ASA 128th Meeting - Austin, Texas - 1994 Nov 28 .. Dec 02

4pPAb10. Retroreflective backscattering of ultrasound due to Rayleigh waves on an elastic solid rectangular parallelepiped.

Karen Gipson

Philip L. Marston

Dept. of Phys., Washington State Univ., Pullman, WA 99164-2814

High-frequency (1.5-MHz) sonar experiments were performed on a solid stainless-steel rectangular parallelepiped block in water in order to determine the most likely mechanism for producing a strongly backscattered signal. When the angle of incidence is near the Rayleigh angle of the steel, a leaky Rayleigh wave is launched on the flat surface of the block. When this wave reaches the edge of the block, part of it is reflected and propagates to the adjacent edge forming the corner, where it is reflected again. The net result is that the wave vector of the Rayleigh wave is reversed, and the resulting leaky radiation gives a pronounced enhancement of the backscattering. For a randomly oriented block, this effect is more likely to be observed than specular reflection, since the latter requires that two of the Euler angles of the surface normal lie in a small range, while for the Rayleigh mechanism only one angle is narrowly constrained. The observed peak amplitude of the retroreflected backscattered signal agrees well with an approximate theoretical model [Marston et al., J. Acoust. Soc. Am. 94, 1861 (A) (1993)] modified for the present case of Rayleigh waves. [Work supported by ONR.]