a n c e . As a r e s u l t  major  original  of  of in  the  determining Since  the  at d e p t h , remote from the  ship  i n s t a n t of detonation.  impulse.  Thus  depth of the shot  e x p l o s i v e impulse  a the  fuse/Seisiaocap assembly, i t i s i m p o s s i b l e  and  no  these  program  a v a i l a b l e for general  and  begun,  correct  p r o g r a m i s now  explosive detonation,  ship  concatenation  with the  original  the  the  the e r r o r s a s s o c i a t e d  the  data  at  a l l  between  marine  Files"  department.  expended i n  2.2  and  the  occasions  "End  data, r e s u l t i n g  files.  with  at  we  plus the  the  must  use  the  known t i m e  arrival  of the  hydrophone  behind  t h e s h i p t o e x t r a p o l a t e back t o t h e o r i g i n  to  of  trailed  t i m e of  the  shot. The  paper c h a r t  recordings  described  i n s e c t i o n 1.3  were  22  the  only data  accuracy  s e t used t o time  was  obtained  the d i r e c t  usinq  these  arrivals.  r e c o r d i n g s and hence the .  back-up data  on t h e FM t a p e s  the  a t t h e h y d r o p h o n e c o u l d be t i m e d  ms  signal from these  ship  to  was n e v e r u s e d .  c h a r t r e c o r d i n g s . The method  shot  d i s t a n c e has been d e s c r i b e d  The  (100  which  was  of  measuring  the  i n s e c t i o n 1.3.  The than  t r a n s l a t e s t o an e r r o r o f 15 i f o r t h e n e a r  shots  used.  Using  distance errors translate  t o be  5  less  m) and 45 m f o r t h e f a r t h e s t s h o t s  method  arrival  to b e t t e r than  of  e r r o r i n t h e r a n g e f i n d e r method i s e s t i m a t e d 15%,  Sufficient  (300 m) i n w h i c h  a water v e l o c i t y  this  o f 1.49 km/s,  these  t o t i m i n g e r r o r s o f 10 ms and 30  ms,  determining  the  respectively. The r a d a r anti-submarine gallon they  projectiles  drums were s o w e i g h t e d barely  them v e r y were  method o f  the  was c o m p l e t e l y down  by  distances unreliable.  the  On s u b s e q u e n t c r u i s e s a r a d a r  The 45  projectiles  showed a b o v e t h e s u r f a c e o f t h e w a t e r .  p o o r r e f l e c t o r s and t h u s t h e r a d a r  useless.  to  ranges  that  T h i s made obtained  reflector  should  be p l a c e d on t h e d r u m s . I was a b l e , h o w e v e r , t o d e v i s e o b t a i n i n g the d i s t a n c e s to these diagram :  an  alternate  shots. Consider  method  of  the following  23  The  method  requires  only  a  difference  between t h e d i r e c t  plus  water  the  depth  t i m e s of a r r i v a l Tz  actual length  depth  D  could  reflection typically surface  o f e i t h e r Tz moved  applying  to  path  have the f o l l o w i n g  the t r a v e l  the  bottom  wave T<  and  bottom  bounce  i s determined  without  knowinq  or T i . I n essence,  the s h o t  the  the  surface  for  assume  that  the  i s t h e same as t h e Tz  T^  T2=  I V-  tl,2 )''  2.2-2  /htl  II"]",-- f f * S * Z * / ' * - ( x % 0 ' ' \ ? / U 7  by  correct  and  v a l u e o f T^-  error  using  method w o u l d was  not  the yield  first  and  or  i n t h e T.j_+66  t h e main s o u r c e o f e r r o r depth.  w o r k i n g depth  we  second  bottom  of  can  use the  I t i s highly sounding  analysis  reflections.  X i n d e p e n d e n t l y o f the depth  correspondingly  water  t h e same t y p e  a b l e to time a c c u r a t e l y  error  we  Ti .  hen'ce c o u l d n o t a p p l y t h i s The  Z,  t o f i n d t h e v a l u e of X y i e l d i n g  I t i s p o s s i b l e t o perform above  path  2.2-3  a r e a s o n a b l e v a l u e f o r the depth trial  surface-to-  : 2.2-1  2.2-3  at  bottom  travel  + d*)**/l-U1  By a s s u m i n g  The  the  (  X  bounce,  to our d a t a .  f,=  l  time  t h e c o r r e c t i o n T c = D * c o s 9 / 1 . 4 9 , where  cos£-0.97. Then i f we travel  and  of  t o make i t a p p l i c a b l e  h e n c e I * - Tt  be  by  arrival  of the d i r e c t  a r e r e c o r d e d and  the  knowledge  the second  Z.  bottom  as  Such a  However bounce  I and  procedure. Ti  method i s e s t i m a t e d t o be  ms  u s i n g a 1.49  i s the lack  km/s  water  o f an a d e q u a t e l y  £100  velocity; defined  recommended t h a t f u t u r e c r u i s e s  equipment as  the l a c k  m  of adequate  have depth  24  knowledge in  was  a hindrance  not o n l y i n t h e s e c a l c u l a t i o n s  s e v e r a l other p l a c e s throughout  the  study.  C o r r e c t i o n s t o o b t a i n the t r u e o r i g i n were  c a l c u l a t e d u s i n g 2.2-1. The ms  f o r the near  time  of  70  £15  £35  ms  f o r t h e f a r t h e s t s h o t s u s i n g b a l l o o n s , and  s h o t s using the  f o r the shots using the anti-submarine  Once t h e  Shot-Receiver  calculated. w a t e r wave From  This  shot-receiver  arrival  d i s t a n c e s can  has  170  ms  been d e t e r m i n e d , each s h o t  time,  water v e l o c i t y  the  must  of the  be  direct  hydrophones.  the o r i g i n {1.49  time  of  the  km/s)  ,  the  be c a l c u l a t e d . I n s o d o i n g , i t i s  assumed t h a t t h e d i f f e r e n c e s i n t h e s h o t and are  250  projectiles.  h y d r o p h o n e s from  assuming a c o n s t a n t  to  1000  the shot at the v a r i o u s  a knowledge of t h i s  s h o t , and  balloons  i s done by t i m i n g t h e a r r i v a l  (D.W.W.) f r o m  shot  Distances  o r i g i n time of the shot  d i s t a n c e of the i n d i v i d u a l  the  r e s u l t i n g c o r r e c t i o n s ranged  from  2.3  but  receiver  depths  insignificant. The  d e m u l t i p l e x e d r e c o r d s o f e a c h s h o t were p l o t t e d a t a  d e n s i t y of digitized plotted  100 ppinti/in v a l u e . The  o f 0.75  each  was  millimeter  direct timed  trace  i n . The  are t y p i c a l of these  The onset  s i x seismic channels  simultaneously,  maximum a m p l i t u d e F i g 2.1  ( i . e . 1s=3.125 i n ) e a c h p o i n t b e i n g  four  and  beinq  normalised  seismograms  then  identified  with respect to the nearest Timinq  were to a  shown  on  records.  w a t e r wave was  ruler.  W.8.V.B.  one  of,first  breaks  and  s e c o n d by  the  phase  usinq  a  f o r a l l c l e a r D.W.B.  25  Four  seismograms  typical water  profile  75-1,  of those used to time the  direct  wave  arrivals.  and  the  D)  seismogram (A), the  first  Approximate  d i s t a n c e s a r e ; A) 94 km,  from  44 km. is  the d i r e c t  0.5 The  the  refraction shot-receiver  km,  B)  9.5  km,  C)  upper  trace of  WWVB  t i m e code.  arrival  each In  i s ' overloading  hydrophone/amplifier combination.  26  27  a r r i v a l s was b e t t e r t h a n 0.5 mm, c o r r e s p o n d i n g 7ms  i n time.  be c a l c u l a t e d kilometers  Naturally this  along  depth,  the direct  bottom  bounce.  observed i.e.  wave  became  being  this  normalise  the surface  the  be t i m e d than  of  profiles  refraction  part  t o b e t t e r than  was n o t  as the hydrophones. I n  any  simply being t o arrivals  bottom r e f l e c t i o n . to better  than  This 1mm  o f the p r o f i l e ,  0.5 mm  (7ms)  for  t h e D.W.W. a l l shots  25 km. I n t h e r a n g e 25-55 km, t h e D.W.W. became i t e a s i e r to time  shot,  i t  was t h e n  was t h e m a j o r  again  of the depth  accuracy.  I t was p o s s i b l e  D.W.W. and t h e b o t t o m to within  being  the bottom depth  bounce  a t the middle  p o s s i b l e to c a l c u l a t e  thelack  source  assumption  agreed  the  shots.  r e c e i v e r d i s t a n c e . Once sounding  and  u s i n g s h o t s a t 45 m d e p t h ,  By a s s u m i n g a v a l u e f o r t h e w a t e r  the  the  from t h e  o f t h e t r a c e c o n t a i n i n g D.W.W.  v e r y e m e r g e n t and I f o u n d instead.  to  T h i s problem  a l l o w e d t h e D.W.W. t o b e t i m e d  Along  closer  compared  o f t h e water  o f the l a r g e amplitude  13 ms f o r t h e s e  could  u s i n g s h o t s at 7 m  was n o t c a t a s t r o p h i c , t h e s o l u t i o n  independently  t h a n two  due t o t h e e n e r g y  t o o deep t o d e t e c t i t .  the section  proceedure  profiles  was p r o b a b l y  on t h e r e f l e c t i o n  greater  indistinct  w i t h t h e s h o t s t h e same d e p t h  case  or  At distances  t h ereflection  s h o t s being trapped near hydrophones  than  not a l l s h o t - r e c e i v e r d i s t a n c e s could  simply.  This  to better  o f adequate  o f error,  thelimiting on some  depth  t h e poor  factor  i n  the  s h o t s t o time both the  bounce. In t h e s e c a s e s ,  50 m f o r a l l c a s e s  with  the shot-  t h e two  t e s t e d . Beyond  results  55 km, t h e  28  D.W.W. a n d . t h e b o t t o m to  reflection  be i n d i s t i n g u i s h a b l e .  a r r i v e d so close together  T h e c o m b i n e d p h a s e s were somewhat  emergent from t h e background n o i s e and  reverberations,  be d e t e r m i n e d The t o t a l incorporates  not only  estimated  therefore  of l a t e  the f i r s t  refraction  t h a n 4ram o r 50 ms.  t h etiming  but a l s o the e r r o r  error  arrivals  break, i n t h e s e c a s e s t o  e r r o r c a l c u l a t e d f o r the s h o t - r e c e i v e r  bottom r e f l e c t i o n s , total  causing  to better  i n the  error  distance  o f t h e D.W.W. and  i n the o r i g i n  shot-receiver  t i m e . The  distances a r e  :  TABLE 2. 1  Reflection  Data  Distance Range km  Shot Depth m  Timing E r r o r ms  Distance E r r o r ra  0-2 2-4 0-4  7 7 45  <22 <30 <22  <30 <45 <30  Refraction  Distance Range km  as  Data  Timing E r r o r ms  Distance Error m  <22 <70 < 120  <30 <100 <200  29  Thus  in a l l cases the d i s t a n c e s  than  1.5%  and  could  be  i n most c a s e s t o b e t t e r t h a n  Record  ZSLH  were c o m p i l e d  using  the  sufficient any  to simply  amplitude  corrections from effect  use.  compile  amplitudes  As  would To  and  refraction  might expect, demultiplexed  be  lost  obtain  i t was  profiles  author  and  i t was  not  data  i f no  meaningful  neccessary  of s p h e r i c a l s p r e a d i n g ,  amplifier  one  t h e raw  information  were a p p l i e d .  the  1%.  p r o g r a m RSEC w r i t t e n by t h e  a v a i l a b l e for general  better  Sect ions  Record s e c t i o n s of a l l r e f l e c t i o n  now  determined to  since  appropriate information  t o compensate f o r  v a r y i n g c h a r g e s i z e and  the  varying  gain.  A m p l i f i e r Gain : The  am'bient n o i s e  due  to the  the  ship.  level  on  impulses  p o s i t i o n i n g of  gain  a t each hydrophone v a r i e d g r e a t l y  the  main c a b l e  order  to  a l l  traces  and  to  prevent  the  at  high  gain)  from  (especially different  (G)  maintain  and  In  amplifiers, The  level  gains  i s defined  by  maneuvering  a s i m i l a r background  of  noise  transient  noise  overloading  the  were u s e d on d i f f e r e n t a m p l i f i e r s . :  G = 20Log (Vout) Vout-=output v o l t a g e  for a  1 volt  input -6  To  remove t h i s  traces, constant  effect  in effect gain.  a c o r r e c t i o n of  normalising  a l l  10 7-0  was  amplitudes  a p p l i e d to a l l to  a  chosen  30  Charge S i z e : For Mueller sea  a  weight  approximately  amplitudes  profiles, single  of  W  as  using  pounds, O ' B r i e n  various  charge  and  recorded  at  sizes  were a p p l i e d  2/  effect  along  the  to a l l traces for a  shot.  ( s h o t 54)  were u s e d source  p r o f i l e s and  on t h e r e f r a c t i o n  as e n e r g y for  out  p r o f i l e s , commercial  each  shot an  consisted  of  equivalent  commercial  t y p e s and  the  correction  e x p l o s i v e s due  steel  for this  packaging difference  three of  yield  67  explosives energy  anti-submarine 282  kg.  There  b e t w e e n t h e bombs  to the d i f f e r e n t e x p l o s i v e  of has  of  however, t h e  yield  p r o b a b l y e x i s t s a d i f f e r e n c e i n energy the  to a distance  s o u r c e s . B e y o n d 67 km,  p r o j e c t i l e s having  and  (1960)  I n o r d e r t o remove t h e  c o r r e c t i o n s o f W~ 3  For t h e r e f l e c t i o n km  of  (1962) have shown t h a t s e i s m i c a m p l i t u d e s  vary  on  charge  the  bombs.  However,  no  been made, b u t i t s h o u l d  be  no t e d .  S p h e r i c a l Spreading Cerveny  and  : Ravindra  amplitudes at l a r g e critical 1/r  3  or  distances  h a v e shown t h a t h e a d wave  decrease  d i s t a n c e , h o w e v e r , t h e drop 1/r*.  Even  so,  amplitude  correction of r  correct  for  a p p r o p r i a t e as angle  (1971)  2  spherical Braille  reflection  and  i t  was  to the  amplitudes  1/r ;  near  2  i n amplitude  refraction This  is  not  as  1/r.  to  uniform  seismograms  (1975) have shown  decrease  the  i s closer  decided to apply a  spreading. Smith  as  to  entirely that  wide  Thus t h e  r  2  31  factor  will  enlarge  refraction applied  these  amplitudes.  to  the  amplitudes However  synthetic  out  of  these  seismograms  proportion to  same  so  the  corrections  the  are  compensation  is  consistent. To  compensate  the  sub-critical  the  data.  Smith  This  (1975)  layered  medium  however  that  amplitudes  for  the  reflection  data  i s also  not  having  shown  drop a  were  effect  correction being  of  used  of  r  was  l  reflection and  l  r  in  spreading  appropriate,  that r ~  spherical  factor  entirely  o f f between  not  a  of  r  - 1  - .  was  1  applied  Braille  was  in  than  a  a  decided  satisfactory  other  to and  amplitudes It  5  on  since  gualitative  manner.  Hydrophone Two others  of  were  varying tend  Sensitivities the  to  from  on  affect  hydrophones after  hydrophones  purchased  extents  each  . amplitude.  As  of  each  was  factor It record varying  to  a  i s desirable  bottom  to  a  i n the  remove  the  topography.  Both  age  and  extent  element;  as  a  been shot this an  effect,  c o m p i l a t i o n of effect  on  have  for a any  travel mentioned  to  of  use the  the  traces  significantly the  appropriate  amplitude  the  Conseguently,  applied differed  used  result,  sensitivities.  by  I  whereas  been  for  constant  unused had  particular  multiplied  and and  had  correction  new  before  trips.  corrections  approximate  section  years  differing  for a  in  were  sensing  of  channel  few  previous  other  channel  a  the  were  a l l the  :  amplitude correction  given  phase.  marine  seismic  times  of  several  the times  Sl  previously cruise. such the  that  we  Without  profile  (see  functioning with  midway  along  the  profile  usable  information  inoperative  on  to  Butterworth  the  one  was  might  (recorded  the the  at  An  filters  was  to  judged  be  noise the  75-1.  the  lack  of  topography of  started  expect, two  first)  of  duration  and  alonq static  irregular  six  did  not  remained  the  profile  so  threatened  however,  intervals.  ignored  with  inoperative  six also  75-1; of  we  became  Channel  start  using  i t  thus  A l l  did  giving  data  during  the  i s given  record  s e c t i o n s , the  remained.  various to  In  as  from  compilation  phase,  of  2.0 of  of  of  order  5.0 to  to  and  remove  of and 15  profile  later  0.8  the 30 hz  data  four the  (1 9 7 6 ) . 100  a considerable  apearance  sections  identify  zero  between  insufficient  limits  a  i n Kanasewich  filtering  general  into  e x c e l l e n t development  were f i l t e r e d between  Filtering  effort  as  filtered  provided  to  application  f i n a l compilation  amplifiers  stage  made  profile  for  filter.  Butterworth  traces  smooth  hydrophones were  bandpass  improve  the  on  sections.  Prior  freguency  point,  profile  75-1R  of  o f f and  were  soundinq  relatively  first  functioning at  record  the  same number. C h a n n e l  operate  the  1.2)  the  duration  cease  belabour  the  hydrophones,  finish  the  to  plus Fig  have adequate depth  unnecessary.  Although  for  not  wishing  information  corrections  to  did  pole  theory The  hz,  for  analog  but  this  amount o f  hiqh  this  noise  and  sections a l l seismic hz.  were  75-1.  arrivals  applied  This  was  embedded  at  done i n in  one an the  reverberations of e a r l i e r guite  successful  arrivals  were  as  arrivals.  shown  by  This  proceedure  proved  F i g 4.8 where c o h e r e n t  later  identified.  The p r o g r a m RSEC i s c a p a b l e o f c o m p i l i n g and p l o t t i n g part of a complete  record  section  w i t h any  desired  time  any and  d i s t a n c e s c a l e , a n d any d e s i r e d a m p l i t u d e . T h i s p r o v e d t o be a most u s e f u l t o o l look  at  desired  any  i n a n a l y s i n g t h e s e i s m i c d a t a . The a b i l i t y  individual  amplitude  p o r t i o n of a c o m p l e t e  saved  a n a l y s i s of both r e f l e c t i o n  considerable and r e f r a c t i o n  time  section  a t any  throughout  profiles.  to  the  2  SUB-CRITICAL  REFLECTION  Methods of  The 1  are  record  shown  contain The  in  Figs  a l l the  first  wave.  sections 3.1  data  and  arrival  seen  the  first  identified,  the  first  at  s.  shown Fig  5.3  on  3.1  the  were  These  of  particularly arrivals and  the  due  to  complete  at  about  ones  defined  to  allow  very  hydrophone  than  could  with  a  they  more  are  "P"  on  a  broad  were  not  individual  signature  be  second  analysis.  timing  Far  be  analysis  in gaining  and  water  can  the  proper  However  identifying  arrivals  for  shown  useful  seguences.  direct  s and  timed  75-  profiles  bounces  2.7  the  overlap,  and  hydrophones.  i s the  bottom  c l u t t e r e d appearance.  time the  3.3  shows t h e  arrivals.  possible time.  operative  profiles  second  were  75-1E  variations detail  obtained  was  off  these  sections.  Fig time  the  the  although  in  trace  profiles  r e s p e c t i v e l y . These  reflections  reflecting  generally  needed  on  the  poorly  useful to  3.2,  starting  sections  the  reflection  and  profiles,  record  overview  A l l of  too  Analysis  a v a i l a b l e from  Both  about  for  DATA  to As  By  type  using  examine s e t s previously  different  ages,  each  one.  This  for  each  of  of  section  these  to  expanded  arrivals  mentioned,  used  for the  identify  sections one  r e s u l t i n g i n d i f f e r e n t freguency differing  channel  frequency  having  response  a>' d i f f e r e n t a r r i v a l  i t  channel  hydrophones  was  and was  at  a  were  of  responses  for  responsible  signature.  With  Record s e c t i o n 1R  using  of r e f l e c t i o n p r o f i l e  shots at  c o r r e c t i o n s have data as filtered section  45m depth. Amplitude  been  per s e c t i o n 5.0-30  75-  applied  2.4  hz.  to  the  and the data The  part  w i t h i n the r e c t a n g l e  of  are the  is  shown on  the r i g h t s i d e with a f i v e - f o l d  increase  in amplitude. the s t r a i g h t six were  l i n e . Curved l i n e s  reflection  a  line  deep  refraction  which  the bottom r e f l e c t i o n . shows  reflector  arrival.  poorly d e f i n e d  along  show the  phase c o r r e l a t i o n s  timed. W i s  The dashed from  The DWW a r r i v a l l i e s  The  the to "?"  reflections.  transition the  first  indicates  38  all  c h a n n e l s p l o t t e d out  found  i t  to  differing signature  the  phases.  extremely  together  However  correlations differing  difficult  with  could  any  individual  3.2,  the  s e c t i o n s , phase  quite  easily.  i s shown c l e a r l y  I due  continuity i n  channel  accomplished  a r r i v a l signature  and  t o phase c o r r e l a t e a r r i v a l s  complicating  the  be  as i n P i g s 3.1  on  Fig  This  3.3.  Even u s i n g t h e s m a l l e r s e c t i o n s I e n c o u n t e r e d a number o f problems, of the  the  m a j o r one  being  the  a r r i v a l signatures. This  bubble  pulse  with  a r r i v a l s e e n on clearly  this  was  F i g 2.1  (B)  and  on  had  records,  f i r s t bubble having  the second  the  original  everything  sufficient  a relative  w i t h i n 100  Various  This  ms  data  by  adaptive  deconvolution  p u l s e e f f e c t and  Knize  methods  would be  D.W.W.  3.2  show  and  second  complicate amplitude  tended  have  The  first  0.9  compared  to  obliterate  arrival. been  attempted was  that  r e q u i r e d t o reduce t h i s  data.  the r e s u l t  This  t h a t no  the  of  to  (1 976) . H i s c o n c l u s i o n  enhance the  of 'this p r o j e c t w i t h  ms.  when  o n s e t of an  length  prominent  and  to  0.4,  effect  of the  deconvolution  similar  100  a relative of  ms  a  3.1  amplitude  amplitude  pulse.  Figs  300  by  problem. Both the  bubble pulses  and  caused  period of approximately  bubble pulse  the  approximately  time  bubble  f a l l s beyond the deconvolution  on  scope  has  been  atteaipted. One decrease  other in  makes l a t e r plotted.  minor problem with the s e c t i o n s i s amplitude  arrivals  This  was  of  the  difficult  solved  s i g n a l along  t o see  quite simply  i f the by  the  the  marked  trace.  This  complete t r a c e i s  splitting  the  section  Two  typical  individual sections  are  correlated and  first  reflectors.  record  sections  reflection filtered phases and  used  to time  arrivals. 5.0-30  hz.  a r e the water second  'Both The  bottom  sub-bottom  i n t o v a r i o u s time segments  and  by  plotting  s e c t i o n i n d i v i d u a l l y w i t h an a p p r o p r i a t e Any  reflection  information w i l l of  interpretation  each  time  amplitude. from  which  be c a l c u l a t e d i s only as good a s  the various a r r i v a l s .  phase c o r r e l a t e  out  velocity  the  timing  As s t a t e d p r e v i o u s l y , I was a b l e t o  arrivals  from  shot  t o shot  using  record  s e c t i o n s showing o n l y one c h a n n e l  a t a time. Phase c o r r e l a t i o n  of  channels  arrivals  between  different  however was not p o s s i b l e . various  Thus  t h e phases  picked  f o r the  c h a n n e l s were sometimes o f f s e t from one a n o t h e r by as  much as 50 ms. C o r r e c t i o n s f o r these overlapping  the v a r i o u s  sections  measuring the t i m e d i f f e r e n c e between each  f o r t h e same shot  channel  r e l a t i v e t o a standard  offsets on  a  the  were  made  light  table  phases  picked  channel.  each  on  phases  channel.  While  i t  approximately breaks.  was  possible  Most  first  to  trace  arrivals  3.0 t o 3.5 kin i t was i m p o s s i b l e arrivals  were  r e v e r b e r a t i o n s and bubble p u l s e s their  and  By t h i s method I  was a b l e t o e l i m i n a t e the problem of p i c k i n g d i f f e r e n t for  by  extremely of  to  emergent  earlier  breaks were e s s e n t i a l l y h i d d e n .  out to  pick  first  from  arrivals.  the Thus  As a r e s u l t , a l l  p i d k s f o r any one a r r i v a l c o u l d be o f f s e t by up t o 50 ms the  first  break.  No  attempt  however, as i t was d i f f i c u l t time  should  be.  This  was made t o c o r r e c t f o r t h i s ,  to identify  j u s t what t h e  offset  o f f s e t , w h i l e not a f f e c t i n g t h e l a y e r  v e l o c i t i e s obtained i n the a n a l y s i s , placed 100  from  m on t h e l a y e r d e p t h s .  limits  of  50  to  Finally, correlating from  the  caused the  although  arrivals,  different  I  at  had  distances  horizons  that  the  the  arrivals  beyond  started  considerable confusion  result  considerable  to  of  km  the  overlap  in trying  possibility  2.5  success  to  a  arrivals  somewhat.  follow  "jumping"  phase  This  phases  phase  with  cannot  be  overlooked. Once problem  of  is  the  that  average to  a  how  to  reflecting  dominated average  by  velocity to  became  essential  this  thick  Kay  as  ray  the  ear-th  to  change  the  obtained.  case  is  the  Method  :  this  little  from  the  effect Three  on  with  basic  shot  receiver is  causing  the  to  layer.  In  time  picks,  i t  various  methods  the  velocity  layer  the  to  and  m)  travel  the  problem  related  average  (2000  of  faced  The  layer  results  layer.  parameter  ray  (p)  in  work.  of  (an  reflection constant  suitable  a  seismic  have  as  a  If  we  assumption  analyses along  tool  ray  can  t u r n i n g about  question.  and  remains  of  velocity  refraction  refraction  our  water  homogeneous  extremely  was  c o n t a i n i n g the  very  remove  laterally  (B.P.)  In  I  arrivals  water  the  to  the  plane  thick  angular  of  of  data  timed  arrivals  been  used  to  this.  Parameter  The  a  been  the  horizon.  analyse  accomplish  1)  time  from  the  order  of  analyse  travel  velocity  had  for  be  the  assume that  ),  then  the  ray  analysing  centre the  path.  ray This  of  of  the  earth  is  i s made  the  both  thought  i n  most  parameter makes i t  reflection  and  43  Consider  the following  :  A)  B)  p = s i n (i) /v  3.1-1  p=dT/dX  3.1-2  According simply  a  to Snell's restatement  Bullen  (1963,  ray  parameter  The  two  X  curves  of  through  of  reflection passing simply  on  arrival  110)  medium  layers  and  of  law.  3.1-2  Hence i s  i s the fundamental  diagram  (B).  one  the second  of  using  Eguation an  Snell's  contributions  through by  of  diagram  both  effect  s i n ( i / ) /V/ = s i n ( i z ) / v * .  3.1-1  is  derived  equation  by  of the  method  layered  comprised  the  p  law  the  3,1-2  The  curve  from and  T  the  two.  vs  X  The  one.  This  can  i n t h e two  defining passage objective  curve,  r a y o f f the bottom  layer egn  (A) r e s u l t s  of be  arrival of  the  i s to  generated layer  T  2,  vs 2 is rays  remove by  the  of the ray  accomplished  guite  3.1-2. states  at a distance  that  X equals  the p value the slope  corresponding  of the t r a v e l  to  time  curve p  (T v s X) a t t h a t p o i n t . I t i s p o s s i b l e t h e n  vs  X  curves  curve  f o r both  arrivals  (T v s X) w i t h a t h i r d  the  derivative  R.P.  value p w i l l  of the fitted  and s e c o n d  times  and d i s t a n c e s o b t a i n e d two T vs X c u r v e s  of i t s passage through arrival  to a simple  the standard  formula  T2= ( X + 4 H ) / V 2  2  p o l y n o m i a l and t h e n  curves.  layers. from  Any p a r t i c u l a r  talcing r a y with  reflecting  off  By s u b t r a c t i n g t h e t r a v e l  the corresponding  we c a n remove t h e e f f e c t  l a y e r o n e . We  have  one l a y e r e d c a s e given  a  one and two by f i t t i n g t h e  at the surface after  the  the  first  arrive  both  on  order  to obtain  then  p  values  on t h e r a y  reduced  the  w h i c h may be a n a l y s e d b y  below 3.1-3  2  Where H=depth o f t h e l a y e r V=velocity of the layer Keen analysis. and  (1976) h a s d e v i s e d an a l g o r i t h m t o p e r f o r m  the above  I have m o d i f i e d  own  have  used  reflection  T -X 2  2  the  primary  method  data  of reducing the  data.  Method : Having  method, using  i t as  t h i s a l g o r i t h m t o f i t my  I  h a d no p r e v i o u s e x p e r i e n c e thought  with the ray  i t b e t t e r t o compare t h e r e s u l t s  i t with r e s u l t s  o b t a i n e d u s i n g t h e more  parameter obtained  familiar  T -X  root-mean  square  2  2  method. Dix (r.ra.s.)  (1955)  has  defined  the  average  velocity  from  t h e s u r f a c e down t o t h e b o t t o m  of  the  nth  layer  as  3. 1-4 *L  To,i  Where Vw i s t h e a v e r a g e r.m.s. V~i i s t h e i n t e r v a l  velocity  velocity  of the i t h layer  To,! i s t h e two-way v e r t i c a l i n c i d e n c e t r a v e l t i m e i n the i t h l a y e r . When from  the t r a v e l  times  and d i s t a n c e s o f a s e t o f a r r i v a l s  one r e f l e c t i n g h o r i z o n a r e p l o t t e d  a r e h y p e r b o l i c . I f , however, curve, they d e f i n e s t r a i g h t  they  average  corresponds  r.m.s. to W  are  plotted  velocity  of eguation  of  kth  -  This  velocity  2  2  being  velocity  from  the  and i t s c o r r e s p o n d i n g  time a s V K - I and  TK-I .  d e f i n e V K a n d T* as b e i n q t h e c o r r e s p o n d i n g  layer  T -X  3.1-4.  way v e r t i c a l i n c i d e n c e t r a v e l  the bottom of t h e k t h l a y e r .  they  b e i n g t h e two  the a r r i v a l .  to the top of the kth layer  further  a  t i m e and t h e i n v e r s e s l o p e  L e t u s now d e f i n e t h e a v e r a g e r.m.s. surface  on  l i n e s , the intercept  way v e r t i c a l i n c i d e n c e t r a v e l the  on a T v s X c u r v e  two  Let  us  values to  Then t h e i n t e r v a l v e l o c i t y  of the  i s g i v e n b y D i x (1955) a s :  ^H* Th  -  J/h-i  3.1-5  T*-i  TH - TH -1 Eguation model  3.1-5 has been u s e d  to  compare  p a r a m e t e r method.  with  the  to  generate  results  a  velocity  depth  obtained using the r a y  3) S t r i p p e d T - X 2  A third stripped  layer  2  obtaining  ray  layers  parameter  method  to  f o r the a r r i v a l  however  3 . 1 - 1 and s i m p l e  stripped  as  the  o f the upper  depths.  the p vs X curve  upper  been  the  to  method, u n l i k e t h e  two, d e p e n d s on t h e p r e v i o u s c a l c u l a t i o n  used  equation  referred  m e t h o d , was a t t e m p t e d . T h i s  2  v e l o c i t i e s and I  The  method o f a n a l y s i s , h e r e b y  T ~X  previous  Method :  2  were  the  extent  branch i n guestion.  stripped  off  by  means o f  g e o m e t r y . Once t h e u p p e r l a y e r s  o f f the remaining  data  of  were a n a l y s e d  had  by use o f  eg u a t i o n 3,1-3 The m a j o r p r o b l e m a s s o c i a t e d w i t h t h i s r e q u i r e s an a c c u r a t e depths  above  the  method i s t h a t  knowledge o f a l l t h e l a y e r v e l o c i t i e s and layer  in  question.  T h e s e v e l o c i t i e s and  d e p t h s a r e a l s o c a l c u l a t e d b y t h i s method, r e s u l t i n g by  step  means the  progression that  velocity  subsequent accumulating that  any  i n the analysis. Unfortunately  e r r o r s encountered during  and  thickness  values from  for  for a  a l l the lower  layer to layer.  We  layer  layer.  e d i f i c a t i o n than  This  will  affect  usable  the  with  the e r r o r s  expect  therefore with  each  method was a t t e m p t e d more f o r my own  f o r the r e s u l t s  t h a t i t would y i e l d  also  the c a l c u l a t i o n of  layers, would  i n a step this  t h i s method w o u l d become l e s s and l e s s a c c u r a t e  additional  i t  obtained.  I t was n o t  s o l u t i o n s from t h e data.  expected  3 ._2 A n a l y s i s o f R e s u l t s  Profile Fig  3.1  shows t h e c o m p l e t e  data  for profile  using  s h o t s a t 7 m depth  little  to  the  s h o t s d i d not shots. not  For  identified  little  this  was  reason  A  total Fig  well  first  t o be  of  bubble  p u l s e , e f f e c t a s i t had  examination of  interpreted The  sort  5.3  one  of proper  clearly  ms  of  45  reflectors  in  are timed  as e i t h e r  and  two  reflector  raoveout are  as  seen  clearly  time  for  their  arrivals. t h r e e was  As a r e s u l t  too  poor to e n a b l e  i t is identified  1;  however,  i t  upon  at a  evidence  w i t h a " ? " . T h i s a r r i v a l c o u l d a l s o be s e e n on again  two.  reflector  bottom m u l t i p l e s b e g i n n i n g  arrival  and  a r e v e r b e r a t i o n or a  3.1  timing.  the  identified  after  s. T h i s g i v e s f u r t h e r  analysis.  once  o f t h e 7 ra  t i m i n g them. T h e r e i s a  t h e same  as s e p a r a t e  t i m i n g o f of  yield  added  u s i n g s h o t s a t 7 m were  in 300  identified  o f the water  approximately  being  are  approximately  reflectors  profile  analyzed.  however,  Both  The  u n f o r t u n a t e l y o n l y 3 c o u l d be  This,  two.  energy  sub-bottom  encountered  was  reflection  however, i t  penetration  five  reflectors  was  arrival  of  of  m depth.  also compiled;  the p r o f i l e s  3.1;  two  trouble  prominent  u s i n g s h o t s a t 45  a n a l y s i s as t h e l o w e r  on  sufficiently  75-1R  record section  a l l o w the depth  analysed.  The  75-1R  a l s o was  not  on  profile  u s a b l e due  any Fig 75-  t o poor  48  Reflector as it  four i s timed  the reflection  very  w e l l and c a n  of  reflection a dashed  4.28  km/s).  refraction  This  t o the r e f r a c t i o n  identified  five  i s  as a s e p a r a t e  time.  arrival  the  last  arrival.  I t i s my o p i n i o n  i d e a c a n n o t be The  vs  x  modes  2  reflector  times  I  are have  of  discussed  i n the l a s t  of  the  results  method r e s u l t s  on  3.4  the  timing  this  (A)  how  and  arrivals  using  (B),  times  of  inadeguately  T a b l e 3.1 c o m p a r e s t h e r e s u l t s  usable  bottom  T v s X and  arrival  plots to i l l u s t r a t e  the  three  of t h e methods  section.  and  differed  T -X 2  methods  2  cumulative.  yield  essentially  c o m p a r a b l e e r r o r s . The s t r i p p e d T - X 2  from t h e p r e v i o u s  The f i r s t  t h a t n e i t h e r method r e q u i r e s any  two  layer. This  what would be e x p e c t e d c o n s i d e r i n g t h a t  velocities  be  from a deeper  f o r t h e water  Fig  plotted  e s p e c i a l l y i n the l a s t  are  can  i t i s t o o weak  adequate  distances  also  r a y p a r a m e t e r and  identical  method  that  Unfortunately  plotted  i s defined.  analysis  The  reflection  t h a t i t i s an a r r i v a l  and  t h r e e on t h e s e  arrival  extent,  the  i s shown on F i q 3.1 w i t h  and r e f l e c t o r s o n e , two a n d f o u r i n b o t h  respectively.  this  from  supported.  arrival  reflection 2  (apparent  transformation  c r u s t a l l a y e r , b u t due t o t h e l a c k  T  arrival  line.  Reflector  to  identified  from t h e sediment/basement i n t e r f a c e , s i n c e  c a n be t r a c e d o u t t o t h e f i r s t  velocity  be  to  a  greater  i s consistent  the  errors  2  in  with this  two methods a r e s i m i l a r i n  previous  knowledqe  of  the  and d e p t h s o f t h e l a y e r s a b o v e t h e one i n g u e s t i o n .  A.  T  vs  profile lines  X plot  75-1R are  used i n  f o r 5 r e f l e c t i o n s from  shown  on  third-order  the  ray  Fig  3.1.  polynomial  parameter  method  determining  v e l o c i t i e s and d e p t h s .  B. T  p l o t s f o r t h e same d a t a .  2  vs X  2  lines  are l e a s t squares f i t s  T -X  method o f  2  2  analysis.  The fits of  The  used i n the  50  F i g 3.5, same as 3.4, f o r p r o f i l e 75-i  TABLE  Profile  Ray Parameter Water ( s u r f a c e t o W) Velocity Thickness  3. 1  75-1fi  T -X 2  2  Stripped T -X 2  2  1. 50 ± 0. 01 1. 95 2 0 . 0 1  Sediments L a y e r J (W t o 1) Velocity 1.79±0.08 Thickness 0.33 + 0 . 0 1 .  1. 761: 0. 05 0.34 + 0.01  1 .68 + 0. 1 1 0.30+0.02  L a y e r 2 (1 t o 2) Velocity 2 . 0 2 t 0. 06 Thickness 0.32±0.01  2. 01 ± 0. 05 0.33S0.01  2.08t0.10 0.35+0.02  L a y e r 3 (2 t o 4) Velocity 2. 14+ 0. 03 Thickness 1. 17+ 0.02  2. 12 ± 0. 05 1. 15 + 0. 01  2. 16 + 1.26 0.74 + 0. 44  All velocities km.  a r e i n km/s a n d a l l t h i c k n e s s e s a r e i n  53  As  a  result,  depth  of a layer  deeper T -X 2  2  inaccuracies will  layers.  next.  would  fairly  number o f l a y e r s  become p r o g r e s s i v l y  c a l c u l a t i n g the v e l o c i t y the  i s in direct  propagates  From t h i s we  methods t o a g r e e the  not a f f e c t  This  method t h a t  in  results contrast  errors  expect  from  with one  the  for  stripped  layer  to  the  the r e s u l t s between a l l t h r e e  w e l l f o r the f i r s t  increases,  calculated  and  layer  or  two.  h o w e v e r , t h e l a s t method  l e s s a c c u r a t e due  to the  As will  accumulation  of  the  of  errors. The sediment  velocity  vs  depth  results  r e f l e c t i o n s f o r p r o f i l e 75-18  TABLE  Layer  Velocity (km/s) '  Water  1.50  from  are given i n t a b l e  3.2.  3.2  Thickness (km)  Depth t o of L a y e r  +  0.02  2.00 t 0.01  0.0  +  0.03  0.33 i 0.01  2.00 2. 33  Layer 1  1.79  Layer  2  2.02  0. 06  0. 32 t 0.01  Layer  3  2. 14  0. 03  1.17  2  Top (km)  2.65  0 .02  3.82  Oceanic basement  By  analysis  placing  t h e s h o t s a t 45m  depth  for  this  profile  have managed t o p e n e t r a t e t o t h e b a s e o f t h e s e d i m e n t s obliterating  the  immediate  sub-bottom  a r r i v a l s . The  we  without profiles  54  using shots  at  information, (2-way). be  7  m  but  depth  provided  essentially  f o r only approximately  For t h i s  reason  I do  may  deeper they  possible  basement  are of so  from for  be  layers.  low  background  a  "?"  noise  suggested  as  we  and  last  that velocity  do  as  to  analysis.  reflections i f they are  be  from present  indistinguishable  bottom m u l t i p l e s .  with r e f l e c t o r  reflector  (w-k)  profiles  t o the  have  the water  the case  the  little  Unfortunately,  an a m p l i t u d e  e x a m p l e , c o u l d be  with  that  same  s of p e n e t r a t i o n  n o t recommend t h a t s u c h  r e c o r d e d i n f u t u r e work a s t h e y add It  1.5  the  on  filtering  five  This,  identified  F i g 3.1.  I t has  been  ( T r i e t e l et  al  1967)  m i g h t e n h a n c e t h e s e a r r i v a l s . T h i s method o f f i l t e r i n g  was  not  initially  has  the  designed  requirements that  for  type  of  data  as  t h a t t h e s p a c i n g b e t w e e n t r a c e s be  i t  uniform,  and  t h e a r r i v a l s h a v e a u n i f o r m moveout p e r c h a n n e l . T h a t i s ,  when  plotted  straight  on  line.  more  or  first  condition  work  the  linear  a  T vs X g r a p h  type  less  free  moving  i s n o t met.  of  Geophysics  results,  Also, for this  thus t h e d a t a does not  data.  and  514, was  This  was  allowed  to  t o expand done  only  w h i l e b e i n g f a r from  be p o s s i b l e t o e x p a n d w-k  type  of  a be  reflection  arc- h y p e r b o l i c and  f i t the t h e w-k  second  as  method t o f i t t h i s  given  by  a feasability  to this  course,  Dr  T.  J.  study.  The  c o n c l u s i v e , do i n d i c a t e filtering  not  criteria.  as a p r o j e c t f o r t h e  "Time S e r i e s A n a l y s i s " , intended  should y i e l d  to l e s s e n the ambient n o i s e , the  T vs X p l o t s o f t h e a r r i v a l s  and  Ulrych  the a r r i v a l s  S i n c e t h e h y d r o p h o n e a r r a y was  Even s o , I h a v e a t t e m p t e d  may  our  that  t y p e of  i t  data,  55  even c o n s i d e r i n g t h e problems j u s t  discussed.  I  t h i s as a legacy f o r a subseguent adventurous The  velocity  considered  as a b s o l u t e .  the v e l o c i t i e s interval greatly  depth  just  The  given-  should  velocities  presence  the v e l o c i t y  of  leave  student.  Due t o t h e n o n - r e v e r s a l  are only "apparent"  velocities. affect  model  shall  of  not the  be data  and n o t t h e t r u e  dipping  layers  measured f o r a l a y e r  as  can  a l l the  m e t h o d s o f a n a l y s i s assume t h a t t h e l a y e r s a r e homogeneuos a n d flat  lying.  This  will  o f t h e 75-1 r e f l e c t i o n results  t o be  this  profiles  i n Chapter  cruise.  line  75-3  approximately presence  the  definite  An  reflection  in  two  correlations  was  profiles  Even  so,  I  was  the  at i t s  line  while  to i t being  clearly  show  the  sediments,  of the l a y e r s . the  with  However,  this  continuity  of  made t o c o r r e l a t e t h e a r r i v a l s w i t h t h e C.S.P. p r o f i l e s . to  able  reflectors,  about to  between p r o m i n e n t a r r i v a l s sub-bottom  acguired  ( F i g 1.3}  Both p r o f i l e s  layering  attempt  were  runs p e r p e n d i c u l a r  however o n l y p e n e t r a t e d  time.  correlation first  75-1  the  for folding  on t h e r e f l e c t i o n  C.S.P. d a t a travel  data  does n o t appear t o have d i s r u p t e d  layers.  timed  parallels  30 km s o u t h e a s t .  of  C.S.P.  line  ( F i g 1.4)  c o n s i d e r a b l e evidence folding  where d i p p i n g l a y e r s c a u s e t h e  1,  C.S.P.  n o r t h w e s t end c l o s e l y C.S.P.  the d i s c u s s i o n  meaningless.  As m e n t i o n e d during  be e x p a n d e d upon d u r i n g  4  make  s a  two  t h e n o r t h w e s t o f t h e end o f C.S.P. l i n e  and  precise  c o u l d n o t be made. P r o f i l e 75-1R i s  way  tentative  on t h e C.S.P. although  The  the layer  somewhat  75-1. For t h i s  to  reason,  56  it  is  not  p o s s i b l e to i d e n t i f y  dips present we  do  in  the  i n the r e g i o n of  the  p r e c i s e m a g n i t u d e of  the  75-1R. From t h e C.S.P. d a t a  that  have t h o u g h , I t h i n k i t i s s a f e t o assume t h a t t h e region  w i l l be  what c h a n g e s t h e s e velocities layering too  is  of the  dips w i l l  difficult  present.  order  o f one  make t o t h e  t o two  t o determine because of the  changes produced though  of  the  mild caution i s placed  degrees.  order  on  the  of  Just  layer thicknesses  Any  great,being  dips  2 t o 4%.  reflection  complex  should For  and  not  be  t h i s reason  results  for  a  profile  75-1H. The to  obtain  main  purpose behind  a reasonable  a n a l y s i s of t h e accurate  been r e p o r t e d (1969).  The  a n a l y s i s of  depth  by  sediment  .profile  75-1R  and  generous  the  is  s e c t i o n , and  has  as  allowance likely  Tiffin  thickness  (1972) obtained  approximately  1.8  d i p p i n g a t an for  t o be  been  in  to obtain a  s e d i m e n t s t h a n has  l a y e r s are  s e d i m e n t t h i c k n e s s i s not 20 0  the  authors  total  that  making  data  for  such  considering  surveys  p i c t u r e o f t h e s e d i m e n t s f o r use  refraction  total  the r e f l e c t i o n  this  more  previously and  Couch  from  the  km.  Even  unknown  angle  possibility,  i n a c c u r a t e by  the  the  more t h a n  m.  Profile The using  complete  shots  directly  a t 45  set  reflection  data  m d e p t h i s shown i n F i g 3.2  t o F i g 3.1,  immediately  of  75-1  t h a t the  the  similar  arrivals  for profile I f we  compare i t  s e c t i o n f o r 75-1R, we timed  on  7 5-1  are  75-1  can  see  somewhat  57  questionable.  Some  of  a p p e a r as c o h e r e n t a s was on  the  arrivals  the c o r r e s p o n d i n g a r r i v a l s  a b l e t o t i m e what I i n t e r p r e t e d  75-1R,  e x c e p t I was  e v e n s e e i t on t h i s beinq  from  similar 1R.  the  travel  The  (Figs  same  reflectors  t i m e s p a c i n g and  in that  and  3.4).  The  they i n d i c a t e that  decreases  with  depth,  lower than  t h a t o f water. 2  the Table  next.  The  slopes  were i d e n t i f i e d  as  and  had  seen  for  both 75-1  velocity  profiles  are to  unusual a  velocities  be s e e n c l e a r l y  lines  2  t h a t they  p r o f i l e s c a n be  w i t h a l l the average T h i s can  as or  for profile  the average  where t h e s l o p e s o f t h e T - X  I  75-  plots  2  results  F i q 3.1.  a m p l i t u d e s as on  b e t w e e n t h e two 2  not  reflector,  on t h e b a s i s  relative  T -X  do  as t h e same r e f l e c t o r s  p r o f i l e . These a r r i v a l s  t h e T v s X and  3.3  on  not a b l e to time the l a s t  major d i f f e r e n c e  by c o m p a r i n g  shown on F i q 3.2  layer being  on F i g  i n c r e a s e f r o m one  layer  3.5 to  i n t e r c e p t s of t h e l i n e s a r e g i v e n  3.3.  TABLE  Velocity  km  Intercept s  2  3.3  Branch  2  Hater  Branch  1.49  1. 42  1. 42  1.45  1. 35  10.06  11.92  15.25  17.78  7.54  1  Branch  3  Branch  4  58  The r e a s o n f o r t h e s e non been  introduced  being  the  presence  analysis layers,  methods  The  only  profile  dipping  discussed  profile  determine the obtained.  layers.  previously n o t be  o f o b t a i n i n g any  A l l of assume used  dips  This  was  sub-bottom  calculation  ten degrees would km/s  average v e l o c i t y  of 75-1  now  profile {Fig  the  three  flat  lying  from  that  would  give  rise  to  the  results  done t o o b t a i n t h e a p p r o x i m a t e d i p f o r t h e  reflector.  A model b a s e d on t h e d i a g r a m  various  values  a velocity  velocity  obtained  t o the f i r s t  1.3)  reflector  of Q ranging  from f i v e  to  f o r the a r r i v a l  similar  to  from the T - X 2  2  p l o t f o r the  reflector.  have d i p s o f t h i s does  not  below  for0,  showed t h a t v a l u e s yield  t o the  r e m a i n s t o be s e e n i f t h e s e d i m e n t s i n t h e 75-1  given  a l s o a p p l i e d h e r e . Then i t i s p o s s i b l e t o  calculated using  It  that  useful information  was  1.42  75- 1R,  this  u s e d and t h e " a p p a r e n t " a v e r a g e v e l o c i t y  the  already  to analyse  was  The  has  i s t o a s s u m e t h a t t h e v e l o c i t i e s and d e p t h s  by t h e 75-1B  first  of  way  results  t h e d i s c u s s i o n of p r o f i l e  f o r t h i s reason they could  profile. this  during  sensical  give  order.  any  Our  clear  own  region  C.S.P.  details  of  line the  59  sediments Tiffin  at  the  southeast  The  of the p r o f i l e .  ( 1 9 7 6 ) , h o w e v e r , have p u b l i s h e d a C.S.P.  1.5) w h i c h a l m o s t d i r e c t l y can  end  be  profile  i s  drastically,  layering  present at  not as evident,  reqion are hiqhly  the  northwest  increasinq  end  As  folded. of  the more  with  depth.  of the dips i n t h i s  reqion,  From  t h e C.S.P. d a t a , c a l c u l a t i o n  range  f r o m a p p r o x i m a t e l y +10 t o -10 d e q r e e s , and  with  the  analysis.  (Fiq  and t h e s e d i m e n t s a r e f o l d e d  the amplitude of foldinq  values of the dips required  apparent v e l o c i t y  profile  o v e r l a p s our r e q i o n o f i n t e r e s t .  seen, the sediments i n t h i s  distinct  M u r r a y and  hence  t o make t h e f i r s t  agree with that determined  from  the  agree layer T -X 2  2  60  H 5EPRACTION DATA ANALYSIS  H-.1 F i r s t Until refraction type  r e c e n t l y the d a t a was  only  t h e so  of a n a l y s i s , w h i l e  does not  relatively  to  as  qenerating initial  a valuable  interpretive  generation arrival  of  dynamic  The  advent of s o p h i s t i c a t e d of  calculatinq of  interpretive The  the  d a t a was  give  first  arrivals 2.3  To  the  and  a distance  breaks  now  use  i s in as  an  model f o r  the  t h i s reason, a the  first  step  first  are  the  d i r e c t w a t e r wave p h a s e s , w h i c h  in  study. the  distances. to that  first  The  arrivals  timing  used f o r the  method  D.W.W.  for (see  2.1). of  55  1, t h e s i g n a l / n o i s e r a t i o first  method  the  identical  Fig  use  as  in  shot-receiver was  as w e l l  model f o r  as a s t a r t i n q  s e i s m o g r a m s used i n t i m i n g  turn  section  and  p r o c e d u r e used i n t h i s  same o n e s u s e d t o t i m e t h e  refraction  arrival  however. I t s major  but  synthetic  marine  first  s y n t h e t i c seismoqrams. For of  quick  This  s e c o n d a r y a r r i v a l s and  v e l o c i t y depth  quide  interpretation  Method".  amplitude information  tool,  a preliminary  Arrival  marine  perform,  c o m p l e t e t r a v e l t i m e d a t a s e t . The  remains  analyse  to  expansion  include  to  easy and  methods  enabled the  interpretations  of  very  data.  inexpensive  s e i s m o q r a m s has  used  called "First  beinq  the  Interpreta tioru  method  take i n t o account the  a m p l i t u d e a s p e c t s of  the  Arrival  to  km was  better  on  profile  high than  75-1R  and  65  km  on  enough to e n a b l e the 1  mm  or  15  ms.  75-  timing The  61  signal/noise over  lessened  t h e r a n g e 55-75 km  profile has  ratio  on  75-1,  an  been a t t r i b u t e d  on  at  75-1R  were t o o l o w  to  degree of  reasonable  p o i n t and for  the  of b e t t e r than  picks.  s i g n a l amplitudes any  and  accuracy  t o the  this  B e y o n d 75  t o enable  conseguently rest  2 mm  km  of or  the 30  ms  on  75-1R  the  arrrivals  t o be  timed  accuracy. i  Fig data  4.1  i s the  f o r both  being 8-28  6 km)  first  p r o f i l e s 75-1  km/s.  An  two  now  thing  to note  two  profiles,  also dip  F i g 4.2.  the  200  inverse  both  ms  profiles,  velocity  time  branch  Over  the  km/s is  branch such  and  difference  as w e l l  being  layer  first  the  The  first-  between an  the  offset  velocities  that there  of  given  is  s e t of a r r i v a l s  f o r about the next  of approximately km/s)  f o r the next  i s extremely distance  the  persist  (5.26  well  4.28  km/s.  becomes t h e  first  15 km  are  little  on b o t h  becomes t h e f i r s t  d e f i n e d as t h e  previous  arrival this  by  beinq  travel 4.2.  the  This  14 and  Fig  profiles,  at  with a  At a b o u t  arrival.  two,  start  5 km,  . Once a g a i n  d e f i n e d , a s shown  r a n g e 29-44 km  t r a v e l time branch not  well  horizons.  least-squares  as  how  o f t h e s l o p e s of t h e l e a s t - s q u a r e s l i n e s  9 km  persists  shows  phases ( i . e .  similarity  b e t w e e n them. The  approximately  the next  two  velocity  defined.  degree of  main  arrival  reducing  of the f i r s t  i n e x c e l l e n t agreement, i n d i c a t i n g  For  for f i r s t  e x a m i n e F i g 4. 1 i n g r e a t e r d e t a i l .  on t h e r e f r a c t i n g  km,'  75-1R, t h e  This plot  are  i s the great  approximately the  and  s e t s of a r r i v a l s us  traveltime plot  expanded p l o t  i s shown on  Let  by  reduced  6.28 branch  defined  by  Reduced squares  travel fitted  profiles. km/s.  The  time  lines  data  f o r both  reducinq  least  refraction  velocity  i s  6  Inverse slopes are the v e l o c i t i e s  in  k i l o m e t e r s per second;  in  seconds.  profile  only  intercepts are  T r i a n g l e s show d a t a  75-1;  sguares  p r o f i l e 75-1R. &t km,  and  one  show  distances  time  f o r the data  beyond  for 24  d i s t a n c e value per  s h o t was u s e d f o r a n a l y s i s .  DISTANCE  (KM)  64  Reduced  travel  time  squares  fitted  l i n e s f o r the f i r s t  phases of p r o f i l e s  75-1  data  and  and  75-1R.  least two  16.0  18.0  D I S T A N C E  20,0 Sll)  66  5 points  only  Travel  time  becomes t h e until  branch first 65  about  arrival, too  w h i c h i s f a r l e s s t h a n f o r b r a n c h e s one  the  the  fact  km  where t h e  to a l l o w  poor data that  considerably  with  a velocity  a t a b o u t 4 if km  arrival  however, i s o n l y  poor  for  four  7.8  km/s  s e e n on  75-1R  the  compared  weather  worse,  leading  c l o s e t o 7.0  km/s  remains  75-1;  75-1  with  as  such  over.  This  data  were  The  probably  75-1R  the  a  the  7 5 - 1R.  profile  during to  two.  branch takes  profile  i t t o be s e e n on  on  and  and  reason lies  profile  in was  much p o o r e r s i g n a l / n o i s e  ratio. After  identifying  the  f i t t e d to the  first  points  arrival  using  line  was  His  procedure i s a l e a s t squares technique  both the  X and  Y co-ordinates  to  usual  method t h a t a l l o w s  the  ordinate. are  the  The  travel  distance  shot-receiver shows t h e  distances  discussed  l e a s t squares f i t t e d l i n e s data  Table  associated  standard  each  There  I  4.1  velocity  gives values,  deviations  i n which e r r o r s i n  into  contrast  start  of t h i s  chapter,  and and  with  2.4.  Section  least  number  the  Fig the  sguares  intercepts, the  co-  proqram  s u p e r i m p o s e d on  the  Y  the  4.1  first  slopes, with  of  the points  line.  are  several  methods  for obtaining  model f r o m t h e l e a s t s q u a r e s v e l o c i t i e s and  decided  in  those associated in  (1969).  f o r e r r o r s i n the  errors input at the  straight  method o f Y o r k  considered,  only  e r r o r s are  to  depth  be  (X)  corrected  defining  (Y)  timing errors discussed  whereas the  arrival  time  can  the  branches, a  t o use  the  "Slope  the  velocity-  intercepts.  I n t e r c e p t " method o f Ewinq e t a l  67  Table  Profile  75-1  75-1R  4.1  T.T. Branch  No. Pnts  Velocity (km/s)  Intercept (s)  1 2 3 4 5  25 26 5 4 7  4.257 i 5. 35 1 + 6.260 ± 7.097i 7.8251  0.010 0.005 0.020 0.007 0.040  4.154 ± 0. 060 4. 84 1 t 0. 038 5.700 ± 0, 080 6. 465 i 0. 100 7. 221 ± 0. 510  1 2 3 4  20 26 5 5  4. 297± 5. 160± 6. 3 3 0 1 7. 023 $  0.009 0.005 0.015 0.0 10  4.001± 4. 535 ± 5. 550 ± 6. 241 +  Q  0.020 0.038 0.065 0.096  For t h e f i r s t two t r a v e l t i m e b r a n c h e s , a l l t r a c e s f o r e a c h shot were t i m e d ; a t g r e a t e r d i s t a n c e s o n l y one t i m e - d i s t a n c e v a l u e p e r s h o t was u s e d .  Table  Layer  1 2 3 4 5 6 7  4.2  Dip  Veloc i t y (km/s)  Thickness 7 5- 1  0 0° 1.49° -2. 3 5 0 1.25°  1.49 2.801 4. 28 5.26 6.28 7. 04 7. 8 2 2  2.00 3.03 1.6 2.74 4.13  0  00 00  1 assumed v e l o c i t y 2 unreversed v e l o c i t y  3. 76  (km) 75-1R 00 ,75 1 ,45 ,98  68  (1939). and  This  method a s s u m e s t h a t  bounded  the  seismic  a b o v e ". are  The  One  d i p s on  the  Fig  4.3  layer  profile  Tiffin  et a l  the  to the  oceanic  direction  data  of  profiles  75-1  and  thus i s c o n s i d e r a b l y  the  reflection  results.  assumption, p r i o r  of  ( 1976)  horizons.  section  75-1  and and  and  continuous  Hinona  roughly  this  basin. lines  perpendicular  accounting  1.8 is  km  As  roughly given  due  reflection  sediments. the  model i s  the  discrepancy  indicates,  l e s s than t h i s ,  for  t h i c k e r than  f o r the  the  For  75—1R.  p r e d i c t e d by  This  the  horizons.  b o t h h a v e C.S.P.  to the a n a l y s i s of t h e  average v e l o c i t y  significantly  obtain  models i s  refracting  basement d i p p i n g  3 km  interpretation  to  velocities  these  available  Hopkins  sediment t h i c k n e s s  km/s  layers  refracting  layer  layer  the  i t  i s consistent with  The  2.8  than i n the  use  concerning  this  (C.S.P.)  that  4.2.  point  (1972) and  w h i c h show t h e  can  various  with  i n Table  interesting  seismic  l a y e r s and  depth models g e n e r a t e d f o r p r o f i l e s  given  uppermost  homogeneous  assume t h a t  we  absence of s u b s t a n t i a l d i p s of t h e the  is  smooth p l a n e  therefore  about the  shown on  thicknesses  layer  i n each l a y e r i s higher  and  velocity  are  b e l o w by  method however d o e s n o t  lying  information  and  velocity  This  flat  75-1R  above  "each  the  average f o r the  to  by the  data,  of  a  reflection  velocity thinner  is  layer  sediments. The  first  the  velocities  of  reversal  point except  is  t o be t h e Pn  regrettable  made a b o u t t h e velocity  model i s t h a t a l l  are reversed.  especially  i n the  This  light  lack of  the  69  lis.  Velocity the  vs d e p t h  first  average sediments analysis 3.2)  models  arrival  d e p t h i s 2000 m. velocity was  Jia.1  analysis.  In  this of  2.8  assumed.  of r e f l e c t i o n data  indicates  derived  t h i s i s too  from  Datum  water  analysis, km/s  an  f o r the  Subsequent (see high.  section  6  8  10  12  75 - 1 14  16  18  75 - IR  71  large  relative  velocity  error  (0.040  in relation  The  first  thickness  of  sub-sediment  to the other  arrival  15 km  Synthetic  seismic  refraction  last  the  limited  by t h e e x p e n s e  the large  work.  More  obtaining has the  more  Wiggins  (1976)  to-use  method a,b)  data  predicts with  a total  roughly  o f data  this  sub-bottom  12  km  being  normally  algorithms information  of the "Disc  has p r o v i d e d  the  implemented their  acquired and  about  use has been as  i n this  type  the  oceanic As an  (DRT)  of  example,  and  s y n t h e t i c seismograms. foundation  of  crust  concept  inexpensive  theoretical  well  neccessity  Ray T h e o r y "  a relatively  only  the  of this.method.  calculating  has p r o v i d e d  been  for interpreting  f o r the computations  the application  of  has  In t h e past  required  detailed  establishment  of  Seismograms  years.  efficient  expedited  (1976  few  amounts  km/s)  velocities.  use o f s y n t h e t i c seismograms  within  as  analysis  0.005  crust).  ii2  marine  with  layer  for the profile  (oceanic  Widespread  compared  by easy-  Chapman for  the  method. At HRGLTZ  U.B.C.  (written  quantities, by  we  Malecek  seismograms  DRT  a r e f o r t u n a t e t o have  the use of t h e program  by  computes,  Wiggins),  which  s y n t h e t i c seismograms. (1976)  and  f o r comparison  data.  The a l g o r i t m  times  and a m p l i t u d e s  on w h i c h  myself with  to  provide  observed  HRGLTZ  for arrivals  I t h a s been  i s based defined  marine  among  other  used  both  theoretical refraction  calculates by  an  travel  input  72  curve. P-A  It also  curve  model  and  layers  includes  modification  interested  i t i s  changes  who  an  must  propose  expected  arrival.  Much  change  more  by  working  following  two  eguations.  -  T(P &) }  Where  T(p,A)  =  P  time,  P-A  curve,  presenting  where  then,  (p,A)  i n travel  time  us  with  thought  (Wiggins  complex  square  root  the  curve  to  purpose  of a  can  as i n d i c a t e d  the area  method  and M a d r i d  be  by t h e  (Bullen  the the  1974).  2-2  arrival slowly  1963).  o f an a r r i v a l of the slope  the P - A curve  under  of changing  4.  function  with  this  of as  a simple  a  the  with  (1/Vsurface)  F(P#A)=  the amplitude  in the  4.2-1  of the  that  this  and a m p l i t u d e  J d P / d A l life  |dP/dA7i/2  lies V-D  dP  amplitude  Working  for  the  curve  use  changes  difficulty i n  A(p,£) =  to  to  a change  c a n be  of a r r i v a l s  2) A ( p , A ) = F  difficult  V-D  time  value  travel  the  t h e P-A c u r v e  A(P)  travel  The  time  low-velocity  in  the desired  This  control  with  p£ * f  Pmax=max  travel  that  finally  reflect  sections.  obtained  1)  of the  v e l o c i t y - d e p t h (V-D)  such  extremely  which  seismogram  interpreter  given  are  HRGLTZ,  t o make  provide  a  integration  considered. we  by  synthetic  Weichert-Herglotz  the corresponding  Whereas  curve  a  to determine  c a n be  produced  performs  then  varying  The l a s t  is directly  o f t h e P-A allows  respect  equation  states  proportional  curve  the  with  at that  interpreter  to  point. much  73  more  control  o v e r t h e t r a v e l t i m e s and a m p l i t u d e s than c o u l d  be o b t a i n e d by u s e o f t h e V-D c u r v e a l o n e . The U.B.C. trial  inexpensive nature of of  an  excellent  o f many d i f f e r e n t  HRGLTZ  turn-around  presence  instantaneous  time.  used t o p r o v i d e t h e p r e l i m i n a r y  input  V-D  model  according  (1963,p 1 1 2 ) . T h i s p r o q r a m future  input  into  v a r i e s over  chanqing  parts  from  to  the  of  equations of Bullen the  P-A  curve  digitizinq  rate  the' P - A  curve,  of the curve r e q u i r i n q  for  (sampling rapidly  a hiqher samplinq  h a p p e n s , i t was n o t p o s s i b l e  perfectly.  arrival  A  trade-off  b r a n c h e s and t r a v e l  one  usually affected  on  fittinq  the  a continuous P-A curve  rate  more s l o w l y v a r y i n g s e c t i o n s . As o f t e n  data  into  This  the ranqe  by W i g g i n s ,  P-A c u r v e f o r i n p u t  then d i q i t i z e s  HRGLTZ.  interval)  than  t h e need  c o m p u t e r p l o t s w h i c h had a c o n s i d e r a b l y l o n g e r  HRGLTZ . T h i s r o u t i n e a p p r o x i m a t e s an  comparison  data, e l i m i n a t i n g  A c o m p u t e r r o u t i n e named MDLPLT, a l s o w r i t t e n was  at  m o d e l s i n a s h o r t t i m e . The u s e o f t h e  between t h e s y n t h e t i c s and t h e r e a l expensive  the  Adaqe G r a p h i c s T e r m i n a l a l l o w e d t h e  g r a p h i c s t e r m i n a l a l l o w e d an a l m o s t  for  and  travel  subjective  the  t i m e s was n e c e s s a r y s i n c e  t i m e s " t h a n on f i t t i n q  determinations  were  t h a n a m p l i t u d e de t e r m i n a t i o n s  real  amplitudes, slopes of  the o t h e r s . G r e a t e r emphasis  the travel time  between  t o model  chanqinq  was  amplitudes,  considerably  placed since less  74  4 ._3  A from  preliminary  V-D  the r e f l e c t i o n  data  structure was  used  This  from  model  starting  model with  of  depended P- A  to  directly  curve.  The  this  HRGLTZ  then  specified  by  getting  MDLPLT  input  model  modelling.  provided The  4.2-2, second  only  to  f i t the  arrivals  altered  important  to  how  be  output  obtain used  of  input  specified  I t  the  which  models f o r the  calls  The  output  difficulty  closely  to  The  P - c u r v e  This  was  were  then  and s y n t h e t i c s .  which  a  t o c a l c u l a t e an  prevously.  model  as  problems  and  finely  the  a s was i n p u t .  model  impossible  use  in  f i t the  for  reguired  i s  actual  however,  problems.  P-Zi c u r v e keeping  was  i n mind  traveltimes  amd  75-1. T h i s  t o f i t 75-1R.  aware  to  least  model  4.2)  calculations.  Several  not the  a P-A c u r v e  for profile  further  (Table  beinq  digitization.  a starting  method,  turn  the input  on  MDLPLT  preliminary  o f 75-1  interval  plots  initial  no s e r i o u s  trial-and-error  the  V-D  to generate  Since  MDLPLT,  as s p e c i f i e d T-A,  made  in  structure  and t h e c r u s t a l  order  synthetics.  uses  depends the  i n  t h e s a m e V-D  program  and produces  model  curve  75-1  3.2)  analysis  the sampling  P-A c u r v e  (Table  f o r the synthetic  between  on  on t h e s e d i m e n t  MDLPLT  t h e program  matching  approximate  then  model  i t s r e f u s a l to output  degree  this  for  based  arrival  input  P-Acurve,  t o Data  f o r 75-1S  the f i r s t  was  encountered  V-D  model  as the s t a r t i n g  preliminary  was  Ajsr>l i c a t i o n  of' t h e  altered  by  eguations amplitudes final  For such  t h e famous 4.2-1  of f i r s t  P-A  curve  and and was  calculations i t  difficulties  and  i s  trade-offs  75  mentioned  previously.  iiii As a  final  seismograms  prelude  and  are  record  respectively, The  amplifier  sections  of  have  been  in section complete As  shown  the  profiles  a simple  first  type of  data.  Such a s t a c k i n g  75-1,  the  resulting  decrease  in  l a r g e decrease this  processing section  was  arrival  t o be  greater  than 6 size,  sensitivity  cumbersome data  along was  stacked  an  amplitude profile using  shown  km  profile  variation  data  (1976)  for  on  This  and  to  As  this  profile  Fig  4.6.  75-1R,  the  resulted in  consequently this  introduced  unacceptable. the  and  a lag t r a j e c t o r y  a m p l i t u d e s beyond  only  ratio  applied  c o r r e l a t i o n between t r a c e s in the  for  aalecek  being on  handle  stacked  optimum  p r o c e d u r e was  35  to  signal/noise  stack  profile  were  deteriorated considerably.  compiled  4.5  75-1R,  charge  handled.  velocity  at approximately  i t made t h e  was  the  linear  by  weather c o n d i t i o n s  and  and  hydrophone  are  increasing  defined  Unfortunately,  and  for  into  2.4.  amount o f d a t a  that  75-1  corrected  a consequence, the  of  the  synthetic  F i g s 4.4  at distances  gain, s p h e r i c a l spreading  purpose  Since  profiles  sections  decreasing  of  were c o m p i l e d  i n s e c t i o n 2.4.  recorded  computationally.  a  calculation  r e f r a c t i o n data  data  These  has  the  for  as d e s c r i b e d  the  the  s e c t i o n s as d e s c r i b e d  the  km.  to  Sections  h e n c e a c o m p l e t e i n t e r p r e t a t i o n b a s e d on a l l  available information, record  Record  a  distance. through  a compromise a  f r o m c h a n n e l 3,  this  Record the  section  critical  crustal  section  distance  the  multiples  the  corrected  filtered  secondary a r r i v a l s  after  probably  and  for  beyond first  a r r i v a l . A l l traces  amplitude 2.4  Prominent s  p r o f i l e 75-1  refraction  h a v e been  10  of  first  as  per  5.0-30  hz.  f r o m 2.5 t o  arrivals  of e a r l i e r  phases.  are  Record after  s e c t i o n of d a t a stacking  per s h o t . corrected discussed amplitude trace  t o form  A l ltraces and in  for profile a  h a v e been  filtered the  single  text,  o f 32  km.  h z . As  additional  c o r r e c t i o n was a p p l i e d  at a distance  trace  amplitude  5.0-30 an  75-1  to the  Record s e c t i o n 3's.  of p r o f i l e  A l l traces  corrected  have  and f i l t e r e d  75-1R been  channel amplitude  5-0-30 h z .  83  channel  having  Whereas t h i s ratio,  particularly  good  procedure d i d l i t t l e  i td i d allow  improved, be  Both p r o f i l e s  having  of  a  much  persists  we  arrival;  amplitude  a  such  of  any  amplitude  arrivals  i t was d e c i d e d  first  a short arrival  the length  however, does n o t  both  profiles,  the  i n complexity,  with  time along  the  36  arrivals by  arrivals  t r a c e h a s an a m p l i t u d e  gen'eral  simple  trace.  increase i n the amplitude  three  refraction  still  squares  f o r only  the  the amplitude,  The o u t s i d e two t r a c e s h a v e of  least  p o i n t . Between 29 and  series  could  e m e r g i n g from t h e  with  on  was n o t  relatively  appear t o i n c r e a s e  at t h i s  whereas t h e i n n e r  km  on F i g  with  varying  f a r the  largest  either  profile,  on  consistent  with  the  t r e n d o f t h e p r o f i l e . On t h e b a s i s o f l a t e r to increase the amplitude  t r a c e t o make i t c o n s i s t e n t w i t h  the outer  two.  of the inner This  can  be  by c o m p a r i n g F i g 4.4 and 4.6 o v e r t h e d i s t a n c e r a n q e 2 9 -  km. T h i s  record  h a n d l i n g and  a b o u t two t i m e s  At a b o u t 2 1 km  again  have  amplitudes.  36  as  g r a d u a l l y i n c r e a s i n g with  of t h e a r r i v a l s  seen  arrival  a l s o a p p e a r s t o be an o v e r a l l  4.4,  a  amplitude,  more c o m p l e x , h a v i n g  arrivals  There  with  17 km on b o t h p r o f i l e s ,  change s i g n i f i c a n t l y .  the amplitude  This  km/s  of the i n i t i a l f i r s t  first  i n data  the signal/noise r a t i o  substantial  4*28  d i s t a n c e . A t about becomes  the signal/noise  begin  water bottom r e f l e c t i o n s . velocity  to increase  f o r most o f t h e s e c t i o n c l e a r a r r i v a l s  followed.  arrival  (see F i g  a considerable saving  hence computing c o s t s . A l t h o u g h  4.7).  recordings  was done  before  s e c t i o n s and b e f o r e  the  compilation  calculatinq  of  the  75-1R  s y n t h e t i c seismoqrams.  84  It  a p p e a r s , however, a f t e r s t u d y i n g  t o 75-1R that the  and  the  after  inner amplitude  outer  two  amplitudes  Once a g a i n about  29  km  where t h e  a b o u t 35  profiles.  background noise  amplitude  point  and  out  calculations,  h a v e been l e f t  as  of  the  The  e f f e c t of the  can  arrival  clearly  level  there  i t was  and  increases  at  be  deteriorating seen  on  Fig  increases considerably  both  a processing  problem.  (11.5  d r o p was  profiles.  s),  physically to  at  break i n the  At f i r s t  this  decided  The  length  considerably  can  be  of  was  being  due  followed  with  little  (see  F i g 4.6);  profile  75-1,  i n t h e b a c k g r o u n d n o i s e on  of  thought to  be  sharp  the  of  later  amplitude  arrivals  also  p o i n t , with  W.A.R.  phases.  t r o u b l e out  75-1R  Past  amplitudes  study  beyond t h i s to  r e m a i n more  profiles.  that this  first  becomes l o s t  both  i t was  amplitude  on  on  careful  m a j o r i t y of the  l a s t shot  km  arrivals  However, a f t e r  real.  increase  arrival  of the  i s a significant  on  arrivals  complexity  t o a b o u t 55  the a r r i v a l s  very  synthetic  corresponding  dropped.  t h e 75-1.R p r o f i l e  or l e s s constant  appear  sections  km.  The  this  the  should  the complexity  on b o t h  weather d u r i n g 4.5  working with  the  the The  to  the  however i t soon past  about  67  km.  1  Perhaps the lack  of  secondary  m u l t i p l e s can distances  most s t r i k i n g  be  arrivals.  seen  (8.5 s and  interpretation  and  clearly 10.5  will  not  f e a t u r e of both p r o f i l e s First on  and  second water  both  profiles  s ) ; however t h e y be  discussed.  add  Fig  the  bottom  at  nothing 4.8  is  short to  shows  the the  85  last  38 km o f 75-1  c o v e r i n g a l l the shots beyond the point  which the amplitudes is  filtered  5.0-30  the W.A.R. energy shows  the  decrease c o n s i d e r a b l y .  hz and shows both the f i r s t  discussed  previously.  same data f i l t e r e d  arrival  starting  at  l e n g t h of  the  4.8.  This  benefit  secondary  i d e n t i f i e d as  arrival  is  as  its  presence  5.0-30 hz p r o f i l e data,  of s t a c k i n g  alone.  hindered the p r o c e s s i n g  observable  starts  at  identified  by a " ? "  role  clear  about  40  on F i g 4.11.  a r r i v a l q u i c k l y d i e s out  in  further  the in that  synthetic  to t h e i r amplitude can  at  on  on  on the on  calculations  the  arrivals  arrival  that  75-1  it  does not  is  and  The amplitude a s s o c i a t e d  presence,  the  l e v e l and the l a c k  profile  first  Fig  filtering  was t r i e d  secondary  and at  a  is  with  appear  a r r i v a l or wide  however,  did  and  be mentioned  will  play  a  section.  The two p r o f i l e s  amplitude  f  and no e g u i v a l e n t  km  does  i n d i c a t i o n of  l i n e up with the c o n t i n u a t i o n of any f i r s t branch. I t s  range  s  various  but the high background n o i s e  anqle r e f l e c t i o n  6.5  branch  The same procedure  An even l a t e r  to  profile  would not have been recognised  could be d i s c e r n e d .  this  a  and  along the remaining  achieved by viewing the data under  limits,  75-1R  is  upper  about  of 61 km, a n d . b e i n g c o r r e l a t a b l e It  arrivals  break i n f o r m a t i o n but i t  distance  profile.  The  profile  2.0-30 h z . T h i s f i l t e r i n g  reduced the c l a r i t y of the f i r s t show a s i g n i f i c a n t  The lower  at  be  then are remarkably s i m i l a r with  structures. either  problems or an i n c r e a s e  Most  directly  of  the  differences  attributed  i n the ambient noise  respect  to  in  processing  l e v e l between  the  Expanded p l o t p r o f i l e 75-1 of  the  of with  final  the  last  40 • km  the t r a v e l time  is filtered  2.0  t o 15 hz ;  section  i s filtered  5,0  t o 30 h z .  on  the  lower  time p i c k s . t h e upper branch  f.  curves  model s u p e r i m p o s e d .  section  section  Upper lower Arrows  give the t r a v e l  Note the c o h e r e n t phases  section)  of  following  travel  (on time  ^7~00  bT0O  6^00  69 00  73.00  77.00  DISTRNCE  (KM!  o  DISTRNCE  (KM)  81.00  85.00  89.00  93.00  97.00  88  two  profiles.  4„_5 C o m p a r i s o n o f Sj/n t h e t i c s w i t h R e a l As was d i s c u s s e d i n variations  between  profiles  difference i n travel  times  amplitude  differences  processing  problems,  the  range  quality  27-35  data  section  t h e only  on  200 ms  t h e two p r o f i l e s .  amplitudes  with  75-1. For t h i s  reason,  the f i t o f the s y n t h e t i c seismograms t o t h e r e a l  for  both  profiles  The  starting  P-^ curve  synthetic  profile  secondary source and  W.A.R.  given  phases  time  close,  by  between t h e s t a r t i n g  the f i r s t a r r i v a l analysis. with f i t t i n g  t h e removal  while s t i l l  of  retaining  these  real  model  possible The  ma-jor  model s y n t h e t i c s  presence  of  large  T h e s e W.A.R. 's a r e  increases  which  By f a r t h e m a j o r  s y n t h e t i c s t o t h e r e a l data  are effort  h a s been  w.A.R. p h a s e s f r o m t h e s y n t h e t i c s ,  a reasonable  travel  s y n t h e t i c seismogram s e c t i o n s f o r both the  starting  alteration.  on t h e s y n t h e t i c s .  and t h e  a r e shown on F i g s  including  f o r p r o f i l e 75-1 i s t h e  present  MDLPLT  f i t between t h e  by t h e d i s c o n t i n u o u s v e l o c i t y  associated in  data  by  seismogram s e c t i o n  75-1 i s e x t r e m e l y  of discrepancy  generated  generated  a r r i v a l s , and r e g u i r e s l i t t l e  the real  data  w i l l be d i s c u s s e d s i m u l t a n e o u s l y .  4.9 a n d 4.10. The t r a v e l and  in  7 5 - 1 , o r a s b e i n g due t o t h e p o o r e r  o f p r o f i l e 75-1R compared  corresponding  What  a r e c a n be e x p l a i n e d e i t h e r a s  see f o r example t h e l a r g e  km  significant  75-1 a n d 75-1R i s a  between  there  4.2  Data  t i m e f i t . The  profiles,  together  final with  d a t a a r e shown on F i g s 4.11 a n d 4.12. The f i n a l P - A  P-A  curve  velocity starting  corresponding layered model  synthetic with p r o f i l e  model. for  seismograms 75-1.  to  the used  calculation for  isoas  a of  comparison  Synthetic  seismograra  iso-velocity the reflection arrival  section  layered  associated  wide a n g l e r e f l e c t i o n s  observed  and  from first  data a n a l y s i s .  the large amplitudes  discontinuities.  the  model d e r i v e d  data a n a l y s i s  refraction  for  from  with  the  Compare  s e c t i o n s o f F i g s 4.'6  Note  with and  the  velocity the 4.7.  92 m "cn  03  m  01  "ID  m  "in  rn  r— a  m "rn  m  "IM  m  4  Z  5  J  (D3S) 0'9/a-L  J  E  93  Fig  Top:  Synthetic  Ujj.11,  seisraoqrains  times computed t o f i t the of  profile  75-1.  results  stacked  data  The i n s e r t shows the  source wavelet which the  and t r a v e l  was convolved  of  the  with  synthetic  calculation. Bottom: synthetic  Data  from  travel  profile times  Lower case l e t t e r s have identify  the  branches.  The  arrival  picks  arrows made  The phase c o r r e l a t i o n  with  superimposed. been  various  75-1  added  travel show  to time  the  first  from the raw d a t a . shown  by  a  "?"  seems to l i n e up with t h e c-d r e f r a c t i o n branch.  LO  ^.OO  fig  lTbl  V3.00  27.00  35~00  43.00  51.00  DISTANCE  (KM)  DISTRNCE  (KM)  59 U o  67.00  4 . 1 2 . Same a s F i g 4 . 1 1 , f o r p r o f i l e 7 5 - 1 R  75.00  83.00  91 .00  99.00  P-A  c u r v e from which t h e s y n t h e t i c s f o r  profile  are  75-1  computed  p-&  Starting  superimposed  curve  {Fig  (Fig  4.11).  4.9)  is  f o r comparison.  Zia B.2.1H'  Same a s F i g 4 . 1 3 f o r p r o f i l e 75- 1R.  Comparison profiles  75-1  of  final and  p-A  75-1E.  curves  for  86  66  100  curves are shown on F i g s 4.13  Sediments It  delineate  the  the  last off  ensure  the  first  with 75-1  Crustal  the  correct  (branch b - c ) .  reason  sediments,  i.e.  , is  shown  on  This branch was shown i n  in  was found  that  set  order  to  times to both the 75-1 and 75-IR a  difference  in  the  sediment was needed,  deeper.  km the f i r s t  The  the  :  being  generated  4.4 and  of  far on  4.5,  refraction  amplitudes  the  these larger  branch  over  the  (c-d)  is  arrivals than  preliminary  the  distance the  are  first quite  corresponding  synthetics  of  Fig  I had very l i t t l e or no success i n m o d e l l i n g these l a r g e  amplitudes on e i t h e r can  For t h i s  only.  arrivals  being the  substantial,  4.10.  first  the f i r s t  travel  are  the  the c o r r e c t amplitude f o r It  to  sediments  of about 200 m between the two p r o f i l e s  5-15  amplitudes  from  the top of the o c e a n i c basement  arrivals  Layers  arrival.  The  reflection  As can be seen on F i g s range  structure.  calculations  and 4 . 1 2 ) .  refraction  thickness  synthetic  (c-d of F i g s 4.11  to c a l c u l a t e  of r e f r a c t i o n  the  the c o r r e c t t r a v e l time to  synthetic sections  an e f f o r t  of  sedimentary  branch  reflection the  aim  only to ensure  refraction only  4.14.  :  was not the  included  and  profile.  That t h i s a r r i v a l i s a head wave  be seen c l e a r l y on F i q 3. 1 where i t can be t r a c e d back  the l a s t  sub-critical  incidence  reflection  that  could  to be  101 timed.  The  fact  significant  in  encounter  an  that  that  this  arrival  nowhere  isolated  else  head  i s a p u r e head wave i s  on  the  wave  with  profiles any  do  we  substantial  a m p l i t u d e . The. l a r g e a m p l i t u d e s on t h e s e c t i o n s a r e due m a i n l y to  energy  much  a s s o c i a t e d w i t h W.A.R.'s; head wave  the start  time o f branch  o f t h e m o d e l l i n g , b o t h t h e s l o p e and  c-d a g r e e d  very w e l l  for  these  arrivals,  considerably  slope  substantial  o f t h e branch  example o f t h e t r a d e - o f f  all  for  the  amplitudes, slope  of  identified  I would  arrivals.  tne the  interesting  was  mentioned  h a v e been l e f t  decreased  In  an  effort  f i t of the t r a v e l  arrival  point  to  on F i g 4.6  branch note  to  i n section  therefore the  between  that first  this  up  that  secondary  arrival  refraction/W.A.R.  t h e two t h o u g h  the  secondary  DRT method. The s i t u a t i o n  It  i s  a t t e m p t i n g t o model, t h u s  and t h e  arrival.  The  refraction arrival  One  arrival with  possible  i s i n some way  related  correlation  To c o n c l u d e t h e n , I do  i s likely  these  well  i s t e n t a t i v e and t h e secondary  c a n model t h i s f i r s t  this  considerably.  w i t h a " ? " now l i n e s up f a i r l y  i s n o t s e e n a t a l l on 7 5 - 1 R . we  build  t i m e s was r e d u c e d  lowered  is  This  w i t h no a m p l i t u d e a t  the c o n t i n u a t i o n o f t h i s t r a v e l time branch.  are  amplitudes  I n o r d e r t o m o d e l t h e t r a v e l t i m e s and t h e s l o p e o f  b r a n c h c, c o r r e c t l y  that  During  a n d t h e t r a v e l t i m e f i t became much p o o r e r .  an e x c e l l e n t  4.3.  the  travel  with the r e a l data.  the process of attempting to generate  to'  are  smaller. At  is  amplitudes  arrival  not  feel  by u s i n g t h e  t o be more c o m p l e x t h a n  causing  the  f i t between  we the  102  synthetics this  and t h e r e a l  i s d i s c u s s e d i n s e c t i o n 5.2. Considerably  travel The  d a t a t o be s o p o o r . The r e a s o n f o r  time f o r t h e second  extremely well.  constrained end  was  achieved  modelling  t i m e s and a m p l i t u d e s f o r t h e r e m a i n d e r  travel  fit  more s u c c e s s  of  As was m e n t i o n e d  by t h e p r e s e n c e  the profile  refraction  on t h i s  figure.  generate region  two W.A.R. b r a n c h e s  the increased  earlier,  this  a r e shown  ( d - e , f - g ) have been used t o  complexity  of  the arrivals  t h e W.A.R.  up somewhat w i t h t h e s e c o n d  on  been  amplitude it.  75-1 h a s a l r e a d y  over-correction  Instead  I  amplitude throughout branches  a n d t h u s no a t t e m p t  attempted  to f i ta  this  region  f - g and h - i were  complexity of the a r r i v a l s being  used t o s t r e t c h It  i s at this  on  used  over t h i s  point,  different  and t o compensate f o r t h i s  profile  i  times  that  The t r a v e l  positions  more both  29-35 km  explained  a s an  was made t o or less  f i t  constant  profiles.  W.A.R.  t o generate the increased region  with  branch f - g  out the a r r i v a l i n time.  slightly.  at  d-e  a r r i v a l shown w i t h a " ? " over the range  profile  i n the  branch  on F i g 4.6. The s u d d e n b u r s t o f e n e r g y real  been  arrivali s  time branches  15-27 km. As a l r e a d y m e n t i o n e d ,  now l i n e s  branch e - f has  ( s e e F i g 4.8, a r r i v a l b r a n c h f ) . The  as w e l l as t h e model t r a v e l  first  of the profile.  o f W.A.R. e n e r g i e s p r e s e n t t o t h e  arrivals  The  both  and k  between  t h e two p r o f i l e s t h e two  were  slightly  I h a d t o move t h e c a u s t i c  a p p r o x i m a t e l y 7 km f u r t h e r  on 7 5 - 1 . T h i s a l l o w e d me t o  differ  f i t the last  along the two 75-1  103  travel  time  branches,  accuracy. This procedure start  another  travel  times.  The  example  provide  arrival  a s l i g h t delay i n the  branches  range  a s s o c i a t e d w i t h each  remains  i s t o s e e how w e l l  There  i s a significant  real  data  at  profiles  model  extremely  this  difficult  feel  that t h i s  The  amplitudes  fit  very well  about  55  identifiable The branch in  of  decrease  as  km  and  well  set  refraction  well.  i n amplitudes  What  as  c a n be  as  t h e W.A.R. b r a n c h e s  decreasing  real  on t h e  s e c t i o n s f o r both expected.  t o model s h a r p a m p l i t u d e  then  The  extremely  km. T h e s y n t h e t i c  I ti s  variations  t h e program  and I allows.  h - i and j - k h a v e been out f a i r l y  until  high  at  they  are  just  the  refraction  a t the end o f the p r o f i l e . a r r i v a l amplitude generated by  k-1 h a s n o t been f i t w e l l . t o decrease  or  km.  with the amplitudes s t a r t i n g  no  o f both  C o n s i d e r a b l e t i m e was s p e n t  t h e amplitude  of  this  s u c c e s s . T h i s i s most l i k e l y  and n o t p h y s i c a l l y basis  times  h a s been done a s w e l l  final  trying  little  45-95  the  the f i n a l  t h e W.A.R. a m p l i t u d e s f i t t h e  data.  61  t o generate  of  one h a v e a l r e a d y been d i s c u s s e d  arrival travel  about  5-7 km,  b r a n c h k-  f o r t h e remainder  They w e r e u s e d  over the  f i tthe f i r s t  by a b o u t  h - i , j-k and the r e f r a c t i o n  amplitudes  profiles.  a r r i v a l energy  and  however caused  o f a t r a d e o f f between a m p l i t u d e s and  W.A.R. b r a n c h e s  synthetic of  w i t h c o n s i d e r a b l y more  o f t h e l a r g e a m p l i t u d e s on p r o f i l e 75-1  being  1  i - j and k-1,  real.  my  own  arrival  with  a program  problem  Such a c o n c l u s i o n was r e a c h e d  on t h e  results  and  the r e s u l t s  o f Malecek  104 (1976).  His  problem to  test  using  a  synthetic  with  the  whether  seismogram  final  or  not  refraction arrival. this  d i f f e r e n t method  Generalised  Ray  artificially  introducing  layer  that The  well,  is a  for  Theory; a Pn  synthetics  for  the  The  procedure  that  does not  the  interpreted  method  also  does  not  two  in  from take  the  surface  topography;  filter make allowed  effect the for  nature of  the  arrivals by  the  of  the  Earth. more  modelling  useful  problem  by  either  synthetics  Helmberger  the  (eg.  1974),  beneath  or  the  by  final  first  and  source A l l of  complex  so  as  and  procedure.  a  situation  shown  arrival  by  analysis.  dispersion  elastic i t  wavelet the  procedure.  layers,  account  i n the  reasonably  modelling  plane  into  variations  variable  be  practice  the  lateral  the  I t would  p r o f i l e s f i t about  attenuation, or  this  arrivals.  completely  dips  show  the  layer  horizontal  slight  layers  and  l i m i t a t i o n s of  assumes hold  computing  further  the  also  programming  Higgins  generates  considering  sections  properties  cannot due  above  to are  the The or of  consider  the  lowpass  likely  reverberatory  than  to is  105  5 INTERPRETATION AND  DISCUSSION  Velocity-Depth Before attempting to i n t e r p r e t velocity-depth it  i s  useful  produced  models produced to  .discuss  Models t h e meaning o f  by t h e s y n t h e t i c  the  significance  the  final  calculations,  of  the  curves  w i t h r e s p e c t t o t h e m o d e l l i n g p r o c e d u r e . The f i n a l  V-  D c u r v e s a r e g i v e n i n F i g 5. 1. The shown  first  should  other  not  be  considered  c-d.  analysis  of  diagrams. associated  a l l the  reflection This  is  a  sedimentary  profile direct  75-1R  are  result  models f o r the s y n t h e t i c s .  sedimentary  layers  and  of  the sediments  synthetic  Other  between  the  travel  for  The  the  difficulties  used t o g e n e r a t e  Probably  because  of  approximations within  the  V-D  model  calculations  for  has  the  the 3.2.  been  approximately  t i m e s o f p r o f i l e s 75-1  than t h i s t h e s y n t h e t i c s  sedimentary  on  f o r p r o f i l e 75-1 by a p p r o x i m a t e l y 200  T h i s h a s been done t o c o r r e c t offset  arrival  that contained a l l the l a y e r s d e f i n e d i n Table  The o n l y e f f e c t o f t h e deepen  of  shown the  was  p r o g r a m , i t was i m p o s s i b l e t o g e n e r a t e a sediments  time  l a y e r s d e f i n e d by t h e  w i t h t h e p r o g r a m MDLPLT, w h i c h  the s t a r t i n g thin  Not  structure  as m e a n i n g f u l i n any s e n s e  than i n the p r e s e r v a t i o n o f the t r a v e l  branch  the  p o i n t t o note i s that the sedimentary  have  had  no  200  to m. ms  and 75-1R.  effect  on  the  5.1  (C)  structure. layer  identified  as  layer  2a  on  Fig  Final  velocity  profiles  75-1  fitting  both  amplitudes.  depth  and  75-1R  travel  Also  model d e t e r m i n e d alone.  vs  curves determined times  shown i s t h e by  for  the t r a v e l  by and  starting time  f i t  107  on  1 — I — 4  h  DEPTH (KM)  i's  1B  I I  I I  <  ca  I  u  108  represents layer.  the t r a n s i t i o n from  This  refraction problems have  layer  arrivals  that  complex curve  been  the  by  the f i r s t  The r e m a i n i n g and  3b  on  synthetic about the  the gross  that  they  procedure  For  arrival  three  have  are used,  however,  for this  layer  major change  has been  curves  possible  and a m p l i t u d e s  d i f f e r e n t V-D structure  more  model.  smooth  2b,  result  of the  be  stressed  of the  modelling  of the synthetic curves.  effect  arrivals  on by a  I t i s possible  have been  of the curves  brought  gradients.  t h e same  could  3a  by t h e  f o r each  i t 'should  to obtain  curve  i s no  the replacement of  o f t h e V-D  a  V-D  t o the curves  structure  As  the  significantly  by  unique.  i s f a r too  as l a y e r s  increases  i t was  the gross  layer reason,  o f d i f f e r e n t c h a n g e s t o t h e p-  a slightly  conclusion  this  altered  The  4.5, t h e  velocity  not  c-d.  amplitudes  identified  been  amplitude  large  homogeneous l a y e r e d  layers,  basaltic  branch  this  i s essentially correct,  t r a v e l times  number  would have  produced; remained  same. The  a  DRT.  the synthetics  5.1  discontinuous  layers  the  using  of  the  the large  4.12,  i n Section  structure  c a l c u l a t i o n s . The  three  that  Fig  and  by t h e u s e o f t h e s y n t h e t i c s  Whereas  the  discussed  t o be m o d e l l e d  than  for  to  i n t r y i n g t o f i t these  actual  generated  valid  responsible  o n F i g 4.11  encountered  previously  being  i s  the sediments  2b-3a  noticeable  transition differs  extent.  overly  important.  travel  time  offset  The a c t u a l  I t was  form  introduced  between  between  t h e two p r o f i l e s t o  of this  difference  t o compensate  the profiles.  This  for a  offset,  i s not slight  however,  109  could  have been  slightly point this  different  t o note  altered data.  the  The d e p t h  about  differ  m,  exception  The  i n  important  i n the nature  extent  although  depth  neccessary  to  The  of  arrival  t o delay  reduced  however,  times.  the  near  not' p h y s i c a l l y  to  7.55  were  as not  was a c h i e v e d  by  t h e Pn a r r i v a l  branch  refraction  branch  observed  on F i g  as s i g n i f i c a n t .  I t was  ( F i g 4.11, branch  i t agree  with  t h e 7.8  km/s.  of a trade  reduced  km/s P n  This  first  velocity  velocity  o f f between  k and a r r i v a l  the  k-1)  branch  travel slope  drop times,  and  i s  real t h e V-D  seismograms  structure  of the curve  study  branch  this,  amplitude  75-1R  layer  last  make  In doing  result  synthetic  recent  arrival  the caustic  conclude,  surprising  f o rthis  been  n o t be r e g a r d e d  o f t h e Pn v e l o c i t y ,  t o approximately  i s  amplitudes  To  this  have  use o f  on p r o f i l e  n o t be r e g a r d e d  100 ms i n o r d e r travel  should  this  branch  75-1R h a s b e e n  j - k ; however,  define  (A, 3 ) , a l s o s h o u l d  about  this  control  lowering  refraction  by t h e a d d i t i o n a l  W.A.R. b r a n c h  completely.  the  o f t h e Pn a r r i v a l )  s i n c e t h e Pn a r r i v a l s Some  neccessary  nor  t o t h e Moho on p r o f i l e  500  manipulating  was  depths  the  t o any g r e a t  observed.  by  resulting  the transition.  t h e two p r o f i l e s  layer  (with  significant  5.1  for  ways,  boundary.,  velocities  is  i n s e v e r a l other  forms  i s that  Neither  by  modelled  curves  are only  considering  produced  valid  with  and not t h e f i n e that  by H e l m b e r g e r  DRT o n l y  (1977)  through  respect  details.  t h e use of  t o the gross This  models t h i c k  h a s shown  that  this  i s not layers. type  A of  110  approach the  i s only  fine structure  number exist  of at  thin  U.B.C. i n  the  which  Wiggins  and  is  DRT  provides gross the  oceanic  form  based  i t i s an  of  that  to  must  use  c a p a b i l i t y to  a  computer  This  more e x p e n s i v e  Ray  than  DRT  a  do  large  this  program Theory  type of  delineate  called  (GRT),  approach  and  does  beyond  see  however the  scope  project. of  calculating  of  a  structure  order  method  of  seismograms utilising  refraction profile  excellent extension  a layer,  synthetic  easy-to-use  structure  i n t e r p r e t a t i o n . In  structure  one  Generalised  (1974).  overall velocity-depth  such  s e n s e and crust,  of  on  inexpensive,  amplitude  broad  l a y e r s . The  method  an  in a  the  Helmberger  present  The  of  is  considerably this  of  very  STPSYN  of  valid  of  of  however,  the  to  oceanic  first  to  delineate  q u a l i t y data  5.2  Discussion  and  delineate crust.  the  high  arrival  the  the  method  the  use  As  of  fine GRT  is  reguired.  Sediments The  : v e l o c i t y depth  the. n o r t h w e s t  end  disappointing. the were  sediments able  obtained  by  between  J.  It  of  the  was  in this  to  model  Knize Tuzo  the  basin  hoped  area  discern  for  (Table  that  could  be  however,  (1975)  for  two  Wilson  Knolls  sedimentary  is  a more d e t a i l e d obtained. aqree  short and  3.2)  structure  the  well  a  little  picture  What  results  with  the  profiles  in  of we  results  the  Queen C h a r l o t t e  of  area Sound,  Ill  a b o u t 60 km n o r t h 75-1R. high  His  of the r e c e i v i n g s h i p  interpretation  and l o w v e l o c i t y  roughly area  300  m  in  velocity  2 km,  Winona  s e q u e n c e s as being coarse  the  sedimentary  very  Basin.  close to  He  two  layers  velocities consistent  they to  can  be  -10°. C o n s i d e r i n g  to t h e sedimentary the  i t  history  o f Winona  the  north . Even  the  is  with  then the c l o s e  area s t u d i e d  Basin  discussed  was t o o b t a i n  may  velocity  in  Winona  by  Knize. the  same  reflecting  end,  dips ranging  although  from  +10°  p r o x i m i t y o f Winona  Basin  by K n i z e ,  assume  be s i m i l a r  by K n i z e  and c o n s i d e r i n g  at  both  that  ends  of  that the  the d e p o s i t i o n a l  t o t h a t of the area  to  (1976).  s t r u c t u r e within the basin,  a more a c c u r a t e  p r e v i o u s l y been r e p o r t e d  thickness  results  (low  t h o u g h i t was h o p e d t o o b t a i n a d e t a i l e d p i c t u r e o f  sedimentary  extent,  to  the  of g l a c i a l  as a t t h e n o r t h e r n  more f o l d e d  reasonable  of  300 m t h i c k a n d have  Basin  same r e f l e c t o r s c a n be i d e n t i f i e d  basin,  depth  due t o p o o r e r g u a l i t y d a t a ,  those found  identified  are considerably  the  are both roughly  with  i n this  and i n t e r - g l a c i a 1 ( h i g h  At t h e s o u t h e r n end o f Winona horizons  the  due t o a l t e r n a t i n g p e r i o d s sediments)  thickness  of  i n t e r p r e t e d the sedimentary  do n o t show t h e same d e t a i l , upper  profile  a s e r i e s of a l t e r n a t i n g  f i n e sediments) d e p o s i t i o n . Although Basin  for  sedimentary l a y e r s , with a thickness  e a c h . The t o t a l  i s approximately  sediments  includes  location  the  study  was  objective  depth f o r the s e d i m e n t s than  (Tiffin a  t h e main  1973, Couch  success.  The  f o r t h e s e d i m e n t s i n Winona B a s i n  1969). total  To  has this  sedimentary  h a s been f o u n d  to  112  be  approximately  to  6  km  The  2 km.  This  p r e v i o u s l y suggested  previous  gravity  data,  lines,  and  the  C.S.P.  for  the  studies, the on  an  assumed  differs  the  afore have  velocity  into  depths.  area  penetrated  only  to  travel  time  ( F i g s 1.3  seconds  from  s on  the  and  depth  2  of  i s a  reported  depths of  Basement  layer  The  profiles  refraction  km  least  the  4 to  squares  observation layer  i s  found  by  region  of not  6  sediments.  qiven  by  which and  can  3.2.  about  were be  On  depths  C.S.P.  to  convert  4  2-way  about  seen  than  lines  s  only  the  at  a  0.5  about  basis of  the  on  on  C.S.P.  i t i s believed that  by  layer  of  i n the  Ridge  obtained  thicknesses  of  and  perpendicular  to  similar  et  velocity region.  f o r the  velocities  which al  good  sediment  previously  for  Similar  This  range  for his  with  2a  have  layer than  The  been  in 250  the m  4.06-4.20  profiles (1976)  the  has  basaltic  results  basement  In  layer  first  were l e s s  areas.  basaltic  for layer  the  Knize  to  i s compatible  (1974)  i n the  Ridge.  i n two  F i g 5.1.  there  0 . 9 7 - 1 . 7 1 km/s  results  on  oceanic  where  Explorer  material  km/s  Peterson  unique  (1976)  2a  4.28  km/s  and  obtained  maximum o f  sedimentary  a 4.28  Explorer He  However, the  4  km.  from  velocity  Malecek of  data,  s t r u c t u r e used  the  authors.  their  basement  Thus they  3.1  from  :  transition  velocity  based  better interpretation  basement i s r e p r e s e n t e d a  1.5).  a c t u a l basement,  reflection  reflection  to  a  qreatly mentioned  observable  times  4.5  travel  by  however,  lack of an  value  area  of km/s  parallel has  also  between  J.  113  Tuzo  Wison  K n o l l s and  transition 2.2  km.  layer  Further,  about  200  km  transition  and at  as arrivals  simple  1.5  km  already  from  this  4.5).  (1976)  this.  On  have  found  fractured These  Winona of  basis  drill  that  i t c o n s i s t s of  illustrate fractured  While Ridge,  the the  unreasonable  to  while  the  deposited, basement  basalts  and  the  us  This  to  large  of  other  f i t  the  amplitude  synthetically  conclusion to  on  that  explain  the  by  Hyndman  l a y e r 2a  porous,  low  a  the  reasons  Basin  and  i n no  resembles  of this  in fact  et  transition  that  mixture  way  Hyndman  layer  sediments.  each  material  transition a.  thicknesses on  with  for they  density,  l a y e r i s c h a r a c t e r i s e d by  from  be  and  penetrating  sediments  would  the  some i n t e r c a l a t e d s e d i m e n t s .  this  assume  that  layers  the  highly  of  basin  three  been  cores  results  Cascadia  km.  some l i g h t  volcanic  Winona  a  depth  M i d - A t l a n t i c Ridqe  with  that  not  and  the  found  found  sufficient  p o s s i b l y shed  nature  being  leads  over  of  2  he  sub-bottom  stacked  not  s t r u c t u r e i s not study  he  km/s  mentioned,  fact  inhomogeneous  created  of  have  highly be  4.43  depth  volcanic material  sediments.  Basin,  and  been  and  a s e r i e s of  the  of  Atlantic  northern  layer  could  results  mixture  of  respectively,  This  velocity  al  region  a sub-bottom  r e s u l t s , ft r e c e n t et  km/s  3.78  has  (section  4.2  of  4.56,  Sound  velocity  i n the east  of  0.40  beginning  with  Queen C h a r l o t t e  layer consisted  velocities 0.41,  the  i f the  has  a l do  nearby  high been  velocity the  Mid-  show  layer. It  Winona  between of  low  continent  and  low  the  would  Basin  the  a  were were  sediments velocity  postulated  by  114  Knize  for  his  reflection  results  results  sediment/basalt two  higher  4.43 be  for  layer  kra/s,  1.5  km  the  area  (3.78  km/s  thick)  Such  by t h e l o n g e r  r e f r a c t i o n part If  this  layer  i s  materials,  explained  as  being  a then  due  to either  to explain  It  reflection  i s  hoped  data,  eg.  of this  Crustal The  three  layer  Layers  least  layers  consistent  w-k  are'  close  the  than  thick;  could  t h e DET  filtering  (section  further  high  not  during  and  signals  a focussing  further  i n Table  or  low  could  be  constructive  modelling  procedure  structure  of  processing 3.1}, t h e  this of the  complex  revealed.  depths of the  4.1.  2b, 3 a , and  These 3b,  have  maximum  values  reported  e t a l 1974).  crustal thickness  of  normally  12 km  However,  i s  considered  remaining velocities  r e s p e c t i v e l y and  A l l o f the sub-basement  (Peterson  t h e 5 t o 7 km  km  recorded  of  amplitude  v e l o c i t e s and  layers as s u c h .  sediment  between  :  identified  section  signals  after  c a n be  been  oceanic  however,  mixture  event  that  are l i s t e d  to  0.41  His  velocity  sandwiched  the velocity-depth  sguares  with  km)  basin.  low  ( 4 . 5 6 km/s,  the large  is  Lower  0.4  a  fine layering  complex  e f f e c t . I n any  nature  showed  wavelength  interference too simple  Cascadia  of h i s profiles.  velocity  layer.  the northern  velocity basaltic layers  delineated  the  in  layer for a  depths standard  the t o t a l  considerably  standard  have  sub-  thicker  f o r an  oceanic  crust. The  velocity gradients  introduced  into  t h e V-D  models  by  115  t h e s y n t h e t i c s have remains  however  structure 3a  r  from  is  somewhat.  What  t o d i s c u s s the s i g n i f i c a n c e of the  gross  o f t h e c u r v e s . The  3b  are  a l l similar.  one  l a y e r t o the  intermixing transition velocity (with  already  of  the  f r o m one  remains  a slight  curves,  i t  strongly  next  nearly  caused  that  the  temperature  c o n d i t i o n s i n each  has  a  Explorer to  depth  to  the  transition  compaction both.  12 km  75-1R  with  information. velocity  On  the  however,  basis  2-3  of  the km  these  i n this region i s layers,  possibly  different  pressure-  layer.  velocity  what  underlying  of  structure  found  Winona  to  sub-sediment  similar Basin.  11 km.  show  thicknesses His a  reversed sub-bottom  Considering that  thickness underlying  to those observed to  both  the basis of t h i s ,  vs d e p t h  crustal  travel and  on p r o f i l e s  t i m e s and  the s i m i l a r i t y  models i n t e r p r e t e d  the  agree  e x a m i n a t i o n of h i s data r e v e a l s  similar  respect  is  i s n o r m a l . In the r e g i o n of  E x p l o r e r Ridge 9  but  cover i n t h i s region i s minimal, these r e s u l t s  with the  or  Once  f o r the next  the  under  (1976) has  mantle  they are remarkably  the  types  than  Winona B a s i n . A d e t a i l e d  and'  On  separate  oceanic  (74-2,2R) c r o s s i n g  sediment well  thicker  observed  or  2b,  i n t h i s s t u d y , t h e c r u s t u n d e r l y i n g Winona  R i d g e , Malecek  those  profile  rock  typical  considerably  layers  by  lower, c r u s t  three  different  Basin  layers  to the next i s complete  reflecting  As i n t e r p r e t e d  either  gradient).  into  for  They i n d i c a t e a g r a d u a l  u n i f o r m with depth  positive  divided  discussed  c u r v e s produced  adjacent  layer  appears  been  that 75-1  amplitude between  f o r Winona B a s i n  and  116  Explorer seem  plate just  of  the  c r u s t i n the  two  thick  crust,  then,  does not  Winona  Basin.  In  that  same  the  beneath  structure shown  beneath  a similar  crust  Whereas  those  by  used  indicate A  the  recorded  1977).  zone  thick  oceanic  These  case  the  i n much  the  both  the  of  the  havinq  a  thick  Pacific  movement  away  from  a piling  up  has  possible  been  and  or  in  this  study,  i t  i s a widespread  the of  suggested  explanation  of  the by for  a  of  reversed the  reqion  analoq  and  is  a direct  sides  of  the  spreadinq newly  formed  Malecek the  centre  and  thick  of  of  the  a  could  oceanic  a  lack  If this  is as This  the  differing  The  Explorer  between the  As  Clowes  to  Ridge.  ridges.  plates.  1.1)  drastically  fievere  result  (Fiq  communication  because  i s trapped  Fuca  does  refraction  similar  playbacks.  Paul  as  Revere-De11wood  personal  however  sub-  feature.  t h i c k n e s s changes  Ridge  km  Explorer Ridqe  Hyndman  the  de  has  quality  crust,  Juan  a l 1976)  same  tentative  opposite  et  crustal  the  (R.D.  be  (Davis  the  of  northern  Explorer  of  only  not  of  crustal  localized  are  south  on  be  10-11  crust in this  are  to  of  and  section  American,  causinq  to  i n thickness could  tectonics plate,  crust  arrivals  though,  crosses  change  idea  would  thickness  data  west  results Pn  a  interpretation  and that  observable  one  i t  formed  study  Ridge,  (1976)  parallel  indicated  the  centre,  was  seem  OBS  Fuca  these  preliminary  standard  of  regions  an  with  Malecek  that  fracture  fact  J u a n de  sediment.  has  spreading  manner. The  line  east  North  result, be  i t s  restricted  material. This (1977)  c r u s t i n the  as  one  region  of  117  Explorer plate not  Ridge.  Winona  terminated isolated  by r o t a t i o n  northwest  t h e movement o f t h e P a c i f i c  this  piling  was c r e a t e d  up  effect-  type  has  implication  a  being  Explorer  of the p l a t e ,  of  crust  Winona  that  Basin  as  and  then  the  Explorer  thicker  ridge.  at the time  f r a c t u r e zone,  of the present  basin  from  Basin  at the Brooks  t h e same  that  hand,  should  observed.  If  The  the other  i s u n r e s t r i c t e d and hence  be  have  On  Ridge.  crust  i s  an  tends  was  This  than  subseguently  plate  i tt o  to  the  i s not the case.  the Pacific created  plate, the  by  spreading  to refute a possible  isolated  Ridge  would expect  Pacific  i t has n o t been This  we  Explorer  section  of  argument  old  Pacific  material. A been  more  reasonable  created  by t h e slow  point.  The  extremely centre crust up  slow  underlying  o f t h e new  '  The basin  Basin the  anomalies is  Knolls)  crustal  trapped  and B r o o k s  Basin  then  material,  lack  has already been  ? This  been  formed  p o i n t , then  possible  i n  has  i s a difficult  that  much  the  my,  happened  question  triple  most l i k e l y the  has  be  spreading The  by a  same  thick piling  way  as  plate. anomaly  i n Chapter 3-4  i f the spreading  the  be formed  maqnetic  i n the l a s t what  would  Basin  f r a c t u r e zone.  Explorer  mentioned  of  between  could  o f any s i g n i f i c a n t  Winona  movement  i n the basin  f o r the c r u s t beneath  has  triple  rate  Winona  i s that  northward  due t o i t b e i n g  (Dellwood  postulated  the  spreading  suggestion  1.  I f  Winona  by t h e movement o f to  the  t o answer.  centre  pattern i n  magnetic  Perhaps i t  creating the basin  118  were no  very  diffuse,  discernible The  (Couch -160 a  mgal  pile  will  qravity  rather  than  Paul-Revere  analysis,  crust is,  across  the  explanation  course,  detailed  analysis  and  hopefully  be  extremely  created.  including  only  reversed  provide and  s h o w n by  to  km  of  gravity  Couch, but chanqes  terminate  What d o e s t h i s  a question  lends  in progress,  qiven  of  additional  complex  the  2  data  c r u s t i s c o n s i s t e n t with  require other  Ridge.  slow,  of  lack  in his  at  Winona  Ridqe  importance  two  of  model.  mean i n t e r m s  added the  the  the  of to  reversed  DSS  the  h e r e of  the  origin  seismic  profiles  of  along  the Winona  thick Basin  h i g h l y s p e c u l a t i v e . Subsequent a n a l y s i s of  two  Ridge,  were  basin.  i n t e r p r e t e d from of  as  tends  currently  remaining  this  low  s t r u c t u r e ? Such  The  A thick  f r e e a i r anomaly,  the  profiles  would  section,  i s required.  Also,  the  pattern  crustal  sediment  crustal  spreadinq  i m p l i e s t h a t a r e c o n s i d e r a t i o n of  1969}  thick  i f the  anomaly  thick  sediments,  and  DSS the  profiles OBS  data  geophysical  more e v i d e n c e  fascinating  i n ' Winona to the  studies to  area.  Basin,  west  in  aid in fully  of  the more  Explorer  progress  will  understanding  119  REFERENCES  A t w a t e r , T. 1970. I m p l i c a t i o n s of p l a t e t e c t o n i c s f o r t h e Cenozoic t e c t o n i c e v o l u t i o n o f western North America. G e o l . Soc. Am. B u l l . 8 _1, pp. 3513-3536. B a r r , S.M. and R.L. Chase 1974. Geology of t h e n o r t h e r n end o f Juan de Fuca Ridge and s e a - f l o o r s p r e a d i n g . Can. J . 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R. a s t r Soc. 46, pp. 1-10.  calculations-II.  , and D.V. Helmberger 1974. S y n t h e t i c seismogram computation by e x p a n s i o n i n g e n e r a l i z e d r a y s . Geophys. J . R. a s t r Soc. 37, pp. 73-90. , and J.A. Madrid 1974. Body wave a m p l i t u d e c a l c u l a t i o n s . Geophys. J . R. a s t r . Soc. 37, pp. 423-433.  122  Y o r k , D. 1969. L e a s t s q u a r e s f i t t i n q of a s t r a i q h t line with c o r r e l a t i n q e r r o r s . E a r t h and P l a n e t a r y S c i . L e t t . 5, pp. 320-324.  >  1  j  HG.  1.3  8 LINE iS-k MOftTUVEST  io ten  i  73-1 LINES  13L  .H.  uwt  15-3  S O U T H E A S T  FlGr. l * t  

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