BOTTOM REFLECTION. SHALLOW-WATER TRANSMISSION 



233 



lb 

 C 



h = Depth of water. 



/ = Frequency of contribution of first normal mode, 

 c = Velocity of sound in water. 

 Ci = Velocity of sound in bottom, assumed uniform. 



Az = Depth below the surface of the bottom above 

 which 99% of the wave energy of the first mode in 

 the bottom is included. 



Figure 34. Typical curves of the frequency dependence of the depth of penetration of the first normal mode into the bot- 

 tom. Density of bottom assumed 2 times density of water. 



out, another disturbance is recorded, weaker than the 

 first and due to the bubble pulse. On the low-fre- 

 quency trace this second water wave looks similar to 

 the first; but on the high-frequency trace it is very 

 different. Frequencies above about 200 cycles are 

 absent in the bubble pulse disturbance but strong 

 in the primary disturbance. This is probably due to 

 the fact that, as indicated in Figure 8 of Chapter 8, 



the pressure deHvered by the bubble in its contracted 

 stage has a duration of several miUiseconds and is 

 thus lacking in high-frequency components. 



A detailed analysis of the dispersion phenomena in 

 the water wave can provide useful information on the 

 characteristics of the upper layers of the bottom.^ 

 Unless shots are made at very short ranges, the arrival 

 times and frequencies of ground waves furnish infor- 



