High-frequency bistatic scattering experiments using proud and buried targets

Emergent technologies such as Autonomous Underwater Vehicles and bistatic sonars offer immense leverage to the modern applications of underwater acoustics. However, high-frequency scattering processes need to be better understood, especially in multiple-target environments (e.g. dumpsites or highly cluttered seabeds). Using our facilities at the University of Bath, we have conducted scaled experiments in a large water tank containing several sediment trays representative of continental margin seabeds. This work is part of the SITAR project, funded by the European Commission and investigating the risks caused by buried toxic waste. We therefore used scaled targets comparable to the toxic waste containers found in many dumpsites at sea (fluid-filled cylinders, empty cylinders, spheres, boxes, etc.). Multiple targets (up to 4) were imaged with a narrow acoustic beam at different incidence angles. The receiving hydrophones were positioned to correspond to a vertical linear array some distance away from the targets. This set-up is a scaled-down version of the SITAR sea trials performed in September/October 2003 in the Stockholm Archipelago, except that it offers a totally controlled laboratory environment. Systematic and thorough bistatic measurements were carried out as a function of both scattering angle and bistatic angle. The targets were arranged in different ways, and the differences and similarities between the results associated to these configurations are presented. Advanced deconvolution methods and search routines have been used to reconstruct acoustic scattering inside and outside the different objects. After discussing the influence of the interior of the targets, their distribution, and the influence of the enclosing seabed, we present the possible implications for multiple-aspect surveys. This is followed with presentations of the early results from the processing of the tank and sea trials data, and a discussion of their significance.