Remote Acoustic Characterization of Seafloor Properties from an AUV

Coastal settings are complicated by seabed sediment and bedform variability that often exist at spatial scales on the order of 1-10s m, well below either numerical model grid size or satellite-based observational scales. This pronounced spatiotemporal heterogeneity is a critical first-order factor in determining sediment transport and seabed geoacoustic properties. Being able to map this heterogeneity using remote acoustic methods is a critical component of predictive sea floor modeling efforts. We present recent results of a small autonomous underwater vehicle (AUV) equipped with an advanced phase measuring bathymetric sonar system (PMBS) for shallow seafloor mapping. The development of multibeam echo-sounders to look at the angular dependence of the acoustic response of the seafloor has opened up new possibilities with respect to seafloor characterization. The angular response of the echo holds important information about seafloor roughness and volume reverberation, as well as acoustic impedance. In this work, we employ a physics-based model known as ARA (Angular Response Analysis) that uses the angular dependence of backscatter for the remote prediction of seafloor properties. The ARA technique corrects sonar backscatter for radiometric and geometric factors, parameterizes the corrected angular response curve, and applies a constrained inversion to solve for seafloor properties such as roughness and grain size. The AUV configuration in this study has been designed specifically with coastal seafloor mapping studies in mind. To this end the AUV is configured with a 500 kHz phase measuring bathymetric sonar (PMBS) for collocated micro-bathymetry and backscatter intensity. One of the key features of the PMBS is that it collects simultaneous digital side-scan data with the bathymetry, which is used in mapping and classifying the seafloor bottom type via implementation of ARA. We present examples of efforts to implement automated classification systems to the AUV gathered bathymetric sonar data. Field data collection was conducted in July 2009 in Nantucket Sound and in the vicinity of the Martha’s Vineyard Coastal Observatory (MVCO). The AUV was operated at depths between 8-20 m deep, flying survey patterns at a constant 6 m altitude and accumulating a total of \u3e50 km of trackline survey over 3 days of missions. The surveys were run with parallel lines up to 1.9 km long spaced at 10 to 30 m. The MVCO location possesses a very broad range of sediment textural composition ranging from well-sorted fine sand to poorly sorted gravelly sand. Having collected data in the same area as part of the ONR Mine Burial Program allows us to leverage the previous data sets of backscatter, bathymetry, and ground-truthing sediment samples for use in comparing our approach using Angular Response Analysis for sediment inversion. The July surveys off of Martha’s Vineyard demonstrated the ability of the AUV to conduct high-resolution detailed seabed mapping in a shallow energetic coast