R. Paul Young

Micromechanical modeling of cracking and failure in brittle rocks

Dynamic micromechanical models are used to analyze crack nucleation and propagation in brittle rock. Models of rock are created by bonding together thousands of individual particles at points of contact. The feasibility of using these bonded particle models to reproduce rock mechanical behavior is explored by comparing model behavior to results from actual laboratory tests on different rock types. The behavior of two granite models are examined in detail to study cracking and failure patterns that occur during compressional loading. Because discontinuum models are being used, the rock models are free to crack and break apart under stress, such that the micromechanics of cracking can be examined. Stress waves are allowed to propagate outward from each crack, and it is shown that these dynamic waves significantly affect the rock behavior. As the peak stress in the modeled rock is approached and many of the bonds are close to breaking, a passing wave from a nearby crack is sufficient to break more bonds. This causes clusters of cracks to be created, and then eventual macroscopic shear failure occurs as these clusters connect to bisect the sample. The failure patterns observed in the granite models are similar to those observed in actual laboratory tests.

Laboratory simulation of volcano seismicity

The physical processes generating seismicity within volcanic edifices are highly complex and not fully understood. We report results from a laboratory experiment in which basalt from Mount Etna volcano (Italy) was deformed and fractured. The experiment was monitored with an array of transducers around the sample to permit full-waveform capture, location, and analysis of microseismic events. Rapid post-failure decompression of the water-filled pore volume and damage zone triggered many low-frequency events, analogous to volcanic long-period seismicity. The low frequencies were associated with pore fluid decompression and were located in the damage zone in the fractured sample; these events exhibited a weak component of shear (double-couple) slip, consistent with fluid-driven events occurring beneath active volcanoes.

Imaging slow failure in triaxially deformed Etna basalt using 3D acoustic-emission location and X-ray computed tomography

We have deformed basalt from Mount Etna (Italy) in triaxial compression tests under an effective confining pressure representative of conditions under a volcanic edifice (40 MPa), and at a constant strain rate of 5 similar to 10(-6) s(-1). Despite containing a high level of pre-existing microcrack damage, Etna basalt retains a high strength of 475 MPa. We have monitored the complete deformation cycle through contemporaneous measurements of axial strain, pore volume change, compressional wave velocity change and acoustic emission (AE) output. We have been able to follow the complete evolution of the throughgoing shear fault without recourse to any artificial means of slowing the deformation. Locations of AE events over time yields an estimate of the fault propagation velocity of between 2 and 4 mm. s(-1). We also find excellent agreement between AE locations and post-test images from X-ray microtomography scanning that delineates deformation zone architecture.

Laboratory simulations of fluid/gas induced micro-earthquakes: application to volcano seismology

Understanding different seismic signals recorded in active volcanic regions allows geoscientists to derive insight into the processes that generate them. A key type is known as Low Frequency or Long Period (LP) event, generally understood to be generated by different fluid types resonating in cracks and faults. The physical mechanisms of these signals have been linked to either resonance/turbulence within fluids, or as a result of fluids ‘sloshing’ due to a mixture of gas and fluid being present in the system. Less well understood, however, is the effect of the fluid type (phase) on the measured signal. To explore this, we designed an experiment in which we generated a precisely controlled liquid to gas transition in a closed system by inducing rapid decompression of fluid-filled fault zones in a sample of basalt from Mt. Etna Volcano, Italy. We find that fluid phase transition is accompanied by a marked frequency shift in the accompanying microseismic dataset that can be compared to volcano seismic data. Moreover, our induced seismic activity occurs at pressure conditions equivalent to hydrostatic depths of 200 to 750 meters. This is consistent with recently measured dominant frequencies of LP events and with numerous models.

Imaging compaction band propagation in Diemelstadt sandstone using acoustic emission locations

We report results from a conventional triaxial test performed on a specimen of Diemelstadt sandstone under an effective confining pressure of 110 MPa; a value sufficient to induce compaction bands. The maximum principal stress was applied normal to the visible bedding so that compaction bands propagated parallel to bedding. The spatio-temporal distribution of acoustic emission events greater than 40 dB in amplitude, and associated with the propagation of the first compaction band, were located in 3D, to within +/- 2 mm, using a Hyperion Giga-RAM recorder. Event magnitudes were used to calculate the seismic b- value at intervals during band growth. Results show that compaction bands nucleate at the specimen edge and propagate across the sample at approximately 0.08 mm s(-1). The seismic b-value does not vary significantly during deformation, suggesting that compaction band growth is characterized by small scale cracking that does not change significantly in scale.

Lithological Controls on Seismicity in Granitic Rocks

Seismicity induced from a tunnel excavation through two lithological units, granite and granodiorite, at the Canadian Underground Research Laboratory (URL) is analyzed in an attempt to understand observed lithological differences in the damage-zone development. The results from seismicity recorded by the 16 triaxial accelerometer array found the damage zone around granite excavations to have more events occurring ahead of the tunnel face and a shorter overall seismic response time than the granodiorite. Petrographic analysis of the rock samples show stress relief microcracking predominantly in the larger quartz crystals, suggesting that these are the weakest mineral grains. We propose that initial in situ cracking occurs in the large quartz crystals, significantly reducing the strength and resulting in rapid formation of the excavation damage zone. The smaller-grained, more homogenous granodiorite shows less stress relief microcracking, probably due to the stresses being distributed over a larger number of grain boundaries. From the seismic and petrographic evidence we propose that the crack initiation stress is lower in the granite than the granodiorite. The events from the granite and granodiorite have a similar range in magnitude ( M w = –2.9 to –4.2) and source dimension ( r = 0.13–0.51 m). They have a mean P - to S -wave corner frequency ratio of 1.0, probably indicating relatively slow rupture velocities. About 25% of the events have an S - to P -wave energy ratio less than 10, agreeing with previous source mechanism studies, which find a number of events at this depth have significant isotropic components.

The Assessment of Damage Around Critical Engineering Structures Using Induced Seismicity and Ultrasonic Techniques

Two large-diameter boreholes have been excavated vertically from the floor of a tunnel at the Aspo Hard Rock Laboratory, Sweden. The two deposition holes will have simulated high-level radioactive waste canisters installed in them in an experiment undertaken to test the retrievability of waste from a proposed repository. Induced seismicity and other acoustic monitoring techniques have been used to investigate the Excavation Damaged Zone (EDZ) around the two holes. High-frequency acoustic emission (AE) monitoring has been used to delineate regions of stress-induced microfracturing on the millimetre scale. This has been shown to locate in clusters around the perimeter of the deposition hole at azimuths orthogonal to the far-field maximum principal stress. Three-dimensional velocity surveys have been conducted along ray paths that pass through the damaged region and through a stress-disturbed zone around the excavation. Induced microfracturing and stress disturbance have been observed as sharp decreases in velocity as the excavation proceeds through the rock mass. The combination of the high-resolution velocity measurements and the AE source locations has allowed the linking of the velocity measurements to a volume of excavation damaged rock. This has provided a quantitative estimate of the effect of the EDZ on the rock mass.

Using Continuous Microseismic Records for Hydrofracture Diagnostics and Mechanics

Summary Using continuous microseismic records is a novel technique for better understanding the mechanics of the fracture network evolution during a hydrofracture treatment, and to provide a tool for diagnostic evaluation of recorded microseismic data. Hydrofracture stimulations are widely used during well completions to optimize production volumes and extraction rates in petroleum reservoirs, enhanced geothermal systems and block-caving mines. Microseismic monitoring is now becoming a standard tool for evaluating the position and evolution of a given treatment, principally by source locating microseismic hypocenters and visualizing these with respect to the treatment volume and infrastructure. The continuous microseismic amplitude record includes the full history of the seismic energy response of the rock mass recorded at a given geophone. We present case studies illustrating the use of this technique for supplementing microseismic locations to better understand the evolution of the fracture treatment, and to diagnose the condition of a given data set, so as to design criteria for more effective processing of the discrete microseismic events.

Modelling Seismic Waves Around Underground Openings in Fractured Rock

The potential for large excavation-induced seismic events may be recognised, even if the timing of an event may be inherently unpredictable. In this case, modelling the wave propagation from a potential event could allow the dynamic motions around an excavation to be projected, and for areas of danger to be anticipated. However, the above and other potential applications require accurate models of wave interaction with the openings, as well as with the fractured rock which surrounds such excavations. This paper considers real recorded waveforms and how well these waveforms are modelled by explicit mechanical models of the source, the medium and the excavation. Models of experiments at three different scales of the problem are presented: small and large amplitude waveforms recorded around a deeplevel mining tunnel in a synthetic rockburst experiment; waveforms from laboratory experiments of waves through plates of steel representing fractures; waveforms from active pulses in an acoustic emission experiment in a small volume of fractured rock at the surface of an underground excavation. The results show that elastic wave propagation around an excavation was a first approximation for small amplitude waves, but was less successful for modelling large amplitude waves and more fractured rock. Fractures in the models were represented explicitly with displacement discontinuities. Waveforms through known fracture geometries were particularly well-reproduced, and indicate the importance of fracture stiffness, the in situ stress state, and stress-dependence of the fractures in such models. Overall, the models are sufficiently successful at representing recorded behaviour, to be encouraging for the goal of representing accurate wave motions around excavations.

Fracture network engineering for hydraulic fracturing

Fracture network engineering (FNE) involves the design, analysis, modeling, and monitoring of infield activities aimed at enhancing or minimizing rock mass disturbance. FNE relies specifically on advanced techniques to model fractured rock masses and correlate microseismic (MS) field observations with simulated microseismicity generated from these models. Hydrofracture stimulation is an example where FNE is playing a role, with hydraulic treatments now being widely used to optimize production volumes and extraction rates in petroleum reservoirs, enhanced geothermal systems, and preconditioning operations in caving mines. MS monitoring is now becoming a standard tool for evaluating the geometry and evolution of the fracture network induced during a given treatment, principally by source locating MS hypocenters and visualizing these with respect to the treatment volume and infrastructure. The integrated use of synthetic rock mass (SRM) modeling of the hydrofracturing with enhanced microseismic analysis (EMA...