Speiss, Mudie and Lowenstein 



improving t±ie signal-to-noise ratio. The record of Fig. 4 was high pass 

 filtered with and without the THRESHER peaks using a cutoff wavelength of 

 400 meters. The result is shown in Fig. 5, in which the lower trace includes 

 the inserted targets. In some parts of the record these show clearly but 

 there are many similar peaks generated by the background. Even with this 

 treatment detection would be far from easy. 



Having found this strong short wavelength background in one area 

 one may ask whether it should be expected elsewhere. The answer is that 

 surface observations show similar lineated structures in much of the deep 

 ocean and that the strong similarities observed among these various occur- 

 rences make it highly likely that deep observations, when they are made 

 elsewhere, will show this same fine structure. Figure 6 shows areas which 

 we estimate will have background of a sort which will render magnetic 

 search difficult. 



Acoustic search for objects on the sea floor can be carried out in 

 either of two basic modes— one utilizing essentially a down-looking system 

 and the other using more or less horizontal paths. If the down-looking mode 

 is used in the deep sea from near the surface a reasonable sweep width can 

 be achieved. Determination of the horizontal coordinates of suspected 

 targets is uncertain without very sophisticated (and expensive) sonar 

 systems. The downward looking technique can give good resolution if near- 

 bottom equipment (towed and in future on free-running vehicles) is used. 

 Figure 7 shows a profile over some small natural irregularities on the sea 

 floor, at about 2200 fathoms depth 500 miles west of San Diego (area B). 

 The upper portion shows the record of the surface-operated echo sounder 

 over the same portion of track as that shown by the lower record from the 

 deep tow. The lower record displays both the down-looking sounder output 

 which gives the height of the towed body off the bottom, and (in the middle 

 of the record) the sum of the down plus up echo sounder travel times, that 

 is, the actual depth of the water. The small hill is very clearly discernible. 



While the near-bottom use of the dovraward path provides good 

 resolution, this good resolution implies a very narrow search path (typically 

 only a few fathoms wide). In fact once one has decided to operate near the 

 sea floor it is immediately obvious that a nearly horizontal path is much 

 more appropriate. Under such circumstances one should naturally utilize a 

 system in which beam width and pulse length are adjusted to provide proper 

 resolution and employ such a beam either fixed in side-looking mode or 

 sweeping in searchlight or scanning sonar fashion. The most natural 

 question to ask as one proceeds with such a search system design is what 

 one might expect in the way of false targets arising from the natural rough- 

 ness of the sea floor. It is this question for which we now have the begin- 

 nings of an answer from the deep sea floor in the northeast Pacific. 



71 



