Impact of flat subsurface approximation on scattering of multilayer media

Subsurface profile is one of the crucial parameters to construct an accurate scattering model of multilayer media. Due to the difficulties of tracing the subsurface profile, the subsurface is often assumed to be flat to simplify the scattering model. The validity of this flat subsurface approximation, however, was examined only by empirical data, and its applicable condition was barely provided in the literatures. The accuracy of multilayer scattering model can be significantly affected without a proper applicable condition. In this paper, two models are investigated: a three-layer media with a rough subsurface (exact geometry model) and a three-layer media with a flat subsurface (approximate geometry model). The scattered fields of these two models are solved by using the Method of Moment combined with the Kirchhoff Approximation. The results indicate that the fields are strongly dependent on the parameters of the roughness of surfaces, the mean thickness of medium, and the angle of incidence. These parameters are assigned with different values to quantitatively evaluate their impacts on the difference in bistatic and backscattering scattering coefficients between the approximate and exact geometry models. Finally, the applicable conditions of the flat subsurface approximation are determined by limiting this difference within 1 dB.