"Science, Faculty of"@en . "Earth, Ocean and Atmospheric Sciences, Department of"@en . "DSpace"@en . "UBCV"@en . "Thorleifson, Allan James"@en . "2010-02-27T00:00:44Z"@en . "1978"@en . "Master of Applied Science - MASc"@en . "University of British Columbia"@en . "During the summer of 1975 a deep seismic sounding survey was carried out over Winona Basin, a deep water sedimentary basin located off the northern end of Vancouver Island. Three reversed refraction profiles were shot, one parallel and two perpendicular to the axis of the basin, with penetration from the ocean bottom to the upper mantle. Several sub-critical reflection profiles were also shot in an attempt to delineate the sedimentary structure of the basin. The two sub-critical reflection profiles shot over the central part of the basin were analyzed using the T\u00B2-X\u00B2 method. The data sets gave layer velocities and thicknesses for 2 km of sediments for one of the profiles and .6 km for the other although petroleum industry data indicate that neither profile penetrated to the volcanic basement. The remaining reflection profiles were shot on the sides of the basin. On the western flank of Paul Revere Ridge, approximately 1 km of sediments with velocity in the range 2.5 to 3.5 km/s overlies volcanic basement. Over the continental slope on the east the seismic energy is strongly scattered below an upper 0.7 km of sediments. Refraction profile 75-1,1R, along the axis of the basin, was analyzed in a previous study using synthetic seismograms. However, the severe lateral inhomogeneities across the basin necessitated the use of ray tracing for the cross basin refraction profiles, 75-2,2R and 75-3,35. The final models are non-unique but they satisfy the seismic data very well and are consistent with profile 75-1,1R, gravity data and current views on plate tectonics. They show deep crustal layers dipping from both sides of the basin towards the center. Evidence for subduction as well as lateral motion between the Explorer and American plates has led to the conclusion that oblique subduction is occurring at Winona Basin."@en . "https://circle.library.ubc.ca/rest/handle/2429/21166?expand=metadata"@en . "_ MARINE DEEP SEISMIC WINONA SOUNDING SURVEY OVER BASIN by ALLAN JAMES THORLEIFSON B. A*Sc. , U n i v e r s i t y o f B r i t i s h Columbia, 1976 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF THE FACULTY OF GRADUATE STUDIES (Department of Geophysics and Astronomy) We accept t h i s t h e s i s as conforming to the r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA MASTER OF APPLIED SCIENCE xn J u l y , 1978 A l l a n James T h o r l e i f s o n , 1978 In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the requ i rement s f< an advanced degree at the U n i v e r s i t y o f B r i t i s h Co lumb ia , I ag ree tha the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s tudy . I f u r t h e r agree 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 c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y pu rpo se s may be g r a n t e d by the Head o f my Department or by h i s r e p r e s e n t a t i v e s . It i s u n d e r s t o o d that c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i thout my w r i t t e n p e r m i s s i o n . Department o f {-r\C<> AbAYQl\Oft\y The U n i v e r s i t y o f B r i t i s h Co lumbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date i i A B S T R A C T During the summer of 1975 a deep s e i s m i c sounding survey was c a r r i e d out over Winona B a s i n , a deep water sedimentary basin l o c a t e d o f f the n o r t h e r n end of Vancouver I s l a n d . Three rev e r s e d r e f r a c t i o n p r o f i l e s were shot, one p a r a l l e l and two p e r p e n d i c u l a r to the a x i s o f the basin, with p e n e t r a t i o n from the ocean bottom to the upper mantle. S e v e r a l s u b - c r i t i c a l r e f l e c t i o n p r o f i l e s were a l s o shot i n an attempt to d e l i n e a t e the sedimentary s t r u c t u r e of the b a s i n . The two s u b - c r i t i c a l r e f l e c t i o n p r o f i l e s shot over the c e n t r a l part of the b a s i n were analyzed using the T 2 ~ X 2 method. The data s e t s gave l a y e r v e l o c i t i e s and t h i c k n e s s e s f o r 2 km of sediments f o r one of the p r o f i l e s and .6 km f o r the other although petroleum i n d u s t r y data i n d i c a t e t h a t n e i t h e r p r o f i l e penetrated to the v o l c a n i c basement. The remaining r e f l e c t i o n p r o f i l e s were shot on the s i d e s of the b a s i n . On the western f l a n k of Paul Revere Ridge, approximately 1 km of sediments with v e l o c i t y i n the range 2.5 to 3.5 km/s o v e r l i e s v o l c a n i c basement. Over the c o n t i n e n t a l s l o p e on the e a s t the s e i s m i c energy i s s t r o n g l y s c a t t e r e d below an upper 0.7 km of sediments. R e f r a c t i o n p r o f i l e 75-1,IR, along the a x i s of the b a s i n , was analyzed i n a previous study using s y n t h e t i c seismograms. However, the severe l a t e r a l inhomogeneities a c r o s s the b a s i n n e c e s s i t a t e d the use o f ray t r a c i n g f o r the c r o s s b a s i n r e f r a c t i o n p r o f i l e s , 75-2,2R and 75-3,35. The f i n a l models are non-unique but they s a t i s f y the s e i s m i c data very w e l l and are consistent with p r o f i l e 75-1,1R, gravity data and current views on plate tectonics. They show deep c r u s t a l layers dipping from both sides of the basin towards the center. Evidence for subduction as well as l a t e r a l motion between the Explorer and American plates has led to the conclusion that oblique subduction i s occurring at Hinona Basin. i v TABLE OF CONTENTS Abstract i i Table Of Contents i v L i s t Of Tables v L i s t Of Figures ......................................... v i Acknowledgements . . . . . . . . . . . . v . . . . . . . . . . . . . . . . . . . . . . . . . . . v i i i 1. I n t r o d u c t i o n 1 1.1 S t r u c t u r e And Tectonics Of Winona Basin ........... 3 1.2 P r o j e c t D e s c r i p t i o n ............................... 8 2. Data A c q u i s i t i o n And Pr e l i m i n a r y A n a l y s i s ............ 10 2.1 Data A c q u i s i t i o n .................................. 10 2.2 P r e l i m i n a r y A n a l y s i s .............................. 11 3. S u b - c r i t i c a l R e f l e c t i o n Data ......................... 17 3.1 Methods Of A n a l y s i s ............................... 17 3.2 Bubble Pulse Deconvolution 19 3.3 Analysis Of Results ............................... 21 4. R e f r a c t i o n A n a l y s i s 35 1.1 The R e f r a c t i o n Data Set 35 4.2 Methods Of Ana l y s i s ............................... 42 4.3 D e s c r i p t i o n Of Ray Tracing Program ,.45 4.4 A p p l i c a t i o n Of Ray Tracing 47 5. Discussion ........................................... 62 5.1 Sediments 62 5.2 Ray Tracing Models ................................ 65 5.3 Tectonic S i g n i f i c a n c e Of Winona Basin 67 References ....................... ........ \u00E2\u0080\u00A2>......... ..... 69 V LIST OF CABLES I R e f l e c t i o n i n t e r p r e t a t i o n r e s u l t s f o r p r o f i l e s 75-2V and 75-3V. ............................ ,23 II R e f l e c t i o n i n t e r p r e t a t i o n r e s u l t s f o r p r o f i l e s 75-2, 75-2R, 75-3 and 75-3S. ........................ 33 v i LIST OF FIG0RES 1.1 L o c a t i o n map f o r Sinona Basin. .................... 2 1.2 L o c a t i o n map f o r DSS p r o f i l e s , ..................., 4 1.3 Continuous s e i s m i c p r o f i l e l i n e 75-2. ............. .7 1.4 Continuous s e i s m i c p r o f i l e l i n e 75-3.............. 8 2.1 The 6 s e i s m i c channels plu s SIVB time code f o r two shots. .......... ....\u00C2\u00BB............. ........ .13 3.1 Eecord s e c t i o n f o r r e f l e c t i o n p r o f i l e 75-2V. ...... 22 3.2 Record s e c t i o n f o r r e f l e c t i o n p r o f i l e 75-3V. ......25 3.3 Record s e c t i o n f o r r e f l e c t i o n p r o f i l e 75-2R, shallow shots. .. , 27 3.4 Record s e c t i o n f o r r e f l e c t i o n p r o f i l e 7 5-2B, deep s h o t s . ............................ ............ 28 3.5 Record s e c t i o n f o r r e f l e c t i o n p r o f i l e 75-2. 29 3.6 Record s e c t i o n f o r r e f l e c t i o n p r o f i l e 75-3. ....... 30 3.7 Record s e c t i o n f o r r e f l e c t i o n p r o f i l e 75-3R. ......31 4.1 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 75-2. ...... .36 4.2 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 75-2K. ...... 37 4.3 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 75-3. .......38 4.4 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 75-3R. ...... 39 4.5 S t a r t i n g model f o r 75-3 ray t r a c i n g . .............. 48 4.6 Ray p l o t and t r a v e l time curve f o r 75-3 s t a r t i n g model. ..... ...........52 4.7 Ray p l o t and t r a v e l time curve f o r 75-3R s t a r t i n g model. .................................... ......53 4.8 Ray p l o t and t r a v e l time curve f o r 75-3 f i n a l model. ............................................ 55 4.9 Ray p l o t and t r a v e l time curve f o r 75-3R f i n a l model. ......... , ............56 4. 10 Ray p l o t and t r a v e l time curve f o r 75-2 f i n a l model. ...\u00C2\u00BB........................................59 v i i 4.11 Say p l o t and t r a v e l time curve f o r 75-2B f i n a l model. 60 v i i i ACKNOWLEDGEMENTS F o r h i s e n c o u r a g e m e n t a nd open h e l p f u l n e s s t h r o u g h o u t a l l p h a s e s o f t h i s p r o j e c t I w o u l d l i k e t o t h a n k my s u p e r v i s o r D r . R.M. C l o w e s . I wou l d a l s o l i k e t o t h a n k my f e l l o w g r a d u a t e s t u d e n t s H e n r y Cheung, Andy J u r k e v i c s a n d B a r r y N a r o d f o r t h e i r s t i m u l a t i n g d i s c u s s i o n s a n d h e l p f u l s u g g e s t i o n s . S p e c i a l t h a n k s a l s o go t o Ken W h i t t a l l f o r h i s e n t h u s i a s t i c work w i t h t h e r a y t r a c i n g p r o g r a m . C h e v r o n S t a n d a r d L t d . p r o v i d e d t h e o p p o r t u n i t y t o e x a m i n e t h e i r r e f l e c t i o n r e c o r d s e c t i o n s i n Winona B a s i n . I w o u l d a l s o l i k e t o e x p r e s s my a p p r e c i a t i o n t o t h e o f f i c e r s and c r e w o f t h e C. F. A. V. ENDEAVOUR and C.F. A. V. LAYMORE a s w e l l a s t h e e x p l o s i v e s e x p e r t s f r o m t h e F l e e t D i v i n g U n i t , P a c i f i c M a r i t i m e Command, E s g u i m a l t , B.C. f o r t h e i r a s s i s t a n c e d u r i n g t h e c r u i s e . F u n d i n g f o r t h i s p r o j e c t was p r o v i d e d t h r o u g h a r e s e a r c h c o n t r a c t a n d r e s e a r c h a g r e e m e n t s w i t h t h e G e o l o g i c a l S u r v e y o f C a n a d a , D e p a r t m e n t o f E n e r g y , M i n e s and R e s o u r c e s and t h r o u g h o p e r a t i n g g r a n t A77 07 o f t h e N a t i o n a l R e s e a r c h C o u n c i l o f Canada. A d d i t i o n a l f u n d s were s u p p l i e d by M o b i l O i l C a nada L t d . , S h e l l Canada R e s o u r c e s L t d . and C h e v r o n S t a n d a r d L t d . 1 U , INTRODUCTION The western Canadian c o n t i n e n t a l margin i s t e c t o n i c a l l y a very complex area. A s e r i e s of a c t i v e s e a - f l o o r spreading r i d g e s connected by transform f a u l t s separates the o c e a n i c c r u s t i n t o the P a c i f i c p l a t e to the west and the much s m a l l e r E x p l o r e r and Juan de Fuca p l a t e s to the e a s t ; east of the l a t t e r p l a t e s i s the l a r g e North American p l a t e , the western pa r t o f which has c o n t i n e n t a l c r u s t (see F i g . 1.1). The Ex p l o r e r and Juan de Fuca p l a t e s are remnants of the once l a r g e r F a r a l l o n p l a t e , and are now separated i n t o two sub-p l a t e s by the Nootka F r a c t u r e Zone (Riddihough and Hyndman, 1976; Hyndman, p e r s o n a l communication, 1977). North of the Dellwood K n o l l s , l a t e r a l motion occurs between the P a c i f i c and North American p l a t e s along the Queen C h a r l o t t e transform f a u l t zone; south o f the Dellwood K n o l l s , convergence of the North American p l a t e with the E x p l o r e r and Juan de Fuca p l a t e s r e s u l t s i n compression of the ocea n i c c r u s t and subduction of the oceanic p l a t e beneath the c o n t i n e n t a l p l a t e (Riddihough and Hyndman, 1976). The p o i n t along the c o n t i n e n t a l margin which s e p a r a t e s the transform f a u l t motion from the convergent motion i s known as the t r i p l e j u n c t i o n between the three p l a t e s . The change i n p o s i t i o n of t h i s t r i p l e j u n c t i o n over the past s e v e r a l m i l l i o n years i s b e l i e v e d to be a t l e a s t p a r t i a l l y r e s p o n s i b l e f o r many of the prominent f e a t u r e s of the r e g i o n , i n c l u d i n g Winona Basin, the area o f i n t e r e s t f o r t h i s deep s e i s m i c sounding survey. .. F i g . 1. 1 3 ls.1 STRUCT0RE AND TECTONICS OF WINONA BASIN Winona Basin i s a deep water sedimentary b a s i n l o c a t e d a t the f o o t of the c o n t i n e n t a l slope o f f the northern t i p of Vancouver I s l a n d (see F i g s . 1.1 & 1.2). I t i s bounded by Paul Revere Ridge t o the southwest, the Brooks F r a c t u r e Zone (adjacent to Brooks Peninsula) to the southeast and the Dellwood K n o l l s to the northwest. The b a s i n i t s e l f , which i s g e n e r a l l y b e l i e v e d to be of P l i o c e n e - P l e i s t o c e n e age, i s d i v i d e d i n t o two s m a l l e r basins by Winona Ridge, which runs o b l i q u e l y down the length of the basin. A -160 mqal f r e e a i r g r a v i t y anomaly l o c a t e d over the e a s t e r n p o r t i o n of the b a s i n has been i n t e r p r e t e d by Couch (1969) as being due to a to 6 km of sediments., Petroleum i n d u s t r y s e i s m i c r e f l e c t i o n data (Chevron Standard L t d . , C a l g a r y , unpublished data) i n d i c a t e up to 4 km of sediments. The basin does not show the t y p i c a l l i n e a r magnetic anomaly p a t t e r n s a s s o c i a t e d with sea f l o o r c r u s t which has been formed at a spreading center, but Riddihough (personal communication, 1978) has sugqested t h a t the o v e r l y i n g sediments c o u l d o b l i t e r a t e such a p a t t e r n . Winona Basin i s l o c a t e d near the j u n c t i o n of t h e P a c i f i c , American, and E x p l o r e r p l a t e s and the main f e a t u r e s of the b a s i n have almost c e r t a i n l y been c o n t r o l l e d by the complex i n t e r a c t i o n of s p r e a d i n g r i d g e readjustment and subduction at the c o n t i n e n t a l margin. In a study o f the magnetic anomaly pa t t e r n s of the area, Riddihough (197 7) has shown t h a t readjustment o f s p r e a d i n g r a t e s and d i r e c t i o n s over the past 4 F i g . 1 .2 5 10 m i l l i o n years has r e s u l t e d i n a complicated motion of the t r i p l e j u n c t i o n near the area of Brooks P e n i n s u l a , T h i s has a l s o been suggested by Murray and T i f f i n <1974) based on evidence of subduction to the southeast of the Brooks F r a c t u r e Zone and s t r i k e - s l i p motion t o the northwest. I f the t r i p l e j u n c t i o n i s now l o c a t e d at t h e Dellwood K n o l l s , i t must have migrated northward over the past few m. y. r e s u l t i n g i n the formation of Winona Basin., I t was suggested by Hyndman and Riddihough (personal communication, 1978) that the t r i p l e j u n c t i o n may have migrated along Winona Ridge, which l i n e s up w e l l with the Queen C h a r l o t t e F a u l t , r a t h e r than f o l l o w i n g the i n d e n t a t i o n i n the c o a s t o f f northern Vancouver I s l a n d . T h i s would imply t h a t the e a s t e r n p o r t i o n of the b a s i n was a t l e a s t t e m p o r a r i l y stuck t o the c o n t i n e n t a l p l a t e r e s u l t i n g i n the e a s t s i d e of t h e basin being c o n s i d e r a b l y o l d e r than the west. However Davis ( p e r s o n a l communication, 1978) has shown, on the b a s i s of heat flow measurements, t h a t both s i d e s of the b a s i n are not l i k e l y more than. 6 m.y. o l d . Another p o s s i b i l i t y i s t h a t Winona Ridge, from which c o n s o l i d a t e d sediments have been dredged (Chase, personal communication, 1978), was formed by slow convergence of the o c e a n i c and c o n t i n e n t a l p l a t e s . The newly formed oceanic c r u s t , being t h i n and s t i l l r e l a t i v e l y warm, may have been more s u s c e p t i b l e t o deformation and compression than to subduction. S i n c e t h e r e very l i k e l y i s subduction to the southeast of the b a s i n and d e f i n i t e l y s t r i k e - s l i p motion t o the northwest ( T i f f i n et a l , 1972; Chase et a l , 1975; Riddihough and Hyndman, 1976), the northwestern edge of t h e 6 subducting p l a t e must be e i t h e r beneath, or j u s t southeast o f the b a s i n . Riddihough (1977) shows i t as being j u s t southeast of the b a s i n . Continuous s e i s m i c p r o f i l e s (C.S.P.*s) show the v o l c a n i c basement d i p p i n g beneath Winona Basin from Paul Revere Ridge (see F i g s . 1. 3 & 1.4), but i t i s not c l e a r i f t h i s i s a c t u a l l y a subducting p l a t e or j u s t the r e s u l t of deformation and u p l i f t along P a u l Revere Ridge. 1.2 PROJECT DJSCJRIPTIOS In order t o g a i n a f u r t h e r understanding of the s i g n i f i c a n c e of Winona Basin, a s e i s m i c survey was c a r r i e d out during the summer o f 1975. The o b j e c t i v e of the study was to provide d e t a i l e d v e l o c i t y and s t r u c t u r a l i n f o r m a t i o n f o r the c r u s t and upper mantle i n the area. Three reversed p r o f i l e s were run; one along the c e n t e r of the e a s t e r n p o r t i o n o f the b a s i n and two across the b a s i n (see F i g . 1.2). To gain f u r t h e r i n f o r m a t i o n about the sediments, short near v e r t i c a l i n c i d e n c e p r o f i l e s were run a t the i n t e r s e c t i o n s of the c r o s s p r o f i l e s with the long p r o f i l e . , The marine s e i s m i c system (Clowes, 1977) records near v e r t i c a l i n c i d e n c e to wide-angle r e f l e c t e d and r e f r a c t e d waves with p e n e t r a t i o n from the ocean bottom to the upper mantle. The f i r s t 16 s h o t s of each p r o f i l e were s e t at shallow depths (7 ra) i n hopes t h a t the gas bubble would blow out a t the s u r f a c e , minimizing the bubble pulse problem. The shooting s h i p then re t u r n e d t o i t s s t a r t i n g p o i n t and proceeded t o shoot the e n t i r e p r o f i l e with the shots a t the C.S. P. LINE 75-2 i 1 F i g . 1.3 Continuous s e i s m i c p r o f i l e l i n e 75-2, p a r a l l e l to r e f r a c t i o n l i n e 75-2E. Superimposed above the s e c t i o n are p r o f i l e l i n e s showing approximate shot l o c a t i o n s {numbers below the l i n e s ) and r e c e i v i n g s h i p l o c a t i o n s f o r these shots (numbers above the l i n e s ) * F i q , 1.4 C o n t i n u o u s s e i s m i c p r o f i l e l i n e 75-3, p a r a l l e l to r e f r a c t i o n l i n e 75-3. For d e s c r i p t i o n of p r o f i l e l i n e s see F i q . 1. 3. 9 optimum depths f o r maximum p e n e t r a t i o n of s e i s m i c energy, these depths depending on the charge s i z e (Shor, 1 9 6 3 ) . P r o f i l e s 75\u00E2\u0080\u00941 and 7 5 - 1 R were analyzed by Lynch ( 1 9 7 7 ) while t h i s t h e s i s p resents an a n a l y s i s of p r o f i l e s 7 5 \u00E2\u0080\u0094 2 , 7 5 - 2 R , 7 5 - 3 , 7 5 - 3 R , 7 5 - 2 V and 7 5 - 3 V , Since t h e r e s u l t s of Lynch*s work are an i n t e g r a l p a r t of t h i s study, many r e f e r e n c e s w i l l be made to h i s t h e s i s . 10 2_. DATA ACMISJTION AN D PRELIMINARY ANALYSIS 2_! DATA ACQUISITION A d e t a i l e d d e s c r i p t i o n of the marine deep s e i s m i c sounding system i s given by Clowes (1977). I t i s s i m i l a r i n p r i n c i p l e to the two-ship r e f r a c t i o n technique described by Shor (1963) , i n which one s h i p , the r e c o r d i n g s h i p , d r i f t s f r e e l y at one end o f t h e l i n e w h i l e the second s h i p proceeds along a predetermined course r e l e a s i n g e x p l o s i v e charges. For our survey, s h i p p o s i t i o n s were determined by LORAN A. On the r e c e i v i n g s h i p , t h e s e i s m i c s i g n a l s were detected by s i x hydrophones suspended to a depth of 45 m from a 610 m c a b l e . The s i g n a l s were p r e - a m p l i f i e d at t h e hydrophone, f i l t e r e d from 0.8 t o 100 Hz and then a m p l i f i e d by i n d i v i d u a l a m p l i f i e r s manually s e t f o r each shot. The s i x analog s i g n a l s p l u s WtfVB time code were d i g i t i z e d at 312.5 Hz and w r i t t e n onto magnetic tape using an I.B\u00C2\u00BBM. compatible, 14 b i t , m u l t i -channel data a c g u i s i t i o n system (see Clowes, 1977). F i v e data channels plus the W5JVB time code were monitored on a 6-channel c h a r t r e c o r d e r f o r g u a l i t y c o n t r o l . On the s h o o t i n g s h i p , the d i r e c t water wave (D.W.H.) was detected by both a hydrophone t r a i l e d d i r e c t l y behind the s h i p and a geophone l o c a t e d on deck. These two s i g n a l s p l u s the \u00E2\u0080\u00A2HflVB time code were recorded on a 4-channel FM tape t r a n s p o r t while the hydrophone and WWVB s i g n a l s were recorded d i r e c t l y onto a 2-channel high speed Brush c h a r t recorder.. For the p r o f i l e s being analyzed i n t h i s t h e s i s , the charges ranged i n 11 s i z e from 2.3 kg to 96 kg of Geogel, a commercial e x p l o s i v e , and were suspended at optimum depths by l a r g e red b a l l o o n s . The s h o t - t o - s h i p d i s t a n c e s were measured with a r a n g e f i n d e r focussed on the b a l l o o n s . 2j.2 PRELIWINAfiY ANALISIS 1) Demultiplexing Since the s i x s e i s m i c channels plus WSVB time code were recorded d i g i t a l l y i n m u l t i p l e x e d form, the f i r s t data p r o c e s s i n g s t e p was d e m u l t i p l e x i n g . The analog r e c o r d s , on which a marker channel i d e n t i f i e d the time i n t e r v a l f o r d i g i t i z a t i o n , were used to e d i t out unnecessary data s i n c e s e v e r a l seconds were reco r d e d both before and a f t e r the a r r i v a l of the shot energy. A computer program w r i t t e n by Lynch (1977) was used t o perform the d e m u l t i p l e x i n g . The program checked the i n p u t tapes f o r missed data and tape e r r o r s and wrote the c o r r e c t e d , demultiplexed data onto new magnetic tapes. The s i x s e i s m i c channels plu s WWVB time code f o r a s i n g l e shot comprise one data f i l e . , 2) Shot O r i g i n Times The D.W.W. recorded at the s h o o t i n g s h i p was used to determine the shot o r i g i n times., The a r r i v a l of the s i g n a l at the hydrophone could be timed t o b e t t e r than 5 ms from the 2-channel c h a r t r e c o r d i n g s . Since the charges were detonated at v a r i o u s depths and d i s t a n c e s from the shooting s h i p , c o r r e c t i o n s were made for the t r a v e l time, g e n e r a l l y of the 1 2 order of 100 ms, from shot to s h i p . T h i s i n t r o d u c e d a f u r t h e r 15 t o 20 ms e r r o r . 3) Shot-Receiver D i s t a n c e s The shot t o r e c e i v e r d i s t a n c e s were determined by measuring the D.w.W. t r a v e l time and assuming a con s t a n t water v e l o c i t y of 1.49 km/s. To measure the a r r i v a l time of the D.W.W., the s i x s e i s m i c and WWVB time channels were p l o t t e d with a common time o r i g i n a t a rate of .126 s/cm {.32 s / i n c h ) , each channel being normalized t o a maximum amplitude of 1.9 cm (see F i g . 2.1). The e r r o r i n measuring the D.W.W. a r r i v a l i s 10 - 20 ms and combined with o r i g i n time e r r o r s t h i s g i v e s a t o t a l e r r o r i n d i s t a n c e o f the order of 50 - 80 m. 4) Topographic C o r r e c t i o n As can be seen from the C. S. P. r e c o r d s ( F i g s . 1. 3 & 1.4), i t was necessary to make c o r r e c t i o n s f o r topography. Since the e a s t e r n p o r t i o n of the b a s i n i s a t a uniform depth of approximately 2.0 km, a l l t r a v e l times were c o r r e c t e d t o t h i s depth, A v e l o c i t y o f 2.0 km/s was assumed f o r the immediate sub-bottom m a t e r i a l , t h i s being r e p r e s e n t a t i v e of shallow sediments. The e f f e c t o f t h i s c o r r e c t i o n i s t o r e p l a c e a l l sub-bottom m a t e r i a l above 2.0 km depth with 1. 49 km/s m a t e r i a l , and a l l water below 2.0 km with m a t e r i a l of v e l o c i t y 2.0 km/s. The c o r r e c t i o n i s <2.1) 13 n O CM I I C\J I L O D.VYW. \u00E2\u0080\u00A21 SEC-H F i q . 2.1 Two seismoqrams t y p i c a l o f those used to time the d i r e c t water wave (D.W.W. ) and the f i r s t r e f r a c t i o n a r r i v a l s . The bubble p u l s e sequence i s c l e a r l y v i s i b l e f o l l o w i n g the D.W.W. a r r i v a l on the upper set of t r a c e s . The lower s e t of t r a c e s shows the a r r i v a l oC r e f r a c t i o n energy at a d i s t a n c e of 19 km. 14 where H i s the height o f the topography from 2.0 km depth, V w i s the water v e l o c i t y , V5 the sub-bottom v e l o c i t y and O the angle of the ray from v e r t i c a l . T h i s c o r r e c t i o n , which i s approximately 50 ms f o r 250 m of topography, was a p p l i e d t o both the sh o o t i n g s h i p and the r e c e i v i n g s h i p water depths. Since the two s i d e s of the b a s i n a re c l o s e to 2.0 km depth, the g r e a t e s t e f f e c t o f the to p o g r a p h i c c o r r e c t i o n i s over Paul Revere Ridge, Winona Ridge and the c o n t i n e n t a l s h e l f and s l o p e . The v e l o c i t i e s beneath these areas are not w e l l known so t h a t the c o r r e c t i o n s c o u l d be i n e r r o r . For example, i f 1.0 km of c o n t i n e n t a l slope m a t e r i a l i s c o r r e c t e d at 2.0 km/s the c o r r e c t i o n i s 170 ms, while i f i t i s c o r r e c t e d a t 4.0 km/s the c o r r e c t i o n i s 4 20 ms. T h i s must be co n s i d e r e d when a n a l y z i n g such areas., During the survey t h e r e was no depth r e c o r d i n g equipment op e r a t i n g on e i t h e r s h i p , although a request f o r workinq echo sounders had been made p r i o r t o the c r u i s e . In order to o b t a i n depth i n f o r m a t i o n , a c h a r t produced by the Canadian Hydrographic S e r v i c e was used. On t h i s c h a r t are p l o t t e d water depths along an ex t e n s i v e s e r i e s of s h i p t r a c k s . The major source of e r r o r i n determining the depths below our s h i p s from t h i s c h a r t i s the u n c e r t a i n t y i n s h i p p o s i t i o n , which f o r LORAN A i n t h e r e g i o n of our survey i s of the order of 1 km. I t was a l s o p o s s i b l e t o c a l c u l a t e the depths below the sho o t i n g s h i p by measuring the d i f f e r e n c e i n t r a v e l time o f the D.W.W. and the f i r s t water-bottom bounce. Con s i d e r the f o l l o w i n g diagram: 15 I f T i i s the a r r i v a l time of the D . W . S . and T2. the a r r i v a l time of the water-bottom bounce then, assuming near-vertical incidence. Tx= ( Z / ( \u00C2\u00A3 ) x * * * -<0/Vw 1 2 . 3 ) where X i s the horizontal distance, d the shot depth, and Z the water depth. Let AT = T* - T| , then 2. = x/(VwAT \u00E2\u0080\u00A2 d <- y^FTc?)1- - XV2- <2.4) The uncertainty involved should be no more than 30 m over a depth of 2000 m. The r e s u l t s obtained using t h i s method agree very well with those taken from the bathymetric chart so i t was assumed that the receiving ship depths taken from the chart were reasonable as well. 16 5) Record S e c t i o n s In order t o present the data i n the best p o s s i b l e way f o r i n t e r p r e t a t i o n , a r e c o r d s e c t i o n was produced f o r each p r o f i l e . The program used to compile the r e c o r d s e c t i o n s makes c o r r e c t i o n s f o r a m p l i f i e r gain, charge s i z e , s p h e r i c a l s p r e ading, and hydrophone s e n s i t i v i t i e s (see Lynch, 1977). The c o r r e c t x o n f o r charge s i z e i s 8 f o r a weight of W pounds (O'Brien, 1960; M u l l e r et a l , 1962) and the c o r r e c t i o n f o r s p h e r i c a l spreading i s X 2 f o r head wave amplitudes at l a r g e d i s t a n c e s (Cerveny and Ravindra, 1971) and X f o r r e f l e c t i o n amplitudes.. Such amplitude s c a l i n g with d i s t a n c e g i v e s a re c o r d s e c t i o n with amplitudes normalized such t h a t a r r i v a l s at a l l d i s t a n c e s can be seen c l e a r l y . , The program a l s o c o n t a i n s a zero phase, f o u r pole Butterworth f i l t e r (Kanasewich, 1976) which was used p r i m a r i l y t o reduce high frequency n o i s e . 17 3_. SUB-CRITICAL REFLECTION DM A -3_.1 METHODS OF ANALYSIS A l l r e f l e c t i o n p r o f i l e s s e r e i n t e r p r e t e d by f i r s t \" p i c k i n g \" the a r r i v a l times vs. d i s t a n c e f o r a l l r e f l e c t i n g h o r i z o n s which could be i d e n t i f i e d . However, d i f f e r e n c e s i n i n d i v i d u a l p r o f i l e s made i t necessary t o use two methods c f a n a l y s i s . P r o f i l e s 75\u00E2\u0080\u00942V and 75\u00E2\u0080\u00943? were shot as s p l i t - d i p p r o f i l e s ( T e l f o r d et a l , 1976) and i t i s obvious from the C S . P . records that t h e r e i s very l i t t l e d i p i n the l a y e r i n g . P r o f i l e s 75-2, 75-2R, 75-3 and 75-3R on the other hand, are not s p l i t - d i p p r o f i l e s and i t i s obvious from the C.S.P. re c o r d s t h a t there i s s i g n i f i c a n t d i p i n the h o r i z o n s over which they were shot. For these reasons, p r o f i l e s 75-2V and 75\u00E2\u0080\u00943V were analyzed u s i n g the standar d T 2-X 2 method f o r f l a t l a y e r s and the other p r o f i l e s were analyzed using the eguation f o r d i p p i n g l a y e r s . 1) T 2 - X 2 Method For plane, h o r i z o n t a l r e f l e c t i n g h o r i z o n s i n a m u l t i l a y e r e d medium, the approximate r e l a t i o n s h i p between t r a v e l time T and d i s t a n c e X i s T ^ i ^ - T V (3.1) where V i s the average rms v e l o c i t y down to a h o r i z o n and T 0 the 2-way v e r t i c a l i n c i d e n c e t r a v e l time. The r i g h t - h a n d s i d e 18 of equation 3. 1 c o n s i s t s of the f i r s t two terms of a T a y l o r s e r i e s expansion f o r T 2 (X) about the point X=0. . A p l o t o f T 2 vs X 2 w i l l y i e l d a s l o p e of 1/V2 and an i n t e r c e p t of T 2., For the case of s e v e r a l l a y e r s , Dix (1955) g i v e s the i n t e r v a l v e l o c i t y V* as x/l;, / ^ T K - ^sx: (3.?) J T\" - Tic-\ where \ and T K are t h e average rms v e l o c i t y and 2-way v e r t i c a l i n c i d e n c e t r a v e l time to the bottom of the k1* l a y e r . The t h i c k n e s s of the k**1 l a y e r i s then g i v e n by 2) Dipping Layer Approach The r e l a t i o n s h i p between t r a v e l time and d i s t a n c e f o r a d i p p i n g plane horizon i s , (from T e l f o r d e t a l , 19 76) Tx= T.Y I + (X** 4 h X S . * 8 i ) I 4K* / (3.4) where \u00C2\u00A9 i s the angle of d i p , and h i s the t h i c k n e s s normal to the bed at the o r i g i n . C onsider 2 p o i n t s , T( and Tz with corresponding X, and X t, 19 from a t r a v e l time curve. For convenience choose X , - 0 , i n which case Ti =T 0=2h/V, the t r a v e l time i n t e r c e p t . T h e r e f o r e , T,a= V o (3. 5) T l = To V 1 Let AT l =T^-T% then (iToX^s.^e)^ - Xaa = O (3.7) AT\" AT1 In t h i s eguation t h e r e are two unknowns, V and 9 . In order to f i n d V, 6 must be known. F o r t u n a t e l y we can estimate \u00C2\u00A9 from the C.S.P. r e c o r d s , although only the f i r s t sub-bottcm l a y e r can be analyzed with any degree of accuracy. Again, the Dix formula can be used t o f i n d the i n t e r v a l v e l o c i t y , which i n turn can be used t o f i n d the l a y e r t h i c k n e s s . 3.2 BOBBLE PULSE DECONVOLUTION In marine s e i s m i c o p e r a t i o n s , the source s i g n a t u r e produced by the e x p l o s i v e energy source i s a s e r i e s of impulses caused by the repeated c y c l e of gas bubble expansion and c o n t r a c t i o n (see Kramer et a l , 1968) . For s u b - c r i t i c a l 20 r e f l e c t i o n data t h i s wavelet can s e r i o u s l y complicate the p r o f i l e i n t e r p r e t a t i o n . An attempt was made to reduce the bubble pulse problem by d e c o n v o l u t i o n f i l t e r i n g . Two methods were t r i e d , both r e q u i r i n g a d e t a i l e d knowledge of the source s i g n a t u r e . The f i r s t method was d i v i s i o n a l d e c o n v o l u t i o n u s i n g the water-l e v e l parameter (Clayton, 1975) . The second method was a technique devised by Hood e t a l (1978), which c o n s i s t s e s s e n t i a l l y of c r o s s - c o r r e l a t i n g the t r a c e with the source s i q n a t u r e . The t r a c e should then c o n t a i n the a u t o c o r r e l a t e d source s i g n a t u r e which can be processed i n t o a d e s i r e d shape by an optimum l a g Wiener i n v e r s e f i l t e r . The advantage of t h i s procedure i s t h a t the a u t o c o r r e l a t i o n f u n c t i o n has a zero phase spectrum, while the o r i g i n a l bubble pulse s i g n a t u r e i s mixed delay, which causes problems when using the c o n v e n t i o n a l Wiener s p i k i n g f i l t e r method. When a p p l i e d t o t h e data, both f i l t e r s produced no n o t i c e a b l e improvement. T h i s i s most l i k e l y because no t r u e source s i g n a t u r e s were a v a i l a b l e . Instead, what was used was the a r r i v a l a t the r e c e i v i n g s h i p of the D.W.W. which i s complicated by the ghost a r r i v a l from the water s u r f a c e . A d e c o n v o l u t i o n method which does not depend on source s i g n a t u r e s would probably g i v e b e t t e r r e s u l t s . Such a method i s Minimum Entropy Deconvolution (Wiggins, 1977) which i s based on s e a r c h i n g each t r a c e f o r seguences which have i d e n t i c a l moveout. T h i s method was not attempted here because of time l i m i t a t i o n s and a l s o because f o r most a r r i v a l s the bubble o s c i l l a t i o n s d i d not present a severe problem. For 21 l a t e r a r r i v a l s i t i s u n l i k e l y t h a t d e c o n v o l u t i o n would s i g n i f i c a n t l y i m p r o v e m a t t e r s , b e c a u s e o f t h e l o w e r s i g n a l t o n o i s e r a t i o s e n c o u n t e r e d . The main a d v a n t a g e w o u l d be t h e e n h a ncement o f p r i m a r y a r r i v a l s w i t h i n 500 ms o f t h e w a t e r b o t t o m a r r i v a l . i _ J ANALYSIS OF j_ES0LTS 1) P r o f i l e s 75-2V a n d 75-3V P r o f i l e s 75-2V a n d 75-3V were b o t h s h o t a s s p l i t - d i p p r o f i l e s . The r e c e i v i n g s h i p d r i f t e d f r e e l y w h i l e t h e s h o o t i n g s h i p r e l e a s e d e x p l o s i v e s , f i r s t i n one d i r e c t i o n f r o m t h e r e c e i v i n g s h i p , and t h e n i n t h e o p p o s i t e d i r e c t i o n . , A l l s h o t s were d e t o n a t e d a t 7 m d e p t h . The r e c o r d s e c t i o n f o r t h e e a s t e r n h a l f o f 75-2V i s shown i n F i g . 3.1, w i t h a l l t r a c e s f i l t e r e d f r o m 5 t o 30 H z . The r e c o r d s e c t i o n f o r t h e w e s t e r n h a l f i s v i r t u a l l y i d e n t i c a l t o t h a t f o r t h e e a s t e r n h a l f , i n d i c a t i n g l i t t l e o r no d i p i n t h e r e f l e c t i n g s u r f a c e s . T h i s a g r e e s w i t h t h e C.S.P. r e c o r d ( F i g . 1.3) w h i c h shows r e l a t i v e l y f l a t l y i n g s e d i m e n t s i n t h i s a r e a of t h e b a s i n . The f i r s t s t r o n g a r r i v a l o f e n e r g y i s t h e w a t e r b o t t o m b o u n c e f r o m t h e i n i t i a l e x p l o s i v e i m p u l s e . , The n e x t s t r o n g a r r i v a l c o r r e s p o n d s t o t h e b o t t o m bounce f r o m t h e f i r s t o s c i l l a t i o n o f t h e g a s b u b b l e c r e a t e d b y t h e u n d e r w a t e r e x p l o s i o n . , The b o t t o m bounce a r r i v a l s f r o m t h e s e c o n d and t h i r d b u b b l e o s c i l l a t i o n s a r e a l s o p r e s e n t , b u t a t r e d u c e d a m p l i t u d e s . The l o n g b u b b l e t r a i n g r e a t l y c o m p l i c a t e s t h e r e c o r d , b u t b e c a u s e t h e b u b b l e p u l s e s h a v e an i d e n t i c a l 22 F i q . 3.1 Record s e c t i o n f o r r e f l e c t i o n p r o f i l e 7 5 - 2 V , A l l t r a c e s have been f i l t e r e d from 5 t o 30 hz. The water bottom r e f l e c t i o n i s marked W and i s f o l l o w e d 2 5 0 ms l a t e r by the water bottom r e f l e c t i o n of the f i r s t babble pulse. A, B, C, and D are c l e a r primary r e f l e c t i o n s while E i s l e s s c e r t a i n . WW i s the f i r s t order m u l t i p l e of Vi. 23 moveout to the i n i t i a l impulse f o r a given r e f l e c t o r , i t i s p o s s i b l e t o i d e n t i f y a r r i v a l s from deeper h o r i z o n s by t h e i r d i f f e r e n t moveouts. Furthermore, s i n c e the time i n t e r v a l T a b l e I R e f l e c t i o n i n t e r p r e t a t i o n r e s u l t s f o r p r o f i l e s 75-2V and 75-3V. t I P r o f i l e \"T\u00E2\u0080\u0094 | Layer ~ i | V e l o c i t y 1 (km/s) -T T-J Thickness! 1 (km) | Depth (km) 1 | 75-2V 1 w 1 1.*>9 1 2.00 | 2.00 1 A I 1.62 I \u00E2\u0080\u00A2 33 | 2. 33 i B I 1.83 \ .39 | 2.72 I c I 2.04 1 .18 \ 2.90 1 D | 2.49 1 .30 | 3. 20 I \u00C2\u00A3 I 3.63 1 .77 | 3.97 l | 75-3V | H | 1.49 1 2.05 I 2.05 i 1 A I 1.67 1 .40 J 2. 45 t 1 B i 1 2.03 -X. 1 .20 | .i _,. j . 2.65 . j between each s u c c e s s i v e bubble pulse decreases along the bubble t r a i n , i t i s n e a r l y always p o s s i b l e t o determine the p o s i t i o n of an a r r i v a l along the t r a i n by measuring the time i n t e r v a l t o the n e a r e s t a r r i v a l with i d e n t i c a l moveout. By i d e n t i f y i n g a l l prominent a r r i v a l s i n t h i s manner i t was p o s s i b l e to d i s t i n g u i s h c l e a r l y the f o u r sub-bottom r e f l e c t o r s A , B, C and D shown i n F i g . 3.1. A f i f t h s e t of a r r i v a l s , l a b e l e d E, shows a d i s t i n c t l y d i f f e r e n t moveout than D but i s p a r t i a l l y obscured by both n o i s e and bubble o s c i l l a t i o n s from 24 e a r l i e r a r r i v a l s . However, the f i r s t bubble p u l s e o f E, 250 ms a f t e r the event marked, can be c o r r e l a t e d a c r o s s most c f the s e c t i o n and g i v e s credence t o the e x i s t e n c e o f t h i s r e f l e c t o r . The T 2 - X 2 r e s u l t s f o r these f i v e l a y e r s are given i n Table I. Beyond E t h e r e appear to be a d d i t i o n a l s e t s o f coherent a r r i v a l s ; however the c a l c u l a t e d i n t e r v a l v e l o c i t i e s f o r these are much too lew, i n d i c a t i n g t h a t they are most l i k e l y r e v e r b e r a t i o n s from e a r l i e r a r r i v a l s . But a t a time of 5.25 s, t h e r e i s an event with s m a l l moveout, not c l e a r enough f o r a n a l y s i s , which suggests at l e a s t one deeper r e f l e c t o r , fit approximately 5.4 s the f i r s t s e t of water bottom m u l t i p l e s i s c l e a r l y v i s i b l e ; i t obscures any p o s s i b l e deep c r u s t a l r e f l e c t i o n s . The r e c o r d s e c t i o n f o r the western h a l f of p r o f i l e 75-3V i s shown i n F i g , 3.2. Only two s e t s of primary a r r i v a l s , A and B, are i d e n t i f i e d , with v i r t u a l l y no coherent energy a f t e r 3.5 s. T h i s i s c o n s i s t e n t with the C.S.P. record (Fig. 1.4) and petroleum i n d u s t r y r e f l e c t i o n data which show deformed sediments with no continuous l a y e r i n g below about 3.5 s. There i s some p o s s i b i l i t y o f a primary a r r i v a l between W and A, but i t i s so badly obscured by bubble o s c i l l a t i o n s t h a t i t cannot be p o s i t i v e l y i d e n t i f i e d . The r e c o r d s e c t i o n f o r the e a s t e r n h a l f o f 75\u00E2\u0080\u00943V i s s i m i l a r to t h a t of the western h a l f except the a r r i v a l s are even more obscured by bubble o s c i l l a t i o n s . S i n c e t h e C S . P , r e c o r d shows very l i t t l e d i p i n the l a y e r i n g , only the r e c o r d s e c t i o n f o r the western h a l f of the p r o f i l e was used i n the a n a l y s i s . The r e s u l t s are summarized i n Table I, F i q . 3.2 Fecord s e c t i o n f o r r e f l e c t i o n p r o f i l e 7 5 - 3 V . 26 2) P r o f i l e s 75-2, 75-2B, 75-3 and 75-3R P r o f i l e s 75-2, 75-2R, 75-3 and 75-3R were each shot twice, f i r s t with s h o t s at 7 m depth and then again with shots at 45 m. None were shot as s p l i t - d i p p r o f i l e s . Comparison of the r e c o r d s e c t i o n s f o r the two shot depths f o r p r o f i l e 75-28 r e v e a l s s i g n i f i c a n t d i f f e r e n c e s (see F i g s , 3.3 S 3 . 4 ) . The major d i f f e r e n c e i s caused by the bubble pulse p e r i o d , which i s much l e s s f o r the deeper s h o t s than f o r the shallow shots. For i d e n t i f y i n g primary a r r i v a l s i t was found t h a t the shallow shot p r o f i l e s were much b e t t e r because the bubble pulse a r r i v a l s were more spread out i n time and were r e l a t i v e l y easy to i d e n t i f y . The bubble pulse period f o r the deeper shots i s c l o s e to the dominant wave p e r i o d o f .06 - .10 s which made i d e n t i f y i n g i n d i v i d u a l bubble a r r i v a l s i m p o s s i b l e and gave the records a much more r e v e r b e r a t o r y nature. The i n i t i a l reason f o r p l a c i n g the shots at 45 m was t o d i r e c t more energy i n t o t h e ocean bottom, but from comparison of the records i t i s not obvious t h a t t h i s r e s u l t e d . The rec o r d s e c t i o n s f o r p r o f i l e s 75-2, 75-3 and 75-3R are shown i n F i g s . 3.5, 3.6 and 3.7 r e s p e c t i v e l y . Another problem which caused some o b s c u r i n g of the re c o r d s was a r e s u l t of the r e c o r d i n g procedure when c o l l e c t i n g data over a d i p p i n g r e f l e c t o r . In c o n t r a s t to normal land s e i s m i c o p e r a t i o n s , marine se i s m i c p r o f i l i n g r e q u i r e s a s t a t i o n a r y array o f r e c e i v e r s and a v a r y i n g sequence of shot p o s i t i o n s . , I f the h o r i z o n i s d i p p i n g i n the d i r e c t i o n o f the s h o o t i n g s h i p t h e o v e r a l l p r o f i l e i s co n s i d e r e d to be a down-dip p r o f i l e and the apparent v e l o c i t y 02 03 04 05 06 07 5-30 Hz Shots at 7m i Csl 7 5 - 2 R 1.0 1.4 1.8 2.2 2.6 3.0 3.4 3.8 4.2 4.6 DISTANCE (KM) r F i g . 3,3 Record s e c t i o n f o r r e f l e c t i o n p r o f i l e 75\u00E2\u0080\u00942R {western end of l i n e ) 1 with shots at 7 m depth. 17 18 19 20 5 - 2 0 * Shots at *5m / C\u00E2\u0080\u0094 I \ ^1.0 1.4 1.8 2.2 2.6 3.0 3.4 3.8 4.2 4.6 5.G DISTANCE (KM) Fig. 3.4 Record section for r e f l e c t i o n p r o f i l e 75-2R with shots at 45 m depth. J0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6 4.0 4.4 4.8 DISTANCE (KM) F i g . 3.5 Record s e c t i o n f o r r e f l e c t i o n p r o f i l e 75-2 {eastern end of l i n e ) . o LD\" in UJ -t - 0 0 \" oo-( M -\u00C2\u00B0 5- 30 Hz Shots at 7m 7 5 - 3 .8 1.2 1.6 2.0 2.4 2.8 3.2 3.6 DISTRNCE (KM) 4.0 4.4 4.8 F i g , 3.6 Record section for r e f l e c t i o n p r o f i l e 7 5 - 3 {eastern end of l i n e ) . o ^.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6 4.0 4.4 4.8 DISTANCE (KM) F i g . 3.7 Record s e c t i o n f o r r e f l e c t i o n p r o f i l e 75-3R (western end of l i n e ) . 32 acro s s the r e c o r d s e c t i o n i s lower than i f the r e f l e c t o r i s h o r i z o n t a l . However, f o r a s i n g l e shot the energy i s t r a v e l i n g up-dip t o the r e c e i v e r s and the apparent v e l o c i t y across the a r r a y of s i x hydrophones i s higher than f o r a h o r i z o n t a l r e f l e c t o r . T h i s r e s u l t s i n a r r i v a l s not being smoothly continuous a c r o s s the s e c t i o n but appearing as a s e r i e s of s h o r t segments o f f s e t from each other by an amount which depends on the degree of r e f l e c t o r d i p . T h i s problem occurred to some extent i n each of p r o f i l e s 75-2, 75-2R, 75-3 and 75\u00E2\u0080\u00943R, which the C.S.P. records show as having d i p p i n g sea f l o o r , and made the c o r r e l a t i n g of coherent phases very d i f f i c u l t i n p l a c e s . I t was p o s s i b l e t o i d e n t i f y only one or two primary r e f l e c t i o n s from the r e c o r d s e c t i o n s of these f o u r p r o f i l e s . T h i s a l s o i s c o n s i s t e n t with the C.S.P. records and petroleum i n d u s t r y r e f l e c t i o n data which show no coherent r e f l e c t i o n s beyond these a r r i v a l times f o r s i m i l a r r e g i o n s . Any deeper c r u s t a l r e f l e c t i o n s , i f present, have low amplitudes and are obscured by n o i s e and bubble pulse r e v e r b e r a t i o n s . , As mentioned i n the p r e v i o u s s e c t i o n these p r o f i l e s are not s p l i t - d i p and must be analyzed using equation 3.1. Examination of the C.S.P. records shows t h a t f o r p r o f i l e s 75\u00E2\u0080\u00942R and 75\u00E2\u0080\u00943R the basement d i p i s about 5\u00C2\u00B0 towards the e a s t , whereas f o r p r o f i l e s 75-2 and 75\u00E2\u0080\u00943, the dip i s not c l e a r . Dsinq the C.S.P.-determined v a l u e s q i v e s a sub-bottom l a y e r 0.8 km t h i c k with v e l o c i t y 2.45 km/s f o r 75-2R and 0.65 km t h i c k with v e l o c i t y 2.42 km/s f o r 75\u00E2\u0080\u00943R. S e v e r a l d i f f e r e n t d i p s were t r i e d with 75-2 and 75-3. Varying the d i p from 2\u00C2\u00B0 33 T a b l e I I R e f l e c t i o n i n t e r p r e t a t i o n r e s u l t s f o r p r o f i l e s 7 5 - 2 , 75-2B, 75-3 & 75-3B. T h r e e s e t s o f p o s s i b l e r e s u l t s a r e shown f o r t h e b o t t o m l a y e r o f e a c h p r o f i l e s i n c e t h e r e i s an a m b i g u i t y b e t w e e n v e l o c i t y , t h i c k n e s s and \" d i p . P r o f i l e L a y e r V e l o c i t y (km/s) T h i c k n e s s (km) D i p (deg) 75-2R \u00E2\u0080\u0094 + 75-3R 7 5-2 \u00E2\u0080\u0094 + B B B B B B H A A A \u00E2\u0080\u00A21.49 2.45 3. 01 3.47 3. 90 1. 49 2. 42 2.66 3. 09 3. 52 1. 49 2.42 2.85 3.24 1.80 .80 \u00E2\u0080\u00A2 30 .35 .39 1.80 .65 .34 .40 .45 1.77 .59 .65 .71 0.0 5.0 3. 0 4.0 5.0 0. 0 5.0 3. 0 4.0 5. 0 0.0 2.0 4.0 6. 0 75-3 A A A 1. 49 2. 51 2. 81 3. 11 1. 75 .78 .84 .91 0.0 2.0 4.0 6.0 34 to 6\u00C2\u00B0 r e s u l t e d i n v e l o c i t i e s v a r y i n g from 2.42 km/s to 3.24 km/s with an average t h i c k n e s s of 0.65 km f o r p r o f i l e 75-2 and v e l o c i t i e s v a r y i n g from 2.51 km/s to 3.11 km/s with an average t h i c k n e s s o f 0.85 km f o r p r o f i l e 75\u00E2\u0080\u00943, For p r o f i l e s 75-28 and 75-38 i t was p o s s i b l e to a r r i v e at v e l o c i t i e s and t h i c k n e s s e s f o r a 2nd sub-bottom l a y e r f o r a range of p o s s i b l e d i p s . These values p l u s a summary of the r e s u l t s of t h i s s e c t i o n are shown i n Table I I . 35 i i i , BEISACTION ANALYSIS HaJ. THE BEFRACTION DATA SET The reduced r e c o r d s e c t i o n s f o r r e f r a c t i o n p r o f i l e s 75-2, 75-2R, 75-3 and 75-3R are shown i n F i g s . 4.1, 4.2, 4,3 and 4.4, r e s p e c t i v e l y . A r e d u c i n g v e l o c i t y of 6.0 km/s has been used and a l l t r a c e s have been f i l t e r e d from 5 to 20 hz. Each se t of 6 t r a c e s {or l e s s i f p a r t i c u l a r channels were d e f e c t i v e ) i s l a b e l e d with i t s shot number, which i s a l s o shewn on the a p p r o p r i a t e C.S. P. r e c o r d ( F i g . 1.3 or 1.4). As d e s c r i b e d p r e v i o u s l y , topography has been c o r r e c t e d t o a constant water depth of 2.0 km using a v e l o c i t y o f 1.49 km/s f o r the water l a y e r and 2.0 km/s f o r the sub-bottom m a t e r i a l . The 2.0 km/s f o r the sub-bottom i s r e p r e s e n t a t i v e of shallow sediments i n t h i s area as determined from the r e s u l t s of Chapter 3.. For shots over the shallower waters of the c o n t i n e n t a l s h e l f and s l o p e , 2.0 km/s i s probably t o o low. T h i s problem was encountered f o r the l a s t few shots of p r o f i l e s 75-2R and 75-3R. F i r s t a r r i v a l p i c k s were made from the same p l o t s as the D.W.H. a r r i v a l s ( F i g . 2.1) and are i n d i c a t e d on the r e c o r d s e c t i o n s by s o l i d t r i a n g l e s . Weaker and more emergent a r r i v a l s are i n d i c a t e d by s l i g h t l y s m a l l e r t r i a n g l e s . A l l f o u r r e c o r d s e c t i o n s show extended wavetrains a f t e r the f i r s t a r r i v a l breaks but few coherent secondary a r r i v a l branches. The f i r s t water bottom m u l t i p l e a t the shoo t i n g s h i p comes i n beyond the end of the t r a c e s f o r most shots but i s v i s i b l e f o r I I F i g . 4.1 Reduced rec o r d s e c t i o n f o r r e f r a c t i o n p r o f i l e 75-2. The s o l i d t r i a n g l e s i n d i c a t e f i r s t a r r i v a l p i c k s ; the s m a l l e r ones correspond to more emergent a r r i v a l s . A l l t r a c e s have been f i l t e r e d from 5 to 20 hz. Each s e t of s i x t r a c e s i s l a b e l e d with i t s corresponding shot number. Amplitude c o r r e c t i o n with d i s t a n c e i s r 2 . F i q , 4 , 2 R e d u c e d r e c o r d s e c t i o n f o r r e f r a c t i o n p r o f i l e 7 5 - 2 E . \" \" \" . . - - I F i g . 4.3 Seduced r e c o r d s e c t i o n f o r r e f r a c t i o n p r o f i l e 7 5 - 3 . F i g . 4 . 4 R e d u c e d r e c o r d s e c t i o n f o r r e f r a c t i o n p r o f i l e 7 5 - 3 R . 40 some of the d i s t a n t shots, f o r example a t 5.6 s f o r shot 54 of p r o f i l e 75-3B. Close examination of the r e c o r d s e c t i o n s r e v e a l s many unusual c h a r a c t e r i s t i c s . Some o f thes e are c o n s i s t e n t with the C.S.P. records while others are not., On p r o f i l e 75-2 (F i g . 4.1) a 0.2 s t r a v e l time advance between shots 38 and 39 corresponds t o a f a u l t at the f o o t of the c o n t i n e n t a l s l o p e , c l e a r l y v i s i b l e on C.S.P. r e c o r d 75-2. & 0.1 s advance a t shot 49 corresponds to rays t r a v e l i n g up through Winona Ridge and p o s s i b l y i n d i c a t e s higher v e l o c i t y m a t e r i a l beneath the r i d g e than on e i t h e r s i d e . The f u r t h e s t f i r s t a r r i v a l branch, s t a r t i n g a t 44 km has an apparent v e l o c i t y of 9.8 km/s which i s c o n s i s t e n t with upper mantle head waves t r a v e l i n g up d i p . T h i s would be expected i f the oceanic c r u s t i s d i p p i n g towards or beneath the c o n t i n e n t a l c r u s t . The amplitudes f o r t h i s p r o f i l e are r e l a t i v e l y uniform although i n c r e a s e s are observed on shot 51 and l a t e r shots. The most s t r i k i n g f e a t u r e o f p r o f i l e 75\u00E2\u0080\u00942R ( F i g . 4.2) i s the 0.25 s t r a v e l time advance centered a t shot 45. This corresponds p r e c i s e l y with Winona Ridge and i s a c e r t a i n i n d i c a t i o n of higher v e l o c i t y m a t e r i a l beneath the r i d g e than cn e i t h e r s i d e . The f u r t h e s t f i r s t a r r i v a l branch, s t a r t i n g at 48 km has a very high apparent v e l o c i t y of 20 km/s. T h i s could be p a r t l y due to the low v e l o c i t y of 2.0 km/s used f o r the l a r g e topographic c o r r e c t i o n necessary. Using a v e l o c i t y of 6.0 km/s f o r the c o r r e c t i o n r e s u l t s i n an apparent v e l o c i t y of 10.1 km/s f o r the branch. Since the proper topographic c o r r e c t i o n v e l o c i t y must be between these two extremes there 41 i s a s t r o n g i m p l i c a t i o n t hat the i n t e r f a c e from which these a r r i v a l s were c r i t i c a l l y r e f r a c t e d must be d i p p i n g towards the west s i n c e the r e f r a c t i n g v e l o c i t y i s not l i k e l y t o be much gre a t e r than 8.0 km/s. Amplitudes f o r t h i s p r o f i l e are r e l a t i v e l y uniform out to about 30 km. They i n c r e a s e between 32 and 45 km beyond which they decrease s i g n i f i c a n t l y to the end of the p r o f i l e . On p r o f i l e 75\u00E2\u0080\u00943 ( F i g . 4.3) a s l i g h t t r a v e l time advance at shot 44 c o u l d correspond to e i t h e r a f a u l t or a p o s i t i v e i h o r i z o n t a l v e l o c i t y g r a d i e n t . The i n f l u e n c e of Winona Ridge i s seen on shots 49 and 50 as a 0.2 s t r a v e l time advance. The f u r t h e s t f i r s t a r r i v a l branch, s t a r t i n g at 52 km has an apparent v e l o c i t y o f 12.0 km/s, c o n s i s t e n t with an eastward d i p p i n g c r u s t . Amplitudes f o r t h i s p r o f i l e show c o n s i d e r a b l e v a r i a t i o n , with weak a r r i v a l s from 22 to 25 km f o l l o w e d by no d e t e c t a b l e a r r i v a l s out to 32 km. They then i n c r e a s e to very s t r o n g at 43 km and fade g r a d u a l l y to the end of the p r o f i l e . T h i s wide v a r i a t i o n i n amplitudes could be the r e s u l t of f o c u s i n g of r a y s by Winona Ridge. P r o f i l e 75-3R ( F i g . 4.4) has a 0.15 s t r a v e l time advance over Winona Ridge and a f i n a l f i r s t a r r i v a l branch s t a r t i n g a t 58 km with an apparent v e l o c i t y o f 20 km/s. As f o r p r o f i l e 75\u00E2\u0080\u00942R the topographic c o r r e c t i o n s are probably too s m a l l . Using 6.0 km/s f o r the c o r r e c t i o n r e s u l t s i n an apparent v e l o c i t y of g r e a t e r than 10.0 km/s, again i n d i c a t i n g a westward d i p p i n g i n t e r f a c e . Large amplitudes are observed from 32 to 46 km and a t 59 km, and very weak a r r i v a l s from 23 to 30 km and from 48 to 54 km., 42 In g e n e r a l the s e t s of p r o f i l e s shot i n the same d i r e c t i o n are c o n s i s t e n t with one another. T h i s i s p a r t i c u l a r l y t rue f o r the e f f e c t of Winona Ridge and the apparent v e l o c i t i e s o f the f u r t h e s t f i r s t a r r i v a l branches. For p r o f i l e s 7 5-2 and 75\u00E2\u0080\u00943 an eastward dipping deep i n t e r f a c e i s i n d i c a t e d f o r the west s i d e of the b a s i n while f o r 75\u00E2\u0080\u00942R and 75\u00E2\u0080\u00943H a westward d i p p i n g deep i n t e r f a c e i s i n d i c a t e d f o r the e a s t s i d e of the b a s i n . H\u00C2\u00B12 METHODS OF ANALYSIS S e v e r a l methods e x i s t f o r i n t e r p r e t i n g s e i s m i c r e f r a c t i o n data. The best one t o use f o r a s p e c i f i c case depends on the p r o f i l i n g method and the p h y s i c a l c h a r a c t e r i s t i c s of the area of i n t e r e s t . Those technigues most a p p l i c a b l e to t h i s type of study are f i r s t a r r i v a l a n a l y s i s , s y n t h e t i c seismogram computation and ray t r a c i n g . 1} F i r s t A r r i v a l A n a l y s i s T h i s method assumes a model which c o n s i s t s o f d i s c r e t e , constant v e l o c i t y l a y e r s s e p a r a t e d by plane, a r b i t r a r i l y d i p p i n g i n t e r f a c e s . There can be no l a t e r a l v a r i a t i o n s normal to the p r o f i l e l i n e . Mota (1954) d e r i v e s equations f o r d i p , v e l o c i t y , depth and t h i c k n e s s f o r the n- l a y e r case. The p r o f i l e must be shot i n both the forward and r e v e r s e d i r e c t i o n s . Since only the a r r i v a l times of the f i r s t r e f r a c t i o n energy are used, much i n f o r m a t i o n , such as r e l a t i v e amplitudes and secondary a r r i v a l s , i s n e g l e c t e d . 4 3 2) S y n t h e t i c Seismogram Computation A method which makes use of the e n t i r e s u i t e of s e i s m i c data i s the computation o f s y n t h e t i c seismograms. Techniques devised t o date, such as \" d i s c - r a y t h e o r y \" (DRT) by Wiggins (1976), impose the r e s t r i c t i o n of l a t e r a l homogeneity. For DRT a s t a r t i n g model i s o f t e n d e r i v e d from f i r s t a r r i v a l a n a l y s i s . The f i r s t a r r i v a l v e l o c i t y - d e p t h curve i s used to generate a ray parameter vs. d i s t a n c e (P-X) curve which i n t u r n i s used to generate a t r a v e l time curve, a v e l o c i t y - d e p t h curve and a s e t of s y n t h e t i c seismograms. The t r a v e l times and amplitudes are compared with the r e a l data and i f necessary, the P\u00E2\u0080\u0094X curve i s a l t e r e d and the process repeated. T h i s method was used by Lynch (1S77) to analyze the set o f r e v e r s e d p r o f i l e s 75\u00E2\u0080\u00941 and 75\u00E2\u0080\u00941R taken along the eastern p o r t i o n of Winona Basin, p a r a l l e l to the c o n t i n e n t a l margin (see F i g . 1.2). L a t e r a l v a r i a t i o n s along these l i n e s are most l i k e l y s m a l l , j u s t i f y i n g t h e assumption of l a t e r a l homogeneity. 3) Ray t r a c i n g A method which imposes a minimum i n model r e s t r i c t i o n s i s ray t r a c i n g . The model can c o n s i s t of any number of a r b i t r a r i l y shaped zones o f any v e l o c i t y or v e l o c i t y g r a d i e n t . Rays are sent out from the o r i g i n a t egual angular increments and \" t r a c e d \" through the model by a p p l y i n g S n e l l * s Law a t the i n t e r f a c e s and f o l l o w i n g the a p p r o p r i a t e t r a j e c t o r y through the g r a d i e n t zones. The t r a v e l time f o r each ray to r e t u r n t o the s u r f a c e i s computed and p l o t t e d on a t i m e - d i s t a n c e curve 44 f o r comparison with the r e a l data. An i n d i c a t i o n of r e l a t i v e amplitudes can be ob t a i n e d by observing on a ray p l o t the spa c i n g of a r r i v a l s a t the s u r f a c e ; many a r r i v a l s i n a short d i s t a n c e correspond to i n c r e a s e d amplitudes. Choosing a s t a r t i n g model f o r a complex area can be very d i f f i c u l t and r e q u i r e s c o n s i d e r a t i o n o f f i r s t a r r i v a l i n f o r m a t i o n , s u r f a c e geology, r e g i o n a l t e c t o n i c s and any i n f o r m a t i o n which can be obtained from other g e o p h y s i c a l methods. Say t r a c i n g has been used by Cl e e e t a l (1974) as an a i d i n the i n t e r p r e t a t i o n o f a d e t a i l e d r e f l e c t i o n - r e f r a c t i o n study near Y e l l o w k n i f e , Northwest T e r r i t o r i e s , and by M i l l e r and Gebrande (1976) and Grubbe (1976) i n r e f r a c t i o n s t u d i e s o f C e n t r a l Europe. As demonstrated i n S e c t i o n 4.1, c l o s e examination o f the record s e c t i o n s i n c o n j u n c t i o n with t h e C.S.P. records i n d i c a t e s strong l a t e r a l v a r i a t i o n s i n both s e t s of p r o f i l e s . The t r a v e l time advance observed c o n s i s t e n t l y over Winona Bidge on a l l p r o f i l e s i n d i c a t e s s i g n i f i c a n t l y higher v e l o c i t i e s beneath the rid g e than on e i t h e r s i d e . A major f a u l t i s i n d i c a t e d by p r o f i l e 75-2 and p o s s i b l y by p r o f i l e 75-3. The high apparent v e l o c i t i e s a t the ends of a l l four r e f r a c t i o n l i n e s i n d i c a t e i n t e r f a c e s deep w i t h i n the c r u s t which dip from both s i d e s o f the basin towards the c e n t e r . These o b s e r v a t i o n s immediately r u l e out s y n t h e t i c seismograms as an i n t e r p r e t a t i o n technique f o r these p r o f i l e s . The l a t e r a l inhomogeneities are much too severe to attempt to use a l a t e r a l l y homogeneous model. Furthermore, f i r s t a r r i v a l 45 a n a l y s i s cannot be used f o r the o v e r a l l s e c t i o n s because there appear to be i n t e r f a c e s which are e i t h e r not plane, or do not extend over the e n t i r e p r o f i l e , as i n d i c a t e d by the high apparent v e l o c i t i e s a t the ends of the r e f r a c t i o n l i n e s . T h i s means that the forward and r e v e r s e l i n e s do not have e a s i l y i d e n t i f i a b l e c o r r e s p o n d i n g f i r s t a r r i v a l branches. T h i s l e a v e s ray t r a c i n g as the o n l y a c c e p t a b l e i n t e r -p r e t a t i o n technique. Information f o r s e l e c t i n g a s t a r t i n g model can be obtained from r e f r a c t i o n p r o f i l e s 75-1 and 75-1B (Lynch, 1977), r e f l e c t i o n r e s u l t s from Chapter 3 and unreversed f i r s t a r r i v a l a n a l y s i s of the s t a r t of each r e f r a c t i o n l i n e . The f i r s t two sources provide w e l l e s t a b l i s h e d r e s u l t s ; however, the f i r s t a r r i v a l a n a l y s i s r e g u i r e s assumptions concerning apparent v e l o c i t i e s of h y p o t h e t i c a l r e v e r s e p r o f i l e s which w i l l g i ve reasonable l a y e r t h i c k n e s s e s and d i p s . 4.3 DESCRIPTION OF M I TR1.CTJS PJOGMI-Having decided upon ray t r a c i n g as the most s u i t a b l e method of i n t e r p r e t a t i o n o f the r e f r a c t i o n data i t was necessary to o b t a i n a computer program to perform the a c t u a l t r a c i n g of the rays and produce t r a v e l time curves and ray p l o t s . The type of program needed was one which could handle s e v e r a l l a y e r s with a r b i t r a r i l y shaped p o l y g o n a l boundaries and any d e s i r e d v e l o c i t y or l i n e a r v e l o c i t y g r a d i e n t . Since no e x i s t i n g programs i n the department met these s p e c i f i c reguirements a d e c i s i o n was made t o develop the program here. 46 The completed program, w r i t t e n by Ken W h i t t a l l , accepts s e v e r a l polygonal shaped l a y e r s as i n p u t . The v e l o c i t y i n a l a y e r i s d e f i n e d as being c o n s t a n t along i t s top boundary and varying l i n e a r l y with depth normal to t h i s boundary. Rays le a v e the o r i g i n at egual angular increments over a s p e c i f i e d range o f an g l e s . , Since a l l l a y e r s have non-zero l i n e a r g r a d i e n t s , a l l ray paths are c i r c u l a r a r c s with r a d i u s and cen t e r depending on the v e l o c i t y and gradient. I f a ray i n t e r s e c t s another boundary, S n e l l ' s law i s a p p l i e d using the v e l o c i t i e s on e i t h e r s i d e of the i n t e r f a c e , and a new c i r c u l a r t r a j e c t o r y i s computed from the new g r a d i e n t . The t r a v e l time f o r each c i r c u l a r segment i s c a l c u l a t e d and when the ray i e v e n t u a l l y r e t u r n s t o the s u r f a c e the t o t a l t r a v e l time i s determined by summation and p l o t t e d a g a i n s t i t s a r r i v a l d i s t a n c e . Constant v e l o c i t y l a y e r s are c l o s e l y approximated by s p e c i f y i n g a very s m a l l g r a d i e n t . The g e n e r a t i o n of head waves i s not p r e d i c t e d by ray theory and r e q u i r e s the theory of wave propagation f o r a f u l l d e s c r i p t i o n . For t h i s reason, pseudo head wave a r r i v a l s are generated a r t i f i c i a l l y by the ray t r a c i n g program. I f a ray i n t e r s e c t s a boundary w i t h i n some s p e c i f i e d angle of the c r i t i c a l a ngle, c r i t i c a l l y r e f r a c t e d rays are generated at r e g u l a r i n t e r v a l s along the boundary t o simulate the upward t r a v e l i n g energy a s s o c i a t e d with true head wave a r r i v a l s . The ray t r a c i n g program produces a computer p l o t of the model with ray paths superimposed and a p l o t o f t r a v e l times vs. d i s t a n c e f o r a l l r ay a r r i v a l s . On the t r a v e l time p l o t s , r e f l e c t e d rays are i d e n t i f i e d by c r o s s e s and head waves by 47 X's. The t r a v e l time p l o t can be reduced t o any v e l o c i t y and p l o t t e d a t any s c a l e f o r easy comparison with record s e c t i o n s of r e a l data. Once a good f i t has been made to f i r s t a r r i v a l t r a v e l times the c o n c e n t r a t i o n of a r r i v a l s on the t r a v e l time and r a y p l o t s can be used as a q u a l i t a t i v e measure of amplitudes. F i t t i n q f i r s t a r r i v a l s f o r a s i n q l e p r o f i l e c e r t a i n l y does not quarantee a unique model; however, i f the t r a v e l times and r e l a t i v e amplitudes can be s a t i s f i e d f o r both a forward and r e v e r s e p r o f i l e , then the r e l i a b i l i t y of the model i s improved s i q n i f i c a n t l y , althouqh i t i s s t i l l not unique. i E \u00C2\u00A3 L I \u00C2\u00A3 A T I Q I Of RAY TRACING In order t o apply the ray t r a c i n g technique t o the i n t e r p r e t a t i o n of a r e f r a c t i o n p r o f i l e , a s t a r t i n g model must f i r s t be chosen. A ray p l o t and t r a v e l time curve a re then generated f o r both the forward and reverse d i r e c t i o n s and compared with the r e a l data. I f necessary, a l t e r a t i o n s a re made to the model u n t i l the t r a v e l time f i t i s acceptable and the r e l a t i v e amplitudes agree q u a l i t a t i v e l y . P r o f i l e s 75 -3 and 75 -3R: To i l l u s t r a t e the d e t a i l e d a p p l i c a t i o n of the r a y t r a c i n g technique the s e t o f r e v e r s e d p r o f i l e s 75\u00E2\u0080\u00943 and 75\u00E2\u0080\u00943R w i l l be used. a) The S t a r t i n g Model The s t a r t i n q model f o r p r o f i l e 75-3 i s shown i n F i q . 4 . 5 . F i g . 4.5 S t a r t i n g model f o r ray t r a c i n g of p r o f i l e 75-3. The i n s e t shows the f i n a l v e l o c i t y - d e p t h curves f o r r e f r a c t i o n p r o f i l e s 75-1 and r ) 75-1R a r r i v e d at by Lynch (1977) u s i n g DRT s y n t h e t i c seismograms. j> \ These were used as a c o n s t r a i n t f o r the c r o s s - b a s i n p r o f i l e s . ! 0 0 Numbers w i t h i n model blocks are v e l o c i t i e s i n km/s. ' \u00E2\u0080\u00A2 ' 49 The r e s u l t s of the f i r s t a r r i v a l i n t e r p r e t a t i o n of the s e t of re v e r s e d p r o f i l e s 75-1 and 75-1S (Lynch, 1977) are represented by the v e r t i c a l l i n e through 24 km, and p r o v i d e the only set of c o n s t r a i n t s t h a t extend throughout the e n t i r e c r u s t . The i n s e t shows Lynch's f i n a l v e l o c i t y - d e p t h curves f o r these p r o f i l e s determined from a t r a v e l t i m e and amplitude i n t e r p r e t a t i o n u sing DRT s y n t h e t i c seismograms. The top few l a y e r s on the eas t end of the s e c t i o n were obt a i n e d by performing a f i r s t a r r i v a l a n a l y s i s on the f i r s t few t r a v e l time branches of p r o f i l e 7.5-3, with an assumption of plane, continuous l a y e r s over the f i r s t 10 t o 20 km., A computer program, based on the eguations f o r d i p , v e l o c i t y , depth and t h i c k n e s s f o r s e v e r a l plane l a y e r s (Mota, 1954) was used to generate s e t s o f l a y e r s with d i f f e r e n t v e l o c i t i e s , t h i c k n e s s e s and d i p s , a l l of which s a t i s f y the t r a v e l times f o r the beginning of the p r o f i l e . A unigue s e t o f l a y e r s cannot be determined because the p r o f i l e segments are e f f e c t i v e l y not r e v e r s e d . , The a r r i v a l s from these l a y e r s on the r e v e r s e p r o f i l e 75-3B, i f present at a l l , are secondary and v i r t u a l l y i m p ossible t o i d e n t i f y . The number of p o s s i b i l i t i e s can be reduced, however, by a p p l y i n g c e r t a i n r e s t r i c t i o n s t o the v e l o c i t i e s or d i p s of the l a y e r s . For example, assuming an average v e l o c i t y of 2.5 km/s f o r the f i r s t sub-bottom l a y e r i s c o n s i s t e n t with the r e f l e c t i o n r e s u l t s of Chapter 3. Choosing a v e l o c i t y of 4.3 km/s f o r the second l a y e r i s c o n s i s t e n t with p r o f i l e 75\u00E2\u0080\u00941 but r e g u i r e s a di p of 11\" to the west f o r the f i r s t sub-bottom i n t e r f a c e and 4\u00C2\u00B0 to the east f o r the second. Conseguently, a v e l o c i t y of 50 3.5 km/s was chosen as the s t a r t i n g v a l u e f o r t h i s l a y e r because i t r e s u l t e d i n both i n t e r f a c e s dipping to the west. Applying the same procedure t o p r o f i l e 75\u00E2\u0080\u00943R r e s u l t e d i n l a y e r s with s i m i l a r v e l o c i t i e s t o those of 7.5\u00E2\u0080\u00941, a l l d i p p i n g towards the e a s t . Since t h i s i s c o n s i s t e n t with the C.S.P. reco r d ( F i g . ..1.4) and a c r u s t t h i c k e n i n g towards the c o n t i n e n t , these l a y e r s were used f o r the west s i d e of the s t a r t i n g model f o r p r o f i l e 75-3. As mentioned i n S e c t i o n 4.2, a t r a v e l time advance i s observed on a l l p r o f i l e s f o r shots over Winona Ridge., T h i s i s represented on the s t a r t i n g model by the block of m a t e r i a l with v e l o c i t y 3.0 km/s. Beyond t h i s t h e r e i s no f u r t h e r d i r e c t s e i s m i c i n f o r m a t i o n which can be used t o complete the s t a r t i n g model, p a r t i c u l a r l y f o r the deeper l a y e r s . The choice of these l a y e r s as shown i n F i g . 4.5 was made simply by having them d i p p i n g towards the c o n t i n e n t and i n t e r s e c t i n g the 75\u00E2\u0080\u00941 l a y e r s . T h i s i s c o n s i s t e n t with subduction and whether c o r r e c t or not, i t w i l l be s u f f i c i e n t f o r a s t a r t i n g model. Since p r o f i l e 75\u00E2\u0080\u00943R was not shot d i r e c t l y over 75-3 (see F i g . 1.2), i t s s t a r t i n g model i s s l i g h t l y d i f f e r e n t than f o r 75\u00E2\u0080\u00943. T h i s i s necessary t o r e f l e c t the d i f f e r e n c e s i n p r o f i l e l e ngth and topography. The same s i t u a t i o n a l s o a p p l i e s t o 75-2 and 75-2R. In summary, the main c o n s t r a i n t s on the models are i ) the r e s u l t s of p r o f i l e 75-1,1R, i i ) the p r o f i l e s being approximately r e v e r s e d , i i i ) the C.S.P. r e c o r d s , i v ) c u r r e n t views on p l a t e t e c t o n i c s and v) the c r u s t t h i c k e n i n g from t y p i c a l o c e a n i c c r u s t to a t h i c k e r c o n t i n e n t a l c r u s t (up to 30 51 km) from Winona Basin to the continental shelf. For any model to be acceptable i t must be consistent with these constraints. b) Testing the Models The ray plots and reduced t r a v e l time curves for ray tracing on the s t a r t i n g models for p r o f i l e s 75\u00E2\u0080\u00943 and 75-38 are shown i n Figs. 4.6 and 4.7. The c i r c l e s connected by dashed l i n e s on the t r a v e l time curves represent the f i r s t a r r i v a l times taken from the record sections (Figs. 4. 3 6 4. 4). For p r o f i l e 75\u00E2\u0080\u00943 the agreement i s quite good over most of the p r o f i l e , the a r r i v a l s being 250 ms late at 24 km and 100 ms la t e from 50 km to 60 km. For 75-3R however, the agreement i s good only at the s t a r t , which i s expected because of the way in which the i n i t i a l model was constructed, and poor over the remainder of the p r o f i l e . To make the a r r i v a l s e a r l i e r over Winona Ridge (11 to 21 km) the ridge velocity was increased to 3.7 km/s, more representative of consolidated sediments. In order to greatly increase the apparent velocity of the f i r s t a r r i v a l s beyond 50 km some of the layers against the east side of the basin were made to dip towards the west. These changes greatly improved the 75-3R f i t without seriously a f f e c t i n g the reasonably qood f i t of the 75-3 model. Many subsequent t r i a l s were made, with the t r a v e l time f i t s gradually improving at each step. c) The F i n a l Models Eventually, models were found which s a t i s f i e d both 75-3 and 75-3R t r a v e l times very well. These models, with rays superimposed, are shown i n Figs. 4.8 and 4,9 along with t h e i r 52 i g . 4,6 Ray p l o t and t r a v e l time curve f o r 75-3 s t a r t i n q model. The dashed l i n e on the t r a v e l time curve r e p r e s e n t s the f i r s t a r r i v a l picks taken from the r e c o r d s e c t i o n . Numbers within b l o c k s o u t l i n e d by heavy dashed l i n e s are v e l o c i t i e s i n km/s. 53 F i g . 4.7 Bay p l o t and t r a v e l time curve f o r 75-3R s t a r t i n g model. The model i s i d e n t i c a l to 75 -3 except f o r d i f f e r e n c e s a r i s i n g from the two p r o f i l e s not being e x a c t l y c o i n c i d e n t . Note t h a t the model i s reversed, west being on the l e f t , compared with F i g . 4 .6. Numbers wi t h i n b l o c k s o u t l i n e d by heavy dashed l i n e s are v e l o c i t i e s i n km/s. 54 r e s p e c t i v e t r a v e l time curves. By i d e n t i f y i n g the boundaries from which a l l head wave branches were r e f r a c t e d , i t was found t h a t head waves from the bottom i n t e r f a c e never appear a l c n e as f i r s t a r r i v a l s , although on the f i n a l branch of each p r o f i l e (beyond 48 km) these a r r i v a l s p l u s a r r i v a l s from the next higher i n t e r f a c e ccme i n almost simultaneously as f i r s t a r r i v a l s . For t h i s reason the bottom i n t e r f a c e i s shown with q u e s t i o n marks s i n c e the f i r s t a r r i v a l t r a v e l times could be s a t i s f i e d without i t . Examination of the model t r a v e l time curve f o r p r o f i l e 75-3 shows t h a t the f i r s t a r r i v a l t r a v e l time f i t i s very good. The s l i g h t advance observed a t 22 km was modeled by p l a c i n g the 3.5 km/s and the 4.3 km/s l a y e r s t o g e t h e r , s i m u l a t i n g a h o r i z o n t a l v e l o c i t y g r a d i e n t i n the v i c i n i t y of t h e i r boundary. T h i s same e f f e c t c o u l d have been produced by a f a u l t i n the sediments, but would have r e g u i r e d a change i n the d i p s and v e l o c i t i e s of these l a y e r s which would have made i t very d i f f i c u l t t o f i t t h e f i r s t a r r i v a l branches s a t i s f a c t o r i l y . The e a r l y a r r i v a l s from 36 to 48 km corresponding t o paths through Winona Ridge agree very w e l l with the observed t r a v e l times. The same r e s u l t s c o u l d have been obtained by d e c r e a s i n g both the t h i c k n e s s of the r i d g e l a y e r and i t s v e l o c i t y . T h i s i m p l i e s an upper l i m i t of approximately 3.7 km/s f o r the r i d g e v e l o c i t y . As f o r p r o f i l e 75-3, the model t r a v e l time curve f o r p r o f i l e 75-3R shows a very good f i t t o the observed f i r s t a r r i v a l s . The t r a v e l time advance over Winona Ridge i s w e l l modeled and the high apparent v e l o c i t y of the f i n a l branch has 55 F i g . 4 . 8 Ray p l o t and t r a v e l time curve f o r 75\u00E2\u0080\u00943 f i n a l model. The g u e s t i o n marks on the bottom i n t e r f a c e i n d i c a t e t h a t the f i r s t a r r i v a l t r a v e l times could be s a t i s f i e d without a r r i v a l s from t h i s boundary; t h e r e f o r e i t s p o s i t i o n i s not d e f i n i t e . Numbers w i t h i n blocks o u t l i n e d by heavy dashed l i n e s are v e l o c i t i e s i n km/s. V e r t i c a l exaggeration i s 2X. I 56 F i g , 4.9 Ray p l o t and t r a v e l time curve f o r 75-3R f i n a l model. The model i s i d e n t i c a l to 75-3 except f o r d i f f e r e n c e s a r i s i n g from the two p r o f i l e s not being e x a c t l y c o i n c i d e n t . Note t h a t the model i s reve r s e d , west being on the l e f t , compared with F i g . 4 .8 . 57 been achieved by the p o s i t i o n of the westward d i p p i n g l a y e r s at the e a s t s i d e o f the b a s i n . The model t r a v e l time curve f o r p r o f i l e 7 5-3 shows a s i n g l e head wave branch as the f i r s t a r r i v a l from 20 to 30 km. Being a pure head wave a r r i v a l , i t s amplitude i s expected to be small i n comparison with other r e f l e c t e d and r e f r a c t e d a r r i v a l s . T h i s branch corresponds w e l l with the weak f i r s t a r r i v a l s observed on the record s e c t i o n of 75-3 over the same range. About 0.3 s l a t e r on the model t r a v e l time curve a r e f l e c t e d and a r e f r a c t e d branch c o i n c i d e t o c r e a t e the s t r o n g e r secondary a r r i v a l which i s observed on the r e c o r d s e c t i o n . S i m i l a r l y f o r p r o f i l e 75-38 the weak a r r i v a l s from 22 t o 32 km are generated by the head wave branch observed over t h i s range on the model t r a v e l time curve and t h e l a r g e amplitudes from 34 to 48 km are generated by the l a r g e c o n c e n t r a t i o n of a r r i v a l s observed there. Due to the d i f f i c u l t y i n i d e n t i f y i n g secondary a r r i v a l s on the record s e c t i o n s , i t was not p o s s i b l e to c o n s t r a i n the models by these a r r i v a l s . However, c l o s e comparison of the r e c o r d s e c t i o n s with the ray t r a v e l time plots f o r 75\u00E2\u0080\u00943 and 75\u00E2\u0080\u00943fi shows t h a t some secondary a r r i v a l s can be t e n t a t i v e l y i d e n t i f i e d . Elsewhere on the s e c t i o n s the amplitude agreement i s g e n e r a l l y good. Of c o u r s e , i t i s not d i f f i c u l t t o f i n d areas of apparent c o n t r a d i c t i o n on both s e t s of p r o f i l e s . , T h i s i s to be expected s i n c e the models c o n s i s t of b l o c k s of constant v e l o c i t y m a t e r i a l while i n a c t u a l f a c t , with the e x c e p t i o n of f a u l t s , the boundaries between l a y e r s are most l i k e l y zones of i n c r e a s e d v e l o c i t y g r a d i e n t with very smooth 58 l a t e r a l v a r i a t i o n s . , P r o f i l e s 75-2 and 7 5-2R: The modeling procedure used f o r p r o f i l e s 7 5-2 and 75-2B was almost i d e n t i c a l t o t h a t used f o r 75\u00E2\u0080\u00943 and 75-3R. The f i n a l models, with rays superimposed, are shown i n F i g s . 4.10 and 4.11 along with t h e i r r e s p e c t i v e t r a v e l time curves. The model t r a v e l time curves f o r p r o f i l e s 75\u00E2\u0080\u00942 and 75-2B agree very w e l l with the observed f i r s t a r r i v a l s , with the e x c e p t i o n of the l a s t branch of 75-2R. The t r a v e l time advance observed between shots 38 and 39 f o r 75-2 was generated by a 0.7 km v e r t i c a l f a u l t i n the sediment l a y e r . As with p r o f i l e s 75-3 and 75-3B a v e l o c i t y of 3.7 km/s f o r the Winona Ridge l a y e r generated the r e q u i r e d t r a v e l time advance from 32 to 40 km on 75\u00E2\u0080\u00942 and from 20 to 32 km on 75-2R. Again, t h i s i s an upper l i m i t on the v e l o c i t y as a t h i n n e r , lower v e l o c i t y l a y e r c o u l d a l s o have been used. Beyond 56 km on the 75-2 model t r a v e l time curve, head wave a r r i v a l s from the deepest i n t e r f a c e appear as f i r s t a r r i v a l s , corresponding t o the weak f i r s t a r r i v a l of shot 54 ( F i g . 4.1). However, t h i s i n t e r f a c e i s s t i l l shown as q u e s t i o n a b l e s i n c e the data could be s a t i s f i e d without i t by v a r y i n g the o v e r l y i n g l a y e r s s l i g h t l y . Steeper d i p s were necessary on the east s i d e of the basin f o r p r o f i l e s 75-2 and 75-2R than f o r 75-3 and 75-3B t o achieve the very high apparent v e l o c i t y r e q u i r e d f o r the l a s t branch of 75-2H, which i s s t i l l not s a t i s f i e d . Steepeninq the d i p even more would c e r t a i n l y f i t the t r a v e l times b e t t e r but s i n c e the l a s t two shots of the p r o f i l e were i n a d v e r t e n t l y F i g . 4. 10 1 60 F i g . 4.11 Bay p l o t and t r a v e l time curve f o r 75\u00E2\u0080\u00942B f i n a l model. The model i s i d e n t i c a l to 75-2 except f o r d i f f e r e n c e s a r i s i n g from the two p r o f i l e s not being e x a c t l y c o i n c i d e n t . Note that the model i s r e v e r s e d , west being on the l e f t , compared with F i g . 4. 10. 61 shot over the continental shelf the large topographic corrections required say be i n error. For t h i s reason and also because the dip already seemed steep, no further changes were made. The amplitudes on the record sections for p r o f i l e s 7 5 \u00E2\u0080\u0094 2 and 7 5 \u00E2\u0080\u0094 2 8 do not show as much v a r i a t i o n as for 7 5 \u00E2\u0080\u0094 3 and 7 5 - 3 R . However, the concentrations of a r r i v a l s on the model t r a v e l time curves do seem to agree i n general with the observed results. For p r o f i l e 7 5 - 2 the head wave branch from 12 t c 16 km corresponds to the weak f i r s t a r r i v a l s observed there and the large amplitudes past 44 km correspond to the simultaneous a r r i v a l of two head wave branches. For p r o f i l e 7 5 - 2 B the model a r r i v a l s are very uniform out to 30 km, becoming more concentrated and extended from 30 to 4 8 km, in good aqreement with the observed amplitudes. Beyond 48 km however, the amplitudes are smaller than would be expected from the concentration of a r r i v a l s on the model t r a v e l time curve. This i s possibly an effect of the continental shelf and slope over which t h i s part of the p r o f i l e was shot. The f i n a l models presented are by no means unique. Other combinations of layers could be found which would s a t i s f y the travel times equally well. However, by r e s t r i c t i n q layer v e l o c i t i e s to accepted values f o r s i m i l a r c r u s t a l sections and by considerinq only g e o l o g i c a l l y and t e c t o n i c a l l y feasible situations, i t would be very d i f f i c u l t to a r r i v e at other suitable models which would not have the same qross properties. 62 5_i DISCUSSION 5,1 SEDIMENTS Very r e c e n t l y the r e s u l t s of an extensive s e t of r e f l e c t i o n p r o f i l e s taken i n 197 2 o f f the west coa s t of Vancouver I s l a n d were made a v a i l a b l e to us by Chevron Standard Ltd . of Calgary, S e v e r a l o f the 2400S& coverage a i r - g u n p r o f i l e s were shot over p a r t s of Winona Ba s i n , and the stacked r e c o r d s e c t i o n s show much deeper p e n e t r a t i o n and f a r more d e t a i l than our r e f l e c t i o n p r o f i l e s . However the v e l o c i t y -depth i n f o r m a t i o n determined from our p r o f i l e s i s more acc u r a t e because our s h o t - t o - r e c e i v e r d i s t a n c e s ranged frcm 0.5 t o 4.5 km while the Chevron data were c o l l e c t e d u sing a multichannel streamer o f l e s s than 2 km l e n g t h . One of the Chevron p r o f i l e s c o i n c i d e s almost e x a c t l y with C.S.P. r e c o r d 75-3 ( F i g . 1.4) and shows 2.5 s of sediments at the l o c a t i o n of p r o f i l e 75-3V, almost 2 s more than our p r o f i l e shows. Down to 0.6 s the sediments appear f l a t l y i n g and undisturbed, but beyond t h i s they are f o l d e d and deformed p o s s i b l y e x p l a i n i n g why there i s l i t t l e p e n e t r a t i o n beyond 0.6 s on both 75-3V and C.S.P. r e c o r d 7 5 - 3 , Towards the c o n t i n e n t much t h i c k e r f l a t l y i n g sediments occur at the foot of the c o n t i n e n t a l s l o p e corresponding to the t h i c k sediment r e g i o n of C. S. P. . r e c o r d 7 5 - 3 . Whereas only 0.9 s of sediments are i n d i c a t e d on the C.S.P. r e c o r d , 2.7 s of very s t r o n g r e f l e c t i o n s are obvious on the Chevron s e c t i o n . From a p r e l i m i n a r y and simple NMO a n a l y s i s . Chevron has i n t e r p r e t e d a 63 s i m i l a r s e c t i o n as c o n s i s t i n g of 1.3 km of 2.05 km/s average v e l o c i t y m a t e r i a l o v e r l y i n g 2.5 km of 3.7 km/s average v e l o c i t y m a t e r i a l . These sediments are t r u n c a t e d s h a r p l y by a deep v e r t i c a l f a u l t at the f o o t of the c o n t i n e n t a l s l o p e . To the e a s t , 0.5 s of r e l a t i v e l y u n d isturbed sediments o v e r l y up t o 2 s of deformed sediments. T h i s agrees w e l l with r e f l e c t i o n p r o f i l e 75-3 which shows only one c l e a r r e f l e c t i o n 0.5 s beyond the water bottom a r r i v a l . The Chevron s e c t i o n i n d i c a t e s a dip of 4\u00C2\u00B0- 6\u00C2\u00B0 i n d i c a t i n g a v e l o c i t y of 2 . 6 - 2.7 km/s and a t h i c k n e s s of 0.8 km f o r t h i s sequence (see Table I I ) . To the southwest the Chevron s e c t i o n c l e a r l y shows the v o l c a n i c basement beneath 0 . 7 s of sediments on P a u l Severe Ridge, i n very good agreement with r e f l e c t i o n p r o f i l e 75\u00E2\u0080\u00943R. The basement can e a s i l y be f o l l o w e d d i p p i n g beneath 1.5 s of sediments to the east of Paul Revere Ridge before becoming obscured by at l e a s t 1 . 3 s of deformed sediments beneath Winona Ridge. No other Chevron p r o f i l e i s c o i n c i d e n t with those i n t h i s study although two others were shot over the e a s t e r n p a r t of the b a s i n , one 10 km south of and p a r a l l e l t o p r o f i l e 75\u00E2\u0080\u00942R and another 10 km n o r t h of and p a r a l l e l t o 7 5 - 2 i As with the f i r s t Chevron p r o f i l e , up t o 3 s of sediments are t r u n c a t e d on the east by a deep v e r t i c a l f a u l t a t the f o o t of the c o n t i n e n t a l s l o p e . To the east of t h i s , approximately 0 . 5 s of r e l a t i v e l y undisturbed sediments o v e r l y up to 2 s of deformed sediments. T h i s i s c o n s i s t e n t with p r o f i l e 75\u00E2\u0080\u00942 ( F i g . 3.5), which shows cne c o r r e l a t a b l e r e f l e c t i o n 0.5 s a f t e r the bottom r e f l e c t i o n . As d i s c u s s e d i n s e c t i o n 3.3, 64 p r o f i l e 75-2V ( F i g , 3.1), over the c e n t r a l p a r t of the b a s i n , shews four r e f l e c t o r s w i t h i n 1.3 s of the s e a - f l o o r a r r i v a l as w e l l as one deeper c o r r e l a t a b l e r e f l e c t i o n (E) and one j u s t before the bottom m u l t i p l e . T h i s corresponds w e l l with the Chevron data. A s e r i e s of s t r o n g r e f l e c t i o n s to t r a v e l t i m e s of about 5.5 s are shown c l e a r l y , and, as mentioned above, a p r e l i m i n a r y a n a l y s i s gave 1.3 km o f m a t e r i a l with average v e l o c i t y of 2.05 km/s. T h i s g i v e s a 2-way t r a v e l t i m e o f 1.3 s. Deeper i n the s e c t i o n . C h e v r o n s data i n d i c a t e 2.5 km c f 3.6 km/s sediments. The i n t e r v a l v e l o c i t y of r e f l e c t o r E (Table I) i s 3.63 km/s while the suggested deep r e f l e c t o r i s a t a t r a v e l t i m e of 5.3 s. Another Chevron p r o f i l e 25 km to the northwest of and p a r a l l e l to p r o f i l e 75-2 shows 0.7 s of sediments o v e r l y i n g v o l c a n i c basement a t Paul Severe Ridge, i n good agreement with r e f l e c t i o n p r o f i l e 75-2JR. , The basement d i p s beneath 2 s cf r e l a t i v e l y undeformed sediments before becoming obscured by up to 2 s of f o l d e d sediments which r e p r e s e n t the b u r i e d northern extension of Winona Ridge. In summary, the Chevron p r o f i l e s c l e a r l y show a steady i n c r e a s e i n sediment t h i c k n e s s from a few hundred meters at Paul Revere Ridge t o about 4 km at the f o o t of the c o n t i n e n t a l s l o p e . T h i s i s i n q u a l i t a t i v e agreement with the 4 - 6 km t h i c k n e s s p r e d i c t e d by Couch (1969) on the b a s i s of g r a v i t y data. S e v e r a l f e a t u r e s of the s e c t i o n s , p a r t i c u l a r l y the deformed sediments of winona Ridge appear t o be the r e s u l t of northeast-southwest compression. T h i s could be caused by e i t h e r slow convergence of the Explorer-American p l a t e s or 65 subduction of the E x p l o r e r p l a t e beneath the American p l a t e . The v e r t i c a l f a u l t a t the f o o t of the c o n t i n e n t a l s l o p e i s s i m i l a r to that observed on C.S.P. r e c o r d s o f f the west c o a s t of the Queen C h a r l o t t e I s l a n d s (Chase e t a l , 1975) and c o u l d i n d i c a t e a component o f l a t e r a l motion r e s u l t i n g from o b l i q u e subduction, .!LL2 HAI 2 1 ICING MODELS Before d i s c u s s i n g the t e c t o n i c i m p l i c a t i o n s o f the ray t r a c i n g models, a few words should be s a i d about the r e s o l u t i o n t o be expected f o r the v a r i o u s l a y e r s . As mentioned p r e v i o u s l y , the only c o n s t r a i n t s which apply t o the e n t i r e c r u s t are those o b t a i n e d from r e f r a c t i o n p r o f i l e s 75\u00E2\u0080\u00941 and 75-1E, The u n c e r t a i n t y i n the p o s i t i o n s of these l a y e r s i s of the order of 1 - 2 km. The remainder of the models i s non-unique and depends on preconceived notions of t e c t o n i c s t r u c t u r e . The deeper l a y e r s have been chosen p r i m a r i l y t o s a t i s f y t r a v e l times and v a r i a t i o n s i n t h e i r p o s i t i o n s are p o s s i b l e . The f i r s t few l a y e r s a t the ends of the models are r e g u i r e d to f i t the t r a v e l times at the s t a r t of the p r o f i l e s and i f the v e l o c i t i e s from 75-1,1H are to be assumed they cannot vary too much. Of course t h i s assumption would be i n v a l i d i f s i g n i f i c a n t h o r i z o n t a l v e l o c i t y g r a d i e n t s are present. In l i g h t o f the data r e c e n t l y acguired from Chevron, i t appears at f i r s t g l ance t h a t the sediment l a y e r chosen i s too t h i n . However a re-examination of p r o f i l e 75-1,1R r e s u l t s 66 shows that the 4.3 km/s l a y e r t h a t Lynch (1977) i n t e r p r e t e d extends from 2.2 to 2.8 s beyond the water bottom a r r i v a l . , One of the Chevron p r o f i l e s runs approximately 5 km to the southeast of and p a r a l l e l t o p r o f i l e 75\u00E2\u0080\u00941B and shows d e f i n i t e sedimentary l a y e r i n g over the e n t i r e p r o f i l e l e n g t h f o r t h i s time i n t e r v a l . At t h i s depth of b u r i a l i t i s not unreasonable f o r sediments t o have a v e l o c i t y of 4.3 km/s. Thus both the 2.5 and the 4.3 km/s l a y e r s c o n s t i t u t e sediments on the r a y t r a c i n g models. A v e l o c i t y of 3.7 km/s has been chosen f o r Winona Bidge although making t h i s l a y e r t h i n n e r would have made p o s s i b l e the use of a lower v e l o c i t y . T h i s i m p l i e s t h a t the v e l o c i t y of the r i d g e i s l i k e l y not g r e a t e r than 3.7 km/s, i n d i c a t i v e of a higher v e l o c i t y sediment than the surrounding sediments and which c o u l d have r e s u l t e d from compression. The most s t r i k i n g , and probably the most important f e a t u r e of the models i s the westward di p p i n g l a y e r s on the east s i d e of the b a s i n . These are r e q u i r e d to s a t i s f y the very high apparent v e l o c i t i e s on the t r a v e l time curves of 75-2B and 75-3B. The deepest p o i n t of the l a y e r s c o u l d be s h i f t e d by up to 5 km t o e i t h e r s i d e and s t i l l s a t i s f y the t r a v e l times, but no c o n f i g u r a t i o n of l a y e r s d i p p i n g to the e a s t would be p o s s i b l e without i n t r o d u c i n g h o r i z o n t a l v e l o c i t y g r a d i e n t s . Since t h i s part of both models i s under the c o n t i n e n t a l s l o p e i t could r e p r e s e n t a t r a n s i t i o n from oceanic to c o n t i n e n t a l c r u s t over t h i s d i s t a n c e . However, i n going from o c e a n i c to c o n t i n e n t a l c r u s t a negative h o r i z o n t a l g r a d i e n t , c o n t r a r y to the model, would be expected. 67 Regardless of the l a y e r boundaries shewn, t h e r e must be a more r a p i d o v e r a l l i n c r e a s e i n v e l o c i t y with depth f o r the s i d e s of the basin than f o r the center i n order to s a t i s f y the t r a v e l times. The eastward d i p p i n g c r u s t on the west s i d e of the models suggests a subducting p l a t e . Perhaps the westward d i p p i n g l a y e r s on the e a s t s i d e of the b a s i n r e p r e s e n t a b u c k l i n g of the c r u s t i n response t o a recent i n i t i a t i o n of subduction or i n c r e a s e i n subduction rate i n t h i s area. In order that the models be a c c e p t a b l e , they must be c o n s i s t e n t with the g r a v i t y data. To check t h i s , Clowes and W h i t t a l l (personal communication, 1978) have performed a p r e l i m i n a r y g r a v i t y c a l c u l a t i o n f o r model 7 5\u00E2\u0080\u00943 to compare with the f r e e a i r anomaly, although much more work i s necessary t o remove unwanted end e f f e c t s they have shown t h a t the model i s capable of s a t i s f y i n g the data. 5a_3 TECTONIC SIGNIFICANCE OF WINONA BASIN The r e s u l t s of t h i s p r o j e c t , coupled with e x i s t i n g g e o p h y s i c a l and g e o l o g i c a l i n f o r m a t i o n , suggest t h a t o b l i q u e subduction c o u l d be o c c u r r i n g at Winona Basin. I f , as has been hypothesized by some authors ( i e . Riddihough, 1977; Murray and T i f f i n , 1974), the t r i p l e j u n c t i o n has r e c e n t l y (within the past 4 m,y.) migrated from the Brooks P e n i n s u l a area to the Dellwood K n o l l s , ( F i g s . 1.1 & 1.2) the subduction zone would a l s o have had to make a corresponding s h i f t to the northwest. As p o i n t e d out by Riddihough (1977) convergence of the E x p l o r e r and American p l a t e s has been o b l i q u e and very 68 slow (1 . 9 to 1.4 cm/yr) over the past 3 m.y.; consequently convergence without subduction i s p o s s i b l e s i n c e the young c r u s t i s presumably s t i l l warm and perhaps unable to subduct. However i f there has been a s l i g h t component of o b l i q u e . subduction along the Queen C h a r l o t t e transform f a u l t , a subducting p l a t e would alr e a d y have been e s t a b l i s h e d p r i o r t o the t r i p l e j u n c t i o n s h i f t . The t h i c k s e d i m e n t - f i l l e d trough with evidence of northeast-southwest compression and the bowl l i k e s t r u c t u r e of the deeper c r u s t i n Winona Ba s i n a re str o n g evidence f o r subducti o n . The westward dippin q l a y e r s on the ea s t s i d e of the basin c o u l d r e p r e s e n t b u c k l i n g o f the c r u s t i n response t o an i n c r e a s e i n the convergence r a t e f o l l o w i n g the t r i p l e j u n c t i o n s h i f t . S i m i l a r l y the deep v e r t i c a l f a u l t at the f o o t of the c o n t i n e n t a l slope i n d i c a t e s a d e f i n i t e component of l a t e r a l motion. These o b s e r v a t i o n s are bes t e x p l a i n e d by o b l i q u e subduction between the E x p l o r e r and American p l a t e s . 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Geophysics 43., pp. 715-729. "@en . "Thesis/Dissertation"@en . "10.14288/1.0052990"@en . "eng"@en . "Geophysics"@en . "Vancouver : University of British Columbia Library"@en . "University of British Columbia"@en . "For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use."@en . "Graduate"@en . "A marine deep seismic sounding survey over Winona Basin"@en . "Text"@en . "http://hdl.handle.net/2429/21166"@en .