Radiowave scattering from surface and subsurface heterogeneities

Finite-difference time-domain (FD-TD) techniques allow practical numerical computation of radiowave scattering by a wide range of objects (e.g., rocks) on or in a planetary regolith. Numerical models are evaluated for two-dimensional (2-D) cases in which burial depth D and object size R of a single scatterer vary and for which the separation distance L between two scatterers varies. A buried object typically has a significantly weaker scattering response than the same object resting on the surface, although the strongest response may occur when the scatterer is partially buried. Large objects scatter strongly and in complex ways, but the bounds on solutions appear to be well-defined; this should be useful in predicting aggregate behavior of ensembles of scatterers. The response of closely spaced objects differs significantly from the response of the objects calculated separately, but the coupling decreases rapidly with separation. Two-dimensional wavelength-scale objects (R similar or equal to lambda(0)) separated by L greater than or equal to 15 R effectively behave as independent scatterers. Computations in 2-D can be adapted for three-dimensions (3-D); preliminary results are consistent with measured radar backscatter cross sections for the Moon and Venus and with estimates of block population densities on the Moon. The FD-TD code generalizes to 3-D, permitting the set of case studies to be expanded for interpretation of data such as those from Magellan. (C) 1996 Academic Press, Inc.