Bistatic Scattering from Buried Targets in Shallow Water

The development of new concurrent detection and classification concepts has the potential of dramatically increasing the coverage rate of shallow water mine countermeasures, in particular for buried targets. The GOATS'98 experiment had as one of its main objectives to explore the possibility of basing such a concept on the measurement of classifying features of the 3-D acoustic scattering by such objects, using bistatic configurations made possible by the emerging autonomous underwater vehicle technology. Using an extensive suite of fixed and mobile monoand bistatic acoustic arrays, the experiment produced an extraordinarily rich data set, which as described elsewhere in this volume is being extensively used for a variety of MCM-related issues, including monoand bistatic synthetic aperture processing and subcritical seabed penetration physics. The analysis has revealed a number of unexpected acoustic phenomena in regard to the subcritical excitation of elastic waves in buried shells, and their radiation back into the water column. For example, a strong, delayed maximum in the bistatic scattering from a flush-buried spherical shell is observed at frequencies an order of magnitude higher than predicted for simple point-targets at the same burial depth. The OASES-3D target modelling framework is being used to investigate this phenomenon, and this paper describes some preliminary results, which demonstrate that full wave theory modelling is necessary to adequately describe scattering of evanescent waves from seabed targets. Thus, the traditional plane-wave, raytracing approach to the propagation to and from the target is demonstrated to be totally inadequate, and must be replaced by a scattering model incorporating a wide spatial spectrum of scattered propagation angles. The analysis also shows that although this scattering mechanism also exists for monostatic geometries, its bistatic enhancement is much stronger than the 'geometric', evanescent scattering. In addition, being due to the excitation of structural waves, the presence of this scattered component can potentially be applied for classification.