Speiss, Mudie and Lowenstein 



In making such background observations it seemed most appro- 

 priate to use a side-looking sonar. This, operated from a deep-towed fish, 

 gives high resolution without unduly complicating the equipment problem (no 

 scanning motors or circuits, easy mounting of the necessarily large trans- 

 ducers, simple telemetry and display). At the same time results of such ob- 

 servations tell of the sizes of irregularities on the sea floor in a form which 

 can be made relevant in terms of other types of sonars. We have, to date, 

 utilized a pair of Westinghouse transducers (shown on the towed body in Fig. 

 1) operating in the non-focussed mode at about 230 kcps. Height off the 

 bottom is variable depending on the circumstances but records useful for our 

 purpose, with objects as small as 5 feet high and 10 feet across in mind as 

 targets, can be obtained at heights off bottom of as much as 200 feet. 

 Normally a half-second or one-second repetition period has been used and 

 most records show useful returns to ranges of 150 to 200 fathoms (0. 15 to 

 0. 2 nautical mile). Speed of advance has typically been about 1. 2 knots. 



In all three of the areas of Fig. 2 a major fraction of the sea floor 

 is essentially featureless to the side looker and would be very easy to search 

 systematically and effectively at a rate in excess of a half square mile per 

 hour. In every one of the areas there were, however, regions of sharp 

 topographic relief in which the associated side-looking sonar record is 

 patchy in a manner which would give rise to possible false targets or un- 

 certainty as to whether or not targets were present. 



Three examples can be shown of side-looking sonar records as- 

 sociated with rough topography. In Fig. 8 is shown a pass across a small 

 canyon in the Hawaiian area (area D). This ditch was crossed at several 

 points and was shown to be about 30 fathoms deep, 0. 1 mile wide and at 

 least 2 miles long. Its walls were quite steep (45°). The crossing shown 

 here was made at an angle of about 60° to the axial line, a trend which is 

 evident in the sonar record. 



The second (Fig. 9) was made in area A as the fish was being 

 towed down the side of a seamount at a depth of about 1900 fathoms. 



Most striking from the viewpoint of possible false targets was 

 Fig. 10 which was made close to the small hill shown in Fig. 7, in area B. 

 In this area, along over 200 miles of track on which the total relief of two 

 abyssal ridges was about 200 fathoms, there was only this one region, less 

 than half a mile across, which showed other than a smooth side-looking 

 record. 



As observed with the air gun and penetration sounder there was 

 significant sediment cover (at least 50 meters) in all these areas, except 

 in the immediate vicinity of steep slopes. The only major exception 



72 



