Time-frequency variations of the bistatic scattering response of proud and buried elastic shells in shallow water: Implication for mine counter-measure sonar systems.

Abstract

For underwater sonar, time-frequency analysis has been shown to be a relevant tool for the detection and classification of man-made targets (shell). For instance, for traditional monostatic systems, a key energetic feature of spherical shell is the coincidence pattern, or midfrequency enhancement, that is created by the coherent addition of antisymmetric Lamb-waves propagating clockwise and counterclockwise around the shell. The development by the Navy of mine countermeasure sonar systems, using a network of autonomous systems in unmanned vehicles, provides a mean for bistatic measurements, and thus potentially bistatic enhancement for target detection. We have investigated the time-frequency variations of the bistatic scattering response of elastic shells. The influence of the medium parameters as well as the source-receiver configuration will be investigated in free space and then extended to the case of a shallow water waveguide. In particular, we studied the bistatic variations of the coincidence pattern for classification purposes. Finally, the design of a robust space-time-frequency bistatic sonar system will be discussed to enhance the target detection of shells with the use of multiple sensors.