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International audienceWe have developed a mechanistic model to interpret spectral induced polarization data of partially saturated clay-rocks. This model accounts for the polarization of the grains through an electrical double layer model with a polarization model of the inner part of the electrical double layer called the Stern layer. The polarization model accounts also for the Maxwell-Wagner polarization at frequencies higher than 100 Hz. The Maxwell-Wagner polarization is modelled by using a conductivity model modified to account for the presence of a non-wetting immiscible phase like air in the pore space. The resulting model is consistent with the first and second Archie's laws in the case where surface conductivity can be neglected. The volumetric charge density of the diffuse layer at saturation is divided by the saturation of the water phase to account for the partial water saturation of the porous material. The model comprises seven fundamental parameters: the formation factor, the second Archie's exponent, a critical water saturation level, the mean electrical potential of the pore space at saturation, the density of the counterions in the Stern layer, and at least two parameters describing the grain size distribution. Most of these parameters can be derived independently using alternative measurements and electrochemical models. Measurements were performed in the frequency range 10 mHz-45 kHz using five samples from the Callovo-Oxfordian formation in the eastern part of the Paris Basin, France. The model agrees fairly well with the experimental data at saturation and for partially saturated clay-rocks down to 1 Hz. Most of the seven physical parameters entering the model were independently evaluated

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International audienceSpectral induced polarization or complex conductivity is a promising electric method in hydrogeophysics because of its sensitivity to water saturation, permeability, and particle size distribution (PSD). However, the physical and chemical mechanisms that generate the low-frequency complex conductivity of clays are still debated. To explain these mechanisms, the complex conductivity of kaolinite, smectite, and clay-sand mixtures was measured in the frequency range 1.4 mHz-12 kHz with various clay contents (100%, 20%, 5%, and 1% in volume of the clay-sand mixture) and salinities (distilled water, 0.1 g L-1, 1 g L-1, and 10 g L-1 of NaCl in solution). The results indicated the strong impact of the cation exchange capacity of smectite upon the complex conductivity of the material. The quadrature conductivity increased steadily with the clay content and was fairly independent of the pore fluid salinity. A mechanistic induced polarization model was also developed. It combined a Donnan equilibrium model of the surface electrochemical properties of clays and sand, a conduction model of the Stern and diffuse layers, a polarization model of the Stern layer, and a macroscopic conductivity model based on the differential effective medium theory. It also included the effect of the PSD. Our complex conductivity model predicted very well the experimental data, except for very low frequencies (<0.1 Hz) at which membrane polarization may dominate the observed respons