The research work submitted in this PhD dissertation deals with the study of soil electrical conductivity and dielectric permittivity in the medium frequency range (100 kHz - 10 MHz). The aim of the study is to develop a new mapping method where these two electrical parameters are simultaneously determined. The electrical conductivity and the dielectric permittivity are sensitive to the soil state parameters, and particularly to its water content. Coupling the information brought by the two electrical parameters leads to the estimation of soil water content at each measurement point. Laboratory measurements were led in order to better understand the evolution of these two parameters contingent on the frequency. Dielectric permittivity and electrical conductivity of sand samples, artificial mixtures of sand and clay (kaolinite and bentonite) in various proportions and natural loamy soil samples were measured for different water content. We studied the effect of the clay content, clay mineralogy and electrolytic salinity. For clay free samples, existing theoretical models are well predicting the high frequency response of the studied material as a function of water content. When the frequency becomes lower than 2 MHz, other types of polarization arise. These effects are all the more important than the clay content is high. They are linked to the surface properties of the clay contained in the soil sample (cation exchange capacity, specific surface area). If complex modeling must be considered in order to physically understand these relations, we first choose to establish an empirical relation that allows the discrimination of the dielectric permittivity linked to the dipolar polarization (HF permittivity due to the rotation of water molecules). An in situ electromagnetic prototype was developed in parallel. It is composed of two fixed perpendicular coils (Slingram geometry) and worked at 1.56 MHz. After calibration, we tested the response of the prototype to objects with known properties. The first in situ measurements were led on experimental sites with different types of soils and different water content variations (artificially created or natural): sandy alluvium on a plot of INRA (French National Institute for Agricultural Research) in Orléans (Centre, France), a clay-loam soil on an experimental site in Estrée-Mons (Picardie, France) and fractured limestone at the vicinity of Grand (Vosges, France). In the case of the sandy alluvium, the values of dielectric permittivity measured are closed to those of HF permittivity and allow the use of existing theoretical models to determine the soil water content. For soils containing higher amount of clay, empirical relations established using the laboratory experimentations are used to calculate a first estimation of the water content. These values are confirmed by the measured mass water contents.
Simultaneous measurement in the medium frequency range and mapping of the soil dielectric permittivity and electrical conductivity