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Surface noise/Aeration
When the waters are rough or the boat
passes over a wake, surface noise may ap-
pear near the zero line. As surface turbulence
is acoustically equivalent to running into a
brick wall, the bottom echo will be displayed
intermittently. Similar noise sometimes ap-
pears when a water temperature difference
(thermocline) exists. Different species of fish
tend to prefer different temperature zones,
so the thermocline may be useful to help iden-
tify target fish. 200 kHz tends to show shal-
low thermoclines better than 50 kHz.
In rough waters the display is occasionally
interrupted due to below-the-ship air bubbles
obstructing the sound path. This also occurs
when the boat makes a quick turn or reverses
movement. Lowering the picture advance
speed may reduce the interruption. However,
reconsideration of the transducer installation
may be necessary if the interruption occurs
frequently.
Surface noise
Caused by
aerated water
Thermocline
Figure 2-34 Surface noise/aeration