The scattering of longitudinal elastic waves at a rough free surface

Abstract

A series of experiments has been performed to measure the conversion coefficient for longitudinal to Rayleigh wave scattering when the former is incident on a rough section of an otherwise plane free surface. The waves are harmonic in time, and the roughness takes the form of linear symmetric grooves. The results are used to test the predictions of perturbation theory, and to find the conditions under which they may be regarded as satisfactory. Apparently the theory breaks down when the surface slope exceeds 25° if the linear extent of the irregularity is small. Outside the range of application of the theory (ie when the extent of the irregularity becomes large) the experimental results are interpreted in terms of energy balance, and approximate conversion coefficients are derived. The conclusion is that Rayleigh waves are attenuated to about a tenth of their energy in travelling a distance of ten wavelengths across regular grooves a wavelength in width and with slopes of 10-25°.