[S-CG50] Rheology, fracture and friction in Earth and planetary sciences

[SCG50-P06] Development of measuring system of electrical resistivity anisotropy during triaxial deformation experiments

*JIEI SO¹, Yuya Akamatsu¹, Ikuo Katayama¹, Takeshi Suzuki², Kazuki Sawayama² (1.Hiroshima university, 2.Kyoto university)

Keywords:Radial resistivity, Microcracks development, Anisotropy

Microcracking of rocks has been the subject of a wide variety of studies because they are closely related to the mechanism of earthquake generation, the strength of the earth's crust, and subsurface resources such as geothermal reservoirs. In field-scale investigations, we often evaluate subsurface structure through geophysical observations using rock physical properties (i.e., electrical resistivity and elastic wave velocity). In particular, electrical resistivity is sensitive to pore connectivity and therefore is an important indicator of the presence and hydraulic characteristics of crustal fluids. This study has developed a method of measuring electrical resistivity in axial and radial directions during triaxial deformation experiments to investigate the electrical resistivity anisotropy associated with microcracking to fracturing process. We conducted hydrostatic pressure experiments and triaxial deformation experiments using the Intra-Vessel Deformation and Fluid-Flow Apparatus with Aji granite as the sample. Electrical resistivity was measured by the two-terminal method in both axial and radial direction. Stainless steel was used as the electrode in the axial direction, while three types of electrodes (copper plate, conductive epoxy, and Ag/AgCl ribbon) were tested as candidates for adhesive electrodes in the radial direction. This measurement system was first tested during hydrostatic pressure experiments under a confining pressure of 10 to 100 MPa and pore pressure of 1 MPa. Having established a measurement system, we then measured electrical resistivity in axial and radial directions simultaneously during triaxial deformation experiments under a confining pressure of 20 MPa, pore pressure of 10 MPa, and strain rate of 3.6 x 10^-6/sec. We also measured the volume change of the syringe pump for pore pressure, which can be converted to changes in porosity. As a result of hydrostatic pressure experiments, the Ag/AgCl ribbon showed the best performance in measuring radial resistivity when it was immersed in the KCl solution. The results of triaxial deformation tests show that the radial resistivity increases in the early stages of deformation and then decreases until failure as in the axial direction. This indicates the possibility of capturing microcracks that develop to the maximum principal stress even in the radial direction. Two differences were also identified in the axial and radial directions. The first is that the radial resistivity reaches its maximum value and begins to decrease earlier than the axial resistivity. The other is that the decrease in radial resistivity is greater than that in axial resistivity. These differences in the response of electrical resistivity in different directions are expected to further elucidate the process of microcrack development. Although further verification is required for the anisotropy of these electrical resistivities, the measurement results might contribute to the interpretation of field observations. Future research on radial electrical resistivity will be conducted based on the established measurement system by changing pressure conditions and the position of the electrodes.