A petrophysical model describing spectral induced polarization (IP) has been developed for clay rocks accounting for the Maxwell-Wagner polarization. It is also used to connect the complex conductivity to the relative permeability of the material. This model is applied to the Callovo-Oxfordian clay rock of the Paris Basin (France) where the Meuse/Haute-Marne Underground Research Laboratory is located. Laboratory experiments are performed using eight clay-rock cores to study the effect of desiccation on their spectral-IP response. The measurements are performed along the foliation plane. Complex conductivity spectra are measured over a frequency range of 1 mHz to 45 kHz. These spectra are fitted with a double Cole-Cole model to extract the evolution of the Cole-Cole parameters with the saturation during the desiccation process. The low-frequency Cole-Cole model corresponds to IP phenomena, whereas the high-frequency Cole-Cole model corresponds to the Maxwell-Wagner contribution. We obtain the value of the first and second Archie’s exponents and we check the relationship between the surface conductivity and the cation exchange capacity of the clay rocks. We are also able to connect the relative permeability curve to the second (saturation) Archie’s exponent. The monitoring of the complex conductivity can be used to predict how the permeability of the clay-rock formation changes with the water content.

Desiccation influences the complex conductivity of porous media with disseminated metallic particles. We expand the mechanistic model developed in the previous papers of this series to include the effect of saturation upon the complex conductivity of mixtures of mineral grains, pyrite, and pore water. During desiccation, the salt is assumed to be segregated in the liquid pore water; therefore, the conductivity of the pore water increases when saturation decreases. We have performed 14 experiments corresponding to 91 complex conductivity spectra. In these experiments, the saturation of the water phase is changed over time by desiccation. The resulting spectra are fitted by a double Cole-Cole model used as the fitting model. We also developed a mechanistic model in which the chargeability and the relaxation time of the low-frequency polarization are expected to change with saturation in a predictable way. We first characterized the properties of the background material made by an illitic clay material. We determine how the Cole-Cole parameters depend on saturation. The Cole-Cole exponent is essentially independent on saturation. When the chargeability of the mixture is dominated by the presence of pyrite, it becomes independent of the saturation but a small effect on the chargeability is observed at low pyrite contents. The instantaneous conductivity of the background decreases with the saturation in a predictable way. The relaxation time depends on the inverse of the instantaneous conductivity and therefore on saturation. This dependence is well-explained through numerical simulations made with the finite-element method. Finally, we analyze the complex conductivity spectra of two clay-rock core samples from the Callovo-Oxfordian formation in the Paris Basin (France). The spectra are shown as a function of their desiccation and explained thanks to the newly developed model.

Concrete resistivity measurements strongly depend on the temperature and the water content of the structure. In this paper, a study of the effect of the temperature and saturation degree on electrical resistivity measurement is carried out using an embedded printed circuit board sensor to estimate water content profiles in concrete structures. Resistivity measurements are performed at temperatures between 20 and 60 °C. Experimental results are presented and analyzed in light of well-established empirical models. Calibration curves that link the electrical resistivity to the degree of saturation at a given temperature are discussed. Arrhenius laws that depend on the degree of saturation can be used to fit our data. In the perspective of the instrumentation and monitoring of concrete structure in real conditions, it is important to master the temperature correction laws of resistivity measurement to evaluate the gradients of water saturation degree.

The anisotropy in the complex conductivity of a clay-rock formation is investigated through laboratory and in situ measurements. Laboratory measurements have been conducted on four samples from two cores of the Callovo-Oxfordian Formation from the Paris Basin (France). This clay-rock formation is anisotropic with a well-defined bedding plane. Two of the cylindrical core samples are taken in the transverse direction with respect to the bedding plane, whereas two other core samples are taken in the in-plane direction. The complex conductivity spectra are performed in the frequency range of 20 mHz to 45 kHz and in a broad pore water saturation range from near saturation to a residual saturation of approximately 25%. The complex conductivity spectra are fitted with a double Cole-Cole model, whereas the low-frequency contribution corresponding to the induced polarization effect and the high-frequency contribution is related to the Maxwell-Wagner polarization. The six Cole-Cole parameters are investigated as a function of saturation, and we evaluate their degree of anisotropy versus the effect of saturation. The low-frequency instantaneous conductivity and normalized chargeability indicate a consistent degree of anisotropy with an anisotropy ratio equal to 3.8 ± 0.5 for one core sample and 6.5 ± 0.9 for the second core sample. These values represent a substantial level of anisotropy. The low-frequency Cole-Cole exponent and relaxation time do not indicate substantial amount of anisotropy. The monitoring of the evolution of the weight in the controlled humidity chamber seems to indicate that their relative permeabilities do not indicate a strong anisotropic behavior. Finally, the laboratory data are discussed in the context of field data obtained with an azimuthal array of 12 electrodes.

Fiber optic sensors are increasingly used to measure dynamic strain fields caused by the propagation of mechanical waves. Their low intrusiveness when embedded within a structure makes them suitable for a wide range of applications. In this paper, the feasibility of integrating fiber Bragg gratings (FBGs) into castable materials for ultrasonic applications is investigated. We employed castable polyurethane resins, which are widely used in industry due to their reproducible and durable mechanical properties. Our study began with an analysis of fiber integration by examining the 1D strain profiles of two polyurethane resins during their polymerization and also the impact of their hardening on the central wavelength value of several FBGs spectra. Subsequently, we assessed the sensitivity of FBGs to ultrasonic waves generated at 100 kHz after resin polymerization. Specifically, we explored how the fiber coating influences the rate of energy transfer from the host material to the fiber core. Our findings demonstrate that the central wavelength shift in the FBG reflectivity spectra, caused by shrinkage during resin polymerization, can reach up to 10 nm. This shift must be considered when selecting FBG wavelengths to prevent the reflectivity spectra from falling outside the permissible range of the interrogation system. We measured exploitable ultrasonic waves propagating in the resin samples. Preliminary observations suggest the presence of early arrivals, which could potentially correspond to crosstalk effects between the FBGs even though they are centered at different wavelengths. Furthermore, we show that in dynamic strain fields caused by ultrasonic wave propagation, both acrylate and polyimide coatings transmit similar amounts of energy to the fiber core. These preliminary results highlight the potential of using FBGs as ultrasonic wave sensors embedded in castable materials such as polyurethane resins.