Characterization and evolution of the EDZ in Opalinus clay by 3D reconstruction of the electrical R by 3D reconstruction of the electrical resistivity

Document available in extended abstract form only. Electrical resistance measurements were completed over four years in the gallery 04 of the Mont Terri underground laboratory. This experiment aims at reconstructing the structure of the EDZ in Opalinus Clay and monitoring its evolution with time. For this purpose three rings of 32 electrodes were set up around the gallery and measurements were carried out before and after the excavation procedure of the gallery and nearby niches. Electrical measurements were also conducted in boreholes located in between the electrodes' rings. The data analysis is performed via a three dimensional reconstruction of the media surrounding the gallery using the software EIDORS. The media is represented by a finite element model, particularly pertinent in this case to construct complex structures. This model may takes into account a front-end face or niches depending on the gallery shape of the acquisition period for a given data set. The mesh is also better refined near the electrodes where higher electrical fields and stronger variations of resistivity are expected. The coarser mesh far form the electrodes enables a faster computation of the measurement prediction. New options were developed in the software consisting in the parameterization of the inversion that may be defined differently from the forward problem. For instance, in the case of the borehole data sets, the inverse problem seek the value of a background resistivity, an EDZ resistivity and the shape of the boundary separating the two media. The values of resistivity estimated is used later to construct the initial model of the electrodes' rings data set analysis. In the case of the rings' data sets the inverse problem aims at estimating the logarithm of the resistivity so the unknowns are then real parameters. Moreover, the shape and the size of the inverse elements are defined in a such way that the spatial resolution reflects the geometrical configuration of the data acquisition network. This allows a reduction of the unknowns and so stabilizes the ill-posed problem. This stability is tested by the analysis of the influence of noise added in a data set on the resulting image. This study shows that the region containing the main uncertainty on the resistivity estimation is limited to the first 30 cm around the gallery walls and further away the results are converging. The reconstructed images showing the distribution of the resistivity around the gallery show the sensitivity of electrical measurements to the EDZ geometrical configuration and its temporal evolution. The EDZ, revealed by positive contrasts of resistivity presents a high heterogeneity and shows a maximal extension of 1.3 m in this section of the Mont Terri laboratory. Straight after the excavation process, the presence of the EDZ emerges. With time the EDZ distribution evolves and a decay of its extension is observed at the end of the experiment. (authors)