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A seismic refraction study of the hecate sub-basin, British Columbia Pike, Christopher James 1986

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A SEISMIC R E F R A C T I O N S T U D Y OF T H E HECATE SUB-BASIN, BRITISH COLUMBIA by CHRISTOPHER JAMES PIKE B.Sc.(Physics), The University of Toronto, 1982  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF T H E REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES Department of Geophysics and Astronomy  We accept this thesis as conforming to the required standard  THE UNIVERSITY OF BRITISH COLUMBIA November 1986 ©Christopher James Pike, 1986  In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l  fulfilment  of  the  requirements f o r an advanced degree at the U n i v e r s i t y o f B r i t i s h Columbia, I agree t h a t the L i b r a r y s h a l l make it  f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y .  I  further  agree t h a t p e r m i s s i o n f o r e x t e n s i v e copying of t h i s  thesis  f o r s c h o l a r l y purposes may be granted by the head o f my department or by h i s or her r e p r e s e n t a t i v e s .  It  understood t h a t copying or p u b l i c a t i o n of t h i s  is  thesis  f o r f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n permission.  Department of  G*<_of)Ky<!>t^5 4J\JL  The U n i v e r s i t y of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3  Date  E-6  (3/81)  A~Sr d^ ^ c  oJi  ii To my grandmother,  Nanny  Pike,  who passed away  three short weeks  before the completion of this thesis  iii  ABSTRACT  The Hecate sub-basin is one of two similar sedimentary structures comprising Queen Charlotte Basin, which is located between the British Columbia mainland and the Queen Charlotte Islands. The Queen Charlotte Basin was the locale of an active but unsuccessful exploration program, including drill holes, in the 1960's. However, recent studies incorporating modern concepts of plate tectonics have indicated a re-evaluation of the resource potential of the area is warranted. The Hecate sub-basin and its southern counterpart, the Charlotte sub-basin, are filled with Tertiary sediments that are underlain by a thick sequence of Tertiary volcanics. Penetration of the latter unit using the reflection method has been difficult. Thus the thickness of the volcanics and the existence or not of more sediments below them has not been established. To address this problem an airgun/ocean bottom seismograph (OBS) refraction survey was carried out across the Hecate sub-basin in 1983. Data from the airgun shots at approximately 160 m spacings were recorded on four OBSs deployed at 20 km intervals to provide a series of reverse profiles extending over 60 km. The principal interpretation procedure involved calculation of theoretical seismograms and travel-time curves for 2-D velocity structure models and comparisons with observed record sections. The interpreted structure model shows significant lateral variations. Low velocity Pleistocene and Pliocene sediments form an upper layer varying between 0.5 and 1.0 km thick. The principal sedimentary unit is the Tertiary Skonun Formation with interpreted velocities of 2.7 km/s and a gradient averaging 0.4 km/s/km, values that are consistent with well log data. These sediments are generally thicker (approximately 2.5 km) on the western side of the sub-basin although they reach their maximum thickness of 3 km in a depression near the central part of the basin. Toward the eastern side of the  iv basin, the Tertiary sediments thin to about 1 km as the underlying Tertiary volcanics rise toward the mainland. The maximum sediment thickness in the basin is about 4 km. The upper surface of the volcanic unit shows a pronounced topography which is consistent with the erosional nature of this surface. Velocities for the volcanics vary between 4.8 and 5.0 km/s; thickness of the unit ranges from about 0.2 km to 1.8 km. Below the Tertiary volcanics on the eastern 20 km of the model, a low velocity zone less than 1 km thick had to be introduced to satisfy the data. This zone is inferred to contain Upper Cretaceous sediments. A unit with a poorly constrained velocity of 5.9 km/s which underlies the Tertiary volcanics and low velocity zone on the eastern side is interpreted to be the Paleozoic Alexander Terrane. Most of the characteristics of this model are similar to those determined from an earlier study in the Charlotte sub-basin. An additional component of this thesis project was the development of an interactive procedure for the inversion of densely spaced seismic refraction data by wavefield continuation to derive a l-D velocity-depth profile, and its application to data derived from 2-D structures. The procedure consists of two steps: a slant stack followed by a downward continuation. The method was found to yield velocity-depth structures which, when compared with an average velocity-depth structure from the 2-D model, have very similiar gradients and velocity increases. In general the velocity depth curve from the inversion had lower velocities at deeper depths than the averaged 2-D structure.  V  T A B L E OF CONTENTS  ABSTRACT  ii  LIST OF TABLES  vi  LIST OF FIGURES  vii  ACKNOWLEDGMENTS  ix  CHAPTER I  1  INTRODUCTION  1.1 Tectonic Evolution of the Queen Charlotte Basin 1.2 Subsidence of the Queen Charlotte Basin 1.3 Stratigraphy 1.3.1 Hydrocarbon potential for the major stratigraphic units . . . 1.4 An Outline of the Seismic Refraction Study CHAPTER II  DATA ACQUISITION AND ANALYSIS  2.1 Data Acquisition 2.1.1 Airgun-OBS experiment 2.1.2 Description of the OBSs and the airgun 2.1.3 Description of the procedure 2.2 Data Processing 2.2.1 Digitizing, editing and demultiplexing the analog data 2.2.2 Basic timing and positioning 2.2.3 Special positioning and related timing corrections 2.3 Data Analysis and Interpretation Procedures 2.3.1 Data analysis 2.3.2 Interpretation procedures  4 10 14 17 18 21  . . .  21 21 21 23 24 24 25 25 28 28 28  CHAPTER III INVERSION BY WAVEFIELD CONTINUATION . . . 3.1 Introduction 3.2 The Linear Transformation 3.2.1 The slant stack procedure 3.2.2 The downward continuation procedure 3.3 Examples 3.3.1 The plane-layered synthetic example 3.3.2 The 2-D synthetic example 3.3.3 A real data example 3.3.4 Summary  33 33 34 34 39 43 44 52 57 64  vi CHAPTER IV MODELLING OF T H E AIRGUN-OBS DATA 4.1 Introduction 4.2 Initial Constraints 4.3 OBS 1 - OBS 2 Submodel 4.3.1 Forward profile 4.3.2 Reverse profile 4.3.3 Summary 4.4 OBS 2 - OBS 3 Sub-model 4.4.1 Forward profile 4.4.2 Reverse profile 4.4.3 Summary 4.5 OBS 3 - OBS 4 Sub-model 4.5.1 Forward profile 4.5.2 Reverse profile 4.5.3 Summary  68 68 70 70 71 73 78 80 83 86 90 92 93 97 104  CHAPTER V DISCUSSION AND CONCLUSIONS 5.1 Discussion of the Final Composite Model  106 106  5.2 Conclusion  Ill  BIBLIOGRAPHY APPENDIX I APPENDIX II  • 115 Summary of Formations for the Queen Charlotte Region . 119 Horizontal and Hydrophone Component Data  121  vii  LIST O F T A B L E S  2.1 O B S l o c a t i o n a n d d e p t h of a i r g u n  24  2.2 O B S p o s i t i o n i n g c o r r e c t i o n s  27  3.1 1-D m o d e l u s e d for i n v e r s i o n t e s t i n g  37  5.1 S u m m a r y of s t r a t i g r a p h i c i n t e r p r e t a t i o n  109  viii  LIST OF F I G U R E S  1.1 Location map for the Queen Charlotte region  2  1.2 Late Mesozoic—Cenozoic basins of the Pacific margin  3  1.3 Tectonic evolution of the Pacific Northwest  7  1.4 Present day relationship between Wrangellia and Alexander terranes  . . .  8  1.5 Isopaeh of the Tertiary sediments for QCB  11  1.6 Rifting sequence for the QCB  12  1.7 Estimated present heat flow for the QCB  13  1.8 Cross section for the offshore QCB  16  1.9 Structural geological model for QCB  19  2.1 Plan of the 1983 airgun/OBS survey  22  2.2 Power spectra noise and signal + noise  29  2.3 Comparison of unfiltered and filtered data  30  3.1 Theoretical seismograms and slant stack for l-D model  38  3.2 Downward continuation for l-D model with exact v-z function  45  3.3 Downward continuation for l-D model  46  3.4 Comparison of p-z functions for l-D model  51  3.5 Theoretical seismograms and slant stack for 2-D model  53  3.6 Downward continuation for 2-D model  55  3.7 Comparison of p-z functions for 2-D model  58  3.8 Seismograms and slant stack for OBS 3 data  59  3.9 Downward continuation for OBS 3 data  61  3.10 Comparison of p-z functions for real data 4.1 Final velocity model  65 69  ix 4.2 OBS 1 data with travel-times and model  72  4.3 OBS 1 seismograms  74  4.4 OBS 2 reverse profile with travel-times and model  76  4.5 OBS 2 reverse profile seismograms  77  4.6 Velocity cube for OBS 1—OBS 2  79  4.7 Comparison of sonic log with results from OBS 1—OBS 2 model  . . . .  81  4.8 Sonic logs from Coho, Tyee and Sockeye wells  82  4.9 OBS 2 forward profile with travel-times and model  84  4.10 OBS 2 forward profile seismograms  85  4.11 OBS 3 reverse profile with travel-times and model  87  4.12 OBS 3 reverse profile seismograms  89  4.13 Velocity cube for OBS 2—OBS 3  91  4.14 OBS 3 forward profile with travel-times and model  93  4.15 OBS 3 forward profile seismograms  95  4.16 OBS 4 data with travel-times and model  98  4.17 OBS 4 seismograms  100  4.18 Comparison of data for the three components of OBS 4  103  4.19 Velocity cube for OBS 3—OBS 4  105  5.1 Final velocity structural model for Hecate Strait  107  5.2 Gravity profile and interpretation for Hecate Strait  112  X  ACKNOWLEDGEMENTS I would like to thank my supervisor, Dr. Ron M. Clowes, for his support and encouragement over the past years. I am particularly grateful for his patient and painstaking reviews of this thesis. I would also like to thank Dr. Robert M. Ellis for critically reviewing this thesis, particularly in light of the severe time constraints. Without the invaluable help from Kathy Penney and Andy Boland during the final stages of this thesis it wo.uld not have been completed in time. This is also true of David Mackie and Joane Berube who offered much needed assistance in the preparation of figures. David Mackie is also responsible for many of the programs used in processing the origional analog tapes. The efforts of Phil Ross and Elaine Wright are also greatly appreciated. The use of TgX files compiled by Sonya Dehler permitted the quick understanding of this text writing package. The people of the Geophysics and Astronomy Department are responsible for making the time spent at UBC very enjoyable. Financial support for data acquisition and analysis was provided by NSERC through Operating Grant A7707 and Strategic (Oceans) Grant G0738, by D.S.S. contract Q5SU.23235-3-1089 from the Earth Physics Branch, E.M.R., and by D.S.S. contracts 06SB.23445-4-1170 and 11SB.23227-5-0127 from the Pacific Geoscience Centre, E.M.R. Additional support was provided by Chevron Canada Resources Ltd., Shell Canada Resources Ltd. and Mobil Oil Canada Ltd..  1  CHAPTER I  INTRODUCTION  H e c a t e S t r a i t is s i t u a t e d b e t w e e n t h e west c o a s t of m a i n l a n d B r i t i s h C o l u m b i a a n d t h e Q u e e n C h a r l o t t e I s l a n d s i n t h e l a t i t u d e r a n g e 5 2 ° t o 54° N . It is u n d e r l a i n by t h e Q u e e n C h a r l o t t e b a s i n , a n o r t h - s o u t h t r e n d i n g d e p r e s s i o n 4 0 0 k m l o n g a n d 100 k m w i d e w h i c h to the northwest also underlies northeastern G r a h a m Island a n d D i x o n E n t r a n c e a n d t o t h e s o u t h Q u e e n C h a r l o t t e S o u n d ( F i g u r e 1.1). T h e Q u e e n C h a r l o t t e b a s i n f o r m s p a r t of t h e c o a s t a l d e p r e s s i o n b o r d e r i n g t h e w e s t e r n m a r g i n of N o r t h A m e r i c a a n d is one o f m a n y s e d i m e n t a r y b a s i n s w i t h ages r a n g i n g f r o m L a t e M e s o z o i e t o C e n o z o i c  (Figure  1.2). In t e r m s of t h e b a s i n ' s s i z e , i t r e s e m b l e s t h e C o o k I n l e t a n d B r i s t o l B a y basins i n A l a s k a a n d t h e S a e r e m e n t o - S a n J o a q u i n b a s i n i n C a l i f o r n i a ( Y o u n g , 1981). T h e h y d r o c a r b o n p o t e n t i a l of t h e a r e a w a s i n v e s t i g a t e d as e a r l y as 1912 w i t h t h e d r i l l i n g of t h e T i a n N o . l w e l l w h i c h r e a c h e d a d e p t h of 490 m a n d b o t t o m e d i n P a l e o c e n e M a s s e t v o l c a n i c s (see T a b l e 1 of A p p e n d i x 1 for f o r m a t i o n n o m e n c l a t u r e ) . T h i s w e l l a n d o t h e r s d r i l l e d o n t h e i s l a n d s d u r i n g t h e p e r i o d 1950-1961 d i d not s h o w a n y s i g n i f i c a n t hydrocarbon  finds.  E x p l o r a t i o n w a s r e n e w e d i n t h e 1960's w h e n S h e l l C a n a d a L t d .  d r i l l e d a n u m b e r of w e l l s off t h e w e s t c o a s t of B r i t i s h C o l u m b i a , i n c l u d i n g eight H e c a t e S t r a i t a n d Q u e e n C h a r l o t t e S o u n d ( F i g u r e 1.1; S h o u l d i c e 1 9 7 1 , 1 9 7 3 ) .  in  The  e x p l o r a t o r y w e l l s i n H e c a t e S t r a i t were d r i l l e d o n s t r u c t u r a l h i g h s d e f i n e d by r e l a t i v e l y p o o r r e f l e c t i o n s e i s m i c d a t a a n d a l s o d i d not r e s u l t in a n y s i g n i f i c a n t oil a n d gas s h o w s . A c t i v i t y e n d e d w i t h t h e m o r a t o r i u m o n e x p l o r a t i o n i n t h e e a r l y 1970's. B e t w e e n 1970 a n d t h e p r e s e n t , s e v e r a l f a c t o r s h a v e c o n t r i b u t e d t o a r e n e w e d interest i n t h e b a s i n s off t h e w e s t c o a s t of B r i t i s h C o l u m b i a , a n d p a r t i c u l a r l y t h e Q u e e n C h a r lotte basin. T h e relationship between plate tectonics and the w o r l d - w i d e  distribution  2  132  Figure 1.1  1  3  0  128  Location map for the Queen Charlotte Islands, Hecate Strait and Queen Charlotte Sound.  Exploratory wells drilled by Richfield Oil Corporation on the Queen Charlottte Islands and Shell Canada Ltd. in Hecate Strait and Queen Charlotte Sound are indicated by boxed labels ( after Young, 1981).  3  LATE  MESOZOIC — CENOZOIC WESTERN  Major  sedimentary  structural  Tertiary — Quaternary Spreading  Figure 1.2  center  AMERICA  basins  Calc - alkaline plutonic Geosynclinal  NORTH  BASINS  rocks trends volcanic  cover  £ Subduction  zone  / Transform  fault  Late Mesozoic—Cenozoic basins of the of the Pacific margin ( after Young, 1981).  4 of h y d r o c a r b o n s h i g h l i g h t e d m a n y p r o s p e c t i v e areas (eg. R o n a , 1 9 8 0 ) . O i l c o m p a n i e s b e g a n t o a d o p t these p r i n c i p l e s i n t h e i r a n a l y s i s o f b a s i n d e v e l o p m e n t . T h e c o n c e p t o f a c c r e t e d t e r r a n e s , w h i c h are f r a g m e n t s of m o r e a n c i e n t m a t e r i a l t h a t has b e e n j u x t a p o s e d a g a i n s t o t h e r t e r r a n e s a n d / o r c r a t o n s w i t h v a s t l y d i f f e r i n g e v o l u t i o n s , w a s easily a c c o m m o d a t e d by plate tectonic theory. T h e A l e x a n d e r a n d W r a n g e l l i a Terranes have b e e n i d e n t i f i e d as e x a m p l e s of t h i s p r o c e s s ( B e r g et al., 1972; a n d J o n e s et al.,  1977  respectively). These accreted terranes originated in more equatorial environments, consequently may have developed organic-rich formations ranes, and thus warrant independent investigation.  unrelated to the adjoining  ter-  T h e recent s t u d i e s of t h e e v o l u t i o n  o f t h e Q u e e n C h a r l o t t e b a s i n a n d s u r r o u n d i n g r e g i o n s by Y o r a t h a n d C h a s e  (1981),  Y o r a t h a n d C a m e r o n (1982), Y o r a t h a n d H y n d m a n (1983), M a c k i e (1985), C l o w e s a n d G e n s - L e n a r t o w i e z (1985)  a n d D e h l e r (1986)  have c o n t r i b u t e d  greatly  t o the  present  u n d e r s t a n d i n g of t h e r e g i o n .  1.1 Tectonic Evolution of the Queen Charlotte Basin Yorath  and  C h a s e (1981)  r e v i e w e d t h e w o r k of S u t h e r l a n d  Brown  (1968)  and  S h o u l d i c e ( 1 9 7 1 , 1 9 7 3 ) , a n d i n t e g r a t e d t h i s w i t h t h e i r o w n a n d o t h e r s t u d i e s t o dev e l o p a m o d e l for t h e e v o l u t i o n  of the Q u e e n C h a r l o t t e b a s i n .  T h e y simplified  geology a n d t e c t o n i c s of the r e g i o n b y d e f i n i n g f o u r basic t e c t o n i c assemblages. Paleozoic  the The  A l e x a n d e r Terrane and the M e s o z o i c Wrangellia Terrane comprise the al-  l o c h t h o n o u s a s s e m b l a g e s ; the U p p e r J u r a s s i c p l u t o n s a n d L o w e r C r e t a c e o u s L o n g a r m F o r m a t i o n ( T a b l e 1, A p p e n d i x I) m a k e u p the s u t u r e A s s e m b l a g e ; the p o s t s u t u r e ass e m b l a g e c o n s i s t s o f the M i d d l e t o U p p e r C r e t a c e o u s Q u e e n C h a r l o t t e G r o u p ( T a b l e 1, A p p e n d i x I); a n d t h e m i d - T e r t i a r y  plutons, M a s s e t volcanics and Neogene Skonun  f o r m a t i o n c o m p r i s e the rift assemblage.  5  Berg et al. (1972) denned the Alexander Terrane as a complex assemblage of sedimentary, igneous and metamorphic rocks ranging from Late Precambrian to Late Paleozoic in age. Rocks of Late Triassic age unconformably overlie Permian limestones in the southern part of the terrane (Berg et al., 1978). The metasedimentary and metavolcanic rocks along the margin of the Coast Mountains have also been included in the Alexander Terrane by Yorath and Chase (1981). Van der Voo et al. (1980) and Van der Voo and Channel (1980) have obtained paleomagnetic results from Ordovician, Devonian and Carboniferous rocks from southeastern Alaska which clearly indicate the exotic nature of the Alexander Terrane. Displacements of 1800 km relative to eratonie North America between Late Carboniferous and Triassic time have been determined. Jones et al. (1977) described the terrane underlain by Middle to Upper Triassic tholeitie basalts and calcareous sedimentary rocks which occur in the Wrangell Mountains and Chichagof Islands as Wrangellia. Yorath and Chase (1981) also included Jurassic volcanic and sedimentary rocks under the definition for Wrangellia. The 4300 m of tholeitie pillow lavas, pillow basalts, equagene tuffs, and the massive basalt flows of the Upper Triassic Karmutsen Formation conformably overlie about 100 m of limestone and argillite of the Upper Triassic to Lower Jurassic Kunga Formation (Sutherland Brown, 1968). This formation, together with the terrigenous elastics and calc-alkaline volcanics of Early to Middle Jurassic age on Queen Charlotte Islands (the Maude and Yakoun Formations), are included in Wrangellia by Yorath and Chase (1981) following upon evidence from Tipper and Cameron (1980), who found ammonite faunas in the Yakoun Formation that developed in a warm-water environment. Paleomagnetic evidence from the Nicolai Greenstone belt in the Wrangell Mountains (Hillhouse, 1977) suggests a northward displacement of 3000 km or 6000 km prior to their accretion to  6 the North American continental margin (the different results depend upon whether a northern or southern hemispherical solution is chosen). The model proposed by Yorath and Cameron (1982) for the evolution of the Pacific margin in the vicinity of the Queen Charlotte Islands is depicted in Figure 1.3. By late Triassic time, the Alexander Terrane had reached its present latitude (Figure 1.3a). There was a cessation of the northward movement of the Alexander Terrane following collision with Wrangellia. By some process, the northward movement of the two terranes was converted to movement in a northeasterly direction (Figure 1.3b). The unconformity between the Longarm Formation, characterizing the suture zone, and the Middle Jurassic rocks may be indicative of the Lower Cretaceous-Upper Jurassic collision event (Table 1, Appendix 1). The Longarm Formation consists of sandstones, siltstones and conglomerates (Sutherland-Brown, 1968). Sutherland Brown (1968) noted large blocks of andesitic volcanic rocks (8 to 10 m in size) occurring closely above the unconformable contact. Such large clasts would have to have been derived from the rapid development of substantia] relief in zones adjacent to the suture zone (Yorath and Chase, 1981). Figure 1.4 shows the major faults in the region of the Queen Chatlotte Islands. The offshore trace of the Rennell Sound Fault, which coincides with the trend of a prominent gravity anomaly and a seismic reflection survey interpreted by Yorath (Yorath and Chase, 1981), has led the authors to suggest that this fault zone represents the boundary between the Alexander and Wrangellia terranes. They propose that the structural depression, as evident from gravity and seismic data, was possibly due to a relocation of compressive stresses in a post-collisional regime. Following the Late Jurassic to Early Cretaceous suturing, the two terranes then docked with North America somewhere between 90 and 40 Ma ago (Figure 1.3c) and may have been responsible for the plutonic uplift within the Coast Mountains. The post  > 140 MILLION YEARS A G O  ABOUT 140 MILLION YEARS A G O  90 TO 40 MILLION YEARS A G O  WRANGELLIA MOVES NORTHWARD FROM THE SOUTHERN HEMISPHERE  WRANGELLIA COLLIDES WITH THE ALEXANDER TERRANE  COMBINED WRANG.ELLIA AND ALEXANDER TERRANE DOCK WITH NORTH AMERICA  Figure 1.3  Schematic diagram for the tectonic evolution of the Pacific Northwest margin showing the collision of  Wrangellia with Alexander and then with the Pacific Margin (Yorath and Cameron, 1982).  8  i |  Figure 1.4  Present day relationship between Wrangellia and Alexander terranes showing the major  faults and the position of the proto-Queen Charlotte Islands ( after Yorath and Hyndman, 1983).  . |  9 s u t u r e a s s e m b l a g e , c o m p o s e d of t h e M i d t o U p p e r C r e t a c e o u s Q u e e n C h a r l o t t e G r o u p , w a s d e p o s i t e d a c r o s s t h e s u t u r e z o n e a n d overlies W r a n g e l l i a , t h e A l e x a n d e r T e r r a n e a n d the suture assemblage. T h e f o u r t h e v o l u t i o n a r y s t a g e is r e p r e s e n t e d by t h e M a s s e t a n d S k o n u n F o r m a t i o n s a n d b y a series o f i n t e r m e d i a t e e p i z o n a l p l u t o n s , a l l of T e r t i a r y a g e , d e n n e d by Y o r a t h a n d C h a s e (1981) as t h e rift a s s e m b l a g e . A l s o r e l a t e d t o t h e rift a s s e m b l a g e are s e v e r a l peralkaline volcanic centres of the A n a h i m volcanic belt. T h e M a s s e t F o r m a t i o n is c o m p o s e d o f p y r o c l a s t i c r o c k s c o n s i s t i n g p r i m a r i l y kalic basalt a n d sodic rhyolite  ( S u t h e r l a n d B r o w n , 1968).  The formation  of a l -  is 1200 t o  5 5 0 0 m t h i c k , e r u p t e d s u b a e r i a l l y for t h e m o s t p a r t , a n d rests u n c o n f o r m a b l y o n a l l older units on the Q u e e n C h a r l o t t e Islands.  T h e age o f t h e f o r m a t i o n v a r i e s b e t w e e n  P a l e o c e n e a n d M i o c e n e (62 a n d 11 M a ) ( Y o u n g , 1981).  T h e m e a n age o f t h e M a s s e t  v o l c a n i c s f o r t h e Q u e e n C h a r l o t t e I s l a n d s is 27 M a ( L a t e O l i g o e e n e ) , w h i l e for H e c a t e S t r a i t a n d Q u e e n C h a r l o t t e S o u n d i t is 35 M a ( E a r l y O l i g o e e n e ) . formation  T h e o r i g i n of t h e  h a s b e e n suggested as u p p e r m a n t l e m a t e r i a l for t h e b a s a l t s a n d r e m e l t e d  P a l e o z o i c p l u t o n i c r o c k s for t h e r h y o l i t e s ( S u t h e r l a n d B r o w n , 1968). T h e S k o n u n F o r m a t i o n consists of m a r i n e a n d non-marine sands, sandstone, shale, l i g n i t e s t r i n g e r s a n d c o n g l o m e r a t e s ( S u t h e r l a n d B r o w n , 1968).  Where found on the  Q u e e n C h a r l o t t e I s l a n d s , it rests u n c o n f o r m a b l y o n t h e M a s s e t v o l c a n i c s . T h e f o r m a t i o n t h i c k e n s t o w a r d s t h e east i n t o H e c a t e S t r a i t a n d Q u e e n C h a r l o t t e S o u n d a n d ranges i n age f r o m E a r l y M i o c e n e to L a t e P l i o c e n e . N o r t h of t h e a s s u m e d s u t u r e z o n e , t h e S k o n u n a n d M a s s e t f o r m a t i o n s overly t h e Cretaceous and/or  J u r a s s i c rocks of the post suture assemblage ( Y o r a t h a n d Chase,  10 1 9 8 1 ) . T h e y are a l s o f o u n d i n f a u l t c o n t a c t w i t h t h e J u r a s s i c r o c k s o f W r a n g e l l i a a n d t h e C r e t a c e o u s s t r a t a of t h e p o s t - s u t u r e a s s e m b l a g e o n t h e Q u e e n C h a r l o t t e I s l a n d s .  1.2 Subsidence of the Queen Charlotte basin T h e Q u e e n C h a r l o t t e s e d i m e n t a r y b a s i n is c o m p r i s e d o f several s u b s i d i a r y b a s i n s , of w h i c h t h e H e c a t e s u b - b a s i n a n d t h e C h a r l o t t e s u b - b a s i n are c o n s i d e r e d t h e m o s t i m p o r t a n t . T h e s e a r e s e p a r a t e d b y a b a s e m e n t t o p o g r a p h i c high k n o w n a s t h e M o r e s b y R i d g e , which runs in a northeasterly  direction from southern Moresby Island  (Figure  1.5). T h e b a s e m e n t r i d g e is l o c a t e d r o u g h l y i n t h e c e n t e r of t h e Q u e e n C h a r l o t t e b a s i n . Y o r a t h a n d H y n d m a n (1983) h a v e p r o p o s e d a m o d e l f o r t h e s u b s i d e n c e of t h e Q u e e n C h a r l o t t e b a s i n w h i c h c o m b i n e s a n i n i t i a l stage o f r i f t i n g , f o l l o w e d b y f l e x u r a l  down-  w a r p i n g a n d s e d i m e n t l o a d i n g , w h i c h r e s u l t e d f r o m t h e o b l i q u e c o n v e r g e n c e of o c e a n i c l i t h o s p h e r e w i t h t h e N o r t h A m e r i c a n continental m a r g i n . T h e y argue t h a t if the trace o f t h e A n a h i m h o t s p o t is e x t r a p o l a t e d b a c k w a r d s i n t i m e , it w o u l d have p a s s e d b e n e a t h Q u e e n C h a r l o t t e S o u n d ( F i g u r e 1.6a).  Furthermore, the rifting and crustal extension  w h i c h o c c u r r e d were responsible for the widespread subareal M a s s e t volcanics.  This  rifting activated or reactivated the R e n n e l l S o u n d - S a n d s p i t Fault, w h i c h allowed the p r o t o - Q u e e n C h a r l o t t e Islands t o m o v e t o t h e i r p r e s e n t l a t i t u d e ( F i g u r e 1.6b).  Heat  flow f r o m t h e Q u e e n C h a r l o t t e b a s i n offshore w e l l s is high for t h e n o r t h e r n three w e l l s ( F i g u r e 1.7), p a r t i c u l a r l y t h e S o c k e y e w e l l , w h i c h c o i n c i d e s w i t h t h e p r o p o s e d s u t u r e b e t w e e n W r a n g e l l i a a n d A l e x a n d e r t e r r a n e s . T h e d e p r e s s e d heat flow i n t h e f o u r s o u t h e r n w e l l s is t h o u g h t t o b e d u e t o r e c e n t u n d e r t h r u s t i n g of o c e a n i c l i t h o s p h e r e . T h e r i f t i n g is considered to have occurred d u r i n g a period of regional uplift a n d to have c o m m e n c e d a b o u t 21 M a ago a n d ceased 17 M a a g o ( Y o r a t h a n d H y n d m a n , 1983).  I  1 1  ISOPACH OF NEOGENE SKONUN FORMATION  OBS •  WELL  0 i  Figure 1.5  SO  1  * 100 I  km  !  1_  133° 132° 131° 130° 129°  Isopach of the Tertiary Skonun sediments for the Queen Charlotte Basin ( after Shouldice,  1971, 1973). Location of the airgun /OBS survey and the three nearest wells is also indicated.  A. 20 TO 17 MILLION YEARS AGO  B. ABOUT 17 MILLION YEARS AGO  Alexander Terrane (A.T.)  Trace of Anahim Hot Spot  » • f• •  •>JpJ>*? p p Paleozoic rocks  m  Wrangellia (W.) rocks  Mesozoic  Rift zones  Figure 1.6  C. M.  Continental Margin  RSF  Rennel Sound Fault  SF  Sandspit Fault  Rifting sequence for the Queen Charlotte Basin ( after Yorath and Cameron, 1982).  13  TOO-,  without cooling b y underthrusting  80-  'e | 6 0 -  o40u.  f-H-  £20-  W  S;  >• Ui  oc  CL  Figure 1.7  >•  O  8;  o  O  Ui _J  SOUTH  X  0  Ui Ui  cc <  r  100  • DISTANCE  :  r  2 0 0 (km)  NORTH 300  Estimated present heat flow based on down hole temperature measurements and thermal  conductivities computed from porosity and mineralogy data from the Queen Charlotte Basin offshore wells. The dashed curve of higher heat flow for the southern wells is a possible profile if no underthrust cooling had occurred ( after Yorath and Hyndman, 1983).  14  Yorath and Hyndham (1983) have interpreted the subsidence curves for all the offshore wells drilled in Queen Charlotte basin. Present depths to biostratigraphic horizons were converted to a basement subsidence history through corrections for sediment compaction and paleowater depths. The sediment compaction correction was derived using an exponential approximation of the decrease in porosity with depth. Tectonic subsidence curves were then estimated, thus correcting for sediment loading. The major uncertainty in their interpretation is in the biostratigraphic data reported by Shouldice (1971). The tectonic curves first showed uplift, which resulted in the unconformity, at the end of the Late Miocene or Early Pliocene. Then 6 Ma ago there was a sudden onset of subsidence which has continued at a decreasing rate to the present. The Queen Charlotte Islands appear to have been uplifted and eroded, particularly on the west coast of the islands.  The flexural model developed to explain this second stage of subsidence is  based upon the assumption of oblique convergence at the North American continental margin.  The subsidence due to the flexural downwarping was further amplified by  sediment loading. The Oshawa rise, west of the islands, and the missing Upper Miocene and Pliocene sediments on Graham Island west of the hinge line where subsidence is balanced by uplift are cited by Yorath and Hyndman (1983) in support of this model.  1.3 Stratigraphy Much of the information on the Neogene Skonun succession was obtained from the exploratory wells drilled on northeastern Graham Island by Richfield Oil Corporation and in Hecate Strait and Queen Charlotte Sound by Shell Canada Ltd. The location of the wells is shown on the map in Figure 1.1 and a cross section derived from the  15  well data is depicted in Figure 1.8. There are several striking features evident on the cross section.  As already noted, there are several smaller basins within the Queen  Charlotte basin that are controlled by the unconformity at the base of the Tertiary Skonun sediments. In the central part of Hecate Strait, the Moresby Ridge, between the Sockeye and Murrelet wells, separates the main part of the Queen Charlotte basin into the Hecate sub-basin and the Charlotte sub-basin (Figure 1.5). The Tertiary sediments are primarily non-marine in the Hecate sub-basin and marine to the south in the Queen Charlotte sub-basin. Within the Hecate sub-basin, the major source was probably the Coast Mountains with small contributions from the Queen Charlotte Islands (Young, 1981). The Charlotte sub-basin, of marine Lower Miocene - Upper Pliocene succession, was probably deposited in water depths ranging from shallow to deep (0-50m to 200-1000m) based on biostratigraphic information. Underlying the Tertiary Skonun sediments and Masset volcanics are the Skidegate Formation conformably resting on the Honna formation. The two formations represent the upper and middle units of the Queen Charlotte Group (Sutherland Brown, 1968); they are included in the post suture assemblage of Yorath and Chase (1981). The Honna Formation consists of conglomerate and coarse arkasic sandstone with minor shale or siltstone. The Skidegate Formation consists of fine grained detrital rocks, siltstone, silty shale, fine to medium sandstone and calcareous shale and sandstone. Both formations are estimated to be Late Cretaceous in age with a combined thickness ranging between 800 to 2100 m. Beneath the Queen Charlotte Group, within Wrangellia, the Kunga, Maude and Yakoun Formations locally contain significant quantities of heavy fraction hydrocarbons locally on the Queen Charlotte Islands. The Kunga Formation is primarily composed  16  C. BALL T Y E E COHO  S.L.  SOCK.  AUK. MUR.  OSP. HARL r  S.L. 1000m  4000-  -2000m  3 N-M. S s  Figure 1.8  H H  N-M. S l t s t - S h  WMl  M. S s  H H  M.Sltst-Sh  •  OIL SHOW  ™ — FLORA <™™™ F L O R A - F A U N A  Cross section for the offshore wells drilled by Shell Canada Ltd. showing the unconformity  controlling the basin. Stratigraphic correlations are based upon flora and fauna from well cores ( after Shouldice, 1971, 1973).  17 of limestone and argillite with its age ranging from Early Upper Triassic to mid Lower Jurassic. The Maude Formation, where present, rests conformably on the Kunga Formation and is composed of argillite, shale, calcareous shale and lithic sandstone. The contact with the Kunga Formation is gradational, extending over many meters. The age of the Formation is estimated to be Late Jurassic with a total thickness of 225 m. It is seen on the southeastern portion of Graham Island, northeastern Moresby Island and Lyell Island. The Yakoun Formation contains some non-marine, as well as marine sediments.  The formation consists primarily of pyroclastic rocks, many of which are  formed largely of porphyritic andesite. It also includes volcanic sandstone, some eonglomerate, shale, siltstone and minor coal. The age of the formation is between Middle Jurassic and earliest Upper Jurassic. Sutherland Brown (1968) described the volcanic cones which rose above sea level during this period to have once been clothed in lush Jurassic forests.  The total thickness is approximatley 900 m. Table 1 of Appendix I  summarizes each formation's lithology, age and thickness.  1.3.1 Hydrocarbon potential for the major stratigraphic units The Upper Triassic and Jurassic rocks of Wrangellia, the Kunga, Maude and Yakoun formations, are the most favourable prospect for hydrocarbon generation. These are the Kunga, Maude and Yakoun Formations. In some places, Yorath and Cameron (1982) report that these rocks have been described as oil shales. Offshore, they probably occur at considerable depths but the oil may have migrated along faults into either the Upper Cretaceous sediments or the Tertiary sediments. The Cretaceous Honna Formation has good porosity and fair permeability (Shouldice, 1971, 1973) and is thought to be a good reservoir rock. The Tertiary sediments contain the necessary stratigraphic and structural traps for containing the oil which may have migrated into these sediments.  18 Another source of oil may be from the Tertiary sediments within the rift zones (Figure 1.6). If the increased heat flow seen in the wells (Figure 1.7) was higher in the past, oil may have been generated. The heat flux is certainly sufficient to have produced more than an immature gas. However if the heat flow was too high for too long, only dry gas would have been generated (Yorath and Cameron, 1982). Below the Hecate sub-basin north of the suture zone, the Alexander Terrane rocks are not known to contain hydrocarbons. Therefore, only the Tertiary rocks could have generated oil. However, the geothermal gradients are thought to have been normal, as no rifting occurred in the Hecate sub-basin and only small amounts of immature gas may be expected (Yorath and Cameron, 1982). The lack of a sufficient thickness for the sediments combined with their primarily non-marine composition would further support this conclusion. Another prospect, albeit costly, exists within the Cretaceous sediments which lie beneath the Tertiary lavas. Exploration, using conventional seismic reflection techniques, would be difficult as the volcanic rocks present a strong acoustic barrier to adequate penetration of sound waves. The Tertiary volcanics are known to thin and disappear in some locations (Figure 1.9; Shouldice 1971, 1973), which may aid in the exploration of this stratigraphic zone.  1.4 A n Outline of The Seismic Refraction Study The refraction experiment was designed to determine the seismic velocity structure of the Hecate sub-basin, particularly below the Tertiary sediments where the thickness of the Masset volcanics was unknown. Also, the existence or not of sediments below the volcanics was important to evaluate this region for any further exploration programs.  19  6000m 24000*  ipi T SEDIMENTS  Mz SED.-VOLC.  Wi T VOLCANIC  PLUTONS  0  u  Mi  100 —i  6 Km 100  Figure 1.0 Structural geological model inferred from the cross section in figure 1.8, industry seismic reflection and refraction data and gravity and magnetics. Projected position of the four OBSs is indicated by arrows ( after Shouldice, 1973).  20 T h e initial m o d e l , inferred  by S h o u l d i c e ( 1 9 7 1 , 1973) f r o m w e l l d a t a a n d a v a i l a b l e  g e o p h y s i c a l d a t a , is s h o w n i n F i g u r e 1.9. T h e u p p e r b o u n d a r y a n d e v e n t h e presence of t h e M e s o z o i c s e d i m e n t s a n d v o l c a n i c s is p o o r l y d e f i n e d b e n e a t h t h e H e c a t e s u b - b a s i n . T h e p o s i t i o n o f t h i s u n i t i n F i g u r e 1.9 is based u p o n l i m i t e d w e l l log i n f o r m a t i o n  and  o u t c r o p s seen o n t h e l a n d on e i t h e r s i d e of t h e b a s i n . T o v e r i f y a n d / o r m o d i f y t h e m o d e l of F i g u r e 1.9, a n a i r g u n / O B S s u r v e y w a s c a r r i e d o u t i n 1983.  F o u r O B S s w e r e d e p l o y e d a t 20 k m i n t e r v a l s across H e c a t e S t r a i t ;  a i r g u n s h o t s at a b o u t 0.2 k m s p a c i n g s were r e c o r d e d to p r o v i d e three r e v e r s e d profiles e x t e n d i n g over 60 k m . ysis procedures.  C h a p t e r II g i v e s t h e d e t a i l s of t h e d a t a a c q u i s i t i o n a n d a n a l -  In C h a p t e r I V , t h e r e c o r d s e c t i o n s a n d t h e i r i n t e r p r e t a t i o n  through  c o m p a r i s o n s w i t h t h e o r e t i c a l s e c t i o n s for 2 - D v e l o c i t y m o d e l s are d e s c r i b e d . T h e t h r e e s u b - m o d e l s r e s u l t i n g f r o m i n t e r p r e t a t i o n o f the i n d i v i d u a l r e v e r s e d profiles are c o m p o s i t e d t o p r o v i d e a c o m p l e t e m o d e l a c r o s s t h e b a s i n . A d i s c u s s i o n of t h e r e l i a b i l i t y of t h i s m o d e l a n d i t s r e l a t i o n s h i p t o t h e l o c a l geology for t h e H e c a t e s u b - b a s i n are p r o v i d e d i n Chapter V . C h a p t e r III  c o n t a i n s a n a d d i t i o n a l c o m p o n e n t t o t h i s t h e s i s - the d e v e l o p m e n t o f  a n i n t e r a c t i v e p r o c e d u r e for i n v e r s i o n of r e f r a c t i o n d a t a by w a v e f i e l d c o n t i n u a t i o n .  The  t h e o r e t i c a l b a s i s is o u t l i n e d a n d a n d i t s a p p l i c a t i o n t o t h e s y n t h e t i c a n d r e a l d a t a o f C h a p t e r I V is s h o w n .  21  C H A P T E R II  2.1 D a t a  2.1.1  DATA ACQUISITION A N D A N A L Y S I S  Acquisition  A i r g u n - O B S experiment  T h e a i r g u n / o c e a n b o t t o m seismograph ( O B S ) refraction program in Hecate Strait w a s u n d e r t a k e n as one c o m p o n e n t of a larger r e f r a c t i o n p r o g r a m . In c o o p e r a t i o n w i t h the E a r t h P h y s i c s B r a n c h ( E P B ) a n d the P a c i f i c G e o s c i e n c e C e n t e r ( P G C ) , the U n i v e r s i t y of B r i t i s h C o l u m b i a c a r r i e d o u t a n o n s h o r e - o f f s h o r e r e f r a c t i o n p r o g r a m d u r i n g A u g u s t , 1983. T h i s large 330 k m p r o f i l e w a s r e c o r d e d f r o m the d e e p o c e a n across n o r t h e r n M o r e s b y I s l a n d a n d H e c a t e S t r a i t , t o t h e m a i n l a n d of B r i t i s h C o l u m b i a . S e v e n t e e n s e i s m o g r a p h s were d e p l o y e d : 11 l a n d b a s e d s e i s m o g r a p h s a n d s i x O B S s ; a n d t w o energy s o u r c e s w e r e e m p l o y e d : T N T e x p l o s i v e s a n d a n a i r g u n . T h e o b j e c t i v e of t h i s thesis w a s t o i n v e s t i g a t e the u p p e r 6 k m of t h e c r u s t b e n e a t h H e c a t e S t r a i t . T h e d a t a , r e c o r d e d o n t h e f o u r O B S s d e p l o y e d across H e c a t e S t r a i t u s i n g a 32 l i t r e ( 2 0 0 0 m ) a i r g u n s o u r c e , 3  w e r e u t i l i z e d to d e v e l o p a s e i s m i c v e l o c i t y s t r u c t u r a l m o d e l t o m e e t t h i s o b j e c t i v e . T h e l o c a t i o n a n d p l a n of t h e a i r g u n / O B S e x p e r i m e n t is s h o w n i n F i g u r e 2.1.  2.1.2  Description of the O B S s a n d the  airgun  T h e O B S s w e r e b u i l t at U B C a n d f o l l o w e d t h e d e s i g n used by A t l a n t i c G e o s c i e n c e C e n t e r (Heffler a n d B a r r e t t , 1979) w i t h some m i n o r m o d i f i c a t i o n s i n c o r p o r a t e d d u r i n g construction.  T h e O B S c o n s i s t s of four m a i n c o m p o n e n t s :  (l)  the d e l i v e r y s y s t e m  c o m p r i s e d of a glass f l o t a t i o n s p h e r e , a p r e s s u r e case a n d a n a n c h o r ; (2) t h e s e i s m i c w a v e d e t e c t i o n s y s t e m r e p r e s e n t e d by a v e r t i c a l a n d a h o r i z o n t a l g i m b a l e d 4.5 H z g e o p h o n e  Figure 2.1  Location and plan of the 1983 airgun/OBS survey. OBSs are indicated by darkened circles  numbered 1 to 4 and the Coho, Tyee and Sockeye wells are marked by stars.  23 h o u s e d w i t h i n t h e p r e s s u r e c a s e , a n d a h y d r o p h o n e , m o u n t e d o n t h e side o f t h e  flotation  s p h e r e ; (3) t h e r e c o r d i n g s y s t e m w h i c h uses a f o u r c h a n n e l s l o w s p e e d d i r e c t r e c o r d t a p e u n i t t o store t h e a m p l i f i e d s i g n a l f r o m t h e t w o g e o p h o n e s a n d h y d r o p h o n e ; (4) t h e c l o c k , r a t e d a g a i n s t W W V B a t t h e t i m e of d e p l o y m e n t a n d r e c o v e r y , g e n e r a t e s a 10 H z a m p l i t u d e m o d u l a t e d t i m e c o d e .  T h e o p e r a t i o n o f t h e O B S is g o v e r n e d by a  microprocessor a n d the signals f r o m the three c o m p o n e n t s along w i t h the time code w e r e r e c o r d e d o n cassette t a p e .  T h e p a s s b a n d o f t h e O B S s lies b e t w e e n 4 . 5 a n d 30  H z ( C l o w e s , 1985). T h e s e i s m i c s o u r c e w a s a 32 l i t r e ( 2 0 0 0 m ) a i r g u n w h i c h p r o v i d e d 3  energy equivalent to a b o u t 4 k g w i t h i n the seismic p a s s b a n d . T h e airgun firing was c o n t r o l l e d by a m i c r o p r o c e s s o r w h i c h t r i g g e r e d ' t h e a i r g u n a t o n e m i n u t e i n t e r v a l s u s i n g a c l o c k w h i c h w a s r a t e d a g a i n s t W W V B t i m e c o d e a t t h e s t a r t of t h e s h o o t i n g .  2.1.3 Description of the procedure T h e O B S s w e r e d e p l o y e d a t a p p r o x i m a t e l y 20 k m s p a c i n g across H e c a t e S t r a i t i n w a t e r d e p t h s r a n g i n g f r o m 22 m f o r O B S 1 i n t h e west t o 162 m f o r O B S 4 i n t h e east ( F i g u r e 2 . 1 ) . T h e d e p t h s w e r e d e t e r m i n e d u s i n g t h e s h i p ' s d e p t h s o u n d i n g s y s t e m as r e c o r d e d o n a n E P C line s c a n r e c o r d e r . T h e l a t i t u d e s a n d l o n g i t u d e s w e r e d e r i v e d f r o m r e a d i n g s of t h e s h i p ' s p r i n c i p a l n a v i g a t i o n s y s t e m , L o r a n C , a t t h e t i m e o f d e p l o y m e n t . T a b l e 2.1 lists t h e d e p t h a n d p o s i t i o n f o r each O B S . T h e s h i p s t a r t e d f r o m t h e p o i n t o f d e p l o y m e n t of O B S 4 i n t h e w e s t . T h e a i r g u n w a s t o w e d b e h i n d t h e s h i p a t a d e p t h o f 18 m e t r e s u n t i l i t neared O B S 2 , w h e r e t h e d e p t h w a s d e c r e a s e d t o 14 m f o r t h e r e m a i n i n g p o r t i o n of t h e profile.  This  firing  rate  of one m i n u t e , combined w i t h the ship's speed, resulted i n a source interval spacing  24  OBS 1 2 3 4  LOCATION (N. Long. : W. Lai.) 53.005 : IS 1.4616 53.8008 : 131.2342 58.8060 : 130.9579 53.5158 : 130.7155  DEPTH 22 82 122 163  AIRGUN  DEPTH  U 14 18 18  T a b l e 2.1 O B S l o c a t i o n a n d d e p t h a n d d e p t h o f a i r g u n ( d e p t h s a r e g i v e n in meters).  of a p p r o x i m a t e l y  160 m .  R e v e r s e d profiles f o r s e g m e n t s o f t h e a i r g u n line  between  n e i g h b o u r i n g O B S s arose n a t u r a l l y from the design of the e x p e r i m e n t .  2.2 D a t a  Processing  2.2.1 D i g i t i z i n g , e d i t i n g a n d d e m u l t i p l e x i n g  the analog data  T h e d i r e c t r e c o r d c a s s e t t e t a p e s were d i g i t i z e d u s i n g t h e P D P l l / 3 4 - b a s e d a n a l o g t o - d i g i t a l c o n v e r s i o n f a c i l i t y . T h e d a t a w e r e c o n v e r t e d a t a 120 H z s a m p l i n g rate w i t h v a r i a t i o n s i n a n a l o g t a p e s p e e d t a k e n i n t o a c c o u n t . T h e d i g i t i z e d d a t a were t h e n edited i n t o 2 5 s e g m e n t s c e n t r e d a b o u t t h e l a r g e s t a u t o m a t i c a l l y d e t e c t a b l e event w i t h i n each 60 s r e c o r d i n g i n t e r v a l .  T h e t i m e o f first s a m p l e f o r each event w a s d e t e r m i n e d w i t h  the a i d o f a c o m p u t e r p r o g r a m w h i c h keyed o n t h e O B S time code.  T h e time code  w a s n o t e x p l i c i t l y r e a d b e c a u s e t h e high pass b a n d r e s p o n s e o f t h e h y d r o p h o n e t o t h e w a t e r wave h a d been cross-fed t o the time code c h a n n e l . T h i s facilitated t h e water wave r e c o g n i t i o n , b u t i n t e r f e r e d w i t h t h e t i m e c o d e s i g n a l over a r a n g e o f d a t a p o i n t s . T h i s  25 p r o b l e m w a s c i r c u m v e n t e d by detecting only m i n u t e m a r k s , keeping track of the time to the p o i n t where the edited d a t a segments b e g a n , a n d t h e n s k i p p i n g over the requisite n u m b e r of d a t a b l o c k s t o b y p a s s t h e w a t e r w a v e i n t e r f e r e n c e .  T h e e d i t e d d a t a were  then d e m u l t i p l e x e d a n d converted to an I B M - c o m p a t i b l e format for p l o t t i n g .  2.2.2 Basic timing and positioning C o r r e c t i o n s w e r e a p p l i e d t o t h e t i m e o f t h e first s a m p l e o f each s e i s m o g r a m for d r i f t i n t h e O B S a n d a i r g u n c l o c k s r e l a t i v e t o W W V B , d e l a y i n t h e a c t u a l firing of t h e a i r g u n after t h e trigger pulse a n d a d j u s t m e n t s for the skewness of the recording head.  Ship  p o s i t i o n s w e r e d e t e r m i n e d by L o r a n C r e a d i n g s l o g g e d a t 10 m i n u t e i n t e r v a l s .  2.2.3 Special positioning and related timing corrections T h e n a v i g a t i o n d a t a s h o w t h a t t h e s h o t line d i d n o t pass d i r e c t l y o v e r t h e O B S s n o r d i d t h e n e a r e s t offset, as d e f i n e d b y these d a t a , agree w i t h t h e offset as d e t e r m i n e d f r o m first  arrival observations.  T o enable compilation of the seismic d a t a into appropriate  r e c o r d s e c t i o n s f o r w h i c h 2 - D profile r e c o r d i n g is a s s u m e d ' , d i s t a n c e a n d t i m e c o r r e c t i o n s w e r e r e q u i r e d . A l s o , t h e m o s t a c c u r a t e d a t a f r o m w h i c h s h o t - r e c e i v e r d i s t a n c e s c a n be d e t e r m i n e d is t h e first a r r i v a l a n d / o r d i r e c t w a t e r w a v e a r r i v a l o n t h e s e i s m o g r a m s , rather than interpolated  navigation  positions.  A full description of the t i m i n g a n d  p o s i t i o n i n g c o r r e c t i o n s a l o n g w i t h a c o m p l e t e set o f n a v i g a t i o n a n d t i m e o f first s a m p l e t a b l e s is g i v e n i n C l o w e s (1985).  T h e special p o s i t i o n i n g a n d t i m i n g w i l l be restated  here as t h e y r e p r e s e n t a d j u s t m e n t s w h i c h i n f l u e n c e a l l t h e t r a c e s p a r t i c u l a r l y f o r t h e s o u r c e / r e c e i v e r d i s t a n c e s t h a t a r e less t h a n 2.0 k m w h e r e i n l i n e c o r r e c t i o n s are m o s t noticeable.  26 C o r r e c t i o n s f o r s h o t / r e c e i v e r p o s i t i o n s a n d d i s t a n c e s were m a d e i n t h r e e s t a g e s , a n d n e c e s s i t a t e d a c o r r e s p o n d i n g a d j u s t m e n t t o t r a v e l t i m e s . T h e f i r s t stage m a d e use of t h e n a v i g a t i o n d a t a t o place t h e s h o t s a n d O B S s a p p r o x i m a t e l y i n line a n d p r o v i d e t h e u s u a l t r a c e s p a c i n g a s s o c i a t e d w i t h a one m i n u t e f i r i n g i n t e r v a l . S i m p l e p l a n a r g e o m e t r y w a s used to project the O B S positions onto the shot line. The  revised (shortened)  shot receiver distances did require that a corresponding  t r a v e l t i m e c o r r e c t i o n for t h e e x t r a t r a v e l p a t h be r e m o v e d . T h i s w a s t h e s u b s t a n c e of t h e s e c o n d stage w h i c h used first a r r i v a l s t o d e t e r m i n e t h e t r a v e l t i m e s t o t h e O B S s . D i r e c t w a t e r w a v e s were n o t i n f a c t t h e first a r r i v a l s , e v e n at s h o r t d i s t a n c e s , d u e to t h e effects of t h e s h a l l o w w a t e r a n d r e f r a c t i o n t h r o u g h the s u b b o t t o m . C o n s e q u e n t l y , a t w o l a y e r m o d e l ( w a t e r p l u s s u b b o t t o m , for w h i c h t h e a p p r o x i m a t e v e l o c i t y w a s d e t e r m i n e d f r o m first a r r i v a l d a t a ) w a s used t o d e t e r m i n e t h e b e s t p r o j e c t e d l o c a t i o n of t h e O B S s r e l a t i v e t o t h e s h o t s a l o n g t h e p r o f i l e . T h i s w a s a c c o m p l i s h e d by i t e r a t i v e a d j u s t m e n t s of c a l c u l a t e d i n t e r c e p t s t o a g r e e w i t h m e a s u r e d t r a v e l t i m e s . K n o w i n g t h e d e p t h of t h e O B S a n d t h e d e p t h o f t h e w a t e r b e l o w t h e a i r g u n ( f r o m 3.5 k H z p r o f i l i n g ) , w h i c h w a s n e a r l y c o n s t a n t for a few k i l o m e t e r s on e i t h e r side of t h e O B S s , a s i m p l e c a l c u l a t i o n using P y t h a g o r a s ' theorem enabled determination  of t h e a p p r o p r i a t e t i m e c o r r e c t i o n .  T h e t i m e c o r r e c t i o n s a p p l i e d t o e a c h O B S a m o u n t e d t o less t h a n 0.45 s for t h e O B S r e q u i r i n g t h e largest i n l i n e c o r r e c t i o n at z e r o offset. ( w h i c h w a s for O B S 2) h a d d r o p p e d t o 0.11 s.  At  1 k m offset t h i s  correction  T h e s e i n l i n e t i m i n g c o r r e c t i o n s were  g e n e r a l l y less for t h e o t h e r t h r e e O B S s a n d were large c o m p a r e d t o the p i c k i n g of  first  b r e a k s . H o w e v e r , t h e large shifts d o not c o r r e s p o n d t o a large u n c e r t a i n t y i n first a r r i v a l p i c k s for t h e affected t r a c e s b u t d o affect s e c o n d a r y a r r i v a l p o s i t i o n s , r e n d e r i n g  them  u n r e l i a b l e w h e r e t h e c o r r e c t i o n s w e r e large (less t h a n 1 k m s o u r c e / r e c e i v e r d i s t a n c e ) . S e c o n d a r y arrivals were only m o d e l l e d b e y o n d the 5 k m range where the corrections  27 for travel time were not significant. This was borne out in the comparison of velocities determined from the corrected traces (2.0 km/s) versus velocities derived from some well logs in the area which yielded velocities of 2.17 km/s at 300 m depths. Details of the calculations can be found in Clowes (1985). The time correction is only applied to the plotting parameter which controls the sample representing zero time. The third stage involved a simple lateral shift to produce agreement between the nearest offset based upon navigation and the nearest offset based upon observed data. Although the relative accuracy of Loran C is approximately + or - 200 m in far offshore regions, it may deteriorate near station locations in near shore areas. Furthermore, there was no check for lane jumps which can result in 1 to 2 kilometer errors in the relative position of the ship. To determine the lateral shift, two methods were applied; one which uses the velocity, and one which used the predicted time-distance intercept 1  for a two layer model. Table 2.2 contains a summary of the positioning corrections for each OBS. The details of the calculations for the lateral shift are given in Clowes (1985). Appendix II contains all the corrected sections plotted in variable area format.  OBS 1 2 S 4  OFFLINE CORRECTION (km) NAVIGATION BASED ARRIVAL BASED 0.810 0.190 1.170 0.619 0.460 0.124 0.656 0.154  Table 2.2 OBS positionin g corrections.  LATERAL SHIFT (km) 0.995 1.156 1.990 1.160  28  2.3 D a t a A n a l y s i s a n d Interpretation Procedures  2.3.1 D a t a analysis T h e f r e q u e n c y c o n t e n t o f t h e s e i s m i c s i g n a l a n d noise r e c o r d e d o n a n O B S is i l l u s t r a t e d by t h e p o w e r s p e c t r a s h o w n i n F i g u r e 2.2 . T h e s i g n a l - t o - n o i s e r a t i o f o r t h e r e c o r d s e c t i o n s w a s v e r y g o o d a n d o n l y c e r t a i n cases r e q u i r e d t h e use of f i l t e r e d r e c o r d sections.  T h e noise is c o n c e n t r a t e d i n t w o b a n d s ( F i g u r e 2.2a), t h e 14 t o 24 H z fre-  q u e n c y b a n d a n d t h e 0 t o 5 H z f r e q u e n c y b a n d . T h e r e is also noise p r e s e n t i n t h e 5 t o 12 H z f r e q u e n c y b a n d b u t , as i l l u s t r a t e d i n F i g u r e 2.2b, t h e s i g n a l c o n t a i n s m o r e p o w e r t h a n t h e noise for m o s t cases. H o w e v e r , s o m e t i m e s noise o b s c u r e s a n y r e c o r d e d s e i s m i c s i g n a l as s h o w n i n F i g u r e 2 . 3 a f o r O B S 4 h y d r o p h o n e c o m p o n e n t . In t h i s ease w i t h i n t h e seismic passband proved helpful. u s i n g t h e s a m e scale f a c t o r s ) after b a n d p a s s  filtering  F i g u r e 2.3b shows the same d a t a (plotted filtering  b e t w e e n 5 a n d 12 H z . A n event  p a r a l l e l t o t h e d i s t a n c e a x i s a t a b o u t 1.8 s is n o w d i s t i n g u i s h a b l e . A r r o w s  highlighting  t h e e v e n t were p u r p o s e l y o m i t t e d t o p e r m i t t h e r e a d e r t o j u d g e t h e r e s u l t s w i t h o u t t o o m u c h bias f r o m the a u t h o r .  T h i s p a r t i c u l a r e x a m p l e p r o v e d t o be a v a l u a b l e g u i d e i n  the eventual interpretation of the d a t a recorded on the vertical a n d horizontal channels o f O B S 4.  2.3.2 Interpretation procedures T h e r e c o r d s e c t i o n s i n A p p e n d i x II are i n a f o r m r e a d y f o r i n t e r p r e t a t i o n .  Con-  ventional interpretation of refraction data usually involves a forward m o d e l in the form of two-dimensional m o d e l l i n g schemes. T h e process begins by d e t e r m i n i n g a starting m o d e l , u s u a l l y based u p o n a d i p p i n g l a y e r e d m o d e l d e r i v e d f r o m s t r a i g h t l i n e fits t o  PERIODOGRRH POUER  PERIODOGRRM POUER  9.000 8000 X 7000 6000 5000 | cr  o 0.  UJ  '£ 4000 i 3000 2000 X 1000 20 30 FREQUENCY (HZ)  40  10  "20 30 FREQUENCY (HZ)  40  Figure 2.2 Power spectra (a) background noise (b) signal + noise for OBS 3. Periodograms were computed two second window (see text for explanation).  50  60  30  OBS 4 Hydrophone  OBS 4 Hydrophone Figure 2.3  Comparison of unfiltered (a) andfiltered(b) record sections for OBS 4 hydrophone. Th<  record sections are plotted with the same scale factors Thefilterused for (b) was a 5-12 Hz bandpas: filter.  31 f o r w a r d a n d r e v e r s e d t r a v e l t i m e s of t h e p r i m a r y a r r i v a l s . T h e s t a r t i n g m o d e l m a y also b e d e r i v e d f r o m m o d e l s o b t a i n e d i n o t h e r e x p e r i m e n t s n e a r the a r e a of i n v e s t i g a t i o n . T h e i n i t i a l m o d e l is t h e n p e r t u r b e d by a d j u s t i n g i t s p a r a m e t e r s t o a c h i e v e a closer  fit  between the c o m p u t e d travel times t h r o u g h the m o d e l , and the observed travel times, b a s e d u p o n p r i m a r y a r r i v a l e v e n t s . T h i s p r o c e d u r e is i n h e r e n t l y n o n - u n i q u e , b u t m a y be c o n s t r a i n e d by adherence to other i n f o r m a t i o n  g a t h e r e d f r o m w e l l s d r i l l e d near t h e  a r e a of s t u d y , o t h e r s e i s m i c d a t a a n d / o r m o d e l s a n d t h e r e g i o n a l geology o f t h e a r e a .  A  f u r t h e r r e f i n e m e n t t o t h e final t r a v e l t i m e m o d e l s m a y b e a c c o m p l i s h e d b y t h e m o d e l l i n g of t h e o b s e r v e d a m p l i t u d e s i n t h e r e c o r d s e c t i o n . M o d e l s are v e r y s e n s i t i v e t o changes i n a m p l i t u d e a n d c a r e m u s t be t a k e n w h e n c o m p a r i n g m o d e l l e d a m p l i t u d e s w i t h o b s e r v e d a m p l i t u d e s w h e r e t h e d a t a h a v e b e e n c o m p i l e d f r o m m a n y different r e c e i v e r s a n d / o r s o u r c e s . F u r t h e r m o r e , a n y p o s i t i o n i n g a n d t i m i n g c o r r e c t i o n s a p p l i e d t o t r a c e s , s u c h as i n - l i n e c o r r e c t i o n s , d o n o t i n c l u d e c o r r e c t i o n s for t h e a m p l i t u d e of t h e s h i f t e d a r r i v a l . T h e d a t a in the present study were c o m p i l e d in a c o m m o n receiver format from a s t a b l e consistent source, w h i c h lends itself well to a m p l i t u d e m o d e l l i n g . T h e inline corr e c t i o n s a p p l i e d t o t h e d a t a p r e c l u d e r e l i a b l e a m p l i t u d e m o d e l l i n g of a r r i v a l s w i t h i n 4 k m of t h e O B S . T h e i n t e r p r e t a t i o n of t h e d a t a i n t h i s s t u d y e m p l o y e d a t w o - d i m e n s i o n a l r a y t r a c i n g r o u t i n e d e v e l o p e d at U B C ( S p e n c e , W h i t t a l l a n d C l o w e s , 1984). T h e c o m p u t e d t r a v e l t i m e s m a y be c o m p a r e d w i t h t h o s e p i c k e d f r o m t h e d a t a by d i r e c t l y o v e r l a y i n g t h e c o m p u t e d v a l u e s o n t o the o b s e r v e d d a t a . T h e a d v a n t a g e of d i r e c t l y o v e r l a y i n g t h e d a t a w i t h the c o m p u t e d t r a v e l t i m e c u r v e is t h a t t h e c o m p u t e d r e s u l t s c a n be c o m p a r e d in the context of the entire record section. V a l u a b l e insights can result f r o m following t h i s p r o c e d u r e as t h e i n t e r p r e t e r is c o n s t a n t l y r e m i n d e d of o t h e r t r e n d s w h i c h m a y e x i s t i n t h e d a t a . T h i s is n o t t h e case w h e n t r a v e l t i m e s a r e r e a d i n t o t h e p r o g r a m since t h e interpreter  is t h e n c o n c e r n e d w i t h  fitting  o n l y a n i s o l a t e d set of p o i n t s .  T h i s i d e a is  32 not new  to refraction modelling, since in refraction interpretation, particularly for poor  data, the interpreter will often return to the data after gaining a clearer insight into the structure under investigation and repick the first arrivals. To refine the model derived from the travel-time fit to the data, theoretical seismograms were calculated for the theoretical travel-time curves. Relative amplitudes across the synthetic record section were visually compared to the observed amplitudes. Amplitudes are particularly sensitive to velocity gradients and this feature provided additional control on the choice of velocity gradients for the model. Velocity gradients for the various layers in the model were altered until the amplitudes for the theoretical seismograms matched those in the data. Amplitude modelling also provided critical information for the analysis of particular phases observed in the data. An alternative method for determining a l-D velocity-depth structure directly from data sections was presented by Clayton and McMeehan (1981). The author followed their procedure and  developed an interactive computer program to implement the  method. The work represents a new  development at UBC.  It also provides examples  additional to, and different from, those illustrated in the original paper and insight into the effects of applying a l-D interpretation to a data set which clearly represents a  2-D  structure. Therefore, the method and its application to the airgun/OBS refraction data from Hecate Strait will be described in the next chapter.  33  C H A P T E R III  I N V E R S I O N B Y WAVEFIELD  CONTINUATION  3.1 Introduction Inversion by wavefield continuation is an image analysis procedure by which a 1-D veloeity-versus-depth structure can be extracted directly from a seismic record section. This is accomplished by projecting the observed wavefield, as represented by arrivals recorded at increasing offsets, back to zero offset. This backward projected wavefield then represents the arrival of wavefronts as a function of two-way travel time at the surface. This delay time function for the arrivals, along with the known stacking lines along which the backward projection was carried, may then be utilized to downward project the wavefield at the surface to the appropriate slowness medium from which it originated. The practical application of this method proceeds through two linear transformations; a slant stack (the back projection) and a downward continuation (the downward projected wavefield).  1  The inversion of refraction data by wavefield continuation has the desirable quality of an unbiased inversion, with the exception of the final velocity depth-pick. The observed data represent the input required by the program and the velocity depth function is obtained directly from the imaged wavefield. Inversion of refraction data by wavefield continuation has been applied to real data examples exhibiting weak lateral velocity variations (Clayton and McMechan, 1981). The theory is based upon the solution to the one dimensional wave equation for the 1-D velocity versus depth model. Clayton and McMechan (1981) demonstrated the robustness of the algorithm in the treatment of earth models with weak two dimensional velocity structures.  34 A c o m p u t e r p r o g r a m based u p o n t h e t h e o r y a s p r e s e n t e d b y C l a y t o n a n d M c M e c h a n (1981) h a s b e e n w r i t t e n w i t h t w o o b j e c t i v e s i n m i n d : ( l ) t o test t h e a p p l i c a b i l i t y o f t h e p r o c e d u r e f o r e x a m p l e s w i t h a g r e a t e r degree o f o f l a t e r a l h e t e r o g e n e i t y ; a n d (2) t o a p p l y t h e p r o c e d u r e t o a s e l e c t e d s e g m e n t of t h e a i r g u n / O B S d a t a set f o r t h e p u r p o s e of c o m p a r i n g t h e results w i t h those obtained from the 2 - D m o d e l l i n g interpretation  in  C h a p t e r I V . T h e d a t a u n d e r i n v e s t i g a t i o n i n t h i s t h e s i s offer a n i d e a l o p p o r t u n i t y  to  m e e t these o b j e c t i v e s f o r t w o r e a s o n s . F i r s t , t h e d a t a set satisfies t h e basic a s s u m p t i o n s o u t l i n e d i n t h e d e v e l o p m e n t of t h e t h e o r y . S e c o n d l y , t h e d a t a h a v e b e e n f u l l y i n t e r p r e t e d u s i n g t h e U B C 2 - D r a y t r a c i n g s c h e m e ( S p e n c e , W h i t t a l l a n d C l o w e s , 1985). T h e r e f o r e t h e r e s u l t i n g i n v e r s i o n , i n v o l v i n g c o m p a r i s o n of t h e o r e t i c a l a n d o b s e r v e d s e i s m o g r a m s , m a y b e e v a l u a t e d b a s e d u p o n these 2 - D r e s u l t s . O t h e r i n v e r s i o n p r o c e d u r e s w i l l n o t b e i n v e s t i g a t e d as t h e y a r e b e y o n d t h e s c o p e of this thesis.  T h e r e a d e r is r e f e r r e d t o v a r i o u s p a p e r s by B e s s o n o v a et al.  1 9 7 6 ) ; G a r m a n y et al.  (1979); W e n z e l et al.  (1982); C a r r i o n et al.  (1974,  (1984) a n d m a n y  o t h e r s for m o r e t h o r o u g h d i s c u s s i o n s c o n c e r n i n g t h e i n v e r s i o n o f r e f r a c t i o n d a t a . T h e f o l l o w i n g d e v e l o p m e n t w a s u n d e r t a k e n a s a s e p a r a t e s t u d y of i n t e r e s t , s u b s e q u e n t t o results derived f r o m the 2 - D modelling interpretation described in C h a p t e r I V .  3.2 The Linear Transformations  3.2.1 The slant stack procedure To obtain the slant stack, the a m p l i t u d e s are s u m m e d along lines of constant slope and intercept;  t h e w a v e f i e l d is t h e n c o n s t r u c t e d by s w e e p i n g t h r o u g h a l l slopes a n d  intercepts on a seismic record section.  T h e slope of the line in the traveltime-offset  35 (t-x)  d o m a i n is t h e i n v e r s e of t h e v e l o c i t y ( ^ ) ,  a l s o k n o w n as t h e ray p a r a m e t e r  t h e i n t e r c e p t is t h e t w o - w a y t r a v e l t i m e a t z e r o offset, u s u a l l y r e f e r r e d t o as r.  p;  The  a m p l i t u d e s u m m e d a l o n g l i n e s of c o n s t a n t s l o p e p a n d i n t e r c e p t r is t h e n s t o r e d or p l o t t e d at i t s n e w c o o r d i n a t e s (p, T). H a v i n g d e s c r i b e d h o w t h e s l a n t s t a c k is p e r f o r m e d , it m a y n o t b e a p p a r e n t h o w it arises.  A n u n d e r s t a n d i n g o f t h e s l a n t s t a c k p r o c e d u r e c a n be a t t a i n e d by c o n s i d e r i n g  t h e s e i s m o g r a m p r e s e n t a t i o n itself. A f u n c t i o n P ( t , x ) , w h e r e P r e p r e s e n t s t h e a m p l i t u d e a t t h e p o i n t (t,x)  in the t — x d o m a i n can be defined.  A s t r a i g h t line t h r o u g h  these  p o i n t s , d e f i n e d by t =  Ti +  pi  (3.1)  x,  d e s c r i b e s t h e t r a v e l t i m e for p o i n t s a l o n g t h e l i n e .  If t h i s line i n t e r c e p t s a n a r r i v a l ,  t h e n t h e t r a v e l t i m e t o a r e c e i v e r at a n offset of x c a n be d e t e r m i n e d .  Furthermore,  t h i s a r r i v a l a l s o has a v e l o c i t y i n t h e f o r m of t h e s l o p e of t h e line t h r o u g h t h a t p o i n t . H o w e v e r , t h i s d o e s n o t h a v e m u c h s i g n i f i c a n c e since a n u m b e r of l i n e s c a n pass t h r o u g h t h i s a r r i v a l d e p e n d i n g u p o n t h e choice of i n t e r c e p t a n d s l o p e . T h e s l a n t s t a c k a l l o w s a w e i g h t t o be a t t a c h e d t o t h e a r r i v a l t o d e t e r m i n e t h e s i g n i f i c a n c e o f t h e a s s o c i a t e d v e l o c i t y . In p r a c t i c e t h i s is d o n e by r e d e f i n i n g P{ i +Pi i ) T  x  x  a  n  P{t,x)  by s u b s t i t u t i n g  t  i n (3.1) t o o b t a i n  d t h e n a d d i n g all the a m p l i t u d e values w h i c h f a l l a l o n g t h e line d e f i n e d  b y Ti + p i x. T h i s w e i g h t i n g f u n c t i o n is d e f i n e d b y  (3.2)  z a n d for a set of a r r i v a l s w i t h t h e s a m e a p p a r e n t v e l o c i t y as t h a t o f t h e slope of t h e l i n e , S w i l l a d d t o large v a l u e s w h i l e u n c o r r e l a t e d e v e n t s w i l l d e s t r u c t i v e l y  interfere.  36 W e i g h t i n g f u n c t i o n s c a n b e d e f i n e d f o r t h e e n t i r e s e i s m o g r a m by v a r y i n g b o t h p a n d r. T h i s i s t h e essence o f t h e s l a n t s t a c k a n d i n i t s p r a c t i c a l f o r m i s g i v e n b y  S{T,P)  =  J2P{T  + X,X).  (3.3)  P  z  M o r e f o r m a l l y s t a t e d , t h e s l a n t s t a c k is g i v e n by  + oo  /  P{T + px,x)dx.  (3.4)  - oo  w h e r e S is t h e (p — r ) w a v e f i e l d . T h e s l a n t s t a c k is m o r e e a s i l y c a r r i e d o u t i n t h e t i m e d o m a i n w h e n t r a c e s p a c i n g is n o t c o n s t a n t  (McMechan and Ottolini,  1980).  A correction  for the frequency de-  p e n d e n c e o f t h e s t a c k m u s t b e m a d e i f w a v e f o r m s a r e t o b e p r e s e r v e d . P h i n n e y et al. (1981) s h o w e d h o w t h e f r e q u e n c y d e p e n d e n c e arises t h r o u g h t h e i r d e r i v a t i o n o f the i n verse slant stack. T h i s dependence c a n be i n t u i t i v e l y realized by c o n s i d e r i n g that t h e s u m m a t i o n o f a m p l i t u d e s along different p o r t i o n s of the wavelet, for m a n y  wavelets,  e f f e c t i v e l y s p r e a d s t h e t r a n s f o r m e d w a v e l e t . H o w e v e r , since t h e i n v e r s i o n s c h e m e u n d e r c o n s i d e r a t i o n is o n l y c o n c e r n e d w i t h t h e t a u c u r v e d e s c r i b e d b y t h e l o c u s i n t h e i — p d o m a i n , t h i s c o r r e c t i o n o f t h e f o r m H(t) t~ ' 1  2  ( P h i n n e y et al., 1981) n e e d n o t b e a p -  p l i e d . F u r t h e r m o r e , t h e n o n - i d e a l 2 - D case r e q u i r e s t h e s l a n t s t a c k a n d i n v e r s i o n t o use a l l v a l u e s o f p a l o n g a p a r t i c u l a r r a y p a t h ( C h a p m a n , 1 9 8 1 ) w h i c h is b e y o n d ' t h e s c o p e o f this study. T h e s l a n t s t a c k d e c o m p o s e s t h e o b s e r v e d s e i s m o g r a m i n t o i t s fixed p c o m p o n e n t s w h i c h m a y then be d o w n w a r d continued separately.  F i g u r e 3.1a s h o w s a t h e o r e t i c a l  s e i s m i c s e c t i o n g e n e r a t e d f r o m the a s y m p t o t i c s y n t h e t i c s e i s m o g r a m r o u t i n e o f S p e n c e et  37 al.  (1984) f o r a p l a n e l a y e r e d e a r t h m o d e l w i t h f o u r l a y e r s . T h e four a r r i v a l b r a n c h e s c a n  b e seen i n t h e t h e o r e t i c a l s e i s m o g r a m . O n l y t h e r e f r a c t e d a r r i v a l s w e r e c o n s i d e r e d ; n o p r e c r i t i c a l n o r w i d e a n g l e r e f l e c t i o n s w e r e i n c l u d e d . T a b l e 3.1 s h o w s t h e c h a r a c t e r i s t i c s of t h e 1-D m o d e l .  LAYER  DEPTH  (km)  I II  0.0 0.7  III IV  4-0  VELOCITY  2.0  T a b l e 3.1  (km/s)  GRADIENT  2.0 2.7 4.8 6.0  (km/s/km) O.S 0.5 0.5 O.S  1 - D m o d e l used f o r i n v e r s i o n t e s t i n g .  T h e a r r i v a l s f r o m e a c h l a y e r h a v e b e e n l a b e l l e d w i t h the c o r r e s p o n d i n g l a y e r n u m b e r i n F i g u r e 3.1a. T h e c o r r e s p o n d i n g s l a n t s t a c k e d a r r i v a l b r a n c h e s h a v e b e e n l a b e l l e d by l o w e r case r o m a n n u m e r a l s . If t h e a r r i v a l b r a n c h e s were r e p r e s e n t e d b y s t r a i g h t l i n e s , e a c h a r r i v a l b r a n c h w o u l d m a p t o a p o i n t c o r r e s p o n d i n g t o t h e i n t e r c e p t a n d slope o f the s t a c k i n g line.  H o w e v e r , e x t e n d e d l o c i o f e n e r g y arise w h e n e x t e n d e d w a v e l e t s a r e  u s e d a n d , as i n t h i s c a s e , w h e n t h e a r r i v a l b r a n c h e s possess c u r v a t u r e d u e t o v e l o c i t y gradients. T h e relationship between t h e branches and their transformed values can be e a s i l y seen.  T h e l a y e r I a r r i v a l b r a n c h t r a n s f o r m s t o t h e p — T l o c u s i w h i c h has i t s  m a x i m u m e n e r g y c o n c e n t r a t e d a t a p o i n t w h i c h r e p r e s e n t s t h e average v e l o c i t y i n t h e l a y e r . S i m i l a r r e l a t i o n s h i p s c a n be seen f o r b r a n c h e s II a n d III . T h e last a r r i v a l b r a n c h ( I V ) w a s i n c l u d e d t o d e m o n s t r a t e t h e effects o f s t a c k i n g l o w a m p l i t u d e s w i t h a p p a r e n t v e l o c i t i e s close t o a p r e c e d i n g b r a n c h .  T h e r a y s b o t t o m i n g i n t h i s l a y e r are f e w a n d  a c o r r e s p o n d i n g l y l o w e r a m p l i t u d e o f t h e i r a r r i v a l s is i n d i c a t e d . T h e l o w g r a d i e n t a n d  T R A V E L TIME C U R V E (True T i m e )  P - T A U CURVE  Distance (km) F i g u r e S . l Slant stacked wavefield (b) for theoretical seismograms (a) computed for l - D model in Table 3.1 . Lower case roman numerals are used to show the corresponding p - T mapping for the equivalent arrivals identified by uppercase arrivals in (a). The slope for slant stack lines was incremented by 0.004 s/km and the intercept times were incremented by 0.0083 s or the reciprocal of the sampling rate (120sps). Stacking velocities ranged between 1.8 km/s and 6.5 km/s.  39 t h e f a r offset also affect the a m p l i t u d e o f these a r r i v a l s .  W h e n these a m p l i t u d e s are  s u m m e d , b e c a u s e n o w i n d o w i n g w a s u s e d , t h e y a r e a b s o r b e d essentially by the effects f r o m b r a n c h III.  T h u s , t h e y are n o t c l e a r l y seen i n t h e t r a n s f o r m e d w a v e f i e l d .  Where  t h e y s h o u l d o c c u r h a s b e e n i n d i c a t e d by i v . T h e y a l s o h a v e the effect of s l i g h t l y l o w e r i n g t h e a m p l i t u d e s for t h e t r a n s f o r m a t i o n of b r a n c h III.  T h e a r t i f a c t s seen as d i a g o n a l , l o w  a m p l i t u d e e v e n t s r u n n i n g f r o m left t o r i g h t r e s u l t f r o m the finite a p e r t u r e u s e d , s p a t i a l a l i a s i n g a n d / o r t h e l a c k of w i n d o w i n g t h e s t a c k .  3.2.2  The downward continuation  procedure  T h e d e v e l o p m e n t f o l l o w s t h a t of C l a y t o n a n d M c M e c h a n (1981), e x c e p t t h e y i m p l e m e n t e d the d o w n w a r d continuation in the frequency d o m a i n , but following  their  r e c o m m e n d a t i o n , t h i s s t u d y i m p l e m e n t s it i n t h e t i m e d o m a i n . T h e d o w n w a r d c o n t i n u a t i o n is s i m i l i a r t o a d e p t h m i g r a t i o n b u t is a p p l i e d i n the offset d o m a i n as o p p o s e d to the c o m m o n m i d p o i n t d o m a i n . C l a e r b o u t (1976) a n d G a z d a g (1978) s h o w t h a t t h e d o w n w a r d c o n t i n u a t i o n of t h e w a v e f i e l d o b s e r v e d a t t h e s u r f a c e c a n be i m p l e m e n t e d by a phase r o t a t i o n i n the freq u e n c y d o m a i n w h e n t h e v e l o c i t y varies o n l y w i t h d e p t h . W r i t i n g the w a v e e q u a t i o n i n t h e f r e q u e n c y d o m a i n gives  d + 4 dh 2  2  2  ] p ( w , M ) = o.  T h e s o l u t i o n t o t h i s e q u a t i o n w a s g i v e n by C l a e r b o u t ( 1 9 7 6 ) a n d G a z d a g (1978)  P[u,,kk,z) = P(w,fcfc,0) e x p - i 2  z  2  v {z) 2  4  (3.5)  40 w h e r e u; is t h e t e m p o r a l f r e q u e n c y a n d kh is t h e h o r i z o n t a l w a v e n u m b e r . T h e first m i n u s s i g n i n e q u a t i o n 3.5 i n d i c a t e s t h a t u p c o m i n g w a v e s a r e b e i n g i m a g e d . T h e F o u r i e r C e n t r a l S l i c e t h e o r e m ( C l a y t o n a n d M c M e c h a n , 1981) is i n v o k e d t o r e c a s t e q u a t i o n 3.4 i n t h e f r e q u e n c y d o m a i n w h i c h d e m o n s t r a t e s , m o r e c l e a r l y , t h e r e l a t i o n s h i p b e t w e e n e q u a t i o n 3.5 a n d t h e s l a n t s t a c k . R e w r i t i n g e q u a t i o n 3.4  S ( w , p ) = P{w,-2wp).  (3.6)  E q u a t i o n 3.5 m a y t h e n be c o n v e r t e d t o i t s s l o w n e s s f o r m by s u b s t i t u t i n g — 2up  for k^.  R e w r i t i n g this equation yields  F(«,-2  W  p^) = P(  W  ,-2wp,0)e-  i u  *( ' p  z )  ,  (3.7)  where * (p, z) = 2 /  - p dz.  y/v- (z)  2  2  (3.8)  U s i n g e q u a t i o n 3.6, e q u a t i o n 3.7 m a y be w r i t t e n as  5(u;,p, ) = 5 ( a ; , p , 0 ) e - ^ * ^ ^ 2  (3.9)  Inverse t r a n s f o r m i n g this equation gives  S{r,p,z) = j  S{oj p,0)e- '^ ^'^dw. iu  1  {p  (3.10)  T h i s e q u a t i o n a l l o w s the s p e c i f i c a t i o n of the T — p w a v e f i e l d a t a n y d e p t h z. S ( a / , p , 0) is t h e s l a n t s t a c k e d w a v e f i e l d , o b t a i n e d i n s e c t i o n 3.2.1, w h i c h r e p r e s e n t s t h e d e l a y t i m e  41 f u n c t i o n for u p c o m i n g w a v e s at t h e s u r f a c e . T h e e x p o n e n t i a l f u n c t i o n is t h e d o w n w a r d c o n t i n u a t i o n o p e r a t o r w h i c h is c o m m o n i n p o t e n t i a l field a n a l y s e s . F o r t h e p r o b l e m d e s c r i b e d , it is not necessary t o h a v e t h e w a v e f i e l d f o r e v e r y d e p t h z, o n l y t h o s e d e p t h s s a t i s f y i n g t h e i m a g i n g c o n d i t i o n for t h i s p r o b l e m m u s t be m e t ; the downward continuation  process must stop w h e n all plane wave c o m p o n e n t s have  r e a c h e d t h e i r m a x i m u m d e p t h o f p e n e t r a t i o n or b o t t o m i n g p o i n t . M a t h e m a t i c a l l y , t h i s occurs when r =  0 ( t h e p of t h e r a y e q u a l s t h e t r u e slowness of t h e  medium  S e t t i n g T = 0 i n e q u a t i o n 3.10 y i e l d s  (3.11)  w h e r e s ( p , z) is d e f i n e d as t h e slowness p l a n e . E q u a t i o n 3.11 is i n t h e f o r m u t i l i z e d by C l a y t o n a n d M c M e c h a n (1981) i n a l g o r i t h m for i m p l e m e n t a t i o n .  their  T h e i r p r o g r a m was run on an array processor but they  suggest r e c a s t i n g e q u a t i o n 3.11 i n t h e t i m e d o m a i n for g e n e r a l p u r p o s e m a c h i n e s . T h e p r o g r a m w r i t t e n for t h i s s t u d y uses t h e t i m e d o m a i n r e p r e s e n t a t i o n of e q u a t i o n  3.11  w h i c h is g i v e n by  s{p,z)  = S[T -  *(p,z),p,0].  (3.12)  E q u a t i o n 3.12 a r i s e s by t a k i n g t h e first m i n u s sign i n e q u a t i o n 3.10 i n s i d e the b r a c k e t s , u s i n g t h e s h i f t r u l e ( C h a p m a n , 1978) a n d w r i t i n g e q u a t i o n 3.12 d i r e c t l y . T h e d o m a i n i n w h i c h e q u a t i o n 3.11 or 3.12 is a p p l i c a b l e has a b r a n c h c u t w h i c h is r e m e d i e d by a l t e r i n g t h e d e f i n i t i o n of  to (3.13)  o  42 which prevents attenuation of the wavefield below depths where p is greater than As recommended by Clayton and McMechan (1981), a phase shift of 57r/4 was then applied to the downward continued wavefield. This phase shift embodies three correction factors to compensate for (l) the far-field radiation condition (Aki and Richards, 1980, p 417); (2) the 2-D representation of wave propagation in three dimensions (Chapman, 1978); and (3) an average factor of 7r/2 to compensate for the range of reflection coefficients expected for reflections and refractions (Clayton and McMechan, 1981). This last point may cause confusion as reflection coefficients have been implied for refraction. The image formed by applying equation 3.11 is composed of wavelets whose shapes are defined by the phase shift associated with the reflection coefficient at each z (Clayton and McMechan, 1981). McMechan and Ottolini (1980) show that the p— r curve for a refraction branch is formed by the envelope of p— T curves for the reflections. Chapman (1978) also states that a refracted ray can be treated as having a reflection coefficient of —isgn{u>)>.  The downward continuation procedure represented by equations 3.11 and 3.12, with the phase shift applied, was implemented on the Amdahl 5850 computer at the UBC Computing Center. Since the downward continuation equation requires a velocity versus depth curve as an input, it is necessarily an iterative procedure. The program has been designed to automatically input an initial velocity versus depth curve for the downward continuation of the p — T wavefield. The velocity versus depth curve used was 1.8 km/s for all depths. The downward continued wavefield is then displayed on the terminal. To operate the procedure in an interactive mode, a means of inputting the subsequent user picked velocity depth curves was required. The user obtained these new curves by picking points along the locus of maximum amplitude, now at minimum depth (z) in the downward continued wavefield or p — z wavefield. This was implemented using the cross  43 hairs on the graphics terminal.  T h e p r o g r a m instructs the computer to accept graphic  i n p u t a n d t h e c o m p u t e r r e s p o n d s by d i s p l a y i n g cross h a i r s . T h e user t h e n p o s i t i o n s t h e c r o s s h a i r s o v e r the d e s i r e d a m p l i t u d e s a n d h i t s a n y key t o i n p u t the values. T h e p i c k i n g session e n d s w h e n t h e user selects a p v a l u e less t h a n 0.55 s / k m (this c a n be a l t e r e d if r e q u i r e d ) . T h e c o n t r o l t h e n r e t u r n s t o t h e p r o g r a m w h i c h c o n v e r t s the p i c k e d p v a l u e s to the velocities and resamples t h e m at the desired s a m p l i n g interval through a simple linear interpolation.  T h i s new c u r v e is used to d o w n w a r d c o n t i n u e the o r i g i n a l p — T  w a v e f i e l d a g a i n . T h u s , a n y new c u r v e d o e s n o t d e p e n d o n t h e r e s u l t s of t h e p r e v i o u s c u r v e s i n c e t h e p r o g r a m a l w a y s r e t u r n s t o the o r i g i n a l p — r w a v e f i e l d . T h e new p - z w a v e f i e l d is t h e n d i s p l a y e d a n d t h e p r o c e s s c a n be c o n t i n u e d . U s u a l l y 4 t o 5 i t e r a t i o n s are n e c e s s a r y t o o b t a i n a, p — z w a v e f i e l d t h a t c o n v e r g e s t o a single s o l u t i o n . T h e a d v a n t a g e o f i m p l e m e n t i n g t h e i n v e r s i o n i n a n i n t e r a c t i v e m a n n e r is t h a t t h e r e s u l t a n t p — z c u r v e , f o r e a c h d o w n w a r d c o n t i n u a t i o n i t e r a t i o n , is i m m e d i a t e l y d i s p l a y e d . H e n c e , the user c a n q u i c k l y g a i n e x p e r i e n c e i n i n t e r p r e t i n g the p — z w a v e f i e l d s .  C h a n g e s o b s e r v e d by t h e  user i n a s h o r t p e r i o d c a n be b e t t e r a s s i m i l a t e d a n d a n a c c e p t a b l e s o l u t i o n is r e a c h e d s o o n e r . E a c h p - d e p t h c u r v e is o u t p u t t o a file for later r e v i e w .  3.3 Examples  M c M e c h a n and Ottolini  (1980) a n d P h i n n e y et al. (1981) p r o v i d e a n e x c e l l e n t  d i s c u s s i o n o n t h e a n a l y s i s of t r a n s f o r m e d r e c o r d s e c t i o n s a n d the reader is r e f e r r e d t o these p a p e r s . H o w e v e r , t w o s p e c i f i c p o i n t s i n the a p p l i c a t i o n of the i n v e r s i o n p r o c e d u r e s h o u l d be n o t e d . T h e r e s u l t s o f t h e i n v e r s i o n d e t e r i o r a t e for cases in w h i c h (i) the d a t a a r e s p a t i a l l y a l i a s e d a n d (ii) w h e r e t h e w a v e f o r m s are not phase c o r r e l a t e d or c o h e r e n t  44 along a slant stack. S p a t i a l aliasing of the d a t a c a n be avoided if the source/receiver s e p a r a t i o n is less t h a n one h a l f of t h e w a v e l e n g t h of t h e h i g h e s t f r e q u e n c y one w i s h e s to resolve.  3.3.1 The plane-layered synthetic example T h e w a v e f i e l d i n F i g u r e 3.2 w a s o b t a i n e d b y d o w n w a r d c o n t i n u i n g t h e p— T w a v e f i e l d of F i g u r e 3.1b. S u p e r i m p o s e d o n t h e figure is t h e p— z f u n c t i o n used f o r t h e d o w n w a r d c o n t i n u a t i o n . T h i s i n v e r s e v e l o c i t y d e p t h f u n c t i o n is also t h e o n e used t o c o n s t r u c t t h e p l a n e - l a y e r e d m o d e l f r o m w h i c h t h e t h e o r e t i c a l s e i s m o g r a m s w e r e d e r i v e d ( F i g u r e 3.1a). T h i s example illustrates the relationship between the d o w n w a r d continued wavefield a n d the true velocity depth function.  In this e x a m p l e the p — z wavefield images the true  p - d e p t h c u r v e b u t t h e r e are s o m e m i n o r d i f f e r e n c e s . T h e p — z c u r v e (solid l i n e ) b e t w e e n 0.5 a n d 0.455 s / k m d o e s n o t c o i n c i d e w i t h t h e large a m p l i t u d e e v e n t . T h i s c o u l d be d u e 1  t o s p a t i a l a l i a s i n g a n d t h e effects of u s i n g a n average p h a s e s h i f t o f 7 r / 2 f o r t h e r e f l e c t i o n c o e f f i c i e n t s (see S e c t i o n 3.2.2). T h e a g r e e m e n t i m p r o v e s for t h e d e e p e r v a l u e s . T h e s m a l l v e l o c i t y j u m p s a t 4.0 k m d e p t h a r e d i f f i c u l t t o see b u t m a y be i n d i c a t e d by t h e s m a l l d o w n w a r d s h i f t i n t h e first a r r i v a l s f o r t h e l a s t t w o t r a c e s . In a n y e v e n t , i t w o u l d b e m i s s e d i n a r e a l d a t a case. T o i l l u s t r a t e t h e i t e r a t i v e n a t u r e of t h e p r o c e d u r e a f u l l r u n u s i n g t h e p l a n e - l a y e r e d s y n t h e t i c s e i s m o g r a m w a s p e r f o r m e d a n d t h e r e s u l t s a r e d e p i c t e d i n F i g u r e 3.3 a t o e. T o o b t a i n these r e s u l t s t h e t h e o r e t i c a l s e i s m i c s e c t i o n w a s r e a d i n t o t h e p r o g r a m , s l a n t s t a c k e d ( d e c o m p o s e d i n t o i t s fixed p c o m p o n e n t s ) a n d t h e s l a n t s t a c k e d w a v e f i e l d w a s t h e n d o w n w a r d c o n t i n u e d (each p s e p a r a t e l y ) u s i n g a c o n s t a n t v e l o c i t y of 1.8 k m / s for the zero-order iteration for all depths.  O n c e the d o w n w a r d continued p — z wavefield  45  P - D E P T H CURVE P (s/km) 0.60 0-0 I  0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 llrlllNlllliytu^AJIlj'llllllllllltlMllllllllll'lllllllllllllllliiillllii'iiiiiiiiiiiiliiii  0.15 i  1.0  2.0  3.0  4.0  5.0  6.0 J  Figure 3.2  Downward continuation of the p— r wavefield infigure3.1b computed for the exact p-depth  function (thick solid line). The relationship between wavefield and input p-depth function is illustrated for this best case in which the true p-depth function is known.  Figure 3.3  The figure panels (a) to (e) on this and the following two pages depict the iterative series for the  downward continuation of the p — T wavefield in figure 3.1b. Each panel shows the p — z wavefield computed from the p — r wavefield using the p — z function (thick solid line) selected from the previous downward continued (p — z) wavefield. An initial p — z wavefield was computed using a constant p or inverse velocity for all depths. This wavefield was then picked and the resulting p — z function (thick solid line in (a)) was used to compute the wavefield in (a). This wavefield was then picked (the dashed line in (a)) for the next iteration (b) where it is now plotted as a thick line. This process continues until convergence is obtained (see text for detailled explanation).  Figure 3.3 c & d Iteration 3 (d).  Downward continued p - r wavefield computed for p - z function from Iteration 2 (c) and  P - D E P T H CURVE  oo  Figure S.S e  Downward continued p - r wavefield computed for p — z function from Iteration 4.  49 w a s d i s p l a y e d o n t h e t e r m i n a l , c o n t r o l r e t u r n e d t o t h e u s e r . T h e user t h e n p r o c e e d e d t o i n p u t n e w v e l o c i t y v e r s u s d e p t h c u r v e s by p i c k i n g v a l u e s as p e r s e c t i o n 3.2.2, u n t i l c o n v e r g e n c e w a s o b t a i n e d . A s e a c h new i t e r a t i o n is c a r r i e d o u t , t h e s o l u t i o n oscillates b e t w e e n h i g h p a t l o w z v a l u e s a n d l o w p at h i g h z v a l u e s ( F i g u r e 3.3a-e). C o n v e r g e n c e c a n be r e c o g n i z e d by t w o m e a n s . T h e first o c c u r r s w h e n a single p — z c u r v e is o b t a i n e d for t w o o r m o r e i t e r a t i o n s .  T h e s e c o n d o c c u r s w h e n t w o c u r v e s or  s u c c e s s i v e i t e r a t i o n s h a v e t h e p r o p e r t y s u c h t h a t i n p u t t i n g t h e h i g h p, l o w z c u r v e w i l l y i e l d t h e p r e v i o u s l o w p, h i g h z c u r v e a n d vice versa. use t h i s p r o p e r t y  C l a y t o n a n d M c M e c h a n (1981)  t o select a s o l u t i o n w h i c h is t h e average of these t w o c u r v e s .  The  t w o c u r v e s f o r m a n e n v e l o p e w h i c h c o n t a i n s a n o p t i m a l s o l u t i o n , b u t t h i s is not to be c o n f u s e d w i t h u n c e r t a i n t y i n t h e r e s u l t w h i c h is b a s e d o n t h e w i d t h of t h e p — z i m a g e at c o n v e r g e n c e ( C l a y t o n a n d M c M e c h a n , 1981). T h i s is s i m i l a r t o o t h e r i n v e r s i o n schemes w h i c h a t t e m p t to define the envelope of all possible solutions ( M c M e c h a n a n d W i g g i n s , 1972; B e s s o n o v a et al., 1974). A f u r t h e r i n d i c a t i o n o f c o n v e r g e n c e is the f o c u s s i n g o f t h e a m p l i t u d e s a l o n g t h e p—z  i m a g e . A s c o n v e r g e n c e is a p p r o a c h e d , t h i s i m a g e ' b r i g h t e n s ' .  F i g u r e 3.3a h a s b e e n l a b e l l e d i t e r a t i o n 1 a n d is t h e r e s u l t o b t a i n e d b y p i c k i n g t h e i m a g e i n t h e i n i t i a l p — z w a v e f i e l d for t h e c o n s t a n t v e l o c i t y d e p t h c u r v e . iteration  Therefore,  1 refers t o t h a t i t e r a t i o n w h i c h f o l l o w s f r o m t h e first user p i c k e d p—z  curve,  i n d i c a t e d by t h e s o l i d l i n e . T h e d a s h e d line i n F i g u r e 3.3a is t h e c u r v e p i c k e d for t h i s d i s p l a y of t h e w a v e f i e l d .  T h i s c o n v e n t i o n is f o l l o w e d for a l l t h e e x a m p l e s g i v e n ; t h e  d a s h e d l i n e r e p r e s e n t s t h e p r e s e n t p i c k w h i l e t h e s o l i d line r e p r e s e n t s the p r e v i o u s p i c k . In t h e p i c k i n g o f t h e p—z i m a g e s for t h i s a n d t h e o t h e r e x a m p l e s , t h e p o i n t c o r r e s p o n d i n g t o t h e m a x i m u m a m p l i t u d e a t m i n i m u m z w a s s e l e c t e d . A l s o , a s m a l l n u m b e r of p o i n t s w e r e p i c k e d a n d t h e p r o g r a m l i n e a r l y i n t e r p o l a t e d b e t w e e n t h e m t o give a r e s u l t s i m i l a r t o t h a t used t o define t h e m o d e l (layers w i t h l i n e a r v e l o c i t y g r a d i e n t s ) .  50 In Figure 3.3a the basis for picking the p - z curve for thefirst0.7 km in depth is clear. At this depth the amplitudes of this first locus continue but it must be remembered that we are looking for the 1-D v — z curve. Thus, the point chosen was the maximum peak at the next monotonically lower z value. This is at a value of 0.370 s/km. The next point chosen was at the lower end of this second major locus, but such that it was not lower than the amplitude for the wavelets in the third major locus. This is the general procedure used for the entire picking process for all the examples given in this chapter. The results of picking the dashed curve in Figure 3.3a are shown in Figure 3.3b. The p — z image now mostly appears above that of the previous pick (the solid line, Figure 3.3b) for depths below 1 km. This process continued for Figure 3.3 b to e. The final panel (Figure 3.3e) was not picked as it represents the final iteration based upon the previous pick. This is how the interactive session would normally terminate since the result of any pick is applied before control returns to the user. During the picking process, a possible guide to jumping the low amplitude regions from one major locus to the next are the "knees" which form at the base of each linear gradient (see Figure 3.3c at z = 0.6 and p — 0.45). The p — z curves from the last two iterations (the solid lines in Figure 3.3d and e) are shown in Figure 3.4 along with the correct velocity depth function. The lack of agreement in the upper 1.0 km of depth is probably due to spatial aliasing of the data. For these depths and velocities, a trace spacing of 110 meters would be required, but for this example a trace spacing of 200 m was used. Between a depth of 1 and 2 km, agreement between the correct curve and the downward continued wavefield is very good. Below this depth the last two iterations form bounds on the correct solution. The resolution of the graphics video display device used in the interactive session also contributed to the poorer results for the velocity  51  P-DEPTH CURVE  Figure 3.4  Comparison of p — z functions from figure 3.3d (line 2) and e (line 3) with exact  function (line 1) from 1-D model.  52 v a l u e s p i c k e d a t d e p t h s g r e a t e r t h a n 2.0 k m .  F u r t h e r m o r e , as n o t e d i n s e c t i o n 3.2.1,  t h e r a y s d i d n o t d e n s e l y s a m p l e t h e l a y e r a n d t h i s c o n t r i b u t e s t o the p o o r l y d e t e r m i n e d v a l u e s at d e e p z.  3.3.2 T h e 2 - D s y n t h e t i c  example  T h e 2 - D s y n t h e t i c e x a m p l e is f r o m t h e m o d e l l i n g r e s u l t s for the O B S 3 t o O B S 2 r e c o r d s e c t i o n i n c h a p t e r I V . I n v e r t i n g t h e 2 - D s y n t h e t i c e x a m p l e s e r v e d as a n i n s t r u c t i v e g u i d e p r i o r t o i n v e r t i n g the r e a l d a t a e x a m p l e . K n o w i n g t h e a c t u a l s t r u c t u r e of t h e m o d e l f r o m w h i c h t h e s e i s m o g r a m s w e r e c a l c u l a t e d a l l o w e d for c o m p a r i s o n s b e t w e e n t h e t w o m e t h o d s , p a r t i c u l a r l y w i t h r e s p e c t t o t h e effect of a p p l y i n g t h i s i n v e r s i o n t o a 2 - D case. 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 , g e n e r a t e d by the a s y m p t o t i c r a y t h e o r y as d e s c r i b e d by S p e n c e et al.  (1984), is s h o w n i n F i g u r e 3.5a.  method  T h e slant stacked  w a v e f i e l d is i l l u s t r a t e d i n F i g u r e 3.5b. T h e t r a n s f o r m e d b r a n c h e s c o r r e s p o n d i n g t o t h e t r a v e l t i m e c u r v e s h a v e b e e n l a b e l l e d as in S e c t i o n 3.3.1. T h e e x t e n d e d n a t u r e of t h e t r a n s f o r m e d b r a n c h e s is d u e t o t h e c u r v a t u r e of t h e t r a v e l t i m e b r a n c h w h i c h r e s u l t s f r o m the velocity gradients in the m o d e l . T h e travel-time branches II and III  appear  t o be a s i n g l e b r a n c h b u t a s l i g h t c h a n g e c a n be o b s e r v e d at a m o d e l d i s t a n c e of 6 . 0 k m . S i m i l i a r l y for t h e t r a n s f o r m e d b r a n c h e s , a s l i g h t c h a n g e c a n be seen b e t w e e n i i a n d i i i . Arrival branch I V m u l t i p l e event V  t r a n s f o r m s t o t h e c l e a r , b u t low a m p l i t u d e , p — r b r a n c h i v .  The  t r a n s f o r m s to the p — T event v a p p e a r i n g lower in the transformed  w a v e f i e l d a n d t h e r e f o r e d o e s not i n t e r f e r e w i t h t h e p r i m a r y p-r  event. B y slant stacking  t h e a m p l i t u d e s w i t h i n a s p e c i f i e d w i n d o w t h e d e s t r u c t i v e i n t e r f e r e n c e effects f r o m t h e e x t e n d e d w a v e l e t s of e v e n t s o u t s i d e t h i s w i n d o w c o u l d be d i m i n i s h e d . T h i s w o u l d h a v e  T R A V E L TIME C U R V E (True T i m e )  P - T A U CURVE  Distance (km)  Figure 3.5 Slant stacked wavefield (b) for theoretical seismograms (a) computed for 2-D velocity sub-model of Chapter IV, section 4.3.2 (see figure 3.2 for explanation of symbols).  54 t h e d e s i r a b l e effect of i n c r e a s i n g t h e range of l a r g e r a m p l i t u d e s for e a c h t r a n s f o r m e d b r a n c h . A n e x a m p l e w o u l d be t h e p — r b r a n c h i v .  T h e a m p l i t u d e s at t h e u p p e r e n d  of t h e b r a n c h c o u l d have b e e n d e p r e s s e d by the i n t e r f e r e n c e f r o m the a m p l i t u d e i n the w a v e l e t for t r a v e l - t i m e b r a n c h III. F o r t h e d o w n w a r d c o n t i n u a t i o n of t h e p — r w a v e f i e l d s h o w n i n F i g u r e 3.5b, t h e s a m e p r o c e d u r e as o u t l i n e d i n s e c t i o n 3.3.2 w a s f o l l o w e d . a m p l i t u d e at m i n i m u m iteration.  The  final  F o r e a c h i t e r a t i o n , the  maximum  z w a s p i c k e d f r o m t h e p — z w a v e f i e l d a n d used for the  three iterations  are d i s p l a y e d i n F i g u r e s 3.6 a to c.  next  T h e solid  l i n e i n F i g u r e 3.6a w a s t h e i n p u t p — z p i c k used t o d o w n w a r d c o n t i n u e t h e w a v e f i e l d presented in this  figure.  T h e dashed line represents the pick made f r o m this wavefield  i n the i n t e r a c t i v e session for t h e n e x t i t e r a t i o n s h o w n i n F i g u r e 3.6b (it n o w a p p e a r s i n this  figure  as t h e solid l i n e ) .  A s i m i l a r d e s c r i p t i o n a p p l i e s t o F i g u r e 3.6e.  w a s t h e n m a d e t o c h o o s e w h i c h of these results r e p r e s e n t e d c o n v e r g e n c e . l i n e i n F i g u r e 3.6b w a s chosen as t h e  first  indicator.  A decision The dashed  T h i s choice was made because  t h e a m p l i t u d e s i n c r e a s e d s i g n i f i c a n t l y b e t w e e n t h e w a v e f i e l d s s h o w n in F i g u r e 3.6a a n d b.  T h e s m a l l v e l o c i t y j u m p , w h i c h r e c u r s a t a d e p t h of 3.8 k m w a s b a s e d u p o n  the  w e a k a m p l i t u d e s e x t e n d i n g h o r i z o n t a l l y , c o m b i n e d w i t h the severe decrease i n a m p l i t u d e observed after the trace i n d i c a t e d by the arrow. m a d e p o s s i b l e by t h e noise-free d a t a .  A n a l y s i s of these a m p l i t u d e s is o n l y  T h e s e c o n d c u r v e p i c k e d (dashed l i n e , F i g u r e  3.6b) w a s c h o s e n as the s e c o n d a n d final i n d i c a t o r f o r c o n v e r g e n c e . It is s h o w n as t h e s o l i d l i n e i n F i g u r e 3.6c. In g e n e r a l , the a m p l i t u d e s are w e a k e r t h a n those seen i n the w a v e f i e l d s for F i g u r e 3.6a a n d b. B a s e d u p o n these t w o o b s e r v a t i o n s , the solid lines of F i g u r e 3.6 b a n d c (or c o n v e r s e l y t h e d a s h e d lines of F i g u r e 3.6 a a n d b) were c h o s e n as t h e t w o c u r v e s d e f i n i n g c o n v e r g e n c e .  P-DEPTH CURVE  P-DEPTH CURVE  Figure 3.6  The figure panels (a) to (c) depict the final three iterations for the downward continuation of the p - r wavefield in figure 3.5b . (see figure 3.3 and text for explanation) /  P-DEPTH CURVE  57 Five p-depth functions were selected from the 2-D model, where it was sampled by the rays, and averaged. The average p-depth function and those for the last two iterations are shown in Figure 3.7. Comparing the three curves in this figure shows how the downward continued wavefield solutions (curves 2 and 3) appear to have averaged the velocities across the model. This result can be described as a l-D kinematic equivalent for the actual 2-D velocity structure. The minor discrepancies observed can be attributed to the effects of not windowing the stack, spatial aliasing, the resolution of the graphics terminal at high p, low z values, and the 2-D nature of the model itself.  A plane-layered model constructed  from an average of these curves could easily be perturbed to fit the 2-D model. We might then expect that for the real data example the imaged result will represent a l-D kinematic equivalent for the 2-D earth model. As the subsurface structure more closely approaches a laterally homogenous model, the result will begin to reflect a more accurate representation of the subsurface velocity distribution.  3.3.3 A real data example In this last example, observed data, the interpretation of which led to the theoretical data used in section 3.3.2, were used in the wavefield transformation procedure. The record section and the slant stacked record section are shown in Figure 3.8a and 3.8b. The primary arrivals transform to the maximum amplitude locus at minimum T. The large amplitude event, lower amplitude event and the multiple event are labelled as in previous diagrams.  The larger amplitude artifacts appearing above the primary  p — T event in Figure 3.8b may in part be due to transformed precritical reflections not clearly evident in the seismogram. Clayton and McMechan (1981) observed similiar  58  P-DEPTH CURVE P (s/km) 0.60  0.55  0.50  0.45  0.40  0.35  0.30  0.25  0.20  0.15  6.0 J  Figure S.7  Comparison of p - z functions from figure 3.3b (line 2) and (c) (line 3) with average p — z  function for 2-D model (line 1). The average p-z  function was calculated for five p-z  curves selected  along the 2-D sub-model in figure 4.11b where it is sampled by simple refracted ray paths.  T R A V E L TIME C U R V E (True T i m e )  P - T A U CURVE P (s/km)  Distance (km)  Figure 3.8 Slant stacked wavefield (b) for OBS 3 reverse profile (a) (see section 4.3.2) plotted from east to west, (see figure 3.1 and text for explanation of figure)  60 f e a t u r e s for t h e i r e x a m p l e s . T h e r e v e r b e r a t o r y n a t u r e of t h e s e i s m i c s i g n a l c o n t r i b u t e s to r e v e r b a t i o n s in the p — T section, but below the t r a n s f o r m e d p r i m a r y arrivals.  The  d a t a g a p s d o n o t a p p e a r t o h a v e h a d a n a d v e r s e effect o n the s l a n t s t a c k . T h e i r m a i n effect w o u l d p r o b a b l y be r e d u c i n g t h e p o t e n t i a l a m p l i t u d e of a r r i v a l s w h i c h fell a l o n g the slant stack line. T h e d o w n w a r d c o n t i n u a t i o n steps are d e p i c t e d i n F i g u r e 3 . 9 a - e . T h e i n t e r p r e t a t i o n of t h e r e a l d a t a set p r e s e n t s s o m e n e w p r o b l e m s . T h e r e c o g n i t i o n of t h e t r u e t r a n s f o r m e d p r i m a r y is s o m e t i m e s c o n f u s e d b y t h e noise i n t h e d a t a . H o w e v e r , one c l e a r gain in t h e r e a l d a t a case is i n t e r m s of t h e f o c u s i n g effect on t h e d o w n w a r d c o n t i n u e d w a v e f i e l d . It is m u c h m o r e p r o n o u n c e d t h a n i n t h e s y n t h e t i c e x a m p l e a n d t h i s helps t o c o u n t e r a c t t h e a d v e r s e n o i s e effect.  In F i g u r e 3.9a, t h e solid line s h o w s t h e p e a k f r o m the z e r o -  o r d e r ( c o n s t a n t v e l o c i t y ) d o w n w a r d c o n t i n u e d w a v e f i e l d . A t a d e p t h of 2.0 k m t h e p d e p t h c u r v e w a s e x t e n d e d t o fill t h e d a t a s p a c e .  T h i s is s i m p l y a r e q u i r e m e n t of t h e  r e s a m p l i n g r o u t i n e i n the p r o g r a m . T h e d a s h e d line s h o w s t h e p i c k for t h e use i n t h e n e x t i t e r a t i o n . T h e p i c k s are based u p o n t h e m a x i m u m a m p l i t u d e at m i n i m u m z. W h e r e t h e a m p l i t u d e s h i f t e d d o w n w a r d i n z for a n a d j a c e n t t r a c e , a p o i n t w a s s e l e c t e d at t h e t o p a n d t h e b o t t o m of t h i s j u m p (see t h e a r r o w i n F i g u r e 3.9a). the previous section was followed.  O t h e r w i s e , t h e s a m e p r o c e d u r e as d e s c r i b e d i n  T h e b l o c k y s t r a i g h t line a p p r o a c h w a s used w h e r e  p o s s i b l e . T h a t is, not e v e r y p e a k w a s p i c k e d , n o r need b e , b u t key p o i n t s i n the c u r v e w e r e p i c k e d a n d t h e p r o g r a m i n t e r p o l a t e d b e t w e e n these p o i n t s . T h e p r o c e s s c o n t i n u e d u n t i l t h e p—z  w a v e f i e l d w a s d e e m e d t o h a v e c o n v e r g e d . I t e r a t i o n s 4 a n d 5 ( F i g u r e 3.9  d a n d e) w e r e i d e n t i f i e d as t h e t w o c o n v e r g e n t i n d i c a t o r s .  In t h i s e x a m p l e t h e c u r v e ,  p i c k e d f r o m t h e w a v e f i e l d i n F i g u r e 3 . 9 d , a c t u a l l y fits t h e d o w n w a r d c o n t i n u e d w a v e f i e l d o f F i g u r e 3.9e.  T h i s result  figured  strongly  i n t h e d e c i s i o n t o t e r m i n a t e the  iterative  Figure 3.9 Figure panels (a) to (e) depict the iterative series for the downward continuation of the p - r wavefield infigure3.8b.(seefigure3.3 and text for explanation) i  P-DEPTH CURVE  P-DEPTH CURVE  d  c 0.80  0.55  0.50  0.45  P (s/km) 0.40  0.35  Figure 3 . 9 c ic d  P (s/km) 0.30  0.25  0.20  0.15  0.60  0.55  0.50  0.45  0.40  0.35  downward continued p — r wavefield for Iteration 3 (c) and Iteration 4 (d).  /  0.30  0.25  0.20  0.15  P-DEPTH CURVE  64 process.  T h e f i n a l t w o r e s u l t s (the s o l i d lines f r o m F i g u r e s 3.9 d a n d e) are s h o w n i n  F i g u r e 3.10 a l o n g w i t h t h e s a m e p - d e p t h c u r v e f r o m t h e p r e v i o u s s e c t i o n (see F i g u r e 3 . 7 ) . T h e v e l o c i t y g r a d i e n t s a n d v e l o c i t y i n c r e a s e s for t h e i n v e r s i o n c u r v e s m a t c h those for t h e a v e r a g e d v e l o c i t y - d e p t h c u r v e f r o m t h e 2 - D m o d e l q u i t e w e l l . i n v e r s i o n g a v e p—z  In general t h e  v a l u e s t h a t w e r e l o w e r t h a n t h e p — z c u r v e for t h e 2 - D m o d e l l i n g .  T h i s c o u l d b e a t t r i b u t e d t o t h e p r e v i o u s l y o u t l i n e d p r o b l e m s a n d for t h i s ease, p r o b l e m s a s s o c i a t e d w i t h t h e a c c u r a t e i d e n t i f i c a t i o n of t h e m a x i m u m e v e n t a n d m i n i m u m d e p t h for t h e t r a n s f o r m e d p r i m a r y a r r i v a l e v e n t . identifying the lower a m p l i t u d e p - z  N o i s e m a y have c o n t r i b u t e d t o i n c o r r e c t l y  e v e n t s d u r i n g the p i c k i n g p r o c e s s . F o r d e p t h s  b e l o w 2.0 k m , t h e s t r o n g l a t e r a l h e t e r o g e n e i t y of t h e 2 - D m o d e l w o u l d i n d i c a t e t h a t t h i s f a c t o r h a s t h e largest effect on the f i n a l r e s u l t (see F i g u r e 4 . 1 1 b ) .  3.3.4 Summary T h e i n v e r s i o n p r o c e d u r e y i e l d e d t h e c o r r e c t v e l o c i t y d e p t h c u r v e for t h e p l a n e l a y e r e d ease.  T h e result was o b t a i n e d q u i c k l y a n d in an unbiased fashion.  When  the  s u b s u r f a c e e a r t h m o d e l d i v e r g e s f r o m a p l a n e l a y e r e d case t h e s o l u t i o n o b t a i n e d f r o m t h e w a v e f i e l d c o n t i n u a t i o n a p p e a r s t o r e p r e s e n t a l - D k i n e m a t i c e q u i v a l e n t for t h e 2 - D v e l o c i t y m o d e l . E f f e c t s of n o i s e , a t least at l o w l e v e l s , a n d r e v e r b e r a t i o n s i n t h e d a t a d o n o t a p p e a r t o d e g r a d e the r e s u l t s for t h e e x a m p l e c o n s i d e r e d . T h i s is p r i n c i p a l l y b e c a u s e t h e first a r r i v a l a m p l i t u d e s i n t h e t— x d o m a i n t r a n s f o r m i n t o a m p l i t u d e s w h i c h h a v e m i n i m u m p a n d z values. W h a t a r e t h e i m p l i c a t i o n s of these  findings?  T h e r e a p p e a r t o be s e v e r a l . A s s h o w n  by C l a y t o n a n d M c M e c h a n (1981), t h e i n v e r s i o n of s e i s m o g r a m s r e c o r d e d over a r e a s k n o w n to have a nearly plane-layered v e l o c i t y structure yields valid results. A second  65  P-DEPTH  CURVE  6.0-1  Figure 3.10  Compasrison of p — z functions from figure 3.9d (line 2) and (e) (line 3) with average p— z  function from 2-D model as per figure 3.7 .  66 r e s u l t f r o m t h i s s t u d y is t h a t t h e m e t h o d c a n be a p p l i e d t o v e l o c i t y s t r u c t u r e s  with  a n e v e n h i g h e r degree of l a t e r a l v a r i a t i o n if a n a p p r o x i m a t e f u n c t i o n is t h e o b j e c t i v e . In o t h e r w o r d s , if o n l y a n i n i t i a l m o d e l is d e s i r e d for f u r t h e r p e r t u r b a t i o n  by a 2 - D  m o d e l l i n g s c h e m e , t h i s m e t h o d c a n p r o v i d e a q u i c k a n d efficient m e a n s of o b t a i n i n g the starting m o d e l .  A d i p p i n g l a y e r e d m o d e l m a y be o b t a i n e d by i n v e r t i n g b o t h t h e  f o r w a r d a n d reverse profiles. T h e d e n s i t y of r a y s s a m p l i n g t h e s u b s u r f a c e a t e i t h e r end o f t h e p r o f i l e w i l l be w e i g h t e d f o r t h a t e n d . If t h e r e is s t r u c t u r e , t h e n t h e r e s u l t s f r o m a f o r w a r d a n d r e v e r s e p r o f i l e i n v e r s i o n w i l l be s l i g h t l y different a n d c o u l d be used as t h e basis for a n i n i t i a l 2 - D m o d e l An  important  o b s e r v a t i o n is t h a t e x t r e m e c a u t i o n is a d v i s e d in t h e  application  o f t h i s i n v e r s i o n m e t h o d t o a d a t a set r e c o r d e d over a n a r e a w h e r e l i t t l e o r is k n o w n a b o u t the s u b s u r f a c e . e x a m p l e as a d e m o n s t r a t i o n  C l a y t o n a n d M c M e c h a n (1981) i n v e r t e d a r e a l d a t a  of t h e r o b u s t n e s s of t h e m e t h o d for cases w h e r e  lateral velocity variations exist. a n d an interpretation  nothing  T h e d a t a i n d i c a t e d only weak local velocity  weak effects  f r o m a n e a r b y profile s u p p o r t e d t h e i r r e s u l t s . If t h e t h e o r e t i c a l  s e i s m o g r a m s of F i g u r e 3.5a are c o n s i d e r e d , t h e p r i m a r y a r r i v a l s are o b s e r v e d t o increase s m o o t h l y along the trajectory  d e s c r i b e d by these a r r i v a l s .  Y e t t h e m o d e l (see F i g u r e  4 . 1 1 b ) f r o m w h i c h these s e i s m o g r a m s w e r e g e n e r a t e d is 2 - D . T h e s e o b s e r v a t i o n s suggest t h a t t h e i n d i s c r i m i n a n t a p p l i c a t i o n of a 1 - D i n v e r s i o n to d a t a j u d g e d by t h e i r o w n m e r i t as r e p r e s e n t i n g a r r i v a l s f r o m a p l a n e l a y e r e d e a r t h w o u l d lead t o an e r r o n e o u s s o l u t i o n . It  is c l e a r t h a t t h e r e a l d a t a set i n v e r t e d i n t h i s c h a p t e r i n d i c a t e d n o severe effects  due to a strongly two-dimensional velocity structure. 2-D modelling indicate otherwise.  Y e t subsequent results from the  A l t h o u g h t h e m o d e l l i n g p r o c e s s is n o n - u n i q u e ,  it  is c o n s t r a i n e d by f o r w a r d a n d r e v e r s e profiles a n d s u p p o r t e d b y r e s u l t s f r o m p r e v i o u s  67 s t u d i e s ( S h o u l d i c e , 1971, 1973; a n d S t a c e y a n d S t e p h e n s , 1 9 6 9 ) as d e s c r i b e d in C h a p t e r IV. B a s e d o n t h e p r o g r a m w r i t t e n for t h i s s t u d y , a n d i t s a p p l i c a t i o n to s y n t h e t i c a n d r e a l d a t a , s e v e r a l r e c o m m e n d a t i o n s c a n be m a d e .  For the slant stack o p e r a t i o n , the  s t a c k s h o u l d be w i n d o w e d t o r e d u c e t h e effects f r o m a r r i v a l s not a s s o c i a t e d w i t h t h e s t a c k i n g v e l o c i t y . S e c o n d l y , for t h e a i r g u n s i g n a t u r e w h i c h has a s t r o n g e r a m p l i t u d e i n t h e s e c o n d c y c l e of t h e a r r i v a l , t h e p r o g r a m c o u l d be d e s i g n e d t o use t h i s a m p l i t u d e for t h e i n v e r s i o n . T h i s c o u l d be a c c o m p l i s h e d b y a p p l y i n g a p h a s e shift to m o v e t h i s p e a k t o t h e r i g h t d e p t h . C l a y t o n a n d M c M e c h a n (1981) d o s o m e t h i n g s i m i l a r w h e n t h e y use m u l t i p l e s to extract the velocity vs d e p t h curve.  T h e y a c c o m p l i s h t h i s by u s i n g t h e  f a c t t h a t t h e m u l t i p l e s h a v e t w i c e t h e r of c o r r e s p o n d i n g p r i m a r i e s a n d i m p l e m e n t t h e a p p r o p r i a t e p h a s e r o t a t i o n by d o u b l i n g t h e f r e q u e n c y . T h i s a l l o w s the m u l t i p l e s t o b e used t o e x t r a c t t h e p - d e p t h c u r v e . T h e r e f o r e , a phase shift of p o s s i b l y a q u a r t e r of a w a v e l e n g t h m a y a l l o w t h e user t o u t i l i z e t h e s t r o n g m a x i m u m i n the a i r g u n s i g n a t u r e to e x t r a c t t h e p-depth c u r v e . F i l t e r e d d a t a a n d p — T sections were tested using a zerophase b a n d p a s s f i l t e r , b u t t h e side l o b e s i n t e r f e r r e d w i t h t h e r e c o g n i t i o n of t h e m a x i m u m a m p l i t u d e s at m i n i m u m z i n t h e d o w n w a r d c o n t i n u e d w a v e f i e l d . p h a s e c h a r a c t e r i s t i c of t h e recommended.  filter  m a y h a v e b e e n the p r o b l e m .  first  T h e zero  F u r t h e r i n v e s t i g a t i o n is  68  C H A P T E R IV  M O D E L L I N G OF T H E A I R G U N - O B S DATA  4.1 Introduction T h e a i r g u n - O B S d a t a were m o d e l l e d i n t h r e e s e g m e n t s u s i n g the 2 - D r a y  trace  s y n t h e t i c s e i s m o g r a m a l g o r i t h m d e s c r i b e d by S p e n c e et al. (1984). T h e s e g m e n t s were d e f i n e d by t h e d a t a sets b e t w e e n O B S 1 a n d O B S 2, O B S 2 a n d O B S 3 a n d O B S 3 a n d O B S 4. T h e f o r w a r d a n d r e v e r s e d profiles w e r e m o d e l l e d s i m u l t a n e o u s l y for each s e g m e n t of t h e a i r g u n l i n e . N o n e of the s e i s m i c r e c o r d sections p r e s e n t e d i n t h i s c h a p t e r h a v e b e e n filtered u n l e s s o t h e r w i s e i n d i c a t e d . A r / 1  2  g e o m e t r i c a l s p r e a d i n g f a c t o r , where  r is t h e offset, has b e e n a p p l i e d t o b o t h the t h e o r e t i c a l a n d o b s e r v e d a m p l i t u d e s for each s e i s m o g r a m . T h e a m p l i t u d e s o n the o b s e r v e d s e i s m o g r a m s were o v e r s u p p r e s s e d by its application  for s e i s m o g r a m s w i t h i n 3.0 k m of t h e r e c e i v e r .  a c t u a l receiver was never directly a 2-D  model.  T h i s occurs because the  b e l o w t h e s o u r c e w h e r e a s t h i s m u s t be the case for  H o w e v e r , t h i s o v e r s u p p r e s s i o n d i m i n i s h e s b e y o n d the 3.0 k m s o u r c e -  r e c e i v e r d i s t a n c e . T h e t r a v e l t i m e fits s h o u l d be r e l i a b l e after a 1.0 k m s o u r c e - r e c e i v e r distance. T h e m o d e l l i n g for e a c h s e g m e n t w i l l be d i s c u s s e d s e p a r a t e l y for t h e f o r w a r d  and  r e v e r s e p r o f i l e s . T h i s w i l l be f o l l o w e d by a s u m m a r y of the m o d e l t h a t w a s d e v e l o p e d for t h a t segment plus a brief geological interpretation.  T h r o u g h o u t the following discussion  o n l y the v e r t i c a l c o m p o n e n t for each d a t a set is p r e s e n t e d . H o w e v e r t h e h o r i z o n t a l a n d hydrophone  c o m p o n e n t s were used w h e n  they  p r o v i d e d c o n s t r a i n t s for  questionable  e v e n t s on t h e v e r t i c a l c o m p o n e n t s e i s m o g r a m . A p p e n d i x II i n c l u d e s r e c o r d s e c t i o n s for a l l c o m p o n e n t s . T h e final v e l o c i t y m o d e l is p r e s e n t e d i n F i g u r e 4.1 for reference d u r i n g the following discussions.  W h e r e v a l u e s for v e l o c i t i e s a n d / o r g r a d i e n t s are  uncertain  OBS 2  OBS 1  OBS 3  2oo (0.25) ^ D i s t a n c e ( k m )  0  10  15  1.80 (0.25)'  20  25  30  35  f  40  45  1  1.49 (0.001)  OBS 4 f .90 (0.25  2.35 (0.25)  s 7.20 (0.30)  Q  F i g u r e 4.1 Final velocity model for airgun/OBS survey. Velocities for each polygonal block apply to the upper boundary or the uppermost boundary segment when indicated. Velocity gradients for the blocks are enclosed in brackets and increase downwards along a line perpendicular to the upper boundary. The location of the OBSs with respect to the final composite model are indicated at the top of the model. The model is plotted with a vertical exageration of 5:1 . Uncertainties in the values derived from the model are indicated by (*) while undefined regions of the model are marked by (?).  —  70 h a v e b e e n i n d i c a t e d by a n a s t e r i s k . Q u e s t i o n m a r k s a r e u s e d t o i n d i c a t e r e g i o n s w h e r e little control exists for the velocities a n d gradients used.  4.2 Initial Constraints T h e i s o p a c h f o r t h e T e r t i a r y S k o n u n s e d i m e n t s ( S h o u l d i e e 1 9 7 1 , 1973) d e r i v e d f r o m industry  reflection a n d refraction seismic d a t a along w i t h wells drilled i n the Queen  C h a r l o t t e basin provided constraints for the m o d e l . p r o v i d e d t h e necessary justification  T h e sonic logs f r o m these w e l l s  f o r t h e c h o i c e of large v e l o c i t y g r a d i e n t s .  d a t a p r o c e s s i n g affected t h e near offset t r a c e s (see C h a p t e r II) such t h a t  Where  interpreted  v e l o c i t i e s were n o t r e l i a b l e , s o n i c logs f r o m w e l l s near t h e a i r g u n line were used t o c o n s t r a i n t h e v e l o c i t y f o r t h e u p p e r s e d i m e n t a r y l a y e r s . A g r a v i t y s u r v e y (Stacey a n d S t e p h e n s , 1969), n e a r l y c o i n c i d e n t w i t h t h e a i r g u n l i n e , also p r o v i d e d j u s t i f i c a t i o n f o r the gross s t r u c t u r a l characteristics o f the m o d e l observed at its eastern e n d . T h e initial m o d e l w a s constructed by projecting t h e position of the O B S s onto the m o d e l presented by S h o u l d i e e (1971) a n d u s i n g t h e v e l o c i t i e s f r o m t h e wells c o m b i n e d w i t h t h e v e l o c i t i e s d e r i v e d f r o m c o n s i d e r i n g t h e f o r w a r d a n d reverse profiles.  4.3 O B S 1— O B S 2 S u b m o d e l F o r t h e p u r p o s e s o f t h i s a n d t h e f o l l o w i n g s e c t i o n s , t h e Forward  profile  refers t o  t h e r e c o r d s e c t i o n w h e r e t h e s o u r c e - r e c e i v e r d i s t a n c e increases f r o m west t o east. T h e Reverse profile  refers t o s o u r c e - r e c e i v e r d i s t a n c e s t h a t increase f r o m east t o west. T h e  r e l a t i v e p o s i t i o n o f t h e O B S t h a t r e c o r d e d t h e s e i s m o g r a m is i n d i c a t e d by t h e insert i n e i t h e r t h e u p p e r left c o r n e r o f t h e r e c o r d s e c t i o n , as f o r t h e Forward profile o r t h e u p p e r r i g h t h a n d c o r n e r , as f o r t h e Reverse profile.  A n y distances quoted in the following  71 discussions will be indicated by the qualifiers MD for model distances and SRD for source-receiver distances. The theoretical travel time curves have been superimposed as solid curves on the observed seismograms and are labelled by lower case letters. The model through which rays are traced appears directly below the seismogram. The ray groups reach the surface below their corresponding travel time curves on the seismogram. In a following figure the data are also presented above the theoretical seismograms derived from the model. In this case, the travel time curves have been omitted from the real data display so as not to detract from the amplitude comparison between theoretical and observed seismograms.  4.3.1 Forward profile The record section for the forward profile is shown in Figure 4.2A. Figure 4.2B shows the ray traced model for the OBS 1 to OBS 2 segment, see Figure 4.1 and Figure 2.1. The analog data were digitized for only the near offset arrivals (first 7.0 km SRD ) as the noise level completely obscured the seismic signal beyond this range. The high noise levels for OBS 1 may be attributed at least partly to the shallow depth of this particular OBS. The model is separated into two basic units, the lower velocity sedimentary unit and the higher velocity crystalline unit although the latter is not defined by the forward profile but by the reversed one (section 4.3.2). The lower velocity sedimentary unit is further divided into three low velocity layers. The energy from the upper layer was modelled by refractions through a layer with a velocity of 1.8 km/s. The theoretical travel time branch, a, matches the observed arrivals but the sonic logs from nearby  4  6 8 10 12 14 16 Source/Receiver Distance (km)  18  20  Model Distance (km)  Bi  o  2-  ffi 4 cu  Ed  r  8 Figure 4.2  The data for OBS 1 (A) are displayed above the ray tracing diagram for the OBS 1—OBS  2 sub-model (B). The source/receiver distance SRD is plotted along the horizontal axis for the data. Model distance MD is plotted along the horizontal axis for the ray tracing diagram. The data are plotted in reduced time format such that arrivals with an apparent velocity of 6.0 km/s appear horizontal. The travel time curves, labelled a to c, superimposed on the data were computed from the corresponding ray groups appearing below each curve. The relative position of the OBS is indicated by the label in the upper part of the record section (see text for discussion).  73 w e l l s i n d i c a t e a v e l o c i t y of 2.0 k m / s for t h e u p p e r s e d i m e n t s . T h e d i s c r e p a n c y m a y be a t t r i b u t e d t o t h e p r o c e s s i n g of t h e d a t a . G o o d fixes o n t h e p o s i t i o n of t h e O B S s i m p r o v e w h e r e s p l i t s p r e a d i n f o r m a t i o n i s a v a i l a b l e . F u r t h e r m o r e t h e n e a r offset t r a c e s are m o r e s t r o n g l y a f f e c t e d by r e s i d u a l 3 - D effects a s s o c i a t e d w i t h t h e i r t r u e offline p o s i t i o n after the 2 - D projection  (see C h a p t e r  Refractions through primary  II).  t h e 2.35 k m / s l a y e r i n t h e m o d e l are not seen t o emerge as  a r r i v a l s b u t t h e p r e c r i t i c a l r e f l e c t i o n s f r o m t h e t o p of t h e 2.75 k m / s l a y e r ,  c u r v e b, d o m a t c h t h e large s e c o n d a r y a r r i v a l s seen b e t w e e n 1.0 a n d 1.7 k m SRD.  The  r e f r a c t i o n s t h r o u g h t h e 2.75 k m / s l a y e r , b r a n c h c , e m e r g e as t h e first set of p r i m a r y a r r i v a l s seen a f t e r b r a n c h a . T h e t h i n n e s s of t h e 2.35 k m / s l a y e r , less t h a n 100 m e t e r s , p r o b a b l y a c c o u n t s for t h i s .  T h i s l a y e r m a y or m a y n o t be present at t h i s e n d of t h e  s u b - m o d e l b u t is o b s e r v e d i n t h e o t h e r s u b - m o d e l s a n d therefore has b e e n i n c o r p o r a t e d into the interpretation  . S i n c e t h e a c c u r a c y of t h e r e s u l t s a t n e a r offset are affected by  t h e d a t a p r o c e s s i n g , e l i m i n a t i o n of t h i s l a y e r w a s n o t j u s t i f i e d . a bandpass  filtered  T h e d a t a are noisy b u t  ( 5 - 1 2 H z ) r e c o r d s e c t i o n p l u s t h e g o o d response o n t h e h o r i z o n t a l  g e o p h o n e , a i d e d i n t h e d e t e r m i n a t i o n of t h e first b r e a k s . T h e t h e o r e t i c a l s e i s m o g r a m s are c o m p a r e d w i t h t h e o b s e r v e d d a t a i n F i g u r e 4.3. A g r a d i e n t of 0.50 k m / s / k m i n t h e 2.75 k m / s l a y e r w a s c h o s e n t o g i v e a g o o d r e l a t i v e m a t c h between the observed a n d the synthetic a m p l i t u d e s .  F o r the Q u e e n C h a r l o t t e  b a s i n , g r a d i e n t s in excess o f 1.0 k m / s / k m are o b s e r v e d f r o m the sonic w e l l logs.  4.3.2 R e v e r s e  profile  T h e r e v e r s e d profile s h o w n i n F i g u r e 4 . 4 a r e c o r d e d s e i s m i c energy o u t t o 17 k m SRD.  O n l y t h i s p r o f i l e e x t e n d e d a c r o s s m o s t of t h e r a n g e b e t w e e n O B S 2 t o O B S 1.  0  2  4  6  8  10  12  14  16  18  20  Source/Receiver Distance (km)  Model Distance (km) Figure 4.3  Data for OBS 1 (A) is compared with the synthetic seismograms (B) computed for the  ray tracing diagram in figure 4.2B. Both the data and the synthetic seismograms are plotted with the same scale factors. The MD plotted along the horizontal axis of the synthetic seismograms corresponds to those of the ray tracing diagram.  75 T h e i s o p a c h of t h e T e r t i a r y s e d i m e n t s a n d t h e k n o w n b a s i n w a r d d i p e x t r a p o l a t e d f r o m t h e g e o l o g y o b s e r v e d on t h e Q u e e n C h a r l o t t e I s l a n d s p r o v i d e d t h e needed c o n s t r a i n t s for t h e s u b - m o d e l . T h e a m p l i t u d e s of t h e first a r r i v a l s are s t r o n g o u t t o 10 k m SRD d r o p off q u i t e s u d d e n l y . B e y o n d 10 k m SRD,  where they then  t h e s e i s m i c s i g n a l s are v e r y w e a k .  o n s e t of a h i g h e r v e l o c i t y b r a n c h c a n b e seen b e t w e e n 10 k m a n d 11 k m Rays traced through  SRD.  t h e 2.0 k m / s l a y e r g e n e r a t e d t h e o r e t i c a l t r a v e l t i m e s  m a t c h e d t h e o b s e r v a t i o n s b e t w e e n t h e 0 a n d 1.0 k m SRD,  The  that  curve a in Figure 4.4A. This  v a l u e is c o n s i s t e n t w i t h t h e v e l o c i t i e s for t h e u p p e r s e d i m e n t s m e a s u r e d by the sonic logs. E n e r g y r e f r a c t e d t h r o u g h t h e 2.35 k m / s l a y e r , s l i g h t l y t h i c k e r n e a r O B S 2, emerge as p r i m a r y  a r r i v a l s b e t w e e n 1.0 k m a n d 1.8 k m SRD,  (curve b ) .  A g r a d i e n t of 0.35  k m / s / k m w a s r e q u i r e d for t h i s l a y e r t o m a t c h t h e o b s e r v e d a m p l i t u d e s a n d p r o d u c e a m o d e l c o n s i s t e n t w i t h t h a t f r o m t h e a d j o i n i n g s u b - m o d e l ( O B S 2 — O B S 3). T h e a r r i v a l s b e t w e e n 2.0 a n d 10.0 k m SRD  w e r e m o d e l l e d by r a y s t r a c e d t h r o u g h a  t h i c k l a y e r w i t h a v e l o c i t y of 2.75 k m / s a n d a g r a d i e n t of 0.50 k m / s / k m ( c u r v e c ) . T h e t h e o r e t i c a l s e i s m o g r a m s a n d t h e o b s e r v e d d a t a are s h o w n i n F i g u r e 4.5. T h e t h e o r e t i c a l a m p l i t u d e s t h a t c o r r e s p o n d t o c u r v e c a c t u a l l y r e m a i n large for a n a d d i t i o n a l n u m b e r of t r a c e s b e y o n d w h e r e t h e a m p l i t u d e s decrease a b r u p t l y .  Considering the amplitudes  a t t h i s offset ( b e t w e e n 6 a n d 8 k m MD ) t h e y a p p e a r t o be l a r g e a n d end m o r e a b r u p t l y t h a n for t h e r e a l d a t a a t e q u i v a l e n t offsets. T h e a c t u a l s t r u c t u r e a l o n g t h e t o p of t h e 5.0 k m / s l a y e r m a y be r e s p o n s i b l e for t h i s b u t t h e u n r e v e r s e d p o r t i o n of t h i s s e g m e n t a n d t h e w e a k a m p l i t u d e s for a r r i v a l s f r o m t h i s l o w e r u n i t d i d n o t j u s t i f y a m o r e c o m p l e x boundary.  Figure 4.4  Data for OBS 2 reverse profile (A) and ray tracing diagram for OBS 1—OBS 2 sub-model  (B). (see figure 4.2 for explanation)  -20  0  -  —r-  2  16 -14 -12 -10 -8 -6 -4 Source/Receiver Distance (km)  -2  4  18  6 8 10 12 14 16 Source/Receiver Distance (km)  20  Figure 4.5 Comparison of data for OBS 2 reverse profile (A) with synthetic seismograms (B) computed for ray tracing diagram infigure4.4B. (see figure 4.3 for explanation)  78 The velocity and depth of the 5.0 km/s and 5.8 km/s layers were constrained by the sonic log from the Tyee well (Figure 2.1) and the isopach trend. The neighbouring sub-model provided an additional constraint by requiring that the model features be continuous beneath OBS 2. The theoretical amplitudes, Figure 4.5B, show the weak amplitudes for the arrivals from these two layers. This indicates that the seismic signal in the data may be obscured by the noise. The theoretical travel time curve, e, coincides with the onset of the higher velocity arrival near the beginning of the curve. This energy can not be seen clearly to extend beyond about 11 km SRD, although some weak coherent energy may be discernible.  4.3.3 S u m m a r y  Figure 4.6 shows a three dimensional representation of the velocity changes along the sub-model between OBS 1 and OBS 2. Velocity and depth profiles were selected from different locations along the model and plotted in a 3-D display. The selected velocity-depth profiles are the solid curves increasing in a particular velocity-depth plane at fixed distance values. This presentation of the velocity information illustrates the overall velocity structure of the model, including velocity gradients. The geological interpretation for each velocity unit is also shown. The thick 2.75 km/s unit is interpreted as the Tertiary Skonun sediments (Sutherland Brown, 1968). Thickening of this unit across the basin is supported by the isopach for the sediments (Figure 1.1). The sonic log from the Tyee well (Figure 2.1) is compared in Figure 4.7 with the velocity versus depth curve from the 15 km MD location. A visually smoothed average of the sonic log is also indicated by the thick solid line. The two compare quite favourably. The velocity from the sonic log has measured the characteristics of  OBS 2 _  Figure 4.6  Water  Velocity cube for O B S 1—OBS 2 sub-model with a brief geological interpretation. Model distance is  plotted at the top of the diagram along the axis projecting into the page. The depth axis is plotted along the vertical axis. Velocity is plotted at the base of the diagram along the axis projecting out of the page. The velocity cube was constructed by selecting five velocity depth profiles from the sub-model at 0, 7, 15 and 20 km MD. The relative position of the two OBSs is indicated at the top of the diagram.  80 v e r t i c a l w a v e p r o p a g a t i o n o v e r d i s t a n c e s of m e t e r s w h e r e a s t h e r e f r a c t i o n m e t h o d t e n d s t o p r o v i d e h o r i z o n t a l v e l o c i t y i n f o r m a t i o n o v e r d i s t a n c e s of k i l o m e t e r s . T h e v e l o c i t y a n d t h e g r a d i e n t s c a n c h a n g e s i g n i f i c a n t l y o v e r s h o r t d i s t a n c e s , a f a c t w h i c h a l s o c o u l d be r e s p o n s i b l e f o r t h e differences i n t h e v e l o c i t y d e p t h f u n c t i o n b e t w e e n t h e t w o d i f f e r e n t l o c a t i o n s . S u c h a c h a r a c t e r i s t i c is o b v i o u s w h e n one c o m p a r e s t h e three sonic w e l l logs for t h e C o h o , T y e e , a n d S o c k e y e w e l l s w h i c h are near the a i r g u n line ( F i g u r e 4.8). G r a d i e n t s i n excess of 1.0 k m / s / k m are o b s e r v e d i n t h e S o c k e y e w e l l . T h i s o b s e r v a t i o n p r o v i d e d s o m e of t h e j u s t i f i c a t i o n for t h e large g r a d i e n t s t h a t were s o m e t i m e s r e q u i r e d to m o d e l the a m p l i t u d e s of the observed arrivals. T h e 5.0 k m / s u n i t u n d e r l y i n g t h e T e r t i a r y s e d i m e n t s of F i g u r e 4.7 is i n t e r p r e t e d as T e r t i a r y volcanics, possibly the M a s s e t formation  ( S u t h e r l a n d B r o w n , 1968). T h e 5.8  k m / s u n i t b e l o w t h i s , a l t h o u g h n o t w e l l d e f i n e d for t h i s s u b - m o d e l , is i n t e r p r e t e d as t h e P a l e o z o i c A l e x a n d e r T e r r a n e ( Y o r a t h a n d C h a s e , 1981). T h e v e l o c i t y j u m p b e l o w t h i s h a s o n l y b e e n i n c l u d e d as a c o n t i n u a t i o n of a u n i t f r o m t h e a d j o i n i n g s u b - m o d e l a n d d o e s not r e p r e s e n t a t r u e s u b s u r f a c e b o u n d a r y for t h i s s u b - m o d e l .  4.4 OBS 2—OBS 3 Sub-model In F i g u r e 4.1 t h e s u b - m o d e l for t h i s s e g m e n t of t h e a i r g u n line c o r r e s p o n d s to t h e r a n g e b e t w e e n 20 a n d 41 k m MD as i n d i c a t e d by p o s i t i o n s of t h e t w o O B S s . P r e - c r i t i c a l r e f l e c t i o n s c o u l d n o t be m o d e l l e d as t h e y were n o t o b s e r v e d i n the d a t a d u e t o t h e i r reverberatory  nature.  Figure 4.7 Comparison of velocity versus depth profile (curve 1) from OBS 1—OBS 2 sub-model at 15 km MD and the sonic log from the Tyee well (curve 2) and its visually smoothed version (curve 3).  82  SHELL ANGLO COHO  2.  3.  DEPTH (KM)  SHELL ANGLO TYEE  2.  3.  DEPTH (KM)  SHELL ANGLO SOCKEYE B - 1 0  1.  2.  3.  DEPTH (KM)  Figure 4.8  4.  Sonic logs from the three wells (Coho, Tyee and Sockey B-10, figure 1.1) nearest the  airgun/OBS line.  83 4.4.1 Forward profile Results of the travel time fit to the data and the corresponding ray trace model are shown in Figure 4.9A and 4.9B, respectively.  Curve a shows the fit to the observed  travel times modelled by tracing rays through the upper 2.0 km/s layer. A gradient of 0.3 k m / s / k m was required to produce the proper curvature and match the amplitudes of these arrivals beyond the 3.0 km 5/?Z?(Figure 4.10b). T h e second theoretical travel time curve, b, modelled the energy arriving from the 2.30 km/s layer. The gap in the data between 3.8 and 5.8 km SRD tion for approximately 20 minutes. between 3.0 and 3.8 km SRD  resulted from an airgun malfunc-  A n emerging higher velocity branch can be seen  which can be followed andi picked up along a continu-  ous curve through the gap at 5.8 km SRD.  Curve c models these arrivals as refractions  through the Tertiary sediments (referred to in the previous sub-model). The amplitudes of the observed arrivals beyond 8.0 km SRD,  Figure 4.9A and 4.10A, are weaker and, at  the level of the noise, degrading the first break picks for this branch. These amplitudes die off sooner and more gradually than their theoretical counterparts.  This may be  due in part to the interference of other arrivals such as refracted multiples of possibly diffracted energy. T h e cut off in amplitudes was modelled by requiring that the volcanic unit (5.00 km/s layer) truncate the ray group for branch c (Figure 4.9). The theoretical travel time, curve d, for the group of rays that have travelled through the Tertiary volcanic unit emerge as a slightly weaker set of arrivals extending to 15.4 km SRD  (Figure  4.9A and 4.10A). The lower apparent velocity for these arrivals was modelled by the drop off in depth of the surface of the volcanics.  Previous studies (Shouldiee,  1971,  1973; Yorath and H y n d m a n , 1983) show the top of the Tertiary volcanics to have been eroded during a period of uplift.  There were even instances where the volcanics were  o Cd  © CO  \ Q I E-<  4  o  4  6  8  10  12  14  16  Source/Receiver Distance (km) Model Distance (km)  6  8  10  12  14  16  18 18  20 20  22 2J  B  CL. H Q  Figure 4.9 Data for OBS 2 forward profile (A) and the ray tracing diagram for OBS 2—OBS 3 su model (B). (see figure 4.2 for explanation)  85  o Cd  cn, o  CO \  Q I' E1  4  6 8 10 12 14 16 18 Source/Receiver Distance (km)  20  22  B 2A  o © eo*  \ Q I  E-  l  A  0  Figure 4.10  0  6  8 10 12 14 16 Model Distance (km)  18  20  22  Comparison of data for OBS 2 forward profile (A) and synthetic seismograms (B)  puted for ray tracing diagram in figure 4.9B (see figure 4.3 for explanation).  86  not present in some of the wells. Shouldice (1973) also indicates that the steep dips seen on the reflection seismic data may also be fault related. The lower unit of the model, 5.9 km/s layer, models the energy thought to have just sampled the upper part of the interpreted Alexander Terrane, curve e. Observed arrivals can not be clearly seen in the data but when these data were bandpass filtered weak correlated arrivals could be discerned at the appropriate range and time, but the location of the first breaks could not be determined. The synthetic amplitudes for curve e Figure 4.9B indicate that these arrivals are weak. The theoretical seismogram, Figure 4.10B, and the observed seismogram, Figure 4.10A, compare well in their overall response.  4.4.2 Reverse profile OBS 3 recorded strong seismic signals, with a good signal to noise ratio (SNR) (Figure 4.1 IA and 4.12A). It was located 21 km from OBS 2 at the eastern end of the sub-model.  The model at this end can be seen to be changing slightly. The upper  two sedimentary units were modelled as for the forward profile, except that the lower unit required a lower velocity (2.25 km/s) beneath OBS 3 to provide some continuity between the adjacent sub-models at this location.  The theoretical travel times for  arrivals from these two layers in the sub-model are shown in Figure 4.11A, curves a and b. The water depth increases towards the east and a corresponding thickening of the uppermost sediments is observed in the sub-model. The theoretical travel time curve, c, modelled observed arrivals as due to ray travel paths through the Tertiary sedimentary layer (2.7 km/s). To match the observed arrivals out to 9.0 km SRD, the sediments were required to thicken towards the west. This thickening of the sediments  87  Source/Receiver Distance (km) Model Distance (km)  8  Figure 4.11  —  —  1  Data for OBS 3 reverse profile (A) and ray tracing diagram for OBS 2—OBS 3 sub-model  (B). Arrivals e and f correspond to the complex set of internally reflecting ray groups, (B), introduced to model the prominent secondary arrivals in (A) (see text for explanation).  88 was accomplished by thinning the underlying volcanic unit. A further thinning of this unit was then required to match the low apparent velocity of the arrivals from this layer, curve d. A velocity of 4.8 km/s was required and would seem reasonable, as the depth of burial decreases beneath OBS 3. At greater depths, the velocity increases to the 5.0 km/s value observed for the forward profile. The theoretical seismograms (Figure 4.12) match the amplitudes for both the c and d arrivals. The cutoff in the amplitude of the arrivals for branch c show very good agreement for this ease. The undulating nature of the arrivals (curve d), as modelled by rays through the volcanic layer, was achieved by slight variations of the upper boundary of the 2.25 km/s layer and the upper surface of the Tertiary volcanics. Unfortunately, the airgun malfunction referred to in the previous sections is also responsible for the premature end of the seismic traces at the western end of the record  1  section. Rays traced through the model, Figure 4.11B, sparsely sample a region of high velocity (5.9 km/s). A portion of the travel-time curve for this sparse group of trays is indicated by the very short solid line near the end of the data. The complete travel time curve for this ray group was omitted beyond the range of the recorded data. The clear distinct secondary arrivals recorded by this OBS provided a good opportunity to investigate the nature of these arrivals (see arrivals along curves e and f). The record sections for this and other marine surveys have been found to exhibit these secondary arrivals. The amplitudes are relatively strong and the arrivals appear as multiples of the near offset primary arrivals. They also exhibit some move out, similiar to that observed for reflections. These events are subsequently interpreted as multiple refractions, generated when the refracted energy was internally reflected during its upcoming travel path from a boundary with a large velocity contrast across it. Three of  Source/Receiver Distance (km)  Model Distance (km)  Figure 4.12  Comparison of data for OBS 3 reverse profile (A) and synthetic seismograms (B) computed  for ray tracing diagram in figure 4.11B. Arrivals labelled d, e and f correspond to similiarly labelled travel time curves in figure 4.11A.  90 these boundaries can be found in typical marine surveys: the air-water interface, the water-sediment interface and the sediment-basement interface. The group of curves, labelled f, are the travel time curves for rays internally reflected from the air-water interface and the sediment-water interface. The amplitudes of these arrivals are strong and persist for all displayed traces (Figure 4.12B). The result of reflecting the energy refracted through the 2.7 km/s layer, from the interface between it and the overlying layer, matches the weaker of the secondary arrivals on the data (see curve e). The arrivals labelled on the synthetic seismogram correspond to the curves shown in Figure 4.11 A . The amplitudes for the e and f arrivals match the relative amplitudes of the secondary arrivals in the data remarkably well. The weaker amplitudes for arrivals labelled e are expected since the contrast for this internally reflected set of rays was smaller than that for the air-water or water-sediment interface. The modelling of these secondary arrivals gives an added degree of confidence to the models since the model is now doubly sampled for some of the upper units. In general, theoretical travel time curves matched observed arrivals to within the error of picking the first breaks for these arrivals. The worst case showed a misfit of no more than ±0.20s for the secondary arrival fits.  4.4.3 Summary The velocity profile cube for this sub-model is shown in Figure 4.13. Seven profiles were selected along the model since it exhibited large lateral velocity variations. Geological interpretation of the various units is shown; the units follow continuously from the OBS 1-OBS 2 sub-model considered in section 4.3 . The lateral velocity variations  92 can be followed easily in this display. The thinning of the Tertiary volcanic layer can be seen as the 4.8 to 5.0 km/s velocities shift to greater depths. Other features also become evident — the upper sedimentary layers thicken and the thicker sedimentary unit begins to show less of a velocity contrast with the overlying thin units. As with the previous sub-model, the lower unit, indicated by the ?, is not defined for this sub-model.  4 . 5 O B S 3 — O B S 4 Sub-model  This third and final sub-model is located between 41 km and 62.5 km MD in Figure 4.1. The forward and reverse profiles recorded across this segment of the airgun line contain several striking anomalous features and a large noise level. One would expect to see unusual events in the data as the subsurface structure has been shown to change rapidly at the eastern margin of the basin (Shouldiee, 1971, 1973; Staeey and Stephens, 1969; Yorath and Cameron, 1982; Young, 1981). However some features were not anticipated. Modelling for a consistent travel time and amplitude match for both the forward and reverse profiles proved to be difficult. This was due to the obvious differences in the data sections recorded at OBS 3 and OBS 4 (Figures 4.14A and 4.16A).  4 . 5 . 1 Forward profile The data for the forward profile and the ray trace model are shown in Figure 4.14. Strong amplitudes extend to 9 km SRD for the refracted arrivals. The amplitude suddenly drops off, then a delayed signal appears between 14 and 18 km SRD. These characteristics are shown more clearly by the hydrophone component (Figure 11.11, Appendix II).  Figure 4.14  Data for OBS 3 forward profile (A) and the ray tracing diagram for OBS 3—OBS 4 sub-  model (B). Some reflected refraction ray paths have been included in an attempt to model particular features (see figure 4.2 and text for explanation).  94 M o d e l l i n g for t h e s e d i m e n t a r y l a y e r s p r o c e e d e d as before a n d t h e t r a v e l t i m e s are s h o w n , c u r v e s a a n d b, i n F i g u r e 4 . 1 4 A . T h e v e l o c i t y of t h e m i d d l e s e d i m e n t a r y u n i t , t h a t is t r u n c a t e d by the thick T h e velocity contrast  Tertiary  u n i t , r e q u i r e d a l o w e r v e l o c i t y of 2.2 k m / s .  between the u p p e r layers and this m i d d l e unit diminishes to  z e r o t o w a r d s t h e e a s t e r n e n d of t h e m o d e l d u e t o t h e v e l o c i t y g r a d i e n t a n d i n c r e a s i n g t h i c k n e s s of t h e u p p e r m o s t l a y e r (see F i g u r e 4 . 1 9 ) . R a y s t h r o u g h t h e l o w e r s e d i m e n t a r y u n i t , the Tertiary sediments, (velocity e.  2.6 k m / s ) g e n e r a t e t h e t h e o r e t i c a l a r r i v a l b r a n c h  In t h i s p a r t of t h e m o d e l , t h e i n t e r p r e t e d  T e r t i a r y v o l c a n i c l a y e r is s h a l l o w e r , as  r e q u i r e d by the t r a v e l t i m e c h a r a c t e r i s t i c s of t h e d a t a a n d t h e i s o p a c h  information  ( S h o u l d i e e , 1971). C u r v e d s h o w s t h e t h e o r e t i c a l t r a v e l t i m e s for t h i s u n i t . T h e s u d d e n decrease i n t h e a m p l i t u d e a t 9 k m SRD  necessitated the  i n t h e m o d e l of a f e a t u r e w h i c h c o u l d cause t h e o b s e r v e d c h a r a c t e r i s t i c .  introduction A  faulted  b l o c k or a l o w v e l o c i t y l a y e r are t y p i c a l m e c h a n i s m s chosen t o c a u s e t h e desired effect. T h e f a u l t s o l u t i o n w a s c o n s i d e r e d b u t d i d n o t y i e l d a m o d e l c o n s i s t e n t w i t h t h e reverse profile.  F u r t h e r m o r e , t h e r e is n o e v i d e n c e t o s u p p o r t m a j o r f a u l t i n g i n t h i s r e g i o n .  A low velocity zone w h i c h pinches out to the west was i n t r o d u c e d i m m e d i a t e l y the Tertiary volcanics.  below  T h e synthetic seismogram section (Figure 4.15B) shows that  t h e a m p l i t u d e for t h e a r r i v a l s for phase d d r o p s off s h a r p l y a t a b o u t 9 k m it S R D , as o b s e r v e d o n the d a t a s e c t i o n . A v e l o c i t y of 3.5 k m / s w a s chosen for t h i s l o w v e l o c i t y zone t o be c o n s i s t e n t w i t h a s i m i l a r l y i n t e r p r e t e d l o w v e l o c i t y l a y e r f r o m the C h a r l o t t e s u b b a s i n ( C l o w e s a n d G e n s - L e n a r t o w i c z , 1985).  T h e low v e l o c i t y / t h i c k n e s s c o m b i n a t i o n  w a s c o n s i s t e n t w i t h t h e d e l a y o b s e r v e d i n t h e a r r i v a l s b e y o n d 11 k m SRD. the introduction  In a d d i t i o n ,  of t h e l o w v e l o c i t y z o n e w a s c o n s i s t e n t w i t h d a t a f r o m t h e reverse  profile as d i s c u s s e d i n t h e n e x t s e c t i o n .  95  Source/Receiver Distance (km)  Model Distance (km)  Figure 4.15  Comparison of data for OBS 3 forward profile (A) and synthetic seismograms (B) com-  puted for ray tracing diagram in figure 4.14B (see figure 4.3 for explanation).  96 T h e s o m e w h a t isolated burst of high a m p l i t u d e  energy o b s e r v e d o n t h e  s e c t i o n at a b o u t 2.5s a n d b e t w e e n 14 a n d 15 k m SRD, hydrophone component  record  is m o r e p r o n o u n c e d on t h e  ( F i g u r e 11.11, A p p e n d i x II). S i m p l e r e f r a c t i o n s  through  the  A l e x a n d e r t e r r a n e (5.9 k m / s l a y e r ) b e l o w t h e l o w v e l o c i t y z o n e d i d n o t generate a m p l i t u d e s c o r r e s p o n d i n g t o t h o s e o n t h e o b s e r v e d s e c t i o n . C o n s e q u e n t l y t h e b u r s t of energy w a s i n t e r p r e t e d t o r e p r e s e n t a l o c a l f o c u s s i n g effect. A n a t t e m p t w a s t h e n m a d e t o f o c u s t h e e n e r g y by s e l e c t i n g a n u m b e r of t r a v e l p a t h s t o g i v e t h e s a m e t r a v e l t i m e s . F i r s t , r a y s w e r e i n t e r n a l l y r e f l e c t e d f r o m t h e base of t h e 2.2 k m / s l a y e r . T r a v e l t i m e s for t h i s r a y g r o u p are s h o w n i n F i g u r e 4 . 1 4 A as c u r v e g . B u t b e c a u s e of t h e t i m e s p e n t in t h e T e r t i a r y s e d i m e n t s a n d t h e l o w v e l o c i t y c o n t r a s t b e t w e e n t h i s l a y e r a n d t h e 2.2 k m / s l a y e r , these a r r i v a l s w e r e d e l a y e d b y t o o m u c h a n d h a d w e a k a m p l i t u d e s ( F i g u r e 4 . 1 4 A a n d 4 . 1 5 B ) . N e x t r a y s w e r e r e f l e c t e d f r o m t h e base of t h e s e d i m e n t - b a s e m e n t b o u n d a r y ( F i g u r e 4 . 1 4 B a n d c u r v e h i n F i g u r e 4 . 1 4 A ) . T h e s e a r r i v a l s r e p r e s e n t t h e p o r t i o n of c u r v e l i o u t t o 16 k m SRD  a n d , as c a n be seen i n F i g u r e 4 . 1 5 B , c o n t r i b u t e the m o s t t o  t h e large a m p l i t u d e s t h a t are o b s e r v e d . T h e d i r e c t a r r i v a l s t h r o u g h t h e A l e x a n d e r t e r r a n e , c u r v e e , are m o d e l l e d for t h e w e a k a m p l i t u d e a r r i v a l s o b s e r v e d at t h i s r a n g e .  T h e h y d r o p h o n e c o m p o n e n t shows  t h e a r r i v a l s m o r e c l e a r l y , b u t t h e first b r e a k s c o u l d not b e d e t e r m i n e d b e c a u s e of t h e low a m p l i t u d e s a n d the noise.  T h e s e l o w a m p l i t u d e s are e v i d e n t as first a r r i v a l s on  t h e t h e o r e t i c a l s e i s m o g r a m s a n d are c l e a r l y n o t r e s p o n s i b l e for t h e f o e u s s e d energy a t t h i s range . T h e r e f o r e , r a y s w e r e r e f l e c t e d f r o m t h e base of the A l e x a n d e r T e r r a n e t o a u g m e n t t h o s e o n c u r v e h. T h e s e r e f l e c t i o n s f o r m c u r v e f a n d t h e rest of c u r v e h, w i t h some overlap between the two groups m a k i n g up curve h .  T h e l o w e r set of reflected  energy c o n t r i b u t e to the overall a m p l i t u d e s observed in the theoretical seismograms. A p o r t i o n of these r a y s , f r o m t h e d e e p r e f l e c t i n g g r o u p , a c t u a l l y r e f r a c t t h r o u g h  the  97 u p p e r m o s t p a r t o f t h e u n i t (7.5 k m / s ) w h e r e v e l o c i t i e s a n d g r a d i e n t s are u n c o n s t r a i n e d by the d a t a . T h e c o n c a v e s h a p e o f t h e l o w e r r e f l e c t i n g b o u n d a r y is s i g n i f i c a n t for t h e r e s u l t i n g travel times.  In o r d e r t o get a n a p p r o p r i a t e set of a r r i v a l s , t h e s t r u c t u r e h a d t o h a v e  a concave shape.  In t h e m o d e l ! w i t h o u t t h i s s h a p e , t h e c o m b i n a t i o n o f t h e  thinning  of t h e s e d i m e n t a r y l a y e r a n d t h e s h a l l o w i n g of t h e T e r t i a r y v o l c a n i c u n i t c a u s e d t h e theoretical arrivals to have larger a p p a r e n t velocities. T o m a t c h the a p p a r e n t velocity of t h e o b s e r v e d a r r i v a l s , a m e a n s o f i n c r e a s i n g t h e t r a v e l t i m e s as t h e offset i n c r e a s e d w a s n e e d e d . T h e c o n c a v e s t r u c t u r e p r o v i d e d t h e necessary m e c h a n i s m a n d f o c u s s e d t h e e n e r g y . H o w e v e r , t h e t r a v e l t i m e s d o n o t a p p e a r t o m a t c h a n y s p e c i f i c set o f a r r i v a l s i n t h e e n e r g y b u r s t ( F i g u r e 4.14A). N e v e r t h e l e s s , t h e t h e o r e t i c a l s e i s m o g r a m s d o m a t c h t h e o v e r a l l c h a r a c t e r i s t i c s of t h e o b s e r v e d s e i s m i c s e c t i o n ( F i g u r e 4.15). It is t h i s o b s e r v a t i o n w h i c h leads t o t h e c o n c l u s i o n t h a t these a r r i v a l s r e p r e s e n t a f o c u s s i n g of e n e r g y , m o s t likely due to c o n s t r u c t i v e a n d / o r d e s t r u c t i v e interference f r o m various arrivals.  4.5.2 R e v e r s e  profile  T h e r e v e r s e d profile w a s m o d e l l e d s i m u l t a n e o u s l y w i t h t h e f o r w a r d  profile.  The  d a t a a n d v e l o c i t y s t r u c t u r e , w i t h t r a c e d r a y s , a r e d e p i c t e d i n F i g u r e 4.16. N o i s e levels w e r e r e l a t i v e l y h i g h for t h i s d a t a set a n d r e q u i r e d a n a l y s i s of r e c o r d s e c t i o n s for all three components.  B a n d p a s s f i l t e r i n g w a s a p p l i e d w h e n n e c e s s a r y . T h e m o d e l l i n g of  t h e p r i m a r y a r r i v a l s b e t w e e n 0 a n d -8 k m SRD f o l l o w e d t h a t for t h e f o r w a r d m o d e l l i n g b u t n o w , a r r i v a l s f r o m t h e u p p e r m o s t s e d i m e n t a r y u n i t , are i m m e d i a t e l y f o l l o w e d by those from the thick Tertiary sediments (curves a middle unit.  a n d b ) d u e t o t h e t r u n c a t i o n of t h e  T o m o d e l the first arrivals, c u r v e a , and generate the necessary velocity  98  Source/Receiver Distance (km)  Model Distance (km)  Figure 4.16  Data for OBS 4 (A) and ray tracing diagram for OBS 3—OBS 4 sub-model (B) (see figure  4.2 for explanation and the text for an explantion of events labelled D).  99 contrast to m o d e l the b u m p - l i k e feature observed in the data between 2 a n d 4 k m  SRD,  t h e v e l o c i t y of t h e s e d i m e n t s b e l o w O B S 4 were d e c r e a s e d t o 1.9 k m / s . B y d i r e c t i n g t h e r a y s t h r o u g h a s t r u c t u r a l h i g h on t h e T e r t i a r y s e d i m e n t a r y l a y e r a f t e r t r a v e l l i n g t h r o u g h t h e 1.9 k m / s l a y e r , t h e a r r i v a l s e x p e r i e n c e a n i n c r e a s e i n a p p a r e n t v e l o c i t y , c u r v e b . T h i s is i m m e d i a t e l y f o l l o w e d b y a decrease i n a p p a r e n t v e l o c i t y as t h e r a y s e n t e r t h e u p p e r 2.0 k m / s layer. T h e t h e o r e t i c a l seismograms m a t c h the b u m p - l i k e feature well in a m p l i t u d e a n d s h a p e of t h e t r a v e l t i m e c u r v e ( F i g u r e 4 . 1 7 ) . T h e 2.2 k m / s l a y e r is p i n c h e d out by t h e rise o f t h e s t r u c t u r e r e p r e s e n t i n g t h e T e r t i a r y s e d i m e n t s . T h e e x t e n s i o n of b as a s e c o n d a r y a r r i v a l r e s u l t s f r o m ray p a t h s t h r o u g h t h e T e r t i a r y s e d i m e n t s , t h e 2.6 k m / s layer. T h e c o m p u t e d t r a v e l t i m e s , c u r v e c, for t h e 4.8 k m / s l a y e r are t r u n c a t e d by t h e e x t e n s i o n of t h e l o w v e l o c i t y l a y e r i n t r o d u c e d i n t h e f o r w a r d profile. T h e a m p l i t u d e of t h e a r r i v a l s i n t h e d a t a d o not a p p e a r t o d r o p off as fast as those f r o m t h e f o r w a r d profile b u t t h i s c u t off is a l s o s u p p o r t e d by t h e 1  filtered  1  h y d r o p h o n e c o m p o n e n t ( F i g u r e 11.12,  A p p e n d i x II). T h e o r e t i c a l a m p l i t u d e s for t h e a r r i v a l s f r o m t h e T e r t i a r y v o l c a n i c s are l a r g e r t h a n t h e first b r e a k a r r i v a l s for t h e o b s e r v e d d a t a . H o w e v e r , for t h e h y d r o p h o n e c o m p o n e n t , t h e a m p l i t u d e s of t h e a r r i v a l s , c, are m o r e c l e a r l y seen even t h o u g h  the  s i g n a l is at t h e noise l e v e l . T h e r e f o r e a g r a d i e n t of 0.70 k m / s / k m w a s c h o s e n t o m a i n t a i n a b a l a n c e b e t w e e n t h e a m p l i t u d e o b s e r v e d on t h e v e r t i c a l c o m p o n e n t a n d those o b s e r v e d on h y d r o p h o n e c o m p o n e n t . A t i m e d e l a y c a n be o b s e r v e d f o r t h e a r r i v a l s m a t c h e d by c u r v e d . T h e low v e l o c i t y l a y e r m a y b e the U p p e r C r e t a c e o u s s e d i m e n t s t h a t w e r e d e p o s i t e d across t h e s u t u r e z o n e d e f i n e d by Y o r a t h a n d C h a s e (1981) as t h e post suture assemblage. A s m e n t i o n e d i n s e c t i o n 4 . 5 . 1 , a s i m i l i a r low v e l o c i t y l a y e r w a s r e q u i r e d i n a m o d e l f r o m t h e C h a r l o t t e  100  -24 -22 -20 -18 -16 -14 -12 -10 -8 -6 Source/Receiver Distance (km) Model Distance (km)  -4  -2  0  Figure 4.17 Comparison of data for OBS 4 reverse profile (A) and synthetic seismograms (B) computed for ray tracing diagram in figure 4.1 IB (see figure 4.3 for explanation).  101 s u b - b a s i n a n d h a s b e e n i n t e r p r e t e d as t h e post suture assemblage ( C l o w e s a n d G e n s L e n a r t o w i c z , 1985). T h e t h e o r e t i c a l t r a v e l t i m e , c u r v e d, w a s c o m p u t e d by t r a c i n g r a y s t h r o u g h t h e b l o c k i n t e r p r e t e d as t h e A l e x a n d e r t e r r a n e . P o o r s i g n a l - t o - n o i s e r a t i o s p r e v e n t e d a n a c c u r a t e d e t e r m i n a t i o n of t h e f i r s t b r e a k s for t h i s b r a n c h . T h e h y d r o p h o n e c o m p o n e n t of the d a t a w a s b a n d p a s s filtered b e t w e e n 5 a n d 12 H z a n d s u c c e e d e d i n b r i n g i n g o u t t h e p r i m a r y a r r i v a l b r a n c h ( F i g u r e 11.12, A p p e n d i x II) w h i c h a i d e d i n t h e final d e t e r m i n a t i o n of its p o s i t i o n ( c u r v e d).  A s s h o w n on the theoretical seismograms ( F i g u r e 4 . 1 7 B ) , the slight  i n c r e a s e i n a m p l i t u d e has b e e n m o d e l l e d f a i r l y s u c c e s s f u l l y by t h e a d d i t i o n of r e f l e c t i o n s f r o m t h e t o p of t h e 7.2 k m / s l a y e r ( c u r v e e o n F i g u r e 4 . 1 6 A ) T h e d a t a i n F i g u r e s 4 . 1 6 A a n d 4 . 1 7 A s h o w a s t r o n g set of s e c o n d a r y a r r i v a l s , event D , w h i c h h a v e not b e e n m o d e l l e d e x p l i c i t l y . T h i s large a m p l i t u d e event ranges f r o m 13 t o 24 k m SRD  at a r e d u c e d t i m e of 2.5 s e c o n d s .  T o e x p l a i n t h e n a t u r e o f t h i s event  m o r e e a s i l y , t h e o r d e r i n w h i c h t h e i n v e s t i g a t i o n p r o c e e d e d w i l l be d e s c r i b e d . I n i t i a l l y t h e e v e n t w a s t h o u g h t t o r e p r e s e n t a s t r o n g set o f p r i m a r y a r r i v a l s a n d w e r e m o d e l l e d as s u c h .  H o w e v e r , the r e s u l t i n g m o d e l was f o u n d to be inconsistent  w i t h t h e i n t e r p r e t a t i o n of t h e f o r w a r d p r o f i l e . T h e general s t r u c t u r e of t h e u n i t s b e l o w t h e s e d i m e n t s w e r e r e q u i r e d t o h a v e a n e a s t w a r d d i p , w h i c h is not i n a c c o r d w i t h t h e a c c e p t e d r e g i o n a l geology o f t h e a r e a .  U p o n a closer e x a m i n a t i o n of t h e d a t a , weak  a r r i v a l s p a r a l l e l l i n g e v e n t D w i t h a n a r r i v a l t i m e a p p r o x i m a t e l y 0.8 s e c o n d s e a r l i e r were n o t e d a n d s u b s e q u e n t l y m o d e l l e d as d e s c r i b e d . T h i s raised t h e q u e s t i o n — w h a t s i t u a t i o n w i l l g i v e rise t o w e a k p r i m a r y a r r i v a l s f o l l o w e d by large a m p l i t u d e s e c o n d a r y a r r i v a l s ? T h e first a t t e m p t t o s o l v e t h i s p r o b l e m w a s t o m o d e l the e a r l i e r events as h e a d waves a n d t h e s e c o n d a r y e v e n t as r e f l e c t i o n s f r o m t h e base of t h e l a y e r g e n e r a t i n g t h e h e a d  102 w a v e s . T h i s s o l u t i o n f a i l e d t o generate t h e o b s e r v e d a m p l i t u d e s a n d t h e p a r a l l e l n a t u r e of the two events. A solution w h i c h adequately explained the observations evolved following e x a m i n a t i o n of t h e t h r e e c o m p o n e n t s for t h i s d a t a set. F i g u r e 4.18 s h o w s t h e t h r e e c o m p o n e n t s o f d a t a for O B S 4 w i t h i n t h e r a n g e a n d t i m e w i n d o w of i n t e r e s t . T h e v e r t i c a l c o m p o n e n t , F i g u r e 4.18c, a n d t h e h o r i z o n t a l c o m p o n e n t , F i g u r e 4 . 1 8 b , were p l o t t e d w i t h t h e s a m e scale f a c t o r w h i c h is t h r e e t i m e s g r e a t e r t h a n t h e f a c t o r used for t h e h y d r o p h o n e c o m p o n e n t , F i g u r e 4 . 1 8 a . C o m p a r i n g F i g u r e 4.18 b a n d c, the large a m p l i t u d e event D is s t r o n g e r a n d m o r e c o h e r e n t on t h e h o r i z o n t a l c o m p o n e n t .  C o m p a r i n g F i g u r e 4.18a  a n d 4.18e t h e w e a k p r i m a r y a r r i v a l s , d are o b s e r v e d t o e x t e n d a c r o s s b o t h record sect i o n s . E v e n t d is n o t seen o n t h e h o r i z o n t a l c o m p o n e n t , w h i l e e v e n t D is n o t present o n t h e h y d r o p h o n e c o m p o n e n t . F i n a l l y t h e p a r a l l e l n a t u r e o f t h e t w o events suggests a rel a t i o n s h i p b e t w e e n t h e m a n d t h e t r a v e l - t i m e difference b e t w e e n t h e m is c o n s i s t e n t w i t h w a v e c o n v e r s i o n at t h e b a s e m e n t s e d i m e n t b a s e m e n t . T h e s e o b s e r v a t i o n s are d i a g n o s t i c of converted S-wave arrivals.  T h e r e is a larger h o r i z o n t a l c o m p o n e n t of t h e i n c o m i n g  w a v e a n d it is n o t t r a n s m i t t e d t h r o u g h t h e w a t e r . A r e v i e w of t h e l i t e r a t u r e s u p p o r t s t h e p r e m i s e t h a t t h e l a t e r , s t r o n g e r a m p l i t u d e a r r i v a l s c o u l d be c o n v e r t e d phases. C o n v e r t e d S - w a v e a r r i v a l s h a v e b e e n s h o w n t o b e c h a r a c t e r i s t i c of a n u m b e r of m a r i n e s e i s m i c r e f r a c t i o n s u r v e y s ( W h i t e et al., m a n u s c r i p t i n p r e p a r a t i o n  1986; C h e u n g a n d  C l o w e s , 1981; A u a n d C l o w e s , 1984), a l t h o u g h a l l o f these were i n deep w a t e r e n v i r o n m e n t s . W h i t e a n d S t e p h e n s (1980) r e v i e w e d t h e p r o p e r t i e s for shear w a v e c o n v e r s i o n . M o d e c o n v e r s i o n b e t w e e n P a n d S w a v e s o c c u r s w h e n t h e P or S w a v e e n c o u n t e r s a n i n t e r f a c e w i t h large v e l o c i t y c o n t r a s t . In m a r i n e e n v i r o n m e n t s t h e r e are t w o s u c h i n t e r faces, the w a t e r - s e d i m e n t interface a n d the b a s e m e n t - s e d i m e n t interface.  The P to S  103  -24  -22  -20 -18 -16 -14 OBS 4 Hydrophone (5-12)  -24  -22  -20 -18 -16 -14 OBS 4Horizontal (5-12 Hz)  -20 -18 -16 OBS 4Vertical (5-12  -12  -10  -14 Hz)  F,gure 4.18 Comparison of filtered hydrophone (a), horizontal (b), and vertical (c) components for OBS 4. The source/receiver distance is plotted along the horizontal axis. The data is plotted from 1 to 3 seconds m reduced time format. Primary arrival events are labelled with d and secondary P to S converted phases are labelled by D (see text for explanation).  104  conversions at the basement-sediment interface have been observed to yield S-wave arrivals that are much larger than the P-wave arrivals ( Cheung and Clowes, 1981; White et al., manuscript in preparation 1986). Cheung and Clowes used these large amplitude  arrivals to determine the position of the weak P-wave arrivals. White et al. modelled these conversions using 1-D WKBJ synthetic seismograms which included an option for phase conversions (Chapman, 1978). Where conditions enhance the conversion of P to S waves for the basement-sediment interface, they reduce the efficiency for S to P wave conversions at the water-sediment interface (White and Stephens, 1980). The lack of a doubly converted phase (P to S to P) on the hydrophone may be due to such an effect.  4 . 5 . 3 Summary Figure 4.19 shows the velocity cube display for this sub-model along with the geological interpretation for the major units. The Tertiary sediments thin substantially as the Tertiary volcanic basement structure rises. This factor may account for the presence of the P to S wave arrivals seen only on OBS 4. The low velocity unit has been interpreted as Upper Cretaceous sediments. This unit pinches out at 5.0 km MD. This pinch-out was required as there was no evidence for a low velocity layer in the adjoining sub-model for OBS 3-OBS 2. The Alexander Terrane is nearly truncated by the lowermost unidentified unit, for which only the position of its upper surface is even partly constrained by the data.  106  C H A P T E R V DISCUSSION A N D CONCLUSIONS  5.1 Discussion of the Final Composite Model T h e f i n a l v e l o c i t y m o d e l w a s c o m p i l e d f r o m t h e t h r e e s u b - m o d e l s e g m e n t s of C h a p t e r IV  . F i g u r e 5.1 s h o w s t h e c o m p o s i t e m o d e l w i t h a 5:1 v e r t i c a l e x a g g e r a t i o n a n d t h e  same m o d e l w i t h no vertical exaggeration. A legend, relating the various units to the velocities, appears below the m o d e l .  D u r i n g t h e c o m p i l a t i o n of t h e f i n a l m o d e l f r o m  t h e t h r e e s u b m o d e l s , m i n o r d i s c r e p a n c i e s o c c u r r e d at t h e i r c o m m o n b o u n d a r i e s b e n e a t h O B S 2 a n d O B S 3 . T h e composite m o d e l was adjusted to m a k e the s u b - m o d e l b o u n d a r i e s all consistent. T h e various i n d i v i d u a l velocity blocks, shown in Figure 4.1, have been r e p l a c e d b y average v e l o c i t i e s a n d g r a d i e n t s r e p r e s e n t i n g t h e m a j o r u n i t s . T h e i n t e r p r e t a t i o n is n o n - u n i q u e , b u t is based o n a c a r e f u l e v a l u a t i o n of t h e c o m p a r i s o n o f o b s e r v e d a n d t h e o r e t i c a l t r a v e l t i m e s a n d a m p l i t u d e s for r e v e r s e d profiles. S m a l l l a t e r a l v a r i a t i o n s i n v e l o c i t y a n d g r a d i e n t a p p e a r t o be necessary a l t h o u g h t h e i r r e p r e s e n t a t i o n by d i s c r e t e b o u n d a r i e s i n F i g u r e 4.1 is a n a r t i f a c t of t h e m o d e l i n p u t for the a s y m p t o t i c ray theory m o d e l l i n g p r o g r a m . T h e t h i c k e n i n g of u n i t D at a b o u t 30 k m d i s t a n c e is a f e a t u r e i n t r o d u c e d t o a c c o u n t for a m p l i t u d e s a n d a p p a r e n t v e l o c i t i e s of p r i m a r y a n d s e c o n d a r y a r r i v a l s o n b o t h t h e f o r w a r d a n d reverse p r o f i l e s for O B S 2 - O B S 3. T h i s s e e m e d t o be a n e c e s s a r y f e a t u r e of t h e m o d e l as d i d t h e i n t r o d u c t i o n of l a r g e v e l o c i t y g r a d i e n t s (up t o 0.5 k m / s / k m ) . C l o w e s a n d G e n s - L e n a r t o w i c z (1985) used s i m i l i a r g r a d i e n t s for a lower s e d i m e n t a r y u n i t f o r one of t h e i r m o d e l s f r o m Q u e e n C h a r l o t t e S o u n d a n d t h e sonic logs (section 4.3.3) s u p p o r t t h e e x i s t e n c e of large a n d v a r y i n g g r a d i e n t s for t h e s e d i m e n t s . S i m i l i a r l y t h e rise o f u n i t E t o t h e east a n d t h e e x i s t e n c e of a l o w v e l o c i t y z o n e ( u n i t F) b e l o w are f e a t u r e s  0  10  20  30  40  50  60  VE 1:1 OBS 1  OBS 2  f 0  \ 5  10  15  20  OBS 3  OBS 4  Distance (km) f 25  30  35  40  f 45  50  55  60  VE 5:1 VELOCITY (km/s) ; GRADIENT (km/s/km) • IZZi a IZZD  1.49 2.00 2.27 2.72  ; 0.001 A ; 0.250 B ; 0.300 C ; 0.400 D  CEI 4.80 ; 0.500 E 3.50 ; 0.100 F KS 5.90 ; 0.230 G WM 7.70 ; 0.300 H ?  Figure 6.1 Final composite structural velocity model. The model is displayed with no vertical exaggeration (top insert) and a vertical exaggeration of 5:1 . The legend at the bottom of the figure related the symbols for the structural units to the velocities and gradients for those units. Features in this model below 5.0km are either poorly constrained or completely unconstrained (see text for explanation).  108 of t h e m o d e l w h i c h w e r e i n t r o d u c e d t o a c c o u n t for p a r t i c u l a r d a t a c h a r a c t e r i s t i c s . T h e v e l o c i t y for u n i t E w a s m o d e l l e d as 4.8 k m / s w i t h a n average g r a d i e n t of 0.5 k m / s / k m . T h i s g r a d i e n t d e c r e a s e s f r o m l a r g e values i n t h e east (0.7 k m / s / k m ) t o s m a l l e r values (0.2 k m / s / k m )  in the west.  T h e v e l o c i t y for t h e u p p e r s u r f a c e has b e e n i n d i c a t e d as  4.8 k m / s , b u t i n c r e a s e s t o 5.0 k m / s w i t h i n c r e a s i n g d e p t h of b u r i a l . T h e i n t r o d u c t i o n of l o w v e l o c i t y l a y e r s i n t o m o d e l s d e v e l o p e d f r o m s e i s m i c 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 p r e s e n t s t h e i n t e r p r e t e r w i t h a g r e a t e r degree of f r e e d o m i n d e t e r m i n i n g t h e final m o d e l . T h i s arises b e c a u s e l o w v e l o c i t y z o n e s r e p r e s e n t a h i d d e n p r o b l e m for seismic refraction methods.  T h e s a m e d e l a y e d t r a v e l - t i m e c a n be g e n e r a t e d for m a n y  v e l o c i t y a n d t h i c k n e s s c o m b i n a t i o n s . T o a c h i e v e a c o m m o n baseline for t h e c o m p a r i s o n o f these r e s u l t s w i t h o t h e r s a v e l o c i t y for t h e l o w v e l o c i t y l a y e r was c h o s e n t o m a t c h t h a t u s e d b y C l o w e s a n d G e n s - L e n a r t o w i c z (1985) i n t h e i r m o d e l .  B y using similiar  v e l o c i t i e s , t h e c o m p a r i s o n of t h e m o d e l d e s c r i b e d here w i t h t h a t for Q u e e n C h a r l o t t e S o u n d w i l l n o t be d i m i n i s h e d by t h e n o n - u n i q u e n e s s a s s o c i a t e d w i t h t h e i n c l u s i o n o f the low velocity zone. O B S 3 - O B S 4 reverse profile ( F i g u r e 4.16 a n d 4.17) s h o w s a n u n e q u i v o c a l a r r i v a l w i t h a n a p p a r e n t v e l o c i t y o f 6.0 k m / s .  A s t h i s i s t h e o n l y profile w h e r e t h i s phase  e x i s t s , u n i t G is p o o r l y d e f i n e d e l s e w h e r e a n d e s s e n t i a l l y i n f e r r e d . U n i t H is n o t d e f i n e d b e t w e e n 0 a n d 40 k m a n d o n l y p o o r l y d e f i n e d b e y o n d t h a t r a n g e . R e f l e c t i o n s f r o m t h e t o p of t h e l a y e r b e y o n d 40 k m w e r e i n t r o d u c e d t o s a t i s f y p a r t i c u l a r  e v e n t s for O B S  3 - O B S 4 i n t e r p r e t a t i o n b u t t h e u n i t i t s e l f w a s n e v e r s a m p l e d . V e l o c i t i e s w e r e chosen t o p r o d u c e a reflection c o e f f i c i e n t necessary t o g e n e r a t e r e a s o n a b l e a m p l i t u d e s to m a t c h t h e e n e r g y b u r s t d i s c u s s e d i n s e c t i o n 4.5.1 . In s u m m a r y , t h e l o w e r 3 t o 4 k m of t h e m o d e l s h o w n i n F i g u r e 5.1 a r e e i t h e r p o o r l y c o n s t r a i n e d or u n c o n s t r a i n e d .  109  UNIT A  STRATIGRAPHIC  INTERPRETATION Water  B C D  Pleistocene Sediments Pleistocene and/or Pliocene Sediments Tertiary Skonun Sediments  E F G  Tertiary Masset Volcanics Upper Cretaceous Sediments Paleozoic Alexander Terrane  H  Plutons ?  T a b l e 5.1 S u m m a r y of stratigraphic T a b l e 5.1 s u m m a r i z e s t h e s t r a t i g r a p h i c  interpretation.  interpretation  of the various units.  Units  B a n d C c a n b e c o n s i d e r e d c o l l e c t i v e l y , e s p e c i a l l y i n l i g h t o f t h e loss o f a v e l o c i t y c o n t r a s t b e t w e e n 50 k m a n d 58 k m w h e r e t h e l o w e r o n e p i n c h e s o u t . T h e s e have b e e n interpreted  as P l e i s t o c e n e a n d / o r P l i o c e n e s e d i m e n t s l a i d d o w n i n a n e a r s h o r e d e p o -  sitional environment.  T h e m a x i m u m thickness reaches a p p r o x i m a t e l y  1.0 k m b e t w e e n  30 a n d 4 0 k m . T h e v e l o c i t i e s a n d t h e c o m b i n e d t h i c k n e s s o f these u n i t s a r e s i m i l a r t o m o d e l s f r o m Q u e e n C h a r l o t t e S o u n d ( C l o w e s a n d G e n s - L e n a r t o w i c z , 1985). U n i t D has b e e n i n t e r p r e t e d as t h e T e r t i a r y S k o n u n F o r m a t i o n ( S u t h e r l a n d B r o w n , 1968; S h o u l d i e e , 1 9 7 1 , 1973).  T h e s e a r e U p p e r M i o c e n e s e d i m e n t s d e p o s i t e d i n near s h o r e m a r i n e a n d  non-marine environments.  T h e Tertiary sediments represent the thickest  sedimentary  u n i t f o r t h e H e c a t e S t r a i t m o d e l , r e a c h i n g a m a x i m u m t h i c k n e s s of a p p r o x i m a t e l y 3.0 km.  T h e c o m b i n e d m a x i m u m t h i c k n e s s f o r a l l t h e s e d i m e n t a r y u n i t s i s a b o u t 4.0 k m ,  s i m i l i a r t o t h a t d e f i n e d by S h o u l d i e e ( 1 9 7 1 , 1973) f o r H e c a t e s u b - b a s i n a n d C l o w e s a n d G e n s - L e n a r t o w i c z (1985) for C h a r l o t t e s u b - b a s i n . U n i t E has been i n t e r p r e t e d as t h e T e r t i a r y M a s s e t f o r m a t i o n w h i c h c o n s i s t s o f s u b a e r i a l l y e r u p t e d v o l c a n i c s . T h i s u n i t also e x h i b i t s f e a t u r e s o f a b u r i e d e r o s i o n a l surface. Using information  c o l l e c t e d f r o m t h e w e l l s i n H e c a t e S t r a i t by S h o u l d i e e ( 1 9 7 1 , 1973),  110 Y o r a t h a n d C h a s e (1981) suggest t h a t t h i s f o r m a t i o n h a d b e e n p r e v i o u s l y u p l i f t e d a n d e r o d e d b e t w e e n U p p e r a n d L o w e r M i o c e n e t i m e s . T h e T y e e w e l l (see F i g u r e l . l )  showed  n o T e r t i a r y M a s s e t v o l c a n i c s ; h o w e v e r it d i d p e n e t r a t e a h i g h v e l o c i t y m a t e r i a l w h i c h has b e e n i n t e r p r e t e d by S h o u l d i c e (1971, 1973) as a P a l e o z o i c i n t r u s i v e . T h i s is c o n s i s t e n t w i t h t h e m o d e l p r e s e n t e d here since S h o u l d i c e (1973) i n d i c a t e d t h a t e r o s i o n a l c h a n nels h a d r e m o v e d t h e o v e r l y i n g m a t e r i a l .  T e r t i a r y M a s s e t v o l c a n i c s h a v e b e e n logged  i n o t h e r w e l l s a n d h a v e b e e n i n f e r r e d f r o m r e f l e c t i o n s e i s m i c d a t a ( S h o u l d i c e , 1971, 1973).  C l o w e s a n d G e n s - L e n a r t o w i c z (1985) h a v e i n f e r r e d T e r t i a r y v o l c a n i c s b e n e a t h  Q u e e n C h a r l o t t e S o u n d based on a similar refraction survey. These range in thickness b e t w e e n 1.5 a n d 3.5 k m as c o m p a r e d w i t h 0.2 a n d 1.8 k m for t h e v o l c a n i c s b e n e a t h Hecate Strait.  T h e v e l o c i t i e s a s s i g n e d t o t h i s u n i t (4.8-5.0 k m / s ) c o m p a r e f a v o u r a b l y  w i t h t h e 5.2 k m / s v e l o c i t y for t h e T e r t i a r y v o l c a n i c s b e n e a t h Q u e e n C h a r l o t t e S o u n d . T h e i s o p a c h for H e c a t e S t r a i t ( F i g u r e 1.5) s h o w s t h i n n i n g of t h e s e d i m e n t s i n t h e west t o less t h a n 0.1 k m .  T h e i s o p a c h is not w e l l d e f i n e d i n t h i s a r e a a n d t h e r e s u l t s here  w o u l d f a v o u r a n e a s t w a r d s h i f t for t h e b a s i n edge. U n i t F , w h i c h o n l y a p p e a r s b e n e a t h the T e r t i a r y v o l c a n i c s at t h e e a s t e r n end of the m o d e l , was interpreted  as t h e U p p e r C r e t a c e o u s Q u e e n C h a r l o t t e G r o u p of t h e  post suture assemblage ( Y o r a t h a n d C h a s e , 1981). T h e y recognized U p p e r C r e t a c e o u s s e d i m e n t s ( t h e i r p o s t s u t u r e a s s e m b l a g e ) i n t h e T y e e w e l l w h i c h lies close t o t h e a i r g u n / O B S line.  T h e y also o b s e r v e t h a t no T e r t i a r y v o l c a n i c s were p e n e t r a t e d by t h i s  w e l l . T h e p r e s e n t s t u d y d i d not r e q u i r e t h e e x t e n s i o n of t h e low v e l o c i t y z o n e f r o m t h e e a s t e r n e n d t o t h e p o r t i o n of t h e m o d e l near t h e w e l l , b u t t h e v e l o c i t y u n i t i n t e r p r e t e d as t h e T e r t i a r y v o l c a n i c s is r e q u i r e d t o e x t e n d a c r o s s t h e b a s i n . T h e T e r t i a r y v o l c a n i c s , where present in the Hecate s u b - b a s i n , u n c o n f o r m a b l y overly the U p p e r C r e t a c e o u s  Ill s e d i m e n t s ( S u t h e r l a n d B r o w n , 1968; S h o u l d i e e , 1 9 7 1 , 1973) a n d it is n o t u n u s u a l for t h e l a t t e r u n i t t o be a b s e n t . U n i t G h a s b e e n i n t e r p r e t e d as t h e P a l e o z o i c r o c k s of the A l e x a n d e r T e r r a n e w h i c h a r e b e l i e v e d t o u n d e r l y t h e H e c a t e s u b - b a s i n . T h e v e l o c i t y a n d g r a d i e n t for t h i s u n i t w e r e m o d e l l e d as 5.9 k m / s a n d 0.23 k m / s / k m , r e s p e c t i v e l y .  The velocity  compares  f a v o u r a b l y w i t h the v a l u e of 6.0 k m / s f r o m t h e sonic log of the T y e e w e l l (see F i g u r e 4 . 2 ) . C l o w e s a n d G e n s - L e n a r t o w i c z (1985) define a v e l o c i t y of 6.0 k m / s for t h e M e s o z o i c W r a n g e l l i a T e r r a n e b e n e a t h Q u e e n C h a r l o t t e S o u n d . B a s e d u p o n these r e s u l t s , it w o u l d a p p e a r t h a t t h e v e l o c i t i e s for t h e W r a n g e l l i a a n d A l e x a n d e r T e r r a n e s are c o m p a r a b l e . U n i t H has b e e n t e n t a t i v e l y i n t e r p r e t e d as r e p r e s e n t i n g p l u t o n s , b u t as n o t e d e a r l i e r , w h i l e i t s p r e s e n c e is i n d i c a t e d , l i t t l e c a n be i n f e r r e d a b o u t i t s p r o p e r t i e s .  The upper  b o u n d a r y of t h i s u n i t b e y o n d 40 k m a p p e a r s t o h a v e a c o m p l e x s t r u c t u r e w h i c h rises a b r u p t l y a t t h e e a s t e r n e n d of t h i s m o d e l . T h i s is c o n s i s t e n t w i t h the r e s u l t s of g r a v i t y f r o m a profile c o i n c i d e n t w i t h t h e a i r g u n / O B S l i n e ( S t a c e y a n d S t e p h e n s , 1969). T h e i r interpretation  is s h o w n i n F i g u r e 5.2 for c o m p a r i s o n w i t h the gross c h a r a c t e r i s t i c s  of  the velocity m o d e l .  5.2 Conclusion A s e i s m i c s t r u c t u r a l v e l o c i t y m o d e l has b e e n d e v e l o p e d for the H e c a t e s u b - b a s i n . T h i s m o d e l w a s f o u n d to be c o n s i s t e n t w i t h p r e v i o u s s t u d i e s f r o m H e c a t e S t r a i t  and  Q u e e n C h a r l o t t e S o u n d . T h e i n t e r p r e t a t o n is also c o n s i s t e n t w i t h the geology e x p e c t e d b e n e a t h H e c a t e S t r a i t b a s e d o n g e o l o g i c a l s t r u c t u r e s e x t r a p o l a t e d f r o m the Q u e e n C h a r l o t t e I s l a n d s . T h e n a t u r e of t h e s u r f a c e of t h e T e r t i a r y v o l c a n i c s suggests a n erosion al s u r f a c e s i m i l i a r t o t h a t for t h e T e r t i a r y M a s s e t v o l c a n i c s on G r a h a m I s l a n d a n d i n t h e  112 OBSERVED BOUGUER ANOMALY REGIONAL ANOMALY  -301  CALCULATED ANOMALY RESIDUAL ANOMALY  COAST CRYSTALLINE BELT  INSULAR TECTONIC BELT kn  Or  UEST  Fignre 5.2  I  HECATE DEPRESSION  200 km  WO  50  DENSITY CONTRAST RELATIVE TO SURROUNOING ROCKS  -0.5 g / c ^  EAST  0.3 g / c r  Proposed geological structure for Hecate depression based on gravity profile coincident with  airgun/OBS line (after Stacey and Stephens, 1969).  113 S h e l l C a n a d a L t d . w e l l s . T h e e x t r e m e t h i n n i n g of t h e v o l c a n i c s b e l o w a n a r r o w d e p r e s s i o n filled w i t h s e d i m e n t s (30 k m d i s t a n c e o n F i g u r e 5.1) also s u p p o r t s t h i s p r e m i s e . T e r t i a r y v o l c a n i c s are o b s e r v e d on t h e Q u e e n C h a r l o t t e Islands t o overlie C r e t a c e o u s sediments.  unconformably  T h e Cretaceous sediments and the Tertiary  volcanics  m a y a p p e a r t o g e t h e r or w i t h e i t h e r one a b s e n t . T h e r e f o r e the l o w v e l o c i t y C r e t a c e o u s s e d i m e n t s o b s e r v e d t o o c c u r o n l y at the e a s t e r n e n d o f the profile is n o t w i t h available  inconsistent  information.  T h e l o w e r m o s t u n i t c a n o n l y be d e s c r i b e d i n t e r m s of i t s presence as a c o m p l e x r i s i n g s t r u c t u r e at t h e e a s t e r n edge of t h e b a s i n . T h e g r a v i t y i n t e r p r e t a t i o n  (Stacey a n d  S t e p h e n s , 1969) is v e r y s i m i l i a r t o t h e gross s t r u c t u r e for t h e v e l o c i t y m o d e l . T h e g r a v i t y interpretation  y i e l d e d a s i m p l e t w o b l o c k m o d e l for the H e c a t e d e p r e s s i o n . T h e b a s i n  i n f i l l w a s m o d e l l e d w i t h a d e n s i t y c o n t r a s t o f -0.5 g / c m  w h i l e the w e d g e - l i k e b l o c k has  3  b e e n m o d e l l e d as h a v i n g a d e n s i t y c o n t r a s t of +0.3g/cm  3  w i t h the surrounding rocks.  T h e r e s u l t s o f t h i s g r a v i t y s u r v e y c o m b i n e d w i t h those f r o m the r e f r a c t i o n  modelling  m a y be i n d i c a t i n g g e o l o g i c a l f e a t u r e s w h i c h a r r i v e f r o m the c o l l i s i o n of A l e x a n d e r a n d W r a n g e l l i a w i t h t h e c o n t i n e n t a l m a r g i n . O n t h e basis of t h i s s u p p o s i t i o n , t h e l o w e r m o s t u n i t of t h e m o d e l has t e n t a t i v e l y b e e n d e s c r i b e d as p l u t o n s . T h e l o w v e l o c i t y l a y e r m a y be r e p r e s e n t a t i v e of t h e p o s t s u t u r e a s s e m b l a g e of Y o r a t h and Chase  (1981).  T h i s a s s e m b l a g e is d i s c u s s e d i n C h a p t e r I as b e i n g c o m p r i s e d  of U p p e r C r e t a c e o u s s e d i m e n t s of the Q u e e n C h a r l o t t e G r o u p .  T h i s group  contains  g o o d r e s e r v o i r r o c k s , t h e H o n n a F o r m a t i o n , a n d t r a p p i n g m e c h a n i s m s for h y d r o c a r b o n s w i t h i n these u n i t s t h e m s e l v e s a n d also a b o v e t h e m . T h e low v e l o c i t y l a y e r , as d e f i n e d , is l o c a t e d i n a f a v o u r a b l e p o s i t i o n for the a c c u m u l a t i o n of h y d r o c a r b o n s . H o w e v e r , the t h i c k n e s s of t h e o v e r l y i n g T e r t i a r y v o l c a n i c s c o u l d m a k e e x p l o r a t i o n  a costly v e n t u r e .  F u r t h e r m o r e , the t h e r m a l h i s t o r y d o e s not s e e m t o f a v o u r the g e n e r a t i o n of o i l b u t o n l y  114 gas a n d u n f o r t u n a t e l y  no s o u r c e r o c k s are k n o w n t o e x i s t i n t h e P a l e o z o i c A l e x a n d e r  T e r r a n e ( Y o r a t h a n d C a m e r o n , 1982).  A refraction survey from the Charlotte sub-  b a s i n ( C l o w e s a n d G e n s - L e n a r t o w i c z , 1985) also f o u n d e v i d e n c e for a low v e l o c i t y zone i n t e r p r e t e d as u p p e r C r e t a c e o u s s e d i m e n t s . T h e H e c a t e a n d C h a r l o t t e s u b - b a s i n s a p p e a r t o be s i m i l a r i n t h e i r m a k e u p for the upper units.  T h e M a s s e t v o l c a n i c s w h i c h o c c u r i n b o t h a r e a s s u p p o r t the b e l i e f  that this volcanic episode was very widespread.  T h e U p p e r C r e t a c e o u s s e d i m e n t s of  the post-suture assemblage variously appear a n d disappear irregularly t h r o u g h o u t the Q u e e n C h a r l o t t e b a s i n . In g e n e r a l t h e f o r m a t i o n s a p p e a r t o be t h i c k e r i n t h e C h a r l o t t e s u b - b a s i n , south of the present study area. In C h a p t e r III, a n i n d e p e n d e n t s t u d y i n v o l v i n g t h e i n v e r s i o n of r e f r a c t i o n d a t a by t h e m e t h o d of w a v e f i e l d c o n t i n u a t i o n w a s u n d e r t a k e n . A s e g m e n t o f d a t a f r o m t h e a i r g u n / O B S s u r v e y , u n d e r s t u d y as t h e m a j o r p a r t of t h i s t h e s i s , w a s i n v e r t e d t o o b t a i n the one-dimensional velocity-depth structure.  T h e m e t h o d w a s f o u n d t o p r o d u c e rea-  s o n a b l e 1-D v e l o c i t y - d e p t h s t r u c t u r e s for these e x a m p l e s . T h e s e f i n d i n g s w o u l d s u p p o r t t h e a p p l i c a t i o n of t h i s i n v e r s i o n m e t h o d t o o b t a i n i n i t i a l v e l o c i t y v e r s u s d e p t h e s t i m a t e s f o r use b y 2 - D m o d e l l i n g s c h e m e s . F o r r e f r a c t i o n l i n e s a l o n g s t r i k e , w h e r e l a t e r a l hete r o g e n e i t y is less p r o n o u n c e d , t h e r e s u l t i n g v e l o c i t y d e p t h m o d e l s w o u l d m o r e closely represent the subsurface s t r u c t u r e .  115 References Aki, K. and Richards, P.G., 1980,  Quantitative Seismology, Theory and Methods, 1:  W.H. Freeman and Company, San Francisco, 535 pp. Berg, B.C., Jones, D.L. and Coney, P.J.,. 1978, Pre-Cenozoie teetonostratigraphie terranes of southeastern Alaska and adjacent areas, United! States Geological Survey Open File Report 78-1085. Berg, H.C., Jones, D.L. and Riehter, D.H., 1972, Gravina- Nutzotin belt-tectonic significance of an upper Mesozoic sedimentary and volcanic sequence in southern and southeastern Alaska, United States Geological Survey, Professional Paper 800-D, pp. D1-D24. Bessonova, E.N., Fishman, V . M . , Ryaboyi, V.Z., and Sitnikova, G.A., 1974, The Tau method for inversion of traveltimes-I. Deep seismic sounding data, Geophysical Journal of the Royal Astronomical Society 3 6 , pp. 377-398. Bessonova, E.N., Fishman, V.M., Shnirman, M.G., Sitnikova, G.A. and Johnson, L.R., 1976, The Tau method for inversion of traveltimes-I I. Earthquake data, Geophysical Journal of the Royal Astronomical Society, 4 6 , pp. 87-108. Carrion, P.M., Kuo, J.T., and Stoffa, P.L., 1984, Inversion method in the slant stack domain using amplitudes of reflection arrivals, Geophysical Prospecting 3 2 , pp. 375-391 Chapman, C.H., 1978, A new method for computing synthetic seismograms, Geophysical Journal of the Royal Astronomical Society, 5 4 , pp.481-518. Chapman, C.H., 1981, Generalized Radon transform and slant stacks. Geophysical Journal of the Royal Astronomical Society, 6 6 , pp.445-453.  116 Claerbout,  J . F . , 1976,  Fundamentals of Geophysical Data Processing, N e w  York,  M c G r a w - H i l l B o o k C o . Inc., 274 p p . C l a y t o n , R . W . a n d M c M e c h a n , G . A . , 1981, I n v e r s i o n o f r e f r a c t i o n  d a t a b y wave field  c o n t i n u a t i o n , G e o p h y s i c s 46, p p . 8 6 0 - 8 6 8 . C l o w e s , R . M . , 1985, S t u d y o f u p p e r c r u s t a l s t r u c t u r e gun/Ocean Bottom  below Hecate Strait from A i r -  Seismograph data, Final Report, D S S Contract  file  number  0 6 S B . 2 3 4 4 5 - 4 - 1 1 7 0 , 16 p p . C l o w e s , R . M . a n d G e n s - L e n a r t o w i c z , E . , 1985, U p p e r c r u s t a l s t r u c t u r e  of southern  Q u e e n C h a r l o t t e B a s i n f r o m s o n o b u o y r e f r a c t i o n s t u d i e s , C a n a d i a n J o u r n a l of E a r t h Sciences 22, pp.  1696-1710.  D e h l e r , S. A . , 1986, A s e i s m i c r e f r a c t i o n s t u d y o f t h e Q u e e n C h a r l o t t e f a u l t z o n e , M . S c . t h e s i s , U n i v e r s i t y o f B r i t i s h C o l u m b i a , V a n c o u v e r , B . C , 112 p p . G a r m a n y , J . , O r c u t t , J . A . , a n d P a r k e r , R . L . , 1979, T r a v e l t i m e i n v e r s i o n : a g e o m e t r i c a l a p p r o a c h , J o u r n a l o f G e o p h y s i c a l R e s e a r c h , 84, p p 3 6 1 5 - 3 6 2 2 . G a z d a g , J . , 1978, W a v e e q u a t i o n m i g r a t i o n  w i t h t h e phase shift m e t h o d , G e o p h y s i c s ,  43, p p . 1 3 4 2 - 1 3 5 1 . H e f f l e r , P . E . a n d B a r r e t t , D . L . , 1979, O B S d e v e l o p m e n t a t B e d f o r d I n s t i t u e o f O c e a n o g r a p h y , M a r i n e G e o p h y s i c a l R e s e a r c h , 4, p p 2 2 7 - 2 4 5 . H i l l h o u s e , J . W . , 1977, P a l e o m a g n e t i s m o f t h e T r i a s s i c N i k o l a i G r e e n s t o n e , M c C a r t h y q u a d r a n g l e , A l a s k a . C a n a d i a n J o u r n a l o f E a r t h S c i e n c e s , 14, p p . 2578-2598. J o n e s , D . L . , I r w i n , W . P . a n d H i l l h o u s e J . , 1977., W r a n g e l l i a — a d i s p l a c e d t e r r a n e i n northwestern 2575.  North America.  C a n a d i a n J o u r n a l o f E a r t h S c i e n c e s , 15, p p .  2565-  117 M a c k i e , D . , 1985., S u b d u c t i o n b e n e a t h t h e Q u e e n C h a r l o t t e I s l a n d s ? T h e r e s u l t s o f a seismic refraction survey. M . S c . thesis, U n i v e r s i t y of B r i t i s h C o l u m b i a , V a n c o u v e r , B . C . , 130 p p . M c M e c h a n , G . A . a n d W i g g i n s , R . A . , 1972, D e p t h l i m i t s i n b o d y w a v e i n v e r s i o n s , G e o physieali J o u r n a l o f t h e R o y a l A s t r o n o m i c a l S o c i e t y , 283, p p . 4 5 9 - 4 7 3 . M c M e c h a n , G . A . a n d O t t o l i n i , R . , 1980, D i r e c t o b s e r v a t i o n o f a p - r c u r v e i n a s l a n t s t a c k e d w a v e f i e l d , B u l l e t i n o f t h e S e i s m o l o g i e a l S o c i e t y o f A m e r i c a 70, p p . 775-789. P h i n n e y , R . A . , C h o w d h u r y , K . R . a n d F r a s e r , L . N . , 1981, T r a n s f o r m a t i o n  a n d analysis  of r e c o r d s e c t i o n s , J o u r n a l o f G e o p h y s i c a l R e s e a r c h , 86, n o . B I , p p . 359-377. R o n a , P . A . , 1980, G l o b a l p l a t e m o t i o n a n d m i n e r a l r e s o u r c e s . In T h e c o n t i n e n t a l c r u s t and its mineral deposits.  Edited  by D . W . S t r a n g w a y .  G e o l o g i c a l A s s o c i a t i o n of  C a n a d a , Special P a p e r 20, p p . 607-622. S h o u l d i c e , D . H . , 1971, G e o l o g y o f t h e w e s t e r n C a n a d i a n c o n t i n e n t a l shelf.  B u l l e t i n of  the C a n a d i a n A s s o c i a t i o n o f P e t r o l e u m G e o l o g y , 19, p p . 405-436. 1973, W e s t e r n C a n a d i a n c o n t i n e n t a l shelf. Canada.  Edited by R . G . M c C r o s s a n .  In F u t u r e p e t r o l e u m p r o v i n c e s o f  C a n a d i a n Society of Petroleum Geologists,  M e m o i r 1, p p . 7 - 3 5 . S p e n c e , G . D . , W h i t t a l l , K . P . a n d C l o w e s , R . M . , 1984, P r a c t i c a l s y n t h e t i c s e i s m o g r a m s for l a t e r a l l y  varying media calculated  by asymptotic  ray theory.  B u l l e t i n of the  S e i s m o l o g i e a l S o c i e t y o f A m e r i c a , 74, p p . 1 2 0 9 - 1 2 2 3 . S t a c e y , R . A . , 1975, S t r u c t u r e o f t h e Q u e e n C h a r l o t t e B a s i n . m a r g i n s a n d offshore p e t r o l e u m e x p l o r a t i o n .  In C a n a d a ' s c o n t i n e n t a l  Edited by C . J . Y o r a t h , E . R . P a r k e r ,  a n d D . J . G l a s s . C a n a d i a n S o c i e t y o f P e t r o l e u m G e o l o g i s t s , M e m o i r 4 , p p . 723-741.  118 S t a c e y , R . A . a n d S t e p h e n s , L . E . , 1969, A n i n t e r p r e t a t i o n of g r a v i t y m e a s u r e m e n t s o n t h e w e s t c o a s t o f C a n a d a , C a n a d i a n J o u r n a l of E a r t h S c i e n c e s , 6, p p . 463-474. S u t h e r l a n d B r o w n , A . 1968. G e o l o g y o f t h e Q u e e n C h a r l o t t e I s l a n d s . B r i t i s h  Columbia  D e p a r t m e n t o f M i n e s a n d P e t r o l e u m r e s o u r c e s , B u l l e t i n 54, 226 p p ; T i p p e r , H . W . , a m d C a m e r o n , B . E . B . , 1980, S t r a t i g r a p h y  a n d paleontology  of the U p -  per Y u k o n F o r m a t i o n (Jurassic) in Alliford B a y syneline, Queen Charlotte  Islands,  B r i t i s h C o l u m b i a , in C u r r e n t R e s e a r c h , part C . Geological Survey of C a n a d a , P a p e r 8 0 - I C , p p . 37-44. V a n d e r V o o , R . a n d C h a n n e l , J . E . T . , 1980, P a l e o m a g n e t i s m i n oregenic b e l t s . R e v i e w s of G e o p h y s i c s a n d S p a c e P h y s i c s , 1 8 ( 2 ) , p p . 455-481. V a n d e r V o o , R . , J o n e s , M . , G r o m m e , C . S . , I b e r l e i n , G . D . a n d C h u r k e n , J r . , M . , 1980; P a l e o m a g n e t i s m and northward drift of A l e x a n d e r terrane,  southeastern  Alaska.  J o u r n a l o f G e o p h y s i c a l R e s e a r c h , 8 5 , p p . 5281-5296. W e n z e l , F . , S t o f f a , P . L . , a n d B u h l , P . , 1982, S e i s m i c m o d e l l i n g i n t h e d o m a i n of intercept time a n d ray parameter, I E E E Transactions on Acoustics, Speech and Signal Processing, A S S P - 3 0 ( 3 ) ,  p p . 406-422.  Y o r a t h , C . J . a n d C a m e r o n , B . E . B . , 1982, O i l off t h e w e s t c o a s t ? G E O S , 1 1 , p p . 13-15. Y o r a t h , C . J . a n d C h a s e , R . L . , 1981. T e c t o n i c h i s t o r y o f t h e Q u e e n C h a r l o t t e I s l a n d s a n d a d j a c e n t a r e a s — a m o d e l . C a n a d i a n J o u r n a l o f E a r t h S c i e n c e s , 1 8 , p p . 1717-1739. Y o r a t h , C . J . a n d H y n d m a n , R . D . , 1983, S u b s i d e n c e a n d t h e r m a l history  of the Queen  C h a r l o t t e B a s i n . C a n a d i a n J o u r n a l o f E a r t h S c i e n c e s , 2 0 , p p 135-158. Y o u n g , I.F., 1981, G e o l o g i c a l d e v e l o p m e n t o f t h e w e s t e r n m a r g i n o f t h e Q u e e n C h a r l o t t e B a s i n . M . S c . thesis, U n i v e r s i t y of B r i t i s h C o l u m b i a , V a n c o u v e r , B . C .  119  Appendix I  Summary of the Formations for the Queen Charlotte Region  120 EPOCH/ STAGE  PERIOD  QUATERNARY  .RAX I MUM GROUP OR FORMATION  LITHOLOGY  RECENT  Alluviu  P1FIST0CENF  Cao« Ball Formation  T i l l , sand, s i l t , cl4y_.  PLEISTOCENE or PLIOCENE  Tow Hill S i l l s  Olivine basalt  PLIOCENE ? Hl-Ul MIOCENE  Fn.  LOWER' MIOCENE OLIGOCENE »— EOCENE  Masset Fin.  ENVIRONMENT (METERS? Non-marine and  Mixed Mbr. Skldegate Formation  INTRUSIVE ROCKS  near-shore marine  Glacial marine and non-marine  . Calcareous Ss; Sandstone, slltstone] 1800* Conqlomerate: Pyroclastic brecclasTisoQ^ Dana; Facies volcanic Ss,pomhvr> Rhyolite tuffs»flows" ows' 1200+ Kootenay Fades dacite; basalt flows Basalt flows, pyro1500+ Basalt Mbr. clasdics, andesite Rhyolite, ash flows Rhyolite Mbr 2100 basalt flows  PALEOCENE ?  TECTONIC OR  H1CKNES! DEPOSITIONAL  Basalt breccias & flows  Near-shore, marine and! non-marine:  Mantle plume ?' Rifting ?  Post Tectonic Plutons Middle Miocene Upper Eocene, Ridge, jubduction V Divergent wrenching 7  Divergent wrenching ?  2000  Slltstone, sandstone  600+  Shallow marine  Syntectonic Bathollths Lower Cretaceous (?)Upper Jurassic Collision event  Yakoun Fm.  C  B. Mbr.  BAJOCIAN TOARCIAN PLEINSBACHIAN S1NEMURIAN HETTANGIAN NORIAH  •az  OR  POWSYLVAHfAH  Table I'.l  Mbr.  A  Mbr.  Maude Formation  Shale,, sandstone  Black Argil- ArgUllte.sltstone, l i t e Mbr-. shale, L s . , Ss. Kunga Fm.  Black Limestone Mbr. 'Grey Llme. stone Mbr.  :  KARNIAN  Andesitic: agglomerates and tuffs Shale. Ss, tuffs Calcareous anu l a p i l l i tuffs  Karmutsen Formation Sicker Group ?  290 30+  Marine  200 225'  Marine  580  Carbonaceous limestone, argillite  270  Limestone-  ISO  Basalt flows & p i l lows, tuffs, minor 4.300 Limestone, shale, basalt, diabase  Volcanic arc  Marine  Ocean crust ? Volcanic are ? Injterarcbasln  Shallow marine inot exposed ?) Volcanic arc ?  Table of formations for Queen C h a r l o t t e Islands ( Y o u n g , 1981).  121  Appendix  II  Horizontal and Hydrophone Component D a t a for 1983 A i r g u n / O B S S u r v e y  Figures II.I  - 11.12  R e c o r d s e c t i o n s for t h e h o r i z o n t a l a n d h y d r o p h o n e c o m p o n e n t s  f o r O B S s 1 t o 4 p l o t t e d u s i n g t h e s a m e p a r a m e t e r s as those i n C h a p t e r I V (see C h a p t e r I V for  explanation).  Source/Receiver  Distance  Figure II.1  (km)  20  -18  -16  -14  -12  -10  Source/Receiver  -8  Distance  Figure II.2  -6 (km)  -4  -2  0  Figure II.4  26 -24 -22 -20 -18 -16 -14 -12 -10  -8  -6  Source/Receiver Distance (km) Figure  II.6  -4  -2  0  Source/Receiver  Distance  Figure II. 7  (km)  Source/Receiver Figure  Distance II..8  (km)  Source/Receiver  Distance  Figure II.9  (km)  Source/Receiver Distance Figure 11.10  (km)  Source/Receiver Figure 11.11  Distance  (km)  Source/Receiver Distance (km) Figure  11,12  

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