BOTTOM REFLECTION. SHALLOW-WATER TRANSMISSION 



229 



-BEGINNING OF GROUND WAVE 



J_ 



-I 

 4 

 o 



0.05 0.10 0.15 0.20 0.25 



TIME FROM BEGINNING OF GROUND WAVE, IN SECONDS 



0.30 



GROUND AND 

 WATER WAVES 

 BLENDING 

 TOGETHER 

 AT END 



0.35 



0.40 0.45 0.50 0.55 



TIME FROM BEGINNING OF GROUND WAVE, IN SECONDS 



0,60 



0.65 



Figure 30. Theoretical contribution of the first mode to the disturbance at a distance from an explosion in shallow water. 

 Source and receiver both assumed to be on the bottom. Range: 9,200 yards. Depth of water: 60 feet. Velocity of 

 sound in bottom = 1.1 X velocity in water. Density of bottom = 2 X densitv of water. (A'^oie, A should appear at point 

 indicating beginning of ground wave.) 



the hydrophone with time should be given by the 

 sum of these contributions from all the normal modes, 

 plus certain additional terms whose magnitude de- 

 creases rapidly with increasing range, so that they 

 become negligible at very long ranges. This complete 

 pressure-time curve would of course show sharp 

 jumps at the positions corresponding to the arrivals 

 predicted by the ray picture. 



In the following section we shall compare these 

 theoretical predictions with observations. In this 

 comparison certain factors have to be taken into 

 consideration which for simplicity have been neg- 

 lected in this section, such as the modification of the 

 received disturbance by the frequency response char- 

 acteristics of the recording equipment, and the fact 

 that instead of delivering a single impulse, an ex- 

 plosion gives out a shock wave followed by several 

 bubble pulses (see Section 8.6.). 



9.4..'5 Analysis of Experimental 

 Records 



The Woods Hole Oceanographic Institution has 

 obtained a large number of oscillographic records of 



sound from explosions in shallow water at distances 

 between 0.25 mile and 30 miles. ^^ Several series of 

 experiments were conducted at widely separated 

 places with bottoms of mud, sand, and coral. The 

 depths of the water at the sending and receiving 

 positions were usually similar and in the range 40 to 

 180 ft; some shots were made at greater depths. 

 The hydrophones used were in all cases placed on the 

 bottom, while the charges were usually on the bottom 

 but sometimes at mid-depth. Charges of J^-lb TNT 

 to 300-lb TNT were used. At all stations the water 

 was very nearly isothermal, so that sound rays in the 

 water were refracted slightly upward. 



Figure 31 shows some typical oscillograms of the 

 sound received in these experiments. Each record 

 consists of eight traces simultaneously recorded. The 

 first of these, labeled "time break," is used merely to 

 record the instant at which the charge was set off ;the 

 others record the disturbance received, as modified 

 by the frequency responses shown at the left for the 

 various recording channels. Most of the interesting 

 features show up best on the two channels labeled 

 "Mark II low frequency." which record the same 



