PAGEOPH, Vol. 129, Nos. 3/4 ( 1 9 8 9 )

0033-4553/89/040609-0951.50 + 0.20/0 9 1989 Birkh/iuser Verlag, Basel

Recognition of the Zones of Seismic Hazard in Polish Coal Mines by Using a Seismic Method JOZEF DUBII~SKI I and JANUSZ DWORAK ~

Abstract--In the Upper Silesian Coal Field the seismic hazard induced by mining operations in collieries is closely related to the rockburst hazard. A seismic method is used for premonitory recognition of the zones of seismic hazard. It takes advantage of the relationship between the velocity of propagation of seismic waves and the state of stress existing in the rock. It consists of the determination of the velocity field of artificially induced seismic waves in a studied fragment of the rock, and of locating the velocity anomaly. The position of the velocity anomaly zones and their intensity are the basis for estimating the seismic hazard in advance of mining works.

Key words: Seismic hazard, rockburst hazard, seismic wave velocity, stress state, Upper Silesian coal mines.

I. Introduction

One o f the geophysical methods for recognition o f the seismic hazard present in hard coal mines of the U p p e r Silesian Coal Field, Poland, is a seismic method. It utilizes the physical relationship between parameters characterizing the artificially induced seismic waves field and the state o f rock medium structure which is determined by lithologic features o f rock, their strength properties, existing stress distribution, etc. The basic assumption o f the m e t h o d is a relationship between the seismic hazard and those rock mass areas which are characterized by a n o m a l o u s stress states. In mines o f the U p p e r Silesian Coal Field the seismic hazard induced by mining operations is one o f the most serious natural hazards. Therefore, the premonitory recognition o f the zones where the seismic hazard is present is an extremely important stage in the whole process o f prevention procedures against the rockburst. Actually, the seismic m e t h o d is a basic surveying m e t h o d for obtaining advanced g o o d knowledge o f seismic hazard ahead o f working face and in dog headings. It, also, plays a role in checking the effectiveness o f prevention works tending to the

1 Central Mining Institute, pl. Gwark6w 1, Katowice, Poland.

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restriction or elimination of local seismic hazard zones. In Poland, the seismic method has, in recent years, been expanded especially, in the field of instrumentation technique and measurement data processing basis.

2. Physical Basis of the Method The kinematic and dynamic parameters of seismic waves propagating in the rock body are very informative quantities which describe its physical state. One of them is the velocity of the seismic waves propagation (EWING et al., 1957). The dependence velocity-stress gives a possibility of following these geomechanical processes which influence the change in state of stress in the rock. In hard coal mines of the Upper Silesian Coal Field the zones of stress anomaly are present as a result of local mining and geologic conditions. Results of laboratorY works obtained by RIZNICHENKO (1957) and many underground measurements (DuBIIqSKI, 1980) lead to the following conclusions: --before the critical limit of loading of coal seam has been reached, the relative increases in velocity up to 20-30% are observed; --beyond the critical limit of loading where the medium structure failure processes are predominant, velocity decreases to 30-40% are also observed.

3. Characteristics of the Method A method of underground seismic measurement applies to the determination of stress anomaly zones consists of an accurate calculation ( T- 5%) of distribution of seismic waves propagation velocities in a studied rock mass zone. According to the kind of works it would be a profile survey or a map of velocity distribution. The input data needed for making them up are always travel times of seismic waves from their source to receiver. The survey works include the following basic stages: --installing the receivers for determining appropriate parameters of seismic waves, --inducing the seismic waves, --recording the seismograms of seismic waves along the path source-receiver. To obtain seismogram records special underground seismic equipment is used. The procedure of measurement comprises two basic versions of measurements realization, namely: --seismic profiling (points of inducing and receiving the seismic waves are situated along a common profile line _in the same mining excavation), --seismic transmission (points of inducing and receiving the seismic waves are situated in different mining excavations, usually parallel to each other).

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The seismic profile is divided into smaller measuring units with lengths of 10-20 m along which the velocities of body waves (longitudinal, transverse) are determined. Next, a cumulative diagram of velocity distribution along the seismic profile is made. In case of a seismic transmission between excavations the interpretation is carried out according to principles of algebraic tomographic reconstruction of velocity field. The basic computational programmes are realized by means of computer technology (DIANISKA et al., 1982). A map of velocity distribution in a coal seam which has undergone seismic transmission is obtained. The resulting distribution of seismic waves propagation velocities are the basis of qualitative and quantitative analysis of recorded velocity anomalies.

4. Seismic Criteria of Recognition of Seismic Hazard Zones Assuming that seismic hazard zones are related to areas of higher stresses in the rock mass, the seismic criteria of their recognition are based on the analysis of measured velocity anomalies. The reference level "V0" is a basis for separating the seismic anomalies. It is a value of velocity established under the conditions of lack of stress state disturbances at a site positioned near the main measurement basis. A velocity of seismic waves which exceeds the average value for given mining and geologic conditions (reference level) indicates an increased stress state and a possibility of seismic hazard occurrence. As an example, for seismic P-wave the average value of reference velocity in coal is, under the conditions of the Upper Silesian Coal Field, between 1700 m/s and 2000 m/s depending upon the measurement depth and the type of coal seam. Based on the distribution o f seismic waves propagation velocities, the parameters of corresponding stress anomalies and seismic hazard zones are distinguished. These are: --exten~ of increased stress zone, --position of a site of maximum stress concentration, --intensity of increased stress. The extent parameters are determined by velocity distribution curve form (profiling method) or by the velocity contour line distribution (transmission method). The intensity of stress increase is estimated from scale elaborated by DtJBIf4SK~ (1980) for mining and geologic conditions of the Upper Silesian Coal Field on the basis of laboratory and mine measurements. The scale is specified in Table 1. The seismic anomaly " A " has been determined for longitudinal seismic waves associated with the unfractured portion of a coal seam. This type of wave can, easily enough and surely, be recorded, especially, in the seismic profile procedure.

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Table 1

The velocity sensitivity of stress increase evaluation for the Upper Silesian Coal Ffeld conditions

Degree of stress

Characteristics o f stress increase

Velocity anomaly for P-wave in coal A, %

Probable stress increase Ap % Po '

1. 2. 3. 4.

None Low Medium High

less than 5 5 15 15 25 more than 25

less than 10 10-50 50-100 more than 100

The seismic criteria of recognition of seismic hazard zones are founded on the following parameters: - - t h e extent and position of seismic wave velocity anomaly zones relative to mining excavations, - - t h e value of velocity anomaly " A " , - - t h e value of reference velocity "V0". A =

Vmax-- Vo 9 100%. v0

When velocity anomaly zones are situated close to mining excavations an existing seismic hazard has considerably more influence on the rockburst hazard because of the possibility of tremor occurrences near the mining excavations. In such a situation the seismic events of lower energy can cause disturbances of excavation stability. Similarly, when the seismic anomaly is higher the seismic hazard degree increases.

5. Practical Application of the Seismic Method One of the most frequent causes of rockburst occurrences in the Upper Silesian Coal Field is mining operations carried out in areas under the interaction between the edges of coal remnants that resulted from discontinued mining in neighbouring seams. These areas are typical of increased stress occurrences and intensive seismic activity during performing mining operations within their limits as well. Typical solution of the practical application of the seismic method to the evaluation of the seismic hazard ahead of working faces in afore-mentioned situations has been presented. In the first case, shown in Figure 1, a seismic hazard zone in coal seam No. 503 ahead of the longwall face No. 737 has been located from the velocity distribution

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curve of the seismic longitudinal wave propagating in this seam. This has been obtained by means of the seismic profiling technique. The edges of stopped mining works in neighbouring seams Nos. 501,504 and 507 as well as goaves in a seam No. 503 (Figure la) create zones of increased stresses in the rock equivalent to the seismic hazard zones. These zones, in Figure l b, denoted by Z1 and Z2, have been determined from the velocity curve using

v(a) >- Vo. A considerably higher velocity anomaly is related to the zone Z 2 and it constitutes 14.5%. According to Table 1 this corresponds to a "low" stress increase up to 50%. A velocity reference level in this area equals to 1800 m/s, as well as velocity values lower than this value along a considerable length of the seismic profile indicate an unelevated stress state and measurable destressing of a certain portion of the seam No. 503. The seismic hazard, in this case, has been estimated as low. To prove that the afore-mentioned evaluation is right, a detailed analysis of the seismicity induced by mining the longwall No. 737 in the seam No. 503 has been carried out. The analysed seismic parameter was an E.R.R. (Energy Release Ratio) parameter computed for monthly advances of the longwall No. 737, equal to the ratio of the seismic energy released in the vicinity of the longwall No. 737 to the area of the seam No. 503 portion extracted by this longwall. In Figure lc an E.R.R. parameter distribution is shown. The well-marked maximum, in this figure, should be related to the entry of the longwall No. 737 into the seismic hazard zone Z 2. A difference between the maximum value of the Z2 anomaly determined by the seismic method (Figure lb) and the maximum value of the E.R.R. parameter (Figure lc) determined from the seismic activity of the longwall No. 737 is, in this case, 65 m. This is caused by the existence of a characteristic stress distribution (JAEGER, 1972), the so-called abutment zone, extending for a distance of several tens of meters ahead of the longwall faces, which interacting with the Z2 anomaly zone present in the longwall No. 737 panel yields a maximum on the curve of the E.R.R. parameter distribution. No such regularities are observed by the Zl anomaly because its value is considerably lower. The premonitory seismic evaluation which shows the seismic hazard 1:o be low is, also, confirmed by the low E.R.R. parameter values. In another case, displayed in Figure 2, to determine seismic hazard zones, a technique of seismic transmission between excavations and the algebraic tomographic reconstruction of the velocity field have been used. Seismic measurements have been carried out in a coal seam No. 509 ahead of the longwall face No. 1193. Numerous edges of stopped mining in coal seams Nos. 501 ab, 503 and 504 overlie the longwall panel. Based on the analysis of the velocity data from the seam No. 509 area, the zone of markedly increased stress state would be a region where the velocity values exceed 2150m/s. Therefore, three such zones--in Figure 2a, denoted by Zl, Z2, Z3--in a studied portion of the longwall No. 1193 panel could be obtained,

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Figure 1 D e t e r m i n a t i o n of zones o f seismic h a z a r d on the basis o f p r e m o n i t o r y seismic profiling in coal seam No. 503. a - - m i n i n g situation, b - - d i s t r i b u t i o n o f p r o p a g a t i o n velocity of l o n g i t u d i n a l seismic wave, c - - d i s t r i b u t i o n of E . R . R . p a r a m e t e r . []]]]]--seismic h a z a r d zone, - F T edge o f coal seam No. 501, T . 9 T 9 9 edge o f coal seam No. 504, T T edge o f coal seam No. 507, A B - - s e i s m i e profile.

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Estimation of Seismic Hazard in Polish Coal Mines

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Figure 2 Determination of zones of seismic hazard on the basis of premonitory seismic transmission in coal seam No. 509. a - - m i n i n g situation, and distribution of propagation velocity of longitudinal seismic wave in coal seam No. 509, ~ l i s t r i b u t i o n o f E.R.R. parameter. E22] --seismic hazard zones, o r x 1 - edge of coal seam Nos. 501 ab, 1 - .. o r edge of c0al seam No. 503, T T edge of coal seam No. 504, - - x 3 6 ~ s h o t points, I o 6I--receiving points, e g o effects of rockburst.

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including the z o n e Z1, as very pronounced, with velocities exceeding 2300 mps. The seismic hazard has been evaluated as visibly increased, especially, in the zone Z 1. These findings have been confirmed by the E.R.R. parameter distribution (Figure 2b) determined from the analysis of seismicity induced by mining the longwall No. 1193. Two maxima occur in it--one of them very well marked. They, undoubtedly, should be related to the seismic hazard zones Z1 and Za, respectively. A difference in distance between the maximum of seismic wave velocity and the maximum of the E.R.R. parameter is, in this case, 40 m. A well marked increase in seismic activity (E.R.R. = 675 J/m 2) which caused a rockburst in the top gate of longwall No. 1193, has occurred, first of all, as a result of interaction between the longwall No, 1193 abutment zone and the seismic zones Z3 and ZI. A section of the occurrence of the rockburst effects is shown in Figure 2a. The two presented examples show the usefulness of the seismic method in the field of premonitory recognition of seismic hazard zones during the operation of the longwall panels.

6. Conclusions

The analyses carried out confirm that the measured seismic anomalies agree well with the subsequent distributions of seismological activity and with the manifestation of stress effects observed in mining excavations. A magnitude of seismic anomaly characterizes an induced seismic intensity which is being recorded when a longwall face approaches the seismic zones. This is manifested by the occurrence of higher values of the E.R.R. parameter in the places where a higher seismic anomaly " A " has been determined. A difference between the position of the zones of maximum propagation velocity of seismic wave and the maximum of E.R.R. parameter has been observed. This is several tens of meters and arises from the existence of characteristic stress distribution ahead of the longwall face and from the phenomenon of superposition of high stress zones. High reliability of the method could be reached provided that the measuring works would be carried out with an adequate technical standard end with a skilled interpretation of data. For that reason, the seismic method is useful for the premonitory recognition of stress anomaly zones ahead of longwall faces and for the determination of seismic hazard areas. REFERENCES DIANISKA,L., HERMANN, L., and VERBOCI,J. (1982), Curved Ray Algebraic Reconstruction Technique Applied in Mining Geophysics, GeophysicalTransactions 28(1), 46-57. DUBIIqSKI,J. (1980), The Seismic Method for Evaluation of Stress State in the Rock under The Rockburst Hazard, Prz. G6rn. 4, 190-197. (in Polish).

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EWING, M., JARDETZKY,W., and PRESS, F., Elastic Waves in Layered Media (McGraw-Hill, New York 1957). JAEGER, J. C., Rock Mechanics and Engineering (Cambridge University Press 1972). RIZNICHENKO, J. W., Investigating the Rock Pressure by Geophysical Methods (Nauka, Moscow 1967) (in Russian). (Received July 21, 1987, revised January 8, 1988, accepted May 8, 1988.)