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Calculation of Electromagnetic Radiation Criterion for Rockburst Hazard Forecast in Coal Mines V. FRID Abstract——Intensive micro-fracturing of rock close to mining operations accompanies an increase in the likelihood of rockbursting. This fracturing causes an increase of the electromagnetic radiation (EMR) level by up two orders of magnitude, depending on the mining environment. Several examples of this enhanced EMR are presented in this paper. We first treat the EMR theoretical criterion of rockburst hazard in coal mines and compare it with the empirical criterion of EMR activity that was revealed on the basis of more than 400 dilTerent hazardous and non-hazardous situations in underground coal mines. Only the following parameters are needed to estimate the EMR criterion of rockburst hazard: limiting value of gum volume, mine working width, coal seam thickness, and coal elastic properties. Key words: Rockburst, electromagnetic radiation, fracture, coal mines 1. Introduction The phenomenon of rockbursting has long been known in mining. The rockburst hazard increases if the load on a given part of a coal seam exceeds some critical level,while the distance to the stress maximum in the zone of influence of a mine working is lower than the critical value (PETUKHOV and LINKOV, 1983). The rockburst hazard is usually determined by some standard geomechanical method, for example, gum volume measurement, measurement of hole diameter or number of disks that are created due to core fracturing as a result of drilling in a highly stressed zone, etc. (PETUKHOV, 1972). The method of gum volume measurement is generally used in coal mines of the former USSR. All of these methods are very time-consuming and sometimes dangerous becausedrilling is required. For these reasons, rockburst hazard forecasting at a mineworking face must be made short-term and safe. Geophysical methods can help toreduce the risks (FALLON et al., 1997). As noted by LOCKNER (1993, 1996), there is a strong parallel between the well-known Gutenberg-Richter relation for seismic events (from macro (earthquake) to micro (rock burst)) and power-law frequency magnitude relationship for acoustic emission (AE) events. This analogy suggests that micro shocks (high frequency and small magnitude) are precursors of macro failure (large magnitude and small frequency) and is the theoretical basis for rockburst forecasting by the AE method (KuKSENKO et al., 1982; MANSUROV, 1994). The EMR frequency range is close enough to the AE band. Therefore, both types of emissions are associated with rock fracture YAMADA et Cll., 1989; OKEEFE and THIEL, 1995; RABINOVITCH et Cll., 1995.Hence, it would be correct to assume that electromagnetic radiation (EMR) could beuseful for rockburst hazard forecasting along with AE. Moreover, being non-contact, the EMR method has advantages over AE. For example, when a rapid and comprehensive prognosis of a short-term mine working region (for example, in a drift face) is needed, the roughness of the mine walls becomes a marked problem for the AE method for rapid data acquisition due to inferior contact between the AE transducer and the mine wall. Numerous investigations have examined different aspects of the EMR (CRESS et Cll., 1987; FUJINAWA et Cll., 1992; NITSAN, 1977; OGAWA et Cll., 1985; WARWICK et al., 1982; YAMADA et al., 1989; YOSHINO et al., 1993). The EMR amplitude is a function of the crack area (RABINOVITCH et al., 1998, 1999). Moreover, an increase of elasticity, strength, and loading rate enhances the EMR amplitude (GoLD et al., 1975; NITSAN, 1977; KHATIASHVILI, 1984; FRID et Cll., 1999). Since the eighties, the interest in EMR has increased in connection with the problem of rockburst forecasting. KHATIASHVILLI et al. (1984) carried out an investigation of EMR in the Tkibulli deep shaft (Georgia) prior to an earthquake of 5.4 magnitude. The registration point (at the shaft position) was located 250 km from the earthquake epicenter. Prior to the earthquake itself, an increase of intensity of the lower part of the spectrum (1—100 kHz) and a corresponding decrease of intensity of higher frequencies (100-1000 kHz) were observed. An increase of the number and the sizes of cracks during the earthquake approach could, perhaps, explain this phenomenon. NESBITT and AUSTIN (1988) registered EMR in a gold mine (2.5 km depth). An EMR signal (1.2 mA/m amplitude) was generated seconds prior to the micro-seismic event (magnitude of -0.4). Registra-tion of EMR activity in Ural bauxite mines showed (ScITOVICH and LAZAREVICH, 1985) that its values sharply increased with rockburst hazard increase. Analogous works in Norilsk polymetal deposit (Krasnoyarsk region) revealed an increase of EMR amplitude (up to 150-200 mV/m) and activity in the rockburst hazardous zones (REDSKIN et al., 1985). MARKOV and IPATOV(1986) investigated EMR activity changes in an apatite underground mine (Khibin deposit, Kola peninsula) and ascertained that EMR amplitude in rockburst hazardous zones was in therange of 8-25 mV/m and EMR activity here was significantly higher than the regular noise level. This very limited overview demonstrates that the EMR is a multi-scale phenomenon that is currently investigated in laboratories and in .situ (before earthquake and rockburst). However, all EMR mine investigations have usually been empirical, and the degree of their theoretical generalization is not enough to be useful for rockburst forecasting. This paper first considers a development of the theoretical EMR criterion for rockburst forecasting. 2. Comparison of EMR and Gum Methods The promotion of a new method for rockburst forecasting is a very responsible undertaking. Hence, the new method must be comprehensively compared with the method which is currently being used. In this section of the paper we consider the methodological foundation of the gum method that has been used for rockburst forecasting before discussing EMR and the EMR methodology. Finally, several examples of EMR and gum investigations are presented. 2.1. Methodology of Gum Measurement Drilling of a highly stressed coal seam leads to an intensive fracturing process in the zone, influenced by the drill hole. The volume of this highly cracked zone depends on the hole diameter, the drilling rate and, especially, the stress level. Hence, if the first two parameters remain invariant for a given coal seam, the stress value in the coal seam (Fig. 1) is responsible for the volume of drilled coal rubble that is recovered from the hole (i.e., from the highly stressed zone drilled by the hole). If the drilling is dry, the drilled coal rubble is called"gum." The non-dimensional diameter，of the highly stressed zone (ratio of the non-elastic deformation zone, diameter D, to the hole diameter d=0.043 m, Fig. 2) can be calculated from the following formula (PETUKHOV, 1972): where n,. is the coefficient of coal loosening on borehole wall that is generally equal to 1.3-1.4, Mo is the gum volume of a borehole (MD=}d2/4A, A is the borehole length), and M,S. is the gum volume induced by drilling in a stressed zone) TUKHOv et al., 1976). The vertical stress in the coal seam can be determined as follows (PETUKHOv et al., 1976) where is the coal shear strength. Forecast boreholes are usually drilled in intervals (the length of each interval is 1 m). Hence, if we determine the gum volume for each meter of the hole, we can predict the vertical stress distribution in the coal seam near the mine working face. where k is the coal shear strength. Forecast boreholes are usually drilled in intervals (the length of each interval is 1 m). Hence, if we determine the gum volume for each meter of the hole, we can predict the vertical stress distribution in the coal seam near the mine working face. After the drilling of each interval, the gum volume is measured and if it exceeds a definite limiting value (experience at the mining works in North Kuzbass shows that the limiting values are 5 to 8 liters per meter at the fourth and the seventh drilling meter from the drift face, respectively (Table 1)), drilling is stopped, and that part of the mine working, is considered rockburst hazardous. 2.2. EMR Methodology for Mine Measurement Figure 3 explains the EMR activity definition. The EMR activity is defined by the number of intersections of the EMR voltage signal (per unit time) with a given volt 100 age level (of a special counter). The EMR activity was measured by a resonance 士1 kHz antenna. Our preliminary mine estimation of electromagnetic patibility conditions showed that the given resonance frequency would allow com-us to accurately extract the useful signal from the industrial background noise. Table 1 Calculation of rockburst hazardous zone paramete Figure 1 The vertical stress distribution in the zone of influence of the mine working (all parameters are discussed in the text).  Figure 2 The zone of non-elastic deformation excited by drilling in the high stressed zone (b is the non-dimensioned diameter of this zone:the ratio of the non-elastic deformation zone diameter n to the hole diameter d=0.043 m) Figure 3 The EMR signal that intersects counter voltage level. 電磁輻射在煤礦沖擊地壓預(yù)測(cè)中的應(yīng)用 韋.弗瑞德 摘要——采礦活動(dòng)引起的強(qiáng)烈?guī)r石破碎導(dǎo)致沖擊礦壓發(fā)生的可能性增加。這些采礦環(huán)境導(dǎo)致的巖石破碎引起電磁放射(EMR)的水平增加。本文闡述了電磁輻射規(guī)律顯現(xiàn)的幾個(gè)例子。我們首先對(duì)煤礦在開(kāi)采期基于400多個(gè)采動(dòng)和非采動(dòng)環(huán)境下礦壓顯現(xiàn)狀況引起的電磁輻射顯現(xiàn)現(xiàn)象作了統(tǒng)計(jì)。其中只需以下參數(shù)來(lái)對(duì)礦壓顯現(xiàn)導(dǎo)致的電磁輻射作評(píng)價(jià)：聲發(fā)射、工作面長(zhǎng)度、煤厚、煤層硬度 關(guān)鍵詞： 沖擊礦壓 電磁輻射 破碎 煤礦 1.簡(jiǎn)介 人們?cè)诓傻V方面認(rèn)識(shí)沖擊礦壓現(xiàn)象已經(jīng)很久了。在部分煤體上的負(fù)荷超過(guò)一定的水平，沖擊礦壓發(fā)生的可能性就大大增加(PETUKHOV和LINKOV, 1983)。對(duì)于沖擊礦壓發(fā)生的預(yù)測(cè)常?？紤]地質(zhì)因素：生發(fā)射的測(cè)量、裂隙大小的測(cè)量、高應(yīng)力區(qū)域開(kāi)采導(dǎo)致的巖石破碎度等(PETUKHOV, 1972)。在應(yīng)用方面，聲發(fā)射法經(jīng)常用于前蘇聯(lián)的煤礦。 所有的這些方法都非常耗時(shí)而且有時(shí)很危險(xiǎn)，因?yàn)樾枰陂_(kāi)采區(qū)域打鉆孔，這將導(dǎo)致巖石的應(yīng)力釋放。因?yàn)檫@些原因，工作面沖擊礦壓危險(xiǎn)性預(yù)測(cè)必須耗時(shí)少并且安全。地質(zhì)方法可以并不能幫助降低這些工作的風(fēng)險(xiǎn)(FALLON等, 1997)。 據(jù)LOCKNER (1993, 1996)記載, 因?yàn)榈卣鹪?，著名的古登寶界面和理查德界面有很大的平行面而且有巨大能量的聲發(fā)射。這個(gè)相似點(diǎn)意味著微震 (高頻率、小振幅) 是大震 (大振幅、小頻率) 的前兆，并且意味著這是聲發(fā)射法預(yù)測(cè)沖擊礦壓的理論基礎(chǔ) (KuKSENKO等，1982; MANSUROV, 1994)。電磁輻射的頻率范圍與聲發(fā)射的關(guān)系極為密切。因此，兩種類型的輻射與巖石破碎有關(guān)(YAMADA等, 1989; OKEEFE和THIEL, 1995; RABINOVITCH 等, 1995) 。因此,電磁輻射可以和聲發(fā)射一起應(yīng)用于沖擊礦壓預(yù)測(cè)。而且，遙控操作使電磁輻射法比聲發(fā)射法占優(yōu)勢(shì)。 舉例來(lái)說(shuō)，如果在一個(gè)快速推進(jìn)的工作區(qū)域 ( 例如,在一個(gè)無(wú)人工作面) 礦井巷道的粗糙影響聲發(fā)射的傳輸和聲發(fā)射所需要的各種精確數(shù)據(jù)構(gòu)成了矛盾。 很多的研究成果已經(jīng)測(cè)試了電磁輻射的不同方面的性能(CRESS等., 1987; FUJINAWA 等., 1992; NITSAN, 1977; OGAWA 等, 1985; WARWICK等, 1982; YAMADA , 1989; YOSHINO , 1993) 。電磁輻射振幅是裂隙區(qū)域運(yùn)動(dòng)的一個(gè)函數(shù)(RABINOVITCH et al., 1998, 1999) 。而且，彈力，壓力以及它們作用的頻率增加會(huì)增大電磁輻射的振幅 (GoLD, 1975; NITSAN, 1977; KHATIASHVILI, 1984; FRID, 1999) 。 自80年代以來(lái), 對(duì)電磁輻射預(yù)測(cè)沖擊礦壓?jiǎn)栴}的研究熱度一直在增加。KHATIASHVILLI . (1984)做了一個(gè)在深井下預(yù)測(cè)出5.4級(jí)地震的研究。他在井底的觀測(cè)點(diǎn)距離震源足足有250千米。伴隨地震的發(fā)生，1-100千赫的低頻波急劇增加，而100-1000千赫的高頻波在減少。在地震過(guò)程種巖石破碎的數(shù)量和大小可以，或者大概可以解釋這些現(xiàn)象。NESBITT 和 AUSTIN（1988）再次在地表2.5千米下的一個(gè)金礦對(duì)電磁輻射作了研究。有記載一次微震前一串電磁輻射信號(hào)提前發(fā)射出來(lái)(ScITOVICH 和 LAZAREVICH,1985)。研究還發(fā)現(xiàn)烏拉山脈的鐵礬土礦的電磁輻射活動(dòng)顯示它的輻射程度迅速地以沖擊地壓增加而增加(REDSKIN 等, 1985)。類似的情況也在多金屬沉淀地層發(fā)生，電磁輻射隨著巖塊破斷產(chǎn)生了巨大能量。MARKOV 和IPATOV(1986)在井工礦調(diào)查一個(gè)磷灰石的電磁輻射活動(dòng)變化是，確定巖石破斷導(dǎo)致的電磁輻射范圍在 8-25 mV/ m之間，而且電磁輻射活動(dòng)在這里也比一般輻射要強(qiáng)。這個(gè)非常有限的觀點(diǎn)表示，電磁輻射現(xiàn)在是一種普遍在實(shí)驗(yàn)室和現(xiàn)場(chǎng)(在地震和沖擊礦壓之前)被研究的現(xiàn)象。然而, 所有的關(guān)于電磁輻射的研究通常是基于經(jīng)驗(yàn)的，而且理論上的不足對(duì)沖擊礦壓預(yù)測(cè)是有限的。本文首次拓展基于電磁輻射理論的沖擊礦壓預(yù)測(cè)標(biāo)準(zhǔn)。 2. 電磁輻射和聲發(fā)射方法的比較 為沖擊礦壓預(yù)測(cè)的方法升級(jí)是一個(gè)非常有意義的事業(yè)。因此，新的方法一定要在各個(gè)方面與現(xiàn)在目前使用的方法進(jìn)行比較。在論文的這一部分中我們?cè)谟懻撾姶泡椛浞ê碗姶泡椛鋺?yīng)用之前，首先討論目前使用的聲發(fā)射法，最后將列舉電磁輻射和聲發(fā)射法應(yīng)用的一些例子。 2.1聲發(fā)射法 對(duì)高應(yīng)力煤層打鉆的過(guò)程導(dǎo)致了一個(gè)受鉆孔影響的巖石破碎過(guò)程。這個(gè)高度破碎區(qū)域的體積受鉆孔直徑，打鉆速度，尤其是應(yīng)力大小的影響。 因此，如果前面兩個(gè)叁數(shù)在給定的開(kāi)采煤層中保持不變，如圖1 壓力值會(huì)對(duì)聲發(fā)射大小有影響。如果干法打鉆, 被打的煤巖被叫做 “箱體”(PETUKHOV, 1972)。高應(yīng)力下的鉆孔直徑β， (強(qiáng)力毀壞區(qū)域的直徑D,洞直徑 d=0.043 m (見(jiàn)圖2) 可以由下式求得： 其中nr是煤的松散系數(shù)，大致為1.3-1.4。Mo是鉆孔的聲發(fā)射值。（TUKHOv et al, 1976). Ms是在高應(yīng)力區(qū)域打鉆時(shí)的聲發(fā)射值，煤層應(yīng)力可由下式確定(PETUKHO等,1976)： 其中k*為作用在煤壁上的剪切力。常常把探測(cè)鉆孔間隔打(間隔距離1 m)。因此，如果我們?yōu)槁暟l(fā)射探測(cè)每公尺布置一個(gè)鉆孔,我們能預(yù)測(cè)工作面煤壁的垂直壓力分配。 在每打鉆之后，就獲得一次聲發(fā)射值，而且到達(dá)一定值就停止鉆孔。試驗(yàn)工作面被衡量為有沖擊礦壓發(fā)生危險(xiǎn)。聲發(fā)射被測(cè)得，而且如果它超過(guò)明確的極限(在North Kuzbass的采礦經(jīng)驗(yàn)聲發(fā)射限值是每公尺 5 到 8 個(gè)單位，出現(xiàn)在第四個(gè)和第七個(gè)鉆孔, (見(jiàn)表 1))。 2.2 電磁輻射法在礦井中的應(yīng)用 圖3 顯示了電磁輻射活動(dòng)規(guī)律。電磁輻射活動(dòng)規(guī)律是由給定(用特殊計(jì)算器)的電磁輻射電壓信號(hào)(每隔單位時(shí)間發(fā)射)決定的。電磁輻射活動(dòng)規(guī)律由一個(gè)士1 kHz測(cè)量?jī)x器測(cè)量。 表1沖擊礦壓危險(xiǎn)區(qū)叁數(shù)計(jì)算 圖1 工作面礦壓影響范圍內(nèi)的垂直應(yīng)力分布圖(所有的叁數(shù)在本文中都已定義) 圖2 在高應(yīng)力區(qū)打鉆將會(huì)對(duì)巖體造成塑性破壞 (b 是該區(qū)域直徑于塑性破壞區(qū)直徑的比 d=0.043 m) 圖 3 不同電壓信號(hào)下的電磁輻射顯現(xiàn)