Experimental results of b-value for AE events of thermally cracked granite during triaxial compression test

Fluid flow-induced seismicity has been observed in various engineering fields such as geothermal system (e.g., Grigoli et al., 2018). Ghassemi and Zhang (2006) showed that initiation and propagation of thermal fractures in rocks were induced by cooling by injection of a fluid into rock mass based on a fully coupled model of a uniformly pressurized and cooled crack. To understand the generation processes of induced earthquakes due to fluid injection, it is necessary to research the propagation process of pre-existing cracks in thermally damaged granites. Spatio-temporal distribution of AEs and estimated b value can infer the distribution and density of cracks, which could be influence to the mechanical and hydrological properties on rocks. In this study, we measure acoustic emission (AE) and P-wave velocity during triaxial deformation of thermally cracked granite.

We conduct a triaxial compression test on granite at confining pressure of 22.5 MPa and strain rate of 10^-5 1/s under dry condition using the loading system at GSJ/AIST (Lei et al., 2011). Cylindrical intermediate-grained Oshima granite (50 mm in diameter and 125 mm in length) which is thermally damaged at 550 ℃ is used as a sample. We measure AEs with twenty-eight piezoelectric transducers (PZT) and strains with six strain gauges.

A preliminary result shows that a sample deforms elastically until at 384 MPa. After the peak stress of 438 MPa, post-yield behavior of the sample indicates brittle deformation with drastic stress drop. AE activity was initiated at around 80 % of the peak stress, and increased rapidly after reaching the peak stress. Initiation and growth of cracks inferred from AE events is consistent with the damaging process proposed by Lei (2006). However, the initial b-value in subcritical crack growth phase (called secondary phase) is larger than that of intact Oshima granite (Lei, 2006), and rapidly decreases with increase in differential stress. The initial b-value in secondary phase depends on the density and size of pre-existing cracks, and reduction in b-value is due to increasing in the crack length (Lei, 2006). The rapid decreasing in b-value indicates the subcritical growth of thermal cracks. Therefore, the number of growing cracks under differential stress can be inferred from the decrease trend of b-value during triaxial deformation.