R2Sonic 2022, Operation Manual

Page: 54 / 133

Page 54 of 133
Version 3.0 Rev r000
Date 25-08-2010
Part No. 96000001
6.5 Ocean Setting
Figure 49: Ocean Characteristics
Ocean Characteristics include Absorption and Spreading loss, which are the main components of the
Time Variable Gain (TVG) computation, and Sound Velocity (for receive beam steering).
6.5.1 Absorption: 0 – 120dB/km
Absorption is influenced primarily by frequency and the chemical compounds of boric acid B(OH)
3
and magnesium sulphate MgSO
4
.
It is highly recommended that the local absorption value be entered. If this is not known, a good on-
line source is: http://resource.npl.co.uk/acoustics/techguides/seaabsorption/
1
Appendix V provides a table of absorption values based on operating frequency.
6.5.2 Spreading Loss: 0 – 60 dB
Spreading loss is the loss of intensity of a sound wave, due to dispersion of the wave front. It is a
geometrical phenomenon and is independent of frequency. The sound wave propagates in a
spherical manner, the area of the wave front increases as the square of the distance from the
source. Therefore, the sound intensity decreases with the square of the distance from the
projector. Spreading loss is not dependent on frequency.
Spreading loss is not a setting that normally needs to be changed except when surveying in deeper
depths. As spreading loss is not dependent on frequency, the setting is unaffected by a change in
operating frequency. A general default value of 20 – 30 is normally sufficient for most survey
conditions.
For more detailed information on absorption and spreading loss, please refer to Appendix V Basic
Acoustic Theory.
1
Linked with the kind permission of the National Physical Laboratory; Teddington, United Kingdom TW11
0LW; NPL reserves the right to amend, edit or remove the linked web page at any time
.