A New Cell for Electrical Conductivity Measurement on Saturated Samples at Upper Crust Conditions

Electrical resistivity soundings are used by geophysicists to determine the structure and composition of the Earth’s crust and mantle and to explore natural resources (ore, oil, gas, water). Their interpretations in terms of composition and in-situ physical conditions depend mainly on laboratory measurements of electrical conductivity of rocks at simulated crustal conditions of temperature, pressure, saturation and pore pressures. These measurements present a numbers of limitations, in particular, in the case where conductive pore fluids are present, as in the case of deep reservoir conditions, where temperature exceeds 250 °C. Here, we present a new cell capable of measuring electrical conductivity of large saturated samples at confining pressure up to 200 MPa, pore pressure up to 50 MPa, and temperature up to 500 °C. The measurement cell has been developed in a commercial, internally heated, gas pressure apparatus (Paterson press). It is based on the concept of “guard ring” electrode, which is adapted to samples that are jacketed by a very conductive, metallic material. Numerical modeling of the current flow in the electrical cell allowed defining the optimal cell geometry. Calibration tests have been performed on Fontainebleau sandstones saturated with electrolytes of different conductivities, up to 350 °C. The resulting electrical formation factor and temperature dependence of electrical conductivity are in very good agreement with previous studies. This new cell will improve the exploration and exploitation of deep fluid reservoirs, as in unconventional, high enthalpy geothermal fields. In particular, the investigations address possible effects of fluid-rock interactions on electrical resistivity of a reservoir host rock.     

Nonlinear EEG Changes Associated with Clinical Improvement in Depressed Patients

Assuming that the depressive syndrome could be related to a cerebral dynamical disease, we attempted to describe in a longitudinal and quantitative manner the modifications of brain electrical activity during depressive episodes treatment. This study tested whether mood improvement during therapy (pharmacological treatment or electroconvulsive therapy) is related to concomitant modifications of brain dynamics. The evolution of brain activity and mood were measured, every two days during treatment for three depressed patients and over three weeks for a control subject. Complexity of brain electrical activity was computed for each site of recording (EEG with 31 electrodes), leading to maps of complexity; depressive mood modulations were quantified using a self-assessment scale. The results show a specific organization of depressed patients' brain dynamics compared to the control's dynamics. Moreover, covariations between nonlinear changes in brain dynamics and mood improvement are observed during the remission of depressive episodes.     

Nonlinear EEG Changes in a 48-Hour Cyclic Manic-Depressive Patient

This research report presents the case study of a 48-hour cyclic manicdepressive patient which further demonstrates the association between nonlinear EEG characteristics and mood variations as previously reported by Thomasson et al. (2000). The evolution of brain dynamics and mood were daily measured during a week. Global complexity of brain electrical activity was estimated by a nonlinear index (entropy) and mood modulations were evaluated by a clinical self-assessment scale (BfS'). Illustrating the concept of dynamical disease, a significant co-variation between the nonlinear EEG index and mood evolution (Spearman correlation coefficient = 0.92, p = .008) was observed. This result strengthens the previous ones and demonstrates a clear association between nonlinear brain dynamics and state of mind in psychopathology.