Simultaneous measurements of elastic wave velocity and electrical conductivity in a brine-saturated granitic rock were conducted under confining pressures of up to 180 MPa. Contrasting changes in velocity and conductivity were observed. As the confining pressure increased to 50 MPa, compressional and shear wave velocities increased by less than 10 %. On the other hand, electrical conductivity decreased by an order of magnitude. Both changes must be caused by the closure of cracks under pressures. Microstructural examinations showed that most cracks were open grain boundaries. In reality, a crack is composed of many segments with different apertures. If crack segments have a similar length, segments with small apertures are closed at low pressures to greatly reduce conductivity, while those with wide apertures are open even at high pressures. The latter must form an interconnected fluid path to maintain the electrical conduction through fluid. A power law distribution of apertures causes a steep decrease in conductivity at low pressures. An empirical relation between the crack density parameter and normalized conductivity was obtained. The normalized conductivity is the ratio of bulk conductivity to the conductivity of a pore fluid. This relation should be a basis for quantitative interpretation of observed seismic velocity and electrical conductivity.

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Pore-fluid pressure is a critical parameter that governs geodynamic processes including seismic activities. Its evaluation through geophysical observations provides us insights into these processes. The quantitative evaluation requires a thorough understanding of the influence of pore-fluid pressure on geophysical parameters, such as seismic velocity and electrical conductivity. To study the influence of pore-fluid pressure on these parameters, we have built a new apparatus with a pore-fluid pressure control system, which is capable of simultaneously measuring elastic wave velocity and electrical conductivity. Our new apparatus employs two sets of plastic piston-cylinders for the electrical insulation and pore-fluid pressure transmission. The pore fluid is electrically isolated from the metal work, and its pressure can be precisely controlled without significant contribution of the friction between the piston and cylinder. Our new apparatus was used for a simultaneous measurement of velocity and conductivity in a brine-saturated Berea sandstone. Elastic wave velocity and electrical conductivity changed in response to the change in confining and pore-fluid pressures, showing the usefulness of the new apparatus.