336 



SUMMARY 



coefficient; J„ is the volume reverberation index; r is 

 the range in yards of the reverberation, r = J^Co<; 

 A is the total one-way transmission anomaly to the 

 range r; Ai is the one-way transmission anomaly to 

 the range r due to the effect of refraction. 



Because of surface reflections, not taken into ac- 

 count in equation (6), observed volume reverbera- 

 tion levels with horizontal beams will average about 

 3 db higher than the levels predicted by that equa- 

 tion. 



Dependence on Range 



According to equation (6), if the transmission 

 anomaly terms { — 2A+ Ai) can be neglected, and 

 if the scattering coefficient m is constant throughout 

 the relevant portion of the ocean, then the intensity 

 of volume reverberation should decay with the square 

 of the range; in other words, its level should drop 

 20 db for each tenfold increase in range. In practice, 

 this simple inverse-square dependence is only seldom 

 observed, because (1) the transmission anomaly 

 terms can rarely be neglected at ranges greater than 

 1,000 yd; and (2) the value of m often depends on 

 position in the ocean. The well-established deep 

 scattering layers off San Diego, which apparently 

 scatter much more strongly than surrounding regions 

 of the ocean, are examples of the dependence of the 

 scattering coefficient on position. 



Though detailed agreement with equation (6) is 

 almost never observed, volume reverberation does 

 tend to decrease rapidly with increasing range, as 

 predicted qualitatively by that equation. 



Dependence on Ping Length 



According to equation (6), as the ping length is 

 increased the intensity of volume reverberation 

 should increase proportionally. Although more data 

 are needed, measurements to date indicate that equa- 

 tion (6) does describe the dependence of reverbera- 

 tion on ping length. This proportional dependence is 

 also predicted theoretically for surface and bottom 

 reverberation intensities; for these types of reverbera- 

 tion also, more data are needed, but apparently the 

 theoretical relationship is fulfilled. 



Dependence on Frequency 



The frequency-dependent terms in equation (6) 

 are the volume reverberation index J„, the transmis- 

 sion anomaly terms —2A + Ai, and the scattering 

 coefficient m. The value of the reverberation index 

 can be determined from the pattern fimction of the 

 transducer by means of equation (27) of Chapter 12. 



The transmission anomaly can be estimated by the 

 methods described in Chapter 5. Available data in 

 the frequency range 10 to 80 kc indicate that on the 

 average the scattering coefficient m increases about 

 as the first power of the frequency. However, the 

 data also do not deny the possibility that m is inde- 

 pendent of frequency, or that it increases as the 

 square of the frequency. 



Magnitude of the Volume-Scattering 

 Coefficient at 24 kc 



Observed values of 10 log m, inferred from ob- 

 served reverberation levels, vary between —50 and 

 — 80 db, with — 60 db as a typical value. The varia- 

 tions of 10 log m have not been correlated, in the stud- 

 ies off San Diego, with variations in any factor other 

 than depth in the ocean. The variation of m with 

 depth off San Diego has not yet been fully explained, 

 but its systematic character seems well established. 

 Using a projector pointed straight down, the meas- 

 ured reverberation off San Diego was found to de- 

 crease down to a range of 600 or 700 ft, but then is 

 frequently observed to rise fairly abruptly, and main- 

 tain a high value for a depth interval of about 700 ft. 

 The inferred values of 10 log m for depths within the 

 deep scattering layer are often 15 db or more greater 

 than the values of 10 log m at other depths. Some of 

 these deep layers of high scattering power persist in 

 a given area for periods as long as a month or even 

 longer. Although the scatterers in these deep layers 

 have not been definitely identified, it seems probable 

 that they are of biological origin. 



17.2.2 Surface Reverberation 



The following subsections summarize the known 

 information concerning reverberation from the sur- 

 face of the ocean. This information applies primarily 

 to reverberation measured with horizontal beams at 

 those ranges where surface reverberation exceeds 

 volume reverberation. 



Theoretical Formula for Surface 

 Reverberation Level i 



The expected surface reverberation level R'{t) at a 

 time t seconds after midsignal is given by the formula 



R'it) = 10 log ^ -MO log y -I- JM 



-30\ogr-2A, (7) 



where m' is the backward scattering coefficient of the 

 surface scattering layer; d is the angle at the trans- 



