Electrical resistivity anisotropy in brine-saturated rocks during triaxial deformation

Abstract We developed a method to measure the anisotropic electrical resistivity of rocks during triaxial deformation under brine-saturated conditions. This method addresses two primary challenges: determining the effective cross-sectional area through which the electrical current flows and selecting electrode materials that can function effectively under the confining pressures typical of crustal environments. To accurately estimate radial resistivity, we employed numerical calculations to assess the effective cross-sectional area of the current. For resistivity measurements, we evaluated the performance of Ag/AgCl electrodes against that of electrodes composed of conductive epoxy adhesive. The Ag/AgCl electrodes demonstrated superior performance, exhibiting minimal phase error across a broad frequency range, even under confining pressure. Our experimental setup enabled simultaneous measurements of both axial and radial resistivities during triaxial deformation, allowing us to monitor the development of resistivity anisotropy throughout the brittle deformation process. The variations in resistivity anisotropy correlated well with the known stages of rock deformation. Notably, the early stages of deformation exhibited resistivity anisotropy due to stress-induced crack formation, independent of sample geometry. This approach provides valuable insights for interpreting field-scale resistivity anisotropy data about crustal stress fields. Graphical abstract