Chapter 14 

 DEEP-WATER REVERBERATION 



IT IS CONVENIENT to begin the study of observed 

 reverberation levels by describing the experi- 

 mental observations in deep water. In deep water it 

 is usually possible to ignore bottom reflections, 

 thereby facilitating comparison of the experimental 

 results with the theoretical formulas of Chapter 12. 

 Also, in deep water, it is frequently possible to elimi- 

 nate surface scattering and reflections, bydirectingthe 

 beam downward at some angle. When reverberation 

 from the surface and bottom is effectively eliminated, 

 the received reverberation can assuredly be called 

 volume reverberation. The first section of this chap- 

 ter describes the experimental facts about volume 

 reverberation, as determined by such unambiguous 

 experiments. 



In ordinary echo ranging, with the main trans- 

 ducer beam horizontal, part of the received reverbera- 

 tion is surface reverberation and part volume rever- 

 beration. It is not easy to make a clear distinction be- 

 tween these two components on observed reverbera- 

 tion records. The distinction between the two is 

 usually made by comparing the measured levels ob- 

 tained with horizontal beams with the levels observed 

 in unambiguous volume reverberation experiments, 

 and also by observing the dependence of the rever- 

 beration levels on sea state. The observed levels with 

 horizontal transducer beams are described in the 

 second section of this chapter. 



14.1 TRANSDUCER DIRECTED 



DOWNWARD 



The experimental method for eliminating surface 

 reverberation in deep water has usually been to point 

 a highly directional transducer downward, away from 

 the surface. In this way the main transducer beam 

 does not strike the surface, and the observed rever- 

 beration levels are then assumed to be due to volume 

 reverberation. Of course this assumption requires 

 verification, since in the absence of any information 

 about the relative values of the surface and volume 



backward-scattering coeflBcients, it is not possible to 

 know ill advance how much directivity is necessary 

 to definitely eliminate surface reverberation. How- 

 ever, it may be accepted as a working hypothesis that 

 pointing the main transducer beam down 30 de- 

 grees, or more, does eliminate surface reverberation, 

 for standard 24-kc echo-ranging gear. It will be seen 

 later that surface reverberation levels are not usually 

 high enough to contribute to the received reverbera- 

 tion under these circumstances. The following sub- 

 sections describe the various experimental studies of 

 volume reverberation which have been carried out 

 in this manner. 



-80 



-too 



-120 

 -140 

 -160 



RANGE IN YARDS 

 40 80 400 800 



40008000 



aoi 005 at as « 



TIME IN SECONDS 



Figure 1. Volume reverberation levels showing in- 

 verse square range dependence. 



14.1.1 Dependence on Range 



According to Chapter 12, equation (22), if the 

 volume scatterers are uniformly distributed, and if 

 the transmission anomaly terms — 2A -|- Ai'm equa- 

 tion (22) can be neglected, then the volume rever- 

 beration intensity should be inversely proportional 

 to the square of the range, or in other words, the 

 reverberation level should decrease 20 db with a ten- 

 fold increase in time. As an example of this depend- 

 ence, we may refer to Figure 1, which is a plot of data 

 obtained on June 3, 1942.' The QCH-3 transducers, 

 projector and hydrophone, were lowered to a depth 

 of 60 ft and tilted downward 60 degrees. The water 

 depth was 600 fathoms and the surface was moder- 



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