Semi-controlled seismogenic experiments in South African deep gold mines (original) (raw)
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Multidisciplinary Monitoring of the Entire Life Span of an Earthquake in South African Gold Mines
2005
deep gold mines. In the second field experiment, the authors successfully monitored the entire strain history within a hundred metres from the hypocentres, associated with a few seismic events with M>2. However, there were no close strong-motion meters available to locate asperities; only a single strainmeter was available, so the authors were not able to locate the strain-change source; no in situ stress measurements were carried out at the site, and no information was available to constrain strength. In order to address these deficiencies, from 2003 to 2004, the authors deployed new experimental instrument arrays at fault bracket/stabilising pillars. Multiple strainmeters, arrays of strong ground-motion meters, sensitive thermometers to monitor seismic heat generation, and fault displacement meters were installed. Successful monitoring began, but the authors learnt that they had to develop instruments for much quicker drilling and installation, especially at highly stressed pil...
Proceedings of the Ninth International Conference on Deep and High Stress Mining, 2019
In 2014, a M5.5 earthquake ruptured the range of depths between 3.5 km and 7 km near Orkney, South Africa. The main and aftershocks were very well monitored in the nearfield by dense, surface, strong motion meters and a dense underground seismic network in the deep gold mines. The mechanism of this M5.5 earthquake was left-lateral strike-slip faulting, differing from typical mining-induced earthquakes with normalfaulting mechanisms on the mining horizons shallower than 3.5 km depth. To understand why such an unusual event took place, the aftershock zone was probed by full-core NQ drilling during 2017-2018, with a total length of about 1.6 km, followed by in-hole geophysical logging, core logging, core testing, and monitoring in the drilled holes. These holes also presented a rare opportunity to investigate deep life. In addition, seismogenic zones of M2-M3 earthquakes were probed on mine horizons that were also very well monitored by acoustic emission networks. This paper reviews the early results of the project.
2012
Mining-induced earthquakes pose a risk to workers in deep mines, while natural earthquakes pose a risk to everywhere, but especially near plate boundaries. A five year Japanese-South African collaborative project entitled 'Observational studies to mitigate seismic risks in mines' commenced in August 2010. Here we report on the achievements of the first 18 months of the project. Faults at Ezulwini, Moab-Khotsong and Driefontein gold mines considered likely to become seismically active during mining activity were modelled using pre-existing geological information supplemented by cores and camera images from new boreholes. As of 30 January 2012, about 90% of about 70 planned boreholes totalling more than 2 km in length had been drilled at project sites to locate fault zones accurately and to deploy sensors. Acoustic emission sensors, geophones, accelerometers, strain-and tilt meters, and controlled seismic sources were installed to monitor the deformation of the rock mass, the accumulation of damage during the earthquake preparation phase, and changes in stress produced by the propagation of the rupture front. The suite of sensors has greater sensitivity and dynamic range than those typically used in civil or mining engineering applications, making it possible to record very small changes in stress and strain as well as violent rock mass deformation associated with large seismic events. These data sets will be integrated with measurements of stope closure, strong ground motion in stopes, and seismic data recorded by the mine-wide network.
Earth, Planets and Space, 2009
This is the first report from the JAGUARS (JApanese-German Underground Acoustic Emission Research in South Africa) project, the overall aim of which is to observe ultra-small fracturing in a more or less natural environment. We installed a local (∼40-m span) network of eight acoustic emission (AE) sensors, which have the capability to observe up to 200 kHz at a depth of 3.3 km in a South African gold mine. Our specific objective was to monitor a 30-m thick dyke that remains as a dip pillar against active mining ∼90 m above our network. An M w 1.9 earthquake whose hypocenter was ∼30 m above the network occurred in the dyke. Although the mineowned geophone (4.5 Hz) network detected only five earthquakes in the surrounding 200×200×150-m 3 volume within the first 150 h following the main shock, our AE network detected more than 20,000 earthquakes in the same period. More than 13,000 of these formed a distinct planar cluster (∼100×80 m 2 ) on which the main shock hypocenter lay, suggesting that this cluster delineates the main shock rupture plane. Most of the aftershocks were presumably very small, probably as low as M ∼ −4. The aftershock cluster dipped ∼60 • . This is consistent with normal faulting under a nearly vertical compression field, as indicated by nearly horizontal breakouts found in a borehole crossing the rupture plane.
Japanese-South African collaboration to mitigate seismic risks in deep gold mines
2009
Mining-induced seismicity poses a hazard to workers in deep South African mines, while natural earthquakes pose a hazard to people living in Japan and other regions of the world that are close to plate boundaries. We introduce a 5-year Japanese-South African collaborative project entitled "Observational study to mitigate seismic risks in mines". The principal investigators are H. Ogasawara (Japan) and RJ Durrheim (South Africa). The project will build on previous studies carried out by Japanese seismologists in South African mines, and will develop human and instrumental capacity in South Africa. This knowledge will contribute to efforts to upgrade schemes to assess seismic hazard and to mitigate the seismic risks in deep mines. The knowledge is also relevant to the study of the mechanisms that generate tectonic earthquakes. The project was conditionally approved in April 2009 by the Japan Science and Technology Agency (JST), an external agency of the Ministry of Education, Culture, Sports, Science and Technology, and the Japan International Cooperation Agency (JICA), an external agency of the Ministry of Foreign Affairs. It is anticipated that the agreement between the Japanese and South African governments will be concluded by the end of the 2009 financial year and that research work will commence in 2010.
2010
Mining-induced earthquakes pose a risk to workers in deep South African mines, while natural earthquakes pose a risk to people living close to plate boundaries. We introduce a 5-year Japanese - South African collaborative project entitled "Observational studies to mitigate seismic risks in mines" that commenced in 2010. The project, which seeks to develop human and instrumental capacity in South Africa, will build on previous studies carried out by Japanese and South African seismologists in deep gold mines. The project has five major work streams: (i) determination of rock properties, (ii) sensitive close monitoring, (iii) seismic hazard assessment methods, (iv) strong ground motion monitoring, and (v) upgrading of the South African National Seismological Network in the mining districts. Some aspects of the study will also cast light on the mechanisms that generate tectonic earthquakes.
11th SAGA Biennial Technical Meeting and Exhibition
Mining-induced earthquakes pose a hazard to workers in deep South African mines, while natural earthquakes pose a hazard to people living close to plate boundaries. We introduce a 5-year Japanese-South African collaborative project entitled "Observational study to mitigate seismic risks in mines". The principal investigators are H. Ogasawara (Japan) and RJ Durrheim (South Africa). The project, which seeks to develop human and instrumental capacity in South Africa, will build on previous studies carried out by Japanese and South African seismologists and rock engineers in deep gold mines. This knowledge will be used in efforts to upgrade seismic hazard assessment schemes and to mitigate the seismic risks in deep mines. The knowledge is also relevant to the study of the mechanisms that generate tectonic earthquakes. The project was conditionally approved in April 2009 by the Japan Science and Technology Agency (JST), an external agency of the Ministry of Education, Culture, Sports, Science and Technology, and the Japan International Cooperation Agency (JICA), an external agency of the Ministry of Foreign Affairs. It is anticipated that the agreement between the Japanese and South African governments will be concluded by the end of the 2009 financial year and that research work will commence in 2010.
Pure and Applied Geophysics, 2020
An integrated geophysical approach using seismics and geoelectrical techniques was employed to investigate the architecture of historic narrow-reef workings and a proposed openpit mine at Lancaster Gold Mine, near Krugersdorp, South Africa. The mining activities in the area were mainly carried out within the Kimberley Reef Package in the upper Central Rand Group of the Witwatersrand Supergroup, which hosts several gold-bearing conglomerates (locally known as reefs). The reefs are generally thin (B 2 m thick) and dip between 28°and 32°south. The low-velocity weathered layer introduces significant static shifts in the reflection seismic data. Moreover, environmental noise from drilling and trucking, and the prominent bedrock-overburden contact that produces various wave conversions (P-S conversion), caused undesirable noise that contaminates the shot gathers as high-amplitude, source-generated and monochromatic noise. The noise was removed from the shot gathers using frequency and velocity filtering techniques. The final depth-migrated sections are characterised by high-resolution images of the subsurface from *10 to *150 m depth, which are constrained by the borehole information. The reflection seismic data delineate the interfaces between different rock layers and the stopes. The refraction and resistivity tomograms, on the other hand, provide more detailed images of the top 20-50 m of the subsurface and depict the approximate shallow geometry of fluid migration paths, mined-out areas, and bedrock-overburden boundaries. The integrated results indicate that the study area is characterised by a weathered surface layer with variable low P-wave velocity (400-1200 m/s) and resistivity (150-800 Xm). The deeper layer reveals an increase in resistivity and velocity, and it's characterized by discontinuities, weak zones, cavities or water-bearing zones due to the mining activities. The combined borehole and geophysical data provide valuable information regarding the physical characteristics of the subsurface and can be helpful for future risk management decisions, environmental and engineering studies in the area.
2008
In this project, we are developing and exploiting a unique seismic dataset to address the characteristics of small seismic events and the associated seismic signals observed at local (<200 km) and regional (<2000 km) distances. The dataset is being developed using mining-induced events from three deep gold mines in South Africa recorded on in-mine networks (<1 km) composed of tens of high-frequency sensors, a network of four broadband stations installed as part of this project at the surface around the mines (1-10 km), and a network of existing broadband seismic stations at local/regional distances (50-1000 km) from the mines. Data acquisition has now been completed and includes: (i) ~2 years (2007 and 2008) of continuous recording by the surface broadband array, and (ii) tens of thousands of mine tremors in the-3.4 < ML < 4.4 local magnitude range. Events with positive magnitudes are generally well recorded by the surface-mine stations, while magnitudes of 3.0 and larger are seen at regional distances (up to ~600 km) in high-pass filtered recordings. We have now completed the quality control of the in-mine data gathered at the three gold mines included in this project. The quality control consisted of: (i) identification and analysis of outliers among the P-and S-wave travel-time picks reported by the in-mine network operator and (ii) verification of sensor orientations. The outliers have been identified through a 'Wadati filter' that searches for the largest subset of P-and S-wave travel-time picks consistent with a medium of uniform wave-speed. We have observed that outliers are generally picked at a few select stations. We have also detected that trigger times were mistakenly reported as origin times by the in-mine network operator, and corrections have been obtained from the intercept times in the Wadati diagrams. Sensor orientations have been verified through rotations into the local ray-coordinate system and, when possible, corrected by correlating waveforms obtained from theoretical and empirical rotation angles.