Boundary enhancement of a solid cylinder backscattering response: Experiments and modeling.

Abstract

The presence of a reflecting boundary modifies the free‐field backscattering response of an object. Boundary reflections increase the number of available paths for scattered sound to return to a source‐receiver. In the present work, high‐frequency tank experiments on solid aluminum cylinders demonstrate a strong boundary related backscattering response that is present for a broad range of aspect angles. To study the effects of close proximity to a flat reflecting boundary, cylinders were suspended in water near the tank’s air‐water interface. Targets were insonified from below at grazing incidence. Monostatic measurements were made as the cylinders were rotated in a plane parallel to the boundary. Some aspects of this response could be understood by modeling the region of water bounded by the cylinder top and the air‐water interface as a quasi‐2‐D waveguide. Sound incident on this region of decreasing water thickness can be reflected as from a seamount. The backscattered amplitude is strongly affected by the threshold frequency for propagation in the gap between the cylinder and the adjacent flat surface. Experiments show that the behavior of this response, as a function of target depth and aspect angle, is in qualitative agreement with this waveguide model. [Work supported by ONR.]