Furuno color video plotter GD1720C - Manual (Page 151)

4. VIDEO SOUNDER OPERATION
4-22
4.15.3 Fish school echoes
Fish school echoes will generally be plotted between the zero line and the bottom. Usually
the fish school/fish echo is weaker than the bottom echo because its reflection property is
much smaller compared to the bottom. The size of the fish school can be ascertained from
the density of the display.
Size of
fish school
Small
school
Large
school
Fish school echoes
4.15.4 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.
200 kHz tends to show shallow 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
Surface noise/aeration