SECT. 1] REFRACTION AND REFLECTION TECHNIQUES AND PROCEDURE 2?5 



the bubble blows out. This is usually done by fastening the charge to a balloon 

 or air-filled plastic bag. 



In the early stages of post-war work and in most of the pre-war operations, 

 charges were detonated by electric caps by a firing impulse from the shooting 

 ship, or in the case of depth charges, by depth-sensitive detonators. Since about 

 1949 almost all operations have been conducted using non-electric caps and 

 slow-burning time fuses for detonation ; this eliminates problems of retrieving 

 the firing line between shots or slowing the shooting ship to fire. Reliability of 

 the method is generally better than 97% if properly waterproofed fuse is used. 

 The lower accuracy of determination of firing time in the fuse method (0.01 sec 

 instead of 0.001 sec or better) has been found to be unimportant in deep-sea work. 



When electric detonation is used, the firing impulse can be recorded or sent 

 by radio from the shooting box to the receiving ship directly. When time fuses 

 or pressure detonators are used, a different technique is required. Shots of 

 sizes varying from i to 300 lb are fired on a pre-determined schedule set by the 

 operator on the receiving ship. The shooting crew consists of an operator and 

 a shooter. The operator announces the shot number, time, weight, fuse length 

 and whether it is to float or sink. On the scheduled minute he instructs the 

 shooter to light the fuse. The shooter lights the fuse, drops the charge over the 

 side (with the aid of a tilt-table for the largest charges), and calls, "Mark". 

 Operators at both ends of the radio circuit start stop-watches at the mark, 

 and shortly before the expected time of detonation start their recording 

 equipment. The operator on the shooting shijD switches the radio circuit to put 

 the signal from the fathometer audio output or a special hull-mounted hydro- 

 phone or a towed hydrophone on the radio in place of the microphone ; he also 

 records the same signal along with time-ticks from a marking chronometer on 

 a pen recorder. When the charge fires, it produces a record aboard both ships 

 of the direct wave from the shot, and under favorable conditions also records 

 the bubble pulse, one or more bottom reflections, and the wave reflected once 

 at the water surface and once at the bottom. When the fathometer output is 

 used, the details other than firing mark and bottom reflection are usually lost 

 because of the narrow pass band of the fathometer receiver. Both operators 

 stop their watches at the time of detonation, and the stop-watch time multiplied 

 by the ratio of ship speed to the velocity of sound in the surface layer (the 

 ship's Mach number!) is used as the firing time correction. If the sinking rate 

 of the charge approaches the ship speed, an additional correction for the slant 

 path is necessary. Usually this correction is less than 0.01 sec and can be 

 neglected. In case radio transmission is missed, shot time can be recovered from 

 the shooting ship's firing record using chronometer corrections determined 

 from the shots immediately before and after. Chronometers are normally 

 checked against WWV time signals at least daily ; when radio communications 

 are unavailable or poor, profiles are sometimes shot on a fixed time schedule 

 with firing marks recorded against chronometer only, with more frequent 

 checks of WWV signals. 



Data recording on the shooting ship is relatively simple. The operator keeps 



