338 



SUMMARY 



The main import of this figure is that it indicates 

 both the expected reverberation level and the possible 

 spread in values at any range and wind speed. From 

 the upper and lower limits in the figure were inferred 

 the values of the surface and volume scattering 

 coefficients given in Section 17.2.2. 



17.3 BOTTOM REVERBERATION LEVELS 



The following subsections summarize the known 

 information concerning reverberation from the ocean 

 bottom. This information is mainly concerned with 

 reverberation from horizontally projected beams in 

 shallow water. Under these circumstances, after a 

 sufficient time has elapsed for the beam to reach the 

 bottom, the received reverberation is preponderantly 

 bottom reverberation. 



17.3.1 



Theoretical Formula 



The expected bottom reverberation level R'{i) at a 

 time t seconds after midsignal is given by the formula 



R'(t) = loiogy + loiog Y +Me) 



-30 log?- -2.4, (8) 



where m" is the bottom scattering coefficient, 6 is the 

 angle at the transducer between the sound returning 

 at the time t and the horizontal plane, Jbid) is the 

 bottom reverberation index, corresponding to the 

 angle of depression 6, and the other quantities have 

 the meanings given in Section 17.2.1. 



With horizontal beams and transducers near the 

 surface, observed bottom reverberation levels will 

 average about 6 db higher than the levels predicted 

 by equation (8), on account of surface reflections. 



17.3.2 



Dependence on Range 



According to equation (8), the bottom reverbera- 

 tion intensity at short ranges, where the transmission 

 anomaly term 2A can be neglected, should be propor- 

 tional to the inverse cube of the range, provided m" 

 and Jb{6) also change negligibly with increasing range. 

 This simple inverse-cube relationship is almost never 

 observed. In the first place, because of the distance 

 between the transducer and the bottom, reverbera- 

 tion from the bottom does not set in until a significant 

 time has elapsed after the emission of the ping. 

 Usually the reverberation then quickly builds up to a 

 peak, corresponding approximately to the time when 

 the edge of the main beam strikes the bottom. After 



the peak, the reverberation intensity falls off rapidly, 

 usually about as the fourth power of the range; how- 

 ever, very large deviations from the inverse-fourth 

 power decay have been observed. 



The range to the bottom reverberation peak de- 

 pends on refraction conditions and water depth. In 

 isothermal water, the peak is expected at a range 

 about 12 times the water depth. When the tempera- 

 ture decrease from projector to bottom is greater 

 than 5 degrees, the peak occurs at a range between 

 4 and 8 times the water depth, depending on the 

 severity of the downward refraction, with a median 

 value of 6 times the depth. 



In general, the quantities m" and Jb(d) in equation 

 (8) are dependent on range. However, at ranges past 

 the reverberation peak, both of these quantities 

 usually depend only slightly on range. 



17.3.3 Dependence on Frequency 



The frequency-dependent terms in equation (8) 

 are the surface reverberation index Jb{9), the trans- 

 mission anomaly 2A, and the bottom scattering 

 coefficient m". The value of Jb(,d) can be determined 

 from the pattern function of the transducer, by equa- 

 tions (53), (41), and (42) of Chapter 12, and, as be- 

 fore, the transmission anomaly can be estimated by 

 the methods of Chapter 5. Measurements on rock 

 bottoms indicate no dependence of the bottom-scat- 

 tering coefficient m" on frequency, in the frequency 

 range 10 to 80 kc. It is probable that other bottoms 

 as well would show no dependence of m" on the 

 frequency of the incident sound, although more data 

 are needed to confirm this point. 



17.3.4 Dependence on Bottom 



Bottom reverberation levels are not the same over 

 all types of bottoms. Although wide variations are 

 observed, in general the highest reverberation levels 

 are observed over ROCK, lower values over MUD 

 and SAND-AND-MUD, and the smallest values 

 over SAND bottoms. These classifications of bottom 

 type depend on the particle size in the material com- 

 posing the bottom and are more fully described in 

 Chapter 6. 



17.3.5 Bottom Scattering Coefficients 



at 24 kc 



The backward scattering coefficient depends on the 

 angle at which the sound is incident on the bottom. 



