Surface And Bottom Returns; Prop Wash; Shadows - Marine Sea Scan Manual

6 Sea Scan® Survey Software Manual
2.2.1.2

Surface and Bottom Returns

Two distinct returns should be noted.
Surface and bottom returns show
up in the waterfall
2.2.1.3

Prop Wash

Prop Wash shows in the waterfall when the churning waves and air bubbles produced by the ship's
propellers are picked up in the transducer's beam.
2.2.1.4

Shadows

Shadows are produced when the sound waves transmitted from the SONAR don't bounce off
anything, so there's no signal to return to the transducer.
First Bottom Return: This is the initial data from the point where
the sound meets the seabed.
First Surface Return: This is the first data from the point where the
meets the water's surface. As in the picture, this is usually a wavy,
sparse, non-solid return, which appears mainly in shallow scanning
situations. In deeper water, the sound beam will not reach the
surface.
The effects of prop wash
The horizontal beam width
This problem occurs mainly when the towfish is in
shallow water. In deeper water, the transducer's
beam should be out of the propeller's wash, and
the picture quality will improve. A shallow-water
solution to this problem is to tow the fish ahead of
the propeller from either the front or sides of the
ship.
© 2013 Marine Sonic Technology, Ltd.