[S-CG55] Dynamics in mobile belts

[SCG55-07] Structure of microcracks under pressure: inferred from elastic wave velocity and electrical conductivity

*Tohru Watanabe¹ (1.Faculty of Sustainable Design, University of Toyama)

Keywords:seismic velocity, electrical conductivity, microcrack, fluid

Aqueous fluids must play important roles in geodynamic processes including seismic activity. Geophysical mapping of fluids will give us insights into these processes. Interpretation of geophysical observations requires a thorough understanding of the structure of cracks under pressure that governs seismic velocity and electrical conductivity. Combining the effective medium theory for cracked solid (e.g., Sayers and Kachanov, 1995) with a theory for the reduction of crack compliance due to contacting asperities (Glubokovskikh et al., 2016), we evaluate the number of contacting asperities in a crack and the radius ratio of contact over crack from elastic wave velocities measured in granitic rocks under pressure. Based on the experiments of Dietrich and Kilgore (1994), we assumed that the contact stress equals to the indentation yield stress. Hundreds of contacting asperities are formed in a crack and the radius ratio is of the order of 10^-3. The number of contacts slightly decreases with increasing pressure, while the radius ratio increases linearly. The area fraction of contacts in a crack is only ~1% at the highest pressure (180 MPa). The connectivity of fluid in a crack is thus hardly affected by contacting asperities. We then evaluated the average aperture of cracks from electrical conductivity measured in brine-saturated rock samples by employing the crack network model proposed by Gueguen and Dienes (1989). The average aperture is estimated to be of the order of 0.1 micrometers at atmospheric pressure and steeply decreases at low pressure (<10 MPa) and then gradually to of the order of 1 nm at the highest pressure (150 MPa). The estimated apertures at atmospheric pressure are consistent with the aperture distribution evaluated through the mercury intrusion method. The structure of microcracks under pressure was successfully evaluated by considering the contacting asperities in cracks. The effective medium theory combined with a model for the reduction of crack compliance due to contacting asperities can be used in the interpretation of observed seismic velocity and electrical conductivity to infer the fluid distribution.