Frequency characteristics of acoustic emission in thermally cracked granite

Abstract:

Fluid flow-induced seismicity has been observed in various engineering fields such as enhanced geothermal system (EGS). Especially, in the Pohang EGS project, the Mw 5.4 earthquake was likely induced by fluid injection after the events in the entire period of stimulations (Kim et al., 2018). To understand the processes of induced earthquakes due to fluid injection, it is necessary to research the propagation process of pre-existing cracks in thermally damaged granites. The acoustic emission (AE) is defined as elastic waves released by rapid cracking, which is a useful tool for investigating rock fractures. The frequency of AE signals from fracturing provides information about internal structure of rock. In this study, we measure AE and P-wave velocity of thermally cracked granite during triaxial deformation under dry and fluid-saturated conditions.

We conduct a triaxial compression test at confining pressure of 22.5 MPa and strain rate of 10^-6 s^-1 under dry and wet conditions using the loading system at AIST. Permeability is measured by a fluid flow method using water as a pore fluid, in which a constant upstream pore pressure was maintained at 4 MPa. Cylindrical coarse-grained Inada granite (50 mm in diameter and 125 mm in length), which are thermally damaged at 550 ℃, are used as samples. We measure AEs with 28 piezoelectric transducers with variable resonance frequencies of 0.25 - 2 MHz.

Our result shows that a wet sample deforms elastically until at 234 MPa, and then shows unelastic behavior. After the peak stress of 439 MPa, post-yield behavior of the sample indicates brittle failure with drastic stress drop. AE activity was initiated at 69 % of the peak stress and increased rapidly after reaching the peak stress. Frequency analysis based on the fast Fourier transform shows that three dominant frequency bands of AE waveforms are revealed during deformation (100, 300 and 450kHz). Ohnaka and Mogi (1982) has shown that low-frequency events were dominant as rock approached failure during compression. In our experiments, no clear variation in frequency band was found from the results of this study. This is most likely due to limited counts of AEs at the initial stage of deformation, and we are going to improve to monitor AEs during a series of compression tests. In the poster, we will present results of hydraulic fracturing experiments.