A new high-frequency (3.5-kHz) sub-bottom profiling system is capable of relatively deep penetration into sedimentary deposits, with good resolution of thin reflecting layers. This system has been operated at cruising speeds up to 11 knots and gives up to 0.15 seconds of penetration...

Kulm, L. D.

Moore, T. C., Jr.

English

Yale University

      P.O. BOX 208118 | NEW HAVEN CT 06520-8118 USA | PEABODY.YALE. EDU   JOURNAL OF MARINE RESEARCH The Journal of Marine Research, one of the oldest journals in American marine science, published important peer-reviewed original research on a broad array of topics in physical, biological, and chemical oceanography vital to the academic oceanographic community in the long and rich tradition of the Sears Foundation for Marine Research at Yale University.  An archive of all issues from 1937 to 2021 (Volume 1–79) are available through EliScholar,  a digital platform for scholarly publishing provided by Yale University Library at  https://elischolar.library.yale.edu/.  Requests for permission to clear rights for use of this content should be directed to the authors, their estates, or other representatives. The Journal of Marine Research has no contact information beyond the affiliations listed in the published articles. We ask that you provide attribution to the Journal of Marine Research.  Yale University provides access to these materials for educational and research purposes only. Copyright or other proprietary rights to content contained in this document may be held by individuals or entities other than, or in addition to, Yale University. You are solely responsible for determining the ownership of the copyright, and for obtaining permission for your intended use. Yale University makes no warranty that your distribution, reproduction, or other use of these materials will not infringe the rights of third parties.  This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. https://creativecommons.org/licenses/by-nc-sa/4.0/   A High-resolution Sub-bottom Profiling System far Use in Ocean Basin/ T. C . M oore, Jr. and L. D. Kulm Department of Oceanography Oregon State University Corvalli s, Oregon 9733r ABSTRAC T A new high-frequency (3.5-kHz) sub-bottom profilin g system is capable of relatively deep penetrati on into sedimentary deposits, with good resolution of thin refl ecting layers. This system has been operated at cruising speeds up to r r knots and gives up to o.r 5 seconds of penetration. Profil er records made aboard the R . V. CAY USE and R . V. ARGO indicate that this system is particularly useful for definin g shallow structures and areas of outcrop and that it will undoubtedly be valuable in sedimentological and stratigraphic studies. Used in combination with a low-frequency profilin g system, a good overall picture of the sediment and structure in ocean basins may be obtained. Introduction. Numerous acousti c systems have been used to obtain con-tinuous seismic refl ection profil es of sedimentary deposits (e.g., Smith 1958, McClure et al. 1958, Beckman et al. 1959, Ewing and Zaunere 1964). On the basis of the frequency of sound employed, these systems can be divided into two general groups: (i) those using low-frequency sound (10-500 Hz) that gives deep penetration of the sediments but poor resoluti on of thin re-fl ectors, and (ii) those using a high-frequency sound source (3.8 to 14 kHz) that gives good resoluti on of thin refl ecting layers but only shall ow penetra-ti on. The high-frequency (high-resoluti on) reflection profiling systems have been used extensively in studies of sediment accumulation in lakes and coastal waters and in investigati ons on continental shelves and slopes (Smith 1958, M oore 1960, van Andel and Sachs 1964 ). In these studies, sedimentary and structural features as well as areas of outcrop are clearly revealed on the profil er records. However, in deep water, the signal strength of most existing high-frequency systems is so greatly reduced by attenuation and absorption that sub-bottom echos are rarely recorded (Hersey 1963). Only under conditions of maxi-mum output power and low ambient noise is even shallow penetrati on obtained. 1. Accepted for publication and submitted to press 24 November 1969. Journal of Marine Research T hus, most deep-water reflection work has made use of the low-frequency systems. These systems often allow penetration to the acoustic "basement" but do not all ow the resolution that is necessary to accurately locate areas of outcrop and defin e structures in near-surface sediments. The growing interest in deep-sea sedimentation and stratigraphy and in the sediments and structure of the lower continental slope and continental ri se have accentuated the need Table I. The 3.5-kHz system-Edo W estern Corporation Model 400 bot-tom-penetrati on system. Acoustic power Pulse rate Pulse length Beam width Receiver bandwidth Weight 8000 watts (max) 4 per second (max) 0.2 milliseconds (min) Conical 30° ± 900 cycles 800 lb. (hull mounted) for a high-resolution reflection profiling system that can be oper-ated effectively at normal cruising speeds in the deep ocean. The 3.5-kHz System. The basic characteristics of the new high-frequency (3.5-kHz) system (Edo Western Corporation, model 400) are given in Table I. The transducer may be either towed or mounted on the hull. In the towed configuration, the record generally contains less noise than records obtained with a hull-mounted transducer. Attached to a nylon-polypropylene line (60,000-lb. breaking strength), the transducer may be towed at speeds in excess of Io knots. If a towed transducer is used, the system may be moved from ship to ship. However, for overall ease of operation, the hull-mounted system is more desirable. On the R. V. ARGO, the maximum speed of operation of the hull-mounted system is about I I knots and seems to be limited by water noise around the hull mounting. Careful design and placement of the hull mounting might increase the maximum speed attainable. The dominant frequency of this system is on the lower end of the high-frequency range used in previous profiling studies. The relatively low fre-quency of this system together with the high acousti c power output (up to 8000 watts) allows penetration in deep-water sediments of up to o. I 5 seconds (two-way travel time). In theory, the minimum pulse length of 0.2 milli-seconds and a frequency of 3.5 kHz should all ow the resolution of layers 0.5 m thick. Features that are usually hidden in the outgoing signal ("bubble") pulse of low-frequency profilers may be easily defined with this system. The re-latively deep penetration of the 3.5-kHz signal often results in the detection of refl ectors just before they merge into the surface echo of a low-frequency profiler. Thus, the combination of a low-frequency (deep-penetration) system with the 3.5-kHz profiler can provide a much more complete picture of sedimentary and structural features than either system by itself. Results. On recent oceanographic expediti ons, the 3.5-kHz system has been used in the towed configuration (aboard the R. V. CAYUSE of Oregon ] J: 1-(L w a 1000 1100 1200 1300 1_400 [5 IO00 3: 110 1200 1300 1500 1600 Moore and Kulm: Profiling System 0 2 4 I +wl ......l (km) X 22.0 2 73 1.3 1.4 1.5 1.6 17 "ti I.B _; w :. f:= .J w :i( 1.3 I= 1.4 1.5 .6 .7 .8 Figure 1. Outcropping sediments on the Walvis Ridge (26.5°S, 6°E). Records taken aboard R. V. ] :i: I-0.. w 0 er w 3: 4800 4900 5000 4800 5000 ARGO. a, 3.5-kHz record; b, air-gun record. 0 2 4 I --' ewwJ (km) a b X 22.0 .6 6.7 'J ., $ w 6.8 i= ...J w ;ii 6.6 er I-6.7 (\J 6.8 Figure 2. Outcropping layers south of Walvis Ridge in the Cape Basin (27.5°S, 7°E). Records taken aboard R . V. ARGO. a, 3.5-kHz record; b, air-gun record. 274 Journal of Marine Research [ 28,2 -E -:c: .,_ 0. w Cl 0::: w :: 5300 5400 5500 5300 5400 5500 5600 .. . ··,~ .. . • •• ,h\ b 0 2 4 X 22.0 (km) 8.3 8.4 8.5 ti Q) 1/) 8.6 .,_ ...J w 0::: 8.3 .,_ 8.4 3:: 8.5 8.6 8.7 I C\J Figure 3. Filled basin on the flank of the mid-Atl antic Ridge (8°S, 3°E). Records taken aboard the R . V. ARGO. a, 3.5-kHz record; b, air-gun record. State University) and as a hull- mounted unit (aboard the R. V. ARGO of Scripps Institution of Oceanography). Profiles taken on the southern flank of the Walvis Ridge (26°S, 6°E) show reflecting layers cropping out. The low-frequency (air-gun) record (Fig. 1 b) shows one probable area of outcrop at a water depth of approximately I I 50 m. The 3.5-kHz record (Fig. I a) also shows this reflector; but in addition, the record shows above it several other reflectors that may crop out. The air-gun record shows few reflectors in the upper part whereas the 3.5-kHz record shows in detail the thickening of sedimentary layers to the north of the crest (right) and the pinching out to the south (left). The air-gun record taken in the Cape Basin to the south of Walvis Ridge (27.5°S, 7°E) gives some indication that reflecting layers pinch out to the Moore and Kulm: Profiling System 4700 2 75 6.4 u .6 ., w l= 4900 ::E 6.7 Cl. w 0 0:: w ::: -Ei I l-a.. w 0 0::: w I-0 2 4 I wwl wwl (km) X 23.0 6.4 65 .6 6.7 ...J w 0:: t--~ (\J F igure 4 . Small filled basin (or channel) in the Cape Basin (29.6°S, 9. 5°E). Records taken aboard R. V. ARGO. a, 3.5-kHz record; b, air-gun record. a 5000 4900 b 0 2 4 (km) .7 .8 'J 6.9 6.7 6.8 6.9 Q> 1/) -Figure 5. Mounds of sediment in the Cape Basin (28.6°S, 7.7°E). Records taken aboard R. V. ARGO. a, 3. 5-kHz record; b, air-gun record. 2850 2900 E I 2950 I-Q_ w C) a:: w i 3000 3050 3100 'Journal of Marine Research 4.1 0 2 3 (km) [28,2 3.9 -0 4.0: -w 1-_J w a:: I-I C\J F igure 6. Edge of continental slope off Oregon (42.5°N, 125.2°W). 3.5-kHz record taken aboard R. V. CAY USE. south (left part of Fig. 2 b ). It is not until this record is compared with the 3.5-kHz record (Fig. 2a) that the nature of the structure is recognized. The high resolution of the 3.5-kHz system reveals layers cropping out over a distance of several kilometers. These layers do not reappear in other profiles taken to the south; they dip gently to the north and terminate abruptly at the foot of Walvis Ridge. The existence of some small fill ed basins can be detected on low-frequency profiler records (Fig. 3 b). However, variations in the layering and thickness of sediments in a basin are shown only in the high-resolution record (Fig. 3a). Other shall ow basins or channels may be completely missed on low-frequency Moore and Kulm : Profiling System 0.5 400 0.6 500 0.7 600 0.8 -(.) Cl> E w -::i: I-0.9 I-CL 700 w __J w 0 0:: w - -- · ·· ····-.·,-.. --:\.:.--- -. -··. 1.0 X 36.0 0:: I-!l: 0.6 3: I 500 (\J 0.7 600 0.8 0.9 700 1.0 0 2 4 X 24 .0 {km) Figure 7. Bench on the upper continental slope off Oregon (42°N, 125.5°W) . Records taken aboard the R. V . C AYUSE. a, 3.5-kHz record; b, sparker record. profiler records. Such a filled basin (or channel), over 2 0 km wide, was crossed off the west coast of Africa (8°S, 3°E). Without the accompanying 3.5-kHz record (Fig. 4a), this feature would have remained undetected in the air-gun profile (Fig. 4 b ). The near-surface layering seen in the 3.5-kHz records is commonly related to the gross structural or topographic features that are revealed in accompanying -E I f-a.. w 0 0:: w f--220 300 Journal of Marine Research o s m ""--=-=--=c...ic===' (km) 400 500 ---r---X 72 [28,2 -0~ 0.4 0 :l: 0,5 w :e ..J w 06 ;ii a:: f--3: I 07 C\J 0.8 Figure 8. Edge of continental shelf off Walvis Bay, Southwest Africa (2.5.5°S, r4.3°E). 3.5-k.Hz record tak.en aboard R. V. ARGO. low-frequency profiler records. However, in some cases, sedimentary struc-tures shown in the 3.5-kHz records do not seem to be related to the basement structure or topography. In the Cape Basin (28.6°S, 7.7°E) the 3.5-kHz record shows several large mounds (2 km wide) in the upper layers of the sedi-ments (Fig. 5 a). Some of these mounds show internal layering that is uncon-formable with the adjacent horizontal reflectors. The mound to the left in Fig. 5 a has been formed on top of a flat-lying refl ector while the large mound to the right appears to have gone through two "growth" stages and has no visible horizontal reflectors beneath it. In the air-gun record, there is no clear indicati on of a basement high or sub-bottom structural feature that might give ri se to these near-surface mounds of sediment; nor is there much indicati on of the mounds themselves (Fig. 5 b ). Most sounding and profiling equipment transmits large cones of sound. Consequently, angular interfaces, such as those often found between the con-tinental slope and the abyssal plain, are usuall y masked by hyperbolic echos and are difficult to examine acoustically. A profile made across the continental slope and abyssal plain off southern Oregon (42.5°N, 125.2°W), using a towed transducer, shows the good resolution of the 3.5-kHz system (Fig. 6). The abyssal deposits are horizontall y bedded except where they are sli ghtly upturned at the base of the slope. Virtually no useful sub-bottom information was obtained from the adjacent steep slope. Sediment cores (up to 8 m long) taken on the plain in the vicinity of the 3.5-kHz profile (Fig. 6) contain thin Moore and Kulm: Profiling System 2 79 (less than Io cm thick) fine-grained sand layers interbedded with thick hemi-pelagic silts and clays. Several topographic benches occur on the upper continental slope off south-ern Oregon. Profiles made in this area, using a low-frequency (sparker) system, show that these benches are the surface expressions of sediment ponding behind anticlinal folds (Fig. 7b). The 3.5-kHz record suggests that even the near-surface deposits have become distorted and folded on the seaward edge of the bench (Fig. 7 a). The use of this profiling system is not limited to deep water. Excellent records, showing up to 0.12 seconds of penetration (Fig. 8), have been ob-tained on the continental shelf south of Walvis Bay, Southwest Africa (25.5°S, 14.3°E). Surface sediment samples taken in this area contain sandy mud and shell fragments. From the 3.5-kHz record it appears that the continental shelf has been extended several kilometers seaward by depositional outbuilding, in the manner described by Moore and Curray ( 1963). No sub-bottom information was obtained with the 3.5-kHz profiler on the Oregon continental shelf. Cores taken off Oregon indicate that thick sand layers cover much of the shelf and are overlain by only a thin layer of mud (a few cm thick) on the middle and outer shelf. Apparently, the signal strength of the 3.5-kHz system is not sufficient to penetrate these sandy deposits. Conclusions. The 3.5-kHz system bridges the gap between conventional echo-sounder and low-frequency refl.ection profiler systems. Records obtained with this new high-resolution profiler show areas of outcrop and layered sedi-ments that are within the reach of existing sampling devices but are not easily discernible in records obtained with low-frequency profiler systems. Near-surface features of great lateral extent, which are often completely masked in low-frequency records, are clearly revealed by the 3.5-kHz profiler. The capability to accurately define near-surface features will undoubtedly prove to be a valuable asset in mapping and sampling sedimentary units and in the study of sedimentation in the oceans. Information obtained from this high-resolution system provides a worthwhile supplement to data obtained with low-fre-quency deep-penetration systems . .Acknowledgments. This work has been supported by contracts of the Office of Naval Research with Scripps Institution of Oceanography (NoNR 2216(23)), Oregon State University (NoNR 1286(10)), the National Science Foundation (GA-3990, GA-1246, and GA-934), and the U.S. Geological Survey (14-08-0001- 10766). Records taken aboard the R. V. ARGO were made during legs 8 and 9 of the CrRCE Expedition; we extend thanks to the Scripps Institu-tion of Oceanography for providing the opportunity to participate in this ex-pedition and to the officers and crew of the R. V. ARGO for their assistance. We also thank the officers and crew of the R. V. CAYUSE for their help during 280 Journal of Marine Research expeditions sponsored by Oregon State University. The towing vehicle and 3.5-kHz tranducer were on loan from the U.S. Geological Survey and the towing vehicle was modified with the assistance of William E. Bales, Robert M . Buehrig, and J ames Trumbull; their help is gratefully acknowledged. The help of Robert M . Buehrig in the preparation of the seismic profiles is greatly appreciated. R E FERENC E S BECKMAN , W . C., A . C. R OBERTS, and B. L USKIN 1959. Sub-bottom depth recorder . Geophysics, 24 (4): 749-760. E WING, JOHN, and ROGER ZAUNERE 1964. Seismic profilin g with a pneumatic sound source. J. geophys. Res., 69 (22) : 4913-4915· H ERSEY, J.B. 1963. Continuous refl ection profil ing, In : T he Sea, vol. 3, pp. 47- 72. M . N . Hill , Editor. Interscience, New York. M cCLURE, C. D., H. F . NELSON, and W. B. H UCKABY 1958. Marine sonaprobe system, new tool for geologic mapping . Bull . Amer. A ss. petrol. Geol., 4 2 (4): 701-716. M ooRE, D. G. 1960. A cousti c-refl ecti on studies of the continental shelf and slope off southern California. Bull. geol. Soc. Amer., 72 (8) : 1121-1136. MOORE, D . G., and J. R . CuRRAY 1963. Structural framework of the continental terrace, northwest Gulf of Mexico. J. geophys. Res., 68 (6) : 1725-1 747. SMITH , W . 0. 1958. Recent underwater surveys using low-frequency sound to locate shallow bedrock. Bull. geo!. Soc. Amer ., 69 (1): 69-98. VAN ANDEL, TJ. H ., and P. L. SACHS 1964. Sedimentati on in the Gulf of Paria during the Holocene Transgression; a sub-surface acoustic refl ecti on study . J. mar. Res., 22 (1): 30-50. 

A high-resolution sub-bottom profiling system for use in ocean basin

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A high-resolution sub-bottom profiling system for use in ocean basin

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