Furuno TZT9 Operator's Manual page 169

7. FISH FINDER
Surface noise
When the sea is rough or the ship moves over a
wake, surface noise can appear at the top of the
screen.
Air bubbles in the water
When the sea is rough or the ship makes a quick
turn, there can be blank spots in the bottom echo
(see the right illustration). These blank spots are
caused by air bubbles which stop the movement
of the sound wave. These air bubbles can occur
with the low frequency ultrasonic waves.
Unstable bottom echo
The bottom echoes can have a saw-tooth ap-
pearance. This occurs in heavy weather be-
cause pitching and rolling changes the direction
of the ultrasonic pulses and the vertical motion of
the ship causes the distance to the bottom to
change.
False echo
When an ultrasonic pulse is transmitted, some energy leaves from each side of the
beam. This energy has the name "sidelobe". Echoes from sidelobes show on the
screen as false images like in the illustration shown below.
7-24
Mainlobe Mainlobe
Sidelobe Sidelobe
Surface noise
Ultrasonic wave
blocked by air bubbles
Unstable bottom echo
False image