Micro Seismic Monitoring Technique and Practice of Rock Blast in Deep Mining

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1 Micro Seismic Monitoring Technique and Practice of Rock Blast in Deep Mining Guo Lijie School of Resources and Safety Engineering, Central South University, Changsha , China Beijing General Research Institute of Mining and Metallurgy, Beijing , China ABSTRACT This paper introduces the features and scope of application of micro seismic monitoring technology and then focuses on the analysis of sensor monitoring network optimization, rock velocity correction and the method of differentiation between in micro seismic and explosion. At the same time, according to the analysis of data of typical mine micro seismic monitoring system, a rock blast prediction method has been presented which is based on a blast accumulated volume and energy index of rock. Through the application of two typical mine show that micro seismic monitoring technology and method can produce better results in the practice of the deep mining. KEYWORDS: Micro seismic, Rock blast, Pressure monitoring, Rock velocity correction INTRODUCTION The phenomenon of stressed rock mass acoustic emission was first discovered by Obert and Duvall of United States Bureau of Mines (USBM) at the end of the 1930s. At the beginning of the 1960s, the researches in South Africa started the research of relevant issues of rock bursts and deep gold mines by making use of the phenomenon. The early research has proved conceivably the feasibility of prediction of rock bursts with the micro seismic monitoring technology. From the middle of the 80s and the early 90s, some mines in Canada experienced severe rock burst accidents and after that, more than 20 mines installed the micro seismic monitoring system to make it as part of the routine safety management. In the past more than 20 years, micro seismic monitoring has

2 Vol. 20 [2015], Bund developed from a pure rock burst research means to a necessary industrialized tool of mines routine safety management with the development of the electronic technology and the research and gradual growth of the micro seismic and acoustic emission theory [1]. As the demand for the domestic mineral resources grows larger and larger while the strip and shallow mining approaches to the end, deep mining has become the only way for exploitation of mineral resources. During the process of deep mining, the earth pressure problems occurs frequently such as roof caving, wall caving and surface movement while the traditional methods monitoring stress, strain and dislocation and other aspects have deficiencies in contact, point and line, but the regional monitoring can achieve monitoring for the locations which people are unable to reach with conventional methods, because of its advantages such as real-time monitoring capacity, event source high-precision positioning, monitoring earthquake magnitude and source mechanism. 1 Micro Seismic Monitoring Technology [2,3,4] Micro seismic monitoring technology is developed from the seismic monitoring technology and it shares the same seismic monitoring and acoustic emission monitoring technology in theory, based on the theory of sound and energy of fracture during the process of rock mass failure due to stress. It makes monitoring the process of underground rock mass and concrete structure failure as its object, collects the micro seismic signals released by failure and assesses the stability and safety conditions of the underground structures with the processing and analysis of the seismic source signals. Micro seismic monitoring technology is mainly used in tunnel surrounding rock stability monitoring, hydropower engineering high and steep slope stability monitoring, large strip mine slope monitoring, large underground fuel storage safety monitoring, underground slip casting engineering monitoring, petroleum engineering fracture monitoring and underground mine earth pressure monitoring, in particular underground mine earthquake and rock burst monitoring, surface movement monitoring, caving range monitoring, high stress and stress redistribution monitoring, burst and its aftershock monitoring, illegal mining monitoring, security and emergency rescue and early warning. Micro seismic monitoring system have the features including real-time monitoring capacity, event source high-precision positioning, monitoring earthquake magnitude and source mechanism and so on, which has become an ideal tool for the research of mines and earthquakes and related land subsidence and other issues. It installs sensors securely in the monitoring area in the form of an array to achieve a 24-hour real-time monitoring for micro-seismic events. It has broke through the traditional monitoring methods with respect to the monitoring modes of point or line on force (stress) and dislocation (strain), achieving the monitoring of the time process on the space

3 Vol. 20 [2015], Bund concept of the rock mass fracture process within the excavation affected areas [5]. It is not a noncontacting measurement but is convenient for the achievement of monitoring of the locations people are unable to reach with conventional methods. It usually adopts multiple channels with multiple sensors for monitoring, builds micro seismic monitoring station networks and achieves high-precision positioning of micro seismic events through the positioning algorithms including the calculation of the time difference between P wave and S wave. 2 Micro Seismic Monitoring System and Its Composition At present, those having technology, equipment and provision of complete services and a certain market share of micro seismic monitoring systems mainly include, globally, South African Institute of Mine Earthquake (IMS), Canada ESG Solution, United States MicroSeismic, Australia CSIRO, UK Semore Seismic and Canada mu-sic and so on. The hardware system of micro seismic monitoring mainly includes micro seismic sensors (speed or acceleration, single-axis or thee-axis), data collectors, communication system, time synchronization time service devices, data acquisition and processing serves and so on. The software functions include micro seismic data real-time acquisition, automatic micro seismic event positioning, interactive three-dimensional event and image display, ray tracing and earthquake modeling, multi-purpose tracing/ multi-component processing, travel time tomography imaging and error analysis, tracking display and interactive interpretation, diffraction stack depth tracing offset and so on. 3 Sensor Monitoring Station Network and Its Optimization The IMS and ESG micro seismic monitoring systems respectively installed in some gold mine in Shandong and some phosphate mine in Hubei have great differences in the purposes of monitoring of the two mines, although both are for monitoring of rock mass damage degree. The former is used to monitor the earth pressure activities during the process of god filling treatment and the gob is located in the middle within the ground mountain body in the Shennongjia Mountainous Area, as drift mining. The latter is mainly used to monitor the micro seismic activities during the process of ore body recovery and the regional depth monitored is -880 m to 1200 m, as underground deep mining. The optimization is made based on the following principles and the sensors are arranged around gobs respectively, after a field trip to the two mines for the engineering geological conditions and in consideration of the mining plan and construction difficulty degree: The measured monitoring data is the most sensitive to the change of monitoring objects; In order to prevent micro seismic signals bypassing the gobs causing time errors and influencing positioning accuracy, the sensors should be arranged in the footwall of ore bodies as far as possible; It is

4 Vol. 20 [2015], Bund possible to make key acquisition for the key locations of earth pressure activities with multi-axis sensors; and it needs to arrange monitoring points at multiple middle-sections to form a threedimensional monitoring network. Altogether 6 single-axis acceleration sensors are arranged in the two roadways chosen of level 1190 and level 1200 around the gob for the phosphate mine in Hubei according to the mentioned optimization method, because its key monitoring regions is small and its gob is slightly inclined with a slope about 5º; 4 single-axis acceleration sensors are arranged in -20 (40 m altitude difference between two middle sections, i.e m above sea level), -22, -24 and -30 for the gold mine in Shandong. The final optimized layout of sensors is as shown in Figure 1 and Figure 2. Middle section 20 Middle section 24 Middle section 22 Middle section 30 Figure 1: Distribution map of IMS micro seismic monitoring system of the mine in Shandong Figure 2: Distribution map of ESG micro seismic monitoring system of the mine in Hubei

5 Vol. 20 [2015], Bund Wave velocity correction After installing the micro seismic devices, there are usually certain errors for positioning with the recommended values because of the factors including lithology and structural difference, so it needs to use the method of fixed blasting, carry out comparative analysis with the actually measured coordinates and the coordinates calculated from the results of blast positioning results and do wave velocity correction according to the results of analysis. The site of blasting should be chosen where the micro seismic events resulted from blasting can be received by all sensors. The time of blasting should be period of less production blasting, trying to reduce the interference of other blasts. The wave velocity correction results of the gold mine in Shandong shown in Table 1 indicate that the error resulted from the positioning with the wave velocity recommended values is about 22 m and also indicate the necessity of wave velocity correction of the micro seismic monitoring system. Table 1: Wave Velocity Correction Fixed Blasting Positioning Results of the Gold Mine in Shandong No. 1 2 Micro seismic system Error/m Coordinate Fixed blasting actual positioning Straight azimuth measured coordinate Coordinate coordinate/ m line N E U N E U Data Analysis After obtaining the micro seismic signals, it requires manual screening of micro seismic and blasting evens and demarcation of the starting time of Wave P and Wave S, and then extraction of micro seismic events and high-precision positioning. The main basis of manual screening includes determination of whether the events occurs within the blasting time or not, blasting oscillogram coda wave monotone decrease, production blasting wave form having multiple sections of delay, blasting energy curve normal monotone increase, blasting frequency spectrum normally not in accordance with Brune Model and blasting event corner frequency>100hz and so on. After extraction of micro seismic events, it established the micro seismic event quantity column diagram through the micro seismic events quantity statistics within the micro seismic monitoring cycle; it establishes the micro seismic event weekly distribution diagram, daily

6 Vol. 20 [2015], Bund distribution diagram and micro seismic activity distribution diagram through the statistics of occurrence time of micro seismic events; and it establishes micro seismic event magnitude distribution diagram, accumulated volume change condition diagram, energy and seismic relationship diagram and accumulated apparent volume energy index time distribution diagram and so on. Accumulated Apparent Volume Energy Index Analysis In the prozone of rock peak strength, stress is in a process of increase and at this time, the deformation of rock mass is relatively small; as it approaches to the peak, its deformation tends to increase because of the increase of non-elastic deformation. Accumulated volume is measurement of seismic source co-seismic deformation and, in the prozone of rock peak strength, its value change is consistent with the deformation regulation before rock peak value. Therefore, the increase of energy index and the slow increase of apparent volume indicate that the rock in the seismic source area is stable, on the hardening stage of energy accumulation. After rock peak strength, the deformation increases because of the decrease of rock carrying capacity and stress. The apparent volume increases and the corresponding energy index decreases, so this indicates that the rock has strain softening and is damaged. Figure 3: Distribution map of Accumulated apparent volume energy index time As shown in Figure 3, the blue curve is accumulated apparent volume curve and the red curve is energy index curve. From August 29 th to September 2 nd, the energy index tends to increase and the accumulated apparent volume maintains relatively stable increase; on September 2 nd, the energy index suddenly reduces and the accumulated apparent volume starts to accelerate increase;

7 Vol. 20 [2015], Bund after September 3 rd, the energy index accelerates decrease and the accumulated apparent volume continues to increase until September 9 th. It is strain hardening area from September 9 th to September 16 th when the energy index and the apparent volume present the change features of increase and during the period, the apparent volume has two significant rapid increases and the energy index has large fluctuation and this indicates that the change tendencies of energy index and apparent volume in the strain hardening area are not consistent and that the larger apparent volume increase may bring a larger release of energy to reduce the increase speed of stress, showing a decrease of the energy index growth ratio. On September 17 th, the energy index suddenly starts to decrease and reach a decreasing amplitude; the accumulated apparent volume reaches a new peak and this is consistent with the large event of seismic magnitude 0.1 monitored on September 18. Micro energy analysis The energy index of a micro seismic event is the ratio of the actually measured radiated micro seismic energy produced by the event and the average micro seismic energy of all events within the region. The larger the energy index, the larger the drive stress of seismic source at the time of the event occurrence it indicates; and the relatively high energy index indicates relative concentration of rock stress. We can see from Figure 4 that the micro seismic events in the phosphate mine in Hubei mainly distributes between the levels of middle section in cross-hole; Line 15 is between the levels of middle section and no event greater than grade 0 is monitored. This is consistent with the situation declared by the mine that there is lots of blasting operation in the two areas and that the gob surrounding rock joint fissure is developed. However, the gob in middle section of Ore Block I at the center of the monitoring range has few micro seismic events, which is directly related to the waste filling carried out currently in the gob.

8 Vol. 20 [2015], Bund Figure 4: Micro seismic events distribution and micro seismic energy of the phosphate mine in Hubei Figure 5 is the view of the micro seismic events quantity distribution of the gold mine in Shandong and Figure 6 is the distribution of the energy indexes of all micro seismic events of the mine on the profile. We can see from the figures that, below middle section 18 and deep production related regions, it tends to be relatively significant stress concentration and reflects the relatively significant stress concentration tendency in the region, especially the north side of middle section 20 ore body. The large events mainly distribute within the rock mass near middle section 26, close to excavating working face, including the maximum event seismic magnitude 1.1 and the second seismic magnitude 0.2. According to preliminary analysis, the reasons for these micro seismic events may be that the stress re-distribution is caused by the developing and excavating activities in rock mass deep in the mine and the strain energy stored in rock mass suddenly releases in the form of micro seismic wave. The monitoring results coincide with the dynamic signs observed on the mine site, such as rock burst and wall caving.

9 Vol. 20 [2015], Bund Figure 5: North view of the event quantity distribution of the gold mine in Shandong Figure 6: Micro seismic energy index We can see from the size and color of the micro seismic events energy balls in Figure 4 and Figure 5, the phosphate mine in Hubei and the gold mine in Shandong are basically the same in the quantity of micro seismic events. But the former s micro seismic event balls are basically green or blue, indicating that the quantity of the micro seismic large events monitored by the phosphate mine in Hubei is basically 0, in other words, no micro seismic large event exceeding grade 0 occurring in the gob and surrounding and there is only local rock micro fracture with concentrated damaged area and small degree of damage. But the micro seismic event balls of the gold mine in Shandong are dark blue, light blue to green, light yellow, dark yellow and red, indicating that it has monitored many micro seismic large events and the micro events damaged areas are scattered with a severe degree of damage. This is closely related to the earth pressure induced by deep mining up to thousand m underground. Through the comparison of the two projects, it indicates that the micro seismic monitoring system works more effectively in the earth pressure monitoring caused by deep mining up to thousand m underground. SUMMARY The paper indicates: (1) With respect to the micro seismic monitoring station network arrangement optimization, the sensors should be arranged in the footwall of ore body as far as possible in order to prevent micro seismic signals bypassing gob causing time error and influencing positioning accuracy; multi-axis sensors should be used in the key acquisition in the key locations of earth pressure activities; and monitoring points are arranged in multiple middle sections to build a threedimensional monitoring network;

10 Vol. 20 [2015], Bund (2) Use accumulated apparent volume and energy index as a rock blast prediction index and find the existing incubation stage and prewarning stage before rock blast and large-scale rock mass fracture. The rapid decrease of energy index and continuous increase of accumulated apparent volume can be regarded as the omen of rock blast and large-scale rock body fracture; (3) The industrial application indicates that the micro seismic monitoring system is indeed an effective means to monitor earth pressure. It provides better results in the monitoring of deep mining operation up to thousand m underground and functions better. REFERENCES [1] Maochen Ge. Efficient mine microseismic monitoring. International Journal of Coal Geology. 64 (2005) [2] Kaiser P K, McCreath D R, and Tannant D D. Canadian rockburst support handbook[r]. Geomechanics Research Centre [3] K.Katsuyama. Application of AE Techniques[M]. Translated by Feng Xiating. Beijing: Metallurgy Industry Press, [4] Li Shulin, Yin Xiangang, Zheng Wenda, et al. Research of multi-channel microseismic monitoring system and its application at Fankou Lead-zinc mine[j]. Chinese Journal of Rock Mechanics and Engineering, 2004, 24(12): 2 048~2 053 [5] Li Shulin. Discussion on Microseismic Monitoring Technology and Its Applications to Underground Projects [J]. Chinese Journal of underground space and Engineering, 2009(1): [6] Tang Lizhong, Yang Chengxiang, Pan Changliang. Optimization of microseismic monitoring network for large-scale deep well mining[j]. Chinese Journal of Rock Mechanics and Engineering, 2004, 25(10): 2 036~ Author introduction Guo Lijie (1980-), male, Ph.D., senior engineer. Research: mining processes and technology, mining waste recycling and disposal technologies. Tel : , ljguo264@126.com 2015 ejge