Simultaneous measurement of electrical and mechanical properties of rock at high temperatures and pressures

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Recently observational studies have revealed detailed crust structure of P-and S-wave velocities (Vp and Vs), Q value, and electric conductivity. However, we cannot extract sufficient information about fracture properties of rock from the observational results. In order to get such information, simultaneous measurements of electrical and mechanical properties of rock is helpful.Measurement of electric conductivity of rock at high temperature and pressure needs metal jacket, with which we coat the rock specimen. In order to measure electric conductivity in this situation, we designed an electrode module with a guard ring. Electric current which mainly flows along the metal jacket is separated from the current through the rock specimen, by using the guard ring and center electrode. We numerically calculated the electric currents flowing into the guard ring and center electrode, and obtained the relation between the measured current and the true conductivity of the specimen. Using the newly developed module, we measured rock conductivities at temperatures up to 250 degree, pressures up to 250 MPa, and frequencies ranging from 1 Hz to 1 MHz. We found, for example, that pelitic hormfels in Hidaka had a clear temperature dependence; the conductivity showed the lowest value around 75 degree.