Boundary effects on the scattering of sound by elastic objects under water

Detection and classification of unexploded ordinance (UXO) in shallow-water environments is a problem of increasing importance in underwater acoustics. UXO are often found resting on the sediment-water boundary (proud). Classification of UXO in the absence of the sediment-water boundary (in the free field) is more simple; as a result, it is desirable to determine the effects of close proximity to a flat reflecting boundary on free-field target responses that can be used in classification. Close proximity to a boundary modifies the frequency response of elastic objects such as UXOs. Understanding these effects is an important aspect to detection and classification of man-made objects located near the water sediment boundary at the bottom of the ocean. Water tank experiments, analytical and numerical modeling, imaging techniques such as supersonic acoustic holography and synthetic aperture sonar (SAS), and backscattering spectroscopy were used to understand the effect of close proximity to a planar boundary on the scattering of sound by elastic objects. To simulate some aspects of scattering when resting on the seabed, targets were suspended just below the air-water interface of a water tank and insonified from below at grazing incidence. Monostatic measurements were made as a function of rotation angle and exposure through the free surface. Bistatic line scan measurements were also obtained to construct SAS and holographic images. Experiments and ray modeling demonstrate that the presence of the boundary increases the number of ways for sound to couple into elastic modes of an object. The boundary also increases the number of directions by which scattered sound can reach the receiver. Several aspects of the scattering by an object near a flat reflecting boundary can be modeled as a superposition of bistatic and monostatic free-field object scattering responses. Several significant features in the proud scattering by solid cylinders could be understood using ray models that involve leaky Rayleigh waves. Coupling to helical waves was especially enhanced by the boundary. Bistatic specular features were also detected for some of the targets