I 2 3 4 5 6 7 



TRAVEL TIME OF DISTURBANCE PROPAGATED THROUGH THE WATER IN SECONDS 



Figure 32. Typical plot of travel time against range for components of different frequencies in the ground wave. Loca- 

 tion, near Jacksonville, Florida, at mean depth of 57 feet, charge and hydrophone both resting on bottom. 



served records by considering the initial or ground 

 wave phase of the disturbance. All the records agree 

 in a general way with the predictions of the theory 

 of reference 23 as outlined in Section 9.4.2 (see 

 Figure 30) in showing a gradual increase of frequency 

 between the beginning of the disturbance and the ar- 

 rival of the water wave. According to equation (25) 

 of Section 9.4.2, when the bottom is uniform the 

 period of the disturbance at its beginning is a func- 

 tion of the velocity ci of sound in the bottom. Exten- 

 sion of the theory to cases where the bottom con- 

 sists of a deep firm stratum overlaid by a slower one 

 gives the result that at sufficiently long ranges the be- 

 ginning of the ground wave should have a frequency 

 dependent in a complicated way upon the velocities 

 in both layers, but that, if the upper layer is suf- 

 ficiently thick in comparison with the depth of the 

 water, a strong new disturbance of distinctly higher 

 frequency will arrive some time later, the arrival 



time and frequency of this new disturbance being 

 approximately the same as for the ground wave which 

 would occur if the upper layer were infinitely thick. 

 These theoretical predictions suggest that noting the 

 frequencies of the first arrival and any subsequent 

 arrivals in the ground wave may provide useful in- 

 formation about the different strata in the bottom. 

 This is illustrated in Figure 32, which may be com- 

 pared with Figure 24. Here many of the records show 

 a recognizable new arrival of different frequency from 

 the first which comes some time later. Complete in- 

 terpretation of the data shown in Figure 32 is diffi- 

 cult, but there is definite evidence for a layer in which 

 the velocity of sound is about 1.5 times the velocity 

 in water, as well as of one or two layers of higher 

 velocity which determine the times of the first ar- 

 rivals. It is noteworthy that this dependence of the 

 frequency of a ground wave arrival upon the velocity 

 of sound in the layer chiefly responsible for the ar- 



