Circular synthetic aperture sonar images and aspect dependent spectral properties of solid elastic cubes

Abstract

While cubes may not be commonly found underwater, studying their aspect and material-dependent backscattering mechanisms may be useful in leads to understanding the scattering physics of other objects. Two solid cubes, made of either steel and or brass, were insonified in water using a circular synthetic aperture sonar (CSAS) system; Fourier based techniques were used to reconstruct target images. Identifiable backscattering mechanisms include edge-diffraction and elastic responses. Rayleigh waves, a class of surface elastic waves, are excited near a material specific incident angle (Rayleigh angle) that depends on the material of the cube. Those waves may retroreflect from the edges of the cube’s facee edges of the cube’s face in such a way as to enhance backscattering [K. Gipson and P. L. Marston, J. Acoust. Soc. Am. 105, 700-710 (1998)]. The steel cube displayed a strong Rayleigh wave response, easily visible in CSAS images; for the smaller brass cube that said mechanism was relatively subtle. Aspect dependent spectral properties can bewere extracted from the images or directly from the time-domain records. When the cube’s top ridge is slanted vertically, an aspect angle dependent splitting in the low frequency spectrum can bewas observed and associated with edge diffraction. The experimental results were compared to Kirchhoff-Integration based simulations, which did notthat don’t include elastic responses. [Work supported by ONR.]