Simulating rough surfaces by periodic and biperiodic gratings

Abstract

The scattering of acoustic and electro-magnetic plane waves by rough surfaces is the subject of many books and papers. For simplicity, we consider the special case, described by a Dirichlet boundary value problem of the Helmholtz equation in the half space above the surface. We recall the formulae of the far-field pattern and the far-field intensity. The far-field can be defined formally for general rough surfaces. However, the derivation as asymptotic limits works only for waves, which decay for surface points tending to infinity. Comparing with the case of periodic surface structures, it is clear that the rigorous model of plane-wave scattering is accurate for the near field close to the surface. For the far field, however, the finite extent of the beams in the planes orthogonal to the propagation direction is to be taken into account. Doing this rigorously, leads to extremely expensive computations or is simply impossible. Therefore and to enable the approximation of waves above the rough surface by waves above periodic and biperiodic rough structures, we consider a simplified model of beams. The beam is restricted to a cylindrical domain around a ray in propagation direction, and the wave is equal to a plane wave inside of this domain and to zero outside. Based on this beam model, we derive the corresponding asymptotic formulae for the wave and its intensity. The intensity is equal to the formally defined far-field intensity multiplied by a simple cosine factor. Under special assumptions, the intensity for the rough surface can be approximated by that for rough periodic and biperiodic surface structures. In particular, we can cope with the case of shallow roughness, where the reflected intensity includes, besides the smooth density function w.r.t. the angular direction, a plane-wave beam propagating into the reflection direction of the planar mirror. Altogether, the main point of the paper is to fix the technical assumptions needed for the far-field formula of a simple beam model and for the approximation by the far fields of periodized rough surfaces. Furthermore, using the beam model, we discuss numerical experiments for rough surfaces defined as realizations of a random field and, to get a more practical case, the Dirichlet condition is replaced by a transmission condition. The far-field intensity function for a rough surface is the limit of intensity functions for periodized rough surfaces if the period tends to infinity. However, almost the same intensity function can be obtained with a fixed period by computing the average over many different realizations of the random field. Finally, we present numerical results for an inverse problem, where the parameters of the random field are sought from measured mean values of the intensities.