Application of geoelectrical methods in environmental and engineering geophysics

In this presentation, we will discuss the increasingly important role of electrical resistivity, induced polarization and self-potential tomography in engineering geophysics and the development of joint approaches that can be applied to image water content, permeability, and water flow at various scales. We will first focus on applications to dams and landslides to demonstrate the usefulness of these methods to locate leaks and get a better understanding of the role of ground water flow in clay-rich landslides and mudflow. Then, we will show case a new model of induced polarization that can be applied to cement and concrete and based on fractal theory. Induced polarization can be used as a non-intrusive and non-destructive technique to image and monitor the evolution of cementitious materials, with the objective of retrieving their water content and hydration state. We will show the performance of the model using a collection of cement paste samples (CEMI and CEMV) and corresponding mortar samples (MORI and MORV), all cured for 60 days, with water-to-cement (w/c) ratios ranging from 0.35 to 0.60. Spectral induced polarization measurements are performed in the frequency range 10 mHz-45 kHz. For the cement pastes, both the in-phase conductivity and the magnitude of the quadrature conductivity increase systematically with increase of the w/c (water-to-cement) ratio. The electrical properties of the mortars scale proportionally with those of the corresponding cement pastes, and the proportionality coefficient can be predicted from the volume fraction of cement. The complex conductivity data are well-fitted by a double Cole Cole model, and the normalized chargeability is found to be proportional to the quadrature conductivity, consistent with theoretical expectations. The experimental data are explained using a dynamic Stern layer model associated with the polarization of the inner component of the double layer coating the surface of the C-S-H minerals. Experiments are also performed to monitor the hydration phase of cement pastes like CEMI. To our knowledge, this is the first mechanistic-based interpretation of the complex conductivity spectra of cement pastes and mortars. These results open new perspectives for non-invasive monitoring of concrete in civil and nuclear engineering applications. We will show some time-lapse tomograms obtained inside the PALLAS project demonstrating this point. Furthermore, we will discuss also some applications to raw earth materials for building construction and how we can use time-lapse tomography to image their water content over time.