O n t h e S e d i m e n t a r y S t r u c t u r e i n S o m e P o r t i o n s o f t h e P a c i f i c O c e a n f r o m d a t a o n S e i s m i c R e f l e c t e d W a v e s (*) S . M . Z V E R E V R i c e v u t o il 7 dicembre 1960. 1. During 1957-1958 the Institute for Physics of the E a r t h of the Ac. Sci. USSR carried out investigations by deep seismic sounding (DSS) of the earth's crust in the north-western part of the Pacific ocean and in the adjoining aquatoria (l-2). The investigation technique consisted mainly in following on the seismograms the seismic waves coming from the deep boundaries in the crust. At the same tune data were -also collected on the structure of the upper layer consisting of ocean sediments. 2. Simultaneously with the seismic waves coming from the deep crustal layers, vertical reflections from the bottom surface and from the boundaries in the sediments were also observed in order to investigate the structure of the sediments. The observations of the reflected waves were carried out by two methods. When heavy charges (above 100 kgr) were fired for DSS observations, a unichannel set on the moving ship was used for recording reflected waves. A piezocrystal secured to the underwater part of the ship's hull served as a receiver. The preamplified signal was recorded in a wide frequency range and then passed through a narrow band-pass filter. The frequency band was so chosen as to increase the effective sensitivity of the channel. I t was known from experience t h a t good records of reflected waves were obtained in the 50-70 c/sec frequency range. A sample seismogram is shown in fig. 1. On this and similar seismograms not only reflections from subbottom boundaries are seen, b u t also pulses due to secondary shocks and possibly also to reflections from the water surface a t the shooting point. (*) P a p e r r e a d a t t h e Helsinky Assembly of t h e I . U . G . G . 1960. 1 8 8 S . M. Z V E R E V B y this method reliable records of waves reflected from boundaries in the sediments were obtained a t ocean depths of 5-6 km. I n the region of the deep-water through the intensity of the reflected waves decreased and this method proved not sensitive enough to produce reliable records. The conditions for firing the charges and for reception were sufficient- ly uniform, so t h a t we were able to compare t h e behaviour of t h e reflect- ing boundaries over large areas. Fig. 1 - A s a m p l e record of waves reflected f r o m t h e b o t t o m s u r f a c e (B) a n d f r o m t h e b o u n d a r i e s in t h e s e d i m e n t s (1,2,3), o b t a i n e d on t h e m o v i n g ship. Traces 4 a n d 5 a t t h e t o p are r e c o r d s of d i f f e r e n t l y amplified signals. 3. During t h e short stops of t h e moving ship recording was m a d e on a floating multichannel set similar to those used in prospecting for oil. I n t h e observations of 1958 a set consisting of six hydrophones distanced at 50 metres was used. Their stretching into line was left to the ship's wind drift. The seismic observations of the reflected waves were carried out simultaneously with oceanologic observations. Charges of 1-5 kgr suspended on rubber floats were used. The amplifiers chosen were sim- ilar to those used in prospecting. Beflected waves having frequencies of 30-35 c/sec were recorded. A sample seismogram is given in fig. 2. Since our reciever contained no volume control nor mixer, we were able to m a k e use of the dynamic characteristics of t h e records. Because of different ways in which the charges were fired and rec- ords made, the absolute times of arrival of waves reflected f r o m t h e same b o u n d a r y obtained from both sets differed considerably. A f t e r corrections had been introduced for the m o v e m e n t of the ship and for mr-wsi JS153 MOB HP 9-0 K PI W / W A W / W A W / W A \A. wo Pig. 2 - A s a m p l e record of reflections f r o m t h e b o t t o m (B) a n d f r o m t h e b o u n d a r i e s in t h e s e d i m e n t s (1, 2, 3) o b t a i n e d on t h e m u l t i c h a n n e l set. Traces 1-5 a t t h e t o p — are records f r o m different h y d r o p h o - nes. Traces 6-9 r e p r e s e n t , records f r o m t h e 1-t, 3-d a n d 5 - t h h y d r o p h o n s h a v i n g a lower s e n s i t i v i t y . o a H B ' H CO M O H H > » H( U1 H » a o H « w is 3 <» o w © w H O 2! 00 o H B •H > o o o a H > 188 S . M . Z V E R E V different shooting depths this difference decreased. However, even t h e n the times of arrival of t h e b o t t o m reflections obtained from the multi- channel set exceeded by 0,01-0,07 seconds those f r o m the records b y the unichannel set. This might have been connected with different depths of penetration into sediments of waves of different frequencies (low- frequency waves p e n e t r a t e deeper). Using t h e multichannel set, we were able to record waves reflected from the boundaries in the sediments in t h e regions of the ocean bed, of the deep-water trough and also in some places on the shelf. 5. I t should be noticed t h a t sections constructed from experimental d a t a on vertical reflections might be distorted by multiple reflections of waves in crustal layers, by interference of waves whose wavelengths are comensurable with t h e thickness of the layers involved and b y other complicating phenomena we are usually confronted with in seismic prospecting. I n our case when studying thin sedimentary layers under- lying a deep water layer, with the time-distance curves extending not far enough, discrimination of interfering phenomena becomes even more difficult t h a n in usual prospecting. T h a t is why the here given sections should be considered as a simplest interpretation depicting to some extent t h e real situation. 6. Vertical reflections were observed on all profiles m a d e in 1958. Let us consider the d a t a obtained on t h e profiles running close t o t h e K a m c h a t k a peninsula and the K o m a n d o r islands Profiles NoNo 7 and 9 crossing t h e continental shelf near t h e K a m c h a t k a peninsula and the K u r i l e - K a m c h a t k a trough, ran into the deep-water bed of t h e Pacific ocean extending a t a distance of 500 k m from t h e shore. Profile No 8 crossed t h e shelf near the K o m a n d o r islands and also t h e Aleutian deep- water trough, ending within t h e boundaries of the Pacific border swell a t a distance of 150 k m f r o m the flat p a r t of the ocean bed. Sections through these profiles are shown in fig. 3, 4, 5. The thin lines whose lengths are proportional to the intensities of the reflections taken from t h e seismograms illustrate the reflecting power of t h e boundary under consideration. The d a t a obtained f r o m the multichannel set are marked out. 7. The sections t h r o u g h the sediments are here represented in times of arrival instead of in thickness. This was done because with the methods used for observations of refracted waves (DSS) a n d of vertical reflections P i g . 3 - T i m e s of a r r i v a l of v e r t i c a l reflections on profile No 9 crossing t h e K u r i l e - K a m c h a t k a d e e p - w a t e r t r o u g h beginning f r o m t h e s o u t h e r n e x t r e m i t y of t h e K a m c h a t k a p e n i n s u l a . 1) t i m e s of a r r i v a l o b t a i n e d f r o m o b s e r v a t i o n s of reflections m a d e on t h e m o v i n g ship; 2) t i m e s of a r r i v a l o b t a i n e d f r o m t h e m u l t i c h a n n e l s e t ; 3) places on t h e profile w h e r e a s h a r p change in t h e t i m e s of arrival of reflections f r o m t h e b o t t o m a n d f r o m t h e b o u n d a r i e s in t h e s e d i m e n t s w a s observed. <00 150 200 250 J 00 350 400 450 5 0 0 550 P i g . 4 - Times of a r r i v a l of v e r t i c a l reflections o b t a i n e d on profile No 7 crossing t h e K u r i l e - K a m c h a t k a deep-water t r o u g h f r o m t h e region of P e t r o p a v l o v s k - K a m c h a t s k i . D e s i g n a t i o n s t h e s a m e as in fig. 3. O N T H E S E D I M E N T A R Y S T R U C T U R E I N S O M E P O R T I O N S O F T H E P A C I F I C O C E A N 1 9 1 the determination of velocities in the sediments becomes extremely difficult. Although some d a t a on the velocities were obtained, this pro- blem cannot be considered as finally solved. However, the here given d a t a can be used for a comparative geologic interpretation. Moreover, t h e velocity range in the sediments might be indicated and the thickness of separate layers roughly calculated. 2 0 0 1 7 5 1 3 0 I 2 J 0 0 7 J J O Fig. 5 - Times of a r r i v a l of vertical reflections on profile No 8 crossing t h e A l e u t i a n d e e p - w a t e r t r o u g h f r o m t h e K o m a d o r islands. 8. In evaluating t h e velocities in t h e sediments use was made of t h e multiple reflections in t h e water layer picked out from the T)SS seis- mograms and also of the reflection coefficient calculated from different vertical reflections. F r o m an analysis of the multiply reflected w a t e r waves, including the regions of growing amplitudes connected with the critical angles (3), on one of the profiles considered below running above t h e ocean bed, the following values of velocities were obtained: in t h e subbottom layer — 1,62-1,67 km/sec (*) and in the deeper layer — 1,89-1,91 km/sec. The way in which t h e experimental d a t a agree with (*) Here a n d below t h e t h i n layer (5-14 m t h i c k ) covering t h e sea bot- t o m is n o t t a k e n i n t o consideration. W e a k reflections f r o m s t h i s layer can be seen on t h e seismograms of fig. 2. 188 S . M . Z V E R E V theory (fig. 6) gives reasons to consider t h e velocity 1,89-1,91 km/sec as belonging t o transversal waves. If so, t h e n the velocity of t h e longi- tudinal waves in t h e deeper layer becomes 3,2-3,5 km/sec. F r o m t h e relation between the amplitudes of the first and double vertical reflections t h e following average reflection coefficients were obtained: for t h e b o t t o m surface — 0,17; for the sharpest b o u n d a r y — 0,58 a n d for t h e deepest reflecting b o u n d a r y — 0,34. Calculations of t h e vertical reflection coefficients b y means of R a y - leigh's formula have shown t h a t there is a satisfactory agreement bet- ween t h e above given d a t a and t h e following values of velocities v and densities o in t h e sediments illustrating their variation with d e p t h : a) ocean b o t t o m sediments (unconsolitated) v = 1,65 km/sec As will be seen below, the thickness of t h e layer with a velocity of 1,65 km/sec is of t h e order of 300-600 m and t h a t of t h e layer with a velocity of 3,5 km/sec a b o u t 150-300 m. This gives for t h e average veloc- i t y t h r o u g h o u t the entire sedimentary layer a b o u t 2 km/sec. All t h e above given velocities are very approximately estimated. I t should be noticed though, t h a t all t h e obtained d a t a are not yet used and in particular, t h e existence of a thin layer of consolidated sed- iments having a velocity of 3,5 km/sec should not be considered as finally proved. F u r t h e r investigations are needed to determine more accurately t h e distribution of velocities in the sediments. 9. W e shall now consider t h e sections t h r o u g h the profiles made. On all three of t h e m a sharp change in the n a t u r e of sediments is observ- ed when crossing t h e deepwater trough. On t h e eastern slope of t h e trough a distinct and relatively stable layering of t h e sediments is ob- served for long stretches of t h e profile. The a m p l i t u d e of the b o t t o m reflection is usually 2-4 times smaller t h a n t h a t of t h e later reflections. This indicates t h a t t h e sea b o t t o m is a less sharp b o u n d a r y t h a n t h e deeper boundaries in t h e sediments. On t h e west slope of t h e trough facing the land t h e layering of t h e sediments is less obvious. The reflect- c) b o t t o m of sediments (crystalline rocks) b) deeper sediments (consolitated) q = 1,35 gr/cm3 v = 3,50 km/sec o = 2,50 gr/cm3 v = 6,40 km/sec ~g = 2,80 gr/cm3 O N T H E S E D I M E N T A R Y S T R U C T U R E I N S O M E P O R T I O N S OF T H E P A C I F I C O C E A N 1 9 3 ing power of t h e sea b o t t o m is very variable and on the whole 2-3 times larger t h a t of the ocean bed region. This means high densities and velo- cities of t h e rocks of the sea b o t t o m whose composition is rapidly carying. 10. On profile No 9 running to south-east of the southern extremity of t h e K a m c h a t k a peninsula, on t h e western slope of the deep-water trough 3 reflecting boundaries were observed. The difference in times of arrival (At) of waves reflected f r o m the first and third boundaries was 0,2-04, second (fig. 3). Fig. 6 - A comparison of e x p e r i m e n t a l a n d calculated ampli- t u d e curves for w a t e r w a v e s t h r i c e reflected in t h e ocean h a v i n g for an object t h e d e t e r m i n a t i o n of velocities in t h e s e d i m e n t s : 1) e x p e r i m e n t a l d a t a ; 2) d a t a calculated for t h e first critical angle ( t h e second m a x i m u m in t h e c u r v e corresponds t o a velocity of 1,65 km/sec in t h e s u b b o t t o m l a y e r ; a n d t h e first m a x i m u m — t o t h e velocity 1,90 km/sec of longitudinal waves in t h e deeper s e d i m e n t a r y l a y e r ; (3) d a t a calculated for t h e second critical angle (the fiist m a x i m u m corresponds t o a velocity 1,90 km/sec of t h e t r a n s v e r s a l waves in t h e deep s e d i m e n t a r y layer). I n t h e region of the deep-water trough 5-6 reflecting boundaries were found, At between successive arrivals having been 1,5-1,8 seconds. Within t h e boundaries of t h e oceanic swell, bordering the south- eastern slope of the deep-water trough, 2-5 reflecting boundaries were observed. Near the trough At varied considerably from 0,2 to 0,9 seconds thus indicating a complex relief of the surface underlying the sediments. At a distance of 130 km from t h e axis of the trough a sharp change in the 20 10 0 o 1 • 2 + 3 188 S . M . Z V E R E V relief of t h e b o t t o m a n d a displacement of t h e reflecting elements was found indicating a faulted surface of t h e rocks underlying t h e sediments. At, longer distances f r o m t h e axis of the trough 3-4 reflecting boundaries were detected. W i t h moving away f a r t h e r f r o m t h e axis the relief of t h e reflecting surface became more uniform a n d t h e time interval between successive reflections decreased, reaching 0,4-0,7 seconds. Assuming and average velocity in t h e sediments of 2 km/sec we obtain t h e follow- ing values for t h e thickness of t h e layered sediments: in t h e region of t h e trough — 1,5-1,8 k m ; in the ocean bed elose to the trough — 0,2- 0,9 k m : along t h e south-eastern p a r t of t h e profile — 0,4-0,7 k m . The thickness of t h e entire sediments within t h e trough might h a v e been larger t h a n 2 k m since to all appearance no reflections from t h e b o t t o m of t h e sediments were obtained. W i t h i n the boundaries of the north-western slope of t h e trough the s u m m a r y thickness of the sediments down to the " granitic " layer could not be obtained f r o m the here considered d a t a . I t might be only stated t h a t t h e thickness of t h e almost horizontally layered sediments, giving appreciable seismic reflections, does not exceed 100-300 metres. Underlying the sediments, according the DSS d a t a , are rocks also having small velocities of seismic waves. The absence of reflections shows t h a t these rocks are of a complex nature. 11. The section through profile No 7 (fig. 4) is basically similar to t h a t of profile No 9. The relief is more uniform and sediments are f o u n d near t h e south-eastern slope of t h e trough. Here At varies between 0,5 and 0,9 second (and t h e thickness f r o m 0,5 to 0,9 km). At a distance of 250 k m f r o m t h e axis of t h e trough a f a u l t was detected cutting all t h e sedimentary layers and appearing in t h e relief as a ledge with an amplitude of a b o u t 800 m. A f a u l t of a smaller amplitude was found a t a distance of 200 k m f r o m the axis of t h e trough. A t t h e south-eastern end of t h e profile t h e thickness of t h e sediments was 0,4-0,7 k m . 12. Let us discuss now t h e d a t a obtained on t h e profile running- south-west of t h e K o m a n d o r islands through the aleutian trough (fig. 5). These d a t a are less reliable because of b a d weather which prevented us f r o m using t h e multichannel set. On t h e side facing t h e shelf 2-4 reflecting boundaries were f o u n d corresponding to At not exceeding 0,5 second. I n t h e region of the trough and to t h e south of it the n u m b e r of reflecting boundaries increased shar- ply reaching 8 and even more. The time interval At between successive O N T H E S E D I M E N T A R Y S T R U C T U R E I N S O M E P O R T I O N S O F T H E P A C I F I C O C E A N 1 9 5 reflections was 2.0-2,5 seconds, i.e. 2-3 times those observed in the ocean bed on profiles NoNo 7 and 9. From these figures follows t h a t in t h e region of the Pacific ocean border swell near t h e Aleutian trough t h e thickness of the sediments was no less t h a n 2,0-2,5 km. C O N C L U S I O N S . 1. The deep water trough divides the horizontally layened sediments of t h e ocean bed only, relatively slightly varying in thickness, from the variabfe sediments of t h e shelf ha ving a complex structure. The thickness of t h e sedimentary rocks of the ocean bed decreases with distance f r o m t h e trough. 2. Within the boundaries of the ocean swell, bordering the Kurile- K a m c h a t k a trough, the surface of the rocks underlying the ocean bed sediments has a complex relief mainly levelled out by sediments. When moving away from the trough the relief of t h e sediments b o t t o m becomes more uniform. A t distances of 150-250 km from t h e axis of the trough faults of large amplitudes are found catting all the sedimentary layers and showing in the relief of the bottom. These faults had been probably produced b y relatively recent tectonic movements occuring on t h e bor- der of t h e ocean bed near the deep-water trough. 3. W i t h i n t h e boundaries of the Pacific border swell near t h e Aleu- tian deep-water trough, the thickness of the sediments is 2-3 times t h a t of t h e sediments in t h e ocean bed, close to the Kurile-Kamchatka trough. This indicates t h a t the conditions for accumulation of sediments in t h e outer p a r t s of t h e Kurile-Kamchatka and of the Aleutian deep-water troughs are essentially different. SUMMARY The special observation of vertical reflected waves, made during the I G Y simultaneously with deep seismic sounding in the Pacific Ocean, allowed to study the structures of sediments, covered the ocean bed near Kurilo-Kamchatlca and Aleutian trenches. The profiles of observations crossed some great faults, disturbing sedimentary complex on the whole its depth. 1 9 6 S . M . Z V E R E V SOMMARIO La particolare osservazione delle onde verticali riflesse, effettuata du- rante VA.G.I., contemporaneamente al sondaggio sismico profondo nell'oceano Pacifico, ha permesso di studiare le strutture dei sedimenti, ricoperte dal fondo delVOeeano presso Kurilo-Kamchatka e le trincee Aleutine. I profili delle osservazioni hanno incontrato alcune grosse faglie, che disturbano il complesso sedimentario in tutta la sua profonditd. R E F E R E N C E S T1) G A L P E R I N , E . I . , G O R J A C H E V A . V . , Z V E R E V S . M . , Investigation of the earth's crust in the zone of transition from the Asiatic continent to the Pacific ocean. P u b l . H o u s e of Ac. Sci. U S S R , M. (1958). (2) VASILOEV V. G. a n d otli., Investigation of the earth's crust in the zone of transition from the Asiatic continent to the Pacific ocean. S y m p . " Seismic i n v e s t i g a t i o n s d u r i n g t h e I G Y " 4, P u b l . H o u s e of Ac. Sci. U S S R , ( 1 9 6 0 ) . (3) ZVEREV S. M., Multiply reflected water waves and their use for the deter- minaion of velocities in the sediments. T r a n s . Ac. Sci. U S S R , g e o p h . s e r . 1 . ( 1 9 6 0 ) .