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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 acoustically equivalent to running into a brick wall, the bottom
echo will be displayed intermittently. Similar noise sometimes appears 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 identify target fish. 200KHz tends to show
shallow thermoclines better than 50KHz.
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
3-2