III-63 



Much of the work has been directed towards studying the variation of m with 

 changes in volume and frequency. The nature of the data collected varies 

 greatly from study to study, but usually the reverberation level is found to 

 peak at certain frequencies. TTiese peaks are believed to be caused by scat- 

 tering from marine organisms. The data also show a gradual decrease in the 

 volume scattering coefficient with increasing depth, except for a pronounced 

 peak corresponding to depths in the range of 200m- 400m. This has led to the 

 discovery of the so-called deep scattering layer which is believed to consist 

 of marine organisms . 



To explain the experimental data, we must take recourse to the models 

 of scattering from a fluid sphere or an air bubble treated earlier in this chapter. 

 Admittedly most marine organisms are not exactly spherical, but for small scat- 

 terers (ka < < 1) shape is not too important, and for larger scatterers (ka ~ 1) 

 whose scattering is shape -dependent we still have no better guide. 



The model based on a fluid sphere does not take into account the flex- 

 ural properties of the skin or carapace of marine organisms. To consider the 

 effect of this, Machlup (Ref. III-25) studied scattering from a spherical elastic 

 shell containing a fluid. His theoretical results indicate that for incident wave- 

 lengths of the order of magnitude of the radius of the sphere, the behavior for 

 thin shells is similar to that of the fluid sphere alone. For longer wavelengths, 

 a correction may be computed, good to terms in the first power of the ratio of 

 the thickness to the diameter of the shell. 



These models, though they apply strictly to spheres, may be used as 

 a qualitative guide for testing the properties of various possible sound scat- 

 terers. For instance, Hersey and Backus'-*-^ cite work which points out that 

 certain euphausids contain a globule of oil which might account for the scatter- 

 ing observed from deep scattering layers. This oil was found to be 15% more 

 compressible than water at the same depth. The globule has a radius of the 

 order of 10"^ m. Based on (III-15), its back- scattering cross sectiom^) would 

 be of the order of 2* 10" ^^m^ at 20 kc/sec. Based on the scattering or rever- 

 beration levels detected, this would require a population density of about 10 

 to 10^ such scatterers per cubic meter. Such concentrations are extremely 

 unlikely, and the euphausids' oil globules are, therefore, a very improbable 

 explanation of the deep scattering layer. 



1. Hill, Ref. Ill- 15 



2. In this case half of the scattered energy is back scattered. 



artbur B.HittleJnir. 



S-7001-0307 



