Furuno GP-7000F Operator's Manual page 111

Zero line
The zero line represents the transducer's position. It moves off the screen when
a shifted range is used, or is shown at draft depth when ship's draft is entered.
Bottom echoes
Bottom echoes are normally strongest and displayed in reddish brown or red, but
colors and width will vary with bottom material, depth, sea condition, installation,
frequency, pulselength and sensitivity.
Color difference
in sea depth
Second bottom
echo
Bottom profile
A hard and rough bottom appears with a longer tail because it reflects more of
the ultrasonic pulse. Because of their stronger return, shallow echoes appear
wider than deep ones even when all bottom conditions are equal. Also, a longer
bottom tail appears on slopes because of the difference in traveling time at both
edges of the beam angle. In the rugged bottom, echoes are reflected on many
different planes, overlapping to present a 3D effect.
Slope
Flat bottom
Range shifted
Zero line
Rugged bottom
Bottom material and bottom profile
Bottom
with many
undulations
Bottom profile
10. VIDEO SOUNDER OPERATION
Zero line
10-25