Beam excitation of doubly leaky Rayleigh wave on layered viscoelastic half‐space

Abstract

The problem of nonspecular reflection of an ultrasonic beam incident at or near the Rayleigh angle onto a thin surface layer overlying a homogeneous viscoelastic half‐space is investigated. An approximate solution to this problem was recently published by Nayfeh and Chimenti [J. Acoust. Soc. Am. 75 1360–1368 (1984)], who presented detailed computational results for a low‐velocity surface layer (loaded half‐space). We concentrate on the case of a high‐velocity surface layer (stiffened half‐space), where propagation is characterized by a leaky Rayleigh wave, which, however, will become doubly leaky (energy leaking into the water as well as into the lower half‐space) when its phase velocity exceeds the shear velocity of the half‐space. For an incident Gaussian beam, we present exact solutions for the reflected field using a recently developed numerical model [H. Schmidt and F. B. Jensen, J. Acoust. Soc. Am. 77, 813–825 (1985)] providing full wave field solutions for incident beams of arbitrary width, reflected off a multilayered viscoelastic structure. It is shown that the surface‐layer thickness can be determined from the reflectivity pattern for layer thicknesses up to 1.5 times the shear wavelength in the surface layer.