Technical Note 3: Absorption coefficient

What are the changes?

Users can now change the attenuation coefficient that should be applied in post-processing in a new "Oceanography" section of the "Adjust" tab of the Processing Parameters dialog. Leaving it at the default value of zero will result in the processing algorithms using the value that is in the input sonar files, i.e. what is used in real-time.

If the user changes the absorption coefficient in the Processing Parameters dialog box, this will trigger a refresh of the processing flags.   If different data needed to be treated with a different absorption value, for example when day to day variations in oceanographic conditions significantly influence the absorption value, then the user will need to create a new settings configuration and then associate it with the appropriate files.

Who does this impact?

This type of correction is often only done in the event that the absorption correction was incorrectly configured during acquisition.  If appropriate absorption coefficients have been applied in real-time, there is no need to pursue further correction in post-processing.  Having an incorrect absorption coefficient can result in an angular and dependent bias in the computed backscatter.  Angular dependent biases will largely be filtered during the AVG stage for users who primarily use FMGT to create mosaics, however, mapping products will still suffer from a depth dependant backscatter bias.  Users who are interested primarily in ARA products should endeavour to have the attenuation coefficient as accurately modeled as possible since the angular dependent biases will directly impact on ARA fidelity.

For guidance in computing an appropriate absorption coefficient, you can find an absorption coefficient calculator online at http://resource.npl.co.uk/acoustics/techguides/seaabsorption/.  Input requirements are the sonar's main operating frequency and oceanographic properties such as the mean water temperature, salinity and depth.  There are different models available with the online calculator, you should strive to use one with limits of validity that match the range of conditions you are working in.  Note that the online calculator can be saved offline for use at sea when an internet connection may not be available.

This new feature is currently implemented for Reson, Kongsberg and R2Sonic in the GSF pipeline and Kongsberg in the ALL pipeline.  Absorption coefficient corrections for Reson sonar models other than 7125 and 7111 are implemented but these can produce biased results since the real-time Time-Varying-Gain (TVG) applied by these models is unknown and FMGT cannot remove the real-time compensation for absorption loss with any degree of fidelity.  We are currently working with the manufacturer to improve understanding of newer 7K models such as the T20P, 7150, 7160 and 7101 and we expect to be able to support improved corrections for these models in the near future.  TVG corrections for older models such as the 8125, 8101, 8111, 8160 and 8150 will not be improvable due to lack of documentation on the TVG functions for these systems.  For these reasons, these models of sonars should not be used for ARA type analyses.