282 



DEEP-WATER REVERBERATION 



-100 



-120 



a; -160 



-180 



0.01 



0.05 



0.5 

 TIME IN SECONDS 



10 



OCEAN SURFACE 



OCEAN FLOOR 



FiGtiBE 2. Volume reverberation levels with deep scattering layer. 



ately calm with the wind velocity averaging 10 mph, 

 and long low ground swells but no whitecaps. A signal 

 length of 10 msec was used. Twenty records were 

 filmed, measured, and averaged, to give the points 

 shown in Figure 1. It is seen that the experimental 

 data fit fairly well the straight-line dependence of R' 

 on log r which is predicted, if all quantities except R' 

 and r are constant, by equation (24) of Chapter 12. 

 In practice, volume reverberation runs usually 

 show even worse agreement with this simple linear 

 range dependence than do the points shown in 

 Figure 1. In the first place, it is known from trans- 

 mission measurements that the transmission anomaly 

 terms can rarely be neglected at ranges greater than 

 1,000 yd (see Chapter 5). Thus the inverse square 

 dependence can be expected only at relatively short 

 ranges. In addition, there is no real reason to expect 

 the volume scatterers to be uniformly distributed in 

 the ocean. However, the fact that an approximately 

 inverse square dependence has been observed in at 

 least a few cases is evidence that our fundamental 

 assumptions about volume reverberation are not 

 altogether wrong. In general, volume reverberation 

 tends to decrease rapidly with increasing range, in at 

 least qualitative agreement with equation (24) of 



Chapter 12. However, the detailed dependence on 

 range is frequently observed to be very different from 

 the simple form of that equation; often the depend- 

 ence of R' on log r is not linear, and when it is linear, a 

 slope of exactly — 20 is quite unusual. Such observa- 

 tions are described in the following subsection. 



14.1.2 Dependence on Depth 



Measurements off San Diego with the transducer 

 pointed downward have frequently shown sudden 

 increases in reverberation level which seemingly 

 could only be explained by assuming that in certain 

 deep layers of the ocean the backward volume-scat- 

 tering coefficient was much larger than at other 

 depths. Figure 2 was drawn from data obtained on 

 July 28, 1942. The QB transducer was pointed down- 

 ward at an angle of 49 degrees relative to the hori- 

 zontal, in 660 fathoms of water. Ten records were 

 averaged to give the points in this figure. At the re- 

 verberation range indicated by A in the illustration 

 there is a sharp rise of more than 10 db in reverbera- 

 tion level. A comparison with equation (24) of Chap- 

 ter 12 makes it seem necessary to ascribe this rise to 

 an increase in the backward scattering coefficient m; 



