Acoustic scattering of partially exposed spheres and cylinders at a flat interface

Aaron Michael Gunderson

Scattering by partially exposed objects breaking through an interface is profoundly different from scattering by the same objects in a free field environment. The presence of the interface both adds additional scattering paths, and can strongly modify or remove preexisting paths, depending upon the level of exposure and grazing angle. Acoustic scattering by partially exposed spheres and cylinders through an interface is considered in the time and frequency domains. Scattering from a handball (a rubber, hollow spherical shell) is also considered, and results are compared to those from solid spheres. Experimentally acquired results are compared to several theoretic models, including the Kirchhoff approximation, partial wave series solutions, ray theory, and finite element simulations. The Kirchhoff approximation has previously been applied to backscattering from a rigid cylinder at a free surface [K. Baik and P.L. Marston, IEEE Journal of Oceanic Engineering 33, 386-396 (2008)], and the analogous process for a rigid sphere is developed here. Scattering path contributions are identified based on timing models and frequency content, and target elastic resonances and interference phenomena are discussed. Two different interfaces are considered in experiments and models: an air-water interface (free surface) and a sand-water interface. Comparisons and distinctions are made between the results at the two interfaces, which are shown to become similar in the low grazing angle limit. Smooth, continuous transitions in scattering amplitude are shown to correlate with scattering path coupling points becoming exposed as the target is lowered through the interface. Scattering in the low target exposure limit is shown to be dominated by Franz wave contributions (surface-guided, nonelastic waves). This contrasts with high exposure and free field scattering signatures from spheres and cylinders, in which the Franz contribution is highly diminished and is not a significant contributor. Bistatic scattering from free field spheres and cylinders is also considered, in the context of understanding the bistatic scattering paths from these targets at partial exposure through an interface. Results from this work have direct applications to understanding the scattering from partially exposed objects resting on or buried within a seafloor environment. [Work supported by the Office of Naval Research.]

Acoustic scattering of partially exposed spheres and cylinders at a flat interface

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