HYDRO 2016 Paper 2A1
Jean-Guy Nistad, Jean-Marie Augustin, Xavier Lurton, Patrick Lajeunesse
Many survey organisations whose primary focus is bathymetry simultaneously collect the co-registered backscatter from multibeam echo sounders, more often than not in an opportunistic manner. While technological innovation and good survey practice have rendered bathymetric data collection more efficient and measurements both precise and accurate, so much cannot be said about backscatter data. Indeed, the sensitivity of the backscatter measurement cycle coupled with limited good survey guidelines leads to real-time backscatter measurements of poor quality requiring intensive post-processing efforts. While accurate geometric and environment compensations cannot be known until time of survey, an accurate system-dependent a priori compensation is possible by properly accounting for the transmit antenna sector pattern(s). Especially with multi-sector multibeam echo sounders however, the transmit sector patterns can strongly modulate the backscatter response. This unwanted effect, inherent to the sensor characteristics, requires an appropriate compensation. A method to minimise this modulating effect has been developed at IFREMER (Brest, France). The method consists in conducting a field calibration survey, modelling or parameterising the transmission sector patterns and injecting the results into the multibeam echo sounder to produce real-time backscatter measurements devoid of system-dependent artefacts. This method was applied successfully on Kongsberg EM302 and EM710 multibeam echo sounders. Results demonstrate a clear improvement in the precision of the backscatter measurements following application of the calibration even when the real-time uncorrected backscatter measurements are of very poor quality. However, the analysis also highlights the difficulty in obtaining consistently accurate results when no prior knowledge of the field calibration survey seabed substrate exists. Such prior knowledge obtained from an auxiliary calibrated echo sounder offers considerable improvement potential.