Abstract
The present study investigated the evolutions of b-value and fractal dimension of acoustic emission (AE) events during shear rupture of a naturally-created rough fracture in a granite specimen under triaxial compression. Acoustic emission signals were monitored by 16 sensors mounted directly on the surface of the specimen, and AE waveforms were sampled at 16 bits and 25 MHz. Reliable hypocenters were determined using P-wave arrival times picked up from the waveforms. Acoustic emission magnitude was determined from the maximum amplitude monitored by two peak detectors, which have a relative magnitude range of 0 to 2.75. A three-dimensional X-ray computed tomography scan was performed after the test to explore the fracture geometry. Acoustic emission activity was initiated during hydrostatic compression. With increasing differential stress, AEs demonstrated an increasing event rate, a decrease (from approximately 1.8 to 1.6) with a subsequent precursory increase (from 1.6 to 1.8) in fractal dimension, a quick decrease in b-value (from 1.0 to approximately 0.5), and a quick increase in fractal dimension (from 1.8 to 2.0). The exponentially increasing event rate, gradually decreasing b-value, and slowly increasing fractal dimension may be an intermediate-term indication of fault reactivation. In contrast, a progressively increasing event rate, a rapid drop in b-value, and a rapid increase in fractal dimension may facilitate short-term prediction of large events, which reflect the rupture of large patches. Acoustic emission hypocenters were clustered on the entire fracture surface. The present study sheds some light on detecting early signs of fault reactivation by monitoring injection-induced seismicity in areas with faults of different maturity.
Highlights
Injection-induced seismicity is a by-product in several scientific and industrial applications, such as tapping deep geothermal energy through an enhanced geothermal system (EGS), fracking shale gas, enhanced oil recovery (EOR), disposal of waste fluid, and geological storage of CO2
The present paper describes experimental results obtained during shear rupture of a granitic specimen containing a pre-existing natural fracture, which was an analogue of immature faults
The computed tomography (CT) images showed that the involved performed after the test, the 3D fracture surface, and stereo plots of well-determined acoustic emission (AE) hypocenters fracture had a rough surface
Summary
Injection-induced seismicity is a by-product in several scientific and industrial applications, such as tapping deep geothermal energy through an enhanced geothermal system (EGS), fracking shale gas, enhanced oil recovery (EOR), disposal of waste fluid, and geological storage of CO2. At sites without large-scale pre-existing faults, induced seismicity was limited in-zone (stimulated volume) and was mainly controlled by injection parameters, and induced seismicity could be better controlled because seismicity faded out quickly after shut-in. For such cases, as demonstrated by an EGS project, near–real-time seismic monitoring of fluid injection has allowed control of induced earthquakes via a well-designed traffic lighting system [5]. Sci. 2019, 9, 2498 for worse cases encountered in damage events, major seismicity resulted from reactivation of large-scale pre-existing faults having different maturity [6,7]. By integrating seismic b-value into the magnitude-frequency distribution and fractal dimension (or spatial correlation length) in the AE hypocenter distribution, early signs of fault reactivation were examined
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.