Evaluation of a physics-based inversion of multibeam echosounder data for sediment properties using data collected in Sequim Bay, WA.

Most approaches to sonar-based remote sensing of the seafloor rely on inversion techniques that are disconnected from the physics of seafloor scattering. These approaches use features such as the scattering level and bathymetric variation of the seafloor to identify areas that have similar empirical characteristics and then use ground-truth measurements to associate those characteristics with a specific seafloor type. In an effort to both eliminate the need for ground-truth measurements and extend the inversion results beyond seafloor type, an inversion algorithm has been developed which incorporates the physics of high-frequency seafloor scattering and propagation. This technique uses the time-series of the backscattered return along each beam and a sonar-equation model is used to generate echo intensity time series including scattering by both seafloor roughness and volume heterogeneity. Model-data fits provide estimates of acoustic attenuation, volume scattering strength, and interface scattering strength. Next, physics-based models are fit to the interface and volume scattering strengths, and finally, regression relations are used to provide a set of geoacoustic parameters. To evaluate the performance of the algorithm for a range of sediment types, sonar and ground-truth data were collected at 10 sites in Sequim Bay, a shallow, protected bay along the northern coast of the Olympic peninsula in Washington State. The sites were chosen to include sediments with varying amounts of clay, silt, sand, and gravel. At each site, diver cores were collected to measure the grain-size distribution, bulk density, and bulk porosity and a diver-deployed set of transmitters and receivers were inserted into the sediment to measure in-situ sound speed and attenuation. A bottom-mounted frame with a conductivity probe and laser-line scanner was also deployed to measure volume heterogeneity and seafloor roughness. Details of the inversion and it’s performance for the different sediments will be discussed.