A 3-WAVE FDTD APPROACH TO SURFACE SCATTERING WITH APPLICATIONS TO REMOTE-SENSING OF GEOPHYSICAL SURFACES

A major difficulty in physical interpretation of radio wave scattering from geophysical surfaces is the lack of detailed information on the signatures of geologically plausible discrete objects, Although the ag gregate response will never be dominated by any single object, differe nces in the population of discrete objects on or near the surface (the ir sizes and shapes, for example) can change the character of a radio echo markedly. When the average surface is modeled as a flat, homogene ous half-space, the field that ''drives'' the scattering process is a composite consisting of the incident plane wave and the reflected and transmitted plane waves, all of which are known quantities; the total field can then be defined as the sum of the driving field and the scat tered field. When a discrete object is near the surface, the total fie ld can be calculated using finite-difference time-domain (FDTD) techni ques, and the scattered near field can be calculated accordingly, The Green's functions for electric and magnetic currents above and below t he surface, obtained by Sommerfeld theory and employed in conjunction with Huygens' principle, transform the local scattered fields to the f ar field. The FI)TD implementation accommodates discrete lossy dielect ric and magnetic scatterers in the vicinity of a dielectric surface; e xtension to a lossy halfspace is straightforward. Two-dimensional resu lts for scattering from perfectly conducting circular cylinders above and below a dielectric surface agree with moment method solutions with in a few percent. Results for scattering from a dielectric wedge exhib it expected forward diffraction and internal reflection phenomena.