The instrumentation to be carried to the comet Wirtanen in the Rosetta mission includes a radio experiment, Comet Nucleus Sounding Experiment by Radio Wave Transmission (CONSERT), designed to reveal the average dielectric properties of the nucleus, and the structure of the interior. The analysis of the data to retrieve information about the internal structure has the character of rudimentary radio tomography. We investigate mathematically the behavior of the surface field under the transition from direct propagation to geometrical shadow to determine possible effects of the electrical properties in the close vicinity of the lander. In particular we investigate the effects of a surface layer of composition different from that of the nucleus to determine whether such a layer can influence the wave propagating into the geometrical shadow zone. It is shown that the behavior of the field in the shadow zone is determined chiefly by the topmost layer properties.