Multiangle Swath Bathymetry Sidescan quantitative performance analysis

Abstract

The recent extension of Swath Bathymetry Sidescan (SBS) sonar to Multiangle Swath Bathymetry Sidescan (MSBS) has rekindled the argument that existing performance limits (spatial resolution, swath width and accuracy) of seafloor and water column acoustic mapping/imaging may still be improved upon. While beamformed systems continue to improve in achievable angular resolution, these systems also continue to encounter difficulties in achieving high spatial accuracy over a wide swath due to the decreasing spatial resolution (increasing beam footprint) at increasingly shallow grazing angles. Furthermore, the large number of acoustic array elements needed for these systems and the associated instrumentation complexity make this approach both physically large and costly. As an alternative to the beamformed approach, simple and inexpensive interferometric sidescan systems have emerged and have successfully demonstrated the potential for wide swath, high resolution seafloor mapping. However, these single angle-of-arrival measurement systems also encounter accuracy difficulties when faced with multiple concurrent arrivals (e.g. due to nontrivial seafloor features, multipath or water column targets). A multiangle-of-arrivals approach on the other hand accommodates progressively more complex scattering geometries and has been shown, even under conditions of severe multipath, to provide excellent qualitative 3D sidescan imaging results. In this paper, we consider the issue of what can be expected from MSBS systems in terms of quantitative seafloor imaging/mapping performance. The paper begins with a brief introduction to MSBS along with a discussion of its advantages and limitations relative to alternative approaches (multibeam and interferometry). Simulation experiments are then conducted for a flat seafloor with uniformly spaced 1m features (blocks). Three MSBS survey scenarios are considered, over-the side mount in shallow water (sonar 5 m above seafloor), deep tow close to the seafloor (sonar 5 m above seafloor) and over-the-side mount in deep water (sonar 40 m above seafloor). The simulations are used to produce single ping depth profiles that are in turn examined for adherence to survey accuracy requirements prescribed by the Fourth Edition International Hydrographic Organization (IHO) Standards.