232 



TRANSMISSION OF EXPLOSIVE SOUND IN THE SEA 



RADIO SIGNAL r*****, ■ ' '^ , "^ i 





MARKIC 

 HIGH FREQUENCY 



GEOPHONE 



MARK IE 

 RECTIFIED 



%/■■ 



MARK I 



MARK 31 

 LOW FREQUENCY 



MARKZ 

 LOW FREQUENCY 









^^^^T'-^^i^-^'^*;,'^ ,\^' 



'aW^. 



\ %*#V»*V>V^:, 



MARK H ^^^.1,. 



RECTIFIED I /» 



V. 



"WtH 



1M Pi y i .UWn ii>i >M.)l^ "*|j'* *1* ' * W "^ 





I ■ laasB 







Figure 33. Dispersion in the water wave produced by an explosion in shallow water. Shot 90, near Jacksonville, Fla.; 

 charge, 5 lb TNT on bottom; mean depth of water, 57 ft; range, 7,000 yd; frequency response of channels as shown in 

 Figure 31. 



rival in question can be observed even in the first 

 arrivals at fairly short range. Thus in the data from 

 which Figure 24 was constructed, the period of the 

 first arrival was between 0.024 and 0.036 sec for the 

 points to the left of the intersection of the two 

 straight lines, and was between 0.050 and 10 sec for 

 the points to the right, except for two very close to 

 the intersection. 



On some records the ground wave dies out quite 

 noticeably before the arrival of the water wave. The 

 theory of reference 23 indicates that if the bottom is 

 uniform to all depths, the ampUtude of the ground 

 wave should increase steadily until the water wave 

 arrives, and that a decrease in the strength of the 

 ground wave in this region implies the presence of 

 layers of different materials. In the latter case, there 

 may be a secondary ground wave arrival of the sort 

 mentioned in the preceding paragraph, which dies 



out considerably before the arrival of the water wave. 



Let us now consider the disturbance after the arrival 

 of the water wave. Figure 33 shows, in more detail 

 than Figure 31, the dispersion phenomena occurring 

 in this stage. The third trace from the bottom shows 

 most clearly the ground wave just before the arrival 

 of the water wave, and the gradual development of 

 the water wave from a disturbance of very -low ampli- 

 tude and high frequency, superposed on the ground 

 wave, to the final, so-called Airey phase where 

 ground wave and water wave fuse at an intermediate 

 frequency and die out. The similarity of this record 

 to Figure 30 is quite striking. The resemblance is not 

 nearly so close for the second trace from the top, 

 since this trace was recorded with more fidelity at the 

 high frequencies, so that many normal modes higher 

 than the first contribute significantly to it. 



About 0.2 sec after the main disturbance has died 



