1、翻译部分Calculation of Electromagnetic Radiation Criterionfor Rockburst Hazard Forecast in Coal MinesV. FRIDAbstractIntensive 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
2、 (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 reve
3、aled 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 propertie
4、s.Key words: Rockburst, electromagnetic radiation, fracture, coal mines1. IntroductionThe 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 zo
5、ne 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 fracturi
6、ng 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
7、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 micr
8、o (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 b
9、y 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 elec
10、tromagnetic 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 rough
11、ness 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
12、., 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, 19
13、84; 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
14、(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 (1100 kHz) and a corresponding decrease of intensity of higher frequencies (100-1000 kHz) were observed. An increase of the number and
15、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 acti
16、vity 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 deposi
