234 



TRANSMISSION OF EXPLOSIVE SOUND IN THE SEA 



0.05 



0.01 



0.005 



0.001 



Theoretical dispei'sion in the first normal 



mode for various uniform bottoms all of den- 

 sity 2. 



Same for layer of thickness 0.1ft and Ci/c = 1.1 



underlain by infinite layer with oijc = 3, both 

 of density 2. 



Same for layer of thickness h and cj/c = 1.1 



underlain by infinite layer with oi/c = 3, both 

 of density 2. 

 Figure 35. Theoretical and observed dispersion in the water wave. Shots were made off Jacksonville, Fla., where the 

 depth of water was 1 15 to 120 feet. Hydrophone was on the bottom for all shots. Observed frequencies are taken from the 

 Mark II low-frequency record whose response is shown in Figures 31 and 33. 



h = Depth of water. 



/ = Frequency of contribution of first normal mode. 

 t = Time between explosion and arrival of frequency/. 

 to = Time between explosion and first water wave ar- 

 rival. 

 V = Group velocity for frequency /. 

 c = Velocity of sound in water, 

 ci, C2 = Velocities of sound in bottom layers. 



mation only on those layers of the bottom which are 

 reasonably thick, in comparison with the depth of the 

 water; the water wave, on the other hand, can supply 

 information on the uppermo.=!t layers even when they 

 are much thinner. This is a consequence of the fact 

 that for normal modes of any order the higher the 

 frequency the more rapidly the disturbance dies out 



with Increasing depth. Since the frequencies in the 

 water wave are much higher than those in the ground 

 wave, the water wave will not penetrate so deeply 

 into the bottom, and will thus be less affected by the 

 characteristics of deep layers and more affected by 

 the characteristics of the top layers. Figure 34 shows 

 how the depth of penetration varies with frequency 



