R. P. Young

Space-time correlations ofb values with stress release

A total of 1503 events for a 2-month period associated with amN 2.6 rockburst is investigated for possible space-time correlations between low magnitude (−1.1 to −0.4)b values and several estimates of stress (static stress drop, apparent stress, and dynamic stress). Spatial variations of decreasingb values were found to be well correlated with increasing stress release estimates for time intervals prior to the rockburst and following the aftershock sequence. The strongest correlation tob value was with the dynamic stress drop, having correlation coefficients of 0.87 and 0.79 for the two intervals, respectively. The rockburst was found to actually occur at the intersection of the spatial coordinates corresponding to the largest gradient inb value. Based on these correlations, we conclude that the low magnitude seismicity is an indicator of the stress state within the rock mass, and can be used to study and forecast stress patterns in the vicinity of an impending major event. Time variations, however, did not show the same clear correlations and these are discussed in terms of departure from steady state conditions. Regardless, our results favour the use ofb values in a spatial, context rather than in a time analysis approach, and we consider thatb values provide valuable information regarding the changing stress conditions within the seismogenic volume.

Ultrasonic imaging and acoustic emission monitoring of thermally induced microcracks in Lac du Bonnet granite

Concurrent ultrasonic tomography and acoustic emission monitoring were employed to study thermally induced microfracturing in an unconfined, 15-cm cube of Lac du Bonnet granite from Atomic Energy of Canada Limiteds Underground Research Laboratory. An electrical resistance cartridge heater, placed in a central vertical borehole, was used to cycle the sample to progressively higher peak temperatures between 75°C and 175°C. Tomography data were collected, at room temperature, before and after each thermal cycle. Acoustic emission monitoring proceeded during both heating and cooling phases of each thermal cycle. Microfractures opened above 80°C and eventually coalesced into a macroscopic fracture plane. The macroscopic fracture originated at the outer edges of the sample and then extended inward, parallel to the fast velocity direction, and eventually intersected the borehole. Both acoustic emission locations and slowness difference tomography clearly delineated the fracture plane. We attribute the development of the macroscopic fracture to a thermal gradient cracking mechanism acting upon a brittle, anisotropic medium.

Ultrasonic tomography and acoustic emission in hydraulically fractured Lac du Bonnet Grey granite

Ultrasonic tomography and acoustic emission (AE) data were obtained during laboratory hydraulic fracturing tests on two large, unconfined cylinders of Lac du Bonnet grey granite. The cylinders were internally pressurized over four to five cycles prior to final failure. Compressional velocities were measured before and after each pressurization cycle with an array of 16 evenly spaced transducers around the central, cross-sectional plane of each sample. Sixteen channels of whole waveform AE data were recorded during most pressurization cycles and for a period of about 1 hour after final failure in one sample. Compressional velocities were found to be strongly anisotropic, with the in situ vertical direction being the most rapid direction in both samples. The velocity anisotropy is related to the rocks preexisting microcrack fabric. Owing to radial penetration of fluid into the rock, compressional velocities rose over the course of the experiment. A regression analysis showed that the velocity changes can be explained by variations in crack density, inferred from initial velocities, and radial distance from the borehole. Saturation levels consistent with the observed velocity changes were calculated on the basis of the OConnell and Budiansky theory. Acoustic emissions reoccurred in a few distinct zones over several pressurization cycles. The AE locations allowed two distinct fracture planes to be sharply delineated in one sample. The fracture plane orientations were controlled by the preexisting microcrack fabric in both samples. AE occurred too rapidly during peak pressure failure to permit us to isolate distinct events. Source mechanism analysis of the AE which occurred prior to peak pressure failure in both samples, and during postfailure monitoring in one sample, showed a predominance of double-couple sources. Compressive sources, thought to be related to crushing of asperities during crack closure, and tensile sources, related to mode I crack growth, were also recorded, as well as more complex sources that could not be modeled by the simple source types listed above.

Mining-induced microseismicity: Monitoring and applications of imaging and source mechanism techniques

The study of microseismicity in mines provides an ideal method for remote volumetric sampling of rock masses. The nature and uniqueness of microseismic monitoring is outlined in the context of acquisition hardware and software requirements. Several topics are used to highlight the potential for novel applications of microseismicity and to outline areas where further study is required. These topics reflect some of the current interest areas in seismology, namelyb values and source parameters, fault-plane solutions, modes of failure and moment tensor inversion, imaging and seismicityvelocity correlations. These studies suggest potential correlations between zones of high seismic velocity, high microseismic activity and maximal stress drops, which can be interpreted spatially to be the locations of highly stressed ground with a potential for rock bursting. Fault-plane solutions are shown to be useful in determining the slip potential of various joint sets in a rock mass. Source parameter studies and moment tensor analysis clearly show the importance of non-shear components of failure, andb values for microseismicity appear to be magnitude-limited and related to spatial rather than temporal variations in effective stress levels.

Geotomographic imaging in the study of mining induced seismicity

Geotomographic imaging is a technique which allows seismic waves to be used to gain information about the internal structure of rock masses, in a way conceptually similar to medical CAT scanning. Traditional approaches to the study of mining-induced seismic phenomena have concentrated on using passive monitoring methods. This paper gives an overview of the developments in the acquisition, processing and interpretation of geotomographic data and outlines how images can be used in conjunction with passive techniques to study mining-induced seismicity.

Analysis of mining-induced microseismic events at Strathcona Mine, Subdury, Canada

Rockbursts and mining-induced seismic events have serious socio-economic consequences for the Canadian mining industry, as their mines are extended to greater depths. Automatic multichannel monitoring systems (Electro-Lab MP250s) are routinely, used to detect the arrival times of seismic waves radiated by mining-induced events and sensed on an array of single component transducers installed throughout a mine. These arrival times are then used to locate the events and produce maps of areas of high activity for use in mine planning and design. This approach has limitations in that, it does not allow a detailed analysis of source mechanisms, which could be extracted if whole waveform signals are recorded and analyzed.A major research project, sponsored by the Natural Sciences and Engineering Research Council of Canada (NSERC) with the collaboration of the Canadian mining industry, is aimed at enhancing existing mine seismic monitoring technology in Canada, in order to carry out more advanced processing of data to obtain fundamental scientific information on mining-induced seismic events This paper describes preliminary results from seismic monitoring experiments carried out in a hard rock nickel mine in Sudbury, Canada. Existing seismic monitoring instrumentation was enhanced with a low cost microcomputer-based whole waveform seismic acquisition system. Some of the signals recorded during this experiment indicate anisotropic wave propagation through the mine rock masses, as observed by the splitting of shear waves and the relative arrival of two shear waves polarized in directions which may be related to the structural fabric and/or state of stress in the rock mass. Analysis of compressional wave first motion shows the predominance of shear events, as indicated by focal mechanism studies and is confirmed by spectral analysis of the waveforms. The source parameters were estimated fro typical low magnitude localized microseismic events during the initial monitoring experiments. The seismic moment of these events varied between 106 N.m and 2.108 N.m. with a circular source radius of between 1 m and 2 m with an estimated stress drop of the order of 1 MPa.

Sequential velocity imaging and microseismic monitoring of mining-induced stress change

Sequential imaging of the temporal changes inP-wave velocity offers a practical tool to monitor a rock mass. Using established correlations between the location of seismic events and velocity structure, the temporal seismic potential characteristics of the rock may be monitored. Furthermore, the temporal velocity differences isolate the time dependent factors effecting velocity such as stress, while cancelling static factors such as lithology. Various sequential imaging techniques were compared with respect to accuracy. Differences between successive velocity images were found to have relatively high associated error estimates. However, images of velocity differences calculated from measured travel time delays between successive velocity surveys were found to have lower error estimates. In particular, travel time delays measured using cross-correlation techniques resulted in the most accurate sequential image. Two 3D sequential imaging studies were carried out at Strathcona Mine in Sudbury, Ontario, Canada. Results of the average static images indicated an association between the location of induced microseismicity and a zone of both high velocity and high gradient. Additional examples are described from the global seismology literature which also show a similar correlation between seismicity and velocity structure. We attribute this association to an interrelated stress and strength effect. The Strathcona Mine sequential images show zones of significantly decreased velocity in regions of concentrated microseismic activity, which are postulated to be indications of localized destressing and relaxation of the clamping forces resulting in the microseismicity. The zones of decreased velocity corresponded to an increase in the velocity gradient. One of the case studies also shows an increase in velocity in a zone of high static velocity, which is later the site of amN 2.5 mining-induced seismic tremor. The increase in velocity is believed to correspond to a region of stress concentration, resulting in the subsequent seismic tremor.

Influence of source region properties on scaling relations forM<0 events

Excavation induced seismic events with moment magnitudesM<0 are examined in an attempt to determine the role geology, excavation geometry, and stress have on scaling relations. Correlations are established based on accurate measurements of excavation geometry and methodology, stress regime, rock mass structure, local tectonics, and seismic locations. Scaling relations incorporated seismic moments and source radii obtained by spectral analysis, accounting for source, propagation, and site effects, and using Madariagas dynamic circular fault model. Observations suggest that the interaction of stresses with pre-existing fractures, fracture complexity and depth of events are the main factors influencing source characteristics and scaling behaviour. Self-similar relationships were found for events at similar depths or for weakly structured rock masses with reduced clamping stresses, whereas a non-similar behaviour was found for events with increasing depth or for heavily fractured zones under stress confinement. Additionally, the scaling behaviour for combined data sets tended to mask the non-similar trends. Overall, depth and fracture complexity, initially thought as second order effects, appear to significantly influence source characteristics of seismic events withM<0 and consequently favour a non-similar earthquake generation process.

Ultrasonic tomography using scanned contact transducers

Ultrasonic tomographic images have been obtained from objects of rectangular cross section by scanning spring‐loaded transducers across the sample faces. Hemispherical brass caps, placed on each transducer, facilitated sliding along the surface. The complete system was under the control of a microcomputer, resulting in an entirely automatic data collection system. Selected images obtained from this system are presented for metal and rock samples, as well as samples with artificial anomalies.

ACOUSTIC TOMOGRAPHY IN SOLIDS USING A BENT RAY SIRT ALGORITHM

Abstract Many techniques have been established that perform tomographic reconstructions of the acoustic velocity profile of a solid object. A SIRT technique was selected as a particular one that was able to handle arbitrary sampling geometries and to incorporate a raybending correction. In this paper, these modifications to the basic algorithm are presented. A fast approximate raybending correction, suitable for use on personal computers, is given based on discretization of the raypaths, and the use of a graph-searching algorithm to obtain the minimum time path. The performance of the algorithm is evaluated in the field of seismic frequencies, imaging an area of backfill in an irregularly shaped rock pillar in a hard rock mine.