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

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_ 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<zc,pV*f$>\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 ....................... ........ •>......... ..... 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. .......... ....»............. ........ .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. ...»........................................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—1 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 — 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»M. 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 •HflVB 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. •1 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 / ( £ ) 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 • 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—2V and 75—3? 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—3V 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 © 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 © 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— | 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 • 33 | 2. 33 i B I 1.83 \ .39 | 2.72 I c I 2.04 1 .18 \ 2.90 1 D | 2.49 1 .30 | 3. 20 I £ I 3.63 1 .77 | 3.97 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—3V 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—2R {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— 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 -° 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—3R, 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—2R and 75—3R the basement d i p i s about 5° towards the e a s t , whereas f o r p r o f i l e s 75-2 and 75—3, 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—3R. 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° 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 — + 75-3R 7 5-2 — + B B B B B B H A A A •1.49 2.45 3. 01 3.47 3. 90 1. 49 2. 42 2.66 3. 09 3. 52 1. 49 2.42 2.85 3.24 1.80 .80 • 30 .35 .39 1.80 .65 .34 .40 .45 1.77 .59 .65 .71 0.0 5.0 3. 0 4.0 5.0 0. 0 5.0 3. 0 4.0 5. 0 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° 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—3, 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—2R ( 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—3 ( 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—2R 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—3 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—2R and 75—3H 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±2 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—X 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—1 and 75—1R 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 £ L I £ 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—3 and 75—3R w i l l be used. a) The S t a r t i n g Model The s t a r t i n q model 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. ' • ' 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—1 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° 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—3R 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—1, 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—1 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—3R 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—3. 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—3 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—3 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—3 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—3 and 75—3fi 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—3 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—2 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—2 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—2B 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 — 2 and 7 5 — 2 8 do not show as much v a r i a t i o n as for 7 5 — 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°- 6° 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—3R. 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—2R 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—2 ( 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—1 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—1B 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—3 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 . In order t o qet a complete understandinq of the s t r u c t u r e of Winona Basin and the t e c t o n i c f o r c e s o p e r a t i n q on i t , more ge o p h y s i c a l i n f o r m a t i o n i s needed. However the r e s u l t s of t h i s study p l a c e important c o n s t r a i n t s on the o v e r a l l p i c t u r e and any c o n c l u s i o n s made must be c o n s i s t e n t with them. 69 Cerveny, V. and fi. Bavindra 1971. Theory of Seismic Head Haves. U n i v e r s i t y of Toronto Press, 312 pp. Chase, R. L., D. L. T i f f i n and J.W. Hurray 1975. The western Canadian c o n t i n e n t a l margin. Canadian S o c i e t y of Petroleum G e o l o g i s t s , Memoir 4, pp. 701-721. Cla y t o n , R.W. 1975. The deconvclution of t e l e s e i s m i c recordings. Unpublished M.Sc. t h e s i s . U n i v e r s i t y of B r i t i s h Columbia. . Clee, T.E., K,G. Barr and M.J. Berry 1974. Fine s t r u c t u r e cf the c r u s t near Y e l l o w k n i f e . Can. J . Earth S c i . 11, pp. 1534- 1549. Clowes, R.M. 1977. A marine deep s e i s m i c sounding system. Can. J . Earth S c i . 14, pp. 1276-1285. Couch, R.W. 1969. G r a v i t y and s t r u c t u r e s of the c r u s t and sub-c r u s t i n the northeast P a c i f i c ocean west of Washington and B r i t i s h Columbia. Unpublished PhD. t h e s i s , Oregon State U n i v e r s i t y , C o r v a l l i s , OR. Dix, C.H. 1955. Seismic v e l o c i t i e s from surface measurements. Geophysics 20, pp. 68-86. Grubbe, K. 1976. S e i s m i c - r e f r a c t i o n measurements along two c r o s s i n g p r o f i l e s i n northern Germany and t h e i r i n t e r p r e t a t i o n by a r a y - t r a c i n g method. I n : Explosion Seismology i n C e n t r a l Europe. P. Giese, C. Prodehl and A. S t e i n (eds.), S p r i n g e r - V e r l a g , B e r l i n 1976, pp. 268-282. Kanasewich, E. R. 1976. Time S e r i e s A n a l y s i s i n Geophysics, 2nd ed., U n i v e r s i t y of A l b e r t a Press, 352 pp. Kramer, F.S., R.A. Peterson and W.C. Walter 1968. Seismic energy sources handbook, 1968. United Geophysical Corporation, 57 pp. Lynch, S. 1977. The c r u s t a l s t r u c t u r e of Sinona Basin as determined by deep seismic sounding. Unpublished M.Sc. t h e s i s , U n i v e r s i t y of B r i t i s h Columbia. M i l l e r , H. and H. Gebrande 1976. C r u s t a l s t r u c t u r e i n southeastern Bavaria derived from s e i s m i c - r e f r a c t i o n measurements by r a y - t r a c i n g methods. I n : Explosion Seismology i n C e n t r a l Europe. P. Giese, C. Prodehl and A. S t e i n (eds.), Springer-Verlag, B e r l i n 1976, pp. 339-3 46. Mota, L. 1954. Determination of dips and depths of g e o l o g i c a l 70 l a y e r s by the s e i s m i c r e f r a c t i o n method. Geophysics 19, pp. 242-254.. fluller, S t . , A. S t e i n and B. V e i s 1962. Seismic s c a l i n g l a s s f o r e x p l o s i o n s on a lake bottom. Z e i t . f . Geophys. .28, pp. 258-280. Murray, J.W. and D.L. T i f f i n 1974. Patterns of deformation, sedimentation and tectonism, southwestern Canadian c o n t i n e n t a l margin. Ann. de l a Soc. Geol, de Belgique 97, pp. 169-183. O'Brien, P.N.S. 1960. Seismic energy from e x p l o s i o n s . Geophys. J.R. a s t r . Soc. 3, pp. 29-44. Riddihough, R.P. and R.D. Hyndman 1976. Canada»s a c t i v e western margin - the case f o r subduction, Geosci, Canada 3, pp. 269-278. Riddihough, R.P. 1977. A model f o r r e c e n t p l a t e i n t e r a c t i o n s o f f Canada»s west c o a s t . Can. J . Earth S c i . J4, pp. 384-396. , Shor, G.G,, J r . 1963. R e f r a c t i o n and r e f l e c t i o n technigues and procedure. I n : The Sea. M.N. H i l l (ed.), I n t e r s c i e n c e , New York, pp. 20-38, T e l f o r d , W.M., L.P. G e l d a r t , B.E. S h e r i f f and D.A. Keys 1S76. Applied Geophysics. Cambridge U n i v e r s i t y Press, Cambridge, 860 pp. T i f f i n , D.L., B.E.B. Cameron and J.W. Murray 1972.„Tectonics and d e p o s i t i o n a l h i s t o r y of the c o n t i n e n t a l margin o f f Vancouver I s l a n d , B r i t i s h Columbia. Can. J. E a r t h S c i . 9, pp. 280-296. T i f f i n , D.L. and D. ,5eeman 1975. Bathymetric map of the c o n t i n e n t a l margin of western Canada. Open f i l e map, Geol. Surv. o f Canada. Wiggins, B.A. 1976. Body wave amplitude c a l c u l a t i o n s I I . Geophys. J.R. a s t r . Soc. ^6, pp. 1-10. Wiggins, R.A. 1977. Minimum entropy d e c o n v o l u t i o n . Proceedings of the I n t e r n a t i o n a l Symposium on Computer Aided Seismic A n a l y s i s and D i s c r i m i n a t i o n , June 9 & 10, 1977, Falmouth, Mass., IEEE Computer S o c i e t y , pp. 7-14. Wood, L.C., R.C. R e i s e r , S. T r e i t e l and P.L. R i l e y 1978. The debubbling of marine source s i g n a t u r e s . Geophysics 43., pp. 715-729. 

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