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ACTIVE SONAR USAGE
SeaBat T-Series Subsea Operator's Manual Page 46 January 31, 2019
Version 7
from rivers etc. may influence sound propagation, but these mechanisms are difficult to measure
and model, as they vary with time and only appear in a limited number of areas. However, as
absorption and geometrical spreading exist everywhere in the underwater environment, they are
considered to be the most important loss mechanisms.
In the SeaBat system it is possible to compensate for loss caused by absorption and geometrical
spreading. In addition, the SeaBat system allows for changes in the speed of sound, a parameter
which is important in the beamforming process.
For instructions on how to adjust this setting, refer to the RESON Sonar UI User Manual ( see
Appendix E Reference Documentation ).
6.4.1 Loss of Sound by Absorption
As sound moves through the water, some of the sound energy is converted into to heat by
absorption. The absorption loss depends on the salinity and temperature of the water as well as
the depth. Finally, it also depends on the acoustic frequency.
Figure 21 shows the absorption loss
for three different salinities and three different temperatures at an average depth of 20 meters
10
.
If the exact absorption loss value is not known, these are the recommended approximate values:

400kHz operation:
o
110dB/km for salt water
o
70dB/km for fresh water

200kHz operation:
o
50dB/km for salt water
o
20dB/km for fresh water
The absorption loss value is part of the formula that the SeaBat system uses to calculate the
amount of gain to apply to the sonar return signal (see
section 6.2.4 above ).
Ideally, the purpose of the TVG is to ensure the receive gain increases with a rate that
compensates for the continuously decaying intensity of the received sound field - a decay which
is caused by geometrical spreading and sound absorption. Meanwhile, the TVG does not need to
perform an exact compensation. In practice, the purpose of the TVG is only to ensure that the
received signals continuously are within the dynamic range of the receiver. Therefore, the applied
settings for the absorption and spreading only need to be approximate.
For instructions on how to adjust this setting, refer to the RESON Sonar UI User Manual ( see
Appendix E Reference Documentation ).
10 Note, that absorption and attenuation of sound in the water column caused by microbubbles and species such
as, e.g., plankton and fish is not included in this consideration.