Asymptotic modeling of three-dimensional radar backscattering from oil slicks on sea surfaces

International audienceThis paper presents new results of a simulation of radar backscatter from oil slick areas1on a real three-dimensional sea surface, based on a physical hydrodynamic model of surface wave2damping in the presence of oil films, the local equilibrium model (MLB). To solve this problem,3the modelling was carried out by using the first-order small-slope approximation (SSA1) model.4It presents the advantage of having a very good compromise between rapidity and accuracy of5the calculation. The choice of the model is justified by solving the two-dimensional problem with6several asymptotic methods and further comparing the results with a rigorous numerical method,7based on the Method of Moments (MoM). Two approaches called “thin-layer” (TL) and “classical”8were used to deal with the double layer (air/oil/sea) problem. The TL approach assumes that this9double-layer problem can be seen locally as a Fabry-Pérot interferometer, which implies that the10Kirchhoff-tangent plane approximation (KA) is valid. The classical approach consists in neglecting11the presence of the oil layer for dealing with electromagnetic backscattering, which is valid for12very thin oil films compared to the electromagnetic (EM) wavelength. It is shown that these two13approaches have rather complementary validity domains: The TL approach is always valid for14small observation angles, which makes it suitable for near nadir sensors like altimeters, whereas15the classical approach is valid for moderate observation angles, which makes it suitable for most16satellite applications. The 3D modelling results are compared with C-band and X-band measured17data (CSK experiment and OOW NOFO experiment) in VV polarization. The calculation takes into18account that the oil film on the sea surface is mainly in an emulsion state. The results highlighted19the relevance of the MLB hydrodynamic model, as well as the SSA1 EM model combined wit the20classical approach, for quantifying NRCS in seas contaminated with marine oil or surfactants. The21agreement is indeed very good in the X-band range