SECT. 4] 



SOUND SCATTERING BY MARINE ORGANISMS 



525 



and over at different gain settings ; the outline of the blackened areas of the 

 resulting recordings give contours of equal wave-analyzer response (Koenig 

 and Ruppel, 1948). Fig. 17 shows a sunset sequence of observations analyzed 

 in this manner. For analysis by the method of equation (40), the tapes were 

 played to an oscilloscope through Rayspan Filters (heterodyne filters with a 

 constant bandwidth of 100 c/s) and photographed. 



A. Frequency Dependence of Sound-Scattering in the Sea 



Hersey and Backus (1956) noted that reverberation from certain scattering 

 layers exhibits a frequency variation which is related to depth migration (i.e. 



Fig. 14. Index chart showing sources of data used for analysis of frequency-dependent 

 characteristics of deep scattering layers. 



change in hydrostatic pressure) in such a manner as to suggest that the scat- 

 terers are highly compressible. In accordance with this hypothesis, particular 

 attention has been paid to the frequency behavior of the reverberation from 

 scattering layers. 



Within the frequency range of the equipment (100 c/s to 32 kc/s) we find that 

 reverberation from deep scattering layers has the following frequency-dependent 

 characteristics : 



(1) At a given time, layers appearing at different depths exhibit strong 

 peaks of scattering intensity at correspondingly different frequencies. These 

 intensity peaks are of the order of 10 dB or more above background. 



(2) At a given time each layer exhibits peak scattering, in all but extremely 

 questionable cases, at only one frequency band within the range under examina- 

 tion. These peak frequencies have been found to range from about 25 kc/s down 

 to about 2.5 kc/s. 



