290 



DEEP-WATER REVERBERATION 



500 



LU 1000 





1500 



2000 



I 



t 



llJ 

 o 



< 

 o 

 in 



500 



1000 



1500 



2000 



-100 



-80 



-80 



-60 -100 

 10 LOG M 



Figure 10. Mean value of 10 log M for all positions 

 in 1943 cruise. 



60 



rapidly with increasing sea state. A later section en- 

 titled "Possible Explanations" advances some fairly 

 plausible qualifications of the simple theory which 

 may explain away, in part, the seeming discrepancies 

 between theory and observation. 



Experimental Results 



Figure 14 illustrates the results of one experiment 

 comparing the reverberation with horizontally and 

 vertically directed beams. At the time of the experi- 

 ment, the sea surface was confused, with whitecaps 

 present and with a wind velocity of 17 mph. A com- 

 parison of the two reverberation level curves shows 

 that for times up to 0.1 sec the horizontal reverbera- 

 tion is more than 20 db above the vertical reverbera- 

 tion; for times between 0.1 and about 0.4 sec, it is 

 more than 10 db above; and for times greater than 

 0.4 sec it is less than 4 db above. Two conclusions are 

 obvious from the figure. One is that on the day of the 

 experiment there was a surface layer of scatterers 

 which was very different in nature from the scatterers 

 in the ocean body. The other is that the reverberation 

 due to surface scatterers decays more rapidly than 

 the reverberation due to volume scatterers. The pres- 

 ence of a deep scattering layer can also be noted at B 

 in Figure 14, and also a shallower scattering layer 

 SitA. 



According to equation (39) of Chapter 12, the sur- 

 face-reverberation intensity at short range, where 2A 

 in equation (39) can be neglected, should be propor- 

 tional to the inverse cube of the range, provided 



500 



1000 



X 



S 1500 



2000 



2500 



f=20 

 2 



-2^ DB/,YO 



-10 t|0 





-10 tio 



-10 ••10 



-10 +10 



Figure 11. Variation with depth of ratio of scattering coefficients. 



