Kongsberg Simrad SN90, Installation Manual

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Reverberation
The energy reflected from other objects than the one we wish to observe is called
reverberation.
Reverberation is generated when the echoes from your sonar transmission are reflected
not only by fish and valid targets in the water, but also from the seabed and from the
sea surface.
We have two kinds of reverberation:
•
Volume reverberation
: This is caused by minute plankton and air-bubbles in the water.
This reverberation often has daily cycles in intensity.
•
Surface and bottom reverberation:
The surface reverberation is dependent on the
weather condition, while the bottom reverberation depends on the roughness of the
bottom.
Reverberation is often a limiting factor for sonars, especially if you are searching for
smaller targets.
The echoes from the bottom will depend on the bottom properties; how uneven it is and
what kind of bottom you have under your vessel, (rock, mud, sand). Your current tilt
setting will also have an effect on the reverberation. The echo from the sea surface will
vary with the current sea state and your current tilt setting.
(CDCD010202_301_001)
The illustration demonstrates how the bottom and the sea surface return echoes back to
your sonar transducer. In situation (A) you will be able to identify the upper school of
fish, but the lower school will be hidden in the shadow of the seabed rock formation. In
situation (B) you will see several weak echoes from the waves and from the rock on the
bottom, but you will still be able to identify the school of fish.
The reverberation is caused by the transmission pulse itself and can only be reduced by
shortening the pulse duration or using a narrower beam width. The bottom reverberation
usually makes it difficult to investigate targets located close to the seabed.
Some means to reduce acoustic noise
Careful planning of the SN90 installation may reduce the acoustic noise.
Unfortunately, it is impossible to simply provide a number of specific procedures to
reduce the noise.
Preparations