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HERSEY AND BACKUS 



[chap. 13 



phenomena had the general properties of the layer first described. Multiple layers 

 were detected at single localities, and midday depth, intensity and migration 

 pattern were noted to vary greatly at widely separated points in the ocean (see, 

 for instance, Dietz, 1948; Hersey and Moore, 1948; Johnson, 1948; Tchernia, 

 1950; Moore, 1950; Boden, 1950; Tucker, 1951 ; and Batzler and Westerfield, 

 1953). Moreover, observations made by different investigators in the same part 

 of the ocean sometimes were at variance (for instance, observations by Dietz 

 (1948) and Tchernia (1950) in Antarctic waters). As later information shows, 

 this was doubtless due to differences in the echo-sounding equipment, especially 

 its operating frequency. In addition a host of other sound-scattering patterns 

 were recorded which did not fit the layer pattern. 



Fig. 1. Scattering groups in shallow water in the region of Nantucket Shoals recorded by 

 a 12 kc/s echo -sounder. The conspicuous bottom features are sand waves. 



In general we may distinguish between "scattering groups" and "scattering 

 layers". By scattering groups we mean phenomena which are discontinuous in 

 the horizontal plane and whose horizontal dimensions on the echo-sounding 

 record are less than or only a few times more, at most, than their vertical 

 dimension. These aggregations are generally of high scattering cross-section 

 and are usually attributed to schooling fishes. They are common features of the 

 continental shelves and are often abundantly observed over known fishing 

 grounds. Often the echo-sounder will resolve individual scatterers (which may 

 be individual organisms or small concentrations of organisms within the larger 

 aggregation), and echo sequences are recorded which form crescentic patterns. 

 (These patterns are formed because the echo-sounder on a moving ship usually 

 moves in nearly a straight horizontal line at uniform speed. As the ship ap- 

 proaches a scatterer, the slant distance to the scatterer varies as a hyperbolic 

 function of the horizontal distance ; hence the crescentic form of the echo 

 sequence. The extent of the crescent is determined by the directional properties 



