SEARCH FOR THE THRESHER — SPIESS AND ]VIAXWELL 379 



metric-ton) lump of iron] had been used for geophysical exploration 

 both on land and at sea, but usually as airborne or shallow-towed 

 instruments. Only the geophysical group at Cambridge University, 

 England, had a magnetometer capable of being towed at great depth, 

 and this particular instrument was then m use in the Indian Ocean. 

 Several laboratories (Lamont, Scripps Institution of Oceanography, 

 and the Naval Ordnance Laboratory) thus began packaging the avail- 

 able magnetometers for use at depths wliich would give the required 

 proximity (about 200 meters) to the hulk during search. More was 

 involved than simple provision of a pressure-proof case ; also required 

 were a strong towing wire having good capability as a conductor of 

 an electrical signal and proper telemetering circuitry to make the 

 signal available on the towing ship. In early attempts there were 

 many electrical problems. Nevertheless, one credible anomaly was 

 found, at about the time of the dredging operations, but it was appar- 

 ently remote from the debris area by more than a kilometer. Some- 

 what later, another signal (fig. 1) was found, several times, by Conrad. 

 Still later this magnetic signal was confirmed by both Gibhs and GiUis 

 (with equipment from Scripps and the Naval Kesearch Laboratory). 

 In each instance, navigational uncertainty and lack of ability to make 

 photographs or view by television at the time the signal was obtained 

 precluded the possibility of identifying these signals with Thresher, 

 or even of being sure that they were all generated by the same object. 

 The amplitude and dimensions of the signals were such that it is highly 

 probable that they were generated by a mass of iron of the approximate 

 dimension of a submarine, but whether this was Thresher, some other 

 wreck, or even natural background is as yet not known. 



High-resolution acoustic techniques, used near the sea floor, were 

 regarded from the start as providing a most promising type of search. 

 Two units were assembled through modification of existing equipment 

 (by Marine Physical Laboratory and Woods Hole Oceanographic In- 

 stitution) , but these units lacked adequate resolution in angle. West- 

 inghouse, mider contract with Hudson Laboratories, built a unit 

 specifically designed for the purpose, and it was operating effectively 

 by July. This unit was towed near bottom, by means of a cable similar 

 to that used with the magnetometers; it had an acoustic transmitter 

 and receiver whose two narrow beams were directed one to each side. 

 The variation in amplitude of the nearly continuous sea-floor reverber- 

 ation from each transmitted pulse was plotted on a facsimile-type 

 recorder. In this way, for each pulse a high-intensity mark was made 

 at the ranges of highly reflecting sea-floor features and virtually no 

 intensity was recorded at ranges corresponding to sliadows. Thus, 

 as the towed unit moved along, from successive pings it created a 

 picture of the sea floor similar to that used by cartographers to show 

 rouglmess of terrain, or similar to the "PPI" (plan position indicator) 



