Chapter 15 

 SHALLOW-WATER REVERBERATION 



SURFACE 



BOTTOM 



BOTTOM 

 REFLECTION 



TIME 



PROJECTOR DOWN 90 DEGREES 



SURFACE 



BOTTOM 



m 



^BOTTOM 

 y^ SCATTERING 



TIME 

 PROJECTOR DOVm 30 DEGREES 



SURFACE 



BOTTOM 



TIME 

 PROJECTOR UP 30 DEGREES 



SURFACE 



TIME 

 PROJECTOR HORIZONTAL 



Figure'I. Expected behavior of bottom reverberation 

 for idealized projector. 



COMPABisoN OF THEORY and experiment in bottom 

 reverberation is complicated by the directivity 

 pattern of the transducer and by uncertainty regard- 

 ing the dependence of the scattering coefficient on the 

 angle of incidence. An additional compUcation is 

 refraction, which is of considerable importance in de- 

 termining bottom-reverberation levels. One way in 

 which bottom reverberation differs from the other 

 types of reverberation we have considered is that 

 bottom reverberation is not heard immediately after 

 the initial ping, but appears some time later, usually 

 coming in as a distinct crash. This delay results from 

 the fact that the bottom, unlike the scatterers re- 

 sponsible for surface and voliune reverberation, is 

 usually a significant distance from the projector. 



15.1 QUALITATIVE DESCRIPTION OF 

 BOTTOM REVERBERATION 



Figure 1 illustrates the expected behavior of the 

 bottom reverberation for an ideaUzed projector hav- 

 ing constant sound output within 5 degrees of the 

 axis and zero output outside 5 degrees. For illustra- 

 tive purposes, we may assume there is no refraction. 

 Then for this simple type of sound beam, scattered 

 soimd is received, at the time instant t, only from 

 those scatterers included within a sector of a spherical 

 shell centered at the projector and bounded by the 

 limits of the sound beam. The mean radius of this 

 shell is ct/2 and its thickness ct/2, as pointed out in 

 Section 12.2. Bottom reverberation is received when- 

 ever this shell cuts off some portion of the bottom. 



Bottom reverberation will set in at the time cor- 

 responding to the shortest range at which the beam 

 strikes the bottom. Since bottom scattering coeffi- 

 cients are usually relatively large, the total received 

 reverberation will increase sharply at the time of on- 

 set of bottom reverberation. Scattering at the bottom 

 will cease, except for soimd which is reflected or scat- 

 tered toward the bottom, at the time the last portion 

 of the beam leaves the bottom. 



The case of vertical incidence on the bottom is 

 illustrated in the first box of Figure 1. In this case, as 

 shown in the box, all portions of the beam strike the 



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