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The purpose of this ISRM Suggested Method is to describe a methodology for in situ acoustic emission monitoring of the rock mass fracturing processes occurring as a result of excavations for tunnels, large caverns in the fields of civil, rock slopes and mining engineering, etc. In this Suggested Method, the equipment that is required for an acoustic emission monitoring system is described; the procedures are outlined and illustrated, together with the methods for data acquisition and processing for improving the monitoring results. There is an explanation of the methods for presenting and interpreting the results, and recommendations are supported by several examples.

The Canadian Nuclear Safety Commission (CNSC), Canada's nuclear regulator, conducts regulatory research in order to develop independent knowledge on safety aspects related to the deep geological disposal of radioactive wastes. In Canada, the Cobourg limestone of the Michigan Basin is currently considered as a potential host formation for geological disposal. The understanding of the hydro-mechanical behavior of such a host rock is one of the essential requirements for the assessment of its performance as a barrier against radionuclide migration. The excavation of galleries and shafts of a deep geological repository (DGR) can induce damage to the surrounding rock. The excavation damaged zone (EDZ) has higher permeability and reduced strength compared to the undisturbed rock and those factors must be considered in the design and safety assessment of the DGR. The extent and characteristics of the EDZ depend on the size of the opening, the rock type and its properties, and the in situ stresses, among other factors. In addition, the extent and characteristics of the EDZ can change with time due to rock strength degradation, evolution of fractures within the EDZ, and the redistribution of pore pressure around the excavation. In this research project initiated by the CNSC, the authors conducted experimental and theoretical research in order to assess the hydro-mechanical behavior of the Cobourg limestone under undamaged and damaged conditions, both in the short and long terms. The short-term behavior was investigated by a program of triaxial tests with the measurement of permeability evolution on specimens of Cobourg limestone. The authors formulate a coupled hydro-mechanical model to simulate the stress-strain response and evolution of the permeability during those triaxial tests. Using creep and relaxation data from a similar limestone, the model was extended to include its long-term strength degradation. The model successfully simulated both the short- and long-term hydro-mechanical behavior of the limestone during those tests. This provides confidence that the main physical processes have been adequately understood and formulated. Keywords: Hydro-mechanical behavior, Excavation damage, Deep geological repository (DGR), Limestone, Poro-elasto-plasticity

Microseismic monitoring, the engineering application of earthquake seismology, provides an insight into the location and extent of fracturing induced by the caving process. The spatial characteristics of the observed microseismicity provide valuable validation for the back analysis of the cave process through numerical models (e.g. Pierce et al. 2006, Sainsbury et al., 2008). It also provides a tool for the validation of predictive cave models that aim to forecast the extent and location of the seismogenic zone and the evolution of damaged rock mass properties. A case study is presented using Northparkes Mine microseismic (MS) events located by forwardcalculating Pand S-wave travel times using ray-tracing and wavefront construction techniques through velocity structures that mimic heterogeneity and damage in the rock. By using these models it is expected to get a better resolution of the extent, position and structure of the seismogenic zone. Conversely, the location of high-quality events provides a means to validate the fit of strain models.

Olkiluoto is an island that was selected as the site for a final nuclear waste disposal facility in Finland. ONKALO underground research facility was constructed for the purpose of rock characterization. A hybrid stress path, incorporating the uncertainties of the near-field stress state estimated for the different Posiva Olkiluoto Spalling Experiment (POSE) phases, was developed as the basis for boundary conditions for a series of true-triaxial tests. Evolution of each principal stress component as a result of excavations and heating of the experimental holes to understand the phenomenon of excavation damage zone (EDZ) development was evaluated. The analysis of the temporal and spatial acoustic emission (AE) activity, construction of 3D damage fracture network images from X-ray CT, and thin section studies performed on tested specimens are presented here. AE events are used to evaluate the chronological occurrence of the damage fracture networks within the induced fractures preferably initiating and propagating within the foliation zones characterized by mica grains, and predominantly avoiding quartz and feldspar grains. Veined gneiss (VGN) samples with a 30° foliation angle relative to the maximum loading direction, and the minimum loading direction perpendicular to the strike of foliation, exhibited the lowest strength, showing shear type fracture initiating parallel to the foliation planes at 55 MPa of axial stress. This value corresponds to the stress state at the POSE experiment hole wall when the first shear type damage was observed to be parallel to the foliation planes. Comparison of the evolution of AE events and seismic velocity measured at the field site for granitic pegmatite (PGR) with its counterpart laboratory specimen was found to be in good agreement. PGR is the strongest member of the rock mass and shows significant AE activity for specimens subjected to the simulated in situ loading without development of macroscale fractures.

Active and passive ultrasonic methods were used to study the evolution of attenuation properties in a sample of Fontainebleau sandstone during true-triaxial deformation. A cubic sample of Fontainebleau sandstone (80 mm $$\times$$ 80 mm $$\times$$ 80 mm) was deformed under true-triaxial stresses until failure. From the stress state: $$\sigma _3 = 5$$ MPa and $$\sigma _1 = \sigma _2 = 35$$ MPa, $$\sigma _1$$ was increased at a constant displacement rate until the specimen failed. Acoustic emission (AE) activity was monitored by 18 piezoelectric sensors and bandpass filtered between 100 kHz and 1 MHz. A source location analysis was performed on discrete AE data harvested from the continuous record where 48,502 events were locatable inside the sample volume. AE sensors were sequentially pulsed during periodic P-wave surveys among 135 raypaths. Analytical solutions for Biot, squirt flow, viscous shear, and scattering attenuation were used to discuss to observed attenuation at various stages of the experiment. We concluded that initial attenuation anisotropy was stress induced and resulted from friction and squirt flow. Later attenuation of the high-frequency spectrum was attributed to scattering as a result of the formation of large macroscopic vertical fractures. Passive (AE) ultrasonic data produced similar information to that from active data but with enhanced temporal and spacial resolution.

In this paper we present results from a series of laboratory hydraulic fracture experiments designed to investigate various components of the energy budget. The experiments involved a cylindrical sample of Westerly granite being deformed under various triaxial stress states and fractured with distilled water, which was injected at a range of constant rates. Acoustic emission sensors were absolutely calibrated, and the radiated seismic energy was estimated. The seismic energy was found to range from 7.02E−8% to 1.24E−4% of the injection energy which is consistent with a range of values for induced seismicity from field-scale hydraulic fracture operations. The deformation energy (crack opening) of the sample during hydraulic fracture propagation was measured using displacement sensors and ranged from 18% to 94% of the injection energy. Our results support the conclusion that aseismic deformation is a significant term in the hydraulic fracture energy budget.

Fontainebleau sandstone specimens have been tested in a true-triaxial geophysical imaging cell under hydrostatic (σ1=σ2=σ3), traditional triaxial (σ1=σ2>σ3) and true-triaxial (σ1> σ2>σ3) stress regimes. The research objectives are to recognize the effect of various stress ratios σ2/σ3 under varying magnitudes of σ2 and σ3 on 3-D stress–strain responses, 3-D transport, 3-D ultrasonic wave velocities, and acoustic emission properties. Comparison of the variation of 3-D directional permeability between the specimens shows that higher magnitude of σ3 and σ2 disrupts the 3-D interconnectivity of the pore spaces and intergranular cracks, irrespective of the magnitude of applied stress ratio σ2/σ3. The observed permeability anisotropy between the two specimens is a function of inherent oriented pores and cracks, stress ratio σ2/σ3 and the magnitude of applied σ3 and σ2 stresses. Temporal and spatial evolution of the AE hypocenter locations combined with the examination of micro-CT and thin section images shows the timely development of polymodal fault systems during peak to post-peak stresses and validates the development of the source mechanism inferred from the acoustic emission events.

Understanding the physics of acoustic excitations emitted during the cracking of materials is one of the long-standing challenges for material scientists and geophysicists. In this study, we report novel results of applications of functional complex networks on acoustic emission waveforms emitted during the evolution of frictional interfaces. Our results show that laboratory faults at microscopic scales undergo a sequence of generic phases, including strengthening, weakening or fast slip and slow slip, leading to healing. For the first time we develop a formulation on the dissipated energy due to acoustic emission signals in terms of short-term and long-term features (i.e., networks' characteristics) of events. We illuminate the transition from regular to slow ruptures. We show that this transition can lead to the onset of the critical rupture class similar to the direct observations of this phenomenon in the transparent samples. Furthermore, we demonstrate the detailed submicron evolution of the interface due to the short-term evolution of the rupture tip. As another novel result, we find that the nucleation phase of most amplified events follows a nearly constant timescale, corresponding to the initial strengthening or locking of the interface. This likely indicates that a thermally activated process can play a crucial role near the moving crack tip.

In this paper, we revisit acoustic emission (AE) data from an in situ rock fracture experiment conducted at the Underground Research Laboratory (URL) in Manitoba, Canada. The Mine-By experiment, a large-scale excavation response test, was undertaken at a depth of 420 m and involved the mechanical excavation of a cylindrical tunnel. During the experiment a small array of 16 Panametrics V103 AE sensors enclosed a 0.7 m × 0.7 m × 1.1 m rectangular prism of Lac du Bonnet granite located in the tunnel wall. The V103 sensors were later calibrated in the laboratory, and a source parameter analysis was undertaken using a spectral fitting method. Corner frequency and moment magnitude were found to be inside the ranges 250 kHz

The results of hydro-mechanical and thermo-hydro-mechanical experiments on Cobourg limestone are presented here. Experiments were performed within geophysically instrumented Hoek-type cell under hydrostatic and deviatoric stresses, controlled at ambient temperature (25 °C) and elevated in-situ temperatures up to 150 °C. Variations and differences in the evolution of VP and VS during various stages of each experiment correspond to the heterogeneous and anisotropic nature of tested specimens. VS is influenced by micro-thermal damages caused by the heating stage more than VP. Whereas the latter proves to be more sensitive to the progressive fracture growth leading to rock failure during deviatoric loading stage. Both the elastic wave velocities are a better means to recognize earlier effect of dilatancy than what is shown by volumetric strain curve. The permeability decreases after heating stage and starts to increase beyond compaction and after emergence of axial cracking. This study shows that there exists a self-consistent behavior among the evolution of seismic wave velocities with the hydrostatic stresses, heating stages, deviatoric stress increments, deformational patterns, and transport properties of tested specimens. These complicated experiments are crucial to model various stages of HM and THM conditions and understand the relationships between various types of stress in relation to the deformational responses of argillaceous rocks.

Increasing the damage and crack porosity in crustal rocks can result in significant changes to various key physical properties, including mechanical strength, elastic and mechanical anisotropy, and the enhancement of transport properties. Using a Non-Interactive Crack Effective Medium (NIC) theory as a fundamental tool, we show that elastic wave dispersion can be inverted to evaluate crack density as a function of temperature and is compared with optically determined crack density. Further, we show how the existence of embedded microcrack fabrics in rocks also significantly influences the fracture toughness (KIC) of rocks as measured via a suite of tensile failure experiments (chevron cracked notch Brazilian disk). Finally, we include fluid flow in our analysis via the Gueguen and Dienes crack porosity-permeability model. Using the crack density and aspect ratio recovered from the elastic-wave velocity inversion, we successfully compare permeability evolution with pressure with the laboratory measurements of permeability.

This paper presents an experimental work aimed at assessing the correlation between fracture toughness (K IC ) and fracture roughness for a series of Westerly granite specimens thermally treated up to 850°C. Mode I fracture toughness as a function of thermal treatment is determined using Cracked Chevron Notched Brazilian Disc specimens. The degree of roughness of the resultant fracture surfaces is analyzed with the aid of a high accuracy, high precision stereo-topometric measurement system. Roughness and toughness values display a negative correlation as a function of temperature. Fracture toughness decreases with increasing temperature due to the gradual opening of grain-grain boundaries in response to thermal stresses. Mode I fractures preferentially follow these weakened grain-grain boundaries, which in addition to the thermal expansion of individual grains, result in rougher failure profiles with increasing temperature. At low temperature, a distinct anisotropy in roughness was observed in all fracture surfaces with higher roughness value perpendicular to the direction of fracture propagation. However, higher treatment temperatures resulted in the homogenization of fracture roughness in all directions. These results confirm the important link among petrofabric analysis, fracture toughness, and fracture roughness in response to thermal treatment.

A numerical modeling study of a heated rock was carried out and validated by a laboratory experiment. In the laboratory experiment, a granite cylinder with a diameter of 30 cm was fractured by a heater placed in a central borehole. Acoustic emission (AE) monitoring during the experiment showed the development of the damage in the specimen. The numerical simulations were made with a commercially available code PFC2D, using a bonded-particle approach. A total of 15 numerical specimens were built with varying mechanical and thermal properties. During the modeling, AE events were simulated and compared against the laboratory data. The numerical models showed similar failure behavior to that observed in the laboratory experiment. The pre-failure AE events were isolated and randomly located in the sample, while the failure itself appeared as a single large AE event comprising many microcracks aligned along a plane. The results of these unique thermal simulations have been so encouraging that we are proceeding with larger scale thermal problems.

A series of 20 chevron cracked notched Brazilian disc (CCNBD) samples of Westerly granite were failed in a standard Mode I tensile test at room temperature in order to evaluate the effect of thermal damage on fracture toughness. The heat treatment involved slowly heating four sets of four samples to 250, 450, 650 and 850 °C. The fifth set of samples was not thermally treated. Thermal cracking not only induced a substantial decrease of the mechanical strength, but also of the dynamic elastic properties of Westerly granite. In particular, normalized P-wave compressional velocities matched remarkably well the decreasing trend of normalized fracture toughness (KIC). Above 450 °C, grain boundary opening and cracking, intragranular cracking and mineral grain dissection linked to the quartz α–β phase transition induced a significant increase in the total crack density. Fracture path interaction with various mineral–mineral contact types showed that fracture branching and total fracture length increased with the amount of temperature of heat treatment. Using non-interactive crack theories, dimensionless crack densities were obtained from wave velocity inversion, up to unusually high values of ∼10 at 850 °C. This geophysical analysis showed to be in close agreement with crack parameters determined optically, such as optical crack density determination, crack aspect ratio evolutions, and the measured sample porosity with temperature. Our results also show that only the non-interactive crack theory can predict KIC relatively well at high crack density, by simply using dimensionless crack densities inverted from velocities. A decrease of 50% for crack densities larger than 1, 80% for crack densities larger than 5 is predicted, in close agreement with our observed experimental variation of KIC. At the microscale, this can be interpreted by the fact that the main fracture is strongly interacting with the pre-existing microcrack fabric. These combined experimental and modeling results illustrate the importance of understanding the details of how the rock microstructure is changing in response to an external stimulus, in order to predict the simultaneous evolution of physical and mechanical properties of rock.

The fast sweeping method has been proved very efficient in calculating the first arrivals in isotropic media. In this study, we extend the fast sweeping method to heterogeneous anisotropic medium by adapting the Lax-Friedrichs local scheme. The new fast sweeping method is able to solve the first arrival traveltime field for both qP and qS waves. By comparing the traveltime field to the full-waveform solution, we demonstrate that the iso-surfaces of the time field follow the constant phase of the wave and form a continuous envelope wrapping the wavefront. The iso-surfaces for the shear wave identify two continuous wavefronts one ahead of the other even in the directions where the triplication of qS-wave is developed. The rays for both qP and qS wave can be traced using the slowness vector from the traveltime field and we compared its accuracy with a two-point ray tracing method in a layered model. We show that rays from the traveltime field is nearly identical to the two-point ray tracing results. This new fast sweeping method not only avoids the multipath and shadow zone issues in complex heterogeneous media but also circumvent the multiple shear branch problems due to anisotropy.

Fracture toughness measurements under static loading conditions have been carried out in Barre and Lac du Bonnet granites. An advanced AE technique has been adopted to monitor real-time crack initiation and propagation around the principal crack in these tests to understand the processes of brittle failure under tension and related characteristics of the resulting fracture process zone. The anisotropy of Mode I fracture toughness has been investigated along specific directions. Microcrack density and orientation analysis from thin section studies have shown these characteristics to be the primary cause of the observed variation in fracture toughness, which is seen to vary between 1.14 MPa.(m)1/2 and 1.89 MPa.(m)1/2 in Barre granite. The latter value represents the case in which the crack is propagated at right angles to the main set of microcracks. The creation of a significant fracture process zone surrounding the propagating main crack has been confirmed. Real-time imaging of the fracture process and formation of fracture process zone by AE techniques yielded results in very good agreement with those obtained by direct optical analysis.

To characterize the damage induced during excavation of a test tunnel in granite, ultrasonic wave velocity and amplitude measurements were made in situ around one quadrant of the tunnel. Sixteen 1-m deep boreholes were drilled into the tunnel wall; these boreholes spanned the region from tensile stress-induced damage in the sidewall to compressive stress-induced damage in the well-developed notch. Measurements were made by pulsing a source transducer at numerous points within a given borehole and on the tunnel surface, and then capturing the transmitted ultrasonic waves at transducers located in the adjacent borehole. The travel times of the waves between sources and receivers were used to compute the average velocities and to construct two-dimensional (2D) tomographic images. Wave velocities and amplitudes in the tensile stress region are low and show significant anisotropy, indicating the presence of oriented microcracks even as deep as 1 m from the tunnel wall. Velocities and amplitudes are higher in the compressive stress region and show lower anisotropy, indicating the rock is relatively undamaged away from the tunnel wall. The velocity anisotropy indicates that the dominant orientation of microcrack planes is everywhere tangential to the tunnel wall, even though the minimum principal stress is tangential and tensile in the sidewall. The distribution of microcracks inferred from these results is consistent with the extensive microseismicity detected during and after excavation and suggests that both the stress history and current stress state have impacted the current state of damage around the tunnel.

A technique is presented to simulate seismicity in brittle rock under stress using a distinct element numerical modelling code. Itasca Consulting Group's particle flow code is used to model rock deformation, damage and the resulting seismicity. With this code, rock is represented by thousands of individual particles bonded together at points of contact. Seismicity results when bonds break under high local stresses and stored strain energy is released as kinetic energy. A full 3D formulation that enables the calculation of seismic event locations, magnitudes and mechanisms (moment tensors) is presented. A simulation of an axial compression test on a granite core sample is used to test the modelling technique. The source parameters of the acoustic emissions produced by the model are considered realistic when compared with similar experiments conducted in the laboratory. The technique is then tested on a field-scale problem by applying the algorithms to the 2D mine-by tunnel excavation simulation (Potyondy and Cundall, Int. J. Rock Mech. Min. Sci., this issue). Seismicity produced by the model is compared to actual seismicity recorded in the field and it is shown that locations, magnitudes and mechanisms match reasonably well. The similarities between the modelled and recorded seismicity provide confidence that the model is behaving in a realistic way. It is then shown how the model could be used to examine the details of the mechanisms behind the recorded seismicity by direct observations of particle forces and motions at the seismic sources.

Methods based on acoustic emission and microseismicity (AE/MS) have proven to be valuable for monitoring microcracking at the Underground Research Laboratory (URL) in Canada. The source locations of the seismic events induced by the excavation have helped to map out the extent of the excavation damage/disturbed zone (EDZ) around tunnels. This paper describes methods for advanced processing and interpretation of AE/MS data, and the quantification of changes in material properties using velocity interferometry. Examples of methods that are generally not used routinely in the analysis of AE/MS data are given. These include (1) long-term spatial and temporal source parameter trends and magnitude–frequency studies; (2) source mechanism analysis; and (3) source location refinement by the use of relative location, and clustering techniques. Results from these studies are shown to aid in the understanding of the effect of confining pressure on AE/MS activity around tunnels, and the mechanics of rock failure in the EDZ. Also, the reduction of source location error is shown to be critical for delineating structures that may exist in clusters of AE/MS data. Finally, examples of the use of seismic sources to produce repeat measurements of seismic velocity along raypaths in 3D orientations through a rock mass are given. This is found to be a highly sensitive method for quantifying changes in rock properties such as Young's modulus, as well as for modelling the change in crack density and saturation occurring in a rock mass with time.

The purpose of this cross-sectional study was to examine the risk associations between obesity indexes [body mass index (BMI) and waist circumference (WC)], cardiovascular risk factors [plasma glucose and lipids, blood pressure and urinary albumin excretion (UAE)] and morbidity conditions (Type 2 diabetes mellitus, hypertension, dyslipidaemia and/or albuminuria) in Hong Kong Chinese. Seven-hundred and two Hong Kong Chinese subjects (18-65 years of age, 59.4% of whom had at least one morbidity condition) were recruited from the Prince of Wales Hospital, Hong Kong SAR. The measurements taken of the subjects included: height; weight; waist and hip circumferences; blood pressure; fasting plasma glucose and lipids; and 24-h UAE. The mean BMI was 22.4 and 25.7 kg m(-2) in healthy subjects and patients, respectively. The mean WC measurements of healthy subjects and patients were 77.1 and 86.4 cm in males and 71.0 and 81.8 cm in females, respectively. There were increasing trends between obesity indexes and the severity of cardiovascular risk factors and the prevalence of morbidity conditions (all P-values for trend0.05). Using 19.0-20.9 kg m(-2) and70 cm as a referent, subjects with a BMI ofor =25.0 kg m(-2) (in both sexes) and/or a WC ofor =85 cm in males andor =75 cm in females had an age-adjusted odds ratio between 3.2 and 4.4 for the occurrence of at least one morbidity condition. Patients with a greater number of comorbidities also had higher BMI and WC measurements (all P-values for the trend were0.05 with adjustment for age and gender). Hence, despite Hong Kong Chinese being less obese than Caucasians, the intimate relationships among obesity, cardiovascular risk factors and morbidity conditions remain. Our data support using lower BMI and WC levels to define obesity and its associated health risks rather than using the criteria established from Caucasians who generally have larger body frames.

This paper argues for a social context for the study of industrial society through the examination of the texture of working people's lives as revealed in sources traditionally ignored by the empirical study of monument types. It suggests that this should include vernacular song, and considers in detail three such songs from the north-east of England coalfield.

Characterizing the fast evolution of microstructural defects is key to understanding “crackling” phenomena during the deformation of solid materials. For example, it has been proposed using atomistic simulations of crack propagation in elastic materials that the formation of a nonlinear hyperelastic or plastic zone around moving crack tips controls crack velocity. To date, progress in understanding the physics of this critical zone has been limited due to the lack of data describing the complex physical processes that operate near microscopic crack tips. We show, by analyzing many acoustic emission events during rock deformation experiments, that the signature of this nonlinear zone maps directly to crackling noises. In particular, we characterize a weakening zone that forms near the moving crack tips using functional networks and we determine the scaling law between the formation of damages (defects) and the traversal rate across the critical point of transition. Moreover, we show that the correlation length near the transition remains effectively frozen. This is the main underlying hypothesis behind the Kibble-Zurek mechanism (KZM) and the obtained power-law scaling verifies the main prediction of KZM.

The tunnel sealing experiment at the Underground Research Laboratory in Canada is testing engineered barriers (concrete and clay bulkheads) that will restrict fluid flow passing through the excavation damage zone (EDZ) in the host granitic rockmass. 3D elastic stress modeling has shown that keyed bulkheads significantly disrupt and reduce the high differential stresses around the excavation perimeter. The use of microseismic (MS) and acoustic emission (AE) methods are shown to be a valuable tool for delineating the extent of the EDZ, substantiating the stress modeling, and also for identifying how much tunnel perimeter confinement is needed to stop the occurrence of micro-cracking. In addition, active ultrasonic surveys using AE sensors are shown to be able to quantify the change in material properties such as crack density that have occurred during the development of the EDZ.

SUMMARY The aim of this study was to investigate the effects of the inconsistency between the model used and the actual structure on the inversion of point-source parameters. Specifically, we studied the effects of ignoring a reflecting interface in the model. Such a situation frequently occurs when events with sources close to mine excavations are recorded and subsequently processed using a simplified model describing the interface only approximately or ignoring it completely. In this case, it is important to ask how reliable the retrieved mechanism is. An inversion algorithm with indirect parametrization of the source (INPAR) was applied. A numerical study was performed, in which the mechanism and source time function of a point source were sought in a model inconsistent with the actual structure. The ray synthetic wavefield generated by a source situated close to a free surface of a homogeneous half-space was treated as the observed data. The free surface was then ignored when constructing the Green’s function necessary for the point-source inversion. Thus, the inconsistent response of the medium including only direct phases was used to invert the observed seismograms, which contain both direct and reflected phases. The configuration of the Underground Research Laboratory (URL) of the Atomic Energy Canada Ltd. was simulated in the synthetic experiments. 16 triaxial sensors were situated around the underground tunnel, the face of which was taken to represent the free surface near the hypocentre. High-frequency seismograms were synthetized with a frequency of around 10 kHz. This is close to the prevailing frequency of the actual URL records. It was found that gross features of the source such as orientation of the double couple and the general features of the source time function can be retrieved satisfactorily when the hypocentre is localized correctly. Formal error analysis, however, yields rather large error estimates due to the omission of the free surface, providing us with acceptably constrained solutions at about 70 per cent confidence level only. Mislocation of the hypocentre of the order of seven to 14 wavelengths and/or contamination of the data by noise with an amplitude amounting to 20 per cent of the data amplitude both distort retrieved source parameters and make them rather uncertain. The retrieved orientation deviates by more than 20u and its 70 per cent confidence region extends several tens of degrees. As a consequence of the mismodelling of the medium represented by the neglect of the free surface, spurious non-double couple (DC) components appear in the mechanism. Two URL events from 1991 September 25 separated by 4 s were processed, and a large majority of the compensated linear-vector dipole (CLVD) was found there. The CLVD along the P-axis at the earlier event was replaced by the CLVD along the T-axis at the following one, which suggests an over-relaxation of the stress during the first event and its partial restoration by the subsequent event.

Acoustic (ultrasonic) tomography is now a well acepted method for investigating the internal structure of rock samples and rock masses. In this article we describe a new parameter to be imaged, which combines the robustness of velocity tomography and the sensitivity of attenuation tomography resulting in Enhanced Velocity Tomography. The theoretical considerations are illustrated by a computer simulation and then by application to data from a granite sample subjected to induced damage by heating. In both cases the Enhanced Velocity Tomography is found to be more sensitive to the internal rock structure than classical velocity tomography and overcomes instability problems characteristic of attenuation tomography.

To study microcracking in a brittle material in response to stress-relief, samples of polycrystalline ice were frozen under hydrostatic pressures up to 19 MPa and subsequently decompressed. Eight cylindrical samples (approximately 56 mm in diameter and 120 mm long) were made by freezing a slurry of crushed ice and degassed water inside a clear Teflon jacket. Four additional tests were done by reloading samples which were previously unloaded, allowing them to re-equilibrate, and then unloading again. Tests were filmed through windows in the confining cell. Acoustic emissions were monitored in some samples. When the stress was released, intense cracking occurred for approximately 0.4 seconds and then tapered off. Thin sections showed that the stress-relief cracks were approximately equal to the facet size (< 5 mm), were primarily along grain boundaries, and were distributed throughout the samples with no obvious preferred orientation. Within a few hours of unloading, grain boundaries became cloudy, possibly a result of air trapped at the grain surfaces. Preliminary results from further tests indicate that this air plays a significant role in crack nucleation.

A p-wave velocity survey was performed after the excavation of a cylindrical, horizontal tunnel through homogenous, visibly unfractured granite. An image of the velocity structure around the tunnel indicated a distinct zone of high velocity corresponding to a zone of increased elastic stresses, where a break-out notch formed. Perpendicular to this zone, a region of decreased velocity was found associated with a region of tensile elastic stress. The stress changes are believed to have preferentially opened and closed pre-existing micro-fractures in the rock, thereby decreasing and increasing the p-wave velocity. Additional velocity decreases are also attributed to creation of new fractures. The excavation-induced microseismicity is found to be generally associated with the high-velocity region adjacent to the notch, and appears to be located along the boundary between the fractured, low-velocity and high-velocity regions.