ROUGH-SURFACE EFFECTS ON INCOHERENT-SCATTERING FROM RANDOM VOLUMETRICSCATTERERS - APPROXIMATE ANALYTIC SERIES SOLUTION

An approximate analytic series solution is obtained for the effects of randomly rough surfaces on the time-dependent ultrasonic backscatter that are due to beam-microstructure interactions. The transmission of sound through the rough surface is modeled by scalar waves by use of t he phase-screen and Fresnel approximations, whereas the transducer is assumed to produce a focused normally oriented Gaussian beam. The beam -microstructure interaction is described by a simple, generic model th at attributes backscattering to inhomogeneities in the elastic constan ts of the sample; density variations are ignored. Key predictions of t he approximate series solution are that (a) acoustic backscatter is re latively insensitive to surface roughness for unfocused probes, (b) ro ughness can dramatically reduce the backscatter noise seen by focused probes, (c) backscatter is increased at early times because of weak lo calization, and (d) backscatter is reduced at late times because of in creased diffraction. The predictions of the series solution are briefl y compared with available experiment.