Axially focused (glory) scattering due to surface waves generated on tungsten carbide spheres

Abstract

When a short tone burst is incident on a tungsten carbide sphere in water, the backscattering response shows a decaying pulse train [L. R. Dragonette, S. K. Numrich, and L. J. Frank, J. Acoust. Soc. Am. 69, 1186–1189 (1981)] which can be attributed to a specular reflection and repeated circumnavigations of waves. We have experimentally investigated the angular dependence of Rayleigh wave contributions as the receiver is moved away from the backscattering axis. It was found that this angular dependence can be approximately modeled by J0(βγ), where β depends on the frequency of the sinusoidal tone burst used (or equivalently the ka of the sphere) and γ measures the angle relative to the backscattering axis. This form of angular dependence may be attributed to a weak axial focusing along the backward direction. We have used scattering theory along with an asymptotic expansion of the Legendre polynomials to obtain a prediction of how β will vary with ka. Experimental results in the range 30 ⩽ ka ⩽ 100 are compared with theory. [Work supported by ONR.]