TRANSDUCER DIRECTED DOWNWARD 



283 



certainly it does not seem possible that any change in 

 transmission anomaly could be sufficiently sudden to 

 account for the rise. The geometry of the experiment 

 is shown in the small box of Figure 2, on the assump- 

 tion that the ray paths are approximately straight 

 lines. The peak at A occurs at a time 0.5 sec after the 

 ping. This corresponds to a reverberatior range of 

 400 yd ; thus, the layer of high scattering power must 

 have been centered at a depth of 400 yd X cos 41°, 

 or about 900 ft. The thickness of the layer, as esti- 

 mated from the thickness of the bulge at A in Figure 

 2, was not less than 500 ft. The large increase in re- 

 verberation level at B corresponds to the point at 

 which the beam strikes the bottom. The rise at C 

 in Figure 2 could have resulted from scattering of 

 bottom-reflected sound by the deep scattering layer. 

 It could also have resulted from sound which was 

 scattered from the bottom toward the surface, re- 

 flected from the surface back to the bottom, then 

 scattered from the bottom back to the transducer. 

 These various possible paths are shown in the small 

 box in Figure 2. 



Another record of the many which show the pres- 

 ence of a deep scattering layer is one made August 5, 



40 



RANGE IN YARDS 

 80 400 800 



4000 8000 



-100 



-140 



m-160 --• 



-ISO 



0.01 



0.05 0.1 0.5 i 



TIME IN SECONDS 



10 



Figure 3. Volume reverberation levels with deep 

 scattering layer. 



1942. The data, plotted in Figure 3, were obtained 

 with the QB transducer pointed vertically downward 

 in 650 fathoms of water. Figure 3 is an average of 10 

 pings each 12 msec long. This experimental curve has 

 several important features. The first portion of the 

 curve decreases as 20 log t, indicating imiform distri- 

 bution of scatterers to a depth of about 500 ft. A 

 deep layer of high scattering power is evident in the 

 vicinity of A in Figure 3 ; this layer appears to have 

 a mean depth of 1,000 ft and a thickness of about 

 750 ft. At the position of highest scattering power 

 Avithin the layer, the volume-scattering coefficient is 

 very much greater than its value in the body of the 



ocean above the layer. If we use equation (24) of 

 Chapter 12 to estimate 10 log m, assuming that the 

 transmission anomaly terms are small, then 10 log m 

 at A is 16 db greater than 10 log m at points on the 

 line denoting inverse square decay; in other words, 

 m at A is 40 times as great as m at points in the first 

 500 ft of the ocean. Once the beam is out of the layer, 

 the reverberation level falls off abruptly. At a depth 

 of 2,250 ft, the calculated value of 10 log m, neglect- 

 ing the transmission anomaly terms in equation (24) 

 of Chapter 12, is 20 db down from the value of 10 log 

 m in the first 500 ft. This difference could not be ac- 

 counted for by ordinary values of the transmission 

 anomaly terms -2A + Ai. It is possible (though not 

 likely) that the sound suffers an abnormally high 

 transmission loss in its two-way passage through the 

 high scattering layer, or there may actually be a 

 layer of low scattering power at the 2,250-ft depth. 

 Echoes from the bottom are noted at B, D, and E in 

 Figure 3. The distance the sound which produces a 

 reverberation peak has traveled can be estimated by 

 noting the time at which the peak appears; it is easily 

 seen that the sound producing the peak at D has 

 gone from the transducer to the bottom, back to the 

 surface, then to the bottom again, and finally back 

 to the transducer. By a similar computation the peak 

 at C is seen to be sound which traversed one of the 

 following two paths: (1) scattered by the layer up 

 to the surface, reflected from the surface to the bot- 

 tom, and then returned to the transducer, (2) re- 

 flected from the bottom up to the surface, reflected 

 back toward the bottom, and then scattered back 

 to the transducer from the deep layer. 



Not all scattering layers are at great depths. For 

 example, a scattering layer at a depth of about 200 ft 

 is evident in the reverberation curve of Figure 4. 

 These records were taken with the sound beam di- 

 rected vertically downward, and with a ping length 

 of 10 msec. Occasionally both shallow and deep scat- 

 tering layers are present in the ocean simultaneously. 

 An example is given in Figure 5, which is made up of 

 reverberation from 8-msec pings projected at a de- 

 pression of 60 degrees below the horizontal in 620- 

 fathom water. In that figure, three scattering layers 

 are noted, a,t A, B, and C. The layer ^ is at a depth 

 of about 100 ft, B at about 600 ft, and C at about 

 1,000 ft. 



Some of these deep scattering layers appeared to 

 persist for relatively long periods of time. In the same 

 area of operation as that for Figure 2, deep scattering 

 layers were observed at about the same depth over a 



