Speiss, Mudie and Lowenstein 



was surveyed in detail. In the fourth area a long east-west traverse (105 

 miles) and a shorter north-south run were made. The present paper will be 

 concerned primarily with acoustic system data from the circle areas and 

 magnetometer data from the long straight tows (although both acoustic and 

 magnetic data were obtained in all areas). 



Magnetic search techniques are limited by inherent noise in mag- 

 netometers, noise associated with their motion or the motion of the conduct- 

 ing sea water in the earth's field, by ionospheric current effects and by 

 naturally occurring magnetic effects of the rocks of thu sea floor. The first 

 few of these items lead to uncertainties of a few tenths of a gamma at most 

 in well-designed systems. (One gamma is 10"^ oersted thus typically the 

 earth's field is about 40, 000 gammas. ) These imply, then, that search for 

 iron objects up to 100 meters across must pass at least within a kilometer 

 or so of the target and thus that such searches at sea must be conducted near 

 the bottom. With this in mind magnetometers were towed near bottom at 

 depths of about 2. 5 kilometers in the THRESHER search. As shown in Fig. 

 3, taken from the paper of Maxwell and Spiess in Science,— and as more 

 extensively documented by Heirtzler,-' the natural geological background in 

 that area, on the continental slope, was smooth. A 100 gamma anomaly, 

 presumably due to THRESHER, is clearly visible against the gradual 30 

 gamma change occurring along the 1. 5 km track shown in the figure. 



It has long been known, however, that anomalies of several 

 hundred gammas can be observed at the sea surface over the deep ocean. 

 One of the best magnetic maps showing these strongly lineated anomalies is 

 that produced by Mason and Raff ^^' as a result of surveys which they con- 

 ducted in the middle fifties with cooperation of the U. S. Coast and Geodetic 

 Survey. These show ridges which have north-south continuity in sections 

 for 500 km or more and east-west wavelengths of 20 to 50 km, with ampli- 

 tudes of 50 to 300 gammas. 



The question as to how these anomalies look when observed near 

 the sea floor has now been answered with the long profiles at the location 

 shown in Fig. 2. The result (as seen from about 80 meters off bottom) is 

 shown in Fig. 4. The long wavelength component which would be measured 

 at the surface is barely discernible, as are the small peaks introduced 

 artificially by adding the anomaly of Fig. 3 repeatedly every 20 km. It is 

 clear that the naturally occurring anomalies, associated with variations of 

 magnetization of material lying just below the thin sediment cover (50 to 

 300 meters), have strong short wavelength components which would make 

 detection of ship-sized iron objects quite difficult. 



Since the anomaly of Fig. 3 has significant short wavelength 

 content we have applied a simple signal processing scheme in hope of 



70 



