Reson SeaBat 7125 SV2, Operator'S Manual

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ACTIVE SONAR USAGE
SeaBat 7125 SV2 Operator‟s Manual
Page 112 July 25, 2011
Version 3
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, see
section 5.7.3.2 Change the Spreading
Loss Coefficient (Spreading) .
6.6.3 Sound Velocity
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 profile provided.
If the local sound velocity is not known, RESON recommends a value of 1480 to
1500m/s for open sea areas and 1425m/s for fresh water. For instructions on how to
adjust this setting, see
section 5.7.3.3 Set the Sound Velocity
.
Figure 49 provides a general reference for typical surface sound velocity at different
temperatures and salinity.
Sound velocity (at surface)
1400
1420
1440
1460
1480
1500
1520
1540
1560
0 5 10 15 20 25 30 35
Temperature (degrees C)
V
e
lo
c
it
y
(
m
/s
e
c
)
0 ppt
5 ppt
10 ppt
15 ppt
20 ppt
25 ppt
30 ppt
35 ppt
Figure 49: Typical Sound Velocity Values