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2.15 Interpreting the Display
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
Figure 2-31Zero line
Fish school echoes
Fish school echoes will generally be plotted
between the zero line and the bottom. Usu-
ally the fish school/fish echo is weaker than
the bottom echo because its reflection prop-
erty is much smaller compared to the bot-
tom. The size of the fish school can be
ascertained from the density of the display.
Small
school
Large
school
Size of
fish school
Figure 2-32 Fish school echoes
Bottom echo
Echoes from the bottom are normally the
strongest and are displayed in reddish-brown
color 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 ma-
terials (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
Figure 2-33 Bottom echoes