Page 26 - Electronic Navigation Cyber Book

sonar system, using sonic waves below 50 kHz, the noise level can
usually be ignored, as it is very small compared with the sea noise.
SEA NOISE

They are of two main kinds, the first are interfering wave action, and may
be thought of as background noise. Sources are fish, other ships, and
noise from one's own ship particularly in bad weather and close to land.
For the most purpose, the amplitude of disturbances at any instances is
unpredictable and taken, as a whole may be considered random. For this
reason, the designer must make sure that the signal is
always recognizable above the noise level.
The second is noise produced by the interaction of the sea and the sonar
system. This is generally called "reverberation noise" and when
transmitted into the water, all the small reflectors in the water such as
bubbles, marine life, and mud and sand particles immediately affect it.

This multiple reflectors produces a return signal (echo), which is
theoretically continuous since they exist at all depths. However, the
intensity of the transmitted pulse is reduced as it moves away from the
transducer and the intensity of the return signal also reduces in
accordance with the same law. The result is that after the end of
transmission, the reverberation signal decreases with time according to
an inverse square law. Its effect can be considerably reduced by the use
of time variable gain or "initial suppression". This circuit is set to reduce
the gain of the receiver to a very low level immediately following
transmission, but then allows the gain of reverberation noise after the
same has fallen below that of background noise.

Interpretation of Sounding

False Bottom Echoes. Second Trace Echoes
Echoes, which are received at a properly adjusted sounder, until after the
stylus has completed one or more passes across the paper and the next
pulse have been, transmitted cause false readings. Example of one
revolution represents 1600 meters, and an indicated depth of 50 meters
could be sounding of 50 or 1650 or even 3250 meters. The correct depth
can be ascertained if the transmission circuit can be switched off with the
stylus still moving. After switching off, on the switch and then count the
number of times the stylus crosses the paper before the echo re-appears.
Reflection echoes

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