[S-CG50] Dynamics in mobile belts

[SCG50-10] Fracture as an electrical conduction path under pressure

*Tohru Watanabe¹, Arina Tomioka² (1.Faculty of Sustainable Design, University of Toyama, 2.Graduate school of science and engineering, University of Toyama)

Keywords:crack, electrical conductivity, fluid

Geophysical mapping of fluids is critical for understanding crustal processes. Interpretation of seismic velocity and electrical resistivity structures requires a thorough understanding of the pore structure that governs physical properties under high pressures. Watanabe et al. (2019) showed that wide aperture parts in cracks remain open to govern physical properties under high pressure. In the crust, there should be cracks with various sizes: from grain boundary to large faults. In order to understand the role of a fracture as a conduction path under high pressure, we have made measurements of elastic wave velocities and electrical conductivity in a brine-saturated granitic rock with single fracture. A cylindrical rock sample (D=26 mm, L=30 mm) with a single fracture was cored from a block (20 cm x 20 cm x 20 cm) of Aji granite, which was artificially fractured. The fracture goes through the sample from top to bottom. 3D structure of a fracture was imaged by X-ray micro CT before measurements. A sample was filled with 0.1 M KCl aqueous solution, and velocity and conductivity were simultaneously measured by using a 200 MPa hydrostatic pressure vessel. The pore-fluid was electrically insulated from the metal work by using plastic devices. Compressional wave velocity was measured in the direction subperpendicular to the fracture, while electrical conductivity in the axial direction.

Compressional wave velocity increases and electrical conductivity decreases with increasing confining pressure. The electrical conductivity at the atmospheric pressure was around one order of magnitude higher than that in intact samples. It greatly decreased from 0.1 MPa to 5 MPa and showed a gradual decrease at higher pressure. The conductivity at 150 MPa is only a few times as high as that in intact samples. Compared with grain boundary cracks, this fracture cannot work as a remarkably good conduction path. A misfit of fracture surfaces must increase conductivity at high pressure.