SECT. 4] 



SOUND SCATTERING BY MARINE ORGANISMS 



501 



of the echo-sounder. These instruments insonify and receive sound from a 

 cone-shaped volume directly below much more strongly than in other directions. 

 Hence the beginning and end of the crescent is thought to be determined as the 

 scatterer enters and leaves this cone. Thus the length and curvature of these 

 crescentic echo sequences are a function of the" directionality of the sounder, its 

 pulse repetition rate, the rate of advance of the chart paper, the speed of the 

 ship, the depth to the scatterer, and other variables.) Fig. 1 shows the resolution 

 of individual scatterers in shallow water. Scattering groups are sometimes 

 observed at a considerable depth in the deep ocean (Fig. 2) but are of relatively 

 rare occurrence there, although in certain restricted localities they may be so 

 continuously observed as to form layers. 



DEPTH ^ 

 {fathoms) 



O 200 



mmiwium 



^^^m^^tmm.^ 



5 minutes 



Fig. 2. Scattering groups in deep water south of New England recorded by a 12 kc/s echo- 

 sounder. An ordinary deep -scattering layer is faintly shown at a depth of about 

 200 fathoms. 



Those features which are more or less continuous in the horizontal plane, 

 their horizontal dimension being many times their vertical one, we call "scatter- 

 ing layers". Commonly these layers appear on the record of a surface echo- 

 sounder as a uniform band of numerous, diffuse, relatively weak echoes. 

 Individual scatterers are generally not resolved, and crescentic patterns of 

 echo sequences are not formed. Such features are encountered over the con- 

 tinental shelves and deep ocean alike. The shallow- water layers are more or less 

 local and transitory. In a passage of a few tens of miles one may see them come 

 and go, and their existence is greatly altered by the seasons. The vertical 

 extent of these layers in shallow water is usually not very great and in some 

 cases may be less than a couple of meters. Their position in the water column 

 is usually correlated with a conspicuous feature of the temperature-depth 

 profile. Examples of such layers are shown in Figs. 3 and 4 and have been 

 reported elsewhere (Tucker, 1951 ; Trout, Lee, Richardson and Harden Jones, 



