Induced seismicity associated with longwall coal mining

Int. J. Rock Mech.Min. Sci. &Geomech.Abstr.Vol.25. No. 5. pp. 253-262. 1988 Printed in Great Britain.All rightsrer~r~ed

0148-9062/88 $3.00+ 0.00 Copyright"~ 1988PergamonPressplc

Induced Seismicity Associated with Longwall Coal Mining K. SATO* Y. FUJII*

This paper describes the influence of mining parameters and geological conditions on seismicities associated with longwall coal mining. Microseismic activities were observed by using a mine-wide seismic array at 13 mining panels in the deepest coal mine in Japan. Most microseismic events were located in the vicinity of coal faces. In this case where a coal face was advanced along the rib-side of an old working, the tail entry T-junction suffered the most severe seismicity in the panel. The seismicity also intensified during the widening of a coal face. The seismicity at a panel was significantly alleviated by the presence of old workings above the panel, but intensified by a coal pillar left in an old working above the panel. The seismicity was also controlled by a fault. Stress analysis using a numerical model was attempted and the results offered good interpretation of the distribution of seismic energy density.

INTRODUCTION

MEASURING SYSTEM

Microseismic activity associated with coal mining has The mine-wide seismic array in this mine consists of been experienced in many parts of the world. It has been 10 stations; 6-8 stations are placed on the surface, and monitored by using an underground array of acceler- the rest are on underground roadways near mining ometers or geophones [1-4], or by using a near surface panels being extracted. The underground seismic array of geophones [5]. The induced seismicity associ- stations are capable of detecting small seismic events ated with hydraulic mining was monitored at a Japanese whose magnitudes are down to -2.0, thereby reducing coal mine by means of a mine-wide seismic array which the errors in focal depths of seismic events. consists of surface and underground stations [6, 7]. Large A block diagram of the monitoring system is given seismic events at coal mines, whose magnitudes were up in Fig. 1. Velocity-type seismometers with built-in to 3.5, were also observed by a regional seismic network preamplifiers, Tokyo Sokushin QS-I 1I B, which is intrin[8-11], and a few of the largest seismic events, whose sically safe and whose proper frequency and damping magnitudes were greater than 4, were recorded at the constants are 1.5 Hz and 0.7 respectively, have been WWSSN stations throughout Europe [12]. used. Seismic signals detected by surface stations are In this investigation, a microseismic technique was immediately digitized (sampling time is 2.5 msec) at the applied to monitor the induced seismicity associated site of each station and the digital signals are transmitted with Iongwall coal mining. A field observation has been (4800 baud, 72 dB) to the recording station, while that carried out since 1980 at Horonai Coal Mine, the deepest detected by an underground station are digitized at the coal mine in Japan. The objectives of the investigation recording station. are (a) to develop an automatic monitoring facility The seismic signal that exceeds a prescribed level is applicable to coal mines and (b) to comprehend the recorded automatically by a triggering method on a influence of mining parameters and geological condi- standard 9-track magnetic tape. The seismic data of an tions on the induced seismicity in a typical Iongwall event consist of receiving time and 10 1.5-sec digital mining. seismograms. The monitoring facility developed during the course of An iterative least squares method assuming an anisothe investigation has been presented in another paper tropic P-wave velocity model which was determined [13]. Therefore, the case study on 13 Iongwall panels from underground blasts [13] was used to locate the related to the second problem are presented in this hypocentres of seismic events. Figure 2 shows the paper. An example of numerical calculation is also difference between the actual blast points and the located shown to compare the distribution of seismic energy sources. Most of them are less than 10 m in all directions. density with the stress distribution. Richter's scale of magnitude and seismic energy were estimated by using the Muramatsu's formula and the Gutenberg-Richter's formula, the same formula used in * Department of Geotechnology,Muroran Instituteof Technology, the investigation of microseismicity in hydraulic mining 27-1 Mizumoto. Muroran 050, Japan. [7]. R.M M.S 2S,~--^

253

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SATO and FUJII:

COAL MINING INDUCED SEISMICITY

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SATO and FUJII:

COAL MINING INDUCED SEISMICITY

255

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MINING CONDITIONS OF LONGWALL PANELS The mining area at Horonai Coal Mine is about 2 km long in the strike direction of the coal bearing formation and about 1 km wide in its dip direction. Mining took place at 985-1125m below the surface during this investigation. Figure 3 shows the plan view of the mining panels and the seismic array. A schematic view of the geological section is shown in Fig. 4. The coal seams incline 20-30 ° to NE. The No. 5 upper, No. 5 main, No. 4 upper, No. 4 main and No. i main seam are usually worked. The distance between each pair of upper and main seams is about 8 m. The immediate roof of No. 5 upper seam consists of thin sandstone or sandy shale and is overlayed by thick shale. The strata intervening between the upper and the main seam consist of shale, sandy shale, sandstone with various grain size, coal shale and bad coal. The uniaxial compressive strengths of the shale, the sandy shale, the sandstone and the coal shale are 50-85, 154-164, 60-175 and 35 MPa, respectively [14]. Results of in situ stress measurements are not consistent with each other, but suggest that the tectonic lateral stress is dominant. The mining technique used in this mine is the advancing longwall extraction with total caving. Each pair of main and tail entries is 120-200m apart in the dip

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direction. A coal face with fully mechanized equipment moves 350-700 m along the strike direction from a set up entry to a recovery entry. Panels in the No. 1 main seam are equipped with hydraulic props because the seam height is not thick enough to adopt powered supports. Paired panels at the same mining depth in No. 5 or No. 4 seams are mined simultaneously, where the coal face in the upper seam is kept 50-100 m ahead of the lower seam. SPATIAL DISTRIBUTION OF SEISMICITY

Distribution of hypocentres around a coal face Figure 5 shows an example of the distribution of the hypocentres at the panel W8-5U. The distribution of epicentres along the line of the face advance is almost symmetrical with respect to a position 10-30 m ahead of the coal face, as shown in Fig. 5a. About two-thirds of the events are located in a bounded zone 60 m ahead of and 20 m behind the face. The focal depth distributions in Fig. 5b are also symmetrical. Most events are distributed on the main roof 20-80 m above the coal seam. On the other hand, seismicity is not active in the foot wall. These results seem to be different from those in South African gold mines [15-17], but partly similar to those of Polish and German coal mines [1,3, 4]. The profile of

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Fig. 5. Distribution of epicentres relative to the face (a) and that of focal depth relative to the coal seam (b) for the panel W8-5U. The closed columns and the open columns represent the frequency of events whose magnitudes are greater than - 1.0 and from - 2 . 0 to -1.0, respectively.

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Fig. 6. The seismic energy contour maps for the panel W8-5U. The x-axis directs towards the direction of the face advance, the y-axis is parallel to the coal face and the z-axis is normal to the coal seam. The maps are projected on the seam plane, on the section normal to the direction of the face advance and on the section normal to the face line, from left to right. Face spans are (a) 0-I00 m, (b) 100-200 m, (c) 200-300 m, (d) 300--400 m and (e) 40ff--450 m, respectively.

SATO and FUJII:

COAL MINING INDUCED SEISMICITY

the fracture zone delineated by the induced seismicity is in harmony with that obtained by surveying settlements and mapping strains of roof strata [18-20].

Distribution of seismic energy density around the coalface The seismic energy contour map, which is the distribution of the seismic energy density accumulated during a given period, was also used to indicate the spatial distribution of the seismicity. The origin was placed at the mid-point of the coal face, so that the density distribution relative to the coal face can be easily recognized. Figure 6 shows the seismic energy contour maps for the panel W8-5U. On the left maps, there were two peaks at the first stage of mining, as shown in Fig. 6a. These peaks were located at the mid-point of the coal "face and at the tail entry T-junction. At the following stages, the peak at the tail entry T-junction disappeared as shown in Fig. 6b and c. In Fig. 6d, the peak which was originally at the mid-point moved towards the tail entry T-junction. At the final stage, Fig. 6e, two peaks appeared again at the same positions as in the first stage. The peaks on the middle maps were 30-70 m above the coal seam during the entire mining period. On the right maps, the peak were situated 30-60 m above the coal seam before the final stage. At the final stage, the peak moved to a position 10 m above the seam as shown in Fig. 6e.

257

When the coal face was moved by about 80 m from the set up entry, the seismic energy release rate was low. Then, the rate rapidly increased with every face advance, and attained the maximum value when the face span was 120 m. The abutment stress in front of the coal face might increase with the face span, and intensify more if the side and the front abutment stresses of the old working are superposed. Therefore, the narrow zone between the panel and the old working is expected to have been subjected to high stress as if it were a coal pillar. In the case where the coal face was widened, the release rate increased as shown in Fig. 8. In this panel the face, which was originally 160 m wide, was widened by 40 m when the coal face advanced by 105 m from the set up entry. Here, the seismic energy release rate was calculated per unit area of coal extraction instead of the unit face advance. The release rate was low at first, then it gradually increased with face advance. When the coal face moved by 30 m from where the face was widened, the release rate reached its highest value during the entire mining period. When the seismic energy release rate reached the highest level, the epicentres were concentrated on the corner formed by the widening. The intense seismicity at this corner may be attributed to stress concentration. These results suggest that the induced seismicity associated with longwall coal mining may be related to the stress concentrations caused by complex mine geometry.

INFLUENCE OF MINE GEOMETRY The intensity of seismicity is represented by the seismic energy release rate, i.e. the total sum of the seismic energy released during a unit face advance, and the cumulative seismic energy. Figure 7 indicates the seismic energy released at the panel W8-5U, where the coal face was advanced towards the old working, then along its rib-side.

INFLUENCE OF MULTIPLE SEAMS EXTRACTION Figure 9 shows the cumulative seismic energy curves of five mining panels in different coal seams at the 8th level. The highest cumulative seismic energy is monitored at panel W8-5U and the lowest at panel W8-5M, i.e. the former is five times bigger than the latter. Those

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at panels W8-4U, W8-4M and W8-1M are about onethird of that at panel W8-5U. This result is similar to the case of the extraction of multiple seams in a hydraulic mine [6], and would be interpreted by the difference of the pre-mining stresses on each seam. According to their stratification and the sequence of excavation, the stresses on the seams except the top seam (No. 5 upper seam) are reduced by old workings. When a coal face in a lower seam proceeded to a region which is outside a mined out area in an upper seam, the seismic energy release rate at the lower panel increased. The mining of panel W7-4M took place below a gob of the panel W7-4U until the coal face moved 420 m from the set-up entry. Then the coal face pro-

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ceeded outside the region below the old working. The seismic energy release rate during the mining of the virgin area was about three times bigger than the average level monitoring during mining below an old working, as shown in Fig. 10. It is also interesting that, when the coal face remains below an old working, the curve of seismic energy release rate was symmetrical with respect to the mid-span of the old working. The stress distribution in an old working panel is expected to be the greatest at its centre, and decreases symmetrically towards the set-up entry and the recovery entry. The curve seems to reflect the stress distribution along the face span in the old working panel.

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SATO and FUJII:

COAL MINING INDUCED SEISMICITY

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INFLUENCE OF COAL PILLAR

Induced seismicity was activated when a coal face in a lower seam passed under a coal pillar in an upper seam as in English coal mine [9, I 1]. Figure I l shows the mine geometry and the curves of the seismic energy release rate and the cumulative seismic energy at the panels C8-5U and C8-5M. In this case, a small triangularshaped coal pillar was left in the panel C8-5U. The upper face was advanced leaving this coal pillar, then the lower face passed under it. The seismic energy release rate at the upper panel decreased when the coal face approached the coal pillar,

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while the release rate at the lower panel increased significantly. As the face was passing under the coal pillar, the release rate at the lower panel amounted to about 70% of the maximum value of the release rate at the upper panel. This result suggests that the induced seismicity in the mining of multiple seams might be considerably controlled by leaving a coal pillar, even if it is small. INFLUENCE OF FAULT In addition to mining parameters, a pre-existing fault also affects induced seismicity, as experienced in a

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S A T O and F U J I I :

COAL MINING

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hydraulic mine [7, 21]. Figure 12 illustrates a thrust fault at panels C7-5U and C7-5M. The fault dipped 20-22 ° to the NNW with a throw of 1.2-4.1 m, and it intersected the upper and the lower panels at 380 and 340 m from the set-up entry, respectively. On the plan, the fault intersected the coal faces of the two panels at 8-18 °. The two coal faces approached the fault from the hanging wall, and passed through it. Since the two seams were extracted almost simultaneously, the seismicities were difficult to distinguish. Therefore, the sum of the seismic energy release rates and that of the cumulative seismic energies at the two panels were calculated. The curves shown in Fig. 13 represent the sum of the seismic energy release rates at the upper and the lower panels. As the face approached the fault (the face span in the upper panel was 245-380 m at that time), the seismic energy release rate was significantly low. However, after the upper face passed through the fault, the seismic energy release rate recovered to a level which was almost the same as that in the initial stage of mining. It implies that when a coal face passes a thrust fault from the hanging wall, the seismicity might be temporarily suppressed before the passage. The same results were obtained in a scale model study [23] and finite element analysis utilizing Goodman's joint element [24]. DISCUSSION The induced seismicity presented in this paper appears to be controlled by the induced stress associated with mining. In order to confirm a relation between the induced stress distribution and the seismicity, the distribution of seismic energy density was compared with the stress distribution obtained numerically. The panel W8-5U was chosen as an example. The stress distribution was determined by using the three-dimensional displacement discontinuity method [22]. The coal seams were considered as plate-like inclusions, with a thickness of 2 m, embedded in an isotropic linear elastic medium, and dipped at 20L The lithostatic pressure due to overburden was assumed as the initial

stress. The Young's modulus and the Poisson's ratio of the surrounding rock and those of the coal seam were assumed 2 GPa, 0.2, l GPa, 0.2, respectively. The rectangular mining region which was 840 m long in the strike direction and 630 m wide in the dip direction was divided into 1200 square elements with sides equal to 21 m. The boundary condition was assigned to all elements with respect to the mine geometry (whether the coal seam had been extracted or not). Figure 14 indicates the distributions of the stress normal to the coal seam along two lines. One is the face line AA' that moves with the face advance, another is the line BB' passing the tail entry that is stationary regardless of the face advance. The mining progress was divided into intervals of 100 m in the similar manner in Fig. 7. At the first stage, a stress peak was located at the mid-point of the face as show in the Fig. 14a. At the subsequent mining steps, the peak appeared at the tail entry T-junction. These results are in harmony with the fact that the peak of seismic energy density often appeared on the tail entry T-junction. This agreement confirms the applicability of numerical method for rough prediction of the spatial distribution of seismicity around a coal face. On the other hand, the microseismic technique is a useful tool in monitoring complex geomechanical properties of strata around a coal face which are not fully considered in numerical models.

CONCLUSIONS The microseismic activities have been monitored at 13 Iongwall panels in Horonai Coal Mine, Japan. These measurement results were analyzed to comprehend the influence of mining and geological parameters on the induced seismicity associated with a typical mechanized longwall mining. The seismicity around coal faces was characterized by means of the seismic energy contour map and the seismic energy release rate, and the former was compared with

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the stress distribution obtained from the numerical calculation. The results obtained from the investigation are as follows: (I) The spatial distribution of the epicentres of seismic events was almost symmetrical to position which are

10-30 m ahead of the coal face. Two-thirds o f t h e seismic events were distributed in a zone which is between 60 m ahead of and 20 m behind the coal face. (2) According to the focal depth distribution, most of the seismic events were located in the roof strata during the whole mining period. The maximum frequency in the

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262

SATOand FUJII:

COAL MINING INDUCED SEISMICITY

distribution was observed at 20-80 m above the coal seam. (3) The seismicity was activated when a coal face approached an old working. (4) When a mining panel was advanced along the rib-side of old workings, the peak of the seismic energy density distribution often appeared on the tail entry T-junction. (5) In the case where a face was widened during a face advance, the induced seismicity was most severe just after widening. A large number of seismic events were located on the corner formed by the widening. (6) The induced seismicity at the panels below old workings was not active. The seismic energy release rates at these panels were only 20-40% of that at a panel in the virgin field at the same depth. (7) In the case where a coal face advanced under an old panel, the seismic energy release rate increased with the increase of face span until the face reached the mid-span, then it decreased symmetrically. (8) Where a coal face at the lower seam passed under a coal pillar or proceeded to a region which is outside a mined out area of the upper seam, the seismic energy release rate increased. (9) In the case where a coal face of a longwall panel passed through a fault from the hanging wall, the seismic energy release rate decreased before the passage, and recovered after it. (10) The peaks of the seismic energy distribution often appeared at the same positions with those of the normal stress distribution determined from the numerical method. This agreement encourages the development of advanced arts of microseismic technique and the firm combination of microseismic research with numerical modelling in the study of strata mechanics in conjunction with longwall mining. The microseismic technique described in this paper has been recently applied to other coal mines in Japan. Acknowledgements--We are grateful to Professor Isobe of Hokkaido University and Dr Fukushima of the Coal Mining Research Center, Japan for their encouragement and valuable suggestions during this investigation. We thank Mr S. Uno and Mr T. Tatsumi of Horonai Coal Mine for their co-operation during the course of the investigation. This investigation has been funded by the Coal Mining Research Center. Japan. Received 6 March 1987; ret'ised 17 September 1987 and 13 March 1988.

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