312 Lecture 16 
Fig. 16.12. Cod echoes displayed on Fish-lupe. Expanded portion of trace is 
about seven fathoms. 
models commercially available, ranging from a short-range, high-resolution 
instrument used by purse-seine fishermen to models with ranges up to 2000 
yards. A record taken with the N.I.O. asdic described in Section 16.2.5 is shown 
in Fig. 16.13. It will be seen that fish close to the ship appear very clearly, but 
at longer ranges where the main beam hits the sea bed, the fish echoes have to 
compete with bottom reverberation and detection is more difficult. The long 
ranges of detection are only obtained where the sea bed is smooth and has com- 
paratively low reverberation. 
16.3.2. Scattering Layers 
Apart from the dense shoals of fish, a sensitive echo-sounder also records 
reverberation from diffuse and more or less continuous layers in the sea. A 
narrow-beam, high-resolution sounder operating in deep water willusually show 
a complex structure of layers, some of which migrate to the surface at night 
and return to deeper water at dawn. A record taken with a narrow-beam sounder 
(the N.I.O. asdic turned with the beam axis vertical) is shown in Fig. 16.14. 
