Multibeam Echo Sounding as a Tool for FIsheries Habitat Studies
Our ability to survey and depict the seafloor radically changed with the introduction of multibeam echosounders and associated data processing techniques that can produce complete-coverage, high-resolution maps of relatively large areas of the seafloor. While first used for military, geophysical, and hydrographic applications, the combination of high spatial coverage along with great bathymetric detail has made multibeam echo-sounding a useful tool for benthic habitat mapping. Like all acoustic systems, both the range (and thus spatial coverage) and the resolution of multibeam echo-sounders scale with frequency, but unfortunately in opposite directions with low-frequencies resulting in long ranges but low resolution and high-frequencies resulting in high-resolution but short ranges. We are thus faced with a fundamental trade-off between spatial coverage and desired resolution and compromises must be sought that address the needs of a particular study. These trade-offs will be examined through the exploration of multibeam echo-sounder data sets collected in the Gulf of Alaska. While the fundamental measurement provided by multibeam echo-sounders (bathymetry) offers, in conjunction with interactive visualization techniques, the possibility of spectacular depictions of the morphology of the seafloor (at many scales), most multibeam echo-sounders also produce a time-series of acoustic backscatter. Changes in acoustic backscatter can be very indicative of changes in seafloor type, but the production acoustic backscatter mosaics is often a difficult and tedious task. New work at the University of New Hampshire is developing approaches for rapidly producing very highquality backscatter mosaics and for quantitatively analyzing the backscatter that comes from multibeam echo-sounders in a way that may lead to the direct identification of seafloor type. The new approaches (called GeoCODER and Angular Domain Analysis -- ADA) first carefully correct the acoustic backscatter collected by a multibeam echo-sounder for a range of radiometric and geometric factors. The corrected backscatter is then geo-referenced to create a high-quality backscatter mosaic. A tool allows the extraction of backscatter vs. angle of incidence from selected regions of the seafloor and these extracted backscatter curves are then compared to a physics-based model of the interaction of sound with the seafloor. The inversion of this model is constrained by well-established physical property interrelationships. The results are geo-referenced plots of acoustic impedance and seafloor roughness for the area surveyed. From the derived acoustic impedance values, other parameters like grain size and porosity can also be estimated. The application of these tools will be demonstrated on Gulf of Alaska multibeam echo-sounder data.
Journal or Conference Title
American Geophysical Union (AGU)
May 21 - May 25, 2007
Fonseca, Luciano E. and Mayer, Larry A., "Multibeam Echo Sounding as a Tool for FIsheries Habitat Studies" (2007). American Geophysical Union (AGU). 404.