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Marine deep seismic sounding off the Coast of British Columbia 1976

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MARINE DEEP SEISMIC SOUNDING OFF THE COAST OF BRITISH COLUMBIA by S t a n i s l a v Knize Eng. of T e c h n i c a l and Nuclear P h y s i c s Czech T e c h n i c a l U n i v e r s i t y , Prague, 1961 THESIS SUBMITTED IN'PARTIAL FULFILMENT OF THE REQUIREMENTS FOB THE DEGREE OF DOCTOR OF PHILOSOPHY ~1 : i n the Department of Geophysics and Astronomy We accept t h i s t h e s i s as conforming to the r e q u i r e d s t a n d a r d The U n i v e r s i t y Of B r i t i s h Columbia May, 1 9 7 6 @ Stanislav Knfze, 1 9 7 6 In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced d e g r e e a t the 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 , I a g r e e t h a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and S t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by the Head o f my Department or by h i s r e p r e s e n t a t i v e s . It i s u n d e r s t o o d t h a t c o p y i n g or p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . | Department o f . G e o p h y s i c s and Ast ronomy The U n i v e r s i t y , o f B r i t i s h Co lumbia Vancouver 8, Canada Date May 3 1 , 1976 ABSTRACT A marine s e i s m i c system f o r r e c o r d i n g n e a r - v e r t i c a l i n c i d e n c e to wide-angle r e f l e c t e d waves and r e f r a c t e d waves with p e n e t r a t i o n to the bottom of the c r u s t (deep s e i s m i c sounding or DSS) has been developed. In a two s h i p - o p e r a t i o n , s i g n a l s from s i x i n d i v i d u a l hydrophones are recorded i n d i g i t a l form on the r e c e i v i n g s h i p . To p r o v i d e o r i g i n * times and f a c i l i t a t e subsequent p r o c e s s i n g , s i g n a l s from a s i n g l e hydrophone are recorded i n FM mode on the shooting ship. During 1973, DSS p r o f i l e s about 20km i n length were recorded i n th r e e r e g i o n s o f f the west coast of Canada: the Hudson .•70,-survey area, west of the Queen C h a r l o t t e I s l a n d s nea-r : .5 v : i 0 N, 133°K; o f f Queen C h a r l o t t e Sound; and i n Cascadia Basin west of c e n t r a l Vancouver I s l a n d . The recorded data were processed with v a r i o u s d i g i t a l techniques such as autopower spectrum a n a l y s i s , band-pass f i l t e r i n g , d e c o n v o l u t i o n , v e l o c i t y spectrum a n a l y s i s , and s t a c k i n g . A f t e r c o m p i l a t i o n i n record s e c t i o n s , the data were i n t e r p r e t e d i n terms of v e l o c i t y - v e r s u s - d e p t h models of the oceanic c r u s t . Two kinds of models were derived..For the r e f r a c t i o n data, models are based on a t r a v e l t i m e and amplitude i n t e r p r e t a t i o n made by comparing the observed data with s y n t h e t i c seismograms. For the r e f l e c t i o n data, the models are based on a T 2-X 2 a n a l y s i s of s e i s m i c phases. The c r u s t a l models d e r i v e d from the two approaches i n d i c a t e the same b a s i c c r u s t a l l a y e r s , but the r e f l e c t i o n models show d e t a i l e d v e l o c i t y changes w i t h i n these l a y e r s . The models show the complexity of the s t r u c t u r e of the oceanic c r u s t and r e l a t e to r e g i o n a l geology. The c r u s t a l model of the Hudson '70 area shows t h i n sediments over the basement which c o n s i s t s o f e i t h e r two l a y e r s or a l a y e r with a v e l o c i t y g r a d i e n t . The model compares well with the r e s u l t s obtained i n the same area by Keen and B a r r e t t (1971). The model f o r the r e g i o n o f f Queen C h a r l o t t e Sound i n d i c a t e s s i x sedimentary l a y e r s of d i f f e r e n t v e l o c i t i e s , basement at a depth of 2.4 km sub-bottom, and the oce a n i c l a y e r at a.: depth of 4.5 km sub-bottom. V e l o c i t y r e v e r s a l s within;, the sediments p o s s i b l y show the i n f l u e n c e of P l e i s t o c e n e g l a c i a t i o n on the d e p o s i t i o n of sediments underneath the c o n t i n e n t a l s l o p e . The model f o r northern C a s c a d i a Basin shows f o u r l a y e r s w i t h i n the sediments of t h i c k n e s s 1.9 km, a v e l o c i t y t r a n s i t i o n between the sediments and the basement, the basement at a depth of 2.7 km sub-bottom, and the oceanic l a y e r at a depth of 4.2 km sub-bottom. Proposed i n t e r b e d d i n g of v o l c a n i c m a t e r i a l with high v e l o c i t y sediments at the top of the basement c o r r e l a t e s with g e o l o g i c a l formative processes observed a t the c r e s t of the near-by Juan de Fuca Ridge. i i i The r e s u l t s have shown that the marine DSS system i s an e f f i c i e n t technigue f o r d e t a i l e d i n v e s t i g a t i o n of the o c e a n i c c r u s t and i s an i n e x p e n s i v e a l t e r n a t i v e t o the m u l t i c h a n n e l common depth p o i n t techniques used i n the o i l i n d u s t r y . iv Contents 1 INTRODUCTION 1 1.1 Why Deep Seismic Sounding at Sea ................ 1 1.2 Review of the Seismic Work at Sea ............... 4 1.3 Project History and Areas of Recording ,,11 2 DATA ACQUISITION ....................................17 2.1 F i e l d Techniques ....17 2.2 Instrumentation and Procedure ...................19 2.3 DSS P r o f i l e s and Their Locations 27 2.4 Examples of Observed Data 31 3 DATA PROCESSING AND ANALYSIS ........................ 44 3.1 F i e l d Data and Corrections ...................... 44 3.2 Autopower Spectra ............................... 47 3. 3. Band-pass F i l t e r i n g 50 3.3 Deconvolution ................................... 55 3.4 Stacking of Refraction Data ..................... 71 3.5 Velocity Analysis of Reflection Data 76 4 INTERPRETATION 83 4.1 Methods of Interpretation ....................... 83 4.2 Velocity-depth Models ........................... 88 4.3 Discussion of the Results ............130 4.4 Relation to Regional Geology ..,.,,......,,......131 5 CONCLUSIONS 137 V L i s t of F i g u r e s F i g u r e 1.1 L o c a t i o n Map f o r the Areas of Recording .,..14 Figu r e 2.1 Schematic Diagram of the DSS System Using Two Ships ............. 18 F i g u r e 2.2 Schematic Diagram of the Instrumentation on the Shooting Ship ............... 22 F i g u r e 2.3 Schematic Diagram of the Seismic Recording System on the R e c e i v i n g Ship ,...25 Fi g u r e 2.4 D e t a i l e d Maps of the L o c a t i o n s of the Seismic P r o f i l e s ..29 Fi g u r e 2.5 A T y p i c a l Source wavelet ................... 33 Figure .2.6 Example of a Seismic R e f l e c t i o n Trace ......36 Figure- 2.7 Example of F i v e R e f r a c t i o n Traces Recorded Simultaneously .............,,......................39 F i g u r e ; 2 , 8 Example of a Record S e c t i o n from the Expanding P r o f i l e i n AREA 3 ...,.,,.,.,,,,..,..,.,.,41 Fi g u r e 3.1 Amplitude Normalized Aotopower Spectra of C h a r a c t e r i s t i c P a r t s of Seismic Traces ............. 49 F i g u r e 3.2 Examples of the E f f e c t of Various Bandpass F i l t e r s 52 F i g u r e 3.3 E f f e c t s of the S p e c t r a l D i v i s i o n a l Deconvolution on a R e f l e c t i o n Trace ................ 60 Fi g u r e 3.4 C h a r a c t e r i s t i c s of the Shaping Operator f o r v i Spike-Deconvolution ...............................,64 Figure 3.5 Example of the application of Spike Deconvolution to a Reflection Seismogram 66 Figure 3.6 Example of the Application of Deconvolution with Variable Wavelet to Seismograms .69 Figure 3.7 Record Sections of the Unstacked and Stacked Data of the Refraction P r o f i l e 73-1 ................ 73 Figure 3.8 Velocity Spectrum f o r Six Seismic Traces of the Expanding Reflection P r o f i l e 73-5 ..,,80 Figure 4.1 Traveltime-Distance Plot of the Refraction P r o f i l e 73-1 from AREA 1 90 Figure 4.2 Reduced Traveltime-Distance Plot of the Refraction P r o f i l e 73-5 from AREA 3 ................ 94 Figure 4.3 Comparison of the Observed and Synthetic Seismograms of the Refraction P r o f i l e 73-5 .,98 Figure- 4.4 Record Section of the Expanding Reflection P r o f i l e 73-5 from AREA 3 ,,..,.,.,..,,.......,,.,,..101 Figure 4.5 Record Section of the Quasi-Continuous S u b c r i t i c a l Reflection P r o f i l e 73-6 from AREA 3 .,.,103 Figure 4.6 T 2 - X 2 Graph for the Expanding P r o f i l e 73-5 107 Figure 4.7 Velocity-Depth Hodel for AREA 3 109 Figure 4.8 Reduced Traveltime-Distance Plot of the Two Reversed Refraction P r o f i l e s 73-2,3 from AREA 2 .,..113 Figure 4.9 Comparison of the Observed and Synthetic Seismograms of the Refraction P r o f i l e 73-2 ....,.,,.116 v i i F i g u r e 4.11 Record S e c t i o n o f the Expanding R e f l e c t i o n P r o f i l e 7 3-2 from AREA 2 ...........................118 F i g u r e 4.11 T 2-X 2 Graph f o r the Expanding R e f l e c t i o n P x o f x l ^ 7 3** 2 ••••• • ••••••*••* •••• •«••••*•••••• • • 12 H F i g u r e 4.12 V e l o c i t y - D e p t h Model from the Base o f C o n t i n e n t a l Slope i n AREA 2 123 F i g u r e 4.13 Quasi-continuous S u b c r i t i c a l P r o f i l e 73- 4 from the C o n t i n e n t a l Slope i n AREA 2 ............. 126 Fi g u r e 4.14 S t r u c t u r a l Model of Sediments on the C o n t i n e n t a l Slope and I t s C o r r e l a t i o n with a CSP P r o f i l e 128 v i i i ACKNOWLEDGMENTS I would l i k e to thank Dr. R.M. Clowes for i n i t i a t i n g t h i s challenging project. Because we were both previously inexperienced with work at sea, on many occasions the project demanded great endeavour and mutual personal encouragement. Considerable assistance was necessary during the f i e l d operations and for t h i s I sincerely appreciated the cooperation of the o f f i c e r s and crews of CFAV Laymore. Endeavour and St. Anthony, who participated i n the operations during the project. My part i c u l a r thanks go to Captain- M. Dyers and o f f i c e r s W. Frame and A. Reid f o r th e i r personal involvment and friendship. I also appreciated the helpful assistance of my fellow mates, Paul Somerville, Larry Lines, and others from the Department of Geophysics and Astronomy, who shared some of the tough moments at sea with me. The suggestions of Dr. R.A. Wiggins i n the data analysis were constructive and together with the p r a c t i c a l help'of my frie n d Rob Clayton during computer programming, were very much appreciated. During part of t h i s project, the author was supported ix by a Graduate Research Fellowship from the University of B r i t i s h Columbia. Financial support for the project was provided by National Research Council equipment grant E3235 and operating grant A7707, Additional funds were contributed by Mobil O i l Canada Limited. 1 1 INTRODUCTION 1.1 Why. Deep Seismic Sounding at S§a In recent years, increased research e f f o r t s have been made toward studies of the ocean. From a geophysical viewpoint, two p r i n c i p a l areas of investigation have evolved, one of a more economic, the other one of a more academic importance. An area of future economic importance i s the location and recovery of natural resources beneath the sea. It appears possible that within the next decades the a c g u i s i t i o n of minerals and petroleum from the regions of the deep oceans w i l l be feasible and economically p r o f i t a b l e . An example of such a promising area i s the Gulf of Mexico where s a l t domes with petroleum indicators have been found i n deep water regions (Hatkins et al.,1975). The second area of interest concerns the study of tectonic processes and the geological history of the earth. For the development of geotectonic theories, a detailed knowledge of the structure of t e c t o n i c a l l y active areas at sea and of the t r a n s i t i o n zones from the oceans to the continents i s of prime importance. However, even this more 2 academic research pursuit cannot be separated from i t s evident economic aspects as discussed recently by Hammond (1975). Volcanism associated with subduction zones and mid- oceanic ridges cr other active centers such as hot spots are now thought to give r i s e to c h a r a c t e r i s t i c types of ore deposits. The clearest examples are the copper s u l f i d e ore occurring in the Troodos area of Cyprus and porphyry copper ores i n the Andes of South America. Progress toward the solution of such p r a c t i c a l and t h e o r e t i c a l problems reguires an extensive knowledge of the structure and physical c h a r a c t e r i s t i c s of the earth's crust and upper mantle under the oceans. Of the many geophysical techniques a v a i l a b l e , the seismolcgical method provides the most detailed information. In university and governmental research, two standard techniques have been applied: continuous seismic p r o f i l i n g (CSF) and seismic r e f r a c t i o n p r o f i l i n g . The p r i n c i p a l advantage of the CSP method i s i t s excellent resolution due to the high frequency content of the r e f l e c t e d s i g n a l ; the p r i n c i p a l disadvantage i s i t s r e l a t i v e l y shallow penetration due to low energy sources with a high freguency content. The CSP method has been used for more than a decade for obtaining detailed information about the uppermost parts of the crust, mainly the sedimentary layers, on the other hand, the penetration of the seismic refraction method as used at sea i s 3 t h e o r e t i c a l l y unlimited hut i t s resolution i s poorer. The lower frequency content of the seismic signals l i m i t s the precision of the method for distinguishing f i n e structures. Since the signal usually travels long distances, only averaged parameters along the horizontal ray path can be obtained. In addition, velocity gradients and inversions cannot be detected without additional analysis including the use of both traveltimes and amplitudes; even then t h i s can be d i f f i c u l t . In the petroleum exploration industry, another technique of investigation has teen developed. The multichannel common depth point (CDP) procedure has the ca p a b i l i t y of providing good resolution with deep penetration. The cost of the eguipment required for t h i s procedure i s so high that i t s use in university research i s u n r e a l i s t i c . However, i t was believed that a technique which would combine the advantages of the seismic r e f r a c t i o n technique and the industry's multichannel CDP technique might be established on a limited budget. Using the name established by Russian seismologists (Zverev,1967), t h i s compromise technique has been ca l l e d marine 'deep seismic sounding' (DSS), Marine DSS refers to a marine seismic procedure for recording near-vertical incidence to wide-angle r e f l e c t e d and refracted waves with penetration from the ocean bottom 4 to the upper mantle. Higher frequency signals than with the usual r e f r a c t i o n method are recorded at the near distances, thus allowing detailed changes in structure to be distinguished even i n the deeper layers of the crust. At greater distances, the seismic r e f r a c t i o n signals provide average c h a r a c t e r i s t i c s along the p r o f i l e s and are used to check the information obtained from the r e f l e c t i o n data. Several factors played important role i n the decision to i n i t i a t e the marine DSS program within the Department of Geophysics and Astronomy at a.B.C. The dominant ones among these were the necessity for detailed information throughout the oceanic crust and the successful use of the DSS method for detailed c r u s t a l studies on land. As well, the location of the University on the west coast, continuing marine studies of the upper crust by the Department of Geological Sciences and complementary wcrk by the Marine Geosciences Group of the Geological Survey of Canada in Vancouver, a l l contributed to the i n i t i a t i o n of the project. This thesis concerns the establishment of the marine DSS technique , methods of data analysis and interpretation, examples cf interpreted velocity-depth structures i n representative areas off the west coast of Canada, and a discussion of t h e i r geological significance. 5 1«2 fieview of the Seismic Work at Sea During the l a s t decade our knowledge of the structure of the crust and upper mantle under the oceans increased remarkably. Much of the information i s due to the use of new seismic recording technigues at sea and the development of more sophisticated methods of analysing recorded data. The o r i g i n a l seismic model of the oceanic crust was based on the results obtained with well established r e f r a c t i o n technigues (Hill,1952; O f f i c e r et al.,1959; and Shor, 1.963) using unreversed, reversed and s p l i t p r o f i l e recording. These technigues are a r e l i a b l e source of information in normal oceanic regions such as deep basins. The model consists of three layers with a sediment layer at the top, a basement or secondary layer beneath, and an oceanic or t h i r d layer at the bottom. The v e l o c i t i e s and thicknesses of the layers can vary widely. The average model for the P a c i f i c Ocean basin (Shor et a l . , 1970) shows that the velocity for the sediments varies i n the range from 1.9 to 2.5 km/sec and the thickness from 0 to 1.6 km. The basement velocity varies from 4.5 to 5.8 km/sec and i t s thickness from 0.5 to 2.5 km. For the oceanic layer the ve l o c i t y i s i n the range from 6.7 to 7.0 km/sec with varying thickness from 3.3 to 5.9 km. Abnormal oceanic regions such as ridges and continental 6 margins are characterized by a more complex c r u s t a l structure. Several d i f f i c u l t i e s are encountered when one t r i e s to obtain r e l i a b l e data with the use of re f r a c t i o n technigues i n such areas. Interfaces distorted by folding or broken by f a u l t i n g give complex traveltime curves which are undulating or broken into short segments. The most r e l i a b l e seismic a r r i v a l s are the f i r s t ones. For operational reasons i t has been d i f f i c u l t with r e f r a c t i o n technigues using explosives to provide spacing close enough to detect s t r u c t u r a l features or layers that appear as f i r s t a r r i v a l s on the records only over a limited distance i n t e r v a l . Thus thin sedimentary layers or a thin basement lying at greater depth can be d i f f i c u l t to detect (Shor and fiaitt, 1969). For second or l a t e r a r r i v a l s the coincidence of two or more phases makes the analysis d i f f i c u l t , unless there i s an appreciable difference in the c h a r a c t e r i s t i c frequencies. New techniques of marine recording and shooting have solved some of the problems. Recording of variable-angle r e f l e c t i o n data from closely spaced series of shots f i r e d at shorter ranges (H. .Ewing,1963; J. Ewing and Nafe,1963) together with records of v e r t i c a l r e f l e c t i o n p r o f i l e s (Hersey,1963) provide valuable assistance in the analysis and interpretation of the traveltime curves. Such data enable determination of the v e l o c i t i e s and thicknesses of sedimentary layers , a r e s u l t which i s not possible with the 7 CSP method. The v e r t i c a l incidence r e f l e c t i o n s provide apparent dip and topographic corrections for the variable- angle r e f l e c t i o n data (Le Pichon et al.,1968). Reliable measurements of the thickness of basement are obtained by comparing the reflected wave results with the position of the top of the deeper oceanic layer as determined from r e f r a c t i o n measurements (Zverev,1970). Another technique with increased s t r u c t u r a l resolution uses expendable sonobuoys and precision echo-recording equipment (Haynard et al.,1969; Ewing and Houtz, 1969). The shot spacing was decreased s i g n i f i c a n t l y through the use of a r e p e t i t i v e a i r gun as a signal source. With t h i s technigue a previously undetected layer in the deep parts of the oceanic crust was i d e n t i f i e d . This high velocity basalt c r u s t a l layer with an average seismic v e l o c i t y of 7.3 km/sec under the normal oceanic layer was discussed by Maynard (1970) and by Sutton et a l . (1971). Variations i n v e l o c i t i e s and velocity gradients within the oceanic layer and upper mantle have been investigated, p a r t i c u l a r l y by Russian researchers. In th e i r work, which was one of the e a r l i e s t applications of the DSS technique for t h i s purpose, they used both the traveltime and amplitude information from long expanding p r o f i l e s with dense explosion spacing. In such a study near the Southern Kurile Islands near Kamchatka, running p r o f i l e s up to the 8 distance of 160 km with shot spacing of about 6 km, they were able to i d e n t i f y v e l o c i t i e s of 8.6 to 9.0 km/sec at a depth of 12 km below the Mohorovicic (M-) discontinuity (Zverev,1970). With the r e a l i z a t i o n of the greater complexity of the crust and upper mantle under the oceans, new shooting procedures and instrumentation for the study of s p e c i f i c problems i n the investigation of seismic structures have been developed. For example, orthogonal and ring surveys bave been conducted to detect lower crust and upper mantle anisotropy (Eaitt et al.,1971; Keen and Barrett,1971; Whitmarsh,1971). In instrumentation, ocean bottom seismometers with explosion and a i r gun sources to detect both compressional and shear wave v e l o c i t i e s d i r e c t l y have been tested (Hussong et al.,1969; Francis and Porter,1973; Carmichael et al.,1973; L i s t e r and Lewis,1974; Prothe.ro, 1974; Qrcutt et a l . , 1975, 1976) . The comparison of t h e i r exact values with the r e s u l t s of laboratory tests made on rock samples from deep sea d r i l l i n g would greatly help i n the interpretation of oceanic structure i n terms of petrology. Attempts to investigate detailed structure of the deep crust and upper mantle with the use of the ne a r - v e r t i c a l incidence r e f l e c t i o n technigue have been reported during the l a s t decade. In a two ship operation in the Skagerrak area 9 north of Denmark, v e r t i c a l incidence r e f l e c t i o n s from the M- discontinuity at a depth of 30 km below the sea surface were observed (Aric,1968). The results were compared with previous r e f r a c t i o n observations and exhibited good agreement. A v e r t i c a l incidence p r o f i l e recorded at the Great Meteor Bank near the Canary Islands (Aric et al.,1970) indicated a r r i v a l s from a depth corresponding to 10 sec (two-way) traveltime. The results when compared with r e f r a c t i o n data recorded with geophones located on the sea bottom at depths between 300 and 800 m showed good agreement. Perkins (1970) observed near-vertical incidence r e f l e c t i o n s from the deep parts of the crust north of Puerto .Rico. Data were recorded in a single ship operation with the use of sonobuoys and 0.8 kg charges as a source of energy. Stacked data show r e f l e c t i o n a r r i v a l s from within the basement, oceanic layer and possibly from the M- discontinuity; Some spec i a l applications of the multichannel CDP procedure with the use of r e p e t i t i v e sources to obtain deep cr u s t a l and upper mantle r e f l e c t i o n s have recently been r e a l i z e d . Limond et a l . (1972) reported mantle r e f l e c t i o n s at n e a r - v e r t i c a l incidence i n the Bay of Biscay north of Spain. Threefold stacked data indicate v e r t i c a l incidence a r r i v a l s of 9.2 to 9,4 sec (two-way) traveltime. The r e s u l t s agreed well with wide-angle r e f l e c t i o n and r e f r a c t i o n data 10 p r e v i o u s l y r e c o r d e d i n the a r e a , Watkins et a l . (1975), u s i n g a s i m i l a r method, r e c o r d e d deep r e f l e c t i o n s i n the G u l f o f Mexico. T h e i r m u l t i f o l d data r e c o r d e d d i g i t a l l y c l e a r l y show a deep r e f l e c t o r ( p o s s i b l y upper mantle) a t 6.5 sec (two-way) t r a v e l t i m e a f t e r the f i r s t water bottom r e f l e c t i o n . I n t h e i n t e r p r e t a t i o n o f marine r e f r a c t i o n d a t a , t h e u s u a l method used was t h e s l o p e - i n t e r c e p t method d e s c r i b e d by J . Ewing (1963). I t i s r e l a t i v e l y s t r a i g h t f o r w a r d , but such d e v i a t i o n s as v e l o c i t y g r a d i e n t s and i n v e r s i o n s were n o t u s u a l l y i n c o r p o r a t e d i n t h e i n t e r p r e t e d s e i s m i c s t r u c t u r e s . Thus i t can l e a d t o major e r r o r s , p a r t i c u l a r l y i n the d e t e r m i n a t i o n of l a y e r t h i c k n e s s e s . As new r e c o r d i n g procedures and methods were a p p l i e d new t e c h n i q u e s c f data a n a l y s i s were developed t o o b t a i n more i n f o r m a t i o n from r e c o r d e d d a t a . For example, a method u s i n g t h e a m p l i t u d e and waveform i n f o r m a t i o n developed by Helmberger (1968), and by Helmberger and M o r r i s (1969,1970) has e n a b l e d i d e n t i f i c a t i o n o f v e l o c i t y g r a d i e n t s and i n v e r s i o n s from r e f r a c t i o n d a t a . O r c u t t e t a l . (1975,1976), a n a l y z i n g s e i s m i c r e f r a c t i o n p r o f i l e s r e c o r d e d d i g i t a l l y by an ocean bottom seismometer on t h e E a s t P a c i f i c R i s e , show th e o c c u r r e n c e o f a low v e l o c i t y l a y e r o f 4.8 km/sec u n d e r l y i n g a h i g h v e l o c i t y l a y e r of 6.7 km/sec. I n t h e a n a l y s i s of the seismograms both t r a v e l t i m e and a m p l i t u d e 11 studies were used. Recent detailed studies indicate that the model of the oceanic crust i s much more complex than was o r i g i n a l l y assumed. Successfully recorded s u b c r i t i c a l r e f l e c t i o n s from the deep parts of the crust suggest that the method of r e f l e c t i o n seismology should be developed further and used to a greater extent as one of the p r i n c i p a l means for detailed investigation of the oceanic crust. 1.3 Project History and Areas of Recording In 1971# the marine deep seismic sounding research program was i n i t i a t e d at the Department of Geophysics and Astronomy, University of B r i t i s h Columbia. The purpose of the project was to: 1) develop and test the instrumentation i n the laboratory and at sea; 2) use the technigue for recording in t e c t o n i c a l l y and economically interesting areas off Canada's west coast; 3) apply data processing technigues such as bandpass f i l t e r i n g , stacking and deconvolution to the d i g i t a l l y recorded data to improve the quality of the seismograms; 12 4) a n a l y s e the r e c o r d e d data i n terms of v e l o c i t y s t r u c t u r e ; 5) i n t e r p r e t t h e s t r u c t u r e t o g i v e a g e o l o g i c a l u n d e r s t a n d i n g ; and t h u s 6) determine the f e a s i b i l t y of t h e t e c h n i g u e f o r d e t a i l e d s e i s m i c s t u d i e s of t h e o c e a n i c c r u s t . Two b a s i c f a c t o r s , a l i m i t e d budget, and a v a i l a b i l i t y o f two s h i p s f o r t h e o p e r a t i o n , s t r o n g l y i n f l u e n c e d t h e d e c i s i o n s c o n c e r n i n g the marine t e c h n i q u e t o be used. A d e s i g n s i m i l a r i n concept to t h a t used by S c r i p p s I n s t i t u t e of Oceanography and d e s c r i b e d by Shor (1963) was chosen. However, the data a c g u i s i t i o n was improved w i t h m u l t i p l e s e n s o r s and d i g i t a l r e c o r d i n g . S e c t i o n 2.1 o f t h i s t h e s i s d e s c r i b e s t h e system i n d e t a i l . D u r i n g the f i r s t h a l f of 1971, the i n s t r u m e n t a t i o n was assembled and t e s t e d i n the l a b o r a t o r y . The f i r s t t e s t o f r e c o r d i n g w i t h t h e designed t e c h n i g u e a t sea i n J u l y 1971 was u n s u c c e s s f u l . Any p o s s i b l e s e i s m i c i n f o r m a t i o n was l o s t due to h i g h n o i s e background caused by water wave motion. A f t e r a few changes i n the s e i s m i c d a t a a c g u i s i t i o n system t o compensate f o r t h i s , a n o t h e r c r u i s e was u ndertaken i n November 1971. D u r i n g an experiment c a r r i e d out 150 km west of Los Angeles an e xpanding s e i s m i c p r o f i l e was s u c c e s s f u l l y r e c o r d e d . The sea o p e r a t i o n had t o be suddenly t e r m i n a t e d due t o a s e r i o u s a c c i d e n t on one of the s h i p s . I n t h i s 13 experiment analog FM magnetic tape r e c o r d i n g was used. E l e c t r i c a l f i r i n g of charges enabled r e c o r d i n g of the exact s h o t - i n s t a n t s . The disadvantage of t h i s shooting procedure was t h a t i t s i g n i f i c a n t l y slowed the o p e r a t i o n . Much of the recorded data showed a low s i g n a l / n o i s e r a t i o , although some i n d i c a t i o n of deep r e f l e c t i o n s was noted. However,the timing s i g n a l was o f t e n unreadable because of poor r a d i o r e c e p t i o n . For these reasons the i n t e r p r e t a t i o n of the data was not c o n s i d e r e d . However, a r e c o r d s e c t i o n of the s u b c r i t i c a l r e f l e c t i o n data was made t o a s c e r t a i n i t s o v e r a l l g u a l i t y and provided enough encouragement t o continue the development. In 1972, no s h i p time was a v a i l a b l e . In the meantime the method of tape r e c o r d i n g was changed from analog FM to m u l t i c h a n n e l d i g i t a l . C o n c u r r e n t l y , the hydrophone suspension was improved by the i n t r o d u c t i o n of a d d i t i o n a l mechanical damping a g a i n s t the p u l l of sea waves. • In the summer of 1973, the f i n a l sea experiment was planned f o r three areas o f f the c o a s t of B r i t i s h Columbia ( F i g . , 1 . 1 ) . AREA 1 i s l o c a t e d west of the southern t i p of the Queen C h a r l o t t e I s l a n d s , I t was the area of the 1 HUDSON 70* experiment where the a n i s o t r o p y of the upper mantle was i n v e s t i g a t e d by Keen and B a r r e t t (1971). By choosing t h i s area the r e s u l t s of our r e c o r d i n g could be compared with the s e i s m i c model determined from p r e v i o u s r e f r a c t i o n work., AREA 2 l i e s at the entrance t o the Queen C h a r l o t t e Figure 1.1 Location Map Fox the Areas of DSS Recording Area 1: 510 2n* - 510 32« 1 3 3 0 30' - 1330 53 • « Area 2 : Sfjo 58 • - 510 1 2 ' N 1300 Oii« _ 130O 2 1 ' I Area 3 : 4 8 0 31 • - 4 8 0 3 4 ' N 1270 2 1 « - 1270 25' B The boxes outline the regions which are shown i n more d e t a i l i n Figs. 2.4a and 2.4b. The numbers 74 and 75 designate p r o f i l e l i n e s for recording i n 1974 and 1975. Map after Chace et al.,1970. Contours are in fathoms.  16 Sound and i n c l u d e s a p a r t of the c o n t i n e n t a l s l o p e . I t i s a t e c t o n i c a l l y complex a r e a . I n such a r e g i o n , i n f o r m a t i o n about th e deeper s t r u c t u r e s would a s s i s t i n t h e u n d e r s t a n d i n g of the o c e a n - c o n t i n e n t t r a n s i t i o n zone and the t e c t o n i c h i s t o r y o f the r e g i o n . Deep s e i s m i c data from t h i s a r e a would a l s o complement the CSP work on t h e c o n t i n e n t a l s h e i f and s l o p e b e i n g done,by marine g e o s c i e n t i s t s of the G e o l o g i c a l Survey and O.E.C. AREA 3 i s l o c a t e d i n the n o r t h e r n C a s c a d i a B a s i n west of the s o u t h e r n p a r t of Vancouver I s l a n d . T h i s r e g i o n i s a deep s e d i m e n t a r y b a s i n where p r e v i o u s CSP p r o f i l e s d i d not show s u f f i c i e n t p e n e t r a t i o n to reach t h e basement. Because o f the t h i c k sediments and t h e r e f o r e a l a c k of i n f o r m a t i o n about t h e depth and form of basement, some i n t e r e s t i n t h i s r e g i o n had been i n d i c a t e d by the o i l i n d u s t r y and by the G e o l o g i c a l Survey of Canada. I n a d d i t i o n to p r o v i d i n g d a t a o f i n t e r e s t to o t h e r groups, the r e g i o n would be a good area f o r t e s t i n g the p e n e t r a t i o n and r e s o l u t i o n o f t h e DSS t e c h n i g u e . A l s o , d e t e r m i n a t i o n of the v e l o c i t i e s of t h e s e d i m e n t a r y l a y e r s would enable an e s t i m a t e o f t h e i r ^ t h i c k n e s s e s . I n J u l y 1973, s e i s m i c p r o f i l e s as i n d i c a t e d i n F i g . 1.1 were s u c c e s s f u l l y r e c o r d e d . The data a c g u i s i t i o n , and t h e i r a n a l y s i s and i n t e r p r e t a t i o n , are p r e s e n t e d i n the remainder o f . t h e t h e s i s . 17 2 DATA ACQUISITION 2 .1 F i e l d Technigues The DSS t e c h n i g u e which was developed r e g u i r e s two s h i p s . During t h i s p r o j e c t CFAV ENDEAVOUR s e r v e d as the r e c e i v i n g v e s s e l and CFAV LAYM0SE as the s h o o t i n g v e s s e l . F i g . 2 .1 i l l u s t r a t e s t h e g e n e r a l f e a t u r e s o f t h e f i e l d t e c h n i g u e . D u r i n g a p r o f i l e run t h e r e c e i v i n g s h i p d r i f t s f r e e l y w h i l e t r a i l i n g t he n e u t r a l l y buoyant main c a b l e . S i x i n d i v i d u a l hydrophone systems suspended from t h i s c a b l e were used f o r r e c o r d i n g t h e d i r e c t water wave and s e i s m i c s i g n a l s . ' The s h o o t i n g s h i p proceeds along a chosen c o u r s e and d e t o n a t e s c h a r g e s a t p r e d e t e r m i n e d d i s t a n c e s ; The l o c a t i o n s of the p r o f i l e l i n e s were determined w i t h an a c c u r a c y o f about 2 km from Loran A f i x e s and checked every 2 km. S h i p - t o - s h i p d i s t a n c e s were determined by r a d a r r e a d i n g s on both s h i p s . Hater depths were r e c o r d e d w i t h echo-sounding systems o p e r a t i n g c o n t i n u o u s l y on both s h i p s d u r i n g the p r o f i l e r u n . 18 Figure 2.1 Schematic Diagram of the DSS System Osing Two Ships Top - The two ship procedure with dir e c t water wave, ref l e c t e d and refracted wave ray paths indicated. Bottom - Sketch of one of the s i x hydrophone systems suspended from the main cable. The battery box contains >i 6-volt Gel-Cel rechargeable batteries to provide power for the preamplifiers. Flotation i s attached to make the 15 m cables with hydrophones neutrally buoyant at depth. The S-shapes of these cables plus the shock cord minimize the e f f e c t s of the surface sea waves.  20 2.2 Instrumentation and Procedure SHOOTING SHIP As a source of seismic energy, Nitrone SM Super commercial explosive was used. This blasting agent gives high seismic energy per unit and was developed s p e c i f i c a l l y for marine seismic shooting. It was used in the form of 0.45 kg cans which were easy to handle on the ship. Charges up to 8.1 kg were assembled for shots at the greatest distances. The shooting procedure was r e l a t i v e l y simple and similar to that outlined by Shor (1963). Charges were prepared for detonation with a timed-fuse/Seismocap assembly and Primacord (Malecek, 1976). After the charge was fastened to a l i n e which had one or more balloons t i e d to the end of i t , the fuse was l i t and the charge was dropped overboard. A four minute fuse allowed time for the charge to sink to the reguired depth and gave the shooting ship time to s a i l a safe distance ahead of the shot. After the shot was detonated, the distance of the balloon from the ship was estimated by visual sighting. The optimum depth for detonating charges with respect to the minimum loss of energy depends on the charge size 21 {.Haiti, 1952) , and for TNT i s given by the empirical formula where H i s the depth (m) of the charge from the sea surface, C 0 i s the sound velocity (km/sec) i n sea water, and 8 i s the weight (kg) of the charge. This formula i s based on a curve r e l a t i n g freguency of bubble o s c i l l a t i o n s to charge weight and the c r i t e r i a that the detonation depth should equal a guarter of the wavelength for the bubble pulse freguency. For the Nitrone SM Super charges (60% TNT) of sizes from 0.45 to 8.1 kg the formula gives the optimum depths i n the range of 30 to 52 m. A preliminary testing at sea confirmed these values, and i t was decided to shoot a l l charges at a uniform depth of 45 m. In order to time the dire c t wave a r r i v a l s , each detonation was received by a single hydrophone located 30m behind the shooting ship. As a back up for the hydrophone, a geophone was placed on the deck of the ship. These signals, together with the WBVE time code s i g n a l , were recorded on an FM tape recorder for subseguent playback. A two channel chart recorder was used to monitor signals being recorded. The instrumentation on the shooting ship i s i l l u s t r a t e d schematically i n Fig. 2.2. (2.2-1) 22 Figure 2 .2 Schematic Diagram of the Ins t r a i S J i t a t ion on the Shooting. Ship SHOOTING SHIP INSTRUMENTATION WWVB t ime code rece iver DEVELCO BRUSH chart recorder W A V E F O R M hydrophone b a c k - u p ' g e o p h o n e on ship deck A M P E X F M 7-channel taperecorder GOULD - CLEVITE CH-24 hydrophone BRUSH - 220 char t r e c o r d e r 24 RECEIVING SHIP During the running of a p r o f i l e , the receiving ship d r i f t e d freely with engines stopped and the main cable stretched behind. The effects of the motion (due to sea waves at the surface) of the main cable on the hydrophone systems were reduced by means cf rubber shock cords extending from the main cable to the battery cases. Additional mechanical damping of the movement of the hydrophones due'to water disturbances was effected by attaching s u f f i c i e n t f l o t a t i o n to the hydrophones and 15 m cables such that they were approximately neutrally buoyant at depth (Fig.2;1,lower part). The instrumentation used on the receiving ship i s shewn in F i g . 2.3. Pressure waves incident on a pi e z o e l e c t r i c c r y s t a l i n the CH-2A hydrophone produce an e l e c t r i c a l signal which i s preamplified and transmitted to an amplifier system aboard ship. In the l a t t e r , the sig n a l i s bandpass f i l t e r e d between 0.8 and 100 Hz, and then amplified with the gain set manually for each shot. The outputs from the six amplifiers plus the HWVB time code signal are recorded on magnetic tape with an IBM-compatible, 14 b i t , multichannel d i g i t a l a c g u i s i t i o n system. Five seismic data channels and the WHVB time signal are monitored on a six-channel chart recorder. This enables one to check data guality and make decisions 25 Figure 2.3 Schematic Diagram of the Seismic Recording Sistem on the Receiving Ship SEISMIC RECORDING S Y S T E M 6 C L E V I T E hydrophone systems 6 G E O T E C H A S 3 3 0 amp l i f i e rs .0.8-100 Hz B R U S H chart recorder W W V B r e c e i v e r A N A L O G I C A / D converter OCEANIC HYDROPHONE SYSTEM K E N N E D Y formatter & digit, recorder C H - 2 A s e n s o r GOULD CE-25L preamplif ier ( 2 0 d b , 2 Hz - 40 kHz ) A N A L O G I C SERIES AN - 5 8 0 0 14 bit conversion system : A N 2814 digital convertor & 16 channel mult iplexor KENNEDY MODEL 8 / 0 8 synchr . digital taperecoraer ( 9 track IBM compat ib le ) & BUFFER FORMATTER 8208 ( sampling interval AOiisec ) 27 concerning changes in gain settings of the amplifiers or changes in charge sizes with increasing distance between ships. 2.3 DSS P r o f i l e s and Their Locations Two kinds of seismic l i n e s were recorded: expanding p r o f i l e s and n e a r - v e r t i c a l incidence p r o f i l e s . For the former, the shooting ship detonated charges at increasing distances from the receiving ship. Shot spacing was 0.2 km for small charges (0.9 kg) at distances where ne a r - v e r t i c a l incidence r e f l e c t i o n s were anticipated, 0.5 km f o r intermediate charges (1.4-2.3 kg) where wide-angle r e f l e c t i o n s were recorded, and approximately 1 km for the large charges (3.2-8. 1 kg) used i n r e f r a c t i o n recording. For the near-vertical incidence p r o f i l e s the shooting ship detonated 0.9 kg (2 lb) charges at 1 . 1 , 0.9, 0.7 and 0.5 km behind the stationary receiving ship. After each set of four detonations the receiving ship moved along the recording l i n e and the procedure was repeated. In AREA 1 , a single expanding p r o f i l e 20 km long was recorded. Because of the d i r e c t i o n of d r i f t of the receiving ship and the desire to have the shots detonated in l i n e with the hydrophone cable, the p r o f i l e was run obligue to the 28 r e f r a c t i o n l i n e s of the * HUDSON 70' experiment as shown i n Fig. 2.4a,upper. In ASEA 2, two p a r a l l e l reversed p r o f i l e s of the lengths 18 and 15 km respectively were recorded. They were run along the base of the continental slope to determine the velocity structure and possible sloping of layers i n the area. As well, a single guasi-continuous near- v e r t i c a l incidence p r o f i l e , 36 km long, was recorded across the expanding p r o f i l e s in order to follow stratigraphic changes of the structure up the continental slope (Fig.2.4b), In ABEA 3 two p r o f i l e s were recorded. An expanding p r o f i l e over the sedimentary basin near the continental slope was run to determine layer v e l o c i t i e s , and a short near-vertical incidence p r o f i l e perpendicular to i t (Fig. 2.4a, lower). The l a t t e r one was recorded i n order to investigate uniformity of the l o c a l stratigraphy. 2.4 Examples of Observed Data During the 1973 cruise, source wavelets and seismograms from 240 shots were recorded. The guality of the acquired data varied depending upon the physical conditions for recording. 29 Figure 2.4 Detailed Majjs of the Locations of the Seismic P r o f i l e s a) Top - The seismic p r o f i l e s of recording i n AREA 1. The s o l i d l i n e designated with 73-1 i s the expanding profile.,The dotted l i n e s are refr a c t i o n p r o f i l e s from HUDSON '70 experiment. D r i f t of the receiving ship i s shown by the s o l i d dots. Bottom - The seismic p r o f i l e s of recording from AREA 3.,Number 5 designates the expanding p r o f i l e , number 6 the near-vertical incidence p r o f i l e . b) The seismic p r o f i l e s of recording i n AREA 2. Number 2 and 3 designate the reversed expanding p r o f i l e s , number 4 the near-vertical incidence p r o f i l e . Line 37 i s CSP p r o f i l e recorded by the Geological Survey of Canada i n 1975. (The map sections are parts of the Bathymetric Map of the Continental Margin of Western Canada, T i f f i n and Seeman,1975. Contours are i n meters.)  31 32 SHOOTING SHIP An example of a t y p i c a l s o u r c e s i g n a t u r e showing the seguence o f bubble p u l s e s i s g i v e n i n F i g . 2.5. T h i s s i g n a t u r e was r e c o r d e d d u r i n g t h e 1971 c r u i s e when f i r i n g was done e l e c t r i c a l l y and e x a c t s h o t i n s t a n t s were known. As shown i n the diagram, the d i r e c t wave from t h e e x p l o s i o n a r r i v e d 6U msec a f t e r t h e shot i n s t a n t . The second l a r g e peak o f o p p o s i t e p o l a r i t y and d e l a y e d by 7 msec, f o l l o w e d i m m e d i a t e l y by h i g h f r e g u e n c y r e v e r b e r a t i o n s of d e c a y i n g a m p l i t u d e s , c o r r e s p o n d s t o t h e wave from the e x p l o s i o n which was r e f l e c t e d i n t o the hydrophone from t h e sea s u r f a c e . A f t e r an e x p l o s i o n , a gas bubble r a p i d l y forms and expands u n t i l i t r e a c h e s i t s maximum d i a m e t e r , depending on t h e charge s i z e and the ambient p r e s s u r e , and b e g i n s c o l l a p s i n g (Kramer e t a l . , 1 9 6 8 ) . Hhen i t r e a c h e s i t s minimum d i a m e t e r and s t a r t s expanding a g a i n i t sends a s i g n a l . The p e r i o d (sec) of the f i r s t bubble c y c l e i s g i v e n by an e m p i r i c a l f o r m u l a T B = 2.I W (H + I 0 ) (2.4-1) where 8 i s the charge weight (kg) of TNT, and H i s t h e depth (m) of the charge d e t o n a t i o n ( W i e l a n d t , 1 9 7 5 ) . The f i r s t bubble p u l s e i s observed i n the f i g u r e 133 msec a f t e r t h e 33 Figure 2,5 A Typical Source Signature The top trace: a source waveform followed by a sequence of bubble pulses. The bottom trace: shot instant signature recorded in e l e c t r i c a l charge f i r i n g i n 1.971. (Both traces are recorded simultaneously on a two channel chart recorder.) B U B B L E P U L S E \fh> 64 — 133 V S H O T TIME B R E A K T R A I N charge 10 lb i i i t I I 7 7 7 97 73 > h 65 »1 msec dep th : 45 m distance : 90 m 35 d i r e c t wave a r r i v a l . The cycle of expansion and collapse repeats a few times. However i n each of the subsequent periods, the difference between the maximum and minimum diameter i s smaller due to the energy loss i n the gas bubble. The bubble pulsates faster and faster, and the signal amplitude decreases rapidly u n t i l the gas bubble, migrating upwards, reaches the water surface, and the remaining energy'is released into the a i r (Kramer et al.,1968). A whole t r a i n of such events can be observed in Fig. 2 . 5 . RECEIVING SHIP As an example of the guality of the near-vertical incidence r e f l e c t i o n seismograms, a single trace together with the WWVB time code signal i s shown in F i g . 2 . 6 . I t can be seen that the signal/noise r a t i o i s very good. The dir e c t water wave arrives f i r s t ; the complexity of the waveform i s due to bubble o s c i l l a t i o n s . The water bottom r e f l e c t i o n a r r i v e s a l i t t l e more than 1 sec l a t e r . It i s followed by a high frequency t r a i n of large amplitudes which correspond to r e f l e c t i o n s from within the sediments. Any r e f l e c t i o n s from beneath the sediments would arrive after second 3 ; they are d i f f i c u l t to i d e n t i f y from a single trace record. Toward the end of the trace, f i r s t order multiple r e f l e c t i o n s from the 36 Figure 2.6 Example of a Seismic Reflection Trace The trace was recorded at the distance of 2.5 km from the detonation i n water of depth 2.1 km. The f i r s t a r r i v a l i s the d i r e c t water wave, the second major wave t r a i n i s composed of r e f l e c t i o n s from the water bottom and beneath, and the t h i r d t r a i n (after second 4) i s due to f i r s t - o r d e r multiple r e f l e c t i o n s . (The trace below the seismic l i n e i s the WHVB time code signal.) 37 38 sea bottom and beneath give the t r a i n of reverberations which give r i s e to a sudden increase in amplitude. Fig . 2.7 shows a section of f i v e r e f r a c t i o n traces recorded simultaneously for one shot. The signal/noise r a t i o varies very l i t t l e with different channels, A few seismic events can be c l e a r l y correlated over a l l the traces. In Fig. 2.8 a complete record section from the expanding p r o f i l e i n ABEA 3 i s shown to demonstrate the quality of seismic data which has been obtained. Fig 2.8a shows mainly the r e f l e c t i o n part of the p r o f i l e while Fig. 2.8b i l l u s t r a t e s the re f r a c t i o n part. Amplitudes of r e f l e c t e d waves from the upper sedimentary layers (Fig. 2.8a) are overloaded as a resu l t of amplifier gain settings chosen to enhance r e f l e c t i o n s from deeper horizons. Coherent r e f l e c t i o n a r r i v a l s for at least 2 sec after the bottom r e f l e c t i o n are c l e a r l y observed on the recorded data. The f i r s t order multiple r e f l e c t i o n s from the bottom and beneath are obvious at a time of about 7 sec and l a t e r . At distances from 7 to 10 km (Fig. 2.8a) the t r a n s i t i o n from r e f l e c t i o n a r r i v a l s to re f r a c t i o n a r r i v a l s i s c l e a r l y shown for a strong r e f l e c t o r as indicated i n the figure. The l a t t e r extend to nearly 22 km (Fig. 2*8b), but the quality of the data deteriorates due to the use of small charges and to poorer physical conditions at sea. 39 Figure 2.7 Example of Five Refraction 2rac.es Recorded Simultaneously Charge of 6.8 kg detonated at the distance of 18 km. Darkened wavelets indicate the c o r r e l a t i o n of possible seismic a r r i v a l s . The d i r e c t water wave and f i r s t bottom r e f l e c t i o n s can be i d e n t i f i e d c l e a r l y by t h e i r high frequency content toward the end of each trace. S E I S M I C R E F R A C T I O N T R A C E S 1 sec W W V B 41 Figure 2.8 Example of a Becord Section from the Expanding P r o f i l e _ i n ABEA 3 a) Reflection part: seismic data shown from the a r r i v a l of the f i r s t bottom r e f l e c t i o n s to beyond the time f o r most f i r s t - o r d e r multiples (7 sec and l a t e r ) . b) Refraction part: the straight l i n e connects the d i r e c t water wave a r r i v a l s and i t s slope gives the ve l o c i t y of sound i n water of about 1.5 km/sec. Numbers near the traces refer to shot numbers. Arrows point out the continuity of the deep c r u s t a l seismic a r r i v a l s acrcss the record section. They can be followed from near - v e r t i c a l incidence through wide-angle to re f r a c t i o n distances. Note the change in the distance scale between a) and b) . 42 7 3 - 5 D km 43 1.5 6 8 10 12 14 16 18 2 0 2 2 D km 44 3 DATA PROCESSING AND ANALYSIS Procedures for handling the f i e l d data and for i t s d i g i t a l processing are discussed in t h i s chapter. By applying traveltime and amplitude corrections, normalized record sections-can be displayed for interpretation. The application of d i g i t a l processing technigues improves the quali t y of the data. This i n turn enables a clearer resolution of the seismic phases and t h e i r c o r r e l a t i o n throughout the record sections. 3.1 f i e l d Data and Corrections The data recorded on the f i e l d tapes were i n an impractical form for immediate processing and storing. The multichannel data had been multiplexed, written i n very short blocks of 512 half-words, and more than a minute of data for each shot had been recorded to ensure a complete HWVB time code. Furthermore, the recording density was 800 bytes per inch (BPI) rather than the more standard 1600 BPI. For the above reasons, the f i e l d data were edited, demultiplexed, and written on new tapes i n a convenient blocking format with the higher recording density. The IBM 45 370/68 computer, operating under the Michigan Terminal System (MTS) at the University of B r i t i s h Columbia, was used for a l l the d i g i t a l processing operations. ORIGIN TIME CORRECTIONS The data reguired corrections before they could be displayed for analysis, o r i g i n times of the shots, obtained from an FM playback of the single hydrophone and WWVB signals onto a two-channel chart recorder, had to be corrected for the shot-to-ship distances. Although the shot instants could be timed to better than 5 msec on the chart recorder, an additional error of up to 15 msec was introduced by the application of the shot-to-ship distances since these were •eyeball 1 estimated. The maximum resultant error in determination of the o r i g i n times was about 20 msec for larger detonation distances (600 to 900 m). Shot-receiver distances for each shot and hydrophone were calculated using dir e c t water wave (DWW) traveltimes and assuming a constant DWW velocity of 1.48 km/sec as recorded for the depth of about 45 m at the regular west coast Ocean Station P by Minkley et al.(1970). The error associated with these calculated distances i s less than 3%. 46 TOPOGRAPHY CORRECTIONS In most cases, topographic corrections were not required since there were no substantial topographical changes along the p r o f i l e l i n e s as was shown by the continuous fathometer records. The single exception was the 73-4 p r o f i l e running up the continental slope. However, i n t h i s case also, the topographic corrections were not applied in order to be able to follow the r i s i n g topography of the bottom and the shape of the layers beneath on the record section. AMPLITUDE CORRECTIONS As i t was intended to do an interpretation based on both traveltime and amplitude information, corrections were necessary f o r variations in amplifier gain settings, charge sizes, and geometrical spreading. In order to keep the magnitude of amplitudes within a d e f i n i t e range during recording along a p r o f i l e , siqnals from d i f f e r e n t distances were amplified with d i f f e r e n t gains. To get true r e l a t i v e amplitudes, a reference gain l e v e l was chosen and a l l amplitudes were normalized to i t . The corrections for charge sizes were calculated using 47 an expression based on studies of underwater explosions by O'Brien (1960). He has shown that f i r s t seismic a r r i v a l 2/3 amplitudes are proportional to 8 , where W i s the weight of the charge (kg). The correction egual to 1/H was applied to a l l shots in each p r o f i l e . As the energy travels away from the explosion, the amplitudes of the seismic a r r i v a l s decrease. With increasing epice n t r a l distance (r) the wide-angle r e f l e c t i o n amplitudes decrease as 1/r- (Braile and Smith,1975), and the head wave amplitudes for<refractions as 1/r 2 (Cerveny and Havindra,1971); The record sections of wide-angle r e f l e c t i o n data (for distances from 4 to about 8 km) were corrected using a correction factor egual to r , and the sections of re f r a c t i o n data (for greater distances than 8 km) using a correction factor equal to r 2 . Amplitude corrections were not applied to the near-vertical incidence r e f l e c t i o n data since amplitude information was not used durinq t h e i r i n t e r p r e t a t i o n . 3« 2 Autopower Spectra In order to determine the frequency content of the recorded waveforms and seismic siqnals, necessary for further data processinq, autopower spectra were computed 48 with the use-of a fast Fourier transform algorithm. Fig. .3,-1 shows amplitude normalized autopower spectra of some c h a r a c t e r i s t i c parts of seismic traces. The frequency content of the background noise, the r e f r a c t i o n part of a seismic trace> r e f l e c t i o n s from beneath the sediments, and f i r s t - o r d e r multiples from the water bottom and beneath are displayed. The main freguency content of the background noise (Fig. 3.1a) i s i n the i n t e r v a l from 0 to 20 Hz with the maximum amplitude at 2 Hz. The r e f r a c t i o n trace (Fig. 3.1b) has the maximum amplitude at about 10 Hz, and both r e f l e c t i o n traces (Fig. 3.1c,d) show maxima between 20 and 30 Hz. Autopower spectra of primary r e f l e c t i o n s from within the sediments were not investigated because of amplitude overloading as mentioned e a r l i e r . 3.3 Eand-pass F i l t e r i n g Since the freguency content of the re f l e c t e d and refracted energy i s confined to a certain band of freguencies, d i g i t a l band-pass f i l t e r i n g was applied as a sign a l enhancement technigue. A zero-phase s h i f t , fourth order, recursive. Butter worth band-pass f i l t e r (Kanasewich,1973) was applied to the data. Effects of the f i l t e r for d i f f e r e n t frequency bands on 49 Figure 3.1 amplitude Normalized flutO£Ower Spectra, of £ f e a £ a s i g £ i § ^ i £ Parts of Seismic Traces a) Background noise, b) refraction trace, c) r e f l e c t i o n s from beneath the sediments, d) f i r s t - o r d e r multiples of the water bottom and sedimentary layers. (Seismic traces are displayed above, autopower spectra below.) 50 51 a single r e f l e c t i o n trace, on a set of six r e f l e c t i o n traces and on a set of six re f r a c t i o n traces are displayed i n Fig, 3,2. The diagrams of r e f l e c t i o n data (Fig. 3.2a,b) show c l e a r l y that passbands of 2.5-30 Hz and 5-30 Hz enhance the seismic signals while maintaining their character. For narrower bands of frequencies, o s c i l l a t o r y e f f e c t s are observed and the i d e n t i f i c a t i o n of the beginnings of seismic phases becomes d i f f i c u l t i f not impossible. Similar effects are seen on the ref r a c t i o n traces . The best enhancement of the signals without additional ringing has been obtained with 0.5-25 Hz and 2.5-30 Hz f i l t e r s . They give traces with c l e a r l y distinguishable r e f r a c t i o n phases. On the basis of the examples just shown and others, passbands of 2.5-30 Hz for r e f l e c t i o n data and 0.5-25 Hz for re f r a c t i o n data were chosen and the f i l t e r s were applied to the data. 3.3 Decogvolution Deconvolution i s used to improve the guality of seismograms with reverberation patterns. The desirable r e s u l t from i t s application i s sharper resolution of seismic a r r i v a l s , achieved by shaping a reverberation into a single spike or pulse. 52 Figure 3.2 Examples of the Effect of Various Jjincl-Pass F i l t e r s a) A single r e f l e c t i o n trace (showing the influence on r e f l e c t i o n wavelet character and signal/noise r a t i o ) . b) Six r e f l e c t i o n traces (showing the change i n gua l i t y of correlation across the traces). c) Six r e f r a c t i o n traces (showing the change i n the amplitude and t h e i r c o r r e l a t i o n with the band-width). Note that as the lower l i m i t of the passband increases, the traces for both r e f l e c t i o n and r e f r a c t i o n data become more o s c i l l a t o r y such that i d e n t i f i c a t i o n of phases becomes more d i f f i c u l t . 53- a) i « t i i 4 5 6 7 8 sec 54 s e c c) • » I I I 7 . 8 9 10 11 s e c 56 I t was decided to apply two di f f e r e n t deconvolution techniques to the r e f l e c t i o n data; one i n the frequency domain and one in the time domain. In the frequency domain, spectral d i v i s i o n a l deconvolution was chosen for i t s s i m p l i c i t y and because source wavelets of a good quality were available. In the time domain, spike deconvolution was chosen because'it can be used to consider changes of the source wavelet with time. SPECTBAL DIVISIONAL DECONVOLUTION The time domain model for the j-th seismic trace i s S j Ct) = W Ct) * Tj (0 t nj (t) (3.3 - 1 ) where w (t) i s the source signature for the pa r t i c u l a r shot, rj (t) i s the impulse response of the transmission path to the j-th hydrophone, nj (t) i s a white noise sequence, and * denotes convolution. In the frequency domain a sinqle seismic trace i s described by 57 S(a) * W(u). R t » + N(<o) (3.3-2) where c a p i t a l l e t t e r s denote the Fourier transforms of the variables in (3.3-1). To obtain the estimated impulse response B, the spectrum of the seismic trace i s divided by the spectrum of the source signature w where w' i s the complex conjugate of W. As the amplitude of the source signature becomes small the factor multiplying the noise component becomes increasingly large. Therefore, to obtain an estimated impulse response B which converges to the impulse response B, i t i s ess e n t i a l to establish a minimum amplitude l e v e l for the source signature in order to l i m i t the gain of the deconvolution in parts of the seismogram where the trace contains l i t t l e or no information. This minimum source signature amplitude W„ i s termed the waterlevel (Helmberger and Wiggins, 197 1) . For convenience a r e l a t i v e waterlevel TOL i s introduced and defined as a f r a c t i o n of the maximum source signature amplitude wmfl)(: TOL=W0/W^, (0-TC1-1). The estimate of impulse response B then becomes z S R = — - W IWI.R +W*N (3.3-3) iwt 2 58 R «3.3-„ Considering only the factor containing the impulse response R, we can see that as the r e l a t i v e waterlevel TOL approaches zero we obtain unrestricted deconvolution of the seismic trace S by the source signature W. As the parameter TOL approaches unity, the estimator i s just a scale factor multiplied by the crosscorrelation of S and W (Clayton, 1975) . Unrestricted deconvolution attempts to remove a l l of the source e f f e c t s from the seismogram, and the estimator R gives the best true impulse response. This type of estimator i s best for resolving traveltimes. The crosscorrelation i s the least-squares estimate of the a r r i v a l amplitude (Helmberger and Wiggins,1971). This means that i f the parameter TOL equals unity the best r e l a t i v e amplitude resolution i s obtained. The waterlevel can therefore be used i n determining the preferred trade-off between a r r i v a l time resolution and r e l a t i v e amplitude resolution. These considerations suggest that spectral d i v i s i o n a l deconvolution of a seismic trace for a range of waterlavels should be attempted. The effectiveness of spectral d i v i s i o n a l deconvolution applied to the r e f l e c t i o n data for d i f f e r e n t waterlevels TOL 59 i s demonstrated i n Fig. 3 . 3 . A seismic r e f l e c t i o n trace was deconvolved by a source signature which consists of the dir e c t source wave a r r i v a l and the f i r s t bubble pulse as recorded for the pa r t i c u l a r shot. The source signature was f i r s t p r e f i l t e r e d for the same passband of frequencies as the seismic trace in order to avoid the introduction of higher frequencies into the deconvolved trace. Deconvolution was performed for f i v e d i f f e r e n t waterlevels TOL of values from 0.01 to 1,0. In the figure, the deconvolved traces are compared with the f i l t e r e d trace. In none of the cases do the deconvolved data show any s i g n i f i c a n t improvement i n quality. - The main reason that the data quality has not improved with the application cf spectral d i v i s i o n a l deconvolution i s that the frequency content of the source siqnature d r a s t i c a l l y changes as i t passes through the sediments and upper layers. Thus deconvolution using a source signature recorded i n the water close to the source i s not p a r t i c u l a r l y e f f e c t i v e . The application of a modified source signature was considered, but since the construction of i t s t h e o r e t i c a l estimate without a detailed knowledge of the upper c r u s t a l structure i s extremely d i f f i c u l t , this idea was not r e a l i z e d . Another reason for negligible data improvement i s the rather high noise l e v e l i n the freguency domain r e l a t i v e to Figure 3 .3 Effects of the Spectral D i v i s i o n a l Deconvolution on Reflection Trace The f i l t e r e d source signature (2.5-30 Hz) consists of a d i r e c t source wave a r r i v a l and the f i r s t bubble pulse. Six various waterlevels (TOI) were used for deconvolving the trace section containing deep c r u s t a l r e f l e c t i o n s and f i r s t - o r d e r multiples from the upper layers. SOURCE WAVELET (FILTERED) SEISMIC TRACE (FILTERED ) T sec 62 the maximum spectral amplitude of the source signature. Naturally, i f the waterlevel i s placed above the noise l e v e l , most of the source signature i s cut o f f and the ef f e c t of deconvolution i s weak. If the waterlevel i s placed below the noise l e v e l the ef f e c t of deconvolution i s la r g e l y l o s t SPIKE DECONVOLUTION In spike deconvolution we look for an inverse f i l t e r which when applied to a seismic trace increases the resolution of seismic a r r i v a l s by shaping the source wavelet into a single spike. Such a f i l t e r i s usually c a l l e d a spike operator, and i t s c o e f f i c i e n t s are determined by the input source wavelet and the desired output of a unit spike. Spike deconvolution was applied with two di f f e r e n t source wavelets: f i r s t , the recorded source signature, and then, varible wavelets chosen from the seismic trace. 63 A) Sgike Deconvolution with a Source Signature Spike deconvolution using the recorded source wavelet was applied to the data in order to obtain a basis for comparison of the effectiveness of spike deconvolutions with constant and variable (time adaptive) wavelets. The spike operator was designed for the f i l t e r e d (2.5-30 Hz) source wavelet from a given shot using Robinson's modified subroutine SPIKE (Robinson,1967, p.79). The shaping quality of the operator was tested by i t s application to the source wavelet and the results are displayed in Fig. 3.4. The operator produced a near-ideal spike as the output as the figure shows. Fig. 3.5 i l l u s t r a t e s the application of the operator to seismic traces. The deconvolved traces are displayed together with the unprocessed and f i l t e r e d data i n order to evaluate the effectiveness of deconvolution. The deconvolved data show some reduction in reverberation compared with the unprocessed data. However, the improvement i n the a r r i v a l time resolution i s not s i g n i f i c a n t when compared with the bandpass f i l t e r e d data. Conseguently, a seismic model employing a variable source wavelet was considered. 64 F i g u r e 3.4 C h a r a c t e r i s t i c s of the Shaping Operator gor Sjaike Deconvolution a) Input - the p r e f i l t e r e d (2.5-30 Hz) source signature (for the 3 lb charge). b) Output - a unit spike delayed 53.2 msec (in the middle between the two maxima on the source wavelet). (The parameters of the operator are: f i l t e r length=90,source wavelet length=50, length of the Parzen window=50, l i m i t for the spike position=18. The lengths are given in samples; sampling i n t e r v a l i s 2.8 msec.) INPUT 66 Figure 3 .5 Example of the Application of Spike £§£2222iu£ipji to a Reflection Seismpgram. The s i g n a l resolution of the deconvolved section i s compared with the resolution of the un f i l t e r e d and f i l t e r e d traces. (The single trace used to exemplify spectral d i v i s i o n a l deconvolution in Fig. 3.4 was one from this seismogram,) S P I K E D E C O N V O L U T I O N BAND PASS FILTER 0.5-30 Hz DECONVOLUTION WITH SOURCE i 6 • 7 8 s e c 68 B) Spike Deconvolution .with Variable lavele,t For spike deconvolution with a variable wavelet, a segment of the seismic trace was used as the source wavelet. Such a wavelet results from two-way transmission of the source signature through the layers of the crust, and changes with each r e f l e c t i n g horizon. The variable wavelet i s the most probable seismic phase picked along the seismogram within a particular time i n t e r v a l . Only those events are chosen which can be correlated over the record section or s i g n i f i c a n t part of i t (at least 3 shots). Thus for a given i n t e r v a l the correlatable phases are chosen, and for each of them a spike operator i s designed. These are then i n d i v i d u a l l y applied to the seismic traces. Such a procedure i s a s i m p l i f i e d approach to more expensive time adaptive deconvolution i n which the wavelet changes continuously with time as the operator i s being applied along the trace. The effects of the deconvolution on the data are displayed i n F i g . 3 . 6 . Six examples of deconvolution with d i f f e r e n t wavelets, chosen from the f i l t e r e d r e f l e c t i o n record, are shown. Analysis of the deconvolved traces reveals an improvement i n the resolution of the seismograms for the i n t e r v a l from B h i c h the wavelet was chosen. In addition, there i s a general improvement in the signal/noise 69 Figure 3.6 Example of the application of 8eson,yo.3,ution with §. Variable Wavelet to Seisicoqrams Six d i f f e r e n t wavelets picked along the uppermost trace of the bandpass f i l t e r e d data were used. The location where a p a r t i c u l a r wavelet was picked and i t s shape are indicated. The numbers on subsequent deconvolved records show the time i n t e r v a l s corresponding to the choice of wavelet. 70 6 DECON 6 71 r a t i o . S i n c e the deep c r u s t a l r e f l e c t i o n s were expected i n the i n t e r v a l b e f o r e the f i r s t water bottom m u l t i p l e a r r i v a l , an attempt to d e l i n e a t e s e i s m i c phases p a r t i c u l a r l y i n t h i s i n t e r v a l was made. The a p p l i c a t i o n o f s p i k e d e c o n v o l u t i o n w i t h v a r i a b l e wavelet i n d i c a t e d t h a t time a d a p t i v e d e c o n v o l u t i o n would be the most a p p r o p r i a t e method f o r such enhancement. I t i s p o s s i b l e t h a t any r e f l e c t i o n a r r i v a l s from deeper h o r i z o n s , such as M - d i s c o n t i n u i t y and upper mantle l a y e r s , are obscured fay t h e bottom m u l t i p l e . To remove t h i s m u l t i p l e , p r e d i c t i v e d e c o n v o l u t i o n (Peacock and Treitel,1969) s h o u l d be a p p l i e d . However, f o r f i s c a l and time reasons t h i s was not attempted as p a r t o f t h i s r e s e a r c h p r o j e c t . 3.4 S t a c k i n g o f R e f r a c t i o n Data S t a c k i n g i s a s i g n a l enhancement t e c h n i g u e which improves the s i g n a l / n o i s e r a t i o and reduces t h e amount of d a t a t o be a n a l y s e d w h i l e m a i n t a i n i n g t h e s e i s m i c i n f o r m a t i o n . I n o r d e r t o s t a c k m u l t i c h a n n e l d a t a , summation a l o n g a predetermined l a g t r a j e c t o r y i s performed. The l a g t r a j e c t o r y i s d e f i n e d by the paths o f s i g n a l s which a r e r e c e i v e d a t d i f f e r e n t d i s t a n c e s . For r e f r a c t i o n a r r i v a l s , 72 the trajectory i s a straight l i n e . For stacking of refraction data, optimum stacking v e l o c i t i e s for each seismic phase should be determined. This implies a time-varying velocity for stacking. As an alternative to such a d i f f i c u l t procedure, stacked traces using various stacking v e l o c i t i e s corresponding to f i r s t and l a t e r seismic a r r i v a l s along the traces were computed. These showed l i t t l e difference between them. This i s a r e s u l t of the fact that the six traces were recorded over a t o t a l distance of only 4 50 m and that the velocity contrast at the ocean bottom r e f r a c t s a l l rays corresponding to head waves into near-vertical paths. Thus the stacking velocity to be used was not c r i t i c a l and so was chosen to correspond to that velocity determined from the f i r s t r e f r a c t i o n a r r i v a l s . It was determined from the slope of the l i n e connecting the f i r s t r e f raction a r r i v a l s which appeared most frequently. The six r e f r a c t i o n traces for each shot were stacked to qive a sinqle trace. F i g . 3.7 compares an unstacked ref r a c t i o n record section with the corresponding section of stacked traces. The seismic signals on the stacked traces are enhanced due to improvement in the signal/noise r a t i o . Seismic phase resolution i s increased along each stacked trace. (As an example, note two d i s t i n c t phases between 7 and 8 sec on the trace for shot 30 compared to a single t r a i n of amplitudes on the corresponding unstacked traces.) 73 Figure 3.7 Unstacked and Stacked E f f r a c t i o n Record Sections from AREA 1 a) Dnstacked section. The f i r s t r e f r a c t i o n a r r i v a l s are seen between 6 and 8 sec. The short high freguency wave t r a i n consists of the direct a r r i v a l s ; the large amplitude wave t r a i n a r r i v i n g l a t e r are r e f l e c t i o n s from the water bottom and below. (Because of the high noise l e v e l only exemplatory traces for each shot were displayed to be able to follow each trace on the section.) b) Section of the stacked data with the stacking velocity of 4.5 km/sec (a t y p i c a l v e l o c i t y for basement). Six traces for each shot were stacked to give a single trace. 73 - 1 a) D km ro - SUBROUTINE: "STACK" 73-1 b) - 2 H LU CO 00 H CNJ Cvl CM C M i n CM C D C M C M 00 C M CO CM o cn u~>. 1^ 8 T 9 —I— 10 r 11 V 12 ' T - IS "T~ 14 — r - 15 ~r- 16 17 -r— 18 DISTANCE (KM) 19 I 20 76 However, because of g r e a t e r i n t e r v a l s between s t a c k e d t r a c e s some of the r e f r a c t i o n a r r i v a l s are more d i f f i c u l t t o c o r r e l a t e a c r o s s the s e i s m i c s e c t i o n . T h e r e f o r e , b oth s t a c k e d and unsta c k e d s e c t i o n s were used f o r f u r t h e r i n t e r p r e t a t i o n , depending on procedures b e i n g f o l l o w e d . 3,5 V e l o c i t y A n a l y s i s o f R e f l e c t i o n Data A c o m p u t a t i o n a l method u s i n g v e l o c i t y s p e c t r a was c o n s i d e r e d t o determine the l a y e r v e l o c i t i e s from t h e r e f l e c t i o n r e c o r d s e c t i o n s of expanding p r o f i l e s . A v e l o c i t y spectrum i s a g r a p h i c a l d i s p l a y o f t h e r e f l e c t i o n energy as a f u n c t i o n o f the normal i n c i d e n c e t r a v e l t i m e and t h e average root-mean-sguare (ms) v e l o c i t y . The r m s - v e l o c i t y was d e f i n e d by Dix (1955) as n z Vn = ' n * (3.5 -1 ) Zr.,; where V n i s t h e average r m s - v e l o c i t y from the s u r f a c e down t o t h e bottom of the n-th l a y e r , vf i s t h e i n t e r v a l v e l o c i t y of t h e i - t h l a y e r , and T0)j i s t h e two-way normal i n c i d e n c e t r a v e l t i m e i n t h e i - t h l a y e r . The v e l o c i t y s p e c t r a e n a b l e us t o measure the power c f r e f l e c t i o n s a r r i v i n g a c c o r d i n g t o v a r i o u s paths determined by t h e i r t i m e - d i s t a n c e 77 relationships. Approximating the lag trajectory for the r e f l e c t i n g parts of expanding p r o f i l e s by a hyperbola, the time- distance relationship can be expressed in the form 2 * X2 TXjn = To,n + —T (3.5-2) where TX)n i s the two-way traveltime f o r a shot-to-receiver distance X, and the layer n, T o n i s the v e r t i c a l two-way traveltime, and Vsf i s the stacking velocity. How much thi s stacking velocity d i f f e r s from the true rms-velocity depends on the value of the spread-length/depth r a t i o . A l Chalabi (1973) has shown that for values less than 1 the discrepancy i s less than 0.5$. and for values less than 2.0 i t i s less than 2%, .In our case, since the depth of water was always more than 2 km, only the r e f l e c t i o n a r r i v a l s from the f i r s t 4 km distance were used for determination of the v e l o c i t i e s . In order to display the r e f l e c t i o n energy in the form of v e l o c i t y spectra, measures of the coherency of the signal along hyperbolic paths defined by (3.5-2) are used. After the alignment of•the input data with respect to a given hyperbolic delay pattern, a simple d i g i t a l f i l t e r that would s e l e c t i v e l y pass events common to a l l traces i s computed for each trace. Then, these are stacked together to give the best estimate of the input sig n a l . The power of t h i s 78 estimate i s then computed within a specified time gate around the reference time and t h i s power i s displayed. In order to generate a velocity spectrum, the seismic traces are swept with various hyperbolas determined by the v e l o c i t i e s i n a-chosen i n t e r v a l f o r the same reference time. Then another reference time point i s chosen at a time i n t e r v a l egual to half the specified time gate and the procedure i s repeated. The coherency measures express i n a guantitive form'the likeness of the data content among data channels. A computer program for deriving and displaying v e l o c i t y spectra with the use of d i f f e r e n t measures of the coherency of signals was written, A fortran l i s t i n g of the program i s i n the Appendix, Three d i f f e r e n t technigues for measurement of coherency along hyperbolic paths were applied: summation, unnormalized crosscorrelation and semblance c o e f f i c i e n t . A l l three are time domain-technigues and the l a t t e r two are well described by Neidel and Taner (1971). Summation i s a coherency measure giving an estimate of t o t a l s i g n a l amplitude within a pa r t i c u l a r time gate. It i s obtained by simple summing of the signal amplitudes or t h e i r powers within the gate. Unnormalized cross-correlation measure i s egual to half the difference between the t o t a l energy and signal energy within the gate. Semblance c o e f f i c i e n t i s defined as the r a t i o of sig n a l energy to t o t a l energy within the gate. The results obtained by t h e i r 79 use were compared. An advantage of semblance i s that i t reguires a smaller dynamic range for the display of the t o t a l signal amplitude than summation or unnormalized cross- c o r r e l a t i o n , when signal/noise r a t i o s remain greater than unity (Neidell and Taner,1971). Since i t also had the best resolution compared with the two other techniques, i t was chosen for the generation of velocity spectra. To i l l u s t r a t e the use of the coherency measures as they were applied to our data, the c h a r a c t e r i s t i c parameters are given here: time gate 56 msec, time step 14 msec, minimum velocity 1.4 km/sec, maximum velocity 10 km, velocity step 0.1 km/sec. The semblance was used at f i r s t for data from a single shot (6 seismic traces), and then for the f i r s t four neighbouring shots (24 seismic traces). The former one i s displayed in F i g . 3.8. The computer program for displaying the velocity spectra was designed to increase the spectrum resolution by spiking the coherency measure amplitude. In spite of that the spectrum resolution i n the figure i s poor. The amplitudes have a tendency to spread along the velocity axis without showing a single peak maximum. This means that the measure of coherency does not have a sharp single maximum within the corresponding time gate. This i s due to the small time differences between the ray-paths to the i n d i v i d u a l hydrophones (the layer depths were about 2500 m and more while the hydrophones sere only 90 m apart). The Figure 3.8 Velocity Spectrum for Six Seismic Reflection Traces of the P r o f i l e 73-5 The spectrum i s shown at the top. It was obtained using semblance c o e f f i c i e n t as the measure of coherency of the s i g n a l across six seismic traces recorded at distances near 2.5 km. One of the traces i s displayed at the bottom of the figure. The section between 5 and 7.5 sec (two-way) traveltime was chosen because deep c r u s t a l r e f l e c t i o n s arrive i n t h i s i n t e r v a l . The seismic a r r i v a l s S1 and S2, and the f i r s t water bottom multiple W from the trace appear c l e a r l y i n the spectrum. The remaining peaks i n the spectrum are mostly deformed and do not give v e l o c i t i e s which could be associated with the deep c r u s t a l r e f l e c t i o n s . 81 82 r e s u l t s obtained by applying the semblance to a group of shots did not show the necessary consistency. The inaccuracy in determination of the o r i g i n times for in d i v i d u a l shots (10-20 msec) limited the effectiveness of the crosscorrelation procedure. Since the computer derived spectra are not sens i t i v e to i n d i v i d u a l seismic traces (displaying the signal coherency of the stack) and their velocity resolution was poor f o r the kind of data we had, i t was abandoned. A simpler method of traveltime sguared versus distance squared (T 2-X 2) graphs (Dix,1955) was adopted for velocity analysis and interpretation of the r e f l e c t i o n data. The method i s explained and presented together with the interpretation cf the data in the following chapter. 83 U INTERPRETATION • 1 Methods of I n t e r pr et a t i on In order to obtain an i n i t i a l model of the oceanic crust in each area of recording, seismic refraction a r r i v a l s were f i r s t interpreted . For t h i s , both traveltime and amplitude information were used by application of traveltime plots and synthetic seismograms . After establishing the i n i t i a l r e f r a c t i o n models, the r e f l e c t i o n data were interpreted, layer v e l o c i t i e s and thicknesses were computed using T 2-X 2 l i n e s with application of the least-sguares method. F i n a l l y , detailed velocity-depth models based on the r e f l e c t i o n and re f r a c t i o n data are presented and the i r geological implications are given. TRAVELTIME INTERPRETATION - REFRACTION DATA The simplest method used for the interpretation of marine r e f r a c t i o n data i s the slope-intercept method (Ewing,1963) which uses traveltime plots. The method i s 84 based on a s e i s m i c model c o n s i s t i n g of homogeneous h o r i z o n t a l l a y e r s where w i t h i n c r e a s i n g depth each subsequent l a y e r has a h i g h e r v e l o c i t y . Because of t h e non- a p p l i c a b i l i t y i n c e r t a i n s i t u a t i o n s of t h e s e assumptions the s l o p e - i n t e r c e p t method sometimes may l e a d to a r t i f i c i a l v e l o c i t y - d e p t h models. The t r a v e l t i m e p l o t s used t o d e r i v e the i n i t i a l v e l o c i t y - d e p t h models were c o n s t r u c t e d from r e f r a c t i o n a r r i v a l p i c k s made on computer p l o t t e d seismograms. The maximum e r r o r i n t i m i n g of the a r r i v a l p i c k s was about 15 msec due t o t h e low s i g n a l / n o i s e r a t i o on some t r a c e s . T h e r e f o r e , t o g e t h e r w i t h the e r r o r a s s o c i a t e d w i t h t h e o r i g i n t i m e s , the e s t i m a t e d maximum t r a v e l t i m e e r r o r was about 35 msec. An example of a t r a v e l t i m e - d i s t a n c e p l o t i s g i v e n i n F i g . 4.1. The s t r a i g h t l i n e s are l e a s t sguare f i t s t o t h e r e f r a c t i o n a r r i v a l p o i n t s . The r e c i p r o c a l of the s l o p e f o r each l i n e i s the r e f r a c t i o n v e l o c i t y of t h e c o r r e s p o n d i n g l a y e r ; t h e t i m e i n t e r c e p t d e t e r m i n e s the l a y e r t h i c k n e s s . The v e l o c i t y - d e p t h models based on the s e parameters were then used as i n i t i a l models f o r the g e n e r a t i o n o f s y n t h e t i c seismograms c o n s i s t e n t w i t h the observed t r a v e l t i m e c u r v e s and a m p l i t u d e s . 85 TRAVELTIME AND AMPLITUDE INTERPRETATION-REFRACTION DATA The interpretation cf seismic body waves consists of finding a range of velocity-depth models which would match the observed t r a v e l t i u e and amplitude information. The number of models which are consistent with the traveltimes of seismic a r r i v a l s i s generally quite large (McMechan and Wiggins, 1972; Wiggins and Helmberger, 1973; Bessonova et al.,1974). However, the number i s considerably reduced by the requirement of th e i r consistency with the observed amplitudes (Helmberger and Wiggins,1971; Wiggins and Helmberger,1973; Fuchs and Muller,1971). The computer programs used for derivation of synthetic seismograms i n t h i s work are based on the disc ray theory introduced by Wiggins (1976). The method was derived from guantized ray theory (Wiggins and Madrid,1974), and named disc ray theory (DRT) because the model of wave propagation i s given i n terms of planar discs guided by rays. In order to generate synthetic seismograms matching both traveltime and amplitude information the i n i t i a l velocity-depth models were expressed in terms of ray parameter versus epicentral distance (p-A) curves. These were derived with the use of a computer routine c a l l e d MDLPLT by Wiggins. Using a spe c i f i e d velocity-depth model and a d i g i t i z e d source wavelet as input, the program g e n e r a t e s t r a v e l t i m e c u r v e s , p-A c u r v e s and s y n t h e t i c seismograms as the o u t p u t . To t e s t f o r c o n s i s t e n c y , the computed t r a v e l t i m e c u r v e s were compared w i t h t h e observed ones. The p-A-values were then used as i n p u t f o r the computer program c a l l e d HBGLTZ , a l s o by Wiggins. For the i n t e r p r e t a t i o n a t r i a l - a n d - e r r o r procedure was f o l l o w e d . From the i n i t i a l p-A c u r v e , t r a v e l t i m e c u r v e s and s y n t h e t i c seismograms were g e n e r a t e d u s i n g HRGLTZ and compared w i t h the observed d a t a . I f n e c e s s a r y , t h e p^A c u r v e s s e r e m o d i f i e d t o match the a m p l i t u d e s and the c a l c u l a t i o n r e p e a t e d u n t i l t h e s y n t h e t i c seismograms were c o n s i s t e n t w i t h t h e r e c o r d e d d a t a . S i n c e both t r a v e l t i m e s and a m p l i t u d e s were t c be matched, a t r a d e - o f f between the two was sometimes n e c e s s a r y . Examples of a s e t o f s t a c k e d seismograms and of a s y n t h e t i c r e c o r d s e c t i o n a re shown i n F i g . 4 . 3 and F i g . 4 i 9 . The r e s u l t a n t v e l o c i t y - d e p t h models d e r i v e d from the r e f r a c t i o n data were used as a check on the models o b t a i n e d from the i n t e r p r e t a t i o n o f the r e f l e c t i o n d a t a . 87 TBAVELTIME INTEBPBETAIION - REFLECTION DATA If seismic traces are displayed according to the i r distances, r e f l e c t i o n a r r i v a l s from common r e f l e c t o r s appear along p a r t i c u l a r hyperbolic paths. When plotted as a traveltime sguared versus distance squared (T 2-X 2) graph, they give straight l i n e s of dif f e r e n t slopes and intercepts. The reciprocals of the slopes determine the average rms- v e l o c i t i e s from the surface to the bottom of a p a r t i c u l a r r e f l e c t o r , and the intercepts determine the normal incidence a r r i v a l times and hence the depth to the r e f l e c t o r s (Dix,1955). It should be mentioned that, since the time a r r i v a l s on the record section are picked v i s u a l l y , the T 2- X 2 method i s a subjective one and i t s accuracy lar g e l y depends on the interpreter's a b i l i t y to recognize and correlate the r e f l e c t i o n a r r i v a l s on seismograms. From the average (surface to ref l e c t o r ) rms-velocities and intercept times determined from the T 2-X 2 graphs, the true rms- v e l o c i t i e s and thicknesses of in d i v i d u a l layers can be calculated. Let the average rms-velocity from the surface down to the top of the k-th layer be Vk_, . The corresponding normal incidence two-way traveltime i s T^, . From the surface to the bottom of the k-th layer they are V k and T k respectively. Then according to the expression (3.5-1) the i n t e r v a l rms- 88 v e l o c i t y for the k-th layer i s given by The detailed velocity-depth models obtained from the r e f l e c t i o n data are based on such analyses. 4 • 2 Velocity-depth Models The data from ARIA 1 were analysed f i r s t i n order to compare the res u l t s with a previously known re f r a c t i o n model. AREA 3 was approached next because the qual i t y of the r e f l e c t i o n data was best and the cr u s t a l structure i n the area was presumably more normal than that of AREA 2 . Therefore, interpretation was expected to be more straightforward. After acguiring experience from these interpretations, the data from the more t e c t o n i c a l l y complex AREA 2 were analysed and interpreted. 89 AREA 1 S i n c e t h e r e f l e c t i o n data from AREA 1 were o f poor g u a l i t y , o n l y t h e r e f r a c t i o n p a r t of the expanding p r o f i l e was i n t e r p r e t e d ; the r e c o r d s e c t i o n i s shown i n F i g . 3.7. A t r a v e l t i m e p l o t based on the a n a l y s i s o f i n d i v i d u a l t r a c e s was used f o r t h e i n t e r p r e t a t i o n and t h e r e s u l t i n g v e l o c i t y - depth model compared wit h the model of Keen and B a r r e t t (1971) from the *HUDSCN 70' s e i s m i c s u r v e y i n t h e a r e a ( F i g . 2.4a) . The a n a l y s i s o f the t r a v e l t i m e p l o t ( F i g . 4.1) shows t h a t : 1) t h e r e f r a c t i o n s from the sediments were never observed as f i r s t a r r i v a l s ; 2) the r e f r a c t i o n a r r i v a l s from the basement g i v e a cu r v e w i t h two branches o f v e l o c i t i e s 4.0 and 5.5 km/sec, the r e f r a c t i o n s of v e l o c i t y 4.0 km/sec appeared as f i r s t a r r i v a l s o v er a s h o r t d i s t a n c e i n t e r v a l (8-10 km); 3) t h e r e f r a c t i o n s from t h e o c e a n i c l a y e r appeared as f i r s t a r r i v a l s f o r most of t h e l e n g t h of p r o f i l e (from 10 t o 20 km) and gave the v e l o c i t y of 6.8 km/sec; 4) the p r o f i l e was t o o s h o r t t o observe upper mantle r e f r a c t i o n s . I n g e n e r a l , t o d i s t i n g u i s h the phases o f the f i r s t a r r i v a l s from those a r r i v i n g i m m e d i a t e l y a f t e r was d i f f i c u l t . I n p a r t i c u l a r t o d i s t i n g u i s h t h e o c e a n i c l a y e r a r r i v a l s from the basement r e f r a c t i o n s f o r d i s t a n c e s from 10 t o 17 km was a problem. T h e i r c l e a r s e p a r a t i o n c o u l d be 90 Figure 4.1 Traveltime - Distance Plot of the I n f r a c t i o n P r o f i l e 12-1 from ABEA J ' The l i n e s correspond to in d i v i d u a l r e f r a c t i n g horizons. The associated v e l o c i t i e s are given i n km/sec at t h e i r ends. Line with the velocity 2.4 km/sec corresponds to the sediments, l i n e s with v e l o c i t i e s of 4.0 and 5.5 km/sec to the upper and lower part of the faesement, and l i n e with the velocity of 6.8 km/sec to the oceanic layer. (The l i n e s are least-sguare f i t s to the observed a r r i v a l times.) 73-1 2.4 Dis tance [km] 92 followed only near the end of p r o f i l e (from 17 to 20 km). There i s a p o s s i b i l i t y of a velocity gradient in the basement layer which i s indicated by a dashed l i n e connecting the branches of the basement traveltime curve between 12 and 15 km. The velocity-depth model based on t h i s traveltime plot i s compared with the 'HUDSON 70* model i n Table 1. The symbols V and H i n the table, are velocity (km/sec) and layer thickness (km)<respectively. The errors given include the timing errors (from picking the in d i v i d u a l a r r i v a l s ) and the standard deviations (from the l i n e f i t s to the picked points). TABLE 1 * ENDEAVOUR 73* *HUDS0N 70* Layer V H V H Sediment 2.4±,2 0.6±.2 2.3** 0.5 Basement 4.7* 2.4 4,5* 1.6-2.4 a 4.0±.2 1.1±.3 b 5.5±.2 1.5±.4 Oceanic 6.8±.2 6.7-7.0 4.7* •Indicates an average value, ** an assumed value. The mean value of 4,5 km/sec for the basement velocity i n 'HUDSON-70* model assumes a velocity gradient for the layer from 4.0 to 5.5 km/sec. In our case, assuming that there i s a velocity gradient (between 4,0 and 5 ,5 km/sec) i n 93 the basement l a y e r , we have obtained a mean v e l o c i t y of 4.7 km/sec. The average value f o r the t h i c k n e s s of the oceanic l a y e r i n *HUDSON 70* model as presented here does not co n s i d e r an anomalous value of 8.5 km recorded a t the north s t a t i o n on one of the p r o f i l e s . T n i s s t a t i o n was the f u r t h e s t from our p r o f i l e . The t h i c k n e s s of the oceanic l a y e r could not be determined i n our model because the p r o f i l e was not long enough to observe mantle r e f r a c t i o n s . In c o n c l u s i o n , i n s p i t e of the small amount and poor g u a l i t y of the recorded data, the v e l o c i t y - d e p t h model of ASIA 1 shows good agreement with the » HUDSON 70• model of Keen and B a r r e t t (1971). AREA 3 The i n i t i a l v e l o c i t y - d e p t h model f o r AREA 3 i n the northern Cascadia Basin was based on the i n t e r p r e t a t i o n of the r e f r a c t i o n part of the expanding p r o f i l e 73-5 shown i n F i g . 2.8b. From the a n a l y s i s of i n d i v i d u a l s e i s m i c t r a c e s a t r a v e l t i m e - d i s t a n c e p l o t was c o n s t r u c t e d ( F i g . 4.2). I t shows that 1) the r e f r a c t i o n s from the sediments were not observed as f i r s t a r r i v a l s ; as secondary a r r i v a l s they i n d i c a t e a p o s s i b i l i t y of two d i s t i n g u i s h a b l e sedimentary l a y e r s with v e l o c i t i e s of 1.9 and 2.4 km/sec; 2) the 94 Figure 4.2 Reduced Traveltime - Distance Plot of the Refraction P r o f i l e 73-5 from AREA 3 A t y p i c a l v e l o c i t y of 4.5 km/sec for basement layer was used to reduce the traveltimes. The small numbers beside the data points are the shot numbers. The associated v e l o c i t i e s i n km/sec are given at the end of each l i n e . K in I CO i 8 ~t 1- L 9 96 re f r a c t i o n a r r i v a l s with the velocity of 4.0 km/sec did not appear as f i r s t a r r i v a l s but are cl e a r l y distinguishable on shots 18,20 and 21; 3) the ref r a c t i o n s giving the velocity of 4.4 km/sec basement showed as f i r s t a r r i v a l s only over a short distance i n t e r v a l between 8 and 11 km; 4) the refr a c t i o n s from the oceanic layer appeared as f i r s t a r r i v a l s at a distances of about 16 km and could be followed to the end of the p r o f i l e at the distance of 22 km; 5) the p r o f i l e was not long enough to observe the ref r a c t i o n s from A the H-discontinuity, From the slopes and intercepts of the traveltime l i n e s , the v e l o c i t i e s and depths of the layers were determined and the i n i t i a l velocity-depth model i s presented in Table 2. TABLE 2 Layer description V H water 1.5 2.5 Sediment a 1.9 1.3 b v 2.4 0.7 Basement a 4.0 0.7 b 4.4 1.7 Oceanic 6.7 The symbols V and fl in the table are velocity (km/sec) and thickness (km) respectively. This i n i t i a l velocity-depth model was used for the generation of synthetic seismograms. 97 The section of the synthetic seismograms i s compared with the stacked recorded data in Fig. 4.3. The r e f r a c t i o n a r r i v a l s with the v e l o c i t i e s of 2.4 km/sec (lower sediments), 4.4 km/sec (basement) and 6.7 km/sec (oceanic layer) are observed on both sections. The upper sediment a r r i v a l s with the velocity of 1.9 km/sec and the refractions from the t r a n s i t i o n between the sediments and the basement do not appear on the synthetic seismograms. The former could be explained by a p o s s i b i l i t y that there i s a small velocity gradient in the sediments (slight changes in the amplitudes caused by such gradient would be d i f f i c u l t to observe on small amplitudes.) The l a t t e r could be explained s i m i l a r l y by a small ve l o c i t y gradient i n the upper part of the basement. Another possible explanation i s that the. velocity corresponds to an i r r e g u l a r t r a n s i t i o n layer with an average thickness less than the l i m i t of the HRG1TZ program resolution for the given velocity (4.0 km/sec) and the input wavelet freguency (12 Hz). The res u l t s of attempts to generate these a r r i v a l s would indicate t h i s . When the layer was modelled 0.7 km thick (value calculated from the traveltime p l o t ) , i t was not possible to match the generated refractions from the oceanic layer with those recorded. The match of these was achieved only when the t r a n s i t i o n was modelled as a thin layer (0.3 km). However, such a thi n layer did not give any observable refractions on the 98 Figure 4.3 Comparison of the Observed and Synthetic Seismograms Sections Of Befraction P r o f i l e 73-5 Left - record section of stacked seismograms. Bight - record section of synthetic seismograms. (5s discussed in the text, the a r r i v a l s with v e l o c i t i e s of 1.9 km/sec and 4.0 km/sec from the section of stacked seismograms could not be included r in the section of synthetic seismograms.) T [SEC) •5 6 1 6 8 10 11 12 - ' 1 1 • 1 1 i o 100 synthetic seismograms. Fig. 4.4 shows the record section of the f i l t e r e d r e f l e c t i o n data from the expanding p r o f i l e 73-5, and Fig. . 4.5 the record section of the f i l t e r e d data from the near- v e r t i c a l incidence r e f l e c t i o n p r o f i l e 73-6. Coherent seismic a r r i v a l s from i n d i v i d u a l r e f l e c t i n g horizons can be distinguished and correlated on parts of either or both p r o f i l e s . A series of sedimentary r e f l e c t i o n s arrives within the time i n t e r v a l of about 1.7 sec (two-way) traveltime a f t e r the f i r s t water bottom r e f l e c t i o n . The f i r s t i d e n t i f i a b l e r e f l e c t i o n s from within the sediments are from the r e f l e c t i n g horizon A. This r e f l e c t o r can be followed c l e a r l y along the section 73-6 and i s characterized by a sudden change in both freguency and amplitude. Following the continuity of t h i s horizon on the expanding p r o f i l e 73-5 i s d i f f i c u l t because the amplitudes in t h i s time i n t e r v a l were often distorted due to the high gain settings of the amplifiers as mentioned before.The a r r i v a l s from r e f l e c t i n g horizon B on the expanding p r o f i l e are easy to i d e n t i f y along the section. The r e f l e c t i n g horizon C can be followed c l e a r l y on section 73-5, but i t s i d e n t i f i c a t i o n on section 73-6 i s more d i f f i c u l t . Horizons D and E which show on the near - v e r t i c a l incidence r e f l e c t i o n seismograms on both sections are impossible to follow beyond the distance of 3 km. 101 Figure 4.4 Record Section of the .Expanding Rgfigction £rof_ile 7 3 - 5 from AREA 3 The water depth was 2 .5 km. The data were bandpass f i l t e r e d ( 2 . 5 - 3 0 Hz). The l e t t e r s designate r e f l e c t i n g horizons: W- r e f l e c t i o n from the water bottom, A to C- r e f l e c t i o n s from sedimentary horizons, D to G- r e f l e c t i o n s from the top of the basement and beneath. The primed l e t t e r s designate f i r s t - o r d e r multiples of these r e f l e c t i o n s . (Note the emergence of the refracted waves as f i r s t a r r i v a l s between 8 and 10 km.) 7 3 - 5 D k m 103 F i g u r e 4 . 5 Record S e c t i o n of the Quasi-continuous S u b c r i t i c a l P r o f i l e 73-6 from AREA 3 The l e t t e r s designate the same primary r e f l e c t i o n s i d e n t i f i e d on the expanding p r o f i l e 7 3 - 5 i n F i g . 4 . 4 . (The i n d i v i d u a l a r r i v a l s are d i s c u s s e d i n d e t a i l i n the text.) 104 T sec b 105 A r r i v a l s from horizon F, which appear i n the i n t e r v a l of possible basement r e f l e c t i o n s , are clear on section 73 -6 and can be correlated to the end of the expanding p r o f i l e , although they are l e s s clear on 7 3 - 6 . Horizon G gives r e f l e c t i o n a r r i v a l s which can be distinguished on the near- v e r t i c a l incidence seismograms, but i t i s d i f f i c u l t to i d e n t i f y them on the wide-angle seismograms, p a r t i c u l a r l y for the-distances between 3 and about 5 km. After that they appear again and i n the i n t e r v a l from 7 to 9 km they arrive almost simultaneously with other r e f l e c t i o n phases and cannot be distinguished from them. The r e f l e c t i o n a r r i v a l s from t h i s horizon appear in the time i n t e r v a l waere the oceanic layer r e f l e c t i o n s could be expected. They also seem to correlate with the refractions from the oceanic layer which follow closely the f i r s t (basement) a r r i v a l s at the end of the expanding p r o f i l e . Also, f i r s t - o r d e r multiples of the r e f l e c t i o n s from the same horizons can be e a s i l y distinguished on the expanding seismic record section 7 3 - 5 . The f i l t e r i n g e f f e c t of multiple paths through layers makes the phases even more distinguishable, however the amplitudes are attenuated. These multiple r e f l e c t i o n a r r i v a l s and their c o r r e l a t i o n with the f i r s t r e f l e c t i o n s were not analysed, although they might be used i n the interpretation to confirm the r e s u l t s 106 A T 2-X 2 graph for the r e f l e c t i o n a r r i v a l s of p r o f i l e 73-5 was constructed to determine the average rms-velocities of the layers. From the average v e l o c i t i e s and intercept times, the i n t e r v a l v e l o c i t i e s and thicknesses of i n d i v i d u a l layers were computed with the use of the least-sguare method. Fig. 4.6 shows straight l i n e f i t s corresponding to the r e f l e c t i o n a r r i v a l s along hyperbolic t r a j e c t o r i e s on the expanding p r o f i l e section in Fig. 4.4. The most r e l i a b l e information should be obtained from the f i r s t part of the curves (to about 9 km 2), where the hyperbolic approximation of lag t r a j e c t o r i e s i s most accurate. The values of the i n t e r v a l v e l o c i t i e s and layer thicknesses derived from the T z-X z graph are presented i n Table 3 of Fig. 4.7. As well, the i n t e r v a l v e l o c i t i e s of the upper crust layers determined from the seismic recording i n the southern Cascadia Basin near 44°N (Seely et al.,1974) are entered i n the table. They compare well with our values. The figure also shows a comparison of the r e f l e c t i o n and the refr a c t i o n velocity-depth models for AREA 3 in the northern Cascadia Basin. The models agree well for the sedimentary seguence. Since i t was not possible tc correlate the a r r i v a l s from the uppermost r e f l e c t i n g horizon A on the expanding p r o f i l e * i t s i n t e r v a l velocity was assumed to be that of the r e f r a c t i o n model for the upper sediments. The seguence of the r e f l e c t i o n horizons A,B and C could be 107 \ F i g u r e 4.6 T£-X£ Graph f o r the Expandinding R e f l e c t i o n P r o f i l e 7 3-5 The i n d i v i d u a l r e f l e c t o r s are d e s i g n a t e d by c a p i t a l l e t t e r s c o r r e s p o n d i n g to the same r e f l e c t o r s on the r e c o r d s e c t i o n i n Fig.4.4. The a r r i v a l p o i n t s were picked v i s u a l l y . The rms average v e l o c i t i e s (km/sec) and i n t e r c e p t times (sec) were computed using the l e a s t - s g u a r e method. The fcrraer are given along the l i n e s and the l a t t e r along the o r d i n a t e . Both are accompanied by t h e i r a s s o c i a t e d standard d e v i a t i o n s . V \ 108 109 Figure 4.7 Velocity - Depth Model for AREA 3 Symbols V and H i n Table 3 , indicate layer v e l o c i t y (km/sec) and thickness (km) respectively. Symbol V* indicates layer v e l o c i t y recorded i n the southern part of the Cascadia Basin near 44°N (seely et al.,1974). The s o l i d l i n e shows the r e f r a c t i o n model, the dashed l i n e the r e f l e c t i o n model. The l e t t e r s designate the bottoms of i n d i v i d u a l r e f l e c t i n g horizons and correspond to the r e f l e c t i o n s observed i n Fig. 4.4 and 4.5. V E L O C I T Y - D E P T H M O D E L V E L . ( k m / s e c ) 1 2 3 4 5 6 7 • i i i i W 's6 1' G 1 TABLE 3 R e f l e c t i n g h o r i z o n V H V W 1. 48 2.5 A 1. 90* .30* 1. 83 B 2. 21 .50 2. 13 C 2.33 .48 D 2. 63 .60 2. 68 E U. 5 6 .41 F 3i 78 .40 3. 96 G 4.'4 3 1.5 • I n d i c a t e s an e s t i m a t e d v a l u e . 111 modelled on the ref r a c t i o n model by a velocity gradient from 1.9 to 2.4 km/sec in the upper sediments, The tops of the r e f l e c t i n g horizons D and E are at the same depth as the tops of the corresponding layers on the r e f r a c t i o n model. The t r a n s i t i o n between the sediments and the basement (previously suggested in the analysis of the re f r a c t i o n traveltime curves)has t«o d i s t i n c t layers in the r e f l e c t i o n model; a high velocity layer (4.56 km/sec) at the top and a low ve l o c i t y layer (3.78 km/sec) at the bottom. (The ve l o c i t y of 3.78 km/sec compares with the velocity of 3.96 km/sec, the highest ve l o c i t y recorded for the sediments i n the Southern Cascadia Basin.) The top of the basement layer i s at the same depth on both models. As well, the velocity and thickness of the basement as determined from the r e f l e c t i o n s and refractions are the same. The deepest r e f l e c t i o n s i n the model are those a r r i v i n g from the top of the oceanic layer; In conclusion, the comparison of both models shows that the velocity structure determined from the r e f l e c t i o n information i s quite detailed yet agrees with the vel o c i t y structures obtained from the r e f r a c t i o n data. 112 AREA2 Three seismic p r o f i l e s were recorded in AREA 2 : two p a r a l l e l reversed expanding p r o f i l e s 15 and 18 km long, and a quasi-continuous near-vertical incidence r e f l e c t i o n cross- p r o f i l e 36 km long (Fig. 2.4b). A reduced traveltime- distance plot based on the analysis of seismograms from the reversed p r o f i l e s i s shown in Fig. 4.8. Four c l e a r l y distinguishable layers with associated v e l o c i t i e s of 1.8, 2.1, 3.0 and 4.2 km/sec were observed on seismograms from both p r o f i l e s . Refraction a r r i v a l s giving a velocity of 6.8 km/sec were observed at the end of the SE p r o f i l e . Only ref r a c t i o n s from the basement and the oceanic layer were observed as f i r s t a r r i v a l s cn the record section. Seismic a r r i v a l s giving a velocity of 3.4 km/sec appeared on the SE p r o f i l e . The i n i t i a l r e f r a c t i o n velocity-depth model was calculated using the traveltime curves of Fig. 4.8 and i t i s presented i n Table 4 on the next page. The symbols V and H i n the table are velocity (km/sec) and layer thicknesses (km) respectively. The r e f r a c t i c n model shows that the o v e r a l l thickness of sediments i s nearly constant along the reversed p r o f i l e . Refraction a r r i v a l s having a phase velocity of 3.4 km/sec might correspond to a high velocity sediment layer l y i n g on the top of the basement. 113 Figure 4.8 Reduced IIayeltime-Distance £lot of the Two Reversed fieffaction P r o f i l e s 73-2a3 from AREA 2 The traveltime i s reduced with the velocity 4.5 km/sec. The numbers give the v e l o c i t i e s (km/sec) derived from the slopes of traveltime l i n e s . Crosses refer to data points for p r o f i l e 73-2 and c i r c l e s refer to data points for p r o f i l e 73-3.  115 TABLE 4 Layer description V H'H-station H SE-station H Water Sediment 1 .5 2. 0 2.0 d Base ment Oceanic c a b 1.8 2. 1 3.0 3.4 4. 2 6. 8 0.7 0.8 0.7 2. 2 0.5 0.6 0.9 0.4 2.1 I t s thickness i s only 0.4 km and i t was observed only on the SE p r o f i l e . The basement i s situated at the depth of 2.4 km below the sea bottom and i t s thickness i s about 2.1 km. The oceanic layer has a velocity of 6.8 km, but i t s thickness could not be determined since the p r o f i l e s were not long enough to obtain refraction a r r i v a l s from the Hi- disc ontinuity. This i n i t i a l r e f r a c t i o n model was used for the generation of synthetic seismograms using HEGLTZ computer program. A l l seismic a r r i v a l s could be modelled with the synthetic seismograms except the a r r i v a l s with v e l o c i t y of 3.4 km/sec. The layer with t h i s velocity was too thin to be modelled for the given input wavelet. The section of synthetic seismograms i s compared with the section of stacked observed seismograms i n Fig. 4.9. The i n i t i a l r e f r a c t i o n model of Table 4 was further used to check the model obtained from the analysis of the r e f l e c t i o n data. The expanding r e f l e c t i o n p r o f i l e 73-2 i s displayed i n Fig. 4. 10. The gua l i t y of the data i s poor and c o r r e l a t i o n 116 Figure 4.9 Comparison of the Observed a.nd Synthetic Seismograms of the Refraction P r o f i l e 73-2 L e f t - record section of the observed seismograms.- Right- record section of synthetic seismograms. 117 T (StC) to IS 118 Figure 4,10 Record section of the Expanding Reflection P r o f i l e 7 3-2 from AREA 2 The l e t t e r s indicate a r r i v a l s from the i n d i v i d u a l horizons, (They are not intended to be correlated with the horizons of AREA 3.) With the possible exception of horizon F, a l l other r e f l e c t i o n s are from within sediments, W* i s the f i r s t - o r d e r multiple of the water bottom r e f l e c t i o n . 119 7 3 - 2 T 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 3 4 5 6 7 D km 120 of i n d i v i d u a l seismic phases along the whole p r o f i l e i s extremely d i f f i c u l t . The guality of the r e f l e c t i o n record section of the reversed expanding p r o f i l e was even worse and the section was not used in the i n t e r p r e t a t i o n . (Bad weather conditions during recording in t h i s area severely limited the guality of the reflections.) In Fig. 4.11, r e f l e c t i o n a r r i v a l s from horizons C and D can be easily i d e n t i f i e d for n e a r - v e r t i c a l incidence r e f l e c t i o n distances. To i d e n t i f y and correlate the phases from the other horizons required a detailed analysis of i n d i v i d u a l seismograms. The o v e r a l l error i n picking the a r r i v a l s was about ±25 msec. The corresponding T 2-X 2 graph i s shown i n Fig. 4.11. The slopes and intercepts of the l i n e f i t s corresponding to the picked a r r i v a l s were computed d i r e c t l y with the use of the l e a s t - sguare method. The comparison of the r e f l e c t i o n and the r e f r a c t i o n velocity-depth models for AREA 2 i s shown in Fig. 4.12, The agreement between the models i s excellent. The r e f r a c t i o n v e l o c i t i e s of 1.9 and 2.4 km/sec average the v e l o c i t i e s of horizons A and B, and C and D. The average v e l o c i t y for the upper sediments, derived from the r e f l e c t i o n model, i s 1.99 km/sec. The v e l o c i t i e s and thicknesses of the lower sediment refract o r s and of the r e f l e c t i n g horizons are almost i d e n t i c a l . They give an average velocity of about 3.1 km/sec for the lower sediments. There are two low v e l o c i t y layers 121 Figure 4.11 ££•-!£ Graph for the Expanding fief l e c t i o n P r o f i l e 73 2 The l e t t e r s designate l i n e s corresponding to the r e f l e c t o r s on the record section i n Fig, 10. The rms average v e l o c i t i e s (km/sec) and the intercept times (sec), both accompanied by the i r associated standard deviations, are given along the li n e s and,the ordinate respectively. (The l i n e s are least-sguare f i t s to the observed data points.)  123 Figure 4.12 Velocity-Depth ijodel from the Bottom of the Continental Slope in AREA 2 In the diagram, the s o l i d l i n e shows the r e f r a c t i o n model, the dotted l i n e the r e f l e c t i o n model. The l e t t e r s designate the bottoms of the i n d i v i d u a l layers. In Table 5, the symbols V and H indicate layer velocity (km/sec) and thickness (km) respectively. \ V E L O C I T Y - D E P T H M O D E L Veloci ty km/sec 1 2 3 4 5 6 7 i i i i i i i 1 TABLE 5 R e f l e c t i n g h o r i z o n V H W 1. 48 2.0 A 1. 92 .35 B 1. 74 . 20 C 2. 25 .31 D 2. 04 .34 E 2. 92 .79 F 3.34 .45 to 125 within the upper sediments, horizon B with the velocity of 1.72 km/sec and horizon D with the velocity 2.04 km/sec. Both are situated below the horizons of higher v e l o c i t y . There i s a pronounced change i n the velocity (0.9 km/sec) at the boundary between the upper and lower sediments. The deepest r e f l e c t i o n s observed are from the top of the basement. The quasi-continuous near-vertical incidence r e f l e c t i o n p r o f i l e 73-6, which crosses the reversed p r o f i l e at r i g h t angles i s displayed i n Fig. 4.13. The r e f l e c t i o n a r r i v a l s observed on the expanding p r o f i l e 73-5 can be i d e n t i f i e d on seismograms of the p r o f i l e 73-6 from the bottom of the continental slope. To follow the a r r i v a l s from i n d i v i d u a l layers going up the slope i s extremely d i f f i c u l t because of the increased number of multiples i n t e r f e r i n g with the r e f l e c t i o n a r r i v a l s . The layer v e l o c i t i e s derived from the expanding r e f l e c t i o n p r o f i l e at the bottom of the slope were used to convert the traveltime section to a depth-varying s t r u c t u r a l model of the continental slope in AREA 2. This model i s presented together with CSP l i n e 37 of the Geological Survey of Canada in F i g . 4.14. The r e l a t i v e location of the DSS p r o f i l e and CSP l i n e no. 37 recorded in 1973 i s shown i n Fig. ,2.4b. The CSP p r o f i l e indicates the presence of a number of horizontally s t r a t i f i e d r e f l e c t i o n horizons within the sediments. The DSS model suggests six 126 Figure 4. 13 p.u a si-continuous Sub c r i t i c a l Reflection Profile, 73- 4 from Along the Continental Slope i n AREA 2 The r e f l e c t i o n a r r i v a l s correlate with the a r r i v a l s on the expanding r e f l e c t i o n p r o f i l e 73-2, f o r which the loca t i o n i s shown, and are designated by the same l e t t e r s . W i s the f i r s t water bottom r e f l e c t i o n , W, »»* and H'1' are the multiples. (Because of the number of multiples the corr e l a t i o n of i n d i v i d u a l r e f l e c t o r s along the record section i s d i f f i c u l t , some places impossible. In spite of that, an attempt has been made.) 127 T (sec) 128 Figure 4.14 Comparison of the Velocity Structure Mod,el of Sediments from the Continental Slope with the CSP P r o f i l e The l e t t e r s i n the model designate the r e f l e c t i n g horizons. Their corresponding v e l o c i t i e s are presented i n Table 5 of F i g . 4.11. The CSP p r o f i l e i s part of Line 37 recorded by the Geological Survey of Canada. (The r e l a t i v e location of the two p r o f i l e s i s shown i n Fig. 2.4b.) 1-29 QUEEN CHARLOTTE SOUND E C -CONTINENTAL SLOPE - o - DISTANCE (km) 10 15 2 0 ZS 30 35 -> 1 1 1 1 i_ 130 such horizons with various v e l o c i t i e s . There i s an ind i c a t i o n of r i s i n g or thickening of the basement with the r i s e of the continental slope i t should be emphasized that the velocity structure presented in Fig. 4.14 i s very tentative. In spite of t h i s , i t does y i e l d some additional information about the approximate thicknesses of the sediment layers observed on the CSP p r o f i l e . , 4.3 Discussion of the Results The interpretation of the data depends on the methods of analysis and the amount and guality of the data. The standard techniques of interpretation, such as f i t t i n g traveltime curves f o r the ref r a c t i o n data , and T 2-X 2 graphs for the r e f l e c t i o n records, determined the f i n a l v e l o c i t y - depth models. The synthetic seismogram amplitude analysis of the re f r a c t i o n data did not give any additional information. The re f r a c t i o n p r o f i l e s were too short for substantial amplitude variation. The computer programs used to generate the synthetic seismograms were near t h e i r l i m i t s of resolution (for the given input wavelet) when some of the thi n c r u s t a l layers were modelled, and did not show the corresponding a r r i v a l s . The synthetics did confirm the r e a l i t y of the 131 basic re f r a c t i n g layers obtained from the observed traveltime curves. The models in a l l three areas extend only to the top of the oceanic layer . The expanding p r o f i l e s were too short to observe M-disccntinuity r e f r a c t i o n s . A l l of the f i n a l r e f r a c t i o n velocity-depth models are credible in s p i t e of the poor guality of the data in some areas. The poorest records were obtained from AREA 1, but since a previously published seismic model agreed well with the presented inter p r e t a t i o n , the results were considered adeguate. A modern method of velocity analysis using computer derived velocity spectra was applied to the r e f l e c t i o n data, but i t proved to be unsuccessful for reasons mentioned i n section 3.5. However, the simpler T z-X 2 method was adeguate for r e f l e c t i o n data of the type and guality obtained i n t h i s thesis. The seismic phases were mostly determined with accuracy ±10 msec. An exception was AREA 2 where the error was about ±25 msec. For the deepest r e f l e c t o r observed in the area (reflector F of the velocity 3.34 km/sec), t h i s amounts to 40 m, a value near the l i m i t s of resolution of the recording. 132 ^•^ Relation to Regional Geology The continental margin of western Canada l i e s within a t e c t o n i c a l l y active part of the earth's crust which includes the t r i p l e junction of the P a c i f i c , American and Juan de Fuca plates. Typical for the area i s : 1) f a u l t i n g p a r a l l e l to the continental margin; 2) changes i n the character of the continental shelf from north to south across the t r i p l e junction; 3) marginal basins to the west of the continental slope; 4) quaternary volcanism at the base of the continental slope; 5) and interaction of tectonic deformation and Pleistocene g l a c i a t i o n . The margin i s d i v i s i b l e into three tectonic regions (Chase et al.,1975): a northern region from Dixon Entrance to Queen Charlotte Sound • (characterized by s t r i k e - s l i p f a u l t i n g ) ; a central region from Queen Charlotte Sound to Brooks Peninsula on Vancouver Island (characterized by f a u l t i n g and f o l d i n g ) ; and a southern region from Brooks Peninsula to Juan de Fuca S t r a i t (characterized by slow subduction). Each of the areas of the DSS recording (Fig. 1.1) was situated in one of the tectonic regions, AREA 1 i s west of the Queen Charlotte Trough i n the northern region; AREA 2 i s west of the central part of Queen Charlotte Sound, i n the central tectonic region; AREA 3 i s in the northern part of 133 Cascadia Basin, i n the southern tectonic region. ABEA 1 served only as a convenient area for the testing of the DSS technigue and was not of particular geological i n t e r e s t to us. Since the recorded data from the area did not y i e l d any new information, the geology of the area w i l l not be discussed i n the thesis. The geophysical c h a r a c t e r i s t i c s of the area are given fay Keen and Barrett (1971); the o v e r a l l geology of the tectonic region i s discussed by Chase et a l . (1975). ABEA 2 i s situated at the base of the continental slope between J. Tuzo Wilson Knolls (to the north) and the southern canyon of Queen Charlotte Sound (to the southeast). Recent findings of young volcanic material (Tiffin,1974,GSC Beport of A c t i v i t i e s ) at the J . Tuzo Wilson Knolls prove that there i s volcanic a c t i v i t y in the area. The southern canyon i s the largest trough i n Queen Charlotte Sound. It cuts the continental slope thus providing a channel f o r transportation of the sediments from across the sound to the base of the slope. The age of the sediments i n Queen Charlotte Sound ranges from Pleistocene (luternauer,1972) through Upper and Lower Pliocene to Miocene (wildcats Harlequin and Osprey cf Shell Canada Ltd.; Shouldice,1971). Geology of the adjacent areas indicates that ABEA 2 could have been subjected to two d i f f e r e n t geological processes, volcanism and Pleistocene g l a c i a t i o n . 134 The velocity-depth model for the base of the continental slope i n AREA 2 (Fig. 4*12) shows six horizons within the sediments. The upper sediments (average velocity of 2.0 km/sec, 1.2 km thick) comprise a seguence of layers with alternately high and low velocity. The thickness of i n d i v i d u a l layers i s almost uniform (about 300 m). The a l t e r a t i o n of the layer v e l o c i t i e s indicates probably two d i f f e r e n t depositional processes. The velocity a l t e r a t i o n together with the r e g u l a r i t y i n the layer thicknesses indicate that the two processes took place a l t e r n a t e l y and occurred with approximately the same time period. It i s suggested that the deposition of the sediments i n AREA 2 occurred during Pleistocene times. During the advances, coarser sediments pushed by a g l a c i e r across the sound were deposited, whereas during retreats, deposition of f i n e r sediments took place. The lower sediments (average v e l o c i t y 3.1 km/sec, thickness 1.2 km) are formed by two high velocity layers. Their v e l o c i t i e s indicate that they are highly compacted dewatered sediments (Nafe and Drake,1957). The basement which l i e s 4.4 km beneath the sea f l o o r i s 2.3 km thick and has a v e l o c i t y of 4.2 km/sec, suggesting i t i s volcanic. Substantial deformation of the basement was not observed. The underlying oceanic layer has velocity of 6.8 km/sec. (both v e l o c i t i e s , f o r the basement and for the oceanic 135 layer, were determined from the re f r a c t i o n recording,) AREA 3 i s located at the foot of the continental slope i n the northern Cascadia Basin. The basin i s a wedge of sediments thickening toward the continental slope and overlying the b a s a l t i c layer of the southeastern flank of Juan de Fuca Ridge (Chase et • a l . f 1975). Sediments of Cascadia Basin merge with those cf Tofino Basin through a zone of folding beneath the continental slope. The volcanic basement, which i s exposed along the crest of Juan de Fuca Ridge, dips gently eastward and disappears beneath the slope (Barr,1974). Its age increases towards the continental slope. ABEA 3 belongs to a region of magnetic anomaly 3 (Heirtzler scale) which indicates the age of between 4 and 5 myr (Barr,1974). The velocity-depth model of AREA 3 (Fig..4.7) shows a sequence of sedimentary layers t o t a l l i n g 1.9 km i n thickness. V e l o c i t i e s increase almost uniformly with depth and range from 1.9 to 2.63 km/sec. This indicates that the process of sedimentation i n the area i s a regular deposition with subseguent compaction. Sediment i s supplied via Vancouver Channel, which extends south from the southern end of Winona Basin, and via canyons cutting the slope between Kyuguoit U p l i f t and Nitinat fan (Carson, 1973). An in t e r e s t i n g feature i n the model i s the occurrence of a t r a n s i t i o n i n velocity between the sediments and the 136 basement: a high velocity layer (4,56 km/sec) overlies a low v e l o c i t y layer (3.78 km/sec), which i n turn overlies the high velocity basement layer (4.43 km/sec). The high v e l o c i t y for the upper layer (4,56 km/sec) indicates that the layer i s made of basalt. The lower ve l o c i t y of the layer beneath suggests that the layer i s composed of compacted sediments, possibly including b a s a l t i c debris. A layer with a similar velocity (3.96 km/sec) was recognized in the southern part of Cascadia Basin near 44°N (Seely et al.,1974). From magnetic anomalies, one deduces that 5 myr ago the present AREA 3 was in the process of formation at Juan de Fuca Ridge. The o r i g i n of the interbedding of the basalt with compacted sediments would be explained by contemporaneous sedimentation and volcanism at the ridge crest. Contemporaneity has been observed at the crest of the northern end of Juan de Fuca Ridge, Barr and Chase (1974) show that f a u l t i n g and d i f f e r e n t i a l u p l i f t at the crest of the ridge formed valleys which were subsequently f i l l e d with t u r b i d i t e s . The volcanic basement in AREA 3 with velocity of 4.43 km/sec, i s situated at the depth of 2.7 km beneath the sea f l o o r . I t i s 1.5 km thick and l i e s over the oceanic layer with velocity of-6.7 km/sec (determined from re f r a c t i o n measurements). 137 5 CONCLUSIONS In this thesis project a marine DSS technigue useful for detailed s t r u c t u r a l studies of the oceanic crust was established. The technigue unites the advantages of the more standard methods of marine seismic recording, such as r e f r a c t i o n and CSP p r o f i l i n g , and i s an inexpensive compromise to the multichannel methods used in the o i l industry. The technigue i s simple in i t s design (using an array of i n d i v i d u a l hydrophones), e a s i l y adaptable to changes (the number of the hydrophones can be increased up to eleven), and f l e x i b l e in i t s application, with the same eguipment, i t can be used for near-vertical incidence r e f l e c t i o n recording, wide-angle r e f l e c t i o n recording, and r e f r a c t i o n recording. . . . . . . The f e a s i b i l i t y of the technigue was studied by testing the DSS procedure at sea i n three t e c t o n i c a l l y d i f f e r e n t areas: ABEA 1 -west of the Queen Charlotte Islands, ABEA 2 - at the continental slope off Queen Charlotte Sound, and A BE A 3 - i n the northern Cascadia Basin west of Vacouver Island. The data were recorded d i g i t a l l y in the freguency range from 0.8 to 100 Hz. The guality of the data varied with the area of recording, but i n general the signal/noise r a t i o was poor. 138 The analysis of the recorded data has demonstrated that the penetration of the DSS technique and the signal/noise r a t i o depends greatly on the thickness and s t r u c t u r a l quality of the sedimentary layer. In regions with l i t t l e or no sediments, most of the signal energy i s re f l e c t e d from the uppermost layer, the seismograms are •noisy 1 and y i e l d seismic information which i s extremely d i f f i c u l t to extract. Such an area of recording was AREA 1 where i d e n t i f i c a t i o n of any r e f l e c t i o n s from beneath the thi n layer of sediments was impossible since t h e i r amplitudes were small and obscured by the noise. The deepest penetration i s achieved i n regions where the density of the sediments increases gradually with depth and 'soft* (low density) sediments are at the top (only a small part of the energy i s reflected from the f i r s t layers and more energy i s available for transmission deeper int o the section). Such an area of recording was ABEA 3 where the deepest r e f l e c t i o n s observed were from the depth of 4.19 km beneath the sea bottom (from the top of the oceanic l a y e r ) . In the regions with velocity reversals i n the sediments, the penetration of the signal decreases. Such an area was AREA 2 where the deepest r e f l e c t i o n s observed were from the depth of 2.44 km beneath the sea bottom (from the top of the basement). From the frequency content of i n d i v i d u a l r e f l e c t i o n a r r i v a l s , the resolution power of the technigue f o r r e f l e c t i o n s within the sedimentary seguence i s 139 calculated to be about 25 msec (frequency 20 Hz), and for the deeper r e f l e c t i o n s about UO msec (freguency 12 Hz). (It should be noted that the parameters, such as penetration and re s o l u t i o n , which specify the f e a s i b i l i t y of the established technigue for detailed studies of the deep c r u s t a l structure are based on the analysis of the f i r s t data recorded with t h i s technigue during t e s t i n g . The results depended greatly on the poor guality and small amount of the data. In the meantime, data of much better guality have been acquired during following cruises, however they have not been analysed yet.) To investigate the p o s s i b i l i t y of i d e n t i f y i n g f i n e r structure and/or deeper r e f l e c t i o n s than those already observed, various methods of data processing and analysis were studied. They can be divided into two groups, stacking technigues and deconvolution technigues. Stacking technigues (such as velocity spectra analysis and stacking of r e f l e c t i o n data) reguire accurate knowledge of the absolute o r i g i n times and s a t i s f a c t i o n of the CDP condition. However, neither of the reguirements can be well s a t i s f i e d with the established technique. Deconvolution techniques do not have these r e s t r i c t i o n s on the data and therefore were more convenient for the application. The results obtained with deconvolution using variable wavelet indicated that time adaptive deconvolution should be applied to the data i n the 140 i n t e r v a l before the f i r s t water bottom multiple a r r i v a l . For the i d e n t i f i c a t i o n of the deep r e f l e c t i o n s obscured by the f i r s t - o r d e r water bottom and upper layers multiples, these should be f i r s t removed and then time adaptive deconvolution used also i n t h i s i n t e r v a l . Within the bounds of the determined signal penetration and resolution, detailed velocity-depth models of the upper and middle part of the oceanic crust have been derived for two regions i n which no such information existed previously. Geological i n t e r p r e t a t i o n of the models contributed to the understanding of the geology of the areas. Velocity reversals within the sediments i n ABEA 2 indicate the e f f e c t s of Pleistocene g l a c i a t i o n on the deposition of the sediments below the continental slope o f f the Queen Charlotte Sound. A velocity reversal within the upper part of the basement in-ABEA 3 indicates the occurrence of interbedding of the volcanic basement with sediments at the top of the basement. The formation correlates with the ge o l o l o g i c a l processes observed recently at the crest of the near by Juan de Fuca Ridge. The main.contribution of t h i s thesis i s that i t established an inexpensive marine recording technigue and analysis procedures which can be used for detailed investigation of the crust i n such a t e c t o n i c a l l y inte r e s t i n g area as the west coast of Canada, No such 141 s i m i l a r technigue convenient for detailed seismic study of the oceanic crust has been used in t h i s area before. The marine DSS program provides additional information about geological structures and assists our understanding of the complex tectonic features off the coast of B r i t i s h Columbia. This l o c a l application i s complementary to the use of other technigues by marine geoscie.nt.ists. 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Problems in seismic studies of the oceanic crust, Izvestiya, Earth Physics,4,49-64, ( t r a n s l . ) , 153 APEESDIX FORTRAN SOURCE LISTING OF THE VELOCITY SPECTRUM COMPUTER PROGRAM The v e l o c i t y spectrum program i s des i g n e d t o d e r i v e and d i s p l a y v e l o c i t y s p e c t r a o f the s e i s m i c r e f l e c t i o n d a t a . The l o g i c of t h e program f o l l o w s the procedure d e s c r i b e d i n c h a p t e r 3.5. Three s u b r o u t i n e s a r e c a l l e d from t h e main program. S u b r o u t i n e RETAP rea d s s e i s m i c data from a tape i n t o the a r r a y A. S u b r o u t i n e SUMMAX (summation), UNNOCR (un n o r m a l i z e d c r o s s c o r r e l a t i c n ) o r SEMBLA (semblance c o e f f i c i e n t ) i s c a l l e d t o compute the s i g n a l coherency o f th e d a t a . S u b r o u t i n e ELVESP p l o t s t h e d e r i v e d v e l o c i t y spectrum. The p r i n c i p l e of each of the measures o f s i g n a l c o h e r e n c y i s p r e s e n t e d i n s e c t i o n 3.5. The m a t h e m a t i c a l e x p r e s s i o n s on which t h e s u b r o u t i n e s SUMMAX, UNNOCR and SEMBLA are based were d e r i v e d and d i s c u s s e d by Taner and Koe h l e r (1969). 154 DIMENSION A(20,3S<0),PLTA(20,1 000) ,STCK(1100) DlNFN'S ION V ( 1 0 0 0 ) , VSO( 1000 ) , VHP T ( 3 C) , VP PT 2 ( 30) , CPH V ( 1 000) PI MENS I ON TDISK6D ) ,01 ST (60 ) ,D1STS 0( 60) , R ST ART ( 10) , * J F ( 10) DIMENSION T ( 6 0 ) - K T ( 6 0 ) READ(5, 1 0 0 0 ) T 1 T G T » DT.NCI,VI,V2,DV RFAD(5,11 O O N S H P FAD ( 5., 1100 M N.H ( I ).»I =.1.». NSH) RFAD(5,3000 )(RSTAFT (I ),I=1,NSH) M = 0 PP 3 T=1.NSH 3 M-M + NHU) READ(f,4000) ( T D I S T U ) ,1=1 ,M) S 1 = 0 .0028 _.. NV=IFIX((V2-V1)/DV+0.5)+1 NG=I FI X(GT/DT+C5) +1 GT?=GT/?.O : : : NG2=(GT2/SI+0.5) IF(NCI-I)11,11,12 11 T2 = T1+GT . . ....... GO TP 13 12 Tl=Ti+GT2/2.0 T?= T I + F L.r.AU.bi.cj J * £ i 2 : : : 13 CPN TINUF WPITE<6,777) WPITF (fc ,66 t ) M, NGt MGI _ ..... .. WRITFt t,777) WFITF(6,2000)T1,T2,V1,V2,GT,DT,DV pp i T =i , M : 1 r i S T ( I ) = TDIST{I 1*1.AE WPITF(6,777 ) V:FITE (6,5000) (DIST (I) ,1=1 , M) . KR ITF( 6,777) MM = M-1 T T=NPI-1 : PP 2 IX=1,KM 2 ri5T5C(IX)=DIST(IX+1J**2-CIST(1X)**2 C A L L R F T A P (A , N 5 H ) T ( 1 ) = T 1 V ( l ) = V3 P P *• T V = 1 . N V : VSO( 1V) = V( IV )**2 V( IV + l )=V( I V)+DV 4- C P N U N U E : : . _ KCT=IF IX(DT/SI ) r.'STAFT-IFI X ( (Tl -P.START (1 ) ) / SI+O .5 ) +1 N F N " = T F T X ( ( T 7 - F : « ; T A P T ( 1) ) / S T + n . » n + v NS = NFNT-N'ST ART WRITFt6,777) W RI Tf ( 6, 2 22 JN START , hi END.... __. 222 FORMAT('NSTART=» ,I 5 ,5X,»NENP=«, ! ) TIK'1=T1 T TN? = TJL+PT . NH(1}=NH(1)-l DO 10 IT=1,NGI _4FJ.T__J 6.,JUJJ 155 WRIT F (6 , 6100 )T IN 1, TIN2 V-,FITF ( 6 , 7 7 7 ) CCHMAX=0.0 LQfiQ : DO 2 0 I V=l , NV IC=1 D_0_e.O_.I.N.= l.jNS t K i l l )= IF IX ( ( T ( 1 ) - R S T A R T ( I N ) ) / S l + 0 . 5 ) + l N=NH(IN) DXL_3J1_JJ^JL,^1 : '. IC=IC+1 T( IC ) = S 0 R T ( T ( I C - 1 ) * * 2 + DIST SO ( I C-1 ) / VSP (I V) ) _ KT ( J C) = 1 F IX ( (T (.1 C) -RST ARj_(J.fc|.U/_SJ+.0.5J+. 1 30 CONTINUE PO CONTIMUF J EJJV_.JC-.JJ-_Grj TP B i C f l l SUKMAX(A ,KT ,KDT ,M,NG,COH) COH'M( IV ) = C'OH I F (COHM( I V) .GT .COHNAX ) I0=IV 1F(C0HM( I V) .GT.COHNAX) COHMAX = CCHM ( IV) GO TO 20 _ e fl_cii.hi!ij.u . a : — 20 CONTINUE DO 40 1V=1 , NV COHM( I V) =CPHM( I V) /CCHN'AX _ 40 CCNTJNUF V F I T r ( 6 , 7 7 7 ) k £ JJ"J_ILJ_9 OQO )_ijD_f_yj JLCJ : I K = I C C F = C P H M ( 1 0 ) V01 = V( I 0) _ . . SU 5=0.1 41 COHM£X = CCHf'( I OJ -SUB : f 0HV, ( T0 ) =r. PHM ( 1 P ) - S U P - 0 . 0 1 r c 45 1V= l , N V I F I C C H M ( I V ) . G E - C O H N A X ) I O = I V I F ( C P Hf-1 ( IV ) .GE , C O H F £ X ) _ CO.HM AX.ELCJHHNLU V ) 4 ? CONTINUE VP2 = V( I Cl IJLLVX2.r o . v n i ) r-n TP -̂6 GO Tp t.i ';6 CONTINUE ... CCH'-:(JK)=.CH W R I T F ( 6 , 7 7 7 ) WFITE ( 6 , ° 0 0 0 ) 1 0 , V ( 10) kJUI^A,_£l_nOJ : , 00 50 I V = l ,NV I F ( I V . G E . N V - 4 ) GO TO 36 . FU_B=0.15 _ SN'X = C.O DO 33 1=1,5 T K = T - 1 _ _ _ 53 S f* X= SMX + C OH M { I V + I K ) C AV= S K X/S . O I F { C Cm ( I V ) .GT . ( C A V - R U B.J. .AND. COHN ( IV ) . L T . ( C&V- .RUB ) ) COHM( I V ) = H . ;~> 156 3 6 C O N T I N U E V>RJ TE ( 6 , 8 0 0 0 ) I V , C C H M I V ) P L T A ( I T , I V ) = C O H M ( I V ) c 0 r O N T T N U F : I F ( I T . E O . N T ) GC TO 1 0 , • T I N 1=T I N 1 + GT 2 _ .•_ J J N2=T .T M2 + G I 2 T ( 1 ) = T ( D + G T 2 1 0 C O N T I N U E 0 P P = C.21 Y S T = 1 . 5 C A L L PL V F . S P ( P L T A , M G I , N V , Y S T , O R D ) C A L L PL jDTND ... : 6 6 6 F O R M A T ( • M = » , I 2 , 2 X , •NG=» , 1 2 , 2 X , • N G I = » , 12 ) 7 7 7 F O R M A T ( • • ) TOOn F P P M A T ( F 5 . 2_ .2JE6^3__L3_ , 2£6_.. 2_, JL5_-.2J 1 1 0 0 F C R M A T ( 3 0 I 2 ) 2 0 0 0 F O R M A T ( • T l = 1 , F 5 . 2 , 2 X , • T 2= • , F 5 . 2 , 2 X , • V 1= • , F 5 . 2 , 2 X , 'V2=' , F 5 . 2 , 2 X , ' T I *M.F GATF . = ' , F 6 . 3 , 2 X , 1 T I ME. S T E P= V, F6 . 3 , 2 X , • V EL OS T C P =.._'» F t . 3 ) . 3 0 0 0 FORM A T ( 1 0 F 7 . 3 ) ^ 0 0 0 F O R M A T ( ( 3 0 X , 6 F 7 . 3 , 8 X ) ) 5 0 0 0 F OF M A T ( ' D I S T ( K M ) ' , 1 X , 1 7 F 7 . 2 , / . ( 9 X . 1 7 F 7 . 2 , / ) ) 6 C 0 0 F O R M A T ( , K T ( I ) = , , 1 X , 1 9 I 6 ) . 1 0 0 F O R M A T C ; O X INT FRV AL T= ( ' , F 4 . 2 , ' , F A . 2 , • ) S E C ' ) 7 C 0 0 F C R M / T ( 9 X , 1 9 I 6 , / , ( 1 I X , 191 6 , /_).) f 0 0 0 F O R M A T ( 3 X , 1 3 , 3 X , F 6 - 3 ) F i l O C F O P MAT ( 1 I X , ' C O H M ' ) 9 0 0 0 F O F M A T ( A R X . ' V ( ' . ? ? . ' ) = ' . F 5 . 7) S T O P E N D 157 C SUPRCtlT INE RET/F.A.NSH) C DIMENSION PATON(56) ,A(20,3840) INTEGEP*2 L EN 1000 FORMAT(313) . ,2000 FORM* TL_F-I LE NO. '.13,' IS ANALYSE C ) 16 PF.AD(5, 1000, END= 1008)NFSK,NPSK , NPEC NF=NFSK MP SK = ftO-NF FC NPTS=NFEC*96 CALL SKIP(NFSK,NRSK,1) _ . DO 60. J=1,NSH NF=NF+1 DO 61 NCH = 1 ,6 DO 63 I=1.NRFC . 11=1-1 CALL FFAD(DAT0N,LEN,0,LNUN,1, C1008 ) DO 64 K=l, 96 _ 64 A INCH.II*96+K) = CATCN(K ) 63 CONTINUE IX.{NCH. EX . 6 ) GO TO 61 CALL SKIP(C,MRSK,1 ) 61 CONTINUE WFITF U ,2000 INF IF(J.EO.NSH) GO TO 60 CALL SKIP. 1 t NF.SK, 1) 6 0 CCNT IJILLF 1008 FETUFN END SUBROUTINE SUMKAX.A,KT,KDT T M ,NG,COH) - C D I_NENSICN A ( 2 0 , 3 8 4 0 ) , K T ( 6 0 ) , P ( 20 ) PMAX=0.0 DO 1 J = 1,NG. .. . P ( J ) = 0.0 DO 2 1=1,M K = K T ( I ) P ( J ) = P ( J ) + A ( I , K ) 2 CONTINUE: . P( J ) =/.BS( P( J). .**N) I F ( p ( J J . G T - P M A X ) PMAX=P ( J ) I F ( J . F O . N G ) GO TO 1 D 0_.lJQ_J__E.Lti_ 10 KT (I ) = KT ( I )+KDT 1 CONTINUE DO 20 I = 1,M . . . 20 K T ( I ) = KT( I ) - ( N G - l ) * K D T COH=PMAX I F_{ C C H . L E . 0.0 )._CD H= 0 .jOJ RETURN END S U E K O U T I K E S E M B L M A , K T , K C T , M , N G , C G H ) C I K E K ' S I C K A (20 , 3E40) ,KT ( 60) , P ( 20) , Q ( N"A"F=1J. 0 D C L ^ O . O DO 1 J=1,NG ' ~ P T J )"=Omt : C ( J ) = 0 . 0 DO 2 1=1,M  Ficmi P ( J ) = P ( J ) * / U , K ) Q ( J ) = 01 J ) •*-A ( I , K ) * * 2 2 C C M T N I / E " P ( J J = P ( J ) * * 2 N A (- = NAH+ P ( J ) C~CL = D C L + C ( J ) I F ( J . E O . N G ) GO TO 1 DO 10 I = 1 , M 10 K T T l ) = K T ( I H K D T 1 C O N T I N U E CQ 20 1 = 1 , M 20 K T ( 1 J = K T . ( I ) - ( N G - l J * K C T £ C=NAH / M * D C L C G H = ( M * S C - 1 ) / ( N - l ) rrcc CTRTC ETCTOT~C _H=~ GTGT RETURN ENC 160 SUBROUT1 NP UNNCCR ( /,KT , KTT , M, NG, CPH) C PI MENS I ON A(20,38AQ), K T( 6 0 ) , P ( 20 ) , 0( 20) UNC = C;.C' DO 1 J=1,MG PJ.J) = p.O QiJ)=0.0 DO 2 1=1,M K = KT ( T ) . ' P ( J ) = P ( J ) + A ( I , K ) 0 ( J )=0(J ) +A{I,K)**2 2. CONTINUE P ( J) =P ( J) **2 UNC=UNC+ ( P (J }-0 (J ) ) IF(J.F.Q.NG) GO TO 1 DO 10 I=1,M 10 KT ( I ) = KT ( I ) +KDT I.. CPNT.I.NUF_ DO 20 1=1iM 20 K.T(I) = KT(IJ-(NG-1)*KDT. CPH =UNC*0. 5 IF(COH.LE.O.O) C0H=0.01 P.FTUPN S U R R 0 U T 1 N E P L V E S P . P L T A , ^ G I . K V , Y S T , C F C ) C nM FNF,T O N PI TA (70. .nnn ) S C = 5 . 0 P Y = 0 . 0 3 XO = _ P R P D O 1 I = 1 » N G I C A L L P L O T . X O + O R D , Y S T , 3 ) YJELYST-DY D O 2 J J = l F N V Y = Y + D Y X = P L T A { I . J J ) / S C + X C M C R D . C A L L P L O T ( X , Y . 2 ) 2 C C N T I N U E xo=xo+npn 1 C O N T I N U E R E T U R N E N D .

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