75 



Chorgt weight in /bs. 



Fig. 9. Optimum charge depth as a 

 function of charge weight. 



10 "goo too TOO "ko iooltiM 

 Depth * 33 Feat 



Fig. 8. Dependence of bubble-pulse 

 frequency. 



Figure 8 shows the dependence of bubble pulse interval on depth for vari- 

 ous sizes of TNT charges. The two curves labeled 4d/c and 2d/c represent 

 twice the time and the time, respectively required for the sound to travel verti- 

 cally to the surface and back to charge depth. Hence the intersections of the 

 upper curve of Figure 8 with the bubble pulse curves determine the depths at 

 which the reflected sound of bubble pulse frequency is in phase with the direct 

 sound sent vertically downward. They define a relation between charge weight 

 and the depth at which maximum energy of bubble pulse frequency is directed 

 downwards. It is the depth to one-quarter wave level of bubble pulse frequency. 



Figure 9 shows a plot of this depth for TNT charges from 0.1 lb. to 1000 

 lb. On the right side of the graph are shown the bubble pulse frequencies cor- 

 responding to the plotted depths. For the charge sizes of \ to I 00 lbs. used in 

 seismic refraction work, the depths and frequencies vary from 80 to 235 feet 

 and from 15 to 5 cps. Examples of the refracted waves obtained by this shoot- 

 ing doctrine are shown in Figure 10 which depicts two pairs of records made at 

 the ranges at which the charge weight was changed from 4 to 50 lbs. The fre- 

 quencies are roughly 10 and 6 cps, respectively, in agreement with the graph in 



Figure 9. 



In order to eliminate undesired 

 sounds outside the frequency range of 

 the refracted waves, the bottom re- 





Fig. 10. Effect of increasing charge 

 weight from 4 to 50 lbs. In each of the 

 two pairs of oscillograms the upper one 

 represents a 4 lb. charge and the lower 

 one a 50 lb. charge. The distances 

 are approximately 25 km. 



Frequency In cps 



Fig. 11. Frequency response of re- 

 cording system. 



