SeaBat T-Series Subsea Operator's Manual
Page 48
January 31, 2019
Version 7
6.4.2 Loss of Sound by Spreading
Spreading of sound through water may be the greatest contributor to the loss of sound. As sound
moves away from its source in all directions, the acoustic pressures will decrease with range.
In deep water, where the acoustic propagation is not bounded by the sea surface or the seafloor,
the spreading loss is spherical. In this case the pressure decrease is proportional to 1 divided by
the range R from the sonar to a given object or surface. Thus, the acoustic pressure decays as
1/R. As the sound is scattered back to the sonar the backscattered field also decreases as 1/R.
Consequently, for a two-travel path the acoustic pressure in a free field decays as 1/R
2
.
In shallow waters, on the other hand, the sound propagation is bounded by the sea surface and
the seafloor, and the spreading loss is cylindrical. In cylindrical spreading the sound pressure only
decreases as 1/R
½
, and the two-way travel path decays as 1/R.
In practical situations the transmission loss will lie between 20 and 40dB; in other words, the
spreading loss will neither be perfectly spherical nor perfectly cylindrical. Finally, if the sonar
operates very close to the seabed or very close to objects in general, near-field effects may yield
a transmission loss less than 20dB.
The spreading 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
). If the exact
spreading loss value is not known, an initial value of 30dB is recommended.
For instructions on how to adjust this setting, refer to the RESON Sonar UI User Manual (
see
Appendix E Reference Documentation
).
6.4.3 Sound Velocity
For instructions on how to adjust this setting, refer to the RESON Sonar UI User Manual (
see
Appendix E Reference Documentation
).
For installation instructions, see
appendix B.4.4 Pin Designations for Host, AUX,
.
6.4.3.1
Sound Velocity and Performance
Online sound velocity readings are crucial in the beamforming process to ensure that the beams
are steered in the correct directions. Ensure that a real-time sound velocity probe (SVP) is set up
to deliver the sound velocity at the sonar head.
Sound velocity varies with pressure, temperature, and salinity. An increase in these three levels
increases the sound velocity. The sonar data presented in the SeaBat system is dependent on
the sound velocity provided.
CAUTION
An online sound velocity probe must be used for providing up-to-date velocities to ensure
optimum performance. A manual velocity may be entered instead but care should be taken to
ensure the value entered is representative of the real velocity.
If the local sound velocity is not known, Teledyne RESON recommends a value of 1480 to 1500m/s
for open sea areas and 1425m/s for fresh water.
provides a general reference for typical
surface sound velocity at different temperatures and salinity.
Summary of Contents for SeaBat T Series
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