Westerly Granite Research Articles

Application of the shear-tensile source model to acoustic emissions in Westerly granite

Traditionally in seismology and in acoustic emission (AE), full moment tensor (MT) is applied as a default model of the mechanism. We present an alternative - an application of shear tensile crack (STC) source model to AEs generated by uniaxial compression loading of Westerly granite. The advantages of STC over the conventional MT are as follows: (i) contrary to the MT, the STC is physical source since it describes straight and simple fracture modes anticipated inside a loaded sample, namely the shear-slip and both of opening and closing tensile cracks; and (ii) the STC is simpler because it is described by fewer parameters (five instead of six required for an unconstrained MT), an essential feature for stabilizing the inverse problem.Better suitability of STC over MT is demonstrated by three exemplary AEs (tensile, shear, and combined). The obtained results were confirmed using a statistical analysis of 1630 reliably determined source mechanisms. The STC, as compared to the MT, provides smaller confidence regions for orientation and even smaller regions for decomposition parameters. Thus, the STC solution appeared to be substantially more useful than the MT, namely for mechanisms with a high content of non-double-couple (non-DC) component, as it allowed better distinction between tensile and shear AEs.Grain scale cracks of tension and combined source type, localized within the middle-height circumferential portion of the specimen, dominated fracturing. Azimuthal distribution of fault planes was found to be approximately uniform for all three source types. The fault dip increased with increasing content of the non-DC component. The average values were 16°, 21°, and 26° for the tensile, combined and shear source types, respectively. The specimen failed by flaking in areas of a high AE activity. AE locations and failure mechanisms indicate a perfect confinement end-boundary conditions between the tested specimen and the loading platens.

Thermo-poroelasticity under constant fluid flux and localized heat source

Coupled thermo-hydraulic-mechanical behavior of saturated porous media with low permeability is of crucial significance to multiple operations including thermal treatment for declogging wellbores, thermal fracturing during production of unconventional shale oil, radioactive-waste disposal, and soft tissue and tumor growth. This work presents new thermo-poroelastic solutions for an isotropic medium subjected to a constant fluid flux and a localized heat source, incorporating two novel components: (a) transient flow transfer between the source and the embedding layer, thus incorporating the temporal thermo-poroelastic changes in the stress state adjacent to the source; and (b) vertical confinement effects on the mechanical response of the target zone, governed by the stiffness of the sealing media using the Winkler model approximation. The Westerly Granite is selected to assess the thermo-poroelastic alterations induced under the imposed fluid flux and heat loads. Results reveal generation of short-term thermal-induced pore pressures subsequent to injection initiation which otherwise cannot be predicted using current solutions. The new solutions capture higher thermal induced pore pressures and a more rapid dissipation under a higher heat transition rate between the source and the target zone. A compassion between the response of the porous layer under constant fluid flux and localized heat source, versus localized heat and flow sources reveals the former loading to be the most effective for localized thermal treatment as it results in: notably higher thermal-induced pore pressures, the maximum thermal-induced pressures to occur in the source vicinity; and particularly slower dissipation of the thermal-induced pore pressures. The stress paths demonstrate slight initial dilation before compaction under the latter loading; while the former results in an initial compaction with higher deviatoric stresses proceeded by dilation once the thermal-induced pore pressures tend to dissipate. Stress paths reveal the vertical confinement to remarkably impact the response of a porous layer. Higher compaction is generated under higher vertical confinement. In case of a mechanically free source-medium interface, lower vertical confinement gives rise to higher deviatoric stresses and lower mean stresses during the initial compaction phase, in addition to causing the formation to undergo dilation much sooner compared to when higher confinement is applied. For a mechanically fixed boundary setting, higher deviatoric stresses are generated under higher vertical confinements.

Numerical analysis of flyer plate experiments in granite via the combined finite–discrete element method

In this study, the combined finite–discrete element method (FDEM), which merges the finite element-based analysis of continua with discrete element-based transient dynamics, contact detection, and contact interaction solutions, is used to simulate the response of a flyer plate impact experiment in a Westerly granite sample that contains a randomized set of cracks. FDEM has demonstrated to be a strongly improved physical model as it can accurately reproduce the velocity interferometer system for any reflector plot and capture the spall region and spall strength obtained from flyer plate experiments in granite. The number and the distributions of preexisting fractures have also been studied to get better understanding of the effect of structural cracks on the mechanical behavior and the failure path of Westerly granite under high strain rate impact. These FDEM capabilities, in the context of rock mechanics, are very important for two main reasons. First, the FDEM can be further applied to many complex large-scale problems such as planetary impact, rock blasting, seismic wave propagation, characterization of material failure around explosive crater formations, and detection of hydrocarbon flow in petroleum industry. Second, it can be used to validate high strain rate impact experiments and essentially, via virtual experimentation, replace these high-cost experiments by very cost- and time-effective simulations.

Spatial Mapping of Fracture Aperture Changes With Shear Displacement Using X‐ray Computerized Tomography

AbstractThe shearing of fractures can be a significant source of permeability change by altering the distribution of void space within the fracture itself. Common methods to estimate the effects of shearing on properties, such as aperture, roughness, and connectivity, are incapable of providing these observations in situ. Laboratory protocols are needed that enable measurements of the spatial structure of the fracture aperture field in the medium, noninvasively. Here, we investigate changes in rough‐walled Brazilian‐induced tensile fracture aperture distribution with progressive shear displacement in Westerly granite and Carrara marble using a novel X‐ray transparent core holder. The so‐called calibration‐free missing attenuation method is applied to reconstruct highly resolved (submillimeter) fracture aperture maps as a function of displacement (0 to 5.75 mm) in induced fractures. We observe that shearing increases the core‐averaged fracture aperture and significantly broadens the distribution of local values, mostly toward higher apertures. These effects are particularly strong in Westerly granite and may be the result of the higher initial roughness of its fracture surfaces. Also, while the correlation length of the aperture field increases in both parallel and perpendicular directions, significant anisotropy is developed in both samples with the progression of shearing. The results on Westerly granite provide a direct indication that fracture aperture remains largely unaffected until 1 mm of displacement is achieved, which is important when estimating permeability enhancement due to natural and induced shear displacement in faults.

The Effect of Dynamic Stress Cycling on the Compressive Strength of Rocks

AbstractQuasi‐static rock strength is a nonconservative property, as fatigue during cyclic loading reduces the macroscopic strength. When strain rate under compressive loading increases above a lithology‐specific threshold, the primary failure mechanism transitions from localized failure along discrete fractures to distributed fracturing. However, the role of load path under high strain rate conditions has not been explored in any detail. We examine the effect of rapid stress cycles on the dynamic compressive strength of Westerly Granite using a modified split Hopkinson pressure bar approach and explore the implications of our results for the formation of pulverized fault zone rocks. Under cyclic loading conditions, the compressive strength can be reduced by a factor of 2, demonstrating that, like the quasi‐static strength, the dynamic rock strength is also a nonconservative property. Therefore, traditional high strain rate experimental approaches utilizing simple load paths may overestimate strength when rocks are subjected to complex load paths.

Investigation of shear slip in hot fractured rock

This paper presents the results of a shear slip experiment in hot rock and its simulation and analysis using 3D coupled thermo-poromechanical model with the aim of assessing the role of pore pressure and temperature change on shear slip in granitic rock. The test is carried out in a triaxial system at representative reservoir pressure and temperature using right circular cylindrical specimens of Westerly granite with a 30° precut and ground surface. After the pore pressure and temperature were stabilized, the differential stress and absolute pore pressure were systematically varied to produce different effective stress states to evaluate the shearing response of the saw-cut fracture. The key objective of the experiment is to test the cooling effect on the fracture slip. The experimental results were then analyzed using a newly developed 3D thermo-poromechanical finite element model which uses traditional 4 node tetrahedron elements for intact rock deformation and transport processes. Zero thickness contact interface element is employed to capture the closure and slip on the fracture surface. The stick/slip state of fracture is determined by the Mohr-Coulomb criterion. Simulation results show that the convective cooling effects on rock/fracture surface promote deformation and slip of the fracture. The cold fluid cools the surrounding rock material and results in normal stress reduction that causes portions of the fracture surface to undergo stick/slip state changes, and induces irreversible slip. The experimental work along with modeling provides a physics-based understanding of the role of coupled processes on shear stimulation phenomenon.

Origin of High‐Frequency Radiation During Laboratory Earthquakes

AbstractWe monitor dynamic rupture propagation during laboratory stick‐slip experiments performed on saw‐cut Westerly granite under upper crustal conditions (10–90 MPa). Spectral analysis of high‐frequency acoustic waveforms provided evidence that energy radiation is enhanced with stress conditions and rupture velocity. Using acoustic recordings band‐pass filtered to 400–800 kHz (7–14 mm wavelength) and high‐pass filtered above 800 kHz, we back projected high‐frequency energy generated during rupture propagation. Our results show that the high‐frequency radiation originates behind the rupture front during propagation and propagates at a speed close to that obtained by our rupture velocity inversion. From scaling arguments, we suggest that the origin of high‐frequency radiation lies in the fast dynamic stress‐drop in the breakdown zone together with off‐fault coseismic damage propagating behind the rupture tip. The application of the back‐projection method at the laboratory scale provides new ways to locally investigate physical mechanisms that control high‐frequency radiation.

The role of fractures, effective pressure and loading on the difference between the static and dynamic Poisson's ratio and Young's modulus of Westerly granite

Fracture in rock is a major factor that affects the rock's elastic properties. Elastic properties can be measured statically during slow loading of a specimen, or dynamically, where the elasticity can be calculated using elastic-wave velocity. However, differences exist between the static and dynamic elastic properties. In this study, the dynamic and static elastic properties were measured under varying effective pressures and differential loads for dry and water-saturated Westerly granite that was thermally treated to 250, 450, 650 and 850 °C. Increasing the temperature produces an increased fracture density that is isotropically distributed. Increased fracture density results in a reduction in the Young's modulus and an increase in the Poisson's ratio, in both the static and dynamic cases, and is very significant for rocks treated above 450 °C. The dynamic Young's modulus is larger than the static value and the difference between them increases when water saturated. Under dry conditions, the static Young's modulus shows good correlation with dynamic values regardless of fracture density, loading or effective pressure. The static and dynamic Poisson's ratio are close to each other under both dry and saturated conditions. However, deviation between the static and dynamic Poisson's ratio occurs when the fracture density is high, differential load is high and effective pressure is low.

Energy delocalization during dynamic rock fragmentation

SUMMARYThe formation of fragments due to avalanche-like growth of damage under impulsive forces is a process central to numerous studies ranging from shaped charge jet break up and rock blasting to bolide impacts, and, more recently, earthquake rupture. In the latter case, pulverized rocks found millimetres to tens of metres from the principal slip zones of large faults have been associated with fast, even supershear, rupture propagation. It has been postulated that earthquake source characteristics directly affect the degree of fragmentation, and the study of fragment size distribution may shed light on the energy budget of individual earthquakes as well as long-term effects on fault zone properties. The actual fragmentation process, and the partitioning of dissipated energy at fast loading rates, however, is still enigmatic. We use modified Split Hopkinson Pressure Bar experiments, in which we can control stressing rate, amplitude and duration, as a laboratory analogue for the complex natural prototype source processes. In our experiments, we characterize the velocity distribution of ejected fragments from Westerly Granite specimens resulting in a range of fragmentation states, from weakly fragmented to pulverized. Analysis of the velocity distributions (and the related kinetic energy) reveals spatial domains that are free of ejected fragments; these so-called ‘zero kinetic energy modes’ are related to the fragmentation state: increasing fragmentation corresponds to a reduction of zero mode domains. The evolution of these zero modes with strain rate reveals that the transition from low strain rate fracturing to high strain rate pulverization is a smooth, continuous transition, rather than a sharp boundary. Furthermore, our results yield important insights into the process of fragmentation in earthquake process zones, including how dissipated energy is partitioned during fragmentation, and indicate that delocalization of energy is systematically coupled with source parameters.

The Role of Shear Stress in Fault Healing and Frictional Aging

AbstractThe seismic cycle involves repeated failure and strength recovery of tectonic faults, known as fault healing. In Earth's crust, where fault strength is dominated by friction, frictional healing is the primary agent of fault healing. Laboratory studies document the role of contact aging and cementation as key mechanisms of frictional healing; however, the role of shear stress is not well understood. We describe a laboratory study of frictional healing as a function of shear stress for bare granite surfaces and simulated fault gouge composed of Westerly granite and quartz. Faults are sheared at normal stresses of 5–25 MPa, 100% relative humidity and room temperature. We quantify frictional healing in slide‐hold‐slide tests for shear stresses from 0 to the sliding friction value and monitor the amplitude of elastic waves propagating across the fault zone. Healing of gouge shows a strong negative dependence on shear stress, whereas healing of bare surfaces does not vary systematically with shear stress. In gouge, frictional healing involves shear stress‐dependent compaction and particle rearrangement, while for bare surfaces frictional strength is derived primarily from contact area evolution. The magnitude of compaction in gouge varies with shear stress, which explains the dependence of healing on shear stress. Elastic wave amplitude increases with hold time and varies inversely with shear stress, consistent with fault zone compaction, shear dilatancy, and their impact on strength evolution and healing. Fault zone compaction during shear unloading and reloading operates with contact aging to dictate frictional healing and fault restrengthening following shear failure.

Friction‐Stability‐Permeability Evolution of a Fracture in Granite

AbstractThe evolution of frictional strength, stability, and fracture permeability is intimately linked both to the seismic cycle and to the impact of hydraulic stimulation for fractured reservoirs. However, despite this importance, the poromechanical relationships between fault permeability and strength remain unclear. The present study explores this relationship via laboratory experiments for concurrent shear‐flow on smooth fractures of Westerly granite. The novelty of these experiments is that the shear velocity is precisely controlled during the measurement of fracture permeability. Results indicate permeability enhancement during velocity‐weakening (potentially unstable) frictional slip. To decipher key processes contributing to this response, we evaluate the state of contacting asperities and of fracture surface asperities via digital rock fracture modeling of statistically equivalent surfaces. We propose two plausible mechanisms constraining the relationship between friction and permeability evolution—one based on changes in asperity contact distribution and one on shear‐induced dilation triggered by changes in fault slip velocity. These mechanisms should be taken into account in interpreting field observation such as the abrupt permeability increase of natural faults at the onset of seismic slip.

Absence of Stress‐Induced Anisotropy During Brittle Deformation in Antigorite Serpentinite

AbstractKnowledge of the seismological signature of serpentinites during deformation is fundamental for interpreting seismic observations in subduction zones, but this has yet to be experimentally constrained. We measured compressional and shear wave velocities during brittle deformation in polycrystalline antigorite, at room temperature and varying confining pressures up to 150 MPa. Ultrasonic velocity measurements, at varying directions to the compression axis, were combined with mechanical measurements of axial and volumetric strain, during direct loading and cyclic loading triaxial deformation tests. An additional deformation experiment was conducted on a specimen of Westerly granite for comparison. At all confining pressures, brittle deformation in antigorite is associated with a spectacular absence of stress‐induced anisotropy and with no noticeable dependence of wave velocities on axial compressive stress, prior to rock failure. The strength of antigorite samples is comparable to that of granite, but the mechanical behavior is elastic up to high stress ( of rock strength) and nondilatant. Microcracking is only observed in antigorite specimens taken to failure and not in those loaded even at 90–95% of their compressive strength. Microcrack damage is extremely localized near the fault and consists of shear microcracks that form exclusively along the cleavage plane of antigorite crystals. Our observations demonstrate that brittle deformation in antigorite occurs entirely by “mode II” shear microcracking. This is all the more remarkable than the preexisting microcrack population in antigorite, is comparable to that in granite. The mechanical behavior and seismic signature of antigorite brittle deformation thus appears to be unique within crystalline rocks.

Coda wave interferometry during the heating of deep geothermal reservoir rocks

Coda wave interferometry (CWI) is a high-resolution technique that aims at tracking small changes in a diffusive medium from the time correlation of seismic waveforms. CWI has been widely used in recent years to monitor the fine-scale evolution of fault zones and more recently of deep reservoirs. However, to provide a quantitative interpretation of the reservoir, direct modeling of physical effects like the influence of temperature on seismic wave scattering is required to investigate temperature effects from measurements of velocity changes. Here, we propose to quantify the impact of thermo-elastic deformation on CWI measurements by comparing experimental results obtained from a previous study on Westerly Granite to a numerical approach based on two combined codes (SPECFEM2D and Code_Aster) for modeling wave propagation in complex media during thermo-elastic deformation. We obtain two major results. First, we show that multiple reflections on the boundaries of our simplified numerical sample reproduce well the wave scattering properties of the experimental granitic sample characterized by a complex mineral assembly and a large set of microcracks. We based our comparison on the wave diffusion model that describes both the experimental and numerical samples (similarity in energy density function and mean free path). We also show that both samples share a similar thermo-elastic behavior, but only after the second heating and cooling cycle. Second, the stretching technique used for CWI measurements on both samples reveals reversible time shifts correlated with the thermo-elastic deformation of the sample. However, the influence of thermo-elastic deformation is different between our numerical proxy and the experimental sample. We discuss the role of irreversible deformation (e.g., microcracking) for the observed discrepancy by introducing temperature dependence of elastic moduli in the model. These results suggest that there are open perspectives to monitor thermal strain in geothermal reservoirs using CWI.

Development and Recovery of Stress‐Induced Elastic Anisotropy During Cyclic Loading Experiment on Westerly Granite

AbstractIn the upper crust, where brittle deformation mechanisms dominate, the development of crack networks subject to anisotropic stress fields generates stress‐induced elastic anisotropy. Here a rock specimen of Westerly granite was submitted to differential stress cycles (i.e., loading and unloading) of increasing amplitudes, up to failure and under upper crustal conditions. Combined records of strains, acoustic emissions, and P and S elastic wave anisotropies demonstrate that increasing differential stress promotes crack opening, sliding, and propagation subparallel to the main compressive stress orientation. However, the significant elastic anisotropies observed during loading (≥20%) almost vanish upon stress removal, demonstrating that in the absence of stress, crack‐related elastic anisotropy remains limited (≤10%). As a consequence, (i) crack‐related elastic anisotropies measured in the crust will likely be a strong function of the level of differential stress, and consequently (ii) continuous monitoring of elastic wave velocity anisotropy along faults could shed light on the mechanism of stress accumulation during interseismic loading.

A Tensile Origin for Fault Rock Pulverization

AbstractThe origin of highly fragmented, but weakly strained rocks found along major strike‐slip faults has been enigmatic since their first recognition. These so‐called pulverized rocks occur up to 100 m away from the principal slip zone of seismogenic faults around the world. Previous dynamic compression experiments have suggested that rock pulverization occurs at strain rates on the order of 102 s−1, pointing to a coseismic origin; however, strain rates during earthquake rupture 100 m from faults is expected to be 4 orders of magnitude smaller. We present evidence from new modified Split‐Hopkinson Pressure Bar experiments that instead supports a tensile origin for coseismic rock pulverization. In the new experimental configuration, the axial compressive load from the Split‐Hopkinson Pressure Bar induces radially isotropic tension in a Westerly Granite disk bonded between two lead cylinders. The isotropic tensile state of stress results in the formation of polygonal fracture arrays that bound axis‐parallel columnar fragments. The tensile strength of Westerly Granite measured at strain rates between ~5 and 50 s−1 bridges the gap between low strain rate and shock strengths reported previously, supporting an interpretation that highly fragmented rocks may form in a state of isotropic tension. The resulting fragment size is independent of strain rate and instead appears to be controlled by elastic strain energy, a strong function of material strength, and fracture toughness. Our results provide a solution to the strain rate‐distance scaling problem between laboratory experiments and field observations of pulverized rocks and also explain the asymmetric distribution of pulverized fault rocks about strike‐slip faults.

Structure of a Deformed Inhomogeneous Material on the Data of Acoustic Emission and X-Ray Computer Microtomography

Accumulation of defects at various stages of the deformation of a structurally inhomogeneous material (granite) has been studied using two nondestructive methods: acoustic emission (AE) and X-ray computer microtomorgaphy (CT). The quasi-static testing of cylindrical samples of a Westerly granite was carried out under uniaxial compression. The control of the defect formation was realized using the real-time monitoring of acoustic emission. For each sample under study, several steps of the loading and tomographic imaging have been performed. We have found that an exponential or power-law function of the energy distribution of the AE signals makes it possible to select a sample region, in which the system of defects has transited into a self-organized criticality state and large cracks have been formed. This result coincides with the data of the X-ray tomography Computer Microtomography.

Elastic potential and pressure dependence of elastic moduli in fluid-saturated rock with double porosity

Models of seismic velocity dispersion and attenuation in porous rock are often based on quantitative relations between empirical moduli by Gassmann, Mavko-Jizba, Sayers-Kachanov, and others. All of these relations have a common origin in the concept of elastic potential; nevertheless, this concept itself has been insufficiently used in the context of multiple porosities and squirt flows. Regardless of the microstructure of a rock, knowledge of its macroscopic elastic potential reveals the complete set of parameters that are necessary and sufficient for characterizing its elasticity and obtaining rigorous equations of deformation. For isotropic rock with double (such as stiff and/or compliant) porosity, this parameter set contains six elements of the bulk-modulus matrix and six moduli for shear. The elastic matrices predict all observable elastic properties, such as bulk, shear and Young’s moduli, pore compressibilities, Skempton coefficients, consolidation parameters, Poisson’s ratios, static (Gassmann’s) and ultrasonic (Mavko-Jizba’s) undrained moduli, as well as the hypothetical high-pressure modulus [Formula: see text] and unrelaxed-frame moduli. By using laboratory observations with Westerly granite, all six elements of the elastic matrix for bulk deformation are inverted for exactly; three moduli for shear are determined with uncertainties due to insufficient data. The inverted matrices of the elastic moduli predict all observations (drained and low- and high-frequency undrained moduli) within measurement accuracy, with minor uncertainties for shear deformation. The secondary pores are approximately 50 times more compressible than are the primary ones, which explains their interpretation as “soft.” Most of the inverted elastic moduli vary with the confining pressure. Despite its usually assumed constancy, the high-pressure modulus [Formula: see text] exhibits an approximately 10% decrease with confining pressure from 0 to 100 MPa, which is opposite of the trend of the solid-grain modulus [Formula: see text].

Three-dimensional strength estimation of intact rocks using a modified Hoek-Brown criterion based on a new deviatoric function

Hoek-Brown criterion has been applied widely in a large number of rock projects around the world and has the main advantage that its input data can be determined from the uniaxial compressive strength, mineralogy and structural properties of the rock. However, there also exist demerits that neglect the effects of the intermediate principal stress and represent a non-smoothness shape in the deviatoric plane. After reviewing the history of the development of the Hoek-Brown criterion briefly, a modified Hoek-Brown criterion was proposed by the combination of a versatile deviatoric function and a meridian function of the original Hoek-Brown criterion. The new deviatoric function, which was proposed by introducing an extra strength parameter α into the one of Matsuoka-Nakai (MN) criterion and behaviors as well as the one proposed by Bigoni and Piccolroaz, is a generalization of deviatoric shapes belonging to several classic strength criteria, i.e. Drucker-Prager (DP), Tresca, Mohr-Coulomb (MC), Lade-Duncan (LD), Matsuoka-Nakai (MN) and Ottosen strength criterion. The procedure for determination of the strength parameters was demonstrated in detail. Comparisons between the derived criterion and true triaxial test data of KTB amphibolite, Westerly granite, Dunham dolomite, Mizuho Trachyte were presented in three-dimensional principal stress space qualitatively and quantitatively. Overall predicted errors were also compared with the ones obtained by the generalized Zhang-Zhu criterion (Z-Z) as well as its modifications (S-D, E-D, H-D), which showed least error for each rock type by the modification in this paper. The effect of the intermediate principal stress was also discussed for these four rock types, confirming that the intermediate principal stress is included in the strength criterion. The derived criterion not only maintains the features in the meridian plane of the original Hoek-Brown criterion, but solves the non-smoothness problem in the deviatoric plane and shows an excellent performance for describing brittle and ductile rock types considered.

Thermal Cracking in Westerly Granite Monitored Using Direct Wave Velocity, Coda Wave Interferometry, and Acoustic Emissions

AbstractTo monitor both the permanent (thermal microcracking) and the nonpermanent (thermo‐elastic) effects of temperature on Westerly Granite, we combine acoustic emission monitoring and ultrasonic velocity measurements at ambient pressure during three heating and cooling cycles to a maximum temperature of 450°C. For the velocity measurements we use both P wave direct traveltime and coda wave interferometry techniques, the latter being more sensitive to changes in S wave velocity. During the first cycle, we observe a high acoustic emission rate and large—and mostly permanent—apparent reductions in velocity with temperature (P wave velocity is reduced by 50% of the initial value at 450°C, and 40% upon cooling). Our measurements are indicative of extensive thermal microcracking during the first cycle, predominantly during the heating phase. During the second cycle we observe further—but reduced—microcracking, and less still during the third cycle, where the apparent decrease in velocity with temperature is near reversible (at 450°C, the P wave velocity is decreased by roughly 10% of the initial velocity). Our results, relevant for thermally dynamic environments such as geothermal reservoirs, highlight the value of performing measurements of rock properties under in situ temperature conditions.

Defect Structure of Deformed Heterogeneous Materials: Acoustic Emission and X-Ray Microtomography

Laboratory investigations of deformation of heterogeneous brittle materials (rocks) by acoustic emission (AE) and X-ray computer microtomography (CT) are presented. The experiments involved loading of cylindrical samples of Westerly granite under the condition of uniaxial compression and recording of AE signals emitted during deformation of samples at different loading-unloading stages. After each unloading of the sample a tomographic survey was performed. Totally, 11 loading-unloading stages and tomographic surveys were carried out. The defect structure evolution is considered in the framework of the concept of self-organized criticality. It has been found that the type of the energy distribution function of AE signals can be used as an indicator of the deformed material state and transition to a critical fracture stage. An exponential function points to a noncritical state of a deformed material, and a power-law function indicates that the defect accumulation has passed to a critical stage. This result is confirmed by X-ray microtomographic data.