5:15 PM - 6:30 PM
[SCG46-P04] Frequency characteristics of acoustic emission in the fracture process of thermally cracked granite
Keywords:acoustic emission, frequency, triaxial compression
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 generation and propagation of 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 triaxial compression tests 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 intact and thermally damaged at 550 ℃, are used as samples. We measure AEs with 28 piezoelectric transducers (PZTs) with variable resonance frequencies of 0.25 - 2 MHz. In the frequency spectrum obtained from the fast Fourier transform, the peak amplitude is defined as the largest amplitude in the spectrum, and the peak frequency is defined as the frequency at the peak amplitude.
Our result shows that wet samples behave elastically until the onset of dilatancy ranging from 27.1 to 32.7 % of the peak stress, and then shows inelastic behavior. After the peak stress of 357-413 MPa, post-yield behavior of the sample indicates brittle failure with drastic stress drop. AE activity was initiated at the crack damage stress ranging from 51.8 to 62.6 % of the peak stress. Frequency analysis based on the fast Fourier transform shows that the peak frequency was mainly distributed in the range of 0 to 0.6 MHz. Chen et al. (2020) has shown that low peak frequency signal with high peak amplitude normalized by the maximum value in the test (LH signal) reflected the formation of macrocracks. In our results of PZTs with resonant frequency of 1 and 0.5 MHz, LH signals were only recorded after 99 % of the peak stress for the thermally cracked sample, while LH signal began to appear after approximately 80 % of the peak stress for the intact sample. A comparison of the results indicates that stress-induced crack growth in the thermally cracked sample was not detected due to the large scale of thermal cracks.
We conduct triaxial compression tests 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 intact and thermally damaged at 550 ℃, are used as samples. We measure AEs with 28 piezoelectric transducers (PZTs) with variable resonance frequencies of 0.25 - 2 MHz. In the frequency spectrum obtained from the fast Fourier transform, the peak amplitude is defined as the largest amplitude in the spectrum, and the peak frequency is defined as the frequency at the peak amplitude.
Our result shows that wet samples behave elastically until the onset of dilatancy ranging from 27.1 to 32.7 % of the peak stress, and then shows inelastic behavior. After the peak stress of 357-413 MPa, post-yield behavior of the sample indicates brittle failure with drastic stress drop. AE activity was initiated at the crack damage stress ranging from 51.8 to 62.6 % of the peak stress. Frequency analysis based on the fast Fourier transform shows that the peak frequency was mainly distributed in the range of 0 to 0.6 MHz. Chen et al. (2020) has shown that low peak frequency signal with high peak amplitude normalized by the maximum value in the test (LH signal) reflected the formation of macrocracks. In our results of PZTs with resonant frequency of 1 and 0.5 MHz, LH signals were only recorded after 99 % of the peak stress for the thermally cracked sample, while LH signal began to appear after approximately 80 % of the peak stress for the intact sample. A comparison of the results indicates that stress-induced crack growth in the thermally cracked sample was not detected due to the large scale of thermal cracks.