Simrad SL 30, Manual

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974-25007001/2.0 SL 30/35 Hull Sonar Operators Manual
Page 4.16 Menu and Software Description
Kongsberg Simrad Mesotech Ltd.
Port Coquitlam, BC - Canada
4.19.4 Shadows
Most targets will block the transmission of sound either by reflecting it or absorbing
it. This will leave a shadowed area beyond the target that is not ensonified and
therefore will not generate any echoes. This is very similar to the shadow formed
when an object is illuminated with a single light source. The shadow behind a target
can often yield more information about the target than the reflections from the target
itself. The shadow will often reveal the shape of the target, but you must remember
that the shape will usually be distorted according to the position of the sonar head
relative to the target and the bottom, and according to the slope of the bottom. It is
often possible to estimate the height of a bottom target based on the length of the
shadow and the known height of the sonar ahead.
4.19.5 Noise and Interference
It is usually easy to recognize interference from other acoustic sources such as echo
sounders, pingers or other sonars. These sources all produce pulses at regularly
timed intervals and will therefore tend to create a regular or symmetrical pattern of
'blips' on the screen. Electrical interference usually occurs at a much higher periodic
rate, and will tend to appear as spiral patterns on the screen. Mechanical noise
sources such as propellers, hydraulic pumps, and thrusters, are usually more
directional and tend to show up only when the sonar head is pointing directly
towards them.
4.19.6 Masking Effects
Sonar signals are easily blocked by air or gas bubbles in the water or on the
transducer face. As an example, the aeration present in the wake of a vessel will
often last for ten to twenty minutes and effectively mask out any sonar returns on the
far side. The aeration blocks the outgoing pulses as well as any returns. Another
source of gas or air bubbles can be found when a sea bottom containing
decomposing organic matter is disturbed by dredging or plowing.