bottom roughness on energy loss are omitted and, therefore, the accuracy of results 

 is improved. Usually these depositional areas contain dramatic stratigraphy, and 

 some contain many sub-bottom acoustic reflectors. 



SURVEY DISCUSSION 



Two sites have been surveyed thus far, one rather insufficiently because of 

 equipment failure. 



In an area about 30 miles south of Bermuda, two core samples of about 5-foot 

 length were obtained about l/2 mile apart. The sub-bottom layering is quite fine, 

 and it is felt that at least two more cores would be required before a cross-sectional 

 diagram of the sub-bottom structure could be constructed with some reliability. 

 Recent Naval Research Laboratory photographs taken by J. Gennari have shown this 

 bottom section to be extremely flat. A study of the area will be continued when the 

 other cores are obtained. 



Five cores were obtained from the Tongue of the Ocean, off Middle Bight (Fig. 1). 

 They were taken from an area less than two miles in diameter and ranged in length 

 from 10 to 16 feet. While stereo-photographic attempts were made, no photographs 

 were obtained. However, the echo sounding trace revealed a flat, though slightly 

 inclined, bottom. 



The in-situ determination of the thin impedance layers in the upper 100 feet of 

 sediments has been a difficult task. In deep water they are not readily identifiable 

 by echo sounding or sub-bottom reflection techniques. The standard 12-kcps hull- 

 mounted or towed UQN source produces very little penetration in deep water. The 

 approach to increased proficiency in acoustic surveying of the ocean bottom has thus 

 far been narrowly limited to two basic interests: (1) detailed study of the water- 

 sediment interface using high-frequency sources, and (2) vertical delineation of the 

 major structural provinces from the bottom interface to the earth's core, using 

 seismic techniques. In the first case very little penetration is sought or achieved. 

 In the second thicknesses of the fine layers are considerably smaller than the wave- 

 lengths of the signals employed and therefore go undetected. 



The 100-kilojoule EG and G sparker source was used about the AGOR-4 with a 

 band pass filter range of 500-2000 cps. The penetration was excellent and apparently 

 the detection of the finer structure was good, since a number of layers were indicated. 

 However, the bubble pulse returns overlapped the fine layer and other bubble pulse 

 returns to the extent that the records could not be interpreted. 



During a recent 3. 5-kcps normal incidence test a magnetostrictive scroll source 

 was towed about 1000 feet above the bottom and the signals were recorded on a PGR. 

 The existence of the individual layers was evident, although the definition was somewhat 



378 



