SEARCH FOR THE THRESHER — SPIESS AND MAXWELL 377 



area, which provided another nearly orthogonal coordinte. Over a 

 single weekend, new charts were prepared and receivers were provided 

 for six ships. This system (combination Loran C and Decca), al- 

 though it lacked accuracy at night, provided the primary navigational 

 reference tliroughout the search. Keproducibility of position, as 

 judged relative to bottom topography and moored buoys, was about 

 100 meters. 



In the beginning of the operation the search area of 18 by 18 kilo- 

 meters (10 by 10 nautical miles) was quartered, and one ship was 

 assigned to each sector. With the availability of the improved naviga- 

 tional system it became apparent, however, that a more systematic 

 approach was required. It was thus decided that four ships {Alle- 

 gheny, Mission Capistrano, Prevail, and Rockville) would make a 

 navigationally controlled, precision exploration of the entire area, 

 with 250 meters between tracks, while Conrad, Gillis, and Atlantis II 

 would move in to investigate possibly significant contacts. The sys- 

 tematic survey required 2 weeks of operating time in the area during 

 which time the data were plotted and contoured aboard ship. The 

 results provided the first quantitative indication of the difficulties of 

 using the echo sounder for this purpose. A model showing the com- 

 plexity of the topography is shown in plate 1, figure 2, in comparison 

 with a model based on previously available data. The result was the 

 conclusion that in about half the area the sea floor was too rough for 

 search by this teclinique. In the other half there were six possible 

 target indications, one of which was point "delta," first observed by 

 Atlantis II. Because "delta" was close to the location deduced from 

 the rough navigational record provided initially by Skylark, and be- 

 cause the echogram (pi. 2, fig, 1) was especially convincing, tliis point 

 was given the highest priority for further investigation. 



During the time the four ships were conducting their detailed 

 sweep of the area, Conrad, Atlantis II, and Gillis had already begun 

 investigation of the most likely locations. They relied principally on 

 photographic equipment built over the years to solve the needs of 

 submarine geologists. With such equipment it was possible to make 

 stereo pair photographs of a strip about 7 to 10 meters wide, with over- 

 lapping coverage for successive exposures, while the vessel was travel- 

 ing at speeds of 1 to 2 knots (1.8 to 3.6 km/hr) . Aside from the result- 

 ing very slow search rate (about 2i/^ km.- per week), this technique 

 has the additional disadvantage of requiring a bottom-referenced navi- 

 gation system accurate to within at least 5 meters to assure that there 

 are no appreciable gaps in coverage. As an investigative tool in a 

 restricted area, however, this is an essential method, since it can provide 

 the detailed view of a wreck that is needed by investigators. 



Underwater television was another device with similar restrictions 

 that was available for optical investigation of the area. At the time 



