Boundary and elastic effects on backscattering by aluminum cylinders: Experiments and modeling for tilted cylinders viewed at grazing incidence.

Abstract

Close proximity to a boundary or partial exposure to the incident sound modifies the frequency response of elastic objects. Understanding these changes in high-frequency scattering is important for detection and classification of metallic objects located near a boundary. Laboratory experiments, ray theory, synthetic aperture sonar (SAS) imaging, and backscattering spectroscopy were used to explore the effect of partial exposure and close proximity to a planar boundary for a solid aluminum cylinder. To investigate these boundary effects, targets were suspended through the air-water interface of a water tank and insonified from below at grazing incidence. Backscattering measurements were made as the target was lowered through the air-water interface, increasing the portion of the target exposed to sound. Backscattering measurements were also made as a function of aspect angle for varying levels of exposure. Line scan measurements were also obtained to construct SAS images. Some of the structure in the frequency response could be explained using a previously developed ray-based theory [K. Gipson and P. L. Marston, J. Acoust. Soc. Am. 106, 1673–1680 (1999); J. Acoust. Soc. Am. 107, 112–117 (2000)] involving leaky Rayleigh waves. The coupling to rays crossing the flat end of the cylinder strongly depends on the amount of exposure. [Work supported by ONR.]