Furuno GP-7000F Operator's Manual page 114

10. VIDEO SOUNDER OPERATION
Surface noise
When the sea is rough or the ship passes over a wake, surface noise may
appear at the top of the screen.
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 echo
Every time 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 echo
10-28
Surface noise
Surface noise
Ultrasonic wave
blocked by
aerated water
Aerated water
Main
Lobe
Side
Lobe