Furuno FCV-1200L Operator's Manual page 71

Surface noise
When the sea is rough or the ship passes over a wake, surface noise may appear at the top of
the screen. It can be suppressed with the CLUTTER function.
Aerated water
When the sea is rough or the ship makes a quick turn, gaps in the bottom echo on the screen
may appear. This is caused by air bubbles which block propagation of the sound wave.
Generally low frequency ultrasonic waves are interrupted more easily than high ones.
False image
Every time the ultrasonic pulse is transmitted, some radiation escapes on each side of the beam,
called "side lobes." Echoes from side lobes show on the display as false images as below.
False Image
Surface Noise
Surface noise
Ultrasonic wave
blocked by aerated
water
Aerated water
False images
Main
Lobe
Side
Lobe
4-5