Laboratory simulation of backscattering by a hemisphere in a boundary wave-guide with an external source

Abstract

While there have been various investigations of backscattering when a source and a target are situated in the same wave-guide, the situation when the source is external to the wave-guide is less well explored. For example, a target buried in a mud layer over sand [K. L. Williams, J. Acoust. Soc. Am. 140, EL504 (2016)], the mud layer may act as a wave-guide. The target signatures when the source is in the water column above the mud are modified. A laboratory experiment has been performed to isolate wave-guide effects on target backscattering, using a simplified wave-guide in water. The wave-guide is defined between a free water surface and a thin partially transparent acrylic plate placed below the water surface. A soft spherical target is half-exposed at the free-surface and source-receiver is below the plate. A geometric method-of-images model gives the timing arrivals of wave-guide echoes from an infinite number of back-scattering paths that undergo multiple reflections in the wave-guide and gives the frequency response of the wave-guide by summation of the contributions. This model gives a closed-form result in the far-field limit, and is compared to the experimental results in the time and frequency domains. [Work supported by ONR.]