Furuno color video plotter GD1720C - Manual (Page 150)

4. VIDEO SOUNDER OPERATION
4-21
4.15.2 Bottom echo
Echoes from the bottom are normally the strongest and are displayed in reddish-brown, but
the color and width will vary with bottom composition, water depth, frequency, sensitivity,
etc.
In a comparatively shallow depth, a high gain setting will cause a second or sometimes a
third or a fourth echo to be displayed at the same interval between them below the first
echo trace. This is because the echo travels between the bottom and the surface twice or
more in shallow depths.
The color of the bottom echo can be used to help determine the density of the bottom
materials (soft or hard). The harder the bottom, the wider the trace. If the gain is set to show
only a single bottom echo on mud, a rocky bottom will show a second or third bottom return.
The range should be chosen so the first and second bottom echoes are displayed when
bottom hardness is being determined.
Intensity difference
in water depth
Second bottom
echo
Rock base
Mud and sand
Bottom echoes