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3.4 Surface Noise/Aeration
When the waters are rough or the boat passes
over a wake, surface noise may appear near
the zero line. As surface turbulence is acous-
tically equivalent to running into a brick wall,
the bottom echo will be displayed intermit-
tently. Similar noise sometimes appears when
a water temperature difference (thermocline)
exists. Different species of fish tend to prefer
different temperature zones, so the ther-
mocline may be useful to help identify target
fish. 200 kHz tends to show shallow ther-
moclines better than 50 kHz.
In rough waters the display is occasionally in-
terrupted 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 3-4 Surface noise/aeration