Furuno GP-7000F Operator's Manual page 116

10. VIDEO SOUNDER OPERATION
Bottom nature
The nature of the bottom is known from the intensity and length of the bottom tail. Gen-
erally, when observing the bottom nature, the lower sounding frequency is used, the
pulselength is set to long, and the gain setting is not disturbed. In the hard and craggy
bottom, the bottom appears in reddish brown with a long tail. In the muddy or sandy
bottom, the bottom appears less reddish and with a short tail. However, the bottom
with sediment may give a long tail if a low frequency sounding is used.
Fish schools
Fish quantity
Fish quantity can be estimated to a certain extent from fish echoes on the screen if
fish school size and fish school density are kept in mind.
Fish school size
Usually the size of fish echoes on the screen is proportional to the actual size of the
fish school. However, if two fish echoes appear at different depths with the same size,
the fish school at shallower depth is smaller because the ultrasonic beam widens as
it propagates and a fish school in deep water is displayed larger.
Size of
fish school
10-26
Rock base
Mud & sand
Bottom nature
Large
Small School depth
school
school
and sounding
time
Fish school size
Deep fish school sounding time
Shallow fish school sounding time