Integrating microseismic and 3 D stress monitoring with 1 numerical modeling to improve ground hazard assessment (original) (raw)
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Routine Micro-Seismic Monitoring in Mines
Routine seismic monitoring in mines enables the quantification of exposure to seismicity and provides a logistical tool to guide the effort into the prevention and control of, and alerts to, potential rock mass instabilities that could result in rock bursts. One can define the fol-lowing five specific objectives of monitoring the seismic response of the rock mass to mining: rescue of personnel, prevention, seismic hazard rating, alerts -including short term response to unexpected strong changes in certain parameters -and back analysis to improve the efficiency of both the mine layout design and the monitoring pro-cess. A quantitative description of seismic events and of seismicity are necessary, but not sufficient, in achieving the above objectives. The paper describes the basis of a modern digital seismic technol-ogy and seismological parameters used to quantify seismic sources and seismicity for seismic hazard assessment and rock mass stability analysis.
International Journal of Rock Mechanics and Mining Sciences, 2020
In this work we investigated seismic and aseismic rock mass behaviors in response to deep underground mining. For this purpose, an area under production of the metal mine of Garpenberg (Sweden) was instrumented with a geophysical and geotechnical monitoring network. In situ monitoring data were analyzed and interpreted considering mining operations and the local geological setting. In addition, a 3D elasto-plastic numerical model was built to better understand the interactions between quasi-static stress changes due to mining and the generation of the induced seismicity. Results of this multiparameter approach show a complex rock mass response. We observed two main types of seismic behaviors: one local and temporally short, directly induced by production, the other long-lasting over time and remote from excavations being mainly controlled by geological heterogeneities. In addition to seismicity, we also observed creep-like phenomena induced by mining. In turn, these time-dependent strains appear to be a third mechanism driving seismicity. All these findings underline the importance of considering both seismic and aseismic deformations when one wants to characterize the rock mass response to mining. This significantly enhances our understanding of the phenomena involved, as well as their interactions, for an improved hazard assessment in deep mining operations.
Pure and Applied Geophysics
Back analysis for evaluation of the merits of the short-term seismic hazard indicators (precursors) used in the mines and their potential application for early warning was carried out for fourteen seismic events that potentially caused damage in Kiirunavaara Mine, Sweden, selected according to our designed criteria. Five short-term hazard indicators: Seismic Activity Rate (SAR), Cumulative Seismic Moment (CSM), Energy Index (EI), Cumulative Apparent Volume (CAV) and Seismic Apparent Stress Frequency (ASF) were tested. The behaviour of the indicators was studied using the parameters of all seismic events within a sphere around the hypocenter location of the analyzed seismic source within one month before the main (damaging) event. The size of the sphere equals the estimated radius of the analyzed seismic source (area of inelastic deformation). mXrap software (Australian Centre for Geomechanics) was used for data visualization, manipulation, analysis and extraction. The results from t...
2020
Kiirunavaara mine, owned by LKAB, is the largest iron ore underground mine. It is about 4 km long and has been mined with sublevel caving since the late 1950s. The current mining levels in the main orebody are 1079 m and 1108 m. The sublevel height is 29 m and it takes about two years to mine one sublevel in the southern half of the mine. As such, the mining-induced stress changes were regarded to be relatively slow. There are 2700 MW ≥ 0 seismic events recorded on average every year, with the largest event recorded on 18 May 2020 MW 4.3. Not many rock mechanical monitoring campaigns have been undertaken during the mine’s history, except for a mine-wide seismic system which has been in operation since 2009. In 2013, 3D stress measurements with the Borre stress cell were undertaken at level 1165 m (production area Block 34), with subsequent installation of 3D CSIRO-HI cells for long-term stress change monitoring. The primary purpose of the installation was to measure the changes in t...
A study of the microearthquake activity observed at the Kaniani-Parnassos mines has performed in order to determine the origin of the several "unknown" shocks observed at the area of the mine. Preliminary analysis of the recordings indicated that the shocks were of tectonic origin. A portable network of three accelerographs (located inside the mine -no absolute time) and five threecomponent digital seismographs (located in surface locations on top of the mine -absolute time available) were installed for a period of nine months. The analysis of impulsive, high frequency (>20 Hz), short duration (2-3 sec) recordings, showed that the observed seismicity is of very local scale, located at a specific section of the mine. The microseismic activity exhibited a very strong time-correlation with the exploitation level, as this is revealed from the very significant variation of the earthquake rate of occurrence after the start of the operation of the mine, as well as the clear coupling on the rate of microearthquake occurrence with the rate of mining explosions. The focal mechanisms of the ruptures and the local stress field were evaluated using the first arrivals of the P-waves along with the P/S spectral ratios. The focal mechanisms are all of thrust type, indicating reverse ruptures caused by a compressional stress field with ESE-WSW direction. This field is almost perpendicular to the strike of the local old syncline-anticline, where the mine deposit is located; hence it can be safely assumed that is the "remnant" stress field that created this compressional mega-structure. The determined ruptures are in excellent agreement with the main tectonic discontinuities mapped inside the mine. A typical dynamic stress drop of 10-20 bars was also calculated for the biggest events (M~1) which is relatively comparable with the typical expected value of the mean effective isotropic stress (~50 bars).
The Practice of Seismic Management in Mines— How to Love your Seismic Monitoring System
Eleven years of seismic monitoring has been the foundation for the current understanding and management of seismicity at Mt Charlotte mine. Knowledge gained by linking seismic monitoring to seismic management has allowed the mine to survive. Seismic events possess elements of both fractal and chaotic behaviour. This gives valid basis firstly to collect seismic data, and secondly to expect that seismicity can be managed by tweaking mine designs. Engineers must collect and analyse both seismic event data and the detailed history of stope performance. This is a joint effort between computers doing the black box work, and engineers doing the value-adding. Suitable computer software and work schedules are vital in the busy mine office scenario. Examples of turning data into expertise are described, via studies of the validity of the links between seismicity and Mt Charlotte's expanded 21 empirical seismic design rules. A Self Organising Map study of the 21-Rules is in progress. Some ...
Applied Sciences
In the case of mining in an inclined intrusion using the block caving method, the highest stress is usually concentrated in the seismogenic and abutment zones, especially in the front of the sloping area. In an inclined intrusion of more than 40°, the seismometer network is usually distributed in the facility area where the footwall area is also located. This causes a limitation in microseismic monitoring due to ray coverage. In this study, we conduct a seismometer deployment outside a mining facilities area with borehole seismometers. The study aims to maximize the resolution and minimize the monitoring uncertainty of underground mines. We created two scenarios of seismometer deployment: (i) seismometers are deployed following the intrusion mining level in the mining facility area; and (ii) additional seismometers are deployed in off-facilities areas. Both areas were tested for their raypath responses and sensitivity using the Checkerboard Resolution Test (CRT). The monitoring reso...