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Sub-critical isonifilcation of buried elastic targets

The detection and classification of buried elastic targets presents a significant challenge to the shallow water mine countermeasures (MCM) community. In the shallow water environment, sub-critical insonification of the seabed is required for an acceptable sonar search rate. Previous experimental and modeling studies have shown that in the case of sub-critical insonification significant evanescent field components can be converted into vertically propagating field components making the plane-wave, ray-tracing approach to the wave propagation inadequate. Furthermore, there has also been empirical evidence that the modal response of the target may be stronger than the direct backscattered return due to the evanescent nature of the sub-bottom field. The GOATS98 experiment had as one of its main objectives to explore the possibility of classifying features of the 3-D acoustics scattering by buried and proud objects using bistatic configurations such as autonomous underwater vehicle (AUV) and horizontal line array (HLA). An unexpected physical phenomenon of a strong delayed flexural return of a flush buried spherical shell was observed in the AUV and the HLA data at frequencies much higher than predicted. A scenario similar to GOATS98 experiment has also been modeled using OASES-3D target modeling framework to further investigate scattering mechanisms of the flush buried spherical shell under evanescent insonification as well as to validate the modeling capability of the package. This paper presents a study of the interaction between the evanescent field and an elastic target, using simulated results with OASES-3D as well as experimental results from the GOATS'98 bistatic detection and imaging experiment.