230 



TRANSMISSION OF EXPLOSIVE SOUND IN THE SEA 



1 RADIO SIGNAL 



2 MARK I HIGH FREQUENCY 



3 GEOPHONE 



4 MARK 1 • 



5 RADIO BUOY GEOPHONE 



6 MARK It LOW FREQUENCY 



7 MARK I RECTIFIED 



2 3 4 

 10 10 10 10 



FREQUENCY 

 IN C 



SHOT 28, NEAR SOLOMONS, MARYLAND 

 CHARGE-. 5 LBS TNT ON BOTTOM 

 DEPTH OF WATER 90 FEET 

 RANGE 3900 YARDS 



1 RADIO SIGNAL 



2' MARK I HIGH FREQUENCY 



3' GEOPHONE 



4' MARK I RECTIFIED 



5' MARK I 



6' MARK or LOW FREQUENCY 



7 MARK I RECTIFIED 



— L I 



2 3 4 

 10 10 10 10 

 FREQUENCY 

 IN C 



SHOT 95, NEAR JACKSONVILLE, FLORIDA 

 CHARGE: 25 LBS TNT ON BOTTOM 

 DEPTH OF WATER 60 FEET 

 RANGE 4200 YARDS 



Figure 31. Typical records of explosive sound transmissions in shallow water. Times marked along the top of each 

 oscillogram are in seconds. Curves at left give relative amplitude response of each channel to the various frequencies. 



signal at two different amplitude levels. However, the 

 rectified traces show most clearly the times of the 

 various water wave arrivals, namely, shock wave and 

 bubble pulses. The arrival time of the first of these is 

 especially useful, since the range can be determined 

 for any shot by multipljang by c the interval between 

 the detonation and this arrival. 



The interpretation of records like those of Figure 

 31 is often complicated by the fact that each ob- 

 served trace represents a superposition of the dis- 

 turbances produced by the shock wave and all the 

 bubble pulses. According to the theory of normal 

 modes, the amplitude of the disturbance produced 

 by any one such pulse of very short duration should 

 be proportional to the impulse ypdt of the pulse; since 

 this quantity is of the same order of magnitude for 

 the shock wave and the first one or two bubble waves, 

 the resulting superposition can become very compli- 



cated. However, as was mentioned in Section 8.6, the 

 bubble pulses are often much weaker when, as was 

 usually the case in the WHOI experiments, the charge 

 is fired in contact with the bottom. Records (A) and 

 (B) of Figure 31 are fairly typical examples of shots 

 on the bottom; the former shows a strong bubble 

 pulse and the latter a very weak one. Note that the 

 separation of the first two high peaks in the ground 

 wave of Figure 31 A is just equal to the bubble period 

 as read from the rectified trace. Since the periods of 

 the oscillations are long compared with the duration 

 of the impulse sent out by the explosions, the only 

 noticeable effects of increasing the size of the charge 

 are to increase the amplitude and to alter the time 

 lag in the arrival of the bubble pulse effects. Chang- 

 ing the position of the charge from bottom to mid- 

 depth also seems to have very little effect. 



Let us begin the detailed discussion of the ob- 



