284 



DEEP-WATER REVERBERATION 



-80 



_, -100 



< -120 



40 eo 



RANGE IN YARDS 



400 800 



4000 8000 



-140 



0.01 



0.05 



0.1 



0.5 

 TIME IN SECONDS 



10 



TEMPERATURE IN DEGREES F 

 46 50 54 58 62 



Figure 4. Volume reverberation levels with shallow scattering layer. 



period from July 9 to August 5, 1942. On the other 

 hand, on June 16 and 17, a layer was observed at 

 1,200 ft; but a week later no such layer was detected. 

 Thus the observations indicate that deep scattering 

 layers, in a given area, may sometimes appear and 

 disappear, and at other times persist for periods as 

 long as a month or even longer. Just what these deep 

 scattering layers consist of is not known ; they may, 

 for example, be concentrations of fish, bubbles or 

 plankton.* The layer of Figure 4 occurs at the same 

 depth as does a temperature inversion on the bathy- 

 thermograph trace shown in the insert of Figure 4. 

 On the other hand, no inversion is noticed at the 

 depth corresponding to 4 in Figure 5. 



14.1.3 Dependence on Frequency 



An extensive series of measurements of volume 

 reverberation in deep water,^ at frequencies of 10, 20, 



» The observations reported in this chapter were made dur- 

 ing daylight hours. More recent studies show evidence of 

 diurnal migration of the deep scatterers and lend support to 

 the theory of biological origin. 



40, and 80 kc have been made by UCDWR. These 

 measurements, described later, were made in water 

 depths ranging from 660 to 1,950 fathoms, in the 

 months of January and February 1943. The area of 

 observations extended southwest of San Diego to 

 Guadalupe Island, which is about 250 miles from 

 San Diego and 200 miles off-shore. The various posi- 

 tions at which observations were taken are marked 

 by roman numerals in Figure 6. 



Deep scattering layers of the type discussed previ- 

 ously were observed on this cruise. Figures 7 and 8 are 

 plots of typical reverberation records obtained at 

 three positions shown in Figure 6. These data were 

 obtained with the transducers directed vertically 

 downward, sending out 10-msec pings at the four 

 frequencies 10, 20, 40, and 80 kc. Each point on the 

 curves for positions III and VIII is an average of 5 

 pings, while points on the curves for position IX are 

 an average of 25 pings. It is evident from Figures 7 

 and 8 that the effective depth of the deep scattering 

 layer does not seem to depend on frequency. This 

 fact is shown somewhat better in Figure 9, which is a 



