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4. VIDEO SOUNDER OPERATION
4-25
Intensity difference
in water depth
Second bottom
echo
Rock base
Mud and sand
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.
Zero line
Shift
Zero line
4.15.2 Bottom echo
Echoes from the bottom are normally the strongest and are displayed in
reddish-brown (color model) or darkest tone (monochrome model) but the color
(tone on the monochrome model) 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 (tone for monochrome model) 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.
Bottom echoes