Microstructural control of permeability, electrical resistivity, and seismic velocity and their relationships in sheared rock fractures under normal stress

Fluid flow through faults considerably influences seismic hazards and geoengineering systems, and it can be detected through geophysical techniques. To indirectly monitor the subsurface fracture permeability using geophysical data, effective rock physics models must be established for fractured rock masses. This study examined coupled changes in permeability, electrical resistivity, P-wave velocity, S-wave velocity, and their ratio (Vp/Vs) under normal stress using a coupled computational approach for various fracture models, which are structurally linked through the fracture microstructures. The results reveal that these properties change with stress, fracture surface roughness, fractal dimensions, and shear displacement. Comparisons of these results with those of the effective medium theory indicate that the permeability is strongly affected by the aperture, while resistivity is controlled by contact area rather than the aperture in the percolation regime. Although seismic properties are influenced by aperture and contacting asperity, these effects can be normalized by applying a power-law scaling. Furthermore, the Vp / Vs ratios obtained for simulated faults are higher than those calculated based on the conventional effective medium theory. Distinct effects of microstructures on each property lead to permeability–electrical resistivity and permeability–seismic velocity relationships showing a bilinear trend, which does not depend greatly on geometric characteristics. The threshold for dividing the linearity is located at ∼20% contact area in our conditions, clearly appearing on the resistivity–velocity relationship. Our findings suggest that resistivity is sensitive to the permeability while the flow paths are connected, whereas seismic velocities are sensitive before reaching to the percolation.