Mining-induced Seismicity Research Articles

Indications for different types of brittle failure due to active coal mining using waveform similarities of induced seismic events

Abstract. Longwall mining activity in the Ruhr coal mining district leads to mining-induced seismicity. For detailed studies the seismicity of a single longwall panel beneath the town of Hamm-Herringen in the eastern Ruhr area was monitored between June 2006 and July 2007 with a dense temporary network of 15 seismic stations. More than 7000 seismic events with magnitudes between –1.7 ≤ ML ≤ 2.0 were detected and localized in this period. Most of the events occurred in the vicinity of the moving longwall face. In order to find possible differences in the brittle failure types of these events an association of the events to distinct clusters is performed based on their waveform characteristics. This task is carried out using a new clustering algorithm utilizing a network similarity matrix which is created by combining all available 3-component single station similarity matrices. The resultant network matrix is then sorted with respect to the similarity of its rows leading to a sorted matrix immediately indicating the clustering of the event catalogue. Finally, clusters of similar events are extracted by visual inspection. This approach results in the identification of several large clusters which are distinct with respect to their spatial and temporal characteristics as well as their frequency magnitude distributions. Comparable clusters are also found with a conventional single linkage approach, however, the new routine seems to be able to associate more events to specific clusters without merging the clusters. The nine largest observed clusters can be tentatively divided into three different groups that indicate different types of brittle failure. The first group consists of the two largest clusters which constitute more than half of all recorded events. Results of a relative relocation using cross-correlation data suggest that these events are confined to the extent of the mined out longwall and cluster close to the edges of the active longwall at the depth of active mining. These events occur in lockstep with the longwall advance and exhibit a high b value of the Gutenberg–Richter relation (GR) of about 1.5 to 2.5 and consist of small magnitude events. Thus, these events represent the immediate energy release adjacent to the mined out area. The second group consists of clusters located either slightly above or below the depth of active mining and occurring at the current position of the longwall face within the confines of the longwall. They consist of generally stronger events and do not follow GR. This activity might be linked to the failure of more competent layers above and below the mined out seam resulting in larger magnitude events. Finally, one cluster represents seismic activity with a rather low b value below 1 and events located partly towards the north of the longwall which are delayed with respect to the advance of the longwall face. These events are interpreted as brittle failure on pre-existing tectonic structures reactivated by the mining activity.

Automated full moment tensor inversion of coal mining-induced seismicity

Seismicity induced by coal mining in the Ruhr region, Germany, has been monitored continuously over the last 25 yr. In 2006, a dense temporary network (HAMNET) was deployed to locally monitor seismicity induced by longwall mining close to the town of Hamm. Between 2006 July and 2007 July, more than 7000 events with magnitudes M-L from -1.7 to 2.0 were detected. The spatiotemporal distribution of seismicity shows high correlation with the mining activity. In order to monitor rupture processes, we set up an automated source inversion routine and successfully perform double couple and full moment tensor (MT) inversions for more than 1000 events with magnitudes above M-L -0.5. The source inversion is based on a full waveform approach, both in the frequency and in the time domain, providing information about the centroid location, focal mechanism, scalar moment and full MT. Inversion results indicate a strong dominance of normal faulting focal mechanisms, with a steeper plane and a subhorizontal one. Fault planes are oriented parallel to the mining stopes. We classify the focal mechanisms based on their orientation and observe different frequency-magnitude distributions for families of events with different focal mechanisms; the overall frequency-magnitude distribution is not fitting the Gutenberg-Richter relation. Full MTs indicate that non-negligible opening tensile components accompanied normal faulting source mechanisms. Finally, extended source models are investigated for largest events. Results suggest that the rupture processes mostly occurred along the subvertical planes.

Mining-Induced Seismicity in the Kola Peninsula Mines

Mining-Induced Seismicity in the Kola Peninsula Mines

Techbits: SPE/SEG Workshop on Injection-Induced Seismicity

Techbits In September, SPE and the Society of Exploration Geophysicists (SEG) cohosted a workshop on injection-induced seismicity in Broomfield, Colorado. The goal of the workshop was to determine industry knowledge about seismicity accompanying injection activities, identify the gaps in knowledge, and to discuss ways to improve understanding of such seismicity. Approximately 115 participants gathered to share knowledge from diverse regions and experiences. Attendees were evenly split between service companies and research organizations, and while a majority of the participants described themselves as geophysicists, about 20% were engineers. Encouraging an open discussion between engineers and geophysicists was one of the reasons for holding the workshop. The opening session set the stage for the workshop, and included presentations about case studies of induced seismicity associated with several types of human activities. The benefits and risks of induced seismicity to subsurface and surface facilities were discussed as a basis for understanding what information is needed to better understand and mitigate potential risks associated with the seismicity. A poster session (chaired by Jamie Rich) highlighted topics related to characterizing and mitigating fluid-induced seismicity. Other sessions included: Underground Mines–In Situ Laboratories for Injection-Induced Seismicity (cochaired by Mark Willis). This session focused on investigations into the established field of mining-induced seismicity, including case studies from fluid injections in underground mines and the opportunity to perform controlled studies. Induced Seismicity Associated with Enhanced Geothermal Systems (EGS)—From Understanding to Hazard Control (chaired by Serge Shapiro and Ahmed Abou-Sayed). This session included discussions about seismicity accompanying the development of EGS reservoirs and how public and regulatory concerns about induced seismicity risk have been addressed particularly through the United States Department of Energy EGS-induced seismicity protocol. Looking Back at Induced Seismicity—Case Histories Examining Wastewater Disposal and Induced Seismicity—What is Known (chaired by Julie Shemeta and Dicky Hall). This session consisted of presentations about wastewater disposal operations, observations of the comparison of seismicity with various injections, and potential interaction with existing faults. What Do We Know About Fraccing-Induced Seismicity (co-chaired by Jamie Rich). This included presentations about characterizing hydraulic fractures through microseismicity, the types of measurements that are made to characterize induced seismicity, and applications of those measurements. Statistical analysis methods for investigating the attribution of induced seismicity were discussed.

A Catalog of Regional Moment Tensors in Utah from 1998 to 2011

Online Material: The electronic supplement includes details for all 48 moment tensor solutions in this study, including waveform fits and a table of event parameters. The University of Utah Seismograph Stations (UUSS) operates regional seismic networks in and around Utah and Yellowstone National Park and provides routine earthquake locations and earthquake catalogs for these two areas (Pechmann et al. , 2007). In this study, we expand the reporting to include moment tensors for seismic events occurring in the Utah region (Fig. 1). This region is a complex seismic environment with diverse source types: tectonic earthquakes, mining‐induced seismicity (MIS) including mine collapses, and man‐made sources such as explosions. Most tectonic earthquakes in the region occur within the Intermountain Seismic Belt (ISB; Smith and Arabasz, 1991). In northern Utah, the ISB roughly delineates the boundary between the Basin and Range and the Middle Rocky Mountains, whereas in central and southern Utah, the ISB separates the actively extending Basin and Range from the more stable Colorado Plateau. The MIS is mostly associated with long‐wall coal mining in central Utah (Fig. 1; Arabasz et al. , 2007). The man‐made blasts occur throughout the state and are related to both surface mining operations and military munitions detonations (Arrowsmith et al. , 2008). Figure 1. UUSS Utah monitoring area (dark gray box) and the surrounding region. Solid black line separates the UUmod velocity model regions between Basin and Range to the northwest and Colorado Plateau to the southeast. Dashed line separates physiographic provinces: Basin and Range (BR), Colorado Plateau (CP), and Middle Rocky Mountains (MR). The transition zone is roughly the overlap between the physiographic and velocity model boundaries between the Basin and Range and Colorado Plateau in Central Utah. Crescent‐shaped and rectangular polygons in central‐eastern Utah are coal mining areas. ECSZ is the Eastern California …

Study on Characteristic of Overburden Movement in Unsymmetrical Isolated Longwall Mining Using Microseismic Technique

Based on the key stratum theory, overlying strata structures above a typical unsymmetrical isolated working face (LW10302) was analyzed, and a microseismic monitoring was also applied to characterize the fracturing propagations associated with overburden movement in mining progress. The results show that the overlying strata above LW10302 can be divided into key strata of different grades, and the formed “O-X” fracturing structure have the main and inferior “O-X” ones. The spatial evolution of seismic events demonstrated that seismic activities fits very well with the overburden fracturing patterns and stress manifestation around the longwall face. In the mining process, most of the events located within the surrounding strata of LW10301 and 10302 while low energy events distributed mainly in multiple roof and floor strata, and the strong tremors occurred almost within the super-thick primary key strata and appeared to be related to shear fracturing of large-scale overburden movement. Additionally, seismic signals corresponding to different failure mechanisms show different characteristics in waveform features. The study in this paper indicates that microseismic monitoring can provide invaluable information to characterize the mining-induced seismicity and reveal the failure patterns within strata associated with mining, which will greatly benefit the alleviation and prevention of rock burst hazards in mines

Omori’s Law Applied to Mining-Induced Seismicity and Re-entry Protocol Development

This paper describes a detailed study of the Modified Omori’s law n(t) = K/(c + t)p applied to 163 mining-induced aftershock sequences from four different mine environments in Ontario, Canada. We demonstrate, using a rigorous statistical analysis, that this equation can be adequately used to describe the decay rate of mining-induced aftershock sequences. The parameters K, p and c are estimated using a uniform method that employs the maximum likelihood procedure and the Anderson–Darling statistic. To estimate consistent decay parameters, the method considers only the time interval that satisfies power-law behavior. The p value differs from sequence to sequence, with most (98%) ranging from 0.4 to 1.6. The parameter K can be satisfactorily expressed by: K = κN1, where κ is an activity ratio and N1 is the measured number of events occurring during the first hour after the principal event. The average κ values are in a well-defined range. Theoretically κ ≤ 0.8, and empirically κ ∈ [0.3–0.5]. These two findings enable us to develop a real-time event rate re-entry protocol 1 h after the principal event. Despite the fact that the Omori formula is temporally self-similar, we found a characteristic time TMC at the maximum curvature point, which is a function of Omori’s law parameters. For a time sequence obeying an Omori process, TMC marks the transition from highest to lowest event rate change. Using solely the aftershock decay rate, therefore, we recommend TMC as a preliminary estimate of the time at which it may be considered appropriate to re-enter an area affected by a blast or large event. We found that TMC can be estimated without specifying a p value by the expression: TMC = aN1b, where a and b are two parameters dependent on local conditions. Both parameters presented well-constrained empirical ranges for the sites analyzed: a ∈ [0.3–0.5] and b ∈ [0.5–0.7]. These findings provide concise and well-justified guidelines for event rate re-entry protocol development.

Mining-Induced Seismicity in the Saarland, Germany

Coal mining in the Saar mine, Germany, is accompanied by mining-induced seismic events. Strong events occur only in certain areas of the mine, other areas exhibit almost no seismicity. Shear events occur simultaneously to non-shear events. The shear events occur in different depths but their epicenters do concentrate in bands. The strike of the bands coincides with the strike of larger regional faults in the area. The seismic events of the Saar mine show some characteristics which distinguish them from seismic events observed in other German coalfields. The Gutenberg–Richter relation, for example, does not hold for these events. Furthermore, radiated seismic energy and extracted coal volume are not correlated. In the Primsmulde field a strong seismic event was observed even before mining in that region started. The event was triggered just by driving roadways into the field. The shear events cannot be explained by the mining process alone. They are presumably induced in certain regions (bands) under tectonic load by an interaction of mining-induced and tectonic stresses. In February 2008, extraction in the Primsmulde field induced a seismic event of magnitude 4, which led to surface vibrations reaching 93 mm/s. After this event, the Primsmulde field had to be abandoned. Future extraction of the Saar mine will be restricted to some small areas not intersected by the event bands found in the Dilsburg Ost and Primsmulde fields. The Saar mine will close in 2011.

Coal Mining Induced Seismicity in the Ruhr Area, Germany

Over the last 25 years mining-induced seismicity in the Ruhr area has continuously been monitored by the Ruhr-University Bochum. About 1,000 seismic events with local magnitudes between 0.7 ≤ ML ≤ 3.3 are located every year. For example, 1,336 events were located in 2006. General characteristics of induced seismicity in the entire Ruhr area are spatial and temporal correlation with mining activity and a nearly constant energy release per unit time. This suggests that induced stresses are released rapidly by many small events. The magnitude–frequency distribution follows a Gutenberg–Richter relation which is a result from combining distributions of single longwalls that themselves show large variability. A high b-value of about 2 was found indicating a lack of large magnitude events. Local analyses of single longwalls indicate that various factors such as local geology and mine layout lead to significant differences in seismicity. Stress redistribution acts very locally since differences on a small scale of some hundreds of meters are observed. A regional relation between seismic moment M0 and local magnitude ML was derived. The magnitude–frequency distribution of a single longwall in Hamm was studied in detail and shows a maximum at ML = 1.4 corresponding to an estimated characteristic source area of about 2,200 m2. Sandstone layers in the hanging or foot wall of the active longwall might fail in these characteristic events. Source mechanisms can mostly be explained by shear failure of two different types above and below the longwall. Fault plane solutions of typical events are consistent with steeply dipping fracture planes parallel to the longwall face and nearly vertical dislocation in direction towards the goaf. We also derive an empirical relation for the decay of ground velocity with epicenter distance and compare maximum observed ground velocity to local magnitude. This is of considerable public interest because about 30 events larger than ML ≥ 1.2 are felt each month by people living in the mining regions. Our relations, for example, indicate that an event in Hamm with a peak ground velocity of 6 mm/s which corresponds to a local magnitude ML between 1.7 and 2.3 is likely to be felt within about 2.3 km radius from the event.

An Engineering Approach to Seismic Risk Management in Hardrock Mines

The problem of mining-induced seismicity in hardrock mines has become significant as underground mines from around the world are pushing production to deeper levels. At many mines, the risk associated with large seismic events and rockburst damage must be managed to ensure the safety of mine workers and minimise production losses. In this paper, an engineering approach to seismic risk management is described. It relies on accepted risk management techniques, which principally include the identification and understanding of hazards from which risk mitigation measures can be developed. This is achieved using simple but effective analysis techniques of high resolution microseismic data.

Rock mechanical investigations of seismic events in a deep longwall coal mine

Seismic events, i.e., fractures in rock that radiate detectable seismic waves, are a worldwide phenomena associated with mining and reservoir exploitation. Longwall mining in German coal mines often causes seismic events, which can be felt at surface where they can lead to a considerable annoyance of residents and sometimes even lead to minor damages of buildings and infrastructure. Strong seismic events are in a large part responsible for a more and more decreasing community acceptance of mining. The mines therefore have to investigate the local conditions, which cause the generation of seismic events, to find ways of avoiding strong mining induced seismic events. This paper describes the research on mining-induced seismicity around a specific active longwall at 1100 m depth under several previously mined longwall panels. Seismic events are recorded by a local network featuring 15 surface and six subsurface stations. Location and fault plane solutions show the occurrence of at least three different event types, which are characterized by distinct modes of failure. Underground core drilling and an extensive rock testing program are used to build up a rock mechanical model featuring a multi-layer strata for use with numerical analyses. The numerical model is able to explain the different failure mechanisms as fault activation, remnant pillar punching and beam failure. The use of rock mass classification, rock mass strength criteria and in situ stresses together with 2D and 3D numerical analyses facilitates the evaluation of the results from seismology approaches, and makes future analysis of longwall-related seismic events possible.

Seismic hazard assessment under complex source size distribution of mining-induced seismicity

It is well-documented that a variety of factors controlling the rockmass fracturing process in mines often results in a complexity of mining event size distribution. In such cases, the estimation of the probability functions of source size parameterizations, with the use of presently known distribution models, brings about an unacceptable and systematic over- or underestimation of the seismic hazard parameters. It is, therefore, recommended that the non-parametric, kernel estimators of the event size distribution functions, be applied to stationary hazard studies in mining seismicity. These data-driven estimators, adapted to seismic source size characterization, accurately fit all kinds of data underlying distributions, regardless of their complexity. Recently, the non-parametric approach to size characterization was supported by a special method of uncertainty analysis based on resampling techniques. At present, it is a fully developed method, which provides point and interval estimates of size distribution functions and related hazard parameters. Two examples of its use in studying mining seismic data are presented and discussed in this paper. The analyzed data sets were recorded in two different copper mines in Poland. The smoothed bootstrap test for multimodality, which is a specialized tool for investigating the shapes of probability densities, provided highly significant proof that in both cases the probability densities of source size parameterization were complex thus implied the superiority of the non-parametric estimation to the classic, model-based approach in the studied cases. The data were then used to construct non-parametric, kernel estimates of the source size cumulative distribution function (CDF), the exceedance probability and the mean return period. Furthermore, confidence intervals for these quantities were also estimated. The intervals for CDF were narrow, showing that the procedures of non-parametric estimation and resampling based uncertainty analysis were precise. Due to the fact that the mean return period is very sensitive to values of the CDF, in particular for larger events sizes, the uncertainty of the return period estimates was not insignificant but remained manageable. The point and interval estimates of source size CDF and hazard parameters so obtained were compared with the respective point estimates achieved from the inappropriate in the case of complex magnitude distributions, model-based approach.

A review of mining-induced seismicity in China

Active seismicity and rockbursting have been an emerging problem in Chinese mines. The distribution and characteristics of mining-induced seismicity in China, and its monitoring, mitigation, and research, are reviewed in this paper. Mining at depth and the activity of current tectonic stress field are the two major factors leading to rockburst hazards. Three critical depths, i.e., critical initiation depth, roof upper-bound depth, and floor lower-bound depth, have been identified based on hypocenter data of seismic events in coalmines. A strong correlation between rockbursts and gas outbursts in coalmines has also been established, and it is recommended to use this correlation for rockburst and gas outburst hazard assessment and warning interchangeably. We find that the key problems of rockburst hazard mitigation in China are the lack of mine seismicity-monitoring networks in most mines, and the need for improvement of the accuracy of the monitoring systems for mines that have been equipped with such systems. Because the demand for minerals resources is extremely high and the mining activities are progressing deeper and deeper, an increasing trend of mining-induced seismicity hazards in China may be anticipated for the near future. Mining-induced seismicities are hazards, but at the same time they have been found useful for studying geophysical problems in deep ground, particularly in the field of earthquake prediction. With the enforcement of relevant laws for the mining industry and the continuous effort to study rockburst problems using rock mechanics and geophysics principles and methods, it is believed that new approaches for rockburst hazard control and mitigation can be developed.

A study of mining-induced seismicity in Czech mines with longwall coal exploitation

A review is performed for the data of local and regional seismographical networks installed in mines of the Ostrava-Karvina Coal Basin (Czech Republic), where underground anthracite mining is carried out and dynamic events occur in the form of rockbursts. The seismological and seismoacoustic observations data obtained in panels that are in limiting state are analyzed. This aggregate information is a basic for determining hazardous zones and assigning rockburst prevention measures.

Mining-induced seismicity: Seismic measurement using multiplet approach and numerical modeling

The volume of influence of mining works is essentially determined from in situ observations, field measurements (particularly surface subsidence measurements) as well as from numerical investigations. Seismic monitoring is the primary requirement for mining in rock-burst environment. A micro-seismic network is systematically used to monitor coal extraction in the Provence colliery (south-eastern France). A significant database of seismic events is now available. A further advancement in this sequence was added by the development of Doublets technique or multiplet selections. The technique uses with the database to identify families of seismic events and to work out the relative locations of these events. The analysis of location of these multiplet families showed a particular spatial distribution. Locations of the epicentre follow the direction in which the coalface is advancing. Numerical models are used in the paper to show the overstress distribution. By comparing the seismic activity and results of the numerical modeling of the advancing coalface with observations, a close correlation can be established between the location of seismic activity and induced stresses in the ground surface of the working areas. By combining micro-seismic monitoring with the numerical modeling, the volume of influence of long-wall mines is determined. Results show that such a coupling can be an efficient way for detecting areas subjected to rock-burst hazards. This can also prove to be a powerful tool for assisting the planning of underground workings in complex geological and mining conditions.

Coal-Mining Seismicity and Ground-Shaking Hazard: A Case Study in the Trail Mountain Area, Emery County, Utah

We describe a multipart study to quantify the potential ground-shaking hazard to Joes Valley Dam, a 58-m-high earthfill dam, posed by mining-induced seismicity (mis) from future underground coal mining, which could approach as close as ∼1 km to the dam. To characterize future mis close to the dam, we studied mis located ∼3–7 km from the dam at the Trail Mountain coal mine. A 12-station local seismic network (11 stations above ground, one below, combining eight triaxial accelerometers and varied velocity sensors) was operated in the Trail Mountain area from late 2000 through mid-2001 for the dual purpose of (1) continuously monitoring and locating mis associated with longwall mining at a depth of 0.5–0.6 km and (2) recording high-quality data to develop ground-motion prediction equations for the shallow mis. (Ground-motion attenuation relationships and moment-tensor results are reported in companion articles.) Utilizing a data set of 1913 earthquakes ( M ≤ 2.2), we describe space-time-magnitude distributions of the observed mis and source-mechanism information. The mis was highly correlated with mining activity both in space and time. Most of the better-located events have depths constrained within ±0.6 km of mine level. For the preponderance (98%) of the 1913 located events, only dilatational P -wave first motions were observed, consistent with other evidence for implosive or collapse-type mechanisms associated with coal mining in this region. We assess a probable maximum magnitude of M 3.9 (84th percentile of a cumulative distribution) for potential mis close to Joes Valley Dam based on both the worldwide and regional record of coal-mining-related mis and the local geology and future mining scenarios.

Development of Ground-Motion Prediction Equations Relevant to Shallow Mining-Induced Seismicity in the Trail Mountain Area, Emery County, Utah

To provide a basis for assessing the seismic hazard to the Joes Valley Dam due to future coal mining in the nearby Cottonwood Tract, central Utah, we developed ground-motion prediction relations using data recorded by a seismic network, established and operated by the University of Utah Seismograph Stations. The network was centered on the Trail Mountain coal mine, located adjacent to the Cottonwood Tract. From late 2000 until early 2001, this network recorded numerous mining-induced events with magnitudes as large as 2.17. The ground motion from these events, recorded at hypocentral distances ranging from about 500 m to approximately 10 km, were well suited to developing new ground-motion prediction relations, especially when augmented by data from a M 4.2 earthquake in the Willow Creek mine, about 50 km north of Trail Mountain. Using a two-stage regression analysis, we determined prediction relations for peak acceleration, peak velocity, and pseudovelocity response spectra, at 5% damping, for periods of 0.1, 0.2, 0.5, 1.0, and 2.0 s. To illustrate the potential seismic hazard at the Joes Valley dam, we used these ground-motion relations to predict a peak velocity of 6.8 cm/s due to an earthquake with the probable maximum magnitude of 3.9, at a hypocentral distance of 1 km, recorded at a rock site typical for this region. This result does not take into account the site response at the dam.

Fractal analysis of earthquake swarms of Vogtland/NW-Bohemia intraplate seismicity

The special type of intraplate microseismicity with swarm-like occurrence of earthquakes within the Vogtland/NW-Bohemian Region is analysed to reveal the nature and the origin of the seismogenic regime. The long-term data set of continuous seismic monitoring since 1962, including more than 26000 events within a range of about 5 units of local magnitude, provides an unique database for statistical investigations. Most earthquakes occur in narrow hypocentral volumes (clusters) within the lower part of the upper crust, but also single event occurrence outside of spatial clusters is observed. Temporal distribution of events is concentrated in clusters (swarms), which last some days until few month in dependence of intensity. Since 1962 three strong swarms occurred (1962, 1985/86, 2000), including two seismic cycles. Spatial clusters are distributed along a fault system of regional extension (Leipzig-Regensburger Störung), which is supposed to act as the joint tectonic fracture zone for the whole seismogenic region. Seismicity is analysed by fractal analysis, suggesting a unifractal behaviour of seismicity and uniform character of seismotectonic regime for the whole region. A tendency of decreasing fractal dimension values is observed for temporal distribution of earthquakes, indicating an increasing degree of temporal clustering from swarm to swarm. Following the idea of earthquake triggering by magma intrusions and related fluid and gas release into the tectonically pre-stressed parts of the crust, a steady increased intensity of intrusion and/or fluid and gas release might account for that observation. Additionally, seismic parameters for Vogtland/NW-Bohemia intraplate seismicity are compared with an adequate data set of mining-induced seismicity in a nearby mine of Lubin/Poland and with synthetic data sets to evaluate parameter estimation. Due to different seismogenic regime of tectonic and induced seismicity, significant differences between b-values and temporal dimension values are observed. Most significant for intraplate seismicity are relatively low fractal dimension values for temporal distribution. That observation reflects the strong degree of temporal earthquake clustering, which might explain the episodic character of earthquake swarms and support the idea of push-like triggering of earthquake avalanches by intruding magma.

Using Neural Networks to Discriminate between Genuine and Spurious Seismic Events in Mines

—Careful observation has shown that mining-induced seismicity follows a multimodal distribution, which we assume to arise from many distinct physical processes. The two major modes however, arise from those seismic events that are associated in some way with geological features on the one hand, and those that are associated, among other things, with fracturing in the volume of extreme stress concentrations ahead of the stope faces, on the other. We call the former "genuine" events and the latter "spurious" events.¶Untangling these modes has been a major problem for those researchers wishing to work with unimodal seismic catalogs. Partial separation of the genuine events from a catalog can be obtained by a careful selection from a scatter diagram of log (radiated seismic energy) against log (scalar seismic moment) or equivalently by selecting a threshold value of magnitude say, from an inspection of the Gutenberg-Richter diagram. This threshold is usually considerably greater than the threshold of completeness that can be achieved by modern seismic networks on mines.¶The main objective of this paper will be the demonstration that a simple neural network can improve this separation. In this study, for example, simple elimination below the threshold of log (scalar seismic moment) = 9.5 resulted in 206 genuine events remaining in the catalog. After running the eliminated events through a trained neural network, an additional 72 genuine events were found, representing an increase of nearly 35%.¶This has important consequences for statistical hazard analysis and for the identification of active geological structures in mines.

Strike stabilizing pillars as a regional support strategy for ultra-deep gold mines

Strike stabilizing pillars are included in the mine layout of a number of deep South African gold mines as a means of providing regional support with the principal aim of controlling rockbursts. Large seismic events associated with stabilizing pillars can cause extensive damage to working areas. Mining-induced seismicity recorded at Western Deep Levels Limited has been analysed in an attempt to improve the design of stabilizing pillars, and thereby reduce their associated seismic hazard. This work revealed that the vast majority of stabilizing pillars, regardless of their dimensions and those of their adjacent stopes, will, at some time, give rise to seismic events of magnitude, M≥2. Contrary to expectations, this work strongly indicates that the rock mass in the near vicinity of the mined out areas does not behave in an elastic manner. Consequently, the currently employed design methodologies, based on elastic principles, should not provide the only criteria when designing strike stabilizing pillars.