Page 27 - Electronic Navigation Cyber Book

a) Double Echoes

Echoes received after reflection from the seabed, but which the hull or
the sea surface back to the bottom and then reflects thence to the
transducer. They produce a second weaker echo at approximately
double the correct depth. It will fade out if sensitivity is reduced (may be
received up to several hundred meters).

b) Multiple Echoes
Echoes received after being reflected several times between the seabed
and the surface or the ship's bottom before the energy is lost. It causes
equally spaced echoes on the trace. Reduce sensitivity to fade out.
Switch on to the first phase and then phase deeper to locate the first
echo.

c) Variable Echoes

These are varying reflecting surfaces on the seabed. In general hard
sand, coral, chalk, and rock are good reflectors and thick mud is a poor
reflector. Stepped formation of rock results in side Echoes from an object
not immediately below the vessel but whose slant depth is less than the
depth of water.

d) Electrical faults or man-made noises.
Other False Echoes

These do not normally obscure the bottom echo and may be caused by

.1 Shoals of fish
.2 Layers of water of differing sounding velocities (salinity etc.)

.3 The deep scattering layer, which is a layer or set of layers, in the
ocean, believed to consist of plankton and which attenuate, scatter and
reflect sound pulses. They lie between about 300-450 meters below the
surface by day, and near the surface between sunset and sunrise (by
day, it is more pronounced when the sky is clear, than when overcast).
.4 Kelp or weed.

.5 Turbulence from the interaction of tidal streams or eddies with
solid particles in suspension.

© 2018 Digital Galaxy Index 27