Bottom Echo - Simrad ES70 - REFERENCE MANUAL REV A Reference Manual

The situation is slightly different when observing the echoes from individual fish. The
transmitted wave undergoes square-law spreading when travelling from the surface and
down to the fish. The swim bladder of the fish scatters a small fraction of the arriving
energy in all directions. Travelling from the fish and back towards the surface the
scattered wave undergoes another square-law spreading. The combined effect is referred
to as quad-law or 40 log TVG spreading.
In the echo sounder's
Bottom Gain, while 40 log TVG spreading is called Fish Gain.
Figure 6 Wave propagation from a fish
The dB (decibel) unit has long traditions in underwater acoustics and other fields in
physics. It is a logarithmic measure for the ratio between two quantities.
Related topics
• About TVG gain on page 205
• Echogram on page 158
• Environment on page 149

Bottom echo

A hard flat bottom reflects the transmitted signal as if it were a mirror. The transmitted
pulse hits the illuminated bottom area at nearly the same instant, and the echo from
different parts of this area arrive back at the surface also at nearly the same instant.
338106/A
dialog 20 log TVG spreading is called School Gain and
Echogram
Propagation losses due to absorption are much
higher in sea water than in fresh water. Absorption
also increases with frequency. At 38 kHz the
absorption is 0.5 dB/km in fresh water and 10
dB/km in sea water. At 200 kHz the absorption
is 10 dB/km in fresh water and 50 dB/km in salt
water. The echo sounder must know which water
type is present in order to compensate for these
losses correctly.
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