Electrokinetic coupling in unsaturated porous media
The aim of this PhD thesis is to study electrokinetics for unsaturated conditions, both through streaming potential (SP) and seismo-electrics conversion. Electrokinetic coupling results from the motion of charges inside the water. Using an experimental approach, (SP) have been measured during drainage experiments in sand. These measurements were combined with hydrodynamics measurements and led to the first continuous recordings of streaming potential coefficient C as a function of water saturation Sw. The normalized streaming potential coefficient was found to be not monotoneously dependent on water saturation. Values of C increases first when Swdecreases until a maximum value for Sw around 65-85 %, and then decreases for decreasing saturation. In addition, experimental electrokinetic coefficient seems to be larger up to two orders of magnitude than classical values measured for saturated conditions. A similar behaviour was observed for three drainage experiments. The second part of this manuscript deals with the modelling of this unexpected behaviour. A new model, including a dynamic component linked to pressure temporal variations, is proposed. Modelling results show that this component is necessary to explain electrokinetic coefficient data of the first experiment, particularly for Sw =[0. 65-0. 85 ; 1]. Processing of the last experiment suggests that hydrodynamic conditions, and particularly the instant fluid flow magnitude, could be involved in the behaviour of C. The third part of this work deals with a field approach using seismoelectrics. Seismoelectric amplitudes have been studied for unsaturated conditions computing transfert functions defined as Ex/üx. First, the soil water saturation map has been deduced from the joint processing of GPR and ERT data. Thus, seismoelectric amplitudes deduced from P wave and surface waves independently, have been compared to water-content and measured electrical resitance. Results suggest that these amplitudes are smaller in unsaturated soils than for saturated conditions. In addition, the independent processing of P wave and surface waves shows that surface waves creates some artefacts in the computed transfert functions, and consequently have to be considered apart.