24 SHOR [CHAP. 2 



a log of shot number, time of day. weight, ex])lo.sive ty])e. fuse length, whether 

 flotation l)alloons are used, sinking time, water de])th at the time the shot is 

 dro])])ed, ship course and s]jeed, and time of passing abeam of the receiving 

 sliip. If necessary he marks the fathometer tape at the time the shot is dropped ; 

 in some systems a mark is created when the fathometer ping is automatically 

 turned off as he shifts his radio circuit, and the time of detonation marks itself 

 as a black line. The operator also identifies and retains all firing mark records. 

 Ship's bridge personnel provide a shooting track with positions of first and last 

 shots, all course and speed changes and navigational fixes, and the range and 

 bearing of the receiving shi]) when ])assing. This track usually has to be re- 

 conciled with that provided independently by the receiving ship to make the 

 abeam positions agree. 



Ex})losive need depends markedly on the length of line to be shot. In current 

 ])ractice a deep-sea station with penetration to the mantle (two profiles, each 

 about 40 miles long) requires about 1600 lb of TNT in about 80 shots. This 

 total includes many small shots fired at short range for a reflection proflle ; 

 the major weight of explosive is used in the few distant shots fired for mantle 

 arrivals. These latter shots are normally between 50 and 100 lb of high ex- 

 plosives each ; in bad weather or when signals are weak, 300-lb shots are not 

 uncommon. The equation 



Maximum range in kilometers = 7Y'(charge weight in j)ounds) 



given by Raitt (1952) for operations in the tropical Pacific continues to be 

 valid in most areas. For work on the continental shelf as much as 4000 lb per 

 station may be required if mantle arrivals are sought. Short lines in shallow 

 water for studies of the shallower layers can be fired with a very few pounds of 

 explosives. 



3. Receiving Techniques 



Receiving techniques have followed three separate patterns. Although many 

 persons have contributed their share, the principal developers of these methods 

 have been Maurice Ewing of Woods Hole and the Lamont Geological Observa- 

 tory, Russell Raitt of the Marine Physical Laboratory of Scrijjps Institution of 

 Oceanography, and Maurice Hill of Cambridge University. The WHOI-LGO 

 methods have been used by Lamont, Woods Hole, Hudson Laboratories, and 

 collaborating groups ; the SIO methods have been followed by Scripps, Pacific 

 Naval Laboratory, and the U.S.S.R. Institute of Oceanology and Institute of 

 Physics of the Earth. Hill's method has been followed by Cambridge and the 

 National Institute of Oceanography. It is described separately in Chapter 

 .'{. The methods discussed here are necessarily those which the author 

 knows best from personal experience, discussions with the users, and the 

 published literature. 



