J. Cent. South Univ. (2012) 19: 2604-2610 

DOI: 10.1007/s11771-012-1317-6

Seismic effort of blasting wave transmitted in coal-rock mass associated with mining operation

CAO An-ye(曹安业)¹, DOU Lin-ming(窦林名)², LUO Xun(骆循)³,

ZHENG Yi-dong(张益东)², HUANG Jun-li (黄军利)⁴, K. Andrew³

1. School of Mines, China University of Mining & Technology, Xuzhou 221116, China;

2. State Key Laboratory of Coal Resource and Mine Safety

(School of Mines, China University of Mining & Technology), Xuzhou 221116, China;

3. Exploration & Mining, CSIRO, Brisbane, Queensland 4069, Australia;

4. Faculty of Safety Engineering, China University of Mining & Technology, Xuzhou 221116, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2012

Abstract: Microseismic effects during the transmission of seismic waves in coal and rock mass associated with mining operation were studied by on-site blasting tests and microseismic monitoring in LW704 of Southern Colliery, Australia, by using spread velocities, amplitudes and frequency contents as the main analysis parameters. The results show that the average P-wave velocity, mean values of combined maximal amplitudes and frequencies of the first arrivals are all reduced significantly along with goaf expanding and intensity weakening of overlying strata during mining process. A full roof fracturing can make the average P-wave velocities, combined maximal amplitudes and frequencies of first arrivals reduce to about 69.8%, 92.2% and 60.0%, respectively. The reduction of the above seismic parameters reveals dynamic effects of the variation of strata structure and property to the wave transmission and energy dissipation of blasting wave. The research greatly benefits further study on stability of surrounding rock under the destructive effort by mine tremor, blasting, etc, and provides experimental basis for source relocation and parameter optimization of seismic monitoring as well.

Key words: seismic effort; blasting wave; transmission and attenuation rule; fracture zone; intensity weakening; geophone station

1 Introduction

The destructive effects of seismic wave on the roadway surrounding rock in underground mine, not only are influenced by failure mode and energy radiation of the source, but also depend on the structure complexity (inhomogeneity, discontinuity, etc) of the coal-rock mass. Because of the isotropical property of energy radiation pattern of calibration shot, it can be specially used as the main experimental method to study the influence of the property variations of the non-source factors (intensity of coal-rock mass, propagation path, etc) on the transmission and attenuation of seismic wave in underground mine [1-3]. There have been some related-literature studies on the transmission rules of blasting waves. YANG [4] discussed the properties of blasting transmission, energy, frequency and damage. CENGIZ [5] studied the blast-induced shock wave attenuation law in relation to the charge weight and the propagation distance based on 60 shots in a sandstone quarry area in Istanbul. MANOJ and SINGH [6] evaluated and predicted the blast-induced ground vibration and frequency by incorporating rock properties, blast design and explosive parameters. XIA et al [7] analyzed the situation of the vibration and the deformation of the roadway under the shock of blasting stress wave. The transmission rules of the stress wave in sandstone [8], halite [9], and limestone [10] have been studied. MEHDI et al [11] studied the blast wave propagation in the medium and the response of the structure to blast loading. MANOJ and SINGH [12] proposed a new neural network for the prediction of ground vibration and frequency by all possible influencing parameters of rock mass, explosive characteristics and blast design. KUZU et al [13] put forward a method of modified scaled distances based on empirical equations considering factors such as  blasting and geological parameters of rock mass for bench blasting in quarries. YE et al [14] carried out an experiment study on the transmission rule of blasting wave in deep underground by using seismic monitoring system. GAO et al [15-16] made a comparative analysis of the transmission rules of blasting wave in several different rock masses on the ground.

The property of coal-rock mass and stress field will change continually during mining process, which makes the transmission and attenuation rule of the seismic wave more complex than that in the laboratory and surface site. Nowadays, there has been little experimental study on the transmission rule of the seismic wave associated with coal mining, while the knowledge of this aspect is useful for the risk evaluation and control of rock burst induced by dynamic effort from mine tremor, blasting, etc.

Based on the blasting experiment and Siroseis microseismic monitoring results in the mining process of Longwall (LW) 704 in Southern Colliery, Australia, the variation characteristics of seismic parameters (wave velocities, amplitudes and main frequencies, etc) influenced by the mining disturbance were studied in this work, for the purpose to reveal the influence factors of transmission and attenuation rule of seismic wave and energy dissipation associated with variations of goaf caving range and rock fracturing extent.

2 Collection of calibration shots during mining process

2.1 Microseismic monitoring layout in LW704

Southern Colliery is located in the Bowen Basin of central Queensland. The German Creek (GC) Seam is mined 150 m below the ground surface. The immediate roof stratum is composed of massive sandstones with bedded siltstones. The massive sandstone roof results in heavy weighting at the longwall face. In order to further determine the fracturing processes and patterns of the heavy roof associated with longwall mining, a micro- seismic monitoring study was carried out at LW704 [17].

The microseismic system consists of 20 triaxial geophones with five in each of four deep boreholes (named by A, B, C and D) across the longwall face  (Fig. 1). This monitoring layout covers an area of    400 m×400 m and provides a better configuration for the detection of roof fracturing occurring in this area.

2.2 Layout of calibration shots

The variation rules of seismic waves along with roof caving were studied using calibration shots during the mining process. These shots were set off at the bottom of the boreholes (named S1, S2 and S3, respectively) located in the microseismic network. In the whole mining process of LW704, three sets of eleven blasting tests (three, four and four tests in S1, S2 and S3, respectively) were carried out while the surrounding rocks around the geophone arrays were undisturbed, partial disturbed and fully disturbed by mining operation. The shots locations are also shown in Fig. 1.

Fig. 1 Layout of LW face, geophones, and calibration shots:  (a) Plan view; (b) Cross section along strike

2.3 Collection of signals of calibration shots and mining-induced seismicities

1) In the first day of the mining operation, a set of three blasting tests were implemented in Borehole S1, S2 and S3, respectively. In addition, only three mining- induced seismicities were recorded by the seismic system, which means that the ray paths between the blasting sources and geophone arrays have not been disturbed by mining operation. The plane view of the blasting sources, seismic events and geophones are shown in Fig. 2(a).

2) When the LW face was retreated to about 110 m, another set of three blasting tests was implemented in Borehole S1, S2 and S3, respectively. At that moment, the surrounding rock around geophone array “A” was partially destroyed by coal mining (Seen in Fig. 2(b)). The seismic features of blasting waves were thus influenced to a certain extent by the characteristic variations (poriness, integrity, hardness, etc) of coal-rock mass in partial areas.

3) When the LW face was retreated to about 450 m, a set of two blasting tests was implemented in Borehole S2 and S3, respectively. At that moment, the surrounding rock around geophone array “A”, “B” and “C” was nearly fully destroyed by coal mining (Seen in Fig. 2(c)). The attenuation extent of the blasting waves was further increased, while the ray paths between the two blasting sources and three geophone arrays were largely destroyed by mining operation.

Fig. 2 Plan view of blasting shots, geophones, and mining-induced seismic events: (a) In first day of LW704 mining; (b) LW704 retreated to about 110 m; (c) LW704 retreated to about 450 m; (d) LW704 retreated to about 550 m

4) When the LW face was retreated to about 550 m, the geophone array “C” was entirely destroyed because of the collapse of surrounding rock around the borehole (Seen in Fig. 2(d)). Besides, the mining-induced seismicities were not recorded by the seismic monitoring system. Until the LW face was fully mined out, the last set of three blasting tests was implemented in Borehole S1, S2 and S3, respectively. At that moment, the surrounding rocks around all the geophone arrays were nearly fully destroyed by coal mining.

3 Seismic efforts of calibration shots associated with fracturing extent of overlying strata

Considering the anisotropy of the real coal and rock mass, the average P-wave velocities, mean values of combined maximal (Comb. Max.) amplitudes and frequencies of the first arrivals were mainly studied, for the purpose to reveal the attenuation rule and energy dissipation of blasting wave associated with characteristic variations of coal and rock mass in mining process.

3.1 Variation rules of average P-wave velocities

In the blasting tests, the locations of all calibration shots and geophone stations are exactly known, and the first arrivals of P-waves are also easily recognized. Therefore, the average P-wave velocities of the calibration shots in different mining periods can be exactly calculated using linear regression method. The average spread velocity can be expressed as

                   (1)

where, which is the distance between the blasting source and each geophone;  is the first arrival of P-wave in each

geophone, and ; n is the number of geophone stations used for calculation.

Therefore, the average P-wave velocities of S1, S2, S3 in different mining periods mentioned above, associated with variations of the goaf caving range and fracturing extent, can be obtained from Eq. (1), seen from Figs. 3-5.

Fig. 3 Velocity variations of S1 along with mining progress

Fig. 4 Velocity variations of S2 along with mining progress

Fig. 5 Velocity variations of S3 along with mining progress

For the blasting borehole S1, when the blasting tests 1 was carried out, the ray paths between the blasting source and geophone arrays haven’t been disturbed by mining operation yet (Fig. 2(a)), and the average spread velocity of blasting wave is comparatively fast (about 4.06 km/s). When the test 2 was carried out, the ray  paths between the blasting source and geophone array “A” have been partially destroyed by coal mining (Fig. 2(b)), and the average P-wave velocity is reduced to 3.89 km/s, with the slight reduction of about 4.2%. In the blasting test 4, the attenuation of blasting waves is increased substantially, while the surrounding rocks around all the geophone arrays are fully destroyed by coal mining (Fig. 2(d)). Thus, the average P-wave velocity is reduced to 2.58 km/s, with the reduction of about 36.5%.

For the blasting borehole S2, when the blasting tests 1 and 2 were carried out, the ray paths between the blasting source and geophone arrays can be regarded to be undisturbed by mining operation yet. The average P-wave velocities are 3.82 km/s and 3.70 km/s, respectively, which are equivalent roughly to each other while considering the calculation error. Meanwhile, when the blasting tests 3 and 4 were carried out, the ray paths between the blasting sources and geophone arrays “B” and “C” have been largely destroyed by mining operation, while the ray paths between the sources and geophone array “A” and “D” are partially destroyed. Due to the comparatively large attenuation of blasting waves, the average P-wave velocities in tests 3 and 4 are reduced to 2.29 km/s and 2.27 km/s, respectively, with the reduction of about 39.4%.

For the blasting borehole S3, when tests 1 and 2 were carried out, the ray paths between the blasting source and geophones haven’t been disturbed yet. The average P-wave velocities are 4.14 km/s and 4.07 km/s, respectively, which are equivalent roughly to each other. When test 3 was carried out, the ray paths between the sources and geophone array “B” and “C” are partially destroyed, and the P-wave velocity is slightly reduced to 3.88 km/s, with the reduction of about 6.3%. In test 4, the surrounding rocks around all geophones are fully destroyed by mining. The average P-wave velocity is substantially reduced to 1.25 km/s, with the reduction of about 69.8%.

Therefore, with the goaf expanding and intensity weakening of overlying strata associated with mining operation, the average P-wave velocities of blasting sources are reduced correspondingly in various degrees, which means that the more integrated and harder the coal-rock mass is, the larger the transmission velocity is, and vice versa.

3.2 Amplitude variations of blasting waveforms

Because of the coal-rock media damping effort, the seismic energy will be dissipated in various degrees during the transmission process. Variations of maximal amplitudes of blasting waveforms recorded by the five geophone stations in each borehole were studied here. Figure 6 shows the typical variation curves of maximal amplitudes as the transmission distance increases. Seen from Fig. 6, the maximal amplitudes of blasting waveforms recorded by different geophones in each geophone array decrease exponentially with the increase of transmission distance, which can be expressed as

                              (2)

where A₀ is the maximal amplitude radiated by blasting source; A_i is the maximal amplitude recorded by geophone after wave transmission; r_i is the distance between blasting source and geophone; η is the amplitude attenuation coefficient, which presents the attenuation degree in the wave transmission process.

Meanwhile, in the view of anisotropy of coal-rock mass, the mean values of combined maximal amplitudes of all five geophone stations in each geophone array are further considered. Then, the variation rules of mean values of combined maximal amplitude of different blasting sources (S1-S3) associated with mining process were discussed. The results are shown in Tables 1-3. From the results of different blasting tests in each borehole, the mean values are almost gradually reduced along with the expanding of goaf caving and fracturing extent during the mining process of LW704. Seen from the tables, taking test 1 (when there is nearly no disturbance by mining) as the basis, when test 4 was carried out, the mean values in each borehole are reduced substantially, with the reduction ranging from 58.49% to 92.19%.

Fig. 6 Typical combined maximal amplitude variations of blasting shots in different geophone stations: (a) S1 in Test 1; (b) S2 in  Test 1

Table 1 Mean values of combined maximal amplitude variations of S1 calculated by different geophone arrays (10^{-6 m/s)}

Table 2 Mean values of combined maximal amplitude variations of S2 calculated by different geophone arrays (10^{-6 m/s)}

Table 3 Mean values of combined maximal amplitude variations of S3 calculated by different geophone arrays (10^{-6 m/s)}

The seismic energy is proportional to the square of waveform amplitude, so the dissipation rules of energies are similar to amplitude variations. According to the amplitude results, with the goaf expanding and intensity weakening of overlying strata, compared to the intact coal-rock mass, the seismic energies transmitted to the recording stations will be reduced to 16% of that at least, and 0.006 at most.

In addition, by the variation rules of velocity and amplitude mentioned above, the variations of dynamic stress drops caused by blasting shots can also be obtained. According to formulas and [18], because of the reductions of spread velocities (v_P, v_S), the maximal amplitudes (normal vibration velocity (PPV)_n and tangential vibration velocity (PPV)_s), and rock densities with the fracturing expanding during mining process, the dynamic disturbance on the surrounding rock caused by stress drop  and  is also weakened largely.

3.3 Frequency variations of blasting waveforms

The main frequency of waveform is determined by failure characteristic of seismic source itself, and the high frequency part is also affected by damping effect of coal-rock medium in its transmission process. Taking the case of borehole S2 as example, Fig. 7 shows the variations of mean frequency of first arrivals. Seen from Fig. 7, the frequencies of first arrivals in each geophone array are also reduced gradually, along with expanding of fracturing extent during mining process. In addition, because of the surrounding rocks around the geophone array “B” are mostly destroyed by mining operation, the mean frequency of first arrivals is reduced to above 60%, while the frequency reductions in other three geophone arrays range from 20% to 30%.

Fig. 7 Sketch map of frequency of first arrival of S2 associated with mining

4 Conclusions

1) With the goaf expanding and intensity weakening of overlying strata associated with LW704 mining, the average P-wave velocities of blasting sources in each borehole are reduced correspondingly in various degrees. Especially, when the surrounding rocks around geophone arrays are largely destroyed by mining operation, the reductions of average P-wave velocities reach 36.5%, 39.4% and 69.8%, respectively.

2) The amplitudes of blasting waveforms recorded by different geophones in each geophone array decrease exponentially with the increase of transmission distance. In addition, the mean values of combined maximal amplitudes in each borehole are also reduced gradually during the mining process of LW704, with the reduction ranging from 58.49% to 92.19% when last tests are carried out.

3) The main frequencies of waveforms are also affected by damping effect of coal and rock medium in its transmission. Taking the case of Borehole S2, a full roof fracturing can make the mean frequency of first arrivals reduce to over 60%.

4) According to the test results, the intensity weakening of coal-rock mass can make the seismic energies radiated by mine tremors or blasting shots dissipate largely, reducing the destructive efforts on the surrounding rocks around roadways or working faces. Additionally, the spread velocity variations mean that the set velocity used for seismic event location needs to be adjusted correspondingly, according to goaf variation during mining process, to ensure the location accuracy of seismic monitoring.

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(Edited by YANG Bing)

Foundation item: Project(2010CB226805) supported by the National Basic Research Program of China; Project(2010QNA30) supported by the Fundamental Research Funds for the Central Universities of China; Project supported by the Priority Academic Development Program of Jiangsu Higher Education, China; Projects(SZBF2011-6-B35, 2012BAK04B06) supported by the National Twelfth Five-year Key Science & Technology Foundation of China

Received date: 2011-07-26; Accepted date: 2011-11-14

Corresponding author: ZHANG Yi-dong, Associate Professor, PhD; Tel: +86-13952118118; E-mail: ydzhang@cumt.edu.cn