Study on the characteristics of coal rock electromagnetic radiation (EMR) and the main influencing factors
Introduction
Coal industry is the basic industry of Chinese national economy, The constituents of coal in primary energy consumption has accounted for more than 60% in China [Ding et al., 2017, Li et al., 2016a, Li et al., 2016b, Wang et al., 2017]. Because of the occurrence of coal seam and complicated geological conditions, in China, more than 95% of coal production is in the underground, so that coal rock dynamic disasters such as coal and gas outburst, rock burst and others occur frequently [Li et al., 2016c, Kong et al., 2017], which results in a large number of casualties and property losses, and has seriously restricts to the increasing of mine production and economic benefit. Besides, with the increasing of mining depth and scale, the problem is becoming more and more serious [Fan et al., 2017, Li et al., 2015, Li et al., 2015, Pan et al., 2003].
Preventing the coal and rock dynamic disaster is an important prerequisite to ensure the safety of mine production and improve the economic efficiency. Studying the economical, reliable and effective forecasting methods is also an important basis for the prevention and control of coal and rock disaster. There are two main type methods, one is the contact static method, drilling, gas pressure measurement [Zhao et al., 1987, Lippmann, 1990, Yin et al., 1997]; the other is the non-contact geophysical methods, micro-seismic, acoustic emission (AE), electromagnetic radiation (EMR) [Cook, 1964, Frid, 1997a, Frid, 1997b, Yamada et al., 1989, Yang et al., 2007, Senfaute et al., 1997]. Static method occupies the working time and space, the project is very large, the operation time is longer, the forecast cost is high; and the accuracy is low. Besides, it is easy to be influenced by the working surface contains the gas coal body structure, the inhomogeneity and instability of stress distribution. EMR method is a new geophysics method, which is treated as a very promising short-term prediction method [Muto et al., 1951, Miura and Nakayama, 2001, Song et al., 2016, Xu et al., 1985].
In the 1950s, Воларович and Пархоменко recorded and studied the piezoelectric phenomena of granite, gneiss and veins, and recorded the light emission in the laboratory [Воларович and Пархоменко, 1955, Yang et al., 2017]. It will produce electromagnetic waves with the fracture of quartz and other piezoelectric materials, The results have been reported by many researchers [Nitsan, 1977, Kong et al., 2016, Masashi et al., 1999, Eftaxias et al., 2007, Hayakawa et al., 2004, Potirakis et al., 2012, Li et al., 1989]. Guo [1997] studied the acoustic emission and EMR characteristics of rock under uniaxial loading and shear conditions. Ohtsuki and Kamogawa [1997] proposed the plasma-induced decay model to explain the EMR mechanism generated in the earthquake center. Kamogawa and Ohtsuki [1997] proposed the dipole mapping theory of electromagnetic waves generated by strong earthquakes. It is believed that the increasing of the high stress, the external electrons would be motivated, the instantaneous dipole moment occurs, then the electromagnetic waves would be radiated. Brady and Rowell [1986] experimentally studied on the electromechanical effect of rock fracture. Through analyzing the spectrum and EMR phenomenon, it is concluded that the mechanism of rock radiation is caused by the high velocity movement and collision of external electrons. Guo et al. [1989] found that some types of rocks may emits the electron while the external force reaches a certain threshold, the number of electrons increases rapidly while the rock ruptures and the electronic energy level distribution is wide. Qian et al. [1998] applied the biaxial pressure to the rock to produce shear rupture and sliding friction, and recorded EMR and AE signals in the meantime. Results show that the signals received in different azimuth and different frequency antennas are not synchronized, and the signal amplitude is also different. The signal amplitude is the largest in the crack. The frequency of the electrical signal and the magnetic signal are not synchronized, and the times of electrical signal and the amplitude are large.
With continuous development of EMR technology in the earthquake, rock rupture and other aspects, the EMR features of coal have gradually attracted the attention in the world. Хамиащвили [1989] measured the EMR spectrum of the mine caused by blasting and the mine collapse during the coal mining. Poturayev et al. [1989] studied on the EMR and AE characteristics of coal, clay, sandstone, granite, quartzite and limestone. Results show that using AE and EMR could monitor the stress state of coal seam. Afanasenko and Shvedov [1991] measured the natural and industrial electromagnetic fields during the salt mining, and found that the EMR generated from the salt layer could be used to evaluate the rockburst. Frid, 1997a, Frid, 1997b did more than 400 coal tests under different outburst hazardous conditions, revealed the relationship between EMR intensity and coal rock outburst dangerousness, It is possible to predict the coal and gas outburst based on EMR method. Cress et al. [1987] carried out uniaxial compression experiments on different types of hard rock, such as granite, basalt and marble, and found that the basalt radiation without quartz emits both light and low frequency electrical signals, which indicates that the piezoelectric effect of quartz contributes little to the total EMR intensity. Yamada et al. [1989] obtained about 10% to 20% of the total EMR events due to the granite constant strain loading, and put forward that the EMR generated of the tensile cracks is more effective than the shear cracks. Afanasenko and Shvedov [1991] found while the roadway rock water content increases, the rock masses strength would decreases, the plastic zone boundary transfers to the deep rock, the plastic zone as a barrier could absorb EMR, then the EMR intensity decreases. He et al., 1999, He et al., 2012 researched on the EMR characteristics and laws of coal rock deeply, and developed KBD5 and KBD7 coal and rock dynamic disaster non-contact EMR monitor, which have been successfully applied in the field [Wang et al., 2012, Wang et al., 2014, Kong et al., 2016]. Dou et al. [2001]; Dou and He [2005] proposed the abrupt change model of rockburst and the acoustic and electrical criterion of rockburst, and applied in the prediction of rockburst successfully [Padhy and Panda, 2017].
To sum up, many scholars have done a lot of research on the characteristics and mechanism of EMR in the process of coal rock deformation and rupture. However, the EMR signals of different coal rock are different. On the one hand, they are affected by external factors, such as loading method, experimental conditions, et al. On the other hand, due to the influence of coal rock attributes, e.g. physical and mechanical properties of coal rock. It shows that the EMR of coal rock is not only closely related to the deformation and rupture of coal rock, but also closely related to the physical and mechanical properties of coal rock mass. If we could obtain the influence of coal rock physical properties such as structure, composition, strength and electrical properties, etc. on the EMR features? Until now, there is few studies in this field. The existence of these problems makes the EMR research have limitations in basic theory and application.
One of the key problems should be solved urgently about EMR is revealing the EMR laws of coal rock with different physical and mechanical properties, establishing the correlation between EMR and coal rock properties, and improving the EMR mechanism of coal rock. In the paper, we studied on the EMR characteristics of different types of coal rock (outburst coal, rockburst coal and rock) under uniaxial loading, and discussed the microstructure, strength, components and other major factors of coal rock influenced on the EMR characteristics. These studies have very important theoretical and practical value for further exploring on the mechanism of coal rock deformation and rupture, using EMR technology to assess the physical and mechanical state of coal and rock, and accurately monitoring the coal and rock dynamic disasters.
Section snippets
Experimental system
The schematic diagram of the experimental system is shown in Fig. 1, which is mainly composed of loading system and data acquisition system.
EMR characteristics of outburst coal
The outburst coal sample taken from YMZ mine, the sample joint is very developed, structure loosed, it shows the obvious anisotropy and heterogeneity. Therefore, at the loading beginning, there is propagation of cracks and generation of new cracks, so that EMR signals are multiple peak area, as shown in Fig. 5.
The intensity of YMZ coal is lower, there is a slight decrease after the first peak 4.9 kN at 189 s. After the plastic hardening process, Load reaches the peak 7.4 kN at 280 s, and then the
Discussion
Different coal rock corresponding to the different EMR, the mainly reason for this is due to the different composition and structure of samples. In the section, we study on the EMR mechanism from micro morphology, strength and composition of coal rock.
Conclusion
In the paper, we studied on the EMR characteristics different coal samples and in the uniaxial loading, and discussed several typical microscopic mechanism influences the EMR characteristics, such as strength, components and microstructures of the samples, the main conclusions are as follows:
- (1)
The coal rock would produce EMR in the loading process, the macroscopic structure of outburst coal is soft, the EMR increases slowly and the EMR peak is smaller. The rockburst coal has the strong
Acknowledgements
This work is supported by the State Key Research Development Program of China (Grant no. 2016YFC0801404), National Natural Science Foundation of China (51674254, 51504097), National Science and Technology Major Project of China (2016ZX05045005), A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). We thank anonymous reviewers for their comments and suggestions to improve the manuscripts.
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