Furuno 1933C - Manual (Page 173)

4. VIDEO SOUNDER OPERATION
4-27
4.15 Interpreting the Sounder Display
4.15.1 Zero line
The zero line (sometimes referred to as the transmission line) represents the
transducer’s position, and moves off the screen when a deep phased range is
used.
Zero line
Shift
Zero line
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