Abstract
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.
Highlights
Geophysical mapping of fluids is critical for understanding geodynamic processes including seismic activities
Based on the empirical relation between electrical conductivity and crack density parameter, we propose a method for the combined interpretation of seismic velocity and electrical resistivity
Measurements were made during the increase in confining pressure
Summary
Geophysical mapping of fluids is critical for understanding geodynamic processes including seismic activities. Seismic velocity and electrical resistivity structures have been constructed to study the fluid distribution in the continental crust. Observations on seismic velocity and electrical resistivity should be combined to make a quantitative inference on fluid distribution. It is impossible to infer the amount of fluid only from observed seismic velocity. Seismic velocity of a fluid-bearing rock depends on the elastic properties of the solid and fluid phases and the geometry and amount of the fluid (e.g., Takei 2002). Even if we know the elastic property and geometry of the fluid, we cannot estimate the amount of fluid. Since the lithology of a study region is usually unknown, elastic properties of the rock matrix must be assumed.
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