In this paper the development of a new method to extract the two complex relative permittivities of a dihedral structure composed of two surfaces in monostatic configuration is exposed. Provided a dominant double bounce mechanism, we only need in theory the copolarised far-field measurements HH and VV at one incidence angle. As the permittivity of natural material is strongly dependent on its moisture content, the method is used here to infer the water content of the soil. To do so we worked with simulated data using the Geometric Optic method implemented in FEKO. First results show that some incidence angle range has to be preferred to accurately determine the permittivity of the horizontal surface.