The use of electromagnetic sensors such as Time Domain Reflectometry (TDR) probes has gained increasing importance for water content measurements since several years, for long term monitoring of structures, among which radioactive waste repositories. TDR probes are basically sensitive to electromagnetic properties of the host material, clayrock in our case Prior to perform in-situ experiments with TDR probes, it is mandatory to have an accurate knowledge of the electromagnetic properties of clayrock as a function of their water content. We developed a new laboratory dielectric measurement device, consisting of a coaxial transmission line, enabling characterization of intact clayrock permittivity over the 50 MHz - 1 GHz frequency range. The study has shown a large variation of complex permittivity with (i) water content, the parameter of interest and (ii) frequency. The frequency dependence is induced by different relaxation processes. In a second step, these data are introduced in an original semi analytical model (Distributed Points Sources Method) in order to obtain a reliable modeling of the TDR probe. Taking into account some experimental aspects of the TDR probe, we propose to introduce a in this paper the effect of an air gap between the TDR antennas and the surrounding media. The effect of this influent parameter is evaluated owing to our DPSM modeling, and some solutions are proposed to overcome the problem.