Furuno COLOR VIDEO PLOTTER 1933C - Manual (Page 171)

4. VIDEO SOUNDER OPERATION
4-27
4.14.3 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
4.14.4 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