SECT. 1] REFRACTION AND REFLECTION TECHNIQUES AND PROCEDURE 21 



by Hill (1952); Russian methods are given in Galperin and Kosminskaya 

 (1958), Andreyeva and Udintsev (1958) and Galperin, Goryachev and Zverev 

 (1958). 



2. Shooting Techniques 



A great impetus to refraction work at sea occurred in 1946 when large 

 quantities of surplus high explosives from World War II became available in 

 many countries. These included such material as aerial bombs, mines, depth 

 charges, cast TNT blocks and diverse types of material intended for demolition 

 work. As most such material deteriorates Avhen stored for extended periods, 

 over-age material is generally dumped at sea. A sizable quantity of this surplus 

 material was made available for scientific work instead ; as a result the cost of 

 operation was materially reduced and the techniques were generally fitted to 

 the tjrpes of material available. These explosives have usually been varying 

 types and packaging arrangements of TNT, Tetryl, Tetrytol and PETN. 



Because all explosives have to be carried long distances aboard small ships, 

 a great deal of work has been devoted to the dual problem of increasing signal 

 strength and decreasing noise in the low-frequency spectrum, in which the best 

 propagation of the refracted waves occurs. The best method found so far for 

 obtaining the maximum signal from a given size of charge is to fire it at a depth 

 equal to a quarter of the wavelength for the bubble-pulse frequency. 



The occurrence of the bubble-pulse phenomenon was observed as early as 

 1925 by Weibull (1954) and has since been described by a number of in- 

 vestigators. Detailed study by a group at WHOI resulted in data such as those 

 given in Fig. 1 (from Raitt, 1952, based on work by Arons and Yennie, 1948), 

 in which it is apparent that the highest intensity occurs at the bubble-pulse 

 frequency. The value of this frequency is determined by the equation (for TNT) : 



^ ~ 4.36 If ^/3 ' 



where H is in feet, W in pounds and / in cycles/sec, from which one can deter- 

 mine the depth of detonation of a charge of given weight for an observed 

 bubble-pulse frequency. In combination with the requirement of quarter-wave 

 depth, this gives 



H = Co/4/ 



where Co is the velocity of sound in sea-water, for optimum shooting. A curve 

 for/ and H as functions of W for normal sound conditions is given in Fig. 2, 



In some cases it has been necessary to place charges at depths other than 

 the optimum. Certain types of explosives such as 300-lb and larger depth 

 charges sink at such a rapid rate that they reach their optimum depth before a 



