An approximate solution to the radiative transfer equation for bistatic scattering from a rough surface covered by a tenuous distribution of particulate (scattering and absorbing) media is derived by means of a series expansion in the scattering coefficient κs of the covering layer up to the first order. The formulation of the successive orders of a scattering series is reviewed, and an analytic solution to the first-order interaction contribution is given by means of a series expansion of the azimuthally averaged product of the bidirectional reflectance distribution function of the surface and the scattering phase function of the covering layer.