Technical Note: Correcting backscatter for seafloor 3D incidence

What are the changes?

It was recently discovered that the backscatter corrections being done in FM Geocoder Toolbox (FMGT) were not correcting for seafloor 3D incidence angle, even when corrected bathymetry was included in the processing routine.  We have added a fix to use a Reference Grid (in Fledermaus SD format) to determine the topographic slope during processing and mosaic creation.  To clear up any potential confusion, it is important to point out that the corrected bathymetry data that has been necessary to import data into FMGT have only ever been used for georeferencing seafloor imagery.  The original Geocoder software from UNH provided the ability to compute the 3D incidence angle with a Reference Grid, however, this was only implemented for sidescan sonar imagery and not for multibeam imagery.  We have followed the same strategy for multibeam imagery now and the Reference Grid is now used to estimate each beam's 3D incidence angle with the seafloor.  This more accurate method of calculating the incidence angle directly improves the accuracy of the ensonified area correction, which is a function of the projected beam footprint size on the seafloor.

While implementing the Reference Grid calculations for multibeam imagery, a small number of errors were noted in the original UNH Geocoder approach.  These were corrected for the multibeam imagery correction algorithms, however, the corrections have not been ported to the sidescan signal level corrections yet.

Who does this impact?

This type of correction is particularly important for ARA users since the input signal to ARA now represents the seafloor's true backscatter value with much higher fidelity than before.  For FMGT users who focus primarily on creating seafloor mosaics only, the new algorithm will dampen topographic effects, such as higher signal levels on inward facing slopes and lower signal levels on outward facing slopes.  As part of our ongoing efforts to improve the underlying Geocoder algorithms from UNH, we have identified the need to propagate the angle and terrain corrections into the AVG stage of processing.  Once this is complete, mosaic imagery will have a much higher fidelity in representing seafloor backscatter.  Look forward to this in a future release.

The terrain correction can be applied for Kongsberg, Reson and R2Sonic multibeam systems that have the sonar beamwidth and pulse length parameters correctly recorded in their data stream.  The corrections are done for both .ALL input format (Kongsberg only) and for .GSF (Reson, R2Sonic and Kongsberg).