Calibrating Broadband Multibeam Seabed Backscatter

Ivan Bodra Guimaraes

M.S. Thesis Defense
Earth Sciences - Ocean Mapping

Thursday, Oct. 8, 2020, 2:00pm
Abstract

Standard calibration procedures for multibeam sonars currently only address the fidelity of the bathymetric data. Equivalent effort is needed to ensure that the acquired seabed backscatter strength measurements are referenced to a similarly precise level. This thesis presents an operational method for acquiring reference seabed backscatter data utilizing multiple pre-calibrated split beam echo sounders covering a wide range (45-450 kHz) of frequencies. This is needed to cover the full range of frequencies utilized by multi-sector multibeam systems operating in continental shelf depths.

The method considers both the frequency and the angle of incidence dependence of the backscatter strength of a homogenous seafloor region. By using a mechanically rotated plate, the split beam transducers, once calibrated, are able to collect the absolute angular response curve of the seafloor for any frequency within the bandwidth of interest. This thesis addresses the design, implementation and required processing to deliver the curves of selected areas. Although not part of this research, the next step would be to calibrate the desired multibeam echosounder for backscatter by comparing the results obtained by the systems over the same seafloor area.

The results obtained, reveal one of the most complete pictures of the continuous variation of the seabed backscatter angular response from 45 to 400 kHz. Significantly, this extends well above the 100 kHz level that normally defines the upper end of surface scattering model fidelity. As the chosen sites cover the main range of expected marine sediment types (gravel to mud), general trends are apparent. Notably, frequency dependence decreases as the wavelength shortens. The greatest variability both in the frequency and grazing angle dependence occurs below 200 kHz. This might impact the choice of frequency used in multi-spectral backscatter imaging.

Bio

Ivan Bodra Guimaraes is a Brazilian Navy Officer specialized in Hydrography since 2011. He worked on Brazilian hydrographic researcher ships from 2009 to 2018, operating with single beam and multibeam echosounders and side scan sonar in offshore, coastal, and shallow waters, and rivers.
After years of an operative life, Ivan dedicated his time at UNH to studying the calibration procedures for seabed backscatter with a lot of support from the family in Brazil and his friends in the US that quickly turned into his local family.