318 Lecture 16 
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Fig. 16. AG Echo- sounder record showing movement of scattering layers caused by 
internal waves. 
of the transducer from the nearest part of the water surface which is effectively 
in the beam, which may be considerably nearerthan the water surface vertically 
above the transducer. Such errors can be quite large with a wide-beam trans- 
ducer, but if the beam is too narrow, there may be no reflecting facet in the 
surface area "illuminated" by the beam, and thus echoes may be lost. 
The other major snag with this type of instrument is that in storms, when it 
is probably most important to record the waves, the water is often so aerated 
that the sound cannot penetrate it andno echoes may be obtained for long periods 
of time. 
16.5.2. Internal Waves 
Internal waves in the sea have both direct and indirect effects on underwater 
sound. The direct effect is refraction of sound, and this has a marked effect on 
asdic records (Fig. 16.17). Here, the sound is focused or dispersed so that some 
areas of the sea floor are more brightly "illuminated" than others and a marked 
pattern is produced on the record. This effect has been studied by Lee [20] and 
is mentioned in Lecture 8 (J. Crease), and might possibly give useful information 
on the waves themselves. The indirect effect is to produce movement of organ- 
isms in the sea which are then picked up on echo-sounders [21]. Figure 16.18, 
