514 



HEKSEY AND BACKUS 



[chap. 13 



Various investigators have used solid spheres as reference targets in scatter- 

 ing studies. Fig. 11, from Smith (1954), is a theoretical plot of scattering cross- 

 section per unit solid angle (sometimes called "target strength") of a brass 

 sphere 6.17 cm in diameter, together with his experimental determinations of 

 the same, in the frequency range where wavelength in the water is comparable 

 to the circumference of the sphere. For the special case where the circum- 

 ference of a rigid sphere is large compared with the wavelength, the scattering 

 cross-section becomes equal to the physical cross-section, nR'^, presented by 

 the sphere to an incident plane wave. 



Anderson's computations (Anderson, 1950), though they apply strictly to 

 spheres, are a useful qualitative guide for testing the properties of various 

 possible sound-scatterers. For example, Benjamin Leavitt {in litt.) of the 

 University of Florida has pointed out that certain euphausids contain a globule 



FREOUEMCY (kc/s) 



Fig. 11. Observations of the target strength of a solid brass sphere 6.17 cm in diameter at 

 various frequencies compared with the theoretical determination. (After Smith, 

 1954.) 



of oil which might account for the scattering observed in deep scattering layers. 

 A cubic centimeter sample of this oil was compressed through the pressure 

 range of the oceans by P. W. Bridgman, who reported {in litt.) that it was 15% 

 more compressible than water throughout this range. The globule has a radius 

 of the order of 1 mm. From Anderson's curves its back-scattering cross- 

 section is of the order of 2 x 10"^^ jn2 ^t 20 kc/s. This implies a population 

 density of about 10^ to 10'^ scatterers per cubic meter to account for observed 

 scattering. Planktonologists estimate such concentrations of euphausids to be 

 most unlikely. Nevertheless, such scatterers are in the ocean and must scatter 

 sound. Their small size will require very high frequencies for appropriate study. 

 Such studies are not now in prospect, but obviously they should be under- 

 taken. In a discussion below about the identity of scatterers we will deal with 

 certain tests of euphausid scattering. 



