Furuno SFD-1010 Operator's Manual page 127

CHAPTER 3 FISH FINDER
Plankton (DSL) layer
A green to blue echo layer sometimes appears
between a zero line and the bottom. This echo is
generated by a mass of microorganisms,
commonly called "dirt" or "plankton (DSL)", which
usually moves down in the day and up at night. As
fish gather to eat them, you can operate the ship
according to this layer's echoes instead of schools
of fish echoes, depending on the fish type.
Current rip
When two ocean currents of different speeds,
directions and water temperatures meet, a current
rip occurs. A current rip appears on the screen as
shown in the right illustration when air bubbles are
caused or plankton has gathered. For such places,
discrimination of current rip is important because
fish gather to eat the plankton.
Surface noise
This noise appears near the sea surface on the
screen when the sea is rough or the ship moves
over other ship's wake. It is mainly caused by the air
bubbles in the surface layer. This noise can be
reduced on this equipment by adjusting the clutter
value.
Air bubbles in the water
When the sea is rough or the ship makes a quick
turn or a backward movement, air bubbles can
occur, interrupting the propagation of ultrasonic
waves and breaking pictures. These air bubbles
can occur with the low frequency ultrasonic waves,
and it is difficult to eliminate the effect of air
bubbles completely. If air bubbles frequently occur
in the situation where they are unlikely to occur, the
installation position of the transducer needs to be
reconsidered.
Unstable bottom echo
The bottom echoes can have a saw-tooth
appearance. This occurs in heavy weather because
pitching and rolling changes the direction of the
ultrasonic pulses and the vertical motion of the ship
causes the distance to the bottom to slightly
change.
3. FISH FINDER
School of fish
Plankton
Current rip
Surface noise
Ultrasonic waves blocked
by air bubbles
Unstable echo
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