The closure of microcracks under pressure: inference from elastic wave velocity and electrical conductivity in granitic rocks

Abstract Measurements of elastic wave velocity and electrical conductivity were conducted on brine-saturated granitic rocks under confining pressure to quantitatively characterize the closure of cracks. The number of contacting asperities and radius ratio of contact over crack were estimated. Contacting asperities were assumed to deform elastically and/or plastically. The number of contacts increases steeply at low pressure (< 10 MPa) and decreases gradually at higher pressure, while the radius ratio of contact over crack linearly increases. Though the area fraction of contacts increases to no more than 30% even at the highest confining pressure (150 MPa), the stiffness of the solid phase is recovered, and the connectivity of fluid is maintained in a crack. Cracks under pressure are closed for elastic properties, but open for transport properties. The effective aperture is of the order of 100 nm at atmospheric pressure, and steeply decreases at low pressure (< 10 MPa) and then gradually at higher pressure. Observed seismic velocity and electrical conductivity in the crust should be interpreted by considering contacts in cracks. Microstructures of closing cracks should be further investigated to seek relationships between structural parameters and effective pressure. Graphical abstract