- Email: [email protected]
Some parameters of rockbursts derived from underground seismological measurements
Available online at www.sciencedirect.com
Tectonophysics 456 (2008) 67 – 73 www.elsevier.com/locate/tecto
Some parameters of rockbursts derived from underground seismological measurements Karel Holub a,⁎, Vladimír Petroš b a
Department of Geophysics, Institute of Geonics AS CR, Studentská 1768, CZ 708 00 Ostrava-Poruba, Czech Republic b VŠB-Technical University Ostrava, 17. listopadu, CZ 708 33 Ostrava-Poruba, Czech Republic Received 31 March 2006; accepted 14 December 2006 Available online 7 February 2008
Abstract A total of 240 three-component recordings from 80 rockbursts, which occurred in various coal mines in the Ostrava-Karviná Coal Basin (Czech Republic) between 1993 and 2005, was used to examine the decrease in maximum particle velocities ui (m/s) with a scaled distance of d⁎ = d/√E (m/√J) or d/3√E (m/3√J) and the rate of predominant frequencies of body waves. The energetic span of rockbursts was within the interval of E = 6.2 × 103 − 5.0 × 108 J, while calculated hypocentral distances d of four underground seismic stations varied from 0.6 to 7 km. The slopes b of regression straight lines for the maximum particle velocities ui (m/s) of P- and S-waves in the bilogarithmic scale correspond to the values of −1.004, − 1.297, −1.183 and − 1.527. The results of the linear regression are as follows: Pmax-waves ui = 1.184 × 10− 4 × d⁎− 1.004 (m/s) (square root scaling) Pmax-waves ui = 3.055 × 10− 3 × d⁎− 1.297 (m/s) (cube root scaling) Smax-waves ui = 5.280 × 10− 4 × d⁎− 1.183 (m/s) (square root scaling) Smax-waves ui = 2.397 × 10− 2 × d⁎− 1.527 (m/s) (cube root scaling). The evaluation of the abovementioned dynamic parameters was based on seismic events data gathered in the database of the regional seismic array, and calculations were carried out either by using special programs applied as part of the automated data processing in the computation center, or by usual linear regression approaches. The aim of the detailed analysis of the maximum particle velocity and predominant frequencies was a) to set up input data from underground seismological observations for laboratory experiments dealing with the comparison of rock mass behaviour under modeled laboratory conditions simulating manifestation of rockbursts, and b) to incorporate particle velocity into the design of support in order to control damage and evident devastation of workings by rockbursts. The investigation of peak particle velocities was based on the recognition that they are the best criterion to assess vibration damage to surface structures and in mines. © 2008 Elsevier B.V. All rights reserved. Keywords: Ostrava-Karviná Coal Basin; Mining-induced seismicity; Rockbursts; Particle velocity
1. Introduction The eastern part of the Ostrava-Karviná Coal Basin (Czech Republic), which is part of the Upper Silesian Coal Basin, has been endangered by mining tremors and rockbursts for almost 100 years now, the first rockburst having occurred here in 1912. At the beginning of mining in this region, sub-surface coal seams in Ostrava partial basin were mined first, while deeper horizons of ⁎ Corresponding author. Tel.: +420 536 979 344; fax: +420 596 919 452. E-mail address: [email protected] (K. Holub). 0040-1951/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2006.12.013
coal seams were gradually mined out afterwards. From the viewpoint of the origin of rockbursts in the Karviná partial basin, two types of events were identified. While in-seam rockbursts occurred down to the depth of about 300–400 m, another more dangerous type of rockbursts was recognized at deeper horizons starting with the coal seam No. 37. This change was influenced by the stratigraphy of the deposit, where the coal seam No. 37 was overlain by thick and hard series of high-strength sandstone strata (up to 175 m). Increasing problems with rockbursts occurrence led to the erection of both local and regional systems of seismic monitoring in mines and adjacent areas (Holub et al., 1995; Holub
68
K. Holub, V. Petroš / Tectonophysics 456 (2008) 67–73
Fig. 1. Map of boundaries of coal basins (▬) and demarcation of mines (—) combined to the Ostrava-Karviná coal mine district. 1—# Dukla (closed), 2—# Lazy, 3—# Doubrava, 4—# ČSA, 5—# Darkov, 6—# František (closed), 7—# 9. květen, 8—# ČSM (acc. to Dopita et al., 1997), ▲ A, D, K and M positions of underground seismic stations and ■ area of the Ostrava-Karviná Coal Field (see larger scale regional map).
and Stodulková, 2001). Stations were situated partly underground, partly on surface observation sites. Since then, almost 50,000 induced seismic events with the energy of up to 108 J have been recorded annually. This study presents an overview describing the geological and tectonic situation of the coal deposit, and our attention was mainly paid to the investigation of the most important parameters of 80 rockbursts observed simultaneously at three underground seismic stations, i.e. the maximum particle velocity ui(m/s) of seismic waves and the range of prevailing frequencies f (Hz) of body waves. The aim of the present study was to gather reasonable input data from underground seismological observations in order to set up experiments under laboratory conditions that would give a better understanding of rock mass behaviour under dynamic loading, which occurs during anomalous geomechanical events, such as rockbursts (Petroš et al., 2003). Another aim of this study is to prepare data for the estimation of correlation between particle velocity observed at the appropriate hypocentral distances, and the range of devastation in underground workings and support damage. The related research, which is still in progress, is based on the recognition that the peak particle velocity is the best criterion for the assessment of level of vibration damage to surface structures (Kaláb and Knejzlík, 2002) and in mines, as was documented by various authors, e.g. Isaac (1981), Hedley (1990), Ortlepp (1993) and Kaiser and Maloney (1997) during blasting operations. 2. Characterization of coal deposit The Ostrava-Karviná Coal Basin is a part of the Upper Silesian Basin, which is situated in the northeast of the Czech Republic. The geological structure of the basin is a result of the development of the continental crust in the Moravian-Silesian region within the adjacent area of the eastern margin of the Bohemian Massif and the Western Carpathians. The whole coalfield is divided into three partial basins—i.e. Ostrava, Petřvald and Karviná basins—by Michálkovice and Orlovská structures
with an anticlinal character. At present, the coal seams in the first two basins are mined out and mines in this area have already been closed, as shown in Fig. 1. Only mines in the Karviná partial basin are still in operation, except for the František and Dukla Mines, which were closed not long ago. The evolution of the basin was seriously affected during the Cadomian, Variscan and Alpine structural stages, which resulted in the disruption of subhorizontally layered strata of sedimentary beds in the Karviná partial basin by numerous faults and fault zones. The predominant orientation of these discontinuities is roughly east–west and north–south, as seen schematically in a slightly modified Fig. 2 (Grygar, 1996). The inner part of the coal-bearing series of the Karviná basin consists of two formations, i.e. the Ostrava Formation in the basement of the basin, and the Karviná Formation, which represents higher horizons. While the age of the Ostrava Formation corresponds to Lower Namurian, the Karviná Formation belongs stratigraphically to the Middle and Upper Namurian and Westphalien age. The Karviná Formation consists of Doubrava, Suchá and Saddle Members. The most important rigid interlayer between the Lower Suchá Member (coal seam No. 33) and the Saddle Member (coal seam No. 37) is a zone where stress accumulates during the coal extraction of coal seams No. 37 through No. 40. The thickness of this interlayer is about 160–175 m and the interlayer itself is very dangerous as a potential source of rockbursts. Therefore, as one of active preventative measures, largescale non-productive blasting operations are performed in the roof in order to disintegrate the strong roof and release the stress accumulated in the rock mass. While the mean thickness of coal seams in the Ostrava Formation is about 1.0–1.1 m, in the Karviná Formation it is approximately 2.5 m. However, the mined height of the coal seam reached, locally, up to 12 m (Dopita and Kumpera, 1993). The present mining depth varies between 800 and 1100 m. The representative types of rocks in the basin under investigation are conglomerates, sandstones, siltstones, claystones and coal-bearing
K. Holub, V. Petroš / Tectonophysics 456 (2008) 67–73
69
Fig. 2. Basic system of regional faults in the eastern part of the Ostrava-Karviná Coal Basin. The shadow strip represents the zone of the Karviná central thrust (modified acc. to Grygar, 1996); ▲ A, D, K and M positions of underground seismic stations.
seams. The Saddle Members consist mostly of solid siltstones, sandstones and conglomerates having the compression strength of 70–100 MPa, while the compression strength of coal varies within the span of 20–25 MPa. As for the mining method, longwall mining with caving or pneumatic backfilling is extensively used in the Ostrava-Karviná mines. 3. Stations and monitoring system For operational purposes of the geomechanical service in mines, two monitoring systems were erected in the eastern part of the Ostrava-Karviná Coal Basin, i.e. a local and a regional array. Only data from three underground regional seismic stations was used in the present study; it is displayed in Figs. 1 and 2. Stations A, K and M were in operation between 1993 and 2004, while in 2005 the station M was substituted by the station D. All stations are equipped with three-component short period sensors to record particle velocity. The primary digital data obtained at individual stations deployed on the surface or underground is automatically pre-processed within microarrays in individual mines; it is then transmitted via modem lines to the central geophysical laboratory, where a comprehensive data processing is carried out. This complex automated evaluation includes data transmission, the foci location and their plots, determination of released seismic energy related to the construction of a Benioff graph, and energyfrequency distribution with the appropriate graphs.
blasting operations relate to the peak particle velocity in dependence on the mass of explosives and the epicentral distance. A similar situation can occur during the investigation of influence of another seismic source, e.g. a rockburst, on stability of mine workings and support. Having assessed the nature of damage criteria that could be tolerable by various structures and mine workings, it was then suitable to confirm the effects of various induced geomechanical events on measured particle velocities. It is generally known that during the passage of vibrations originating in consequence of a rockburst impact, the motion of individual particles has a spatial character of the elliptical type. In order to define and describe this motion in an exact way, we need to record it using three-component sensors that enable to investigate particle velocities in three mutually perpendicular planes. The orientation of these planes can be used either as vertical (Z), transversal (Ht) and radial (Hr), or, according to the geographical directions, as vertical (Z), horizontal (E–W) and (N–S). It is evident that none of these components is permanently predominant, and, therefore, each time history depends first of all on local seismogeological conditions; this is why the individual peaks of particle velocity occur at different times. Regarding the fact that our setup of measurements was based on geographically oriented seismometers, the maximum value of particle velocity for each event was calculated according to the following Eq. (1) h iO 2 2 2 uimax ðm=sÞ ¼ uZi þ uNi þ uEi ; ð1Þ
4. Data processing and analysis
where: uiZ, uiN, uiE represent particle velocities recorded on the corresponding components of time histories.
4.1. Particle velocity
4.2. Maximum particle velocity vs. scaled distance
Structures, tunnels and/or mine workings are usually considered safe if a limit of several parameters controlling the ground motion, e.g. the maximum particle velocity of vibrations in their vicinity, is not exceeded. For instance, most empirical relations in
Considering that we are dealing with three dependent parameters, i.e. particle velocity ui (m/s), hypocentral distance d (m) and energy E (J) a simplified approach for displaying mutual dependences was to be searched. As the simplest
70
K. Holub, V. Petroš / Tectonophysics 456 (2008) 67–73
Fig. 3. Maximum particle velocity vs. scaled distance for P-waves (cube root scaling).
approach it seemed the introducing of so called scaled distance d⁎ = d/n√E (m/n√J). The further step was to search of reasonable value of the root n. In blasting tests different values of n-root of charges Q were applied for observed data best fitting, mostly n = 3 (cube root scaling) which used, e.g., Isaac (1991), Hendron (1997), Dowding (1992) and Jiao et al. (2003). It should be also noted here that other authors preferred the square root scaling in different situations, for instance n = 2 (square root scaling) was applied in studies by Siskind et al. (1980) and by Egan et al. (2001). After this procedure we chose and implemented in calculation of scaled distance both values, i.e. n = 2 and n = 3. All available data obtained between 1993 and 2005 was plotted on a bilogarithmic grid shown in Figs. 3 and 4 to present the relationship between the maximum particle velocity ui (m/s) and the scaled distances d⁎ (m/n√J) in groups of P- and Swaves. In Figs. 3 and 4 only cube root scaling is shown. Since the rockburst foci were irregularly distributed in the rock mass, seismic rays propagating from the focus arrived to seismic stations from different azimuths. Although the distances and azimuths were very variable, all the data surprisingly aligned in straight-lines if n = 2 or n = 3 were used, which was also
documented by the correlation coefficients R2 for all data sets determined by the Eqs. (2–5). Pmax wavesðSRSÞ ui ðm=sÞ ¼ 1:184 104 d41:004 R2 ¼ 0:7989
ð2Þ Pmax wavesðCRSÞ ui ðm=sÞ ¼ 3:055 103 d41:297 R2 ¼ 0:8293
ð3Þ Smax wavesðSRSÞ ui ðm=sÞ ¼ 5:280 104 d41:183 R2 ¼ 0:8627
ð4Þ Smax wavesðCRSÞ ui ðm=sÞ ¼ 2:397 102 d41:527 R2 ¼ 0:9164:
ð5Þ The hypocentral distance d was calculated according to the formula (6) h iO d ðmÞ ¼ ðxF xS Þ2 þðyF yS Þ2 þðzF zS Þ2 ; ð6Þ where: xF, yF and zF are coordinates of a focus, yS, yS and zS are coordinates of seismic station.
Fig. 4. Maximum particle velocity vs. scaled distance for S-waves (cube root scaling).
K. Holub, V. Petroš / Tectonophysics 456 (2008) 67–73
71
Fig. 5. Histogram of predominant frequencies for the first arrivals of P-waves.
4.3. Predominant frequency The way in which rockbursts transfer their released energy from the focus to the surrounding rock mass, and in which different factors affect the propagation of the induced stress wave, are difficult problems to solve, because the radiation pattern of seismic waves is not a stable process but a process during which various changes of time history are observed. Apart from the obvious separation of individual types of seismic waves, which propagate with different velocities, properties of these waves also influence both amplitudes and frequencies of vibrations. The properties of rock mass and its deformation play an essential role in this sense. Significant faults in the Karviná basin can be seen in Fig. 2; they represent the basic barriers against propagation, and, simultaneously, they partly influence amplitudes of vibrations and change the frequency content of the wave train due to dissipation of energy on these discontinuities. Since the laboratory experiments needed to acquire input data of characteristic frequencies generated by a rockburst impact, the concept of the present study was based on the assessment of predominant frequencies within each group of P- and S-waves at the arrival time of the maximum amplitude of particle velo-
cities. It should be noted that the frequencies measured in seismograms recorded at the seismic stations under investigation differ in two ways. While predominant frequencies of P-waves are manifested by a broader bandwidth of 3–10 Hz, this bandwidth is narrower in the case of S-waves, having the distinct maximum value at the frequency of 3 Hz. Resulting values of observed frequencies are displayed in the form of histograms given in Figs. 5 and 6. 4.4. Energy Released seismic energy is understood here as a part of mechanical energy that is radiated into the space during a mining seismic event, and it then spreads through a geological medium in all directions in the form of seismic waves, namely of body waves. Elastic strain waves transform to other kinds of energy during this process, the small part of which (approx. 0.1–1.5%) is represented by seismic energy. The focal region is then defined as a limited area in the rock massif where irreversible deformation occurs. The rationale of automated energetic classification of seismic events is based on the evaluation of the wave train recorded at the respective site of observation under the assumption that the energy is
Fig. 6. Histogram of predominant frequencies for the first arrivals of S-waves.
72
K. Holub, V. Petroš / Tectonophysics 456 (2008) 67–73
proportional to the square limited by the function f = v2 (t) within the time interval (0;T). The following formula has been adopted for energetic calculations: Z T E ¼ KT v2 dt ð7Þ 0
where: K
is a constant defined by characteristics of transmission conditions T = 1.5 s is the time interval accepted for the area of coal fields under investigation, which simultaneously represents the duration of P- and S-waves. In the process of a comprehensive evaluation, the seismic energy of body waves is calculated separately for each station. The maximum and minimum values are then excluded from the data set, and the resulting value is computed as the modus of the set of remaining observations. Rockbursts with energies falling into the interval of E = 6.2 × 103 − 5.0 × 108 J were included in the set of recorded seismic events. 5. Discussion and conclusions Long-term underground seismic observations in the Karviná Coal Basin, which are being continuously carried out, documented the existence of mining-induced seismicity. The origin of this phenomenon is closely linked to the existence of strata series with a relatively high strength, namely of thick and rigid overlying rocks, which are prone to rockburst occurrence and which overlay coal seams, as mentioned in Section 2. The failure process may then be triggered either by long-lasting effects of mining-induced stresses exceeding the strength of rocks, or by a passage of seismic waves from a distant source. Taking into account various amounts of released seismic energy, only the events that caused evident and permanent deformation in workings and at roadways support are classified as rockbursts, as far as this support is not designed to withstand a higher loading generated by a rockburst. According to the current classification of all induced seismic events observed between 1993 and 2005, data from 80 rockbursts were analyzed in this study. Many seismic field experiments documented that the damage to both underground workings and surface structures, caused by blasting operations, could be determined by means of one parameter—particle velocity—which, in fact, seems to be the best criterion for the damage estimation. Since particle velocity during blasting operations proved to be a reliable criterion, the same parameter was accepted for the evaluation of underground damage induced by rockbursts. Investigations presented here focused on the measurement of particle velocity at underground seismic stations, which could be useful in research dealing with the study of physical and mechanical properties of rocks under dynamic loading, which simulates the influence of rockbursts to workings. Moreover, geomechanical input data describing the extent, striking-distance and character of damage caused by rockbursts is being currently gathered. Consequently, particle velocity corresponding to the devastations in workings at various hypocentral distances from rockburst foci will be determined.
The seismic data recorded at seismic stations was used to estimate the attenuation of the maximum particle velocity ui of Pand S- waves in dependence on scaled distance d⁎, and therefore, all available data was plotted in the log–log scale. Two parameters of scaled distance were then applied in the least-square regression method, i.e. d⁎ = d/3√E and d/√E, respectively. Typical examples of the attenuation of particle velocity with increasing scaled distance d⁎, given in Figs. 3 and 4, show general trends described by Eqs.(2–5) and scatter in separate datasets. It is apparent that the graphs of P-waves show a higher scatter of individual measured values, which could be the consequence of a lower accuracy in readings considering the low level of amplitudes. This fact was documented by the correlation coefficient R2 in Eqs. (2–5). According to our opinion, the application of the square root or the cube root scaling did not, in principle, substantially influence the results of the data approximation using the linear regression. It should be made sure that the parameters calculated at the same scaled distance using an identical root for the linear regression process are compared. The numbers of measured frequency values for P- and Swaves are displayed in Figs. 5 and 6. These figures indicate that the P-waves comprise a wider range of frequencies in the first arrivals, i.e. the interval of 3 Hz to 12 Hz, while the S-waves are manifested by a narrower interval of frequencies, namely from 2 Hz to 6 Hz, with the distinct maximum at 3 Hz. It can be concluded that the measured and calculated values of proper parameters will be a useful research tool in both the laboratory experiments and the geomechanical practice, whenever damage to workings, roadways and/or mining support is assessed. Acknowledgements This research was sponsored by the Research Programme of Academy of Sciences of the Czech Republic, No. OZ 30860518 and partly supported the grant project No. 105/05/0883. The authors are grateful to anonymous reviewers for their valuable comments. References Dopita, M., Kumpera, O., 1993. Geology of the Ostrava-Karviná coalfield, Upper Silesian Basin, Czech Republic, and its influence on mining. Int. J. Coal Geol. 23, 291–321. Dopita, M., et al., 1997. Geology of the Czech Part of the Upper Silesian Basin. Ministry of the Environment of the Czech Republic, pp. 278 (in Czech). Dowding, C.H., 1992. Suggested method for blast vibration monitoring. Int. Rock Mech. Min. Sci. Abstr. 29 (2), 143–156. Egan, J., Kermode, J., Skyrman, M., Turner, L.L., 2001. Ground vibration monitoring for construction blasting in urban areas. Final Report, Caltrans, Sacramento, CA, pp. 1–11. Grygar, R., 1990. The Karviná Central Thrust of the Ostrava-Karviná District (Upper Silesian Basin)—kinematic and genesis studies. Geol. Výzk. Mor. Slez. in 1995. Brno, pp. 82–86 (in Czech). Hedley, D.G.F., 1990. Peak particle velocity for rockbursts in some Ontario mines. In: Fairhurst, C. (Ed.), Rockbursts and Seismicity in Mines. Balkema, Rotterdam, pp. 345–348. Hendron Jr., H.J., 1997. Engineering of rock blasting on civil projects. In: Hall, W.J. (Ed.), Structural and Geotechnical Mechanics. Prentice-Hall, NJ, pp. 242–277.
K. Holub, V. Petroš / Tectonophysics 456 (2008) 67–73 Holub, K., Stodulková, S., 2001. Analysis of long-term monitoring of coal mining-induced seismicity. Proc. 14th Int. Conf. Automation in Mining. Tampere, pp. 161–168. Holub, K., Slavík, J., Kalenda, P., 1995. Monitoring and analysis of seismicity in the Ostrava-Karviná coal mining district. Acta Geophys. Pol. XLII (1), 11–31. Isaac, I.D., 1981. A study of blast vibrations—Part 1. Tunn. Tunn. 35–41. Isaac, I.D., 1991. Effects of constructional vibrations upon an urban environment. Earthquake, Blast and Impact—Measurement and Effects of Vibration. Elsevier Applied Science, pp. 442–462. Jiao, Y.Y., Zhao, J., Cai, J.G., 2003. Consideration for 2-D and 3-D modeling of shock wave propagation in jointed rock masses. ISRM 2003-Technology roadmap for rock mechanics. South African Institute of Mining and Metallurgy, pp. 583–586. Kaiser, P.K., Maloney, S.M., 1997. Scaling laws for the design of rock support. Pure Appl. Geophys. 150, 415–434.
73
Kaláb, Z., Knejzlík, J., 2002. Systematic measurement and preliminary evaluation of seismic vibrations provoked by mining induced seismicity in Karviná area. Pub. Inst. Geophys. Pol. Acad. Sci. M-24 (340), 95–103. Ortlepp, W.D., 1993. High ground displacements velocities associated with rockbursts damage. 3rd Int. Symp. Rockbursts and Seismicty in Mines. Balkema, Rotterdam, pp. 101–106. Petroš, V., Bagde, M.N., Holub, K., Michalčík, P., 2003. Comparison of changes in the strength and the deformation behaviour of rocks under static and dynamic loading. ISRM 2003—Technology roadmap for rock mechanics. South African Institute of Mining and Metallurgy, pp. 899–902. Siskind, D.E., Stagg, M.S., Kopp, J.W., Dowding, C.H., 1980. Structure Response and Damage Produced by Ground Vibrations from Surface Blasting. Report of Investigations, vol. 8507. U.S. Bureau of Mines.
Tectonophysics 456 (2008) 67 – 73 www.elsevier.com/locate/tecto
Some parameters of rockbursts derived from underground seismological measurements Karel Holub a,⁎, Vladimír Petroš b a
Department of Geophysics, Institute of Geonics AS CR, Studentská 1768, CZ 708 00 Ostrava-Poruba, Czech Republic b VŠB-Technical University Ostrava, 17. listopadu, CZ 708 33 Ostrava-Poruba, Czech Republic Received 31 March 2006; accepted 14 December 2006 Available online 7 February 2008
Abstract A total of 240 three-component recordings from 80 rockbursts, which occurred in various coal mines in the Ostrava-Karviná Coal Basin (Czech Republic) between 1993 and 2005, was used to examine the decrease in maximum particle velocities ui (m/s) with a scaled distance of d⁎ = d/√E (m/√J) or d/3√E (m/3√J) and the rate of predominant frequencies of body waves. The energetic span of rockbursts was within the interval of E = 6.2 × 103 − 5.0 × 108 J, while calculated hypocentral distances d of four underground seismic stations varied from 0.6 to 7 km. The slopes b of regression straight lines for the maximum particle velocities ui (m/s) of P- and S-waves in the bilogarithmic scale correspond to the values of −1.004, − 1.297, −1.183 and − 1.527. The results of the linear regression are as follows: Pmax-waves ui = 1.184 × 10− 4 × d⁎− 1.004 (m/s) (square root scaling) Pmax-waves ui = 3.055 × 10− 3 × d⁎− 1.297 (m/s) (cube root scaling) Smax-waves ui = 5.280 × 10− 4 × d⁎− 1.183 (m/s) (square root scaling) Smax-waves ui = 2.397 × 10− 2 × d⁎− 1.527 (m/s) (cube root scaling). The evaluation of the abovementioned dynamic parameters was based on seismic events data gathered in the database of the regional seismic array, and calculations were carried out either by using special programs applied as part of the automated data processing in the computation center, or by usual linear regression approaches. The aim of the detailed analysis of the maximum particle velocity and predominant frequencies was a) to set up input data from underground seismological observations for laboratory experiments dealing with the comparison of rock mass behaviour under modeled laboratory conditions simulating manifestation of rockbursts, and b) to incorporate particle velocity into the design of support in order to control damage and evident devastation of workings by rockbursts. The investigation of peak particle velocities was based on the recognition that they are the best criterion to assess vibration damage to surface structures and in mines. © 2008 Elsevier B.V. All rights reserved. Keywords: Ostrava-Karviná Coal Basin; Mining-induced seismicity; Rockbursts; Particle velocity
1. Introduction The eastern part of the Ostrava-Karviná Coal Basin (Czech Republic), which is part of the Upper Silesian Coal Basin, has been endangered by mining tremors and rockbursts for almost 100 years now, the first rockburst having occurred here in 1912. At the beginning of mining in this region, sub-surface coal seams in Ostrava partial basin were mined first, while deeper horizons of ⁎ Corresponding author. Tel.: +420 536 979 344; fax: +420 596 919 452. E-mail address: [email protected] (K. Holub). 0040-1951/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2006.12.013
coal seams were gradually mined out afterwards. From the viewpoint of the origin of rockbursts in the Karviná partial basin, two types of events were identified. While in-seam rockbursts occurred down to the depth of about 300–400 m, another more dangerous type of rockbursts was recognized at deeper horizons starting with the coal seam No. 37. This change was influenced by the stratigraphy of the deposit, where the coal seam No. 37 was overlain by thick and hard series of high-strength sandstone strata (up to 175 m). Increasing problems with rockbursts occurrence led to the erection of both local and regional systems of seismic monitoring in mines and adjacent areas (Holub et al., 1995; Holub
68
K. Holub, V. Petroš / Tectonophysics 456 (2008) 67–73
Fig. 1. Map of boundaries of coal basins (▬) and demarcation of mines (—) combined to the Ostrava-Karviná coal mine district. 1—# Dukla (closed), 2—# Lazy, 3—# Doubrava, 4—# ČSA, 5—# Darkov, 6—# František (closed), 7—# 9. květen, 8—# ČSM (acc. to Dopita et al., 1997), ▲ A, D, K and M positions of underground seismic stations and ■ area of the Ostrava-Karviná Coal Field (see larger scale regional map).
and Stodulková, 2001). Stations were situated partly underground, partly on surface observation sites. Since then, almost 50,000 induced seismic events with the energy of up to 108 J have been recorded annually. This study presents an overview describing the geological and tectonic situation of the coal deposit, and our attention was mainly paid to the investigation of the most important parameters of 80 rockbursts observed simultaneously at three underground seismic stations, i.e. the maximum particle velocity ui(m/s) of seismic waves and the range of prevailing frequencies f (Hz) of body waves. The aim of the present study was to gather reasonable input data from underground seismological observations in order to set up experiments under laboratory conditions that would give a better understanding of rock mass behaviour under dynamic loading, which occurs during anomalous geomechanical events, such as rockbursts (Petroš et al., 2003). Another aim of this study is to prepare data for the estimation of correlation between particle velocity observed at the appropriate hypocentral distances, and the range of devastation in underground workings and support damage. The related research, which is still in progress, is based on the recognition that the peak particle velocity is the best criterion for the assessment of level of vibration damage to surface structures (Kaláb and Knejzlík, 2002) and in mines, as was documented by various authors, e.g. Isaac (1981), Hedley (1990), Ortlepp (1993) and Kaiser and Maloney (1997) during blasting operations. 2. Characterization of coal deposit The Ostrava-Karviná Coal Basin is a part of the Upper Silesian Basin, which is situated in the northeast of the Czech Republic. The geological structure of the basin is a result of the development of the continental crust in the Moravian-Silesian region within the adjacent area of the eastern margin of the Bohemian Massif and the Western Carpathians. The whole coalfield is divided into three partial basins—i.e. Ostrava, Petřvald and Karviná basins—by Michálkovice and Orlovská structures
with an anticlinal character. At present, the coal seams in the first two basins are mined out and mines in this area have already been closed, as shown in Fig. 1. Only mines in the Karviná partial basin are still in operation, except for the František and Dukla Mines, which were closed not long ago. The evolution of the basin was seriously affected during the Cadomian, Variscan and Alpine structural stages, which resulted in the disruption of subhorizontally layered strata of sedimentary beds in the Karviná partial basin by numerous faults and fault zones. The predominant orientation of these discontinuities is roughly east–west and north–south, as seen schematically in a slightly modified Fig. 2 (Grygar, 1996). The inner part of the coal-bearing series of the Karviná basin consists of two formations, i.e. the Ostrava Formation in the basement of the basin, and the Karviná Formation, which represents higher horizons. While the age of the Ostrava Formation corresponds to Lower Namurian, the Karviná Formation belongs stratigraphically to the Middle and Upper Namurian and Westphalien age. The Karviná Formation consists of Doubrava, Suchá and Saddle Members. The most important rigid interlayer between the Lower Suchá Member (coal seam No. 33) and the Saddle Member (coal seam No. 37) is a zone where stress accumulates during the coal extraction of coal seams No. 37 through No. 40. The thickness of this interlayer is about 160–175 m and the interlayer itself is very dangerous as a potential source of rockbursts. Therefore, as one of active preventative measures, largescale non-productive blasting operations are performed in the roof in order to disintegrate the strong roof and release the stress accumulated in the rock mass. While the mean thickness of coal seams in the Ostrava Formation is about 1.0–1.1 m, in the Karviná Formation it is approximately 2.5 m. However, the mined height of the coal seam reached, locally, up to 12 m (Dopita and Kumpera, 1993). The present mining depth varies between 800 and 1100 m. The representative types of rocks in the basin under investigation are conglomerates, sandstones, siltstones, claystones and coal-bearing
K. Holub, V. Petroš / Tectonophysics 456 (2008) 67–73
69
Fig. 2. Basic system of regional faults in the eastern part of the Ostrava-Karviná Coal Basin. The shadow strip represents the zone of the Karviná central thrust (modified acc. to Grygar, 1996); ▲ A, D, K and M positions of underground seismic stations.
seams. The Saddle Members consist mostly of solid siltstones, sandstones and conglomerates having the compression strength of 70–100 MPa, while the compression strength of coal varies within the span of 20–25 MPa. As for the mining method, longwall mining with caving or pneumatic backfilling is extensively used in the Ostrava-Karviná mines. 3. Stations and monitoring system For operational purposes of the geomechanical service in mines, two monitoring systems were erected in the eastern part of the Ostrava-Karviná Coal Basin, i.e. a local and a regional array. Only data from three underground regional seismic stations was used in the present study; it is displayed in Figs. 1 and 2. Stations A, K and M were in operation between 1993 and 2004, while in 2005 the station M was substituted by the station D. All stations are equipped with three-component short period sensors to record particle velocity. The primary digital data obtained at individual stations deployed on the surface or underground is automatically pre-processed within microarrays in individual mines; it is then transmitted via modem lines to the central geophysical laboratory, where a comprehensive data processing is carried out. This complex automated evaluation includes data transmission, the foci location and their plots, determination of released seismic energy related to the construction of a Benioff graph, and energyfrequency distribution with the appropriate graphs.
blasting operations relate to the peak particle velocity in dependence on the mass of explosives and the epicentral distance. A similar situation can occur during the investigation of influence of another seismic source, e.g. a rockburst, on stability of mine workings and support. Having assessed the nature of damage criteria that could be tolerable by various structures and mine workings, it was then suitable to confirm the effects of various induced geomechanical events on measured particle velocities. It is generally known that during the passage of vibrations originating in consequence of a rockburst impact, the motion of individual particles has a spatial character of the elliptical type. In order to define and describe this motion in an exact way, we need to record it using three-component sensors that enable to investigate particle velocities in three mutually perpendicular planes. The orientation of these planes can be used either as vertical (Z), transversal (Ht) and radial (Hr), or, according to the geographical directions, as vertical (Z), horizontal (E–W) and (N–S). It is evident that none of these components is permanently predominant, and, therefore, each time history depends first of all on local seismogeological conditions; this is why the individual peaks of particle velocity occur at different times. Regarding the fact that our setup of measurements was based on geographically oriented seismometers, the maximum value of particle velocity for each event was calculated according to the following Eq. (1) h iO 2 2 2 uimax ðm=sÞ ¼ uZi þ uNi þ uEi ; ð1Þ
4. Data processing and analysis
where: uiZ, uiN, uiE represent particle velocities recorded on the corresponding components of time histories.
4.1. Particle velocity
4.2. Maximum particle velocity vs. scaled distance
Structures, tunnels and/or mine workings are usually considered safe if a limit of several parameters controlling the ground motion, e.g. the maximum particle velocity of vibrations in their vicinity, is not exceeded. For instance, most empirical relations in
Considering that we are dealing with three dependent parameters, i.e. particle velocity ui (m/s), hypocentral distance d (m) and energy E (J) a simplified approach for displaying mutual dependences was to be searched. As the simplest
70
K. Holub, V. Petroš / Tectonophysics 456 (2008) 67–73
Fig. 3. Maximum particle velocity vs. scaled distance for P-waves (cube root scaling).
approach it seemed the introducing of so called scaled distance d⁎ = d/n√E (m/n√J). The further step was to search of reasonable value of the root n. In blasting tests different values of n-root of charges Q were applied for observed data best fitting, mostly n = 3 (cube root scaling) which used, e.g., Isaac (1991), Hendron (1997), Dowding (1992) and Jiao et al. (2003). It should be also noted here that other authors preferred the square root scaling in different situations, for instance n = 2 (square root scaling) was applied in studies by Siskind et al. (1980) and by Egan et al. (2001). After this procedure we chose and implemented in calculation of scaled distance both values, i.e. n = 2 and n = 3. All available data obtained between 1993 and 2005 was plotted on a bilogarithmic grid shown in Figs. 3 and 4 to present the relationship between the maximum particle velocity ui (m/s) and the scaled distances d⁎ (m/n√J) in groups of P- and Swaves. In Figs. 3 and 4 only cube root scaling is shown. Since the rockburst foci were irregularly distributed in the rock mass, seismic rays propagating from the focus arrived to seismic stations from different azimuths. Although the distances and azimuths were very variable, all the data surprisingly aligned in straight-lines if n = 2 or n = 3 were used, which was also
documented by the correlation coefficients R2 for all data sets determined by the Eqs. (2–5). Pmax wavesðSRSÞ ui ðm=sÞ ¼ 1:184 104 d41:004 R2 ¼ 0:7989
ð2Þ Pmax wavesðCRSÞ ui ðm=sÞ ¼ 3:055 103 d41:297 R2 ¼ 0:8293
ð3Þ Smax wavesðSRSÞ ui ðm=sÞ ¼ 5:280 104 d41:183 R2 ¼ 0:8627
ð4Þ Smax wavesðCRSÞ ui ðm=sÞ ¼ 2:397 102 d41:527 R2 ¼ 0:9164:
ð5Þ The hypocentral distance d was calculated according to the formula (6) h iO d ðmÞ ¼ ðxF xS Þ2 þðyF yS Þ2 þðzF zS Þ2 ; ð6Þ where: xF, yF and zF are coordinates of a focus, yS, yS and zS are coordinates of seismic station.
Fig. 4. Maximum particle velocity vs. scaled distance for S-waves (cube root scaling).
K. Holub, V. Petroš / Tectonophysics 456 (2008) 67–73
71
Fig. 5. Histogram of predominant frequencies for the first arrivals of P-waves.
4.3. Predominant frequency The way in which rockbursts transfer their released energy from the focus to the surrounding rock mass, and in which different factors affect the propagation of the induced stress wave, are difficult problems to solve, because the radiation pattern of seismic waves is not a stable process but a process during which various changes of time history are observed. Apart from the obvious separation of individual types of seismic waves, which propagate with different velocities, properties of these waves also influence both amplitudes and frequencies of vibrations. The properties of rock mass and its deformation play an essential role in this sense. Significant faults in the Karviná basin can be seen in Fig. 2; they represent the basic barriers against propagation, and, simultaneously, they partly influence amplitudes of vibrations and change the frequency content of the wave train due to dissipation of energy on these discontinuities. Since the laboratory experiments needed to acquire input data of characteristic frequencies generated by a rockburst impact, the concept of the present study was based on the assessment of predominant frequencies within each group of P- and S-waves at the arrival time of the maximum amplitude of particle velo-
cities. It should be noted that the frequencies measured in seismograms recorded at the seismic stations under investigation differ in two ways. While predominant frequencies of P-waves are manifested by a broader bandwidth of 3–10 Hz, this bandwidth is narrower in the case of S-waves, having the distinct maximum value at the frequency of 3 Hz. Resulting values of observed frequencies are displayed in the form of histograms given in Figs. 5 and 6. 4.4. Energy Released seismic energy is understood here as a part of mechanical energy that is radiated into the space during a mining seismic event, and it then spreads through a geological medium in all directions in the form of seismic waves, namely of body waves. Elastic strain waves transform to other kinds of energy during this process, the small part of which (approx. 0.1–1.5%) is represented by seismic energy. The focal region is then defined as a limited area in the rock massif where irreversible deformation occurs. The rationale of automated energetic classification of seismic events is based on the evaluation of the wave train recorded at the respective site of observation under the assumption that the energy is
Fig. 6. Histogram of predominant frequencies for the first arrivals of S-waves.
72
K. Holub, V. Petroš / Tectonophysics 456 (2008) 67–73
proportional to the square limited by the function f = v2 (t) within the time interval (0;T). The following formula has been adopted for energetic calculations: Z T E ¼ KT v2 dt ð7Þ 0
where: K
is a constant defined by characteristics of transmission conditions T = 1.5 s is the time interval accepted for the area of coal fields under investigation, which simultaneously represents the duration of P- and S-waves. In the process of a comprehensive evaluation, the seismic energy of body waves is calculated separately for each station. The maximum and minimum values are then excluded from the data set, and the resulting value is computed as the modus of the set of remaining observations. Rockbursts with energies falling into the interval of E = 6.2 × 103 − 5.0 × 108 J were included in the set of recorded seismic events. 5. Discussion and conclusions Long-term underground seismic observations in the Karviná Coal Basin, which are being continuously carried out, documented the existence of mining-induced seismicity. The origin of this phenomenon is closely linked to the existence of strata series with a relatively high strength, namely of thick and rigid overlying rocks, which are prone to rockburst occurrence and which overlay coal seams, as mentioned in Section 2. The failure process may then be triggered either by long-lasting effects of mining-induced stresses exceeding the strength of rocks, or by a passage of seismic waves from a distant source. Taking into account various amounts of released seismic energy, only the events that caused evident and permanent deformation in workings and at roadways support are classified as rockbursts, as far as this support is not designed to withstand a higher loading generated by a rockburst. According to the current classification of all induced seismic events observed between 1993 and 2005, data from 80 rockbursts were analyzed in this study. Many seismic field experiments documented that the damage to both underground workings and surface structures, caused by blasting operations, could be determined by means of one parameter—particle velocity—which, in fact, seems to be the best criterion for the damage estimation. Since particle velocity during blasting operations proved to be a reliable criterion, the same parameter was accepted for the evaluation of underground damage induced by rockbursts. Investigations presented here focused on the measurement of particle velocity at underground seismic stations, which could be useful in research dealing with the study of physical and mechanical properties of rocks under dynamic loading, which simulates the influence of rockbursts to workings. Moreover, geomechanical input data describing the extent, striking-distance and character of damage caused by rockbursts is being currently gathered. Consequently, particle velocity corresponding to the devastations in workings at various hypocentral distances from rockburst foci will be determined.
The seismic data recorded at seismic stations was used to estimate the attenuation of the maximum particle velocity ui of Pand S- waves in dependence on scaled distance d⁎, and therefore, all available data was plotted in the log–log scale. Two parameters of scaled distance were then applied in the least-square regression method, i.e. d⁎ = d/3√E and d/√E, respectively. Typical examples of the attenuation of particle velocity with increasing scaled distance d⁎, given in Figs. 3 and 4, show general trends described by Eqs.(2–5) and scatter in separate datasets. It is apparent that the graphs of P-waves show a higher scatter of individual measured values, which could be the consequence of a lower accuracy in readings considering the low level of amplitudes. This fact was documented by the correlation coefficient R2 in Eqs. (2–5). According to our opinion, the application of the square root or the cube root scaling did not, in principle, substantially influence the results of the data approximation using the linear regression. It should be made sure that the parameters calculated at the same scaled distance using an identical root for the linear regression process are compared. The numbers of measured frequency values for P- and Swaves are displayed in Figs. 5 and 6. These figures indicate that the P-waves comprise a wider range of frequencies in the first arrivals, i.e. the interval of 3 Hz to 12 Hz, while the S-waves are manifested by a narrower interval of frequencies, namely from 2 Hz to 6 Hz, with the distinct maximum at 3 Hz. It can be concluded that the measured and calculated values of proper parameters will be a useful research tool in both the laboratory experiments and the geomechanical practice, whenever damage to workings, roadways and/or mining support is assessed. Acknowledgements This research was sponsored by the Research Programme of Academy of Sciences of the Czech Republic, No. OZ 30860518 and partly supported the grant project No. 105/05/0883. The authors are grateful to anonymous reviewers for their valuable comments. References Dopita, M., Kumpera, O., 1993. Geology of the Ostrava-Karviná coalfield, Upper Silesian Basin, Czech Republic, and its influence on mining. Int. J. Coal Geol. 23, 291–321. Dopita, M., et al., 1997. Geology of the Czech Part of the Upper Silesian Basin. Ministry of the Environment of the Czech Republic, pp. 278 (in Czech). Dowding, C.H., 1992. Suggested method for blast vibration monitoring. Int. Rock Mech. Min. Sci. Abstr. 29 (2), 143–156. Egan, J., Kermode, J., Skyrman, M., Turner, L.L., 2001. Ground vibration monitoring for construction blasting in urban areas. Final Report, Caltrans, Sacramento, CA, pp. 1–11. Grygar, R., 1990. The Karviná Central Thrust of the Ostrava-Karviná District (Upper Silesian Basin)—kinematic and genesis studies. Geol. Výzk. Mor. Slez. in 1995. Brno, pp. 82–86 (in Czech). Hedley, D.G.F., 1990. Peak particle velocity for rockbursts in some Ontario mines. In: Fairhurst, C. (Ed.), Rockbursts and Seismicity in Mines. Balkema, Rotterdam, pp. 345–348. Hendron Jr., H.J., 1997. Engineering of rock blasting on civil projects. In: Hall, W.J. (Ed.), Structural and Geotechnical Mechanics. Prentice-Hall, NJ, pp. 242–277.
K. Holub, V. Petroš / Tectonophysics 456 (2008) 67–73 Holub, K., Stodulková, S., 2001. Analysis of long-term monitoring of coal mining-induced seismicity. Proc. 14th Int. Conf. Automation in Mining. Tampere, pp. 161–168. Holub, K., Slavík, J., Kalenda, P., 1995. Monitoring and analysis of seismicity in the Ostrava-Karviná coal mining district. Acta Geophys. Pol. XLII (1), 11–31. Isaac, I.D., 1981. A study of blast vibrations—Part 1. Tunn. Tunn. 35–41. Isaac, I.D., 1991. Effects of constructional vibrations upon an urban environment. Earthquake, Blast and Impact—Measurement and Effects of Vibration. Elsevier Applied Science, pp. 442–462. Jiao, Y.Y., Zhao, J., Cai, J.G., 2003. Consideration for 2-D and 3-D modeling of shock wave propagation in jointed rock masses. ISRM 2003-Technology roadmap for rock mechanics. South African Institute of Mining and Metallurgy, pp. 583–586. Kaiser, P.K., Maloney, S.M., 1997. Scaling laws for the design of rock support. Pure Appl. Geophys. 150, 415–434.
73
Kaláb, Z., Knejzlík, J., 2002. Systematic measurement and preliminary evaluation of seismic vibrations provoked by mining induced seismicity in Karviná area. Pub. Inst. Geophys. Pol. Acad. Sci. M-24 (340), 95–103. Ortlepp, W.D., 1993. High ground displacements velocities associated with rockbursts damage. 3rd Int. Symp. Rockbursts and Seismicty in Mines. Balkema, Rotterdam, pp. 101–106. Petroš, V., Bagde, M.N., Holub, K., Michalčík, P., 2003. Comparison of changes in the strength and the deformation behaviour of rocks under static and dynamic loading. ISRM 2003—Technology roadmap for rock mechanics. South African Institute of Mining and Metallurgy, pp. 899–902. Siskind, D.E., Stagg, M.S., Kopp, J.W., Dowding, C.H., 1980. Structure Response and Damage Produced by Ground Vibrations from Surface Blasting. Report of Investigations, vol. 8507. U.S. Bureau of Mines.