Seafloor Acoustic Backscatter Measurements With Multibeam Echosounders

Abstract

Abstract This paper is a tutorial on the fundamentals of seafloor acoustic backscatter measurements made with multibeam echosounders. Such measurements are used to infer seafloor geoacoustic properties, including: acoustic impedance contrast at the water/sediment interface, mean sediment grain size, spectral characteristics of the roughness of the water/sediment interface, contributions from subbottom layers or inhomogeneities in the sediment volume. Introduction From the 1960s to the mid-1990s, acoustic backscatter imaging of the seafloor was primarily done with towed sidescan sonar systems (e.g. Belderson et al., 1972; Mazel, 1985; Fish and Carr, 1990 & 2001), some of which had bathymetric capabilities (e.g. Blondel and Murton, 1997). Meanwhile, the hull-mounted multibeam echosounder technology has gradually improved since the early 1970s from a purely swath bathymetric survey function to include backscatter imagery (de Moustier, 1988; Lurton, 2002). Today, most modern swath mapping sonar systems used in seafloor surveying collect swath bathymetry and concurrent seafloor acoustic backscatter data. When processed together, these data can be used for remote sensing of seafloor characteristics, with applications in geotechnical surveys (e.g. offshore drilling and mining, dredging and disposal, subsea cable and pipeline routes), in resource management (e.g. fisheries habitat assessment, coastal zone monitoring), in navigation and homeland security surveys (e.g. harbor entrances, waterways, Q routes), and in basic and applied research (e.g. sonar modeling, seafloor acoustic processes, mine warfare). In this paper we shall focus on the particular application of multibeam swath mapping sonars to remote sensing of seafloor geoacoustic properties. The development is tutorial in nature and we begin with a short review of the relevant seafloor acoustic backscatter processes. Then, we consider how acoustic backscatter is measured with a multibeam echosounder, and what processing steps are required before inferences may be made about the underlying geoacoustic parameters. Finally, we review various inference and interpretation methods and conclude with cautionary notes on comparison of results from different sonar systems. Seafloor acoustic backscatter processes Seafloor remote sensing with multibeam echosounders relies on the acoustic backscatter processes that take place at and below the seafloor. These processes depend on the type and distribution of sediments in the survey area and on the corresponding geoacoustic parameters. Among these parameters, we focus on the acoustic impedance contrast at the interface, the roughness of the seafloor relative to the acoustic wavelength, attenuation and scattering within the sediment volume, and the angular dependence of the bottom backscatter process.