526 



HERSEY AND BACKUS 



[chap. 13 



(3) For most layers the peak frequency increases with increasing depth 

 during the course of a diurnal migration. This phenomenon, which may be 

 referred to as "frequency migration", has been found to vary widely with 

 different layers and in different parts of the ocean. 



Layers which have been observed in the western North Atlantic fall loosely 

 into three categories with respect to peak frequency. These may be identified 

 as "low-frequency", meaning about 3-5 kc/s, or the lowest frequency at which 

 peak scattering has been found; "high-frequency", referring to the highest 

 frequencies at which peaking has been closely examined, or about 20 kc/s ; and 

 "mid-frequency", meaning frequencies between these extremes, or about 

 15 kc/s. The peak frequency of all layers varies diurnally in some manner so 

 that the identifying frequencies just mentioned must be regarded as median 

 values for convenience of description. 



Fig. 18 shows peak frequency and corresponding depth versus time relative 

 to sunset for a typical migration in waters south of Nova Scotia north of 

 the Gulf Stream (area A of Fig. 14). In these waters depth migration and 



190" 



180° 



170° 



DIRECTIVITY PATTERN 

 QBG 

 5 kc/s 



(a) 



60° 

 70° 

 80° 

 90° 



100° 

 I 10° 



