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

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THE CRUSTAL STRUCTURE OF WI NONA BASIN AS DETERMINED BY DEEP SEISMIC SOUNDING by STEVEN LYNCH B.Sc., U n i v e r s i t y o f G u e l p h , 1975 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES ( D e p a r t m e n t o f G e o p h y s i c s and A s t r o n o m y ) We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA J u n e , 1977 © S t e v e n L y n c h , 1977 In presenting th is thes is in p a r t i a l fu l f i lment of the requirements for an advanced degree at the Un ivers i ty of B r i t i s h Columbia, I agree that the L ibrary sha l l make it f ree ly ava i lab le for reference and study. I fur ther agree that permission for extensive copying of th is thes is for scho la r ly purposes may be granted by the Head of my Department or by h is representat ives . It is understood that copying or pub l ica t ion of th is thes is fo r f i n a n c i a l gain sha l l not be allowed without my wri t ten permission. Depa rtment The Univers i ty of B r i t i s h Columbia 2075 wesbrook Place Vancouver, Canada V6T 1W5 Date UfZ ^ 1 / ? 7 7 i ABSTRACT D u r i n g Auqust 1975, t h r e e r e v e r s e d deep s e i s m i c s o u n d i n g p r o f i l e s were run o v e r Winona B a s i n , a deep water s e d i m e n t a r y b a s i n l o c a t e d west of the n o r t h e r n end of Vancouver I s l a n d . T h i s t h e s i s r e p o r t s on the a n a l y s i s o f the d a t a f o r r e v e r s e d p r o f i l e s 75-1 and 75-1R, which were r e c o r d e d t o d i s t a n c e s o f g r e a t e r t h a n 90 km a l o n g the a x i s of the b a s i n . The s e i s m i c s i g n a l s from e x p l o s i v e c h a r g e s a r e d e t e c t e d by s i x i n d i v i d u a l hydrophones, a m p l i f i e d and r e c o r d e d on a multi-channel d i g i t a l a c q u i s i t i o n system. The s u b - c r i t i c a l r e f l e c t i o n d ata and t h e r e f r a c t i o n d a t a are a m p l i t u d e c o r r e c t e d f o r s p h e r i c a l s p r e a d i n q , a m p l i f i e r g a i n , charge s i z e and v a r y i n g hydrophone s e n s i t i v i t y . I m m e d i a t e l y p r i o r t o c o m p i l a t i o n i n t o r e c o r d s e c t i o n s , the d a t a a re bandpass f i l t e r e d t o improve t h e i r g e n e r a l appearance. Three methods o f a n a l y s i s were used t o o b t a i n v e l o c i t y -depth i n f o r m a t i o n from the s u b - c r i t i c a l r e f l e c t i o n d a t a ; the r a y parameter, T 2 - X 2 and t h e s t r i p p e d T 2 - X 2 methods. A co m p a r i s o n of the r e s u l t s o b t a i n e d from t h e t h r e e t e c h n i q u e s shows t h a t t h e r a y parameter and T 2 - X 2 methods y i e l d e s s e n t i a l l y t h e same r e s u l t , whereas t h e s t r i p p e d T 2 - X 2 method i s o f no use i n a n a l y s i n g the data below the second sub-bottom l a y e r . The a n a l y s i s of p r o f i l e 75-1H d a t a at the no r t h w e s t end of t h e b a s i n gave a s e d i m e n t a r y s t r u c t u r e d i v i d e d i n t o t h r e e i i prominent horizons , with v e l o c i t i e s ranging from 1.7 to 2.1 km/s. The t o t a l depth to the ba sa l t i c layer was determined to be 1.8 km. Thi s thickness i s considerably less than that suggested by other authors on the basis of gravi ty studies and assumptions concerning continuous seismic p r o f i l e s . Due to the presence of s i g n i f i c a n t dips on the r e f l e c t i n g horizons in the southeast, no ve loc i ty-depth information could be obtained from the ana lys i s of p r o f i l e 75-1. Ca l cu l a t ion of a prel iminary ve loc i ty-depth model based on f i r s t a r r i v a l t r a v e l times was the f i r s t step in the ana lys i s of the r e f r a c t i o n data. These i s o - v e l o c i t y layered models provided an i n i t i a l i n t e r p r e t i v e guide and a s t a r t ing place for the c a l c u l a t i o n of synthet ic seismograms. In order to u t i l i z e t ravel t imes of f i r s t and secondary a r r i v a l s and the r e l a t i v e amplitude c h a r a c t e r i s t i c s of the seismograms, the f i n a l i n t e r p r e t a t i o n made use of synthet ic seismogram sect ions for comparison with the r e a l data. Based on such an a n a l y s i s , the re f rac t ion data ind ica te that the crust underlying Winona Basin i s separated i n t o four sub-sediment l a y e r s , having s i g n i f i c a n t v e l o c i t y gradients . Average v e l o c i t i e s and thicknesses for the layers are ; 4.28 km/s, 1.6 km; 5.26 km/s, 2.75 km; 6.28 km/s, 4.13 km; 7.04 km/s, 3.76 km. The t o t a l sub-sediment thickness of th i s c r u s t a l sec t ion i s thus 12 km. An unreversed mantle v e l o c i t y of 7.8 km/s was interpreted from 75-1 r e s u l t s . On the basis of the v e l o c i t y values , the layers have been i d e n t i f i e d with oceanic c r u s t a l l ayers 2a, 2b, 3a, 3b. i i i The t h i c k c r u s t i s p o s t u l a t e d t o be the r e s u l t of complex p l a t e i n t e r a c t i o n s o c c u r r i n g i n the r e g i o n . A comparison of the data from t h i s work with p r e v i o u s s t u d i e s i n the r e g i o n of E x p l o r e r and Juan de Fuca Ridges has l e d to the s p e c u l a t i v e c o n c l u s i o n t h a t Winona Basin has been c r e a t e d w i t h i n the l a s t 3 t o ii my, by the slow northward p r o g r e s s i o n of the P a c i f i c -North America-Explorer p l a t e t r i p l e p o i n t . i v TABLE OF CONTENTS ABSTRACT TABLE OF CONTENTS LIST OF TABLES LIST OF FIGURES ACKNOWLEDGEMENTS Pa qe la. I N T R O D U C T I O N 1. 1 Area of Study 1 1.2 T e c t o n i c S i g n i f i c a n c e of Winona B a s i n 13 1.3 Data A c q u i s i s t i o n and P r o j e c t D e s c r i p t i o n 16 li. . PRELIMINARY ANALYSIS 2.1 D e m u l t i p l e x i n g 20 2.2 O r i g i n Times .....21 2.3 S h o t - r e c e i v e r d i s t a n c e s 24 2.4 Record S e c t i o n s 29 Is. SUBzCRITICAL REFLECTION DATA 3.1 Methods o f A n a l y s i s 34 3.2 A n a l y s i s o f R e s u l t s 47 i i .REFRACTION DATA 4.1 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 60 4.2 S y n t h e t i c Seismograms 71 4.3 A p p l i c a t i o n t o Data ...74 '4.4 Record S e c t i o n s IS 4.5 Comparison of S y n t h e t i c s w i t h R e a l Data 88 5. INTERPRETATION ANE> DISCUSSION 5.1 V e l o c i t y - D e p t h Models 105 5.2 D i s c u s s i o n 110 REFERENCES 119 V LIST OF TABLES T a b l e Paqe 2 .1 S h o t - r e c e i v e r d i s t a n c e e r r o r s .....28 3 .1 Comparison of r a y par a m e t e r , T 2 - X z and s t r i p p e d T 2 - X 2 methods o f r e f l e c t i o n a n a l y s i s 52 3.2 V e l o c i t y vs depth model f o r r e f l e c t i o n p r o f i l e 75 -1R 53 3.3 T 2-X 2 s l o p e s and i n t e r c e p t s f o r r e f l e c t i o n p r o f i l e 75 -1 57 4.1 L e a s t s q u a r e s v e l o c i t i e s and i n t e r c e p t s f o r r e f r a c t i o n f i r s t a r r i v a l a n a l y s i s .....67 4.2 V e l o c i t y vs depth model from r e f r a c t i o n f i r s t a r r i v a l a n a l y s i s 67 v i L i s t of F i g u r e s F i q ' Paqe 1.1 L o c a t i o n map f o r Winona B a s i n .„ 4 1.2 B a t h y m e t r i c map of Winona B a s i n .6 1.3 C o n t i n u o u s s e i s m i c p r o f i l e l i n e 75-1 p l u s i n t e r p r e t a t i o n ~8- t\ potjj^T 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 75-2 - I 4r p l u s i n t e r p r e t a t i o n l*> J**«* 1.5 A i r qun p r o f i l e over deep water p o r t i o n o f Winona b a s i n .11 2.1 Four t y p i c a l seismograms from p r o f i l e 75-1 ......26 3.1 P r o c e s s e d r e c o r d 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 75-1R ...36 3.2 Same as above f o r p r o f i l e 7 5-1 ......37 3.3 Two t y p i c a l s e c t i o n s used t o time r e f l e c t i o n a r r i v a l s , 39 3.4 T vs x and T 2-X* p l o t s f o r 75-1R r e f l e c t i o n d a t a ..50 3.5 Same as 3.4 f o r p r o f i l e 75-1 51 4.1 T vs X p l o t f o r r e f r a c t i o n p r o f i l e s 75-1 and 75-1R f i r s t a r r i v a l a n a l y s i s 63 4.2 Expanded T vs X p l o t showing t h e f i r s t two phases of F i g 4.2 o n l y 65 4.3 V e l o c i t y vs depth c u r v e 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 ...70 4.4 P r o c e s s 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 75-1 77 4.5 Same as 4.4 f o r p r o f i l e 7 5-1 R 78 4.6 S t a c k e d r e c o r d s e c t i o n f o r p r o f i l e 75-1 80 4.7 P r o f i l e 75-1R r e c o r d s e c t i o n showing o n l y c h a n n e l t h r e e data 82 4.8 Expanded p l o t o f l a s t 40 km of p r o f i l e 75-1 ....87 v i i 4.9 P-A c u r v e generated from i s o - v e l o c i t y l a y e r e d model .90 4.10 S y n t h e t i c seismograms f o r i s o - v e l o c i t y l a y e r e d model .92 4.11 F i n a l s y n t h e t i c seismograms and the r e a l d a t a f o r p r o f i l e 7 5 - 1 94 4.12 Same as above f o r p r o f i l e 7 5-1R 9 5 4.13 F i n a l p -A c u r v e f o r p r o f i l e 7 5 - 1 9 7 4.14 F i n a l p-& c u r v e f o r p r o f i l e 7 5 - 1 R 98 4.15 Comparison o f f i n a l p -A c u r v e s f o r 7 5 - 1 and 7 5 - 1 R 9 9 5.1 F i n a l v e l o c i t y vs depth models f o r p r o f i l e s 7 5 - 1 and 7 5 - 1 R 1 0 7 v i i i ACKNOWLEDGEMENTS For h i s encouragement, p a t i e n c e and s u p p o r t d u r i n g t h e e n t i r e t y o f t h i s p r o j e c t , I w i s h t o e x p r e s s my deepest a p p r e c i a t i o n t o Dr. R.M. Clowes. I am a l s o i n d e b t e d t o W.B. Cumming who p r o v i d e d me w i t h g r e a t a s s i s t a n c e d u r i n g t h e more t e c h n i c a l a s p e c t s of the work. T h i s p r o j e c t has been a i d e d g r e a t l y by t h e e x c e l l e n t q u a l i t y of the d a t a , due i n no s m a l l way t o the e f f o r t s of Dr. Bo Chandra, C h r i s West and B i l l Cumming on the r e c e i v i n g s h i p , and George Spence, P a t r i c k Shore and Rob C l a y t o n on the s h o o t i n g s h i p . T h e i r d i l i g e n c e , e nthusiasm and d e d i c a t i o n a r e g r a t ' e f u l l y acknowledged. F u r t h e r , I wish t o e x p r e s s my a p p r e c i a t i o n t o the o f f i c e r s and crew o f the C. F. A.V. ENDEAVOUR and C F . A.V. LAYMORE f o r t h e i r a s s i s t a n c e d u r i n g the c r u i s e . The a s s i s t a n c e g i v e n by the e x p l o s i v e s e x p e r t s from the 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 i s a l s o g r a t e f u l l y acknowledged. The a n t i - s u b m a r i n e p r o j e c t i l e s were s u p p l i e d by t h e Seismology D i v i s i o n of t h e E a r t h P h y s i c s Branch. Funding f o r t h i s p r o j e c t was s u p p l i e d by t h e N a t i o n a l Research C o u n c i l o f Canada, o p e r a t i n g g r a n t A 7707. A d d i t i o n a l funds were s u p p l i e d by M o b i l O i l Canada L t d . , S h e l l Canada Res o u r c e s L t d . and Chevron S t a n d a r d . The Department o f Energy Mines and Resources ( G e o l o g i c a l Survey o f Canada) p r o v i d e d the c o n t r a c t t o a c q u i r e the d a t a and a r e s e a r c h agreement to a s s i s t i n i t s a n a l y s i s . 1 Jt INTRODUCTION The t e c t o n i c s t r u c t u r e west of Vancouver I s l a n d i s , to say the l e a s t , extremely c o m p l i c a t e d . T h i s r e g i o n i s the area of i n t e r a c t i o n between two manor p l a t e s , the P a c i f i c and North American, and two minor p l a t e s , the Exp l o r e r and Juan de Fuca p l a t e ( F i g 1.1). These s m a l l e r p l a t e s a re the remnant of a much l a r g e r and a n c i e n t p l a t e , t h e F a r a l l o n p l a t e , which a c c o r d i n g t o Atwater (1970) has been consumed by the o v e r r i d i n g North American p l a t e . Of s p e c i a l i n t e r e s t to t h i s study i s the l o c a t i o n o f the t r i p l e p o i n t between the three p l a t e s . I t i s thought to be somewhere near the r e g i o n of 51° N 131° W; however, i t s a b s o l u t e p o s i t i o n i s q u e s t i o n a b l e . Winona Bas i n , the area of i n t e r e s t f o r t h i s study, i s l o c a t e d near the r e g i o n of t h i s t r i p l e p o i n t and i s surrounded by the P a c i f i c , North American and E x p l o r e r p l a t e s . What i t s r o l e i s , and how i t i s r e l a t e d to the complex p l a t e t e c t o n i c s , provided much of the impetus f o r the marine deep s e i s m i c sounding survey which .was c a r r i e d out i n 1975. 1.1 Area of Study • The name Winona Basin was f i r s t a p p l i e d t o the deep sedimentary basin l o c a t e d at t h e foot of the c o n t i n e n t a l margin o f f the northwest t i p of Vancouver I s l a n d , by S r i v a s t a v a et a l (1971). The southernmost boundary i s taken to be at the Brooks f r a c t u r e zone, an area of i n t e n s e f r a c t u r i n q and u p l i f t , whereas the no r t h e r n boundary i s taken to be some 2 150 km to the northwest at the Dellwood K n o l l s . The b a s i n i s bounded on the west by Paul Revere Ridge and on the e a s t by the edge of the c o n t i n e n t a l s l o p e . F i g 1.2 i s a bathymetric map of the area d e t a i l i n g the major topographic f e a t u r e s of the b a s i n . The only major f e a t u r e l o c a t e d w i t h i n the i n t e r i o r of the basin i t s e l f i s a broad l o w - r e l i e f s t r u c t u r e named Winona Ridge. T h i s f e a t u r e t r e n d s somewhat o b l i g u e l y t o Paul Revere Ridge and i s u n d e r l a i n by the s t r a t i f i e d sediments of the b a s i n . Other than t h i s f e a t u r e , the i n t e r i o r of the b a s i n i s f l a t l y i n g with a mean water depth of approximately 2000 m. A g r a v i t y survey over the area by Couch (1969) has r e v e a l e d a -160 ragal f r e e a i r anomaly l o c a t e d between Winona Ridge and the c o n t i n e n t a l s h e l f . He i n t e r p r e t e d t h i s as being due t o 4 to 6 km o f sediments i n t h i s p o r t i o n of the basin. T h i s depth i s c o n s i s t e n t with l a t e r work by T i f f i n e t a l (1973), who suggests t h a t "the basin i s u n d e r l a i n by approximately 4 km of raudstone, sandstone, conglomerate and minor c o a l of d e f i n i t e P l i o c e n e - P l e i s t o c e n e age". On the b a s i s of h i s g r a v i t y data, Couch (1969) generated a c r u s t a l and s u b - c r u s t a l c r o s s s e c t i o n along a l i n e running from 48° N, 134° W to 52° N, 126° W. T h i s s e c t i o n c r o s s e s Winona Basin at an o b l i g u e angle at roughly the midpoint of the b a s i n and shows an approximate depth t o the mantle of 15 km sub-bottom. The b a s i n i s thought t o be a r e l a t i v e l y young s t r u c t u r e , probably o f P l i o c e n e - P l e i s t o c e n e age. T h i s proposal has been given by T i f f i n et a l ( 1972) on the basi-: of l i m i t e d 3 F i g J L I. L o c a t i o n o f Winona B a s i n w i t h r e s p e c t t o t h e P a c i f i c , N o r t h A m e r i c a n , E x p l o r e r and J u a n de F u c a p l a t e s . The a r e a w i t h i n t h e r e c t a n g l e i s shown e n l a r g e d on F i q 1.2. PA P, P a c i f i c p l a t e ; AM P, N o r t h A m e r i c a n p l a t e ; EX P, E x p l o r e r p l a t e ; J F P, J u a n de F u c a p l a t e . The h a t c h u r e d l i n e shows t h e assumed c o n v e r g e n t b o u n d a r y between E x p l o r e r / J u a n de Fu c a p l a t e s and t h e N o r t h A m e r i c a n p l a t e . 4 132 130 128 126 L o c a t i o n o f 1975 marine deep s e i s m i c s o u n d i n g p r o f i l e s i n Winona B a s i n . Open c i r c l e s show t h e d r i f t t r a c k o f t h e r e c e i v i n g s h i p d u r i n g t h e p r o f i l e r u n ; h e a v y l i n e s show t h e t r a c k o f t h e s h o o t i n g s h i p . P r o f i l e s 7 5 - 1 and 7 5 - l f i a l o n g t h e b a s i n a r e t h e s u b j e c t o f t h i s s t u d y . B a t h y m e t r i c c o n t o u r s i n m e t e r s (from T i f f i n and Seemen, 1 9 7 5 ) . 7 f o r m a n i f e r a l e v i d e n c e . The s e d i m e n t s i n t h e b a s i n a r e r e l a t i v e l y undeformed i n t h e n o r t h w e s t , as shown by F i g s 1.3 and 1.4 (pocket on the r e a r c o v e r ) ; however, they a r e s e v e r e l y f o l d e d and f a u l t e d i n t h e s o u t h e a s t , w i t h the f a u l t s t r e n d i n g n o r t h w e s t (see F i g 1.5). C o n t i n u o u s s e i s m i c p r o f i l e (C.S.P.) l i n e 75-2 ( F i g 1.4), shows the o c e a n i c basement d i p p i n g e a s t w a r d from P a u l - R e v e r e Ridge. I t t h e n d i p s beneath Winona r i d g e and can not be i d e n t i f i e d f u r t h e r on the" p r o f i l e . E p i c e n t r a l l o c a t i o n s f o r e a r t h q u a k e s i n the r e g i o n have s u g g e s t e d the p o s s i b i l i t y t h a t a c t i v e d e f o r m a t i o n i s p r e s e n t l y t a k i n g p l a c e i n the b a s i n (Tobin and Sykes 1968). M i l n e e t a l (1977), however,have r e c e n t l y p o i n t e d out t h a t t h e r e i s a s y s t e m a t i c b i a s i n e p i c e n t r a l l o c a t i o n s f o r the e a r t h q u a k e s o f f Canada's west c o a s t . On the b a s i s o f t h i s work and r e s e a r c h c u r r e n t l y i n p r o g r e s s i t appears l i k e l y t h a t the m a j o r i t y of e a r t h g u a k e s are l o c a t e d on t h e E x p l o r e r R i d g e / R e v e r e - D e l l w o o d f r a c t u r e zone and n o t i n t h e b a s i n i t s e l f . The area of Winona B a s i n was p a r t o f the o r i q i n a l m aqnetic s u r v e y by R a f f and Mason (1961) which p r o v i d e d much of . the e v i d e n c e f o r the i n t r o d u c t i o n o f t h e s e a - f l o o r s p r e a d i n g c o n c e p t . I n Winona B a s i n however, t h e a n o m a l i e s show none of t h e magnetic l i n e a t i o n s n o r m a l l y a s s o c i a t e d w i t h a s p r e a d i n g c e n t r e , the magnetic s t r u c t u r e of the b a s i n b e i n g r e l a t i v e l y smooth. There a r e normal magnetic l i n e a t i o n s to the west i n t h e r e g i o n o f E x p l o r e r Ridge; however, they t e r m i n a t e A. C o n t i n u o u s s e i s m i c p r o f i l e l i n e 75-1, p a r a l l e l t o DSS p r o f i l e 75-1. P e n e t r a t i o n i s a c h i e v e d o n l y t o a maximum o f 4 s two-way t r a v e l t i m e . B. I n t e r p r e t a t i o n of C.S.P. l i n e 75-1, c o u r t e s y o f Hopkins (1976). Note t h e c o n t i n u i t y a l o n g t h e p r o f i l e of the i n t e r p r e t e d r e f l e c t i n g h o r i z o n s ( l o c a t e d i n p o c k e t on t h e back c o v e r ) . 9 Fig. JL 4 A. C o n t i n u o u s s e i s m i c p r o f i l e l i n e 75-2 p a r a l l e l i n g DSS p r o f i l e 75-2. Note the r e l a t i v e l y undeformed upper s e d i m e n t s , and t h e i n c r e a s e i n f o l d a m p l i t u d e w i t h d epth. B. I n t e r p r e t a t i o n of C.S.P. l i n e 75-2, a l s o from Hopkins (1 976). On t h e l o w e r r i g h t c o r n e r , basement • has been i n t e r p r e t e d as d i p p i n g t o t h e e a s t ( a l s o i n p o c k e t on back cover) . An a i r gun s e i s m i c p r o f i l e over the deep water p o r t i o n of Winona Basin beyond the base of the c o n t i n e n t a l slope showing a t h i c k f o l d e d and f a u l t e d sedimentary sequence, f=fault„ Area between the arrows i n d i c a t e s the approximate p o s i t i o n of r e f l e c t i o n p r o f i l e 75-1. Taken from Hurray and T i f f i n (1974). 11 12 f o r m a n i f e r a l evidence. The sediments i n the basin are r e l a t i v e l y undeformed i n the northwest, as shown by F i q s 1.3 and 1.4 (pocket on the r e a r c o v e r ) ; however, they are s e v e r e l y f o l d e d and f a u l t e d i n the s o u t h e a s t , with the f a u l t s t r e n d i n g northwest (see F i q 1.5). Continuous s e i s m i c p r o f i l e (C.S.P.) l i n e 75-2 (Fig 1.4), shows the o c e a n i c basement d i p p i n g eastward from Paul-Severe Ridge. I t then dips beneath Winona r i d g e and can not be i d e n t i f i e d f u r t h e r on the p r o f i l e . E p i c e n t r a l l o c a t i o n s f o r earthquakes in the r e g i o n have suggested the p o s s i b i l i t y t h a t a c t i v e deformation i s p r e s e n t l y t a k i n g p l a c e i n the b a s i n (Tobin and Sykes 1968). Milne e t a l (1977), however, have r e c e n t l y pointed out that t h e r e i s a s y s t e m a t i c b i a s i n e p i c e n t r a l l o c a t i o n s f o r the earthguakes o f f Canada's west c o a s t . On the b a s i s of t h i s work and r e s e a r c h c u r r e n t l y i n progress i t appears l i k e l y that the m a j o r i t y of earthquakes are l o c a t e d on the E x p l o r e r Ridge/Revere-Dellwood f r a c t u r e zone and not i n the basin i t s e l f . The area of Winona Basin was p a r t of the o r i g i n a l magnetic survey by Raff and Mason (196 1) which provided much of 1 the evidence f o r the i n t r o d u c t i o n of the s e a - f l o o r s p r e a d i n g concept. In Winona Basin however, the anomalies show none of the magnetic l i n e a t i o n s normally a s s o c i a t e d with a s p r e a d i n g c e n t r e , the magnetic s t r u c t u r e of the b a s i n being r e l a t i v e l y smooth. There are normal magnetic l i n e a t i o n s to the west i n the r e g i o n of E x p l o r e r Ridge; however, they terminate 13 a t P a u l Revere Ridge and do not p e n e t r a t e t h e b a s i n i t s e l f . J.s-2 Tect on i c S i g n i f i c a n c e of Winona B a s i n The p o s i t i o n of t h e t r i p l e p o i n t between the P a c i f i c -N o r t h American and E x p l o r e r p l a t e s has been a s o u r c e of c o n j e c t u r e f o r some t i m e . E a r l y work by S r i v a s t a v a et a l (1971) and B e r t r a n d (1972) have s u g g e s t e d t h a t t h e Dellwood K n o l l s , the n o r t h e r n boundary o f Winona B a s i n , marks t h e p r e s e n t l o c a t i o n o f the t r i p l e p o i n t . Chase (1975) on the o t h e r hand has s u g g e s t e d t h a t the t r i p l e p o i n t does not e x i s t as a d i s c r e t e p o i n t but r a t h e r i t e x i s t s as a broad a r e a of d e f o r m a t i o n . B a s i n g t h e i r c o n c l u s i o n s on the. l a c k o f d e f o r m a t i o n of t h e s h e l f s e d i m e n t s n o r t h of Brooks f r a c t u r e zone as opposed t o t h e extreme d e f o r m a t i o n of t h e s h e l f s e d i m e n t s t o the s o u t h of t h e f r a c t u r e zone, Murray and T i f f i n (1974) have proposed t h a t the t r i p l e p o i n t was s t a b l e a t t h e Brooks f r a c t u r e zone u n t i l a p p r o x i m a t e l y 4 mya. T h i s d i v i d e s t h e c o n t i n e n t a l marqin o f f n o r t h e r n Vancouver I s l a n d i n t o two d i f f e r e n t r e g i m e s : s u b d u c t i o n , u p l i f t and d e f o r m a t i o n t o the s o u t h of 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 n o r t h . They have s u g g e s t e d , however, t h a t the t r i p l e p o i n t has m i g r a t e d n o r t h i n t h e l a s t 4 -my-,- r e s u l t i n g i n the f o r m a t i o n of the Winona' B a s i n by the moving t r i p l e p o i n t . T h i s would r e q u i r e the c r u s t u n d e r l y i n g t h e b a s i n t o be l e s s than 4 my o l d . Such a s u g g e s t i o n i s s u p p o r t e d by r e c e n t work of R i d d i h o u g h (1977). From a ' d e t a i l e d r e - e x a m i n a t i o n o f the 14 e x i s t i n g magnetic anomaly p a t t e r n s , he has c a l c u l a t e d t h e E x p l o r e r - J u a n de Fuca p l a t e motions f o r the past 10 my. H i s r e s u l t s show t h a t the t r i p l e p o i n t remained 'remarkably s t a b l e near th e Brooks f r a c t u r e zone u n t i l about 2 to 3 mya whereupon i t s t a r t e d a slow n o r t h w a r d m i g r a t i o n . I t i s p o s s i b l e t h e n t h a t Winona B a s i n has been c r e a t e d very r e c e n t l y by t h e slow northward p r o g r e s s i o n o f the P a c i f i c - N o r t h A m e r i c a n - E x p l o r e r (P-A-E) p l a t e t r i p l e p o i n t . Another matter of debate i s the p o s i t i o n of t h e P a c i f i c - ^ N o rth America p l a t e boundary s o u t h of 51° N. North of 5 1° t h e boundary i s d e f i n e d by t h e Queen C h a r l o t t e t r a n s f o r m f a u l t zone. To the s o u t h , however, i t s l o c a t i o n i s l e s s w e l l e s t a b l i s h e d . I f i n d e e d t h e Dellwood K n o l l s do form the p o s i t i o n of the P-A-E t r i p l e p o i n t , t h e n s o u t h o f 5 1° the P a c i f i c and American p l a t e s a r e s e p a r a t e d by Winona B a s i n . T h i s i d e a r e q u i r e s t h a t Winona B a s i n be p a r t o f t h e E x p l o r e r p l a t e , or a t l e a s t t h a t i t be i n d e p e n d e n t of t h e American p l a t e . B a r r and Chase (1974) s u g g e s t e d t h a t the P a c i f i c - A m e r i c a n p l a t e boundary now l i e s a l o n g a l i n e c o n n e c t i n g t h e n o r t h e r n edge of Juan de Fuca Ridge w i t h t h e Queen C h a r l o t t e f a u l t i n the. r e g i o n o f 51° N, 131° W. T h i s was s u g g e s t e d on the b a s i s o f the a v a i l a b l e e a r t h q u a k e data and r e q u i r e s t h a t the f a u l t e x t e n d t h r o u g h Winona B a s i n . T h i s would e s s e n t i a l l y i s o l a t e E x p l o r e r Ridqe and t h u s r e n d e r i t i n a c t i v e . However, r e c e n t ocean bottom seismometer r e s u l t s (G.C. R o q e r s , p e r s o n a l c o mmunication, 1977) have e s t a b l i s h e d t h e presence o f s e i s m i c 15 a c t i v i t y on t h e n o r t h e r n branch of E x p l o r e r Ridge. T h i s , p l u s t h e h i g h heat f l o w v a l u e s d e t e c t e d i n t h e r e g i o n by S r i v a s t a v a e t a l (1971), and the f r e s h b a s a l t dredged by B e r t r a n d (1972) i n t h e r e g i o n o f E x p l o r e r R i d g e , make the c o n c l u s i o n o f i n a c t i v i t y of the r i d g e u n a c c e p t a b l e . I n a d d i t i o n t o t h i s , a r e c e n t r e - e x a r a i n a t i o n of t h e e a r t h q u a k e d a t a f o r t h e a r e a ( M i l n e e t a l 1977) has shown s e r i o u s s y s t e m a t i c b i a s e s i n the dat a w i t h t h e r e s u l t t h a t most of the e a r t h q u a k e s a r e now b e l i e v e d t o f o l l o w the Ee v e r e - D e l l w o o d f r a c t u r e zone system. I t i s u n l i k e l y then t h a t the Queen C h a r l o t t e F a u l t e x t e n d s t h r o u g h Winona B a s i n to t h e Juan de Fuca R i d g e . The r e c e n t r e l o c a t i o n o f the e a r t h q u a k e d a t a f o r t h e a r e a s u p p o r t s t h e i d e a o f Chase et a l (1975) t h a t t h e Revere-Dellwood f r a c t u r e zone i s a t r a n s f o r m f a u l t . U n f o r t u n a t e l y t h i s does not c l a r i f y t he p o s i t i o n of Winona B a s i n t o any g r e a t e x t e n t . The b a s i n c o u l d be an i s o l a t e d s e c t i o n of o l d P a c i f i c p l a t e m a t e r i a l , an i n t e q r a l p a r t o f the North American p l a t e o r a r e c e n t l y formed a d d i t i o n to the E x p l o r e r - J u a n de Fuca p l a t e . I n any e v e n t , i n o r d e r t o c o m p l e t e l y u n d e r s t a n d t h e r o l e Winona B a s i n p l a y s i n the complex t e c t o n i c s of t h i s r e q i o n , a d d i t i o n a l g e o l o g i c a l and q e o p h y s i c a l d a t a a re r e q u i r e d . I t was w i t h t h i s purpose i n mind t h a t a deep s e i s m i c s o u n d i n q s u r v e y was r u n over t h e a r e a o f Winona B a s i n d u r i n q the summer of 1975. 16 l i . 2 Data Acgu i s i t i o n and P r o f i l e D e s c r i p t i o n P r o j e c t D e s c r i p t i o n : The main o b j e c t i v e o f t h i s p r o j e c t i s to s u p p l y 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 beneath Winona B a s i n , i n o r d e r t h a t i t s r o l e i n the l o c a l t e c t o n i c s may be f u r t h e r u n d e r s t o o d . To t h i s end t h r e e r e v e r s e d deep s e i s m i c s o u n d i n g (DSS) p r o f i l e s were r u n over t h e b a s i n , t h e i r l o c a t i o n s b e i n g shown on F i g 1.2. The deep s e i s m i c s o u n d i n g method (Clowes, 1977), e n a b l e s the r e c o r d i n q of s u b - c r i t i c a l r e f l e c t i o n s , wide a n g l e r e f l e c t i o n s and r e f r a c t i o n a r r i v a l s , w i t h p e n e t r a t i o n b e i n q a c h i e v e d from the ocean bottom t o the upper mantle. As a method o f d e l i n e a t i n g t h e s h a l l o w s e d i m e n t a r y s t r u c t u r e of t h e b a s i n , s h o r t sub-c r i t i c a l i n c i d e n c e r e f l e c t i o n p r o f i l e s . w i t h the s h o t s at 7 m depth were r e c o r d e d p a r a l l e l t o t h e s t a r t of each complete DSS p r o f i l e and at t h e p o i n t s o f i n t e r s e c t i o n of the cross> p r o f i l e s w i t h p r o f i l e s 75-1 and 75-1R. I t was hoped t h a t by p l a c i n g t h e s h o t s a t t h i s s h a l l o w depth t h e y would blow out and m i n i m i s e t h e b u b b l e p u l s e problem. To d e l i n e a t e t h e deep s e d i m e n t a r y s t r u c t u r e and p o s s i b l y o b t a i n sub-basement r e f l e c t i o n s , more energy was needed than c o u l d be o b t a i n e d by p l a c i n g t h e s h o t s a t 7 m. As a r e s u l t , s u b - c r i t i c a l i n c i d e n c e p r o f i l e s were run u s i n g t h e s o u r c e a t 45 m depth. These p r o f i l e s c o m p r i s e d the s t a r t o f each DSS p r o f i l e , thus e n a b l i n g the i n t e r p r e t e r t o t r a c e the t r a n s i t i o n from 17 r e f l e c t i o n t o r e f r a c t i o n a r r i v a l s . C o n t i n u o u s s e i s m i c p r o f i l e s were a l s o run a l o n g the l e n g t h o f each o f t h e t h r e e r e v e r s e d r e f r a c t i o n l i n e s . A 300 cu i n a i r gun was used as the s o u r c e w i t h t h e r e c o r d i n g s b e i n g on s t a n d a r d e l e c t r o s t a t i c paper. These p r o f i l e s s u b s e q u e n t l y were a n a l y s e d by Hopkins (1976). Data A c q u i s i t i o n : The method of d a t a a c q u i s i t i o n f o r t h e deep s e i s m i c sounding p r o j e c t i s s i m i l a r i n p r i n c i p l e t o t h e two s h i p r e f r a c t i o n t e c h n i q u e d e s c r i b e d by Shor (1963). O p e r a t i o n s at sea r e g u i r e one s h i p , i n t h i s c a s e C.F.A.V. Endeavour, to d r i f t f r e e l y and a c t as t h e r e c o r d i n g s h i p w h i l e the second v e s s e l , C.F.A.V. Laymore, proceeds a l o n g a p r e d e t e r m i n e d path r e l e a s i n g t h e e x p l o s i v e s . D e t a i l e d d e s c r i p t i o n s of the s h o o t i n g p r o c e d u r e s and d a t a a c q u i s i t i o n system a r e p r o v i d e d by Malecek (1976) and Clowes (1977). For t h i s r e a s o n o n l y a b r i e f d e s c r i p t i o n of t h e methods and system w i l l be q i v e n here. Two t y p e s o f s h o o t i n g p r o c e d u r e s were used d u r i n g the p r o f i l e s . G e o g e l , a c o m m e r c i a l e x p l o s i v e , was used as the e x p l o s i v e s o u r c e f o r t h e r e f l e c t i o n p r o f i l e s and out t o a d i s t a n c e o f 70 km on the r e f r a c t i o n p r o f i l e s . The c h a r g e s ranged i n s i z e from 2.3 kg (5 l b ) to 96 kg (200 l b s ) and were suspended i n t h e water by t w i n e a t t a c h e d t o l a r g e r e d p a r t y b a l l o o n s . D e t o n a t i o n was a c c o m p l i s h e d by use o f a timed f u s e / S e i s m o c a p assembly and P r i m a c o r d . For t h e s e s h o t s , the 18 s h o t - t o - s h i p d i s t a n c e was measured by use of a r a n g e f i n d e r f o c u s s e d on t h e p a r t y b a l l o o n s . Beyond 70 km the e x p l o s i v e s o u r c e c o n s i s t e d of t h r e e Hark IV H.E. a n t i - s u b m a r i n e p r o j e c t i l e s per s h o t . Each p r o j e c t i l e c o n t a i n e d the e q u i v a l e n t o f 94 kg o f M i n o l h i q h e x p l o s i v e , r e s u l t i n q i n an e q u i v i l e n t c h a r qe s i z e o f 282 kq per s h o t . Due to t h e i r 68 kq c a s t i n g s t h e p r o j e c t i l e s were f a r t o o heavy t o be suspended by t h e p a r t y ballo.bns. As an a l t e r n a t i v e , t h e y were suspended from a r a f t made o f empty 45 q a l l o n drums. D e t o n a t i o n was s i m i l a r t o t h a t used f o r t h e commercial e x p l o s i v e s w i t h t h e e x c e p t i o n t h a t the bombs were primed w i t h p l a s t i c e x p l o s i v e . As a s a f e t y p r e c a u t i o n , the bombs were d e t o n a t e d at d i s t a n c e s i n excess o f 1 km from the s h o o t i n q s h i p . As a r e s u l t , t h e s h o t - t o - s h i p d i s t a n c e had to be deter m i n e d by the s h i p ' s r a d a r . The s h i p - t o - s h i p d i s t a n c e s were d e t e r m i n e d by r a d a r o u t to a d i s t a n c e of a p p r o x i m a t e l y 22 km. Beyond t h i s , LORAN A f i x e s had t o be used t o deter m i n e the s h i p ' s r e l a t i v e p o s i t i o n s . The d i r e c t water wave (D.W.W.) was d e t e c t e d a t the s h o o t i n g s h i p by means o f a hydrophone t r a i l e d i m m e d i a t e l y b e h i n d the s h i p , and by a geophone l o c a t e d on the s h i p ' s deck. The two s i g n a l s were r e c o r d e d s i m u l t a n e o u s l y w i t h t he WWVB time code on a 4 c h a n n e l FM tape t r a n s p o r t . The hydrophone and WWVB s i g n a l s were r e c o r d e d d i r e c t l y on a 2-ch a n n e l Brush c h a r t r e c o r d e r p l a y e d a t a speed of 125 mm/s. These r e c o r d i n q s were used t o tim e t h e D.W.W. and the data r e c o r d e d on ta p e was used 19 as a backup. The r e c e i v i n g s h i p t r a i l e d a 610 m c a b l e from which, at i n t e r v a l s of 91 n, 6 i n d i v i d u a l hydrophone systems and b a t t e r y boxes were suspended t o a depth of 45 m. The b a t t e r y box was de-c o u p l e d from t h e main c a b l e and hence s u r f a c e wave a c t i o n by _ shock c o r d . In o r d e r t o p r o v i d e a d d i t i o n a l m e c h a n i c a l damping, t h e hydrophone and a 15 m c a b l e l e a d i n g t o the b a t t e r y box were made n e u t r a l l y bouyant by a t t a c h i n g f l o t a t i o n m a t e r i a l . The s i g n a l o u t p u t from the hydrophone element i s p r e -a m p l i f i e d by 20 db and t r a n s m i t t e d t o a m p l i f i e r s i n the s h i p ' s l a b o r a t o r y . They a r e f i l t e r e d u s i n g l i m i t s o f 0.0 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 f i x e d g a i n a m p l i f i e r s , m a n u a l l y s e t f o r each s h o t . The s i x an a l o g s i g n a l s p l u s WWVB ti m e code a r e d i g i t i z e d on board the r e c e i v i n g s h i p at a fr e g u e n c y of 312.5 hz and then w r i t t e n onto magnetic tape u s i n g an I.B.M. c o m p a t i b l e , 14 b i t , m u l t i - c h a n n e l d a t a a c q u i s i t i o n system (Clowes 1977). F i v e of the s i x da t a c h a n n e l s p l u s t h e WWVB t i m e code a r e monitored on a s i x -c h a n n e l c h a r t - r e c o r d e r t o ensure good g u a l i t y c o n t r o l o f the da t a . U n f o r t u n a t e l y , n e i t h e r t h e r e c o r d i n g s h i p nor the s h o o t i n g s h i p had o p e r a t i o n a l depth s o u n d i n g eguipment, a l t h o u g h t h i s was supposed t o be a v a i l a b l e . Thus no r e c o r d of th e sea f l o o r topography was o b t a i n e d d u r i n g the c r u i s e . 20 2 PRELIMINARY ANALYSIS ZsA D e m u l t i p l e x i n c j As d i s c u s s e d i n S e c t i o n 1.4, t h e seven c h a n n e l d a t a s e t used i n t h i s s t u d y were r e c o r d e d i n m u l t i p l e x e d form on two d i g i t a l f i e l d t a p e s . A s a m p l i n g r a t e o f 312.5 hz was used to d i g i t i z e the a n a l o g s i g n a l s on board t h e s h i p . Thus th e s a m p l i n g i n t e r v a l of t h e d a t a i s .0032 s and t h e N y g u i s t f r e g u e n c y i s 156 hz. T h i s high s a m p l i n g r a t e was chosen to e l i m i n a t e any a l i a s i n g problems a s s o c i a t e d w i t h the h i g h - c u t f i l t e r (100 hz w i t h a 12db/octave r o l l o f f ) of t h e a n a l o g a m p l i f i e r s . F a r more data were s t o r e d on the f i e l d t a p e s t h a n was a c t u a l l y needed. For t h i s r e a s o n the d a t a were d e m u l t i p l e x e d and e d i t e d s i m u l t a n e o u s l y as a p r e l i m i n a r y s t e p t o the a n a l y s i s . A p p r o x i m a t e l y t h r e e seconds o f d a t a were kept b e f o r e t h e f i r s t a r r i v a l f o r b o t h r e f l e c t i o n and r e f r a c t i o n d a t a . The t e r m i n a t i o n f o r t h e r e f l e c t i o n d ata was chosen to be i m m e d i a t e l y a f t e r t h e end of t h e second water bottom r e f l e c t i o n whereas the c u t o f f f o r the r e f r a c t i o n data was chosen to be i m m e d i a t e l y a f t e r t h e a r r i v a l o f the d i r e c t water wave and s u p e r - c r i t i c a l water bottom r e f l e c t i o n s . A number of problems were e n c o u n t e r e d d u r i n g the d e m u l t i p l e x i n g o f t h e d a t a . D u r i n g t h e p r o c e s s of d i g i t i z i n g and w r i t i n g t h e a n a l o g s i g n a l s from the hydrophones onto t a p e , t h e d i g i t i z e r o c c a i s i o n a l l y " l o s t " d a t a . Clowes (1977) has 21 d e s c r i b e d how such e r r o r s can o c c u r w i t h t h e marine data a c g u i s i t i o n s y s t e m . A l s o on a number of o c c a s i o n s the s h i p board o p e r a t o r s f a i l e d t o w r i t e "End o f F i l e s " a t the t e r m i n a t i o n of the d a t a , r e s u l t i n g i n the c o n c a t e n a t i o n of s e v e r a l d a t a f i l e s . At t h e time t h i s s t u d y was begun, no d e m u l t i p l e x i n g program t h a t would i d e n t i f y and c o r r e c t t h e s e e r r o r s was a v a i l a b l e i n the department. As a r e s u l t , c o n s i d e r a b l e e f f o r t was expended i n w r i t i n g a program t h a t would handle a l l t h e e r r o r s a s s o c i a t e d w i t h t h e f i e l d d a t a . T h i s program i s now a v a i l a b l e f o r g e n e r a l use. 2.2 O r i g i n Times I n a l l s e i s m i c work a c c u r a t e d e t e r m i n a t i o n o f the time between the o r i g i n a l s e i s m i c i m p u l s e , i n our c a s e the e x p l o s i v e d e t o n a t i o n , and the a r r i v a l of the s i g n a l a t a c e r t a i n d i s t a n c e i s o f p r i m a r y i m p o r t a n c e . As a r e s u l t of the f i e l d t e c h n i q u e used, t h e major s o u r c e of e r r o r i n the t r a v e l t i m e c a l c u l a t i o n s i s t h a t a s s o c i a t e d w i t h d e t e r m i n i n g t h e o r i g i n t i m e o f t h e o r i g i n a l s e i s m i c i m p u l s e . S i n c e the e x p l o s i v e c h a r g e s a r e d e t o n a t e d a t d e p t h , remote from th e s h i p and by a timed f u s e / S e i s i a o c a p assembly, i t i s i m p o s s i b l e to know the e x a c t i n s t a n t o f d e t o n a t i o n . Thus we must use the known d i s t a n c e and depth of t h e s h o t p l u s the known t i m e of a r r i v a l of the e x p l o s i v e i m p u l s e a t the hydrophone t r a i l e d b e h i n d the s h i p t o e x t r a p o l a t e back t o the o r i g i n time of the s h o t . The paper c h a r t r e c o r d i n g s d e s c r i b e d i n s e c t i o n 1.3 were 22 t h e o n l y d a t a s e t used to time t h e d i r e c t a r r i v a l s . S u f f i c i e n t a c c u r a c y was o b t a i n e d u s i n q t h e s e r e c o r d i n g s and hence the . back-up d a t a on the FM t a p e s was never used. The a r r i v a l of the s i g n a l a t t h e hydrophone c o u l d be timed to b e t t e r than 5 ms from these c h a r t r e c o r d i n g s . The method of measuring t h e s h i p t o s h o t d i s t a n c e has been d e s c r i b e d i n s e c t i o n 1.3. The e r r o r i n t h e r a n g e f i n d e r method i s e s t i m a t e d t o be l e s s t h a n 15%, which t r a n s l a t e s to an e r r o r of 15 i f o r the near s h o t s (100 m) and 45 m f o r the f a r t h e s t s h o t s (300 m) i n which t h i s method was used. U s i n g a water v e l o c i t y o f 1.49 km/s, t h e s e d i s t a n c e e r r o r s t r a n s l a t e to t i m i n g e r r o r s o f 10 ms and 30 ms, r e s p e c t i v e l y . The r a d a r method o f d e t e r m i n i n g the d i s t a n c e s t o the a n t i - s u b m a r i n e p r o j e c t i l e s was c o m p l e t e l y u n r e l i a b l e . The 45 g a l l o n drums were so w e i g h t e d down by the p r o j e c t i l e s t h a t they b a r e l y showed above the s u r f a c e o f the water. T h i s made them v e r y poor r e f l e c t o r s and thus the r a d a r ranges o b t a i n e d were u s e l e s s . On subsequent c r u i s e s a r a d a r r e f l e c t o r s h o u l d be p l a c e d on t h e drums. I was a b l e , however, to d e v i s e an a l t e r n a t e method of o b t a i n i n g the d i s t a n c e s t o t h e s e s h o t s . C o n s i d e r t h e f o l l o w i n g diagram : 23 The method r e q u i r e s o n l y a knowledge o f t h e t r a v e l time d i f f e r e n c e between t h e d i r e c t a r r i v a l and the bottom bounce, p l u s the water depth t o make i t a p p l i c a b l e to our d a t a . The t i m e s of a r r i v a l o f t h e d i r e c t wave T< and t h e bottom bounce Tz a r e r e c o r d e d and hence I * - Tt i s determined w i t h o u t knowinq t h e a c t u a l l e n g t h of e i t h e r Tz o r T i . I n e s s e n c e , the s h o t a t depth D c o u l d be moved t o the s u r f a c e f o r the bottom r e f l e c t i o n by a p p l y i n g t h e c o r r e c t i o n Tc=D*cos9/1.49, where t y p i c a l l y cos£-0.97. Then i f we assume t h a t the s u r f a c e - t o -s u r f a c e t r a v e l path T^ i s the same as the Tz t r a v e l path we have t h e f o l l o w i n g : f,= ( + d*)**/l-U1 2.2-1 T2= I V- tl,2l)''X /htl 2.2-2 II"]",-- f f * S * Z * / ' * - ( x % 0 ' ' \ ? / U 7 2.2-3 By assuming a r e a s o n a b l e v a l u e f o r the depth Z, we can use 2.2-3 by t r i a l and e r r o r t o f i n d t h e v a l u e of X y i e l d i n g the c o r r e c t v a l u e of T^- T i . I t i s p o s s i b l e t o perform t h e same t y p e of a n a l y s i s as above u s i n g the f i r s t and second bottom r e f l e c t i o n s . Such a method would y i e l d X i n d e p e n d e n t l y o f t h e depth Z. However I was not a b l e to t i m e a c c u r a t e l y t h e second bottom bounce and hen'ce c o u l d not a p p l y t h i s p r o c e d u r e . The e r r o r i n the T.j_- T i method i s e s t i m a t e d t o be £100 m or c o r r e s p o n d i n g l y +66 ms u s i n g a 1.49 km/s water v e l o c i t y ; t h e main s o u r c e of e r r o r i s t h e l a c k of an a d e q u a t e l y d e f i n e d water depth. I t i s h i g h l y recommended t h a t f u t u r e c r u i s e s have w o r k i n g depth s o u n d i n g equipment as the l a c k o f adequate depth 24 knowledge was a h i n d r a n c e not o n l y i n t h e s e c a l c u l a t i o n s but i n s e v e r a l o t h e r p l a c e s t h r o u g h o u t the s t u d y . C o r r e c t i o n s t o o b t a i n the t r u e o r i g i n t ime o f t h e s h o t were c a l c u l a t e d u s i n g 2.2-1. The r e s u l t i n g c o r r e c t i o n s ranged from 70 £15 ms f o r t h e near s h o t s u s i n g the b a l l o o n s t o 250 £35 ms f o r t h e f a r t h e s t s h o t s u s i n g b a l l o o n s , and 1000 170 ms f o r t h e s h o t s u s i n g t h e a n t i - s u b m a r i n e p r o j e c t i l e s . 2.3 S h o t - R e c e i v e r D i s t a n c e s Once the o r i g i n t i m e o f t h e s h o t has been d e t e r m i n e d , the d i s t a n c e o f t h e i n d i v i d u a l hydrophones from each s h o t must be c a l c u l a t e d . T h i s i s done by t i m i n g the a r r i v a l of the d i r e c t water wave (D.W.W.) from the s h o t at the v a r i o u s hydrophones. From a knowledge o f t h i s a r r i v a l t i m e , the o r i g i n time of the s h o t , and assuming a c o n s t a n t water v e l o c i t y {1.49 km/s) , the s h o t - r e c e i v e r d i s t a n c e s can be c a l c u l a t e d . I n so d o i n g , i t i s assumed t h a t t h e d i f f e r e n c e s i n the shot and r e c e i v e r depths a r e i n s i g n i f i c a n t . The d e m u l t i p l e x e d r e c o r d s of each sh o t were p l o t t e d a t a d e n s i t y of 100 ppinti/in ( i . e . 1s=3.125 i n ) each p o i n t b e i n g one d i g i t i z e d v a l u e . The s i x s e i s m i c c h a n n e l s and W.8.V.B. were p l o t t e d s i m u l t a n e o u s l y , each t r a c e b e i n q n o r m a l i s e d to a maximum a m p l i t u d e o f 0.75 i n . The f o u r seismograms shown on F i g 2.1 a r e t y p i c a l o f t h e s e r e c o r d s . The d i r e c t water wave was t h e n i d e n t i f i e d and t h e phase on s e t was t i m e d w i t h r e s p e c t t o t h e n e a r e s t second by u s i n q a m i l l i m e t e r r u l e r . T i m i n q o f , f i r s t b r e a k s f o r a l l c l e a r D.W.B. 25 Four seismograms from p r o f i l e 75-1, t y p i c a l o f t h o s e used t o t i m e t h e d i r e c t water wave and t h e f i r s t r e f r a c t i o n a r r i v a l s . Approximate s h o t - r e c e i v e r d i s t a n c e s a r e ; A) 0.5 km, B) 9.5 km, C) 94 km, D) 44 km. The upper t r a c e o f each seismogram i s the WWVB t i m e code. I n ( A ) , t h e d i r e c t a r r i v a l i s ' o v e r l o a d i n g the h y d r o p h o n e / a m p l i f i e r c o m b i n a t i o n . 26 27 a r r i v a l s was b e t t e r than 0.5 mm, c o r r e s p o n d i n g t o b e t t e r than 7ms i n time . N a t u r a l l y not a l l s h o t - r e c e i v e r d i s t a n c e s c o u l d be c a l c u l a t e d t h i s s i m p l y . At d i s t a n c e s g r e a t e r t h a n two k i l o m e t e r s a l o n g t h e r e f l e c t i o n p r o f i l e s u s i n g s h o t s at 7 m dep t h , t h e d i r e c t wave became i n d i s t i n c t compared t o the bottom bounce. T h i s was p r o b a b l y due to t h e energy from the s h o t s b e i n g t r a p p e d near the s u r f a c e of the water and the hydrophones b e i n g too deep to d e t e c t i t . T h i s problem was not observed on the r e f l e c t i o n p r o f i l e s u s i n g s h o t s a t 45 m depth, i . e . w i t h the s h o t s t h e same depth as the hydrophones. I n any case t h i s was not c a t a s t r o p h i c , t h e s o l u t i o n s i m p l y b e i n g to n o r m a l i s e the s e c t i o n of the t r a c e c o n t a i n i n g D.W.W. a r r i v a l s i n d e p e n d e n t l y of t h e l a r g e a m p l i t u d e bottom r e f l e c t i o n . T h i s p r o c e e d u r e a l l o w e d t h e D.W.W. t o be tim e d to b e t t e r t h a n 1mm or 13 ms f o r these s h o t s . Along t h e r e f r a c t i o n p a r t o f the p r o f i l e , the D.W.W. c o u l d be timed t o b e t t e r than 0.5 mm (7ms) f o r a l l s h o t s c l o s e r t h a n 25 km. In the range 25-55 km, the D.W.W. became ve r y emergent and I found i t e a s i e r to t i m e the bottom bounce i n s t e a d . By assuming a v a l u e f o r the water depth a t the middle of t h e s h o t , i t was then p o s s i b l e to c a l c u l a t e the s h o t -r e c e i v e r d i s t a n c e . Once a g a i n the l a c k o f adequate depth s o u n d i n g was the major s o u r c e o f e r r o r , w i t h the poor as s u m p t i o n of the depth b e i n g t he l i m i t i n g f a c t o r i n the ac c u r a c y . I t was p o s s i b l e on some s h o t s to t i m e both the D.W.W. and the bottom bounce. In t h e s e c a s e s , the two r e s u l t s agreed to w i t h i n 50 m f o r a l l c a s e s t e s t e d . Beyond 55 km, the 28 D.W.W. and.the bottom r e f l e c t i o n a r r i v e d so c l o s e t o g e t h e r as to be i n d i s t i n g u i s h a b l e . The combined phases were somewhat emergent from the background n o i s e of l a t e r e f r a c t i o n a r r i v a l s and r e v e r b e r a t i o n s , c a u s i n g t h e f i r s t break, i n t h e s e c a s e s to be de t e r m i n e d t o b e t t e r than 4ram or 50 ms. The t o t a l e r r o r c a l c u l a t e d f o r the s h o t - r e c e i v e r d i s t a n c e i n c o r p o r a t e s not o n l y t h e t i m i n g e r r o r o f the D.W.W. and bottom r e f l e c t i o n s , b u t a l s o the e r r o r i n the o r i g i n t i m e . The t o t a l e s t i m a t e d e r r o r i n t h e s h o t - r e c e i v e r d i s t a n c e s a r e t h e r e f o r e : TABLE 2. 1 R e f l e c t i o n Data D i s t a n c e Range km Shot Depth m Timing E r r o r ms D i s t a n c e E r r o r ra 0-2 2-4 0-4 7 7 45 <22 <30 <22 <30 <45 <30 R e f r a c t i o n Data D i s t a n c e Range km Timi n g E r r o r ms D i s t a n c e E r r o r m <22 <70 < 120 <30 <100 <200 29 Thus i n a l l c a s e s the d i s t a n c e s c o u l d be d e t e r m i n e d t o b e t t e r than 1.5% and i n most c a s e s to b e t t e r than 1%. ZSLH R e c o r d Sect i o n s Record s e c t i o n s of a l l r e f l e c t i o n and r e f r a c t i o n p r o f i l e s were c o m p i l e d u s i n g t h e program RSEC w r i t t e n by the author and now a v a i l a b l e f o r g e n e r a l use. As one might e x p e c t , i t was not s u f f i c i e n t t o s i m p l y c o m p i l e the raw d e m u l t i p l e x e d d a t a s i n c e any a m p l i t u d e i n f o r m a t i o n would be l o s t i f no a p p r o p r i a t e c o r r e c t i o n s were a p p l i e d . To o b t a i n m e a n i n g f u l i n f o r m a t i o n from the a m p l i t u d e s i t was n e c c e s s a r y t o compensate f o r t h e e f f e c t of s p h e r i c a l s p r e a d i n g , v a r y i n g c h a r g e s i z e and v a r y i n g a m p l i f i e r g a i n . A m p l i f i e r G a i n : The am'bient n o i s e l e v e l a t each hydrophone v a r i e d g r e a t l y due to the p o s i t i o n i n g o f the main c a b l e and maneuvering o f t h e s h i p . In o r d e r t o m a i n t a i n a s i m i l a r background n o i s e l e v e l on a l l t r a c e s and t o p r e v e n t the t r a n s i e n t n o i s e i m p u l s e s ( e s p e c i a l l y at h i g h gain) from o v e r l o a d i n g the a m p l i f i e r s , d i f f e r e n t g a i n s were used on d i f f e r e n t a m p l i f i e r s . The g a i n (G) i s d e f i n e d by : G = 20Log (Vout) Vout-=output v o l t a g e f o r a 1 v o l t i n p u t -6 To remove t h i s e f f e c t a c o r r e c t i o n o f 10 7-0 was a p p l i e d to a l l t r a c e s , i n e f f e c t n o r m a l i s i n g a l l a m p l i t u d e s to a chosen c o n s t a n t g a i n . 30 Charge S i z e : For a c h a r g e weight o f W pounds, O'Brien (1960) and Mueller (1962) have shown t h a t s e i s m i c a m p l i t u d e s r e c o r d e d at sea v a r y a p p r o x i m a t e l y as In o r d e r to remove the e f f e c t on a m p l i t u d e s of u s i n g v a r i o u s charge s i z e s a l o n g t h e p r o f i l e s , c o r r e c t i o n s o f W~2/3 were a p p l i e d t o a l l t r a c e s f o r a s i n g l e s h o t . For t h e r e f l e c t i o n p r o f i l e s and out t o a d i s t a n c e of 67 km (shot 54) on t h e r e f r a c t i o n p r o f i l e s , c o m m e r c i a l e x p l o s i v e s were used as energy s o u r c e s . Beyond 67 km, however, t h e energy s o u r c e f o r each s h o t c o n s i s t e d of t h r e e a n t i - s u b m a r i n e p r o j e c t i l e s h a v i n g an e q u i v a l e n t y i e l d of 282 kg. There p r o b a b l y e x i s t s a d i f f e r e n c e i n energy y i e l d between t h e bombs and the c o m m e r c i a l e x p l o s i v e s due to the d i f f e r e n t e x p l o s i v e t y p e s and the s t e e l p a c k a g i n g of the bombs. However, no c o r r e c t i o n f o r t h i s d i f f e r e n c e has been made, but i t s h o u l d be no t e d . S p h e r i c a l S p r e a d i n g : Cerveny and R a v i n d r a (1971) have shown t h a t head wave a m p l i t u d e s at l a r g e d i s t a n c e s decrease as 1 / r 2 ; near the c r i t i c a l d i s t a n c e , however, t h e drop i n a m p l i t u d e i s c l o s e r to 1 / r 3 or 1/r*. Even so, i t was d e c i d e d t o a p p l y a u n i f o r m a m p l i t u d e c o r r e c t i o n o f r 2 t o t h e r e f r a c t i o n seismograms to c o r r e c t f o r s p h e r i c a l s p r e a d i n g . T h i s i s not e n t i r e l y a p p r o p r i a t e as B r a i l l e and Smith (1975) have shown t h a t wide a n g l e r e f l e c t i o n a m p l i t u d e s d e c r e a s e as 1/r. Thus the r 2 31 f a c t o r w i l l e n l a r g e t h e s e a m p l i t u d e s o u t o f p r o p o r t i o n t o t h e r e f r a c t i o n a m p l i t u d e s . However t h e s e same c o r r e c t i o n s a r e a p p l i e d t o t h e s y n t h e t i c s e i s m o g r a m s s o t h e c o m p e n s a t i o n i s c o n s i s t e n t . To c o m p e n s a t e f o r t h e e f f e c t o f s p h e r i c a l s p r e a d i n g on t h e s u b - c r i t i c a l r e f l e c t i o n d a t a a f a c t o r o f r l was a p p l i e d to t h e d a t a . T h i s i s a l s o n o t e n t i r e l y a p p r o p r i a t e , B r a i l l e and S m i t h (1975) h a v i n g shown t h a t r e f l e c t i o n a m p l i t u d e s i n a l a y e r e d medium d r o p o f f between r ~ l and r - 1 - 5 . I t was d e c i d e d however t h a t a c o r r e c t i o n of r 1 was s a t i s f a c t o r y s i n c e a m p l i t u d e s were n o t b e i n g used i n o t h e r t h a n a g u a l i t a t i v e manner. Hydr o p h o n e S e n s i t i v i t i e s : Two o f t h e h y d r o p h o n e s were new and unused whereas t h e o t h e r s were p u r c h a s e d a few y e a r s b e f o r e and had been used t o v a r y i n g e x t e n t s on p r e v i o u s t r i p s . B o th age and e x t e n t of use t e n d t o a f f e c t t h e s e n s i n g e l e m e n t ; as a r e s u l t , t h e h y d r o p h o n e s were o f d i f f e r i n g s e n s i t i v i t i e s . C o n s e g u e n t l y , a f t e r a l l t h e o t h e r c o r r e c t i o n s had been a p p l i e d t h e t r a c e s from e a c h c h a n n e l f o r a p a r t i c u l a r s h o t d i f f e r e d s i g n i f i c a n t l y i n . a m p l i t u d e . As a c o r r e c t i o n f o r t h i s e f f e c t , t h e a m p l i t u d e o f e a c h c h a n n e l was m u l t i p l i e d by an a p p r o p r i a t e c o r r e c t i o n f a c t o r t o a p p r o x i m a t e a c o n s t a n t a m p l i t u d e f o r a g i v e n p h a s e . I t i s d e s i r a b l e i n t h e c o m p i l a t i o n o f any m arine s e i s m i c r e c o r d s e c t i o n t o remove t h e e f f e c t on t r a v e l t i m e s o f t h e v a r y i n g bottom t o p o g r a p h y . I have m e n t i o n e d s e v e r a l t i m e s Sl p r e v i o u s l y t h a t we d i d n o t h a v e a d e q u a t e d e p t h s o u n d i n q on t h e c r u i s e . W i t h o u t w i s h i n g t o b e l a b o u r t h e p o i n t , t h e l a c k o f s u c h i n f o r m a t i o n p l u s t h e r e l a t i v e l y s m o o t h t o p o g r a p h y a l o n q t h e p r o f i l e ( s e e F i g 1.2) made t h e a p p l i c a t i o n o f s t a t i c c o r r e c t i o n s u n n e c e s s a r y . A l t h o u g h t h e f i r s t p r o f i l e was s t a r t e d w i t h s i x f u n c t i o n i n g h y d r o p h o n e s , a s one m i g h t e x p e c t , we d i d n o t f i n i s h w i t h t h e same number. C h a n n e l two became i n o p e r a t i v e midway a l o n g p r o f i l e 75-1R ( r e c o r d e d f i r s t ) and r e m a i n e d so f o r t h e d u r a t i o n o f p r o f i l e 75-1. C h a n n e l s i x a l s o t h r e a t e n e d t o c e a s e f u n c t i o n i n g a t t h e s t a r t o f 75-1; h o w e v e r , i t d i d o p e r a t e o f f a n d on f o r t h e d u r a t i o n o f t h e p r o f i l e t h u s g i v i n g u s a b l e i n f o r m a t i o n a t i r r e g u l a r i n t e r v a l s . A l l d a t a f r o m i n o p e r a t i v e h y d r o p h o n e s w ere i g n o r e d d u r i n g t h e c o m p i l a t i o n o f t h e r e c o r d s e c t i o n s . P r i o r t o f i n a l c o m p i l a t i o n i n t o r e c o r d s e c t i o n s , t h e d a t a w e r e b a n d p a s s f i l t e r e d u s i n g a z e r o p h a s e , f o u r p o l e B u t t e r w o r t h f i l t e r . An e x c e l l e n t d e v e l o p m e n t o f t h e t h e o r y f o r B u t t e r w o r t h f i l t e r s i s g i v e n i n K a n a s e w i c h (1 976). The a n a l o g a m p l i f i e r s p r o v i d e d f i l t e r i n g b e t w e e n 0.8 and 100 h z , b u t t h i s was j u d g e d t o be i n s u f f i c i e n t as a c o n s i d e r a b l e amount o f h i q h f r e g u e n c y n o i s e r e m a i n e d . I n o r d e r t o remove t h i s n o i s e and i m p r o v e t h e g e n e r a l a p e a r a n c e o f t h e s e c t i o n s a l l s e i s m i c t r a c e s were f i l t e r e d b e t w e e n 5.0 and 30 h z . F i l t e r i n g l i m i t s o f 2.0 t o 15 hz were a p p l i e d a t one s t a g e t o v a r i o u s s e c t i o n s o f p r o f i l e 75-1. T h i s was done i n an e f f o r t t o i d e n t i f y l a t e r a r r i v a l s embedded i n t h e r e v e r b e r a t i o n s o f e a r l i e r a r r i v a l s . T h i s proceedure proved g u i t e s u c c e s s f u l as shown by F i g 4.8 where c o h e r e n t l a t e r a r r i v a l s were i d e n t i f i e d . The program RSEC i s c a p a b l e o f c o m p i l i n g and p l o t t i n g any p a r t o f a complete r e c o r d s e c t i o n with any d e s i r e d t i m e and d i s t a n c e s c a l e , and any d e s i r e d a m p l i t u d e . T h i s proved t o be a most u s e f u l t o o l i n a n a l y s i n g t h e s e i s m i c d a t a . The a b i l i t y t o l o o k a t any i n d i v i d u a l p o r t i o n of a complete s e c t i o n at any d e s i r e d a m p l i t u d e saved c o n s i d e r a b l e t i m e t h r o u g h o u t the a n a l y s i s of both r e f l e c t i o n and r e f r a c t i o n p r o f i l e s . 2 S U B - C R I T I C A L REFLECTION DATA M e t h o d s o f A n a l y s i s The r e c o r d s e c t i o n s f o r r e f l e c t i o n p r o f i l e s 75-1E and 7 5 -1 a r e shown i n F i g s 3.1 a n d 3.2, r e s p e c t i v e l y . T h e s e p r o f i l e s c o n t a i n a l l t h e d a t a a v a i l a b l e f r o m t h e o p e r a t i v e h y d r o p h o n e s . The f i r s t a r r i v a l s e e n on t h e p r o f i l e s i s t h e d i r e c t w a t e r wave. B o t h t h e f i r s t a n d s e c o n d b o t t o m b o u n c e s can be i d e n t i f i e d , t h e f i r s t s t a r t i n g a t a b o u t 2.7 s and t h e s e c o n d a t a b o u t 5.3 s. A l l o f t h e r e f l e c t i o n s t i m e d f o r a n a l y s i s a r e shown on t h e p r o f i l e s , a l t h o u g h t h e o nes shown w i t h a "P" on F i g 3.1 were t o o p o o r l y d e f i n e d t o a l l o w p r o p e r a n a l y s i s . T h e s e r e c o r d s e c t i o n s were v e r y u s e f u l i n g a i n i n g a b r o a d o v e r v i e w o f t h e r e f l e c t i n g s e g u e n c e s . However t h e y were n o t p a r t i c u l a r l y u s e f u l i n i d e n t i f y i n g and t i m i n g i n d i v i d u a l a r r i v a l s due t o t r a c e o v e r l a p , h y d r o p h o n e s i g n a t u r e v a r i a t i o n s and t h e g e n e r a l l y c l u t t e r e d a p p e a r a n c e . F a r more d e t a i l was n e e d e d t o t i m e t h e a r r i v a l s t h a n c o u l d be o b t a i n e d o f f t h e s e c o m p l e t e s e c t i o n s . F i g 3.3 shows t h e t y p e of s e c t i o n u s e d t o i d e n t i f y a nd t i m e a r r i v a l s . By u s i n g t h e s e e x p a n d e d s e c t i o n s i t was p o s s i b l e t o e x a m i n e s e t s o f a r r i v a l s f o r one c h a n n e l a t a t i m e . As p r e v i o u s l y m e n t i o n e d , t h e h y d r o p h o n e s were o f d i f f e r e n t a g e s , r e s u l t i n g i n d i f f e r e n t f r e g u e n c y r e s p o n s e s f o r e a c h one. T h i s d i f f e r i n g f r e q u e n c y r e s p o n s e was r e s p o n s i b l e f o r e a c h c h a n n e l h a v i n g a>' d i f f e r e n t a r r i v a l s i g n a t u r e . W i t h Record sect ion of r e f l e c t i o n p r o f i l e 75-1R using shots at 45m depth. Amplitude cor rec t ions have been applied to the data as per sec t ion 2.4 and the data are f i l t e r e d 5.0-30 hz. The part of the sec t ion within the rectangle i s shown on the r i g h t side with a f i v e - f o l d increase in amplitude. The DWW a r r i v a l l i e s along the s t r a i gh t l i n e . Curved l i n e s show the s i x r e f l e c t i o n phase c o r r e l a t i o n s which were timed. W i s the bottom r e f l e c t i o n . The dashed l i n e shows the t r a n s i t i o n from a deep r e f l e c t o r to the f i r s t r e f r a c t i o n a r r i v a l . The "?" ind ica te s poorly defined r e f l e c t i o n s . 38 a l l c h a n n e l s p l o t t e d out t o g e t h e r as i n P i g s 3.1 and 3.2, I found i t e x t r e m e l y d i f f i c u l t t o phase c o r r e l a t e a r r i v a l s due t o t h e d i f f e r i n g s i g n a t u r e c o m p l i c a t i n g any c o n t i n u i t y i n the phases. However w i t h t h e i n d i v i d u a l c h a n n e l s e c t i o n s , phase c o r r e l a t i o n s c o u l d be a c c o m p l i s h e d q u i t e e a s i l y . T h i s d i f f e r i n g a r r i v a l s i g n a t u r e i s shown c l e a r l y on F i g 3.3. Even u s i n g t h e s m a l l e r s e c t i o n s I e n c o u n t e r e d a number of problems, the major one b e i n g the a p p r o x i m a t e l y 300 ms l e n g t h o f t h e a r r i v a l s i g n a t u r e s . T h i s was caused by a prominent bubble p u l s e w i t h p e r i o d o f a p p r o x i m a t e l y 100 ms. The D.W.W. a r r i v a l seen on F i g 2.1 (B) and on F i g s 3.1 and 3.2 show c l e a r l y t h i s bubble p u l s e problem. Both the f i r s t and second bubble p u l s e s had s u f f i c i e n t a m p l i t u d e t o c o m p l i c a t e the r e c o r d s , the f i r s t b u b b l e h a v i n g a r e l a t i v e a m p l i t u d e of 0.9 and t h e second a r e l a t i v e a m p l i t u d e of 0.4, when compared to t h e o r i g i n a l p u l s e . T h i s e f f e c t tended to o b l i t e r a t e e v e r y t h i n g w i t h i n 100 ms of t h e onset of an a r r i v a l . V a r i o u s d e c o n v o l u t i o n methods have been attempted on s i m i l a r d ata by K n i z e (1 976) . H i s c o n c l u s i o n was t h a t time a d a p t i v e d e c o n v o l u t i o n would be r e q u i r e d t o reduce t h i s bubble p u l s e e f f e c t and enhance t h e d a t a . T h i s f a l l s beyond the scope of ' t h i s p r o j e c t w i t h the r e s u l t t h a t no d e c o n v o l u t i o n has been a t t e a i p t e d . One o t h e r minor problem w i t h t h e s e c t i o n s i s the marked d e c r e a s e i n a m p l i t u d e o f the s i g n a l a l o n g the t r a c e . T h i s makes l a t e r a r r i v a l s d i f f i c u l t t o see i f the complete t r a c e i s p l o t t e d . T h i s was s o l v e d q u i t e s i m p l y by s p l i t t i n g the s e c t i o n Two t y p i c a l r e c o r d s e c t i o n s used t o time i n d i v i d u a l r e f l e c t i o n a r r i v a l s . 'Both s e c t i o n s are f i l t e r e d 5.0-30 hz. The c o r r e l a t e d phases a r e the water bottom and f i r s t and second sub-bottom r e f l e c t o r s . i n t o v arious time segments and by p l o t t i n g out each time s e c t i o n i n d i v i d u a l l y with an appropriate amplitude. Any r e f l e c t i o n i n t e r p r e t a t i o n from which v e l o c i t y i n f o r m a t i o n w i l l be c a l c u l a t e d i s only as good as the timing of the various a r r i v a l s . As stated p r e v i o u s l y , I was able to phase c o r r e l a t e a r r i v a l s from shot to shot using record s e c t i o n s showing only one channel at a time. Phase c o r r e l a t i o n of a r r i v a l s between d i f f e r e n t channels f o r the same shot however was not p o s s i b l e . Thus the phases picked f o r the various channels were sometimes o f f s e t from one another by as much as 50 ms. C o r r e c t i o n s f o r these o f f s e t s were made by overlapping the various s e c t i o n s on a l i g h t t a b l e and measuring the time d i f f e r e n c e between the phases picked on each channel r e l a t i v e t o a standard channel. By t h i s method I was able to e l i m i n a t e the problem of p i c k i n g d i f f e r e n t phases fo r each channel. While i t was p o s s i b l e to t r a c e a r r i v a l s out to approximately 3.0 to 3.5 kin i t was impos s i b l e to pick f i r s t breaks. Most a r r i v a l s were extremely emergent from the re v e r b e r a t i o n s and bubble pulses of e a r l i e r a r r i v a l s . Thus t h e i r f i r s t breaks were e s s e n t i a l l y hidden. As a r e s u l t , a l l pidks f o r any one a r r i v a l could be o f f s e t by up to 50 ms from the f i r s t break. No attempt was made to c o r r e c t f o r t h i s , however, as i t was d i f f i c u l t to i d e n t i f y j u s t what the o f f s e t time should be. This o f f s e t , while not a f f e c t i n g the la y e r v e l o c i t i e s obtained i n the a n a l y s i s , placed l i m i t s of 50 to 100 m on the l a y e r depths. F i n a l l y , a l t h o u g h I had c o n s i d e r a b l e s u c c e s s phase c o r r e l a t i n g a r r i v a l s , a t d i s t a n c e s beyond 2.5 km t h e a r r i v a l s f r o m t h e d i f f e r e n t h o r i z o n s s t a r t e d t o o v e r l a p somewhat. T h i s c a u s e d c o n s i d e r a b l e c o n f u s i o n i n t r y i n g t o f o l l o w p h a s e s w i t h t h e r e s u l t t h a t t h e p o s s i b i l i t y o f " j u m p i n g " a phase c a n n o t be o v e r l o o k e d . Once t h e a r r i v a l s had been t i m e d I was f a c e d w i t h t h e p r o b l e m o f how t o a n a l y s e t h e d a t a o b t a i n e d . The b a s i c p r o b l e m i s t h a t t h e t r a v e l t i m e o f t h e a r r i v a l s i s r e l a t e d t o t h e a v e r a g e v e l o c i t y f r o m a p l a n e c o n t a i n i n g t h e s h o t and r e c e i v e r t o a r e f l e c t i n g h o r i z o n . In our c a s e t h i s a v e r a g e v e l o c i t y i s d o m i n a t e d by t h e t h i c k water l a y e r (2000 m) c a u s i n g t h e a v e r a g e v e l o c i t y t o change v e r y l i t t l e from l a y e r t o l a y e r . In o r d e r t o a n a l y s e t h e r e s u l t s o f t h e t r a v e l t i m e p i c k s , i t became e s s e n t i a l t o remove t h e e f f e c t on t h e v a r i o u s a r r i v a l s o f t h i s t h i c k w a t e r l a y e r . T h r e e methods h a v e been used t o a c c o m p l i s h t h i s . 1 ) Kay P a r a m e t e r Method : The r a y p a r a m e t e r (p) o f a s e i s m i c r a y can be t h o u g h t of as t h e a n g u l a r v e l o c i t y o f t u r n i n g a b o u t the c e n t r e o f t h e ear-th o f t h e r a y i n q u e s t i o n . I f we assume t h e e a r t h i s l a t e r a l l y homogeneous (an a s s u m p t i o n t h a t i s made i n most r e f r a c t i o n and r e f l e c t i o n a n a l y s e s ) , t h e n t h e r a y p a r a m e t e r (B.P.) r e m a i n s c o n s t a n t a l o n g t h e r a y p a t h . T h i s makes i t e x t r e m e l y s u i t a b l e a s a t o o l f o r a n a l y s i n g b o t h r e f l e c t i o n and r e f r a c t i o n work. 43 C o n s i d e r t h e f o l l o w i n g : A) B) p = s i n ( i ) /v 3.1-1 p=dT/dX 3.1-2 A c c o r d i n g t o S n e l l ' s law s i n ( i / ) /V/ = s i n ( i z ) / v * . Hence 3.1-1 i s s i m p l y a r e s t a t e m e n t o f S n e l l ' s law. 3.1-2 i s d e r i v e d by B u l l e n (1963, p 110) and i s t h e f u n d a m e n t a l e q u a t i o n o f t h e r a y p a r a m e t e r method The two l a y e r e d medium o f d i a g r a m (A) r e s u l t s i n t h e two T vs X c u r v e s o f d i a g r a m ( B ) . The c u r v e d e f i n i n g a r r i v a l 2 i s c o m p r i s e d o f c o n t r i b u t i o n s from t h e p a s s a g e of t h e r a y s t h r o u g h b o t h l a y e r s one and two. The o b j e c t i v e i s t o remove t h e e f f e c t on t h e s e c o n d T v s X c u r v e , g e n e r a t e d by the r e f l e c t i o n o f t h e r a y o f f t h e bottom of l a y e r 2, o f t h e r a y p a s s i n g t h r o u g h l a y e r one. T h i s c a n be a c c o m p l i s h e d g u i t e s i m p l y by u s i n g egn 3.1-2. E g u a t i o n 3,1-2 s t a t e s t h a t t h e p v a l u e c o r r e s p o n d i n g to an a r r i v a l a t a d i s t a n c e X e q u a l s t h e s l o p e of t h e t r a v e l t i m e c u r v e (T vs X) a t t h a t p o i n t . I t i s p o s s i b l e then t o o b t a i n a p vs X c u r v e f o r both a r r i v a l s one and two by f i t t i n g the c u r v e s (T vs X) w i t h a t h i r d o r d e r p o l y n o m i a l and then ta l c i n g t h e d e r i v a t i v e o f t h e f i t t e d c u r v e s . Any p a r t i c u l a r r a y w i t h R.P. v a l u e p w i l l a r r i v e at t h e s u r f a c e a f t e r r e f l e c t i n g o f f both the f i r s t and second l a y e r s . By s u b t r a c t i n g the t r a v e l t i m e s and d i s t a n c e s o b t a i n e d from the c o r r e s p o n d i n g p v a l u e s on the two T vs X c u r v e s we can remove t h e e f f e c t on the ray of i t s passage t h r o u g h l a y e r one. We have then reduced the a r r i v a l t o a s i m p l e one l a y e r e d case which may be a n a l y s e d by the s t a n d a r d f o r m u l a g i v e n below T2= ( X 2 + 4 H 2 ) / V 2 3.1-3 Where H=depth of the l a y e r V = v e l o c i t y o f t h e l a y e r Keen (1976) has d e v i s e d an a l g o r i t h m t o perform the above a n a l y s i s . I have m o d i f i e d t h i s a l g o r i t h m t o f i t my own da t a and have used i t as the p r i m a r y method o f r e d u c i n g the r e f l e c t i o n d a t a . T 2 - X 2 Method : Having had no p r e v i o u s e x p e r i e n c e w i t h the ray parameter method, I thou g h t i t b e t t e r t o compare the r e s u l t s o b t a i n e d u s i n g i t w i t h r e s u l t s o b t a i n e d u s i n g t h e more f a m i l i a r T 2 - X 2 method. Dix (1955) has d e f i n e d the average root-mean square (r.ra.s.) v e l o c i t y from the s u r f a c e down to t h e bottom o f the nth l a y e r as 3. 1-4 *L To,i Where Vw i s t h e average r.m.s. v e l o c i t y V~i i s the i n t e r v a l v e l o c i t y of the i t h l a y e r To,! i s the two-way v e r t i c a l i n c i d e n c e t r a v e l t ime i n the i t h l a y e r . When the t r a v e l t i m e s and d i s t a n c e s of a s e t of a r r i v a l s from one r e f l e c t i n g h o r i z o n a r e p l o t t e d on a T vs X c u r v e they a r e h y p e r b o l i c . I f , however, they a re p l o t t e d on a T 2 - X 2 c u r v e , they d e f i n e s t r a i g h t l i n e s , the i n t e r c e p t b e i n g the two way v e r t i c a l i n c i d e n c e t r a v e l time and the i n v e r s e s l o p e b e i n g t h e average r.m.s. v e l o c i t y of the a r r i v a l . T h i s v e l o c i t y c o r r e s p o n d s t o W of e g u a t i o n 3.1-4. L e t us now d e f i n e t h e average r.m.s. v e l o c i t y from the s u r f a c e t o t h e t o p o f t h e k t h l a y e r and i t s c o r r e s p o n d i n g two way v e r t i c a l i n c i d e n c e t r a v e l time as V K - I and T K - I . Let us f u r t h e r d e f i n e VK and T* as bei n q t h e c o r r e s p o n d i n g v a l u e s t o th e bottom of t h e k t h l a y e r . Then the i n t e r v a l v e l o c i t y of the kt h l a y e r i s g i v e n by Dix (1955) as : - ^H* Th - J / h - i T*-i 3.1-5 TH - TH -1 E g u a t i o n 3.1-5 has been used to generate a v e l o c i t y depth model t o compare w i t h t h e r e s u l t s o b t a i n e d u s i n g the r a y parameter method. 3) S t r i p p e d T 2-X 2 Method : A t h i r d method o f a n a l y s i s , hereby r e f e r r e d t o as the s t r i p p e d T 2 ~ X 2 method, was attempted. T h i s method, u n l i k e the p r e v i o u s two, depends on the p r e v i o u s c a l c u l a t i o n o f the upper l a y e r v e l o c i t i e s and de p t h s . I used the r a y parameter method t o t h e e x t e n t of o b t a i n i n g the p vs X c u r v e f o r t h e a r r i v a l branch i n g u e s t i o n . The upper l a y e r s however were s t r i p p e d o f f by means of e q u a t i o n 3.1-1 and s i m p l e geometry. Once the upper l a y e r s had been s t r i p p e d o f f the r e m a i n i n g data were a n a l y s e d by use of eg u a t i o n 3,1-3 The major problem a s s o c i a t e d w i t h t h i s method i s t h a t i t r e q u i r e s an a c c u r a t e knowledge o f a l l t h e l a y e r v e l o c i t i e s and depths above the l a y e r i n q u e s t i o n . These v e l o c i t i e s and depths a r e a l s o c a l c u l a t e d by t h i s method, r e s u l t i n g i n a s t e p by s t e p p r o g r e s s i o n i n t h e a n a l y s i s . U n f o r t u n a t e l y t h i s a l s o means t h a t any e r r o r s e n c o u n t e r e d d u r i n g the c a l c u l a t i o n of the v e l o c i t y and t h i c k n e s s f o r a l a y e r w i l l a f f e c t the subsequent v a l u e s f o r a l l the lower l a y e r s , w i t h the e r r o r s a c c u m u l a t i n g from l a y e r to l a y e r . We would e x p e c t t h e r e f o r e t h a t t h i s method would become l e s s and l e s s a c c u r a t e w i t h each a d d i t i o n a l l a y e r . T h i s method was attem p t e d more f o r my own e d i f i c a t i o n t h a n f o r the r e s u l t s o b t a i n e d . I t was not ex p e c t e d t h a t i t would y i e l d u s a b l e s o l u t i o n s from t h e d a t a . 3 ._2 A n a l y s i s of R e s u l t s P r o f i l e 75-1R F i g 3.1 shows the c o m p l e t e r e c o r d s e c t i o n of r e f l e c t i o n d a ta f o r p r o f i l e 75-1R u s i n g s h o t s at 45 m depth. The p r o f i l e u s i n g s h o t s a t 7 m depth was a l s o c o m p i l e d ; however, i t added l i t t l e to t h e a n a l y s i s as the lower energy y i e l d of the 7 ra s h o t s d i d not a l l o w t h e depth o f p e n e t r a t i o n of the 45 in s h o t s . For t h i s r e a s o n the p r o f i l e s u s i n g s h o t s a t 7 m were not a n a l y s e d . A t o t a l o f f i v e sub-bottom r e f l e c t o r s a r e i d e n t i f i e d on F i g 3.1; u n f o r t u n a t e l y o n l y 3 c o u l d be timed s u f f i c i e n t l y w e l l t o be a n a l y z e d . The f i r s t two r e f l e c t o r s a re c l e a r l y i d e n t i f i e d and l i t t l e t r o u b l e was e n c o u n t e r e d i n t i m i n g them. There i s a prominent a r r i v a l a p p r o x i m a t e l y 300 ms a f t e r r e f l e c t o r two. T h i s , however, was i d e n t i f i e d as e i t h e r a r e v e r b e r a t i o n or a b u b b l e p u l s e , e f f e c t as i t had the same raoveout as r e f l e c t o r two. Both r e f l e c t o r s one and two are seen c l e a r l y upon e x a m i n a t i o n o f the water bottom m u l t i p l e s b e g i n n i n g a t a time of a p p r o x i m a t e l y 5.3 s. T h i s g i v e s f u r t h e r e v i d e n c e f o r t h e i r b e i n g i n t e r p r e t e d as s e p a r a t e a r r i v a l s . The t i m i n g of of a r r i v a l t h r e e was t o o poor to enable any s o r t o f proper a n a l y s i s . As a r e s u l t i t i s i d e n t i f i e d on F i g 3.1 w i t h a " ? " . T h i s a r r i v a l c o u l d a l s o be seen on p r o f i l e 75-1; however, once a g a i n i t a l s o was not u s a b l e due t o poor t i m i n g . 48 R e f l e c t o r f o u r i s t i m e d v e r y w e l l and can be i d e n t i f i e d as t h e r e f l e c t i o n from the sediment/basement i n t e r f a c e , s i n c e i t can be t r a c e d out t o the f i r s t r e f r a c t i o n a r r i v a l (apparent v e l o c i t y o f 4.28 km/s). T h i s t r a n s f o r m a t i o n from the r e f l e c t i o n t o the r e f r a c t i o n a r r i v a l i s shown on F i q 3.1 w i t h a dashed l i n e . R e f l e c t o r f i v e i s the l a s t r e f l e c t i o n t h a t can be i d e n t i f i e d as a s e p a r a t e a r r i v a l . U n f o r t u n a t e l y i t i s t o o weak to t i m e . I t i s my o p i n i o n t h a t i t i s an a r r i v a l from a deeper c r u s t a l l a y e r , but due t o the l a c k of adequate t i m i n g t h i s i d e a can not be s u p p o r t e d . The a r r i v a l t i m e s and d i s t a n c e s f o r the water bottom r e f l e c t i o n and r e f l e c t o r s one, two and f o u r i n both T vs X and T 2 vs x 2 modes a r e p l o t t e d on F i g 3.4 (A) and ( B ) , r e s p e c t i v e l y . I have a l s o p l o t t e d the a r r i v a l t i m e s of r e f l e c t o r t h r e e on t h e s e p l o t s t o i l l u s t r a t e how i n a d e g u a t e l y t h i s a r r i v a l i s d e f i n e d . T a b l e 3.1 compares the r e s u l t s of the a n a l y s i s of the u s a b l e a r r i v a l s u s i n g the t h r e e methods d i s c u s s e d i n t h e l a s t s e c t i o n . The r ay parameter and T 2 - X 2 methods y i e l d e s s e n t i a l l y i d e n t i c a l r e s u l t s and comparable e r r o r s . The s t r i p p e d T 2 - X 2 method r e s u l t s d i f f e r e d from t h e p r e v i o u s two t o a g r e a t e r e x t e n t , e s p e c i a l l y i n the l a s t l a y e r . T h i s i s c o n s i s t e n t w i t h what would be e x p e c t e d c o n s i d e r i n g t h a t the e r r o r s i n t h i s method a r e c u m u l a t i v e . The f i r s t two methods a r e s i m i l a r i n t h a t n e i t h e r method r e q u i r e s any p r e v i o u s knowledqe o f the v e l o c i t i e s and depths of the l a y e r s above t h e one i n g u e s t i o n . A. T vs X p l o t f o r 5 r e f l e c t i o n s from p r o f i l e 75-1R shown on F i g 3.1. The l i n e s a r e t h i r d - o r d e r p o l y n o m i a l f i t s used i n the r a y parameter method of d e t e r m i n i n g v e l o c i t i e s and de p t h s . B. T 2 vs X 2 p l o t s f o r t h e same d a t a . The l i n e s a re l e a s t s q u a r e s f i t s used i n the T 2 - X 2 method of a n a l y s i s . 50 Fig 3.5, same as 3.4, for p r o f i l e 75-i T A B L E 3. 1 P r o f i l e 75-1fi Ray T 2 - X 2 S t r i p p e d Parameter T 2 - X 2 Water ( s u r f a c e t o W) V e l o c i t y 1. 50 ± 0. 01 T h i c k n e s s 1. 95 2 0.01 Sediments Layer J (W t o 1) V e l o c i t y 1.79±0.08 1. 761: 0. 05 1 .68 + 0. 1 1 T h i c k n e s s 0.33 + 0.01. 0.34 + 0.01 0.30+0.02 La y e r 2 (1 to 2) V e l o c i t y 2 . 0 2 t 0. 06 2. 01 ± 0. 05 2 . 0 8 t 0 . 1 0 T h i c k n e s s 0.32±0.01 0.33S0.01 0.35+0.02 La y e r 3 (2 to 4) V e l o c i t y 2. 14+ 0. 03 2. 12 ± 0. 05 2. 16 + 1.26 T h i c k n e s s 1. 17+ 0.02 1. 15 + 0. 01 0.74 + 0. 44 A l l v e l o c i t i e s a r e i n km/s and a l l t h i c k n e s s e s a r e i n km. 53 As a r e s u l t , i n a c c u r a c i e s i n c a l c u l a t i n g the v e l o c i t y and depth o f a l a y e r w i l l not a f f e c t the r e s u l t s c a l c u l a t e d f o r deeper l a y e r s . T h i s i s i n d i r e c t c o n t r a s t w i t h t h e s t r i p p e d T 2 - X 2 method t h a t p r o p agates e r r o r s from one l a y e r t o the n e x t . From t h i s we would expect the r e s u l t s between a l l t h r e e methods t o agree f a i r l y w e l l f o r the f i r s t l a y e r o r two. As t h e number o f l a y e r s i n c r e a s e s , however, the l a s t method w i l l become p r o g r e s s i v l y l e s s a c c u r a t e due t o the a c c u m u l a t i o n of e r r o r s . The v e l o c i t y vs depth r e s u l t s from the a n a l y s i s of sediment r e f l e c t i o n s f o r p r o f i l e 75-18 are g i v e n i n t a b l e 3.2. TABLE 3.2 L a y e r V e l o c i t y T h i c k n e s s Depth t o Top (km/s) (km) of L a y e r (km) Water ' 1.50 + 0.02 2.00 t 0.01 0.0 L a y e r 1 1.79 + 0.03 0.33 i 0.01 2.00 L a y e r 2 2.02 0. 06 0. 32 t 0.01 2. 33 L a y e r 3 2. 14 0. 03 1 .17 2 0 .02 2.65 Oceanic basement 3.82 By p l a c i n g t h e s h o t s at 45m depth f o r t h i s p r o f i l e we have managed t o p e n e t r a t e to the base of t h e s e d i m e n t s w i t h o u t o b l i t e r a t i n g the immediate sub-bottom a r r i v a l s . The p r o f i l e s 54 u s i n g s h o t s a t 7 m depth p r o v i d e d e s s e n t i a l l y the same i n f o r m a t i o n , but f o r o n l y a p p r o x i m a t e l y 1.5 s of p e n e t r a t i o n (2-way). For t h i s r e a s o n I do not recommend t h a t such p r o f i l e s be r e c o r d e d i n f u t u r e work as they add l i t t l e t o the a n a l y s i s . I t may be p o s s i b l e t h a t we do have r e f l e c t i o n s from deeper basement l a y e r s . U n f o r t u n a t e l y , i f they a r e p r e s e n t t h e y a r e of so low an a m p l i t u d e as t o be i n d i s t i n g u i s h a b l e from background n o i s e and the water bottom m u l t i p l e s . T h i s , f o r example, c o u l d be the case w i t h r e f l e c t o r f i v e i d e n t i f i e d w i t h a " ? " as the l a s t r e f l e c t o r on F i g 3.1. I t has been s u g g e s t e d t h a t v e l o c i t y (w-k) f i l t e r i n g ( T r i e t e l et a l 1967) might enhance these a r r i v a l s . T h i s method of f i l t e r i n g was not i n i t i a l l y d e s i g n e d f o r our t y p e o f d a t a as i t has the r e q u i r e m e n t s t h a t the s p a c i n g between t r a c e s be u n i f o r m , and t h a t the a r r i v a l s have a u n i f o r m moveout per c h a n n e l . That i s , when p l o t t e d on a T vs X graph the a r r i v a l s s h o u l d y i e l d a s t r a i g h t l i n e . S i n c e t h e hydrophone a r r a y was a l l o w e d to be more or l e s s f r e e moving to l e s s e n the ambient n o i s e , the f i r s t c o n d i t i o n i s not met. A l s o , f o r t h i s t y p e of r e f l e c t i o n work the T vs X p l o t s o f t h e a r r i v a l s arc- h y p e r b o l i c and not l i n e a r and thus t h e d a t a does not f i t the second c r i t e r i a . Even so, I have a t t e m p t e d to expand the w-k method to f i t t h i s t y p e of d a t a . T h i s was done as a p r o j e c t f o r the c o u r s e , G e o p h y s i c s 514, "Time S e r i e s A n a l y s i s " , g i v e n by Dr T. J . U l r y c h and was i n t e n d e d o n l y as a f e a s a b i l i t y s t u d y . The r e s u l t s , w h i l e b e i n g f a r from c o n c l u s i v e , do i n d i c a t e t h a t i t may be p o s s i b l e t o expand w-k f i l t e r i n g t o t h i s t y p e of d a t a , 55 even c o n s i d e r i n g t h e problems j u s t d i s c u s s e d . I s h a l l l e a v e t h i s as a l e g a c y f o r a subseguent a d v e n t u r o u s s t u d e n t . The v e l o c i t y depth model j u s t given- s h o u l d not be c o n s i d e r e d as a b s o l u t e . Due t o t h e n o n - r e v e r s a l of the data t h e v e l o c i t i e s are o n l y " a p p a r e n t " v e l o c i t i e s and n o t the t r u e i n t e r v a l v e l o c i t i e s . The p r e s e n c e o f d i p p i n g l a y e r s can g r e a t l y a f f e c t the v e l o c i t y measured f o r a l a y e r as a l l the methods o f a n a l y s i s assume t h a t the l a y e r s a r e homogeneuos and f l a t l y i n g . T h i s w i l l be expanded upon d u r i n g the d i s c u s s i o n o f t h e 75-1 r e f l e c t i o n p r o f i l e s where d i p p i n g l a y e r s cause the r e s u l t s t o be m e a n i n g l e s s . As mentioned i n Chapter 1, C.S.P. data were a c g u i r e d d u r i n g t h i s c r u i s e . C.S.P. l i n e 75-1 ( F i g 1.3} a t i t s no r t h w e s t end c l o s e l y p a r a l l e l s t h e r e f l e c t i o n l i n e w h i l e C.S.P. l i n e 75-3 ( F i g 1.4) runs p e r p e n d i c u l a r t o i t b e i n g a p p r o x i m a t e l y 30 km s o u t h e a s t . Both p r o f i l e s c l e a r l y show the presence of d e f i n i t e l a y e r i n g i n the s e d i m e n t s , w i t h c o n s i d e r a b l e e v i d e n c e f o r f o l d i n g o f the l a y e r s . However, t h i s f o l d i n g does not appear t o have d i s r u p t e d t h e c o n t i n u i t y of the l a y e r s . An attempt was made t o c o r r e l a t e the a r r i v a l s t i m e d on t h e r e f l e c t i o n p r o f i l e s w i t h t h e C.S.P. p r o f i l e s . The C.S.P. d a t a however o n l y p e n e t r a t e d t o about 4 s two way t r a v e l time. Even so, I was a b l e to make a t e n t a t i v e c o r r e l a t i o n between prominent a r r i v a l s on t h e C.S.P. and the f i r s t two sub-bottom r e f l e c t o r s , a l t h o u g h p r e c i s e l a y e r c o r r e l a t i o n s c o u l d not be made. P r o f i l e 75-1R i s somewhat to the n o r t h w e s t of t h e end o f C.S.P. l i n e 75-1. For t h i s r e a s o n , 56 i t i s not p o s s i b l e t o i d e n t i f y t h e p r e c i s e magnitude of the d i p s p r e s e n t i n the r e g i o n o f 75-1R. From the C.S.P. d a t a t h a t we do have though, I t h i n k i t i s s a f e t o assume t h a t the d i p s i n t h e r e g i o n w i l l be of the o r d e r o f one t o two d e g r e e s . J u s t what changes t h e s e d i p s w i l l make to t h e l a y e r t h i c k n e s s e s and v e l o c i t i e s i s d i f f i c u l t t o d e t e r m i n e because of the complex l a y e r i n g p r e s e n t . Any changes produced though s h o u l d not be t o o g r e a t , b e i n g of the o r d e r o f 2 t o 4%. For t h i s reason a m i l d c a u t i o n i s p l a c e d on t h e r e f l e c t i o n r e s u l t s f o r p r o f i l e 75-1H. The main purpose b e h i n d t h e r e f l e c t i o n s u r v e y s has been t o o b t a i n a r e a s o n a b l e p i c t u r e o f t h e s e d i m e n t s f o r use i n the a n a l y s i s of t h e r e f r a c t i o n d a t a s e c t i o n , and t o o b t a i n a more a c c u r a t e t o t a l depth f o r the sediments than has p r e v i o u s l y been r e p o r t e d by such a u t h o r s as T i f f i n (1972) and Couch (1969). The t o t a l sediment t h i c k n e s s o b t a i n e d from the a n a l y s i s of . p r o f i l e 75-1R i s a p p r o x i m a t e l y 1.8 km. Even c o n s i d e r i n g t h a t the l a y e r s are d i p p i n g a t an unknown a n g l e and making generous a l l o w a n c e f o r t h i s p o s s i b i l i t y , the sediment t h i c k n e s s i s not l i k e l y t o be i n a c c u r a t e by more than 20 0 m. P r o f i l e 75-1 The c o m p l e t e s e t of r e f l e c t i o n d a t a f o r p r o f i l e 75-1 u s i n g s h o t s a t 45 m depth i s shown i n F i g 3.2 I f we compare i t d i r e c t l y t o F i g 3.1, the s i m i l a r s e c t i o n f o r 75-1R, we can see i m m e d i a t e l y t h a t the a r r i v a l s t i med on 7 5-1 a r e somewhat 57 q u e s t i o n a b l e . Some of t h e a r r i v a l s shown on F i q 3.2 do not appear as c o h e r e n t as the c o r r e s p o n d i n g a r r i v a l s on F i q 3.1. I was a b l e to t i m e what I i n t e r p r e t e d as the same r e f l e c t o r s as on 75-1R, e x c e p t I was not a b l e to time the l a s t r e f l e c t o r , o r even see i t on t h i s p r o f i l e . These a r r i v a l s were i d e n t i f i e d as b e i n q from the same r e f l e c t o r s on the b a s i s t h a t they had s i m i l a r t r a v e l t i me s p a c i n g and r e l a t i v e a m p l i t u d e s as on 75-1R. The major d i f f e r e n c e between th e two p r o f i l e s can be seen by comparing the T vs X and T 2 - X 2 p l o t s f o r both p r o f i l e s ( F i g s 3.3 and 3.4). The r e s u l t s f o r p r o f i l e 75-1 are u n u s u a l i n t h a t they i n d i c a t e t h a t t h e average v e l o c i t y t o a l a y e r d e c r e a s e s w i t h d e p t h , w i t h a l l t h e average v e l o c i t i e s b e i n g l o w e r than t h a t o f water. T h i s can be seen c l e a r l y on F i g 3.5 where th e s l o p e s o f the T 2 - X 2 l i n e s i n c r e a s e from one l a y e r to t h e n e x t . The s l o p e s and i n t e r c e p t s of t h e l i n e s a r e g i v e n T a b l e 3.3. TABLE 3.3 Hater Branch 1 Branch 2 Branch 3 Branch 4 V e l o c i t y km 1.49 1. 42 1. 42 1.45 1. 35 I n t e r c e p t s 2 7.54 10.06 11.92 15.25 17.78 58 The r e a s o n f o r t h e s e non s e n s i c a l r e s u l t s has a l r e a d y been i n t r o d u c e d d u r i n g the d i s c u s s i o n of p r o f i l e 75- 1R, t h a t b e i n g the presence o f d i p p i n g l a y e r s . A l l o f the t h r e e a n a l y s i s methods d i s c u s s e d p r e v i o u s l y assume f l a t l y i n g l a y e r s , f o r t h i s r e a s o n t h e y c o u l d not be used t o a n a l y s e t h i s p r o f i l e . The o n l y way of o b t a i n i n g any u s e f u l i n f o r m a t i o n from t h i s p r o f i l e i s t o assume t h a t the v e l o c i t i e s and d e p t h s g i v e n by t h e 75-1B p r o f i l e a l s o a p p l i e d here. Then i t i s p o s s i b l e to d e t e r m i n e the d i p s t h a t would g i v e r i s e t o t h e r e s u l t s o b t a i n e d . T h i s was done to o b t a i n the a p p r o x i m a t e d i p f o r the f i r s t sub-bottom r e f l e c t o r . A model based on the diagram below was used and t h e " a p p a r e n t " average v e l o c i t y t o the r e f l e c t o r was c a l c u l a t e d u s i n g v a r i o u s v a l u e s f o r 0 , The c a l c u l a t i o n showed t h a t v a l u e s o f Q r a n g i n g from f i v e t o t e n degrees would y i e l d a v e l o c i t y f o r the a r r i v a l s i m i l a r to the 1.42 km/s v e l o c i t y o b t a i n e d from the T 2 - X 2 p l o t f o r the average v e l o c i t y t o the f i r s t r e f l e c t o r . I t now remains t o be seen i f the s e d i m e n t s i n the r e g i o n of p r o f i l e 75-1 have d i p s of t h i s o r d e r . Our own C.S.P. l i n e 75-1 { F i g 1.3) does not g i v e any c l e a r d e t a i l s o f the 59 s e d i m e n t s a t the s o u t h e a s t end of the p r o f i l e . Murray and T i f f i n (1976), however, have p u b l i s h e d a C.S.P. p r o f i l e ( F i q 1.5) which a l m o s t d i r e c t l y o v e r l a p s our r e q i o n o f i n t e r e s t . As can be seen, the s e d i m e n t s i n t h i s r e q i o n are h i q h l y f o l d e d . The d i s t i n c t l a y e r i n g p r e s e n t at the n o r t h w e s t end o f the p r o f i l e i s not as e v i d e n t , and the s e d i m e n t s a r e f o l d e d more d r a s t i c a l l y , t he a m p l i t u d e o f f o l d i n q i n c r e a s i n q w i t h depth. From the C.S.P. d a t a , c a l c u l a t i o n o f t h e d i p s i n t h i s r e q i o n , range from a p p r o x i m a t e l y +10 t o -10 deqrees, and hence agree w i t h the v a l u e s of t h e d i p s r e q u i r e d t o make the f i r s t l a y e r a p p a r e n t v e l o c i t y a g r e e w i t h t h a t d e t e r m i n e d from the T 2-X 2 a n a l y s i s . 60 H 5EPRACTION DATA ANALYSIS H-.1 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 r u U n t i l r e c e n t l y t h e o n l y method used t o a n a l y s e marine r e f r a c t i o n d a t a was t h e so c a l l e d " F i r s t A r r i v a l Method". T h i s t y p e of a n a l y s i s , w h i l e b e i n q very easy and q u i c k t o perform, does not t a k e i n t o account the secondary a r r i v a l s and dynamic a m p l i t u d e a s p e c t s of t h e d a t a . The advent of s o p h i s t i c a t e d but r e l a t i v e l y i n e x p e n s i v e methods of c a l c u l a t i n q s y n t h e t i c seismoqrams has e n a b l e d t h e e x p a n s i o n of marine r e f r a c t i o n i n t e r p r e t a t i o n s t o i n c l u d e a m p l i t u d e i n f o r m a t i o n as w e l l as t h e complete t r a v e l t i m e d a t a s e t . The f i r s t a r r i v a l method remains as a v a l u a b l e t o o l , however. I t s major use now i s i n q e n e r a t i n g a p r e l i m i n a r y v e l o c i t y depth model f o r use as an i n i t i a l i n t e r p r e t i v e quide and as a s t a r t i n q model f o r the g e n e r a t i o n of s y n t h e t i c seismoqrams. For t h i s r e a s o n , a 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 o f the data was the f i r s t s t e p i n the i n t e r p r e t i v e p r o c e d u r e used i n t h i s s t u d y . The seismograms used i n t i m i n g the f i r s t a r r i v a l s a r e the same ones used t o t i m e t h e d i r e c t water wave phases, which i n t u r n g i v e the s h o t - r e c e i v e r d i s t a n c e s . The t i m i n g method f o r f i r s t a r r i v a l s was i d e n t i c a l t o t h a t used f o r the D.W.W. (see s e c t i o n 2.3 and F i g 2.1). To a d i s t a n c e o f 55 km on p r o f i l e 75-1R and 65 km on 75-1, t h e s i g n a l / n o i s e r a t i o was h i g h enough to e n able the t i m i n g o f f i r s t breaks t o b e t t e r than 1 mm o r 15 ms. The 61 s i g n a l / n o i s e r a t i o l e s s e n e d a t t h i s p o i n t and c o n s e g u e n t l y over the range 55-75 km on 75-1R and f o r the r e s t o f the p r o f i l e on 75-1, an a c c u r a c y of b e t t e r than 2 mm o r 30 ms has been a t t r i b u t e d t o the p i c k s . Beyond 75 km on 75-1R t h e s i g n a l a m p l i t u d e s were too low t o e n a b l e a r r r i v a l s t o be timed to any r e a s o n a b l e degree of a c c u r a c y . i F i g 4.1 i s t h e reduced t r a v e l t i m e p l o t f o r f i r s t a r r i v a l data f o r both p r o f i l e s 75-1 and 75-1R, the r e d u c i n g v e l o c i t y b e i n g 6 km/s. An expanded p l o t of the f i r s t two phases ( i . e . 8-28 km) i s shown on F i g 4.2. T h i s p l o t shows how w e l l the f i r s t two s e t s of a r r i v a l s a r e d e f i n e d . L e t us now examine F i g 4. 1 i n g r e a t e r d e t a i l . The f i r s t -t h i n g t o note i s t h e g r e a t degree of s i m i l a r i t y between the two p r o f i l e s , t h e main d i f f e r e n c e b e i n g an o f f s e t of a p p r o x i m a t e l y 200 ms between them. The l a y e r v e l o c i t i e s g i v e n by the i n v e r s e of the s l o p e s of the l e a s t - s q u a r e s l i n e s are a l s o i n e x c e l l e n t agreement, i n d i c a t i n g t h a t t h e r e i s l i t t l e d i p on the r e f r a c t i n g h o r i z o n s . For both p r o f i l e s , t h e f i r s t s e t of a r r i v a l s s t a r t a t a p p r o x i m a t e l y 9 km and p e r s i s t f o r about the next 5 km, w i t h a l e a s t - s q u a r e s v e l o c i t y o f a p p r o x i m a t e l y 4.28 km/s. At about 14 km,' the next branch (5.26 km/s) becomes the f i r s t a r r i v a l and p e r s i s t s as such f o r the next 15 km . Once a g a i n t h i s t r a v e l t i m e branch i s e x t r e m e l y w e l l d e f i n e d , as shown by F i g 4.2. Over the d i s t a n c e range 29-44 km on b o t h p r o f i l e s , the 6.28 km/s t r a v e l t ime b r a n c h becomes the f i r s t a r r i v a l . T h i s branch i s not as w e l l d e f i n e d as t h e p r e v i o u s two, b e i n q d e f i n e d by Reduced t r a v e l time data and l e a s t s q u a r e s f i t t e d l i n e s f o r b o t h r e f r a c t i o n p r o f i l e s . The r e d u c i n q v e l o c i t y i s 6 km/s. I n v e r s e s l o p e s a r e t h e v e l o c i t i e s i n k i l o m e t e r s per second; i n t e r c e p t s a re i n seconds. T r i a n g l e s show d a t a f o r the p r o f i l e 75-1; s g u a r e s show d a t a f o r p r o f i l e 75-1R. &t d i s t a n c e s beyond 24 km, o n l y one time d i s t a n c e v a l u e per s h o t was used f o r a n a l y s i s . DISTANCE (KM) 64 Reduced t r a v e l t i m e d a t a and l e a s t s q u a r e s f i t t e d l i n e s f o r the f i r s t two phases of p r o f i l e s 75-1 and 75-1R. 16.0 18.0 D I S T A N C E 20,0 S l l ) 66 o n l y 5 p o i n t s which i s f a r l e s s than f o r b r a n c h e s one and two. T r a v e l time branch f o u r w i t h a v e l o c i t y c l o s e t o 7 . 0 km/s becomes t h e f i r s t a r r i v a l at about 4 if km and r e m a i n s as such u n t i l about 65 km where the 7 .8 km/s branch t a k e s o v e r . T h i s a r r i v a l , however, i s o n l y seen on p r o f i l e 7 5 - 1 ; the data were too poor to a l l o w i t t o be seen on p r o f i l e 7 5 - 1R. The reason f o r the poor data on 75-1R compared wit h 75-1 p r o b a b l y l i e s i n t h e f a c t t h a t the weather d u r i n g the 75-1R p r o f i l e was c o n s i d e r a b l y worse, l e a d i n g t o a much p o o r e r s i g n a l / n o i s e r a t i o . A f t e r i d e n t i f y i n g the f i r s t a r r i v a l b r a n c h e s , a s t r a i g h t l i n e was f i t t e d to t h e p o i n t s u s i n g t h e method of York ( 1 9 6 9 ) . H i s procedure i s a l e a s t s q u a r e s t e c h n i q u e i n which e r r o r s i n both the X and Y c o - o r d i n a t e s can be c o n s i d e r e d , i n c o n t r a s t to the u s u a l method t h a t a l l o w s o n l y f o r e r r o r s i n the Y co-o r d i n a t e . The t r a v e l t i me (Y) e r r o r s i n p u t i n t o t h e proqram a r e the t i m i n g e r r o r s d i s c u s s e d a t the s t a r t of t h i s c h a p t e r , whereas t h e d i s t a n c e (X) e r r o r s a r e t h o s e a s s o c i a t e d w i t h the s h o t - r e c e i v e r d i s t a n c e s d i s c u s s e d i n S e c t i o n 2 . 4 . F i g 4 .1 shows the l e a s t s q u a r e s f i t t e d l i n e s superimposed on the f i r s t a r r i v a l d a t a T a b l e 4.1 g i v e s t h e l e a s t s g u a r e s s l o p e s , c o r r e c t e d t o v e l o c i t y v a l u e s , and i n t e r c e p t s , w i t h the a s s o c i a t e d s t a n d a r d d e v i a t i o n s and the number of p o i n t s d e f i n i n g each l i n e . There a r e s e v e r a l methods f o r o b t a i n i n g the v e l o c i t y -depth model from t h e l e a s t s q u a r e s v e l o c i t i e s and i n t e r c e p t s . I d e c i d e d t o use the " S l o p e I n t e r c e p t " method of Ewinq e t a l 67 T a b l e 4.1 P r o f i l e T.T. Branch No. Pnts V e l o c i t y (km/s) I n t e r c e p t (s) 75-1 1 2 3 4 5 25 26 5 4 7 4.257 i 0.010 5. 35 1 + 0.005 6.260 ± 0.020 7 . 0 9 7 i 0.007 7.8251 0.040 4.154 ± 0. 4. 84 1 t 0. 5. Q700 ± 0, 6. 465 i 0. 7. 221 ± 0. 060 038 080 100 510 75-1R 1 2 3 4 20 26 5 5 4. 297± 0.009 5. 160± 0.005 6. 3301 0.015 7. 023 $ 0.0 10 4.001± 0.020 4. 535 ± 0.038 5. 550 ± 0.065 6. 241 + 0.096 For the f i r s t two t r a v e l time branches, a l l t r a c e s f o r each s h o t were t i m e d ; at g r e a t e r d i s t a n c e s o n l y one t i m e - d i s t a n c e v a l u e per sho t was used. T a b l e 4.2 L a y e r Dip V e l o c i t y (km/s) Thickness 7 5- 1 (km) 75-1R 1 2 3 4 5 6 7 0 0 0° 1.49° -2. 3 5 0 1.25° 00 00 1.49 2.801 4. 28 5.26 6.28 7. 04 7. 822 2.00 3.03 1.6 2.74 4.13 3. 76 00 ,75 1 ,45 ,98 1 assumed v e l o c i t y 2 u n r e v e r s e d v e l o c i t y 68 (1939). T h i s method assumes t h a t "each l a y e r i s homogeneous and bounded above and below by smooth p l a n e l a y e r s and t h a t t h e s e i s m i c v e l o c i t y i n each l a y e r i s h i g h e r than i n the l a y e r above ". T h i s method however does not assume t h a t the l a y e r s a r e f l a t l y i n g and t h e r e f o r e we can use i t to o b t a i n i n f o r m a t i o n about th e d i p s on t h e v a r i o u s r e f r a c t i n g h o r i z o n s . The v e l o c i t y depth models g e n e r a t e d f o r p r o f i l e s 75-1 and 75-1R are shown on F i g 4.3 w i t h t h e 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 g i v e n i n T a b l e 4.2. One i n t e r e s t i n g p o i n t c o n c e r n i n g t h e s e models i s the absence of s u b s t a n t i a l d i p s of t h e r e f r a c t i n g h o r i z o n s . For t h e uppermost l a y e r t h i s i s c o n s i s t e n t w i t h the c o n t i n u o u s s e i s m i c p r o f i l e (C.S.P.) d a t a a v a i l a b l e f o r Hinona b a s i n . T i f f i n e t a l (1972) and Hopkins ( 1976) both have C.S.P. l i n e s which show t h e o c e a n i c basement d i p p i n g r o u g h l y p e r p e n d i c u l a r to t h e d i r e c t i o n o f p r o f i l e s 75-1 and 75—1R. The sediment t h i c k n e s s p r e d i c t e d by t h i s model i s r o u g h l y 3 km and thus i s c o n s i d e r a b l y t h i c k e r than the 1.8 km g i v e n by t h e r e f l e c t i o n r e s u l t s . T h i s d i s c r e p a n c y i s due t o the a s s u m p t i o n , p r i o r to the a n a l y s i s of t h e r e f l e c t i o n d a t a , of a 2.8 km/s average v e l o c i t y f o r the s ediments. As the r e f l e c t i o n i n t e r p r e t a t i o n s e c t i o n i n d i c a t e s , the average v e l o c i t y i s s i g n i f i c a n t l y l e s s than t h i s , a c c o u n t i n g f o r the t h i n n e r l a y e r o f s e d i m e n t s . The f i r s t p o i n t t o be made about the model i s t h a t a l l the v e l o c i t i e s except the Pn v e l o c i t y a r e r e v e r s e d . T h i s l a c k of r e v e r s a l i s r e g r e t t a b l e e s p e c i a l l y i n the l i g h t of the 69 lis. Jia.1 V e l o c i t y vs depth models d e r i v e d from t h e f i r s t a r r i v a l a n a l y s i s . Datum water depth i s 2000 m. I n t h i s a n a l y s i s , an average v e l o c i t y o f 2.8 km/s f o r the s e d i m e n t s was assumed. Subsequent a n a l y s i s o f r e f l e c t i o n d a t a (see s e c t i o n 3.2) i n d i c a t e s t h i s i s t o o h i g h . 6 8 10 12 14 16 18 75 - 1 75 - IR 7 1 l a r g e r e l a t i v e e r r o r (0.040 compared w i t h 0.005 km/s) o f t h i s v e l o c i t y i n r e l a t i o n t o t h e o t h e r l a y e r v e l o c i t i e s . The f i r s t a r r i v a l a n a l y s i s p r e d i c t s a t o t a l s u b - b o t t o m t h i c k n e s s o f 15 km f o r t h e p r o f i l e w i t h r o u g h l y 12 km b e i n g s u b - s e d i m e n t ( o c e a n i c c r u s t ) . i i 2 S y n t h e t i c Seismograms W i d e s p r e a d u s e o f s y n t h e t i c s e i s m o g r a m s f o r i n t e r p r e t i n g m a r i n e s e i s m i c r e f r a c t i o n d a t a h a s been i m p l e m e n t e d o n l y w i t h i n t h e l a s t few y e a r s . I n t h e p a s t t h e i r use has been l i m i t e d by t h e e x p e n s e r e q u i r e d f o r t h e c o m p u t a t i o n s as w e l l as t h e l a r g e amounts o f d a t a n o r m a l l y a c q u i r e d i n t h i s t y p e o f work. More e f f i c i e n t a l g o r i t h m s and t h e n e c c e s s i t y o f o b t a i n i n g more d e t a i l e d i n f o r m a t i o n a b o u t t h e o c e a n i c c r u s t has e x p e d i t e d t h e a p p l i c a t i o n o f t h i s . m e t h o d . As an example, t h e e s t a b l i s h m e n t o f t h e " D i s c Ray T h e o r y " (DRT) c o n c e p t by W i g g i n s (1976) has p r o v i d e d a r e l a t i v e l y i n e x p e n s i v e and e a s y -t o - u s e method o f c a l c u l a t i n g s y n t h e t i c s e i s m o g r a m s . Chapman (1976 a,b) has p r o v i d e d t h e t h e o r e t i c a l f o u n d a t i o n f o r t h e method. At U.B.C. we a r e f o r t u n a t e t o have t h e use o f t h e program HRGLTZ ( w r i t t e n by W i g g i n s ) , which computes, among o t h e r q u a n t i t i e s , DRT s y n t h e t i c s e i s m o g r a m s . I t has been used b o t h by M a l e c e k (1976) and m y s e l f t o p r o v i d e t h e o r e t i c a l s e i s m o g r a m s f o r c o m p a r i s o n w i t h o b s e r v e d marine r e f r a c t i o n d a t a . The a l g o r i t m on which HRGLTZ i s based c a l c u l a t e s t r a v e l t i m e s and a m p l i t u d e s f o r a r r i v a l s d e f i n e d by an i n p u t 72 c u r v e . I t a l s o p e r f o r m s a W e i c h e r t - H e r g l o t z i n t e g r a t i o n o f t h e P-A c u r v e t o d e t e r m i n e t h e c o r r e s p o n d i n g v e l o c i t y - d e p t h (V-D) model and i n c l u d e s a m o d i f i c a t i o n such t h a t l o w - v e l o c i t y l a y e r s c a n be c o n s i d e r e d . Whereas we a r e i n t e r e s t e d f i n a l l y i n t h e V-D c u r v e p r o d u c e d by HRGLTZ, i t i s e x t r e m e l y d i f f i c u l t t o use t h i s c u r v e t o make c h a n g e s which r e f l e c t t h e d e s i r e d c h a n g e s i n t h e s y n t h e t i c s e i s m o g r a m s e c t i o n s . T h i s d i f f i c u l t y l i e s w i t h t h e i n t e r p r e t e r who must p r o p o s e a c h a n g e i n t h e V-D c u r v e t o p r o v i d e an e x p e c t e d change i n t r a v e l t i m e and a m p l i t u d e o f a g i v e n a r r i v a l . Much more c o n t r o l f o r t h i s p u r p o s e c a n be o b t a i n e d by w o r k i n g w i t h t h e P-A c u r v e a s i n d i c a t e d by t h e f o l l o w i n g two e g u a t i o n s . 1) T(P}&) - p £ * f A(P) dP 4.2-1 Where T(p,A) = t r a v e l t i m e Pmax=max P v a l u e ( 1 / V s u r f a c e ) The t r a v e l t i m e , t h e n , c an be t h o u g h t o f a s t h e a r e a u n d e r t h e P-A c u r v e , p r e s e n t i n g us w i t h a s i m p l e method of c h a n g i n g t h e t r a v e l t i m e o f a r r i v a l s ( W i g g i n s and M a d r i d 1974). 2) A (p,A)=F (p,A) J dP/dAl life 4. 2-2 where A(p,£) = a m p l i t u d e o f t h e a r r i v a l F(P#A)= a complex f u n c t i o n s l o w l y v a r y i n g w i t h r e s p e c t t o |dP/dA7i / 2 ( B u l l e n 1963). The l a s t e q u a t i o n s t a t e s t h a t t h e a m p l i t u d e o f an a r r i v a l i s d i r e c t l y p r o p o r t i o n a l t o t h e s q u a r e r o o t o f t h e s l o p e o f t h e P-A c u r v e a t t h a t p o i n t . W o r k i n g w i t h t h e P - A c u r v e t h e n a l l o w s t h e i n t e r p r e t e r much 73 more c o n t r o l o v e r t h e t r a v e l t i m e s and a m p l i t u d e s than c o u l d be o b t a i n e d by use o f t h e V-D c u r v e a l o n e . The i n e x p e n s i v e n a t u r e of HRGLTZ and the p r e s e n c e at U.B.C. o f an e x c e l l e n t Adaqe G r a p h i c s T e r m i n a l a l l o w e d the t r i a l o f many d i f f e r e n t models i n a s h o r t t i m e . The use of the g r a p h i c s t e r m i n a l a l l o w e d an almost i n s t a n t a n e o u s comparison between t h e s y n t h e t i c s and t h e r e a l d a t a , e l i m i n a t i n g t h e need f o r e x p e n s i v e computer p l o t s which had a c o n s i d e r a b l y l o n g e r t u r n - a r o u n d t i m e . A computer r o u t i n e named MDLPLT, a l s o w r i t t e n by Wig g i n s , was used t o p r o v i d e t h e p r e l i m i n a r y P-A c u r v e f o r i n p u t i n t o HRGLTZ . T h i s r o u t i n e a p p r o x i m a t e s a c o n t i n u o u s P - A c u r v e from an i n p u t V-D model a c c o r d i n g to t h e e q u a t i o n s o f B u l l e n (1963,p 112). T h i s proqram then d i q i t i z e s t h e P-A c u r v e f o r f u t u r e i n p u t i n t o HRGLTZ. T h i s d i g i t i z i n q r a t e ( sampling i n t e r v a l ) v a r i e s over the ranqe of the' P - A c u r v e , r a p i d l y c h a n q i n g p a r t s o f t h e c u r v e r e q u i r i n q a h i q h e r s a m p l i n q r a t e than more s l o w l y v a r y i n g s e c t i o n s . As o f t e n happens, i t was not p o s s i b l e t o model the r e a l data p e r f e c t l y . A t r a d e - o f f between a m p l i t u d e s , s l o p e s of a r r i v a l branches and t r a v e l t i m e s was n e c e s s a r y s i n c e c hanqinq one u s u a l l y a f f e c t e d the o t h e r s . G r e a t e r emphasis was p l a c e d on f i t t i n q t h e t r a v e l t i m e s " t h a n on f i t t i n q a m p l i t u d e s , s i n c e t h e t r a v e l t i m e d e t e r m i n a t i o n s were c o n s i d e r a b l y l e s s s u b j e c t i v e t h a n a m p l i t u d e de te rminat i o n s 74 4 ._3 Ajsr>l i c a t i o n t o Data A p r e l i m i n a r y V-D model b a s e d on t h e s e d i m e n t s t r u c t u r e f r o m t h e r e f l e c t i o n d a t a f o r 75-1S ( T a b l e 3.2) and t h e c r u s t a l s t r u c t u r e from t h e f i r s t a r r i v a l a n a l y s i s o f 75-1 ( T a b l e 4.2) was us e d as t h e s t a r t i n g model f o r t h e s y n t h e t i c c a l c u l a t i o n s . T h i s model was i n p u t t o MDLPLT i n o r d e r t o o b t a i n t h e p r e l i m i n a r y P - A c u r v e , t h i s c u r v e i n t u r n b e i n q used a s a s t a r t i n g model f o r 75-1 s y n t h e t i c s . S e v e r a l p r o b l e m s were e n c o u n t e r e d w i t h t h e program MDLPLT, n o t t h e l e a s t o f w h i c h was i t s r e f u s a l t o o u t p u t t h e same V-D model as was i n p u t . The d e g r e e o f m a t c h i n g between t h e o u t p u t and i n p u t models depended d i r e c t l y on t h e s a m p l i n g i n t e r v a l s p e c i f i e d f o r t h e P- A c u r v e . The program u s e s t h e i n p u t model t o c a l c u l a t e an a p p r o x i m a t e P-A c u r v e as s p e c i f i e d p r e v o u s l y . I t t h e n c a l l s HRGLTZ and p r o d u c e s T-A, V-D p l o t s and s y n t h e t i c s . The o u t p u t V-D model t h e n d epends on how f i n e l y t h e P - c u r v e i s s p e c i f i e d by t h e i n i t i a l d i g i t i z a t i o n . T h i s d i f f i c u l t y i n g e t t i n g MDLPLT t o g e n e r a t e a P-A c u r v e which c l o s e l y f i t t h e i n p u t model made MDLPLT i m p o s s i b l e t o use f o r a c t u a l m o d e l l i n g . S i n c e o n l y a s t a r t i n g model was r e g u i r e d however, t h i s p r o v i d e d no s e r i o u s p r o b l e m s . The p r e l i m i n a r y P-Zi c u r v e was a l t e r e d by t h e famous t r i a l - a n d - e r r o r method, k e e p i n g i n mind e g u a t i o n s 4.2-1 and 4.2-2, t o f i t t h e t r a v e l t i m e s amd a m p l i t u d e s o f f i r s t and s e c o n d a r r i v a l s f o r p r o f i l e 75-1. T h i s f i n a l P-A c u r v e was t h e n a l t e r e d f u r t h e r t o f i t 75-1R. F o r s u c h c a l c u l a t i o n s i t i s i m p o r t a n t t o be aware of' t h e d i f f i c u l t i e s and t r a d e - o f f s 75 mentioned p r e v i o u s l y . i i i i Record S e c t i o n s As a f i n a l p r e l u d e t o t h e c a l c u l a t i o n of s y n t h e t i c seismograms and hence a complete i n t e r p r e t a t i o n based on a l l a v a i l a b l e i n f o r m a t i o n , the r e f r a c t i o n data were c o m p i l e d i n t o r e c o r d s e c t i o n s as d e s c r i b e d i n s e c t i o n 2.4. F i g s 4.4 and 4.5 a r e t h e r e c o r d s e c t i o n s of p r o f i l e s 75-1 and 75-1R, r e s p e c t i v e l y , f o r d a t a r e c o r d e d a t d i s t a n c e s g r e a t e r than 6 km. The s e c t i o n s have been c o r r e c t e d f o r charge s i z e , a m p l i f i e r g a i n , s p h e r i c a l s p r e a d i n g and hydrophone s e n s i t i v i t y as d e s c r i b e d i n s e c t i o n 2.4. These complete p r o f i l e s a r e cumbersome to handle c o m p u t a t i o n a l l y . As a consequence, t h e data were s t a c k e d f o r t h e purpose of i n c r e a s i n g the s i g n a l / n o i s e r a t i o and d e c r e a s i n g t h e amount o f d a t a to be h a n d l e d . a a l e c e k (1976) has shown t h a t a s i m p l e l i n e a r s t a c k a l o n g a l a g t r a j e c t o r y d e f i n e d by the f i r s t a r r i v a l v e l o c i t y was optimum f o r t h i s t y p e of d a t a . Such a s t a c k i n g procedure was a p p l i e d t o p r o f i l e 75-1, t h e r e s u l t i n g p r o f i l e b e i n g shown on F i g 4.6. U n f o r t u n a t e l y , a t a p p r o x i m a t e l y 35 km on p r o f i l e 75-1R, the weather c o n d i t i o n s d e t e r i o r a t e d c o n s i d e r a b l y . T h i s r e s u l t e d i n a d e c r e a s e i n c o r r e l a t i o n between t r a c e s and c o n s e q u e n t l y a l a r g e d e c r e a s e i n the s t a c k e d a m p l i t u d e s beyond t h i s d i s t a n c e . S i n c e t h i s was an a m p l i t u d e v a r i a t i o n i n t r o d u c e d t h r o u g h p r o c e s s i n g i t made the p r o f i l e u n a c c e p t a b l e . As a compromise a s e c t i o n was c o m p i l e d u s i n g o n l y t h e data from c h a n n e l 3, t h i s Record s e c t i o n o f p r o f i l e 75-1 beyond the c r i t i c a l d i s t a n c e f o r t h e f i r s t c r u s t a l r e f r a c t i o n a r r i v a l . A l l t r a c e s have been a m p l i t u d e c o r r e c t e d as per s e c t i o n 2.4 and f i l t e r e d 5.0-30 hz. Prominent s e c o n d a r y a r r i v a l s from 2.5 t o 10 s a f t e r t h e f i r s t a r r i v a l s a r e p r o b a b l y m u l t i p l e s o f e a r l i e r phases. Record s e c t i o n of d a t a f o r p r o f i l e 75 -1 a f t e r s t a c k i n g t o form a s i n g l e t r a c e per s h o t . A l l t r a c e s have been a m p l i t u d e c o r r e c t e d and f i l t e r e d 5.0-30 hz. As d i s c u s s e d i n t h e t e x t , an a d d i t i o n a l a m p l i t u d e c o r r e c t i o n was a p p l i e d to t h e t r a c e a t a d i s t a n c e of 32 km. Record s e c t i o n o f p r o f i l e 75-1R c h a n n e l 3's. A l l t r a c e s have been a m p l i t u d e c o r r e c t e d and f i l t e r e d 5-0-30 hz. 83 c h a n n e l h a v i n g p a r t i c u l a r l y good r e c o r d i n g s (see F i g 4 . 7 ) . Whereas t h i s p r o c e d u r e d i d l i t t l e t o i n c r e a s e the s i g n a l / n o i s e r a t i o , i t d i d a l l o w a c o n s i d e r a b l e s a v i n g i n d a t a h a n d l i n g and hence computing c o s t s . A l t h o u g h the s i g n a l / n o i s e r a t i o was not improved, f o r most o f t h e s e c t i o n c l e a r a r r i v a l s c o u l d s t i l l be f o l l o w e d . Both p r o f i l e s b e g i n w i t h a r e l a t i v e l y s i m p l e a r r i v a l h a v i n g a s u b s t a n t i a l a m p l i t u d e , emerging from the water bottom r e f l e c t i o n s . T h i s a r r i v a l w i t h l e a s t s q u a r e s v e l o c i t y o f 4*28 km/s p e r s i s t s as such f o r o n l y a s h o r t d i s t a n c e . At about 17 km on both p r o f i l e s , t he f i r s t a r r i v a l becomes much more complex, h a v i n g about two times the l e n g t h o f t h e i n i t i a l f i r s t a r r i v a l ; t he a m p l i t u d e , however, does not change s i g n i f i c a n t l y . At about 2 1 km on both p r o f i l e s , t he f i r s t a r r i v a l s a g a i n appear t o i n c r e a s e i n c o m p l e x i t y , w i t h t h e a m p l i t u d e g r a d u a l l y i n c r e a s i n g w i t h time a l o n g the t r a c e . There a l s o appears t o be an o v e r a l l i n c r e a s e i n the a m p l i t u d e of t h e a r r i v a l s a t t h i s p o i n t . Between 29 and 36 km on F i g 4.4, we have a s e r i e s of t h r e e a r r i v a l s w i t h v a r y i n g a m p l i t u d e s . The o u t s i d e two t r a c e s have by f a r the l a r g e s t a m p l i t u d e of any r e f r a c t i o n a r r i v a l s on e i t h e r p r o f i l e , whereas the i n n e r t r a c e has an a m p l i t u d e c o n s i s t e n t w i t h the gen'eral a m p l i t u d e t r e n d o f t h e p r o f i l e . On the b a s i s of l a t e r a r r i v a l s i t was d e c i d e d t o i n c r e a s e the a m p l i t u d e o f t h e i n n e r t r a c e t o make i t c o n s i s t e n t w i t h t h e o u t e r two. T h i s can be seen by comparing F i g 4.4 and 4.6 over t h e d i s t a n c e ranqe 29-36 km. T h i s was done b e f o r e the c o m p i l a t i o n of the 75-1R r e c o r d s e c t i o n s and b e f o r e c a l c u l a t i n q s y n t h e t i c seismoqrams. 84 I t a p p e a r s , however, a f t e r s t u d y i n g the s e c t i o n s c o r r e s p o n d i n g t o 75-1R and a f t e r w o r k i n g w i t h t h e s y n t h e t i c c a l c u l a t i o n s , t h a t the i n n e r a m p l i t u d e s h o u l d have been l e f t as i t was and the o u t e r two a m p l i t u d e s dropped. Once a g a i n t h e c o m p l e x i t y o f t h e a r r i v a l i n c r e a s e s a t about 29 km on both p r o f i l e s . The e f f e c t of the d e t e r i o r a t i n g weather d u r i n g t h e 75-1.R p r o f i l e can c l e a r l y be seen on F i g 4.5 where the background n o i s e l e v e l i n c r e a s e s c o n s i d e r a b l y a t about 35 km. The a m p l i t u d e and c o m p l e x i t y o f t h e a r r i v a l s r e m a i n more o r l e s s c o n s t a n t out to about 55 km on b oth p r o f i l e s . P a s t t h i s p o i n t t h e r e i s a s i g n i f i c a n t break i n the a m p l i t u d e s of t h e a r r i v a l s on both p r o f i l e s . At f i r s t t h i s was t h o u g h t t o be a p r o c e s s i n g problem. However, a f t e r c a r e f u l s t u d y o f l a t e r a r r i v a l s (11.5 s ) , i t was d e c i d e d t h a t t h i s s h a r p a m p l i t u d e drop was p h y s i c a l l y r e a l . The l e n g t h o f the a r r i v a l s a l s o appear t o i n c r e a s e c o n s i d e r a b l y beyond t h i s p o i n t , w i t h the m a j o r i t y o f t h e a m p l i t u d e b e i n g due t o W.A.R. phases. The f i r s t a r r i v a l can be f o l l o w e d w i t h l i t t l e t r o u b l e out to t h e very l a s t s h o t on p r o f i l e 75-1, (see F i g 4.6); however i t soon becomes l o s t i n t h e background n o i s e on 75-1R p a s t about 67 km.1 Perhaps the most s t r i k i n g f e a t u r e o f b o t h p r o f i l e s i s the l a c k of s e c o n d a r y a r r i v a l s . F i r s t and second water bottom m u l t i p l e s can be seen c l e a r l y on both p r o f i l e s a t s h o r t d i s t a n c e s (8.5 s and 10.5 s ) ; however they add n o t h i n g to the i n t e r p r e t a t i o n and w i l l not be d i s c u s s e d . F i g 4.8 shows the 85 l a s t 38 km of 75-1 covering a l l the shots beyond the point at which the amplitudes decrease considerably . The lower p r o f i l e i s f i l t e r e d 5.0-30 hz and shows both the f i r s t a r r i v a l s and the W .A.R. energy discussed prev ious ly . The upper p r o f i l e shows the same data f i l t e r e d 2.0-30 hz. This f i l t e r i n g range reduced the c l a r i t y of the f i r s t break information but i t does show a s i g n i f i c a n t a r r i v a l s t a r t ing at about 6.5 s at a distance of 61 km, and.being cor re l a t ab le along the remaining length of the p r o f i l e . I t i s i d e n t i f i e d as branch f on Fig 4.8. This secondary a r r i v a l i s a c lear i n d i c a t i o n of the benefit achieved by viewing the data under various f i l t e r i n g l i m i t s , as i t s presence would not have been recognised on the 5.0-30 hz p r o f i l e alone. The same procedure was t r i e d on the 75-1R data, but the high background noise l e v e l and the lack of stacking hindered the processing and no eguivalent a r r i v a l s could be discerned. An even l a t e r secondary a r r i v a l that i s observable s t a r t s at about 40 km on p r o f i l e 75-1 and i s i d e n t i f i e d by a "?" on Fig 4.11. The amplitude associated with th i s a r r i v a l quickly dies out and at f i r s t i t does not appear to l i n e up with the cont inuat ion of any f i r s t a r r i v a l or wide anqle r e f l e c t i o n branch. I t s presence, however, did play a ro le in the synthet ic ca l cu la t ions and w i l l be mentioned further in that s e c t i o n . The two p r o f i l e s then are remarkably s i m i l a r with respect to t h e i r amplitude s t ructures . Most of the di f ferences in amplitude can be e i ther d i r e c t l y a t t r ibuted to processing problems or an increase in the ambient noise l e v e l between the Expanded p l o t o f t h e l a s t 40 • km o f p r o f i l e 75-1 w i t h the t r a v e l t i m e c u r v e s o f t h e f i n a l model superimposed. Upper s e c t i o n i s f i l t e r e d 2.0 to 15 hz ; l o w e r s e c t i o n i s f i l t e r e d 5,0 t o 30 hz. Arrows on the lower s e c t i o n g i v e the t r a v e l t i m e p i c k s . Note t h e c o h e r e n t phases (on t h e upper s e c t i o n ) f o l l o w i n g t r a v e l time branch f . ^7~00 bT0O 6 ^ 0 0 69 00 73.00 77.00 81.00 85.00 89.00 93.00 97.00 DISTRNCE (KM! o DISTRNCE (KM) 88 two p r o f i l e s . 4„_5 Comparison o f Sj/n t het i c s w i t h R e a l Data As was d i s c u s s e d i n s e c t i o n 4.2 t h e o n l y s i g n i f i c a n t v a r i a t i o n s between p r o f i l e s 75-1 and 75-1R i s a 200 ms d i f f e r e n c e i n t r a v e l t i m e s between t h e two p r o f i l e s . What a m p l i t u d e d i f f e r e n c e s t h e r e a r e can be e x p l a i n e d e i t h e r as p r o c e s s i n g problems, see f o r example t h e l a r g e a m p l i t u d e s i n the range 27-35 km on 75-1, o r as b e i n g due t o t h e poorer q u a l i t y data o f p r o f i l e 75-1R compared w i t h 75-1. For t h i s r e a s o n , the f i t o f the s y n t h e t i c seismograms to t h e r e a l d a t a f o r both p r o f i l e s w i l l be d i s c u s s e d s i m u l t a n e o u s l y . The s t a r t i n g P-^ c u r v e g e n e r a t e d by MDLPLT and t h e c o r r e s p o n d i n g s y n t h e t i c seismogram s e c t i o n a re shown on F i g s 4.9 and 4.10. The t r a v e l t i m e f i t between the s t a r t i n g model and p r o f i l e 75-1 i s e x t r e m e l y c l o s e , i n c l u d i n g p o s s i b l e s e c o n d a r y a r r i v a l s , and r e g u i r e s l i t t l e a l t e r a t i o n . The ma-jor s o u r c e of d i s c r e p a n c y between the s t a r t i n g model s y n t h e t i c s and t h e r e a l d a t a f o r p r o f i l e 75-1 i s t h e presence o f l a r g e W.A.R. phases p r e s e n t on t h e s y n t h e t i c s . These W.A.R. 's a r e ge n e r a t e d by the d i s c o n t i n u o u s v e l o c i t y i n c r e a s e s which a r e g i v e n by th e f i r s t a r r i v a l a n a l y s i s . By f a r t h e major e f f o r t a s s o c i a t e d w i t h f i t t i n g s y n t h e t i c s t o t h e r e a l d a t a has been i n t h e removal of these w.A.R. phases from t h e s y n t h e t i c s , w h i l e s t i l l r e t a i n i n g a r e a s o n a b l e t r a v e l time f i t . The f i n a l s y n t h e t i c seismogram s e c t i o n s f o r both p r o f i l e s , t o g e t h e r w i t h t h e r e a l d a t a a r e shown on F i g s 4.11 and 4.12. The f i n a l P - A P-A curve corresponding to the i so -v e l o c i t y layered model. used as a s t a r t i n g model for c a l c u l a t i o n of synthet ic seismograms for comparison with p r o f i l e 75-1. S y n t h e t i c seismograra s e c t i o n f o r the i s o - v e l o c i t y l a y e r e d model d e r i v e d from t h e r e f l e c t i o n d a t a a n a l y s i s and f i r s t a r r i v a l r e f r a c t i o n d a t a a n a l y s i s . Note t h e l a r g e a m p l i t u d e s a s s o c i a t e d w i t h the wide a n g l e r e f l e c t i o n s from the v e l o c i t y d i s c o n t i n u i t i e s . Compare w i t h t h e o b s e r v e d s e c t i o n s of F i g s 4.'6 and 4.7. 9 2 m "cn 03 m 01 "ID m "in rn r— a m "rn m "IM m 4 Z 5 J J E (D3S) 0'9 /a-L 93 Fig Ujj.11, Top: Synthetic seisraoqrains and t r a v e l times computed to f i t the stacked data of p r o f i l e 75-1. The inse r t shows the source wavelet which was convolved with the r e s u l t s of the synthetic c a l c u l a t i o n . Bottom: Data from p r o f i l e 75-1 with synthetic trave l times superimposed. Lower case l e t t e r s have been added to i d e n t i f y the various t r a v e l time branches. The arrows show the f i r s t a r r i v a l picks made from the raw data. The phase c o r r e l a t i o n shown by a "?" seems to l i n e up with the c-d r e f r a c t i o n branch. LO ^ . O O lTbl V3.00 27.00 35~00 43.00 51.00 59 U o 67.00 75.00 83.00 91 .00 99.00 DISTANCE (KM) DISTRNCE (KM) f i g 4 . 1 2 . Same as F i g 4 . 1 1 , f o r p r o f i l e 7 5 - 1 R P - A c u r v e from which t h e s y n t h e t i c s f o r p r o f i l e 75-1 are computed {Fig 4 . 1 1 ) . S t a r t i n g p-& c u r v e ( F i g 4.9) i s superimposed f o r c o m p a r i s o n . Zia B.2.1H' Same as F i g 4 . 1 3 f o r p r o f i l e 75- 1R. Comparison o f f i n a l p - A c u r v e s f o r p r o f i l e s 75-1 and 75-1E. 86 66 100 curves are shown on Figs 4.13 and 4.14. Sediments : I t was not the aim of the synthet ic c a l c u l a t i o n s to del ineate the sedimentary s t ruc ture . The sediments are included only to ensure the correct t r a v e l time to the f i r s t r e f r a c t i o n branch (c-d of Figs 4.11 and 4.12). For t h i s reason only the l a s t r e f l e c t i o n from the sediments, i . e . the r e f l e c t i o n off the top of the oceanic basement , i s shown on the synthet ic sect ions (branch b - c ) . This branch was shown in an e f for t to c a l c u l a t e the correct amplitude for the f i r s t set of r e f r a c t i o n a r r i v a l s only. I t was found that i n order to ensure the correct t r a v e l times to both the 75-1 and 75-IR f i r s t r e f r a c t i o n a r r i v a l s a di f ference in the sediment thickness of about 200 m between the two p r o f i l e s was needed, with 75-1 being the deeper. Crus ta l Layers : As can be seen on Figs 4.4 and 4.5, over the distance range 5-15 km the f i r s t r e f rac t ion branch (c-d) i s the f i r s t a r r i v a l . The amplitudes of these a r r i v a l s are quite s u b s t a n t i a l , being far l arger than the corresponding amplitudes generated on the prel iminary synthet ics of F ig 4.10. I had very l i t t l e or no success in modelling these large amplitudes on e i ther p r o f i l e . That t h i s a r r i v a l i s a head wave can be seen c l e a r l y on Fiq 3. 1 where i t can be traced back to the l a s t s u b - c r i t i c a l incidence r e f l e c t i o n that could be 101 t i m e d . The f a c t t h a t t h i s a r r i v a l i s a pure head wave i s s i g n i f i c a n t i n t h a t nowhere e l s e on the p r o f i l e s do we enc o u n t e r an i s o l a t e d head wave w i t h any s u b s t a n t i a l a m p l i t u d e . The. l a r g e a m p l i t u d e s on the s e c t i o n s are due mainly t o energy a s s o c i a t e d w i t h W.A.R.'s; head wave a m p l i t u d e s a r e much s m a l l e r . At t h e s t a r t o f t h e m o d e l l i n g , both t h e s l o p e and t r a v e l time o f branch c-d agreed very w e l l w i t h the r e a l d a t a . D u r i n g t h e p r o c e s s o f a t t e m p t i n g t o g e n e r a t e s u b s t a n t i a l a m p l i t u d e s f o r t h e s e a r r i v a l s , t h e s l o p e o f t h e branch was d e c r e a s e d c o n s i d e r a b l y and the t r a v e l time f i t became much p o o r e r . T h i s i s an e x c e l l e n t example o f t h e t r a d e - o f f mentioned i n s e c t i o n 4.3. In o r d e r t o model the t r a v e l t i m e s and the s l o p e of t h i s branch c, c o r r e c t l y I would have been l e f t w i t h no a m p l i t u d e a t a l l f o r t h e a r r i v a l s . I n an e f f o r t to b u i l d up t h e s e a m p l i t u d e s , tne f i t of the t r a v e l t i m e s was re d u c e d and t h e s l o p e of t h e a r r i v a l branch lowered c o n s i d e r a b l y . One i n t e r e s t i n g p o i n t t o note i s t h a t the secondary a r r i v a l i d e n t i f i e d on F i g 4.6 w i t h a " ? " now l i n e s up f a i r l y w e l l w i t h the c o n t i n u a t i o n o f t h i s t r a v e l time branch. I t i s p o s s i b l e t h e r e f o r e t h a t t h i s s e c o n d a r y a r r i v a l i s i n some way r e l a t e d t o ' t h e f i r s t r e f r a c t i o n / W . A . R . a r r i v a l . The c o r r e l a t i o n between t h e two though i s t e n t a t i v e and t h e s e c o n d a r y a r r i v a l i s not seen a t a l l on 7 5 - 1 R . To c o n c l u d e t h e n , I do not f e e l t h a t we can model t h i s f i r s t r e f r a c t i o n a r r i v a l by u s i n g t h e DRT method. The s i t u a t i o n i s l i k e l y t o be more complex than we are a t t e m p t i n g to model, t h u s c a u s i n g the f i t between the 102 s y n t h e t i c s and t h e r e a l d a t a t o be so poor. The reason f o r t h i s i s d i s c u s s e d i n s e c t i o n 5.2. C o n s i d e r a b l y more s u c c e s s was a c h i e v e d m o d e l l i n g both t r a v e l t imes and a m p l i t u d e s f o r t h e remainder o f t h e p r o f i l e . The t r a v e l t i me f o r t h e second r e f r a c t i o n branch e-f has been f i t e x t r e m e l y w e l l . As was mentioned e a r l i e r , t h i s a r r i v a l i s c o n s t r a i n e d by t h e presence of W.A.R. e n e r g i e s p r e s e n t t o the end o f t h e p r o f i l e (see F i g 4.8, a r r i v a l b r a n c h f ) . The a r r i v a l s as w e l l as t h e model t r a v e l t i m e branches a r e shown on t h i s f i g u r e . The f i r s t two W.A.R. branches (d-e,f-g) have been used to ge n e r a t e the i n c r e a s e d c o m p l e x i t y o f t h e a r r i v a l s i n the r e g i o n 15-27 km. As a l r e a d y mentioned, the W.A.R. branch d-e now l i n e s up somewhat w i t h t h e second a r r i v a l shown w i t h a " ? " on F i g 4.6. The sudden b u r s t o f energy over the range 29-35 km on r e a l p r o f i l e 75-1 has a l r e a d y been e x p l a i n e d as an a m p l i t u d e o v e r - c o r r e c t i o n and th u s no attempt was made t o f i t i t . I n s t e a d I attempted t o f i t a more or l e s s c o n s t a n t a m p l i t u d e throughout t h i s r e g i o n on both p r o f i l e s . W.A.R. branches f -g and h - i were used t o generate the i n c r e a s e d c o m p l e x i t y of the a r r i v a l s o v e r t h i s r e g i o n w i t h branch f -g be i n g used t o s t r e t c h out th e a r r i v a l i n ti m e . I t i s a t t h i s point, t h a t t h e two p r o f i l e s d i f f e r s l i g h t l y . The t r a v e l t i m e s between t h e two were s l i g h t l y d i f f e r e n t and t o compensate f o r t h i s I had t o move the c a u s t i c a t p o s i t i o n s i and k a p p r o x i m a t e l y 7 km f u r t h e r a l o n g the p r o f i l e on 75-1. T h i s a l l o w e d me t o f i t the l a s t two 75-1 103 t r a v e l t i me b r a n c h e s , i - j and k-1, w i t h c o n s i d e r a b l y more a c c u r a c y . T h i s procedure however caused a s l i g h t d e l a y i n the s t a r t of t h e l a r g e a m p l i t u d e s on p r o f i l e 75-1 by about 5-7 km, b e i n g a n o t h e r example of a t r a d e o f f between a m p l i t u d e s and t r a v e l t i m e s . The W.A.R. branches h - i , j - k and t h e r e f r a c t i o n branch k-1 p r o v i d e a r r i v a l a m p l i t u d e s f o r t h e rem a i n d e r o f t h e s y n t h e t i c p r o f i l e s . They were used t o generate the f i n a l s e t of a r r i v a l energy over the range 45-95 km. The r e f r a c t i o n b r anches a s s o c i a t e d w i t h each one have a l r e a d y been d i s c u s s e d and f i t t h e f i r s t a r r i v a l t r a v e l t i m e s e x t r e m e l y w e l l . What rema i n s i s t o see how w e l l the W.A.R. a m p l i t u d e s f i t t h e r e a l d a t a . There i s a s i g n i f i c a n t d e c r e a s e i n a m p l i t u d e s on the r e a l d a t a at about 61 km. The s y n t h e t i c s e c t i o n s f o r both p r o f i l e s model t h i s as w e l l as can be ex p e c t e d . I t i s e x t r e m e l y d i f f i c u l t t o model s h a r p a m p l i t u d e v a r i a t i o n s and I f e e l t h a t t h i s has been done as w e l l as t h e program a l l o w s . The a m p l i t u d e s o f the W.A.R. branches h - i and j - k have been f i t v e r y w e l l w i t h t h e a m p l i t u d e s s t a r t i n g out f a i r l y h i g h a t about 55 km and then d e c r e a s i n g u n t i l they a r e j u s t i d e n t i f i a b l e a t t h e end o f the p r o f i l e . The f i n a l a r r i v a l a m p l i t u d e g e n e r a t e d by t h e r e f r a c t i o n b ranch k-1 has n o t been f i t w e l l . C o n s i d e r a b l e t i m e was spent i n t r y i n g t o d e c r e a s e t h e a m p l i t u d e o f t h i s a r r i v a l w i t h l i t t l e or no s u c c e s s . T h i s i s most l i k e l y a program problem and not p h y s i c a l l y r e a l . Such a c o n c l u s i o n was reached on the b a s i s o f both my own r e s u l t s and the r e s u l t s of Malecek 104 ( 1 9 7 6 ) . H i s s y n t h e t i c s e i s m o g r a m s e c t i o n s a l s o show t h i s p r o b l e m w i t h t h e f i n a l r e f r a c t i o n a r r i v a l . I t w o u l d be u s e f u l t o t e s t w h e t h e r o r n o t t h i s i s a p r o g r a m m i n g p r o b l e m b y e i t h e r u s i n g a d i f f e r e n t method f o r c o m p u t i n g t h e s y n t h e t i c s ( e g . G e n e r a l i s e d Ray T h e o r y ; H i g g i n s and H e l m b e r g e r 1 9 7 4 ) , o r by a r t i f i c i a l l y i n t r o d u c i n g a f u r t h e r l a y e r b e n e a t h t h e f i n a l l a y e r t h a t g e n e r a t e s t h e Pn a r r i v a l s . The s y n t h e t i c s f o r t h e two p r o f i l e s f i t a b o u t r e a s o n a b l y w e l l , c o n s i d e r i n g t h e l i m i t a t i o n s o f t h e m o d e l l i n g p r o c e d u r e . The p r o c e d u r e a s s u m e s h o r i z o n t a l p l a n e l a y e r s , a s i t u a t i o n t h a t d o e s n o t h o l d c o m p l e t e l y i n p r a c t i c e a s shown by t h e s l i g h t d i p s i n t e r p r e t e d f r o m t h e f i r s t a r r i v a l a n a l y s i s . The m ethod a l s o does n o t t a k e i n t o a c c o u n t d i s p e r s i o n o r a t t e n u a t i o n , l a t e r a l v a r i a t i o n s i n t h e e l a s t i c p r o p e r t i e s o f t h e l a y e r s o r s u r f a c e t o p o g r a p h y ; a n d so i t c a n n o t c o n s i d e r t h e v a r i a b l e n a t u r e o f t h e s o u r c e w a v e l e t due t o t h e l o w p a s s f i l t e r e f f e c t o f t h e E a r t h . A l l o f t h e a b o v e a r e l i k e l y t o make t h e a r r i v a l s more c o m p l e x and r e v e r b e r a t o r y t h a n i s a l l o w e d f o r by t h e m o d e l l i n g p r o c e d u r e . 105 5 INTERPRETATION AND DISCUSSION V e l o c i t y - D e p t h Models B e f o r e a t t e m p t i n g t o i n t e r p r e t the meaning of the f i n a l v e l o c i t y - d e p t h models produced by t h e s y n t h e t i c c a l c u l a t i o n s , i t i s u s e f u l t o . d i s c u s s the s i g n i f i c a n c e o f the c u r v e s produced w i t h r e s p e c t t o t h e m o d e l l i n g p r o c e d u r e . The f i n a l V-D c u r v e s a r e g i v e n i n F i g 5. 1. The f i r s t p o i n t t o note i s t h a t the s e d i m e n t a r y s t r u c t u r e shown s h o u l d not be c o n s i d e r e d as m e a n i n g f u l i n any sense o t h e r than i n the p r e s e r v a t i o n o f the t r a v e l time of a r r i v a l branch c-d. Not a l l t h e s e d i m e n t a r y l a y e r s d e f i n e d by t h e a n a l y s i s of r e f l e c t i o n p r o f i l e 75-1R a r e shown on the dia g r a m s . T h i s i s a d i r e c t r e s u l t of the d i f f i c u l t i e s a s s o c i a t e d w i t h t h e program MDLPLT, which was used t o g e n e r a t e t h e s t a r t i n g models f o r the s y n t h e t i c s . P r o b a b l y because of t h e t h i n s e d i m e n t a r y l a y e r s and a p p r o x i m a t i o n s w i t h i n the program, i t was i m p o s s i b l e t o g e n e r a t e a V-D model f o r the se d i m e n t s t h a t c o n t a i n e d a l l the l a y e r s d e f i n e d i n T a b l e 3.2. The o n l y e f f e c t o f t h e s y n t h e t i c c a l c u l a t i o n s has been to deepen the s e d i m e n t s f o r p r o f i l e 75-1 by a p p r o x i m a t e l y 200 m. T h i s has been done t o c o r r e c t f o r the a p p r o x i m a t e l y 200 ms o f f s e t between t h e t r a v e l t i m e s o f p r o f i l e s 75-1 and 75-1R. Other than t h i s t h e s y n t h e t i c s have had no e f f e c t on the s e d i m e n t a r y s t r u c t u r e . The l a y e r i d e n t i f i e d as l a y e r 2a on F i g 5 .1 (C) F i n a l v e l o c i t y vs depth c u r v e s f o r p r o f i l e s 75-1 and 75-1R d e t e r m i n e d by f i t t i n g both t r a v e l t i m e s and a m p l i t u d e s . A l s o shown i s the s t a r t i n g model d e t e r m i n e d by the t r a v e l time f i t a l o n e . 107 on 1 — I — 4 h i's 1B I I I I I DEPTH (KM) < ca u 108 r e p r e s e n t s t h e t r a n s i t i o n f r o m t h e s e d i m e n t s t o t h e b a s a l t i c l a y e r . T h i s l a y e r i s r e s p o n s i b l e f o r t h e l a r g e a m p l i t u d e r e f r a c t i o n a r r i v a l s on F i g 4.11 and 4.12, b r a n c h c - d . The p r o b l e m s e n c o u n t e r e d i n t r y i n g t o f i t t h e s e l a r g e a m p l i t u d e s h a v e p r e v i o u s l y been d i s c u s s e d i n S e c t i o n 4.5, t h e c o n c l u s i o n b e i n g t h a t t h e a c t u a l s t r u c t u r e o f t h i s l a y e r i s f a r too c o m p l e x t o be m o d e l l e d u s i n g DRT. F o r t h i s r e a s o n , t h e V-D c u r v e g e n e r a t e d by t h e s y n t h e t i c s f o r t h i s l a y e r i s n o more v a l i d t h a n t h e f i r s t a r r i v a l homogeneous l a y e r e d m o d e l . The r e m a i n i n g t h r e e l a y e r s , i d e n t i f i e d as l a y e r s 2b, 3a an d 3b on F i g 5.1 h a v e b e e n a l t e r e d s i g n i f i c a n t l y by t h e s y n t h e t i c c a l c u l a t i o n s . The m a j o r c h a n g e t o t h e c u r v e s b r o u g h t a b o u t by t h e u s e o f t h e s y n t h e t i c s has b e e n t h e r e p l a c e m e n t o f t h e d i s c o n t i n u o u s v e l o c i t y i n c r e a s e s by smooth g r a d i e n t s . W h ereas t h e g r o s s s t r u c t u r e o f t h e V-D c u r v e s f o r e a c h o f t h e t h r e e l a y e r s i s e s s e n t i a l l y c o r r e c t , i t ' s h o u l d be s t r e s s e d t h a t t h e y a r e n o t u n i q u e . As a r e s u l t o f t h e m o d e l l i n g p r o c e d u r e u s e d , i t was p o s s i b l e t o o b t a i n t h e same e f f e c t on t h e t r a v e l t i m e s and a m p l i t u d e s o f t h e s y n t h e t i c a r r i v a l s by a number o f d i f f e r e n t c h a n g e s t o t h e p- c u r v e s . I t i s p o s s i b l e t h a t a s l i g h t l y d i f f e r e n t V-D c u r v e c o u l d h a v e been p r o d u c e d ; h o w e v e r , t h e g r o s s s t r u c t u r e o f t h e c u r v e s w o u l d h a v e r e m a i n e d t h e same. The 2b-3a t r a n s i t i o n d i f f e r s b e t w e e n t h e two p r o f i l e s t o a n o t i c e a b l e e x t e n t . The a c t u a l f o r m o f t h i s d i f f e r e n c e i s n o t o v e r l y i m p o r t a n t . I t was i n t r o d u c e d t o c o m p e n s a t e f o r a s l i g h t t r a v e l t i m e o f f s e t b e t ween t h e p r o f i l e s . T h i s o f f s e t , h o w e v e r , 109 c o u l d h a v e b e e n m o d e l l e d i n s e v e r a l o t h e r ways, r e s u l t i n g i n s l i g h t l y d i f f e r e n t f o r m s f o r t h e t r a n s i t i o n . The i m p o r t a n t p o i n t t o n o t e i s t h a t t h e two p r o f i l e s d i f f e r i n t h e n a t u r e o f t h i s b o u n d a r y . , N e i t h e r t h e l a y e r d e p t h s n o r t h e r e f r a c t i o n b r a n c h v e l o c i t i e s ( w i t h t h e e x c e p t i o n o f t h e Pn a r r i v a l ) h a v e been a l t e r e d t o any g r e a t e x t e n t by t h e a d d i t i o n a l u s e o f a m p l i t u d e d a t a . The d e p t h t o t h e Moho on p r o f i l e 75-1R has b e e n r e d u c e d by a b o u t 500 m, a l t h o u g h t h i s s h o u l d n o t be r e g a r d e d as s i g n i f i c a n t s i n c e t h e Pn a r r i v a l s on p r o f i l e 75-1R were n o t o b s e r v e d . Some d e p t h c o n t r o l f o r t h i s l a y e r was a c h i e v e d by m a n i p u l a t i n g W.A.R. b r a n c h j - k ; h o w e v e r , t h e Pn a r r i v a l b r a n c h i s n e c c e s s a r y t o d e f i n e t h i s l a s t r e f r a c t i o n b r a n c h c o m p l e t e l y . The l o w e r i n g o f t h e Pn v e l o c i t y , o b s e r v e d on F i g 5.1 (A, 3 ) , a l s o s h o u l d n o t be r e g a r d e d as s i g n i f i c a n t . I t was n e c c e s s a r y t o d e l a y t h i s a r r i v a l b r a n c h ( F i g 4.11, b r a n c h k-1) by a b o u t 100 ms i n o r d e r t o make i t a g r e e w i t h t h e f i r s t a r r i v a l t r a v e l t i m e s . I n d o i n g t h i s , t h e 7.8 km/s Pn v e l o c i t y was r e d u c e d t o a p p r o x i m a t e l y 7.55 km/s. T h i s v e l o c i t y d r o p h o w e v e r , i s t h e r e s u l t o f a t r a d e o f f b e t w e e n t r a v e l t i m e s , a m p l i t u d e s n e a r t h e c a u s t i c k and a r r i v a l b r a n c h s l o p e a n d i s n o t ' p h y s i c a l l y r e a l To c o n c l u d e , t h e V-D c u r v e s p r o d u c e d t h r o u g h t h e use o f s y n t h e t i c s e i s m o g r a m s a r e o n l y v a l i d w i t h r e s p e c t t o t h e g r o s s s t r u c t u r e o f t h e c u r v e and n o t t h e f i n e d e t a i l s . T h i s i s n o t s u r p r i s i n g c o n s i d e r i n g t h a t DRT o n l y m o d e l s t h i c k l a y e r s . A r e c e n t s t u d y by H e l m b e r g e r (1977) h a s shown t h a t t h i s t y p e o f 110 a p p r o a c h i s o n l y v a l i d i n a b r o a d s e n s e and t h a t t o d e l i n e a t e t h e f i n e s t r u c t u r e o f t h e o c e a n i c c r u s t , one must u s e a l a r g e number o f v e r y t h i n l a y e r s . The c a p a b i l i t y t o do t h i s d o e s e x i s t a t U.B.C. i n t h e f o r m o f a c o m p u t e r p r o g r a m c a l l e d STPSYN w h i c h i s b a s e d on G e n e r a l i s e d Ray T h e o r y (GRT), s e e W i g g i n s and H e l m b e r g e r ( 1 9 7 4 ) . T h i s t y p e o f a p p r o a c h h o w e v e r i s c o n s i d e r a b l y more e x p e n s i v e t h a n DRT and b e y o n d t h e s c o p e o f t h i s p r e s e n t p r o j e c t . The DRT method o f c a l c u l a t i n g s y n t h e t i c s e i s m o g r a m s p r o v i d e s an i n e x p e n s i v e , e a s y - t o - u s e method o f u t i l i s i n g t h e g r o s s a m p l i t u d e s t r u c t u r e o f a r e f r a c t i o n p r o f i l e t o d e l i n e a t e t h e o v e r a l l v e l o c i t y - d e p t h s t r u c t u r e o f t h e o c e a n i c c r u s t . As s u c h i t i s an e x c e l l e n t e x t e n s i o n o f t h e f i r s t a r r i v a l method o f i n t e r p r e t a t i o n . I n o r d e r h o w e v e r , t o d e l i n e a t e t h e f i n e s t r u c t u r e o f a l a y e r , h i g h q u a l i t y d a t a a n d t h e use o f GRT i s r e g u i r e d . 5.2 D i s c u s s i o n S e d i m e n t s : The v e l o c i t y d e p t h model f o r t h e s e d i m e n t a r y s t r u c t u r e o f t h e . n o r t h w e s t end o f t h e b a s i n ( T a b l e 3.2) i s a l i t t l e d i s a p p o i n t i n g . I t was hoped t h a t a more d e t a i l e d p i c t u r e o f t h e s e d i m e n t s i n t h i s a r e a c o u l d be o b t a i n e d . What r e s u l t s we were a b l e t o d i s c e r n h o w e v e r , a q r e e w e l l w i t h t h e r e s u l t s o b t a i n e d by K n i z e (1975) f o r two s h o r t p r o f i l e s i n t h e a r e a b e t w e e n J . Tuzo W i l s o n K n o l l s and t h e Queen C h a r l o t t e Sound, I l l about 60 km n o r t h of the r e c e i v i n g s h i p l o c a t i o n f o r p r o f i l e 75-1R. H i s i n t e r p r e t a t i o n i n c l u d e s a s e r i e s of a l t e r n a t i n g h i g h and low v e l o c i t y s e d i m e n t a r y l a y e r s , w i t h a t h i c k n e s s of r o u g h l y 300 m each. The t o t a l s e d i m e n t a r y t h i c k n e s s i n t h i s a r e a i s a p p r o x i m a t e l y 2 km, v e r y c l o s e to t h e depth o f the s e d i m e n t s i n Winona B a s i n . He i n t e r p r e t e d t h e s e d i m e n t a r y sequences as being due t o a l t e r n a t i n g p e r i o d s of g l a c i a l (low v e l o c i t y c o a r s e sediments) and i n t e r - g l a c i a 1 ( h i g h v e l o c i t y f i n e s e diments) d e p o s i t i o n . A l t h o u g h t h e r e s u l t s i n Winona B a s i n do not show the same d e t a i l , due t o poorer g u a l i t y d a t a , t h e upper two l a y e r s a r e both r o u g h l y 300 m t h i c k and have v e l o c i t i e s c o n s i s t e n t w i t h those found by K n i z e . At the s o u t h e r n end o f Winona B a s i n the same r e f l e c t i n g h o r i z o n s can be i d e n t i f i e d as at the n o r t h e r n end, a l t h o u g h they are c o n s i d e r a b l y more f o l d e d w i t h d i p s r a n g i n g from +10° t o -10°. C o n s i d e r i n g then the c l o s e p r o x i m i t y o f Winona B a s i n to t h e s e d i m e n t a r y a r e a s t u d i e d by K n i z e , and c o n s i d e r i n g t h a t t h e same r e f l e c t o r s can be i d e n t i f i e d a t both ends o f t h e b a s i n , i t i s r e a s o n a b l e to assume t h a t the d e p o s i t i o n a l h i s t o r y of Winona B a s i n may be s i m i l a r t o t h a t of t h e area t o t h e n o r t h d i s c u s s e d by K n i z e (1976). . Even though i t was hoped t o o b t a i n a d e t a i l e d p i c t u r e of the s e d i m e n t a r y s t r u c t u r e w i t h i n the b a s i n , the main o b j e c t i v e was t o o b t a i n a more a c c u r a t e depth f o r the s e d i m e n t s than has p r e v i o u s l y been r e p o r t e d ( T i f f i n 1973, Couch 1969). To t h i s e x t e n t , t h e s t u d y was a s u c c e s s . The t o t a l s e d i m e n t a r y t h i c k n e s s f o r the se d i m e n t s i n Winona B a s i n has been found to 112 be a p p r o x i m a t e l y 2 km. T h i s v a l u e d i f f e r s q r e a t l y f r o m t h e 4 t o 6 km p r e v i o u s l y s u g g e s t e d by t h e a f o r e m e n t i o n e d a u t h o r s . The p r e v i o u s s t u d i e s , h o w e v e r , h a v e b a s e d t h e i r d e p t h s on g r a v i t y d a t a , t h e l a c k o f an o b s e r v a b l e basement on C.S.P. l i n e s , and on an assumed v e l o c i t y s t r u c t u r e u s e d t o c o n v e r t t h e C.S.P. t r a v e l t i m e s i n t o d e p t h s . H o w e v e r , t h e C.S.P. l i n e s f o r t h e a r e a p e n e t r a t e d o n l y t o a maximum of a b o u t 4 s 2-way t r a v e l t i m e ( F i g s 1.3 t o 1 . 5 ) . T hus t h e y were o n l y a b o u t 0.5 s e c o n d s f r o m t h e a c t u a l b a s e m e n t , w h i c h c a n be s e e n a t a b o u t 4.5 s on r e f l e c t i o n p r o f i l e s 3.1 and 3.2. On t h e b a s i s o f good r e f l e c t i o n a nd r e f r a c t i o n d a t a , i t i s b e l i e v e d t h a t a s e d i m e n t d e p t h o f 2 km i s a b e t t e r i n t e r p r e t a t i o n t h a n t h e p r e v i o u s l y r e p o r t e d d e p t h s o f 4 t o 6 km. Basement l a y e r : The t r a n s i t i o n f r o m s e d i m e n t a r y m a t e r i a l t o b a s a l t i c b a s e m e n t i s r e p r e s e n t e d by l a y e r 2a on F i g 5.1. T h i s l a y e r h a s a l e a s t s q u a r e s v e l o c i t y o f 4.28 km/s w h i c h i s c o m p a t i b l e w i t h t h e v e l o c i t y q i v e n by P e t e r s o n e t a l (1974) f o r l a y e r 2a The o b s e r v a t i o n o f a 4.28 km/s v e l o c i t y f o r t h e f i r s t b a s a l t i c l a y e r i s n o t u n i q u e i n t h e r e g i o n . S i m i l a r r e s u l t s h a v e been f o u n d by M a l e c e k (1976) f o r t h e o c e a n i c basement l a y e r i n t h e r e g i o n o f E x p l o r e r R i d g e where t h e r e w e r e l e s s t h a n 250 m o f s e d i m e n t s . He o b t a i n e d v e l o c i t i e s i n t h e r a n g e 4.06-4.20 km/s a n d t h i c k n e s s e s o f 0.97-1.7 1 km/s f o r h i s p r o f i l e s p a r a l l e l a n d p e r p e n d i c u l a r t o E x p l o r e r R i d g e . K n i z e (1976) h a s a l s o o b t a i n e d s i m i l a r r e s u l t s i n two a r e a s . I n t h e a r e a b e t ween J . 113 T u z o W i s o n K n o l l s and t h e Queen C h a r l o t t e Sound he f o u n d a t r a n s i t i o n l a y e r w i t h v e l o c i t y 4.2 km/s and s u b - b o t t o m d e p t h 2.2 km. F u r t h e r , i n t h e r e g i o n o f n o r t h e r n C a s c a d i a b a s i n a b o u t 200 km e a s t o f Winona B a s i n , he f o u n d t h a t t h e t r a n s i t i o n l a y e r c o n s i s t e d o f a s e r i e s o f t h r e e l a y e r s w i t h v e l o c i t i e s o f 4.56, 3.78 and 4.43 km/s and t h i c k n e s s e s o f 0.41, 0.40 and 1.5 km r e s p e c t i v e l y , s t a c k e d on e a c h o t h e r b e g i n n i n g a t a s u b - b o t t o m d e p t h o f 2 km. a s h a s a l r e a d y b e e n m e n t i o n e d , t h e l a r g e a m p l i t u d e a r r i v a l s f r o m t h i s l a y e r h a v e n o t been f i t s y n t h e t i c a l l y ( s e c t i o n 4 . 5 ) . T h i s f a c t l e a d s u s t o t h e c o n c l u s i o n t h a t a s i m p l e v e l o c i t y s t r u c t u r e i s n o t s u f f i c i e n t t o e x p l a i n t h e r e s u l t s , ft r e c e n t s t u d y o v e r t h e M i d - A t l a n t i c R i d q e by Hyndman e t a l (1976) c o u l d p o s s i b l y s h e d some l i g h t on t h e r e a s o n s f o r t h i s . On t h e b a s i s o f d r i l l c o r e s p e n e t r a t i n g l a y e r 2a t h e y h a v e f o u n d t h a t i t c o n s i s t s o f h i g h l y p o r o u s , l o w d e n s i t y , f r a c t u r e d v o l c a n i c m a t e r i a l w i t h some i n t e r c a l a t e d s e d i m e n t s . T h e s e r e s u l t s i l l u s t r a t e t h a t t h i s l a y e r i s c h a r a c t e r i s e d by a m i x t u r e o f f r a c t u r e d v o l c a n i c m a t e r i a l and l o w v e l o c i t y s e d i m e n t s . W h i l e t h e Winona B a s i n i n no way r e s e m b l e s t h e M i d -A t l a n t i c R i d g e , t h e r e s u l t s o f Hyndman e t a l do show t h e h i g h l y i n h o m o g e n e o u s n a t u r e o f t h i s t r a n s i t i o n l a y e r . I t w o u l d n o t be u n r e a s o n a b l e t o assume t h a t i f Winona B a s i n were c r e a t e d w h i l e t h e s e d i m e n t s f r o m t h e n e a r b y c o n t i n e n t were b e i n g d e p o s i t e d , t h e t r a n s i t i o n l a y e r b e t w e e n t h e s e d i m e n t s a n d b a s e m e n t w o u l d be a. m i x t u r e o f h i g h and l o w v e l o c i t y b a s a l t s and s e d i m e n t s . T h i s i n f a c t h a s been p o s t u l a t e d by 114 K n i z e f o r h i s r e s u l t s i n t h e n o r t h e r n C a s c a d i a b a s i n . H i s r e f l e c t i o n r e s u l t s f o r t h e a r e a showed a low v e l o c i t y s e d i m e n t / b a s a l t l a y e r (3.78 km/s 0.4 km) s a n d w i c h e d b e t w e e n two h i g h e r v e l o c i t y b a s a l t i c l a y e r s (4.56 km/s, 0.41 km t h i c k ; 4.43 kra/s, 1.5 km t h i c k ) S u c h f i n e l a y e r i n g h o w e v e r , c o u l d n o t be d e l i n e a t e d by t h e l o n g e r w a v e l e n g t h s i g n a l s r e c o r d e d d u r i n g t h e r e f r a c t i o n p a r t o f h i s p r o f i l e s . I f t h i s l a y e r i s a c o m p l e x m i x t u r e o f h i g h a n d l o w v e l o c i t y m a t e r i a l s , t h e n t h e l a r g e a m p l i t u d e s i g n a l s c o u l d be e x p l a i n e d a s b e i n g due t o e i t h e r a f o c u s s i n g o r c o n s t r u c t i v e i n t e r f e r e n c e e f f e c t . I n any e v e n t t h e DET m o d e l l i n g p r o c e d u r e i s t o o s i m p l e t o e x p l a i n t h e v e l o c i t y - d e p t h s t r u c t u r e o f t h i s l a y e r . I t i s h o p e d t h a t a f t e r f u r t h e r p r o c e s s i n g o f t h e r e f l e c t i o n d a t a , e g . w-k f i l t e r i n g ( s e c t i o n 3 . 1 } , t h e c o m p l e x n a t u r e o f t h i s l a y e r c a n be f u r t h e r r e v e a l e d . L o w e r C r u s t a l L a y e r s : The l e a s t s g u a r e s v e l o c i t e s and d e p t h s o f t h e r e m a i n i n g t h r e e l a y e r s a r e l i s t e d i n T a b l e 4.1. T h e s e have v e l o c i t i e s c o n s i s t e n t w i t h l a y e r s 2b, 3 a , and 3b, r e s p e c t i v e l y a nd have been i d e n t i f i e d a s s u c h . A l l o f t h e s u b - b a s e m e n t l a y e r d e p t h s a r e ' c l o s e t o t h e maximum v a l u e s r e p o r t e d f o r a s t a n d a r d o c e a n i c s e c t i o n ( P e t e r s o n e t a l 1 9 7 4 ) . H o w e v e r , t h e t o t a l s u b -s e d i m e n t c r u s t a l t h i c k n e s s o f 12 km i s c o n s i d e r a b l y t h i c k e r t h a n t h e 5 t o 7 km n o r m a l l y c o n s i d e r e d s t a n d a r d f o r an o c e a n i c c r u s t . The v e l o c i t y g r a d i e n t s i n t r o d u c e d i n t o t h e V-D m o d e l s by 115 t h e s y n t h e t i c s have a l r e a d y been d i s c u s s e d somewhat. What rem a i n s however i s t o d i s c u s s the s i g n i f i c a n c e o f the g r o s s s t r u c t u r e of the c u r v e s . The c u r v e s produced f o r l a y e r s 2b, 3 a r 3b a r e a l l s i m i l a r . They i n d i c a t e a g r a d u a l t r a n s i t i o n from one l a y e r t o the next caused e i t h e r by compaction or i n t e r m i x i n g o f t h e a d j a c e n t l a y e r s or both. Once the t r a n s i t i o n from one l a y e r t o t h e n e x t i s complete however, th e v e l o c i t y remains n e a r l y u n i f o r m w i t h depth f o r t h e next 2-3 km ( w i t h a s l i g h t p o s i t i v e g r a d i e n t ) . On the b a s i s of t h e s e c u r v e s , i t appears t h a t t h e lower, c r u s t i n t h i s r e g i o n i s s t r o n g l y d i v i d e d i n t o t h r e e s e p a r a t e l a y e r s , p o s s i b l y r e f l e c t i n g d i f f e r e n t r o c k t y p e s under d i f f e r e n t p r e s s u r e -t e m p e r a t u r e c o n d i t i o n s i n each l a y e r . As i n t e r p r e t e d i n t h i s s t u d y , the c r u s t u n d e r l y i n g Winona B a s i n has a t y p i c a l o c e a n i c v e l o c i t y s t r u c t u r e but i s c o n s i d e r a b l y t h i c k e r t h a n what i s n o r m a l . In the r e g i o n of E x p l o r e r R i d g e , Malecek (1976) has found s i m i l a r t h i c k n e s s e s t o t h o s e o b s e r v e d u n d e r l y i n g Winona B a s i n . H i s r e v e r s e d p r o f i l e (74-2,2R) c r o s s i n g E x p l o r e r Ridge show a sub-bottom depth to the mantle o f 9 t o 11 km. C o n s i d e r i n g t h a t the sediment c o v e r i n t h i s r e g i o n i s m i n i m a l , t h e s e r e s u l t s agree w e l l w i t h the 12 km sub-sediment c r u s t a l t h i c k n e s s u n d e r l y i n g Winona B a s i n . A d e t a i l e d e x a m i n a t i o n of h i s data r e v e a l s t h a t they are r e m a r k a b l y s i m i l a r t o those o b s e r v e d on p r o f i l e s 75-1 and' 75-1R w i t h r e s p e c t t o both t r a v e l t i m e s and a m p l i t u d e i n f o r m a t i o n . On t h e b a s i s o f t h i s , and t h e s i m i l a r i t y between t h e v e l o c i t y vs depth models i n t e r p r e t e d f o r Winona B a s i n and 116 E x p l o r e r p l a t e j u s t e a s t o f t h e s p r e a d i n g c e n t r e , i t w o u l d seem t h a t t h e c r u s t i n t h e two r e g i o n s was f o r m e d i n much t h e same manner. The t h i c k c r u s t , t h e n , d o es n o t seem t o be l o c a l i z e d o n l y b e n e a t h Winona B a s i n . I n f a c t an OBS s t u d y o f t h e c r u s t a l s t r u c t u r e b e n e a t h J u a n de F u c a R i d g e , ( D a v i s e t a l 1976) has shown a s i m i l a r c r u s t w i t h a t h i c k n e s s o f 10-11 km s u b -s e d i m e n t . Whereas t h e s e d a t a a r e n o t o f t h e same q u a l i t y a s t h o s e u s e d by M a l e c e k (1976) o r i n t h i s s t u d y , i t does i n d i c a t e t h a t t h e t h i c k c r u s t i s a w i d e s p r e a d f e a t u r e . A p r e l i m i n a r y i n t e r p r e t a t i o n o f a r e v e r s e d r e f r a c t i o n l i n e r e c o r d e d p a r a l l e l t o and s o u t h o f t h e Revere-De11wood f r a c t u r e zone and w e s t o f n o r t h e r n E x p l o r e r R i d q e ( F i q 1.1) h a s i n d i c a t e d t h a t t h e c r u s t i n t h i s r e q i o n i s s i m i l a r t o a s t a n d a r d o c e a n i c s e c t i o n (R.D. Hyndman p e r s o n a l c o m m u n i c a t i o n 1 9 7 7 ) . T h e s e r e s u l t s a r e t e n t a t i v e h owever b e c a u s e o f t h e l a c k o f o b s e r v a b l e Pn a r r i v a l s on t h e a n a l o q p l a y b a c k s . I f t h i s i s t h e c a s e t h o u g h , t h e c r u s t a l t h i c k n e s s c h a n g e s d r a s t i c a l l y as one c r o s s e s b o t h E x p l o r e r R i d g e and P a u l fievere R i d g e . T h i s c h a n g e i n t h i c k n e s s c o u l d be a d i r e c t r e s u l t o f t h e d i f f e r i n g t e c t o n i c s o f t h e o p p o s i t e s i d e s of t h e r i d g e s . The E x p l o r e r p l a t e , h a v i n q a t h i c k c r u s t , i s t r a p p e d b e t w e e n t h e N o r t h A m e r i c a n , P a c i f i c and J u a n de F u c a p l a t e s . As a r e s u l t , i t s movement away f r o m t h e s p r e a d i n q c e n t r e c o u l d be r e s t r i c t e d c a u s i n q a p i l i n g up o f t h e n e w l y f o r m e d o c e a n i c m a t e r i a l . T h i s i d e a h a s been s u g g e s t e d by M a l e c e k and C l o w e s (1977) a s one p o s s i b l e e x p l a n a t i o n f o r t h e t h i c k c r u s t i n t h e r e g i o n o f 117 E x p l o r e r R i d g e . On t h e o t h e r h a n d , t h e movement o f t h e P a c i f i c p l a t e i s u n r e s t r i c t e d and h e n c e t h i s p i l i n g up e f f e c t - s h o u l d n o t be o b s e r v e d . I f W i n o n a B a s i n was c r e a t e d a t t h e t i m e E x p l o r e r R i d g e t e r m i n a t e d a t t h e B r o o k s f r a c t u r e z o n e , and was s u b s e g u e n t l y i s o l a t e d by r o t a t i o n o f t h e p l a t e , t h e n we w o u l d e x p e c t i t t o h a v e t h e same t y p e o f c r u s t as t h e P a c i f i c p l a t e t o t h e n o r t h w e s t o f t h e p r e s e n t E x p l o r e r R i d g e . T h i s i s n o t t h e c a s e . The b a s i n h a s a t h i c k e r c r u s t t h a n t h e P a c i f i c p l a t e , t h e i m p l i c a t i o n b e i n g t h a t i t has n o t been c r e a t e d by s p r e a d i n g f r o m E x p l o r e r r i d g e . T h i s t e n d s t o r e f u t e a p o s s i b l e a r g u m e n t t h a t W inona B a s i n i s an i s o l a t e d s e c t i o n o f o l d P a c i f i c m a t e r i a l . A more r e a s o n a b l e s u g g e s t i o n i s t h a t Winona B a s i n h a s been c r e a t e d by t h e s l o w n o r t h w a r d movement o f t h e t r i p l e p o i n t . The s p r e a d i n g r a t e i n t h e b a s i n w o u l d most l i k e l y be e x t r e m e l y s l o w due t o i t b e i n g t r a p p e d b e t w e e n t h e s p r e a d i n g c e n t r e ( D e l l w o o d K n o l l s ) and B r o o k s f r a c t u r e z o n e . The t h i c k c r u s t u n d e r l y i n g W i n o n a B a s i n t h e n c o u l d b e f o r m e d by a p i l i n g up o f t h e new c r u s t a l m a t e r i a l , i n much t h e same way a s p o s t u l a t e d f o r t h e c r u s t b e n e a t h E x p l o r e r p l a t e . ' The l a c k o f any s i g n i f i c a n t m a q n e t i c a n o m a l y p a t t e r n i n t h e b a s i n h a s a l r e a d y been m e n t i o n e d i n C h a p t e r 1. I f Winona B a s i n h a s been f o r m e d i n t h e l a s t 3-4 my, by t h e movement o f t h e t r i p l e p o i n t , t h e n what has happened t o t h e m a g n e t i c a n o m a l i e s ? T h i s i s a d i f f i c u l t q u e s t i o n t o a n s w e r . P e r h a p s i t i s p o s s i b l e t h a t i f t h e s p r e a d i n g c e n t r e c r e a t i n g t h e b a s i n 118 were v e r y d i f f u s e , and i f t h e s p r e a d i n q were e x t r e m e l y s l o w , no d i s c e r n i b l e a n o m a l y p a t t e r n w o u l d be c r e a t e d . The t h i c k c r u s t a l s e c t i o n , i n c l u d i n g o n l y 2 km o f s e d i m e n t s , i m p l i e s t h a t a r e c o n s i d e r a t i o n o f t h e g r a v i t y d a t a (Couch 1969} i s r e q u i r e d . A t h i c k c r u s t i s c o n s i s t e n t w i t h t h e -160 mgal f r e e a i r a n o m a l y , as shown by C o u c h , b u t t h e l a c k o f a t h i c k s e d i m e n t p i l e w i l l r e q u i r e o t h e r c h a n q e s i n h i s m o d e l . A l s o , t h e q r a v i t y l o w t e n d s t o t e r m i n a t e a t Winona R i d q e r a t h e r t h a n P a u l - R e v e r e R i d g e . What does t h i s mean i n t e r m s o f c r u s t a l s t r u c t u r e ? S u c h a q u e s t i o n l e n d s a d d e d i m p o r t a n c e t o t h e a n a l y s i s , c u r r e n t l y i n p r o g r e s s , o f t h e two r e v e r s e d DSS p r o f i l e s a c r o s s t h e b a s i n . The e x p l a n a t i o n q i v e n h e r e o f t h e o r i g i n o f t h e t h i c k c r u s t i n t e r p r e t e d f r o m t h e s e i s m i c p r o f i l e s a l o n g Winona B a s i n i s , o f c o u r s e , h i g h l y s p e c u l a t i v e . S u b s e q u e n t a n a l y s i s o f t h e r e m a i n i n g two r e v e r s e d DSS p r o f i l e s i n ' Winona B a s i n , more d e t a i l e d a n a l y s i s o f t h e OBS d a t a t o t h e west o f E x p l o r e r R i d g e , and a d d i t i o n a l g e o p h y s i c a l s t u d i e s i n p r o g r e s s w i l l h o p e f u l l y p r o v i d e more e v i d e n c e t o a i d i n f u l l y u n d e r s t a n d i n g t h i s c o m p l e x and f a s c i n a t i n g a r e a . 119 REFERENCES Atwater, T. 1970. 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Geophys. Union, Trans.), 54, no. 3, pp. 140-141. , and D. Seeman 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. Canada. • Tobin, D.G. and L.R. Sykes 1968. S e i s m i c i t y and t e c t o n i c s of the northeast P a c i f i c ocean. J. Geophys. Res. 72, pp. 3821-3846. T r e i t e l , S., J.L. Shanks and CW. F r a s i e r 1967. Some aspects ' of fan f i l t e r i n g . Geophysics 32, pp. 789-800 Wiggins, R.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. 46, pp. 1-10. , and D.V. Helmberger 1974. S y n t h e t i c seismogram computation by expansion i n g e n e r a l i z e d rays. Geophys. J. R. a s t r Soc. 37, pp. 73-90. , and J.A. Madrid 1974. Body wave amplitude c a l c u l a t i o n s . Geophys. J . R. a s t r . Soc. 37, pp. 423-433. 122 York, D. 1969. Least squares f i t t i n q of a s t r a i q h t l i n e with c o r r e l a t i n q e r r o r s . Earth and Planetary S c i . L e t t . 5, pp. 320-324. > 1 j H G . 1 . 3 8 LINE iS-k MOftTUVEST 73-1 LINES 1 3 L . H . uwt 15-3 S O U T H E A S T io ten i FlGr. l * t 

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