The premise and foundation of electromagnetic radiation is the separation of charges, that is, it shows positive and negative charges in macro, and the process of final recovery of this separated charge makes coal and rock mass show electrical neutrality in macro, that is, electromagnetic radiation. The questions to be answered by electromagnetic radiation mechanism are: ① How to separate charges; (2) How the separated charges disappear and produce electromagnetic radiation. Therefore, it is necessary to study the electromagnetic radiation mechanism in the process of deformation and fracture of coal and rock under load from these two aspects.
Through the experimental study of electromagnetic radiation in the process of rock fracture, Xu Weimin and others think that it is appropriate to explain the mechanism of electromagnetic radiation by piezoelectric effect of rock. However, some experiments show that rocks with and without piezoelectric materials will produce electromagnetic radiation. For example, Cress, Sun, Zhu and Wang Chilun think that piezoelectric is not the real mechanism of electromagnetic radiation or has little influence. According to the previous research results, although the piezoelectric effect is not the only cause of electromagnetic radiation, it is indeed one of the reasons. When the piezoelectric body exists, it will produce electromagnetic radiation. The piezoelectric effect of piezoelectric body can be qualitatively explained as follows. When there is no external force, the positive and negative charges in the piezoelectric body overlap, the total electric moment of the crystal is equal to zero, and the surface of the crystal is uncharged. However, when the mechanical force acts on the crystal along the direction, the positive and negative centers of the crystal will not overlap due to deformation, and the crystal torque is no longer zero, so that charges can appear on some corresponding surfaces of the crystal. The amount of electricity generated by a piezoelectric body depends on the stress. Through the analysis of coal ash, it is found that there are more or less silica minerals with piezoelectric effect in coal. This shows that the electromagnetic radiation carried by coal also contributes to piezoelectric effect.
During the loading process of coal and rock, because its composition is very complex and uneven, substances with different dielectric constants rub against each other and become charged, and a dipole layer will be formed at the interface between the two substances, that is, a space charge layer will be formed on the surface. The essence of triboelectrification is charge transfer, which can be explained by the work function of condensed matter. When triboelectrification or the charge generated by dipole layer accumulates to a certain extent, it will also generate electrostatic discharge and emit visible light, which is also a reason for coal and rock to emit light under load.
In addition, triboelectrification is one of the important mechanisms of crack formation and propagation to form new surfaces and generate charges. When the crack is separated, the charges on both sides of the dipole layer will not completely disappear, thus forming charge separation. According to the above-mentioned deformation and failure process of coal, in the initial stage of loading, the friction electrification between coal and rock particles accounts for a large proportion, that is, the closure of fracture surface accounts for a large proportion of mutual friction. In the process of shear failure of coal and rock, the influence of shear angle on electromagnetic radiation is different. The smaller the angle, the higher the friction ratio, and the resulting electrical phenomenon accompanies the whole shear failure process. Triboelectrification plays an important role in the electromagnetic radiation of coal and rock.
To sum up, the deformation and fracture process of coal and rock mass under load provides a charge source for electromagnetic radiation through piezoelectric effect and friction. The microscopic root causes of deformation and fracture charge of coal and rock mass under load are:
1) The extranuclear electron clouds among the atoms that make up the coal and rock mass overlap to form various chemical bonds (including ionic bonds, valence bonds, etc. ). Therefore, the essential reason for the charge generated by the loaded rock is the change of basic groups, in which electrons with positive molecular orbital energy level become free electrons, coal with a certain limit particle size, hanging bonds at the edge of free electric particles (limit particle size) produced by the distortion of electron clouds, and electrons formed by intergranular fracture when the crack speed is high.
2) When coal and rock are locally heated or have extremely high electric field due to friction, electrons will be generated. The extremely high electric field will increase the energy of bound electrons and turn them into free electrons due to the tunneling effect.
3) The coal and rock mass containing piezoelectric body generates free charge through piezoelectric effect.
4) During the loading process of coal and rock materials, coal and rock particles, mineral particles and cement are dislocated, and cracks will slip and rub due to closure, thus generating free charges. Triboelectric electromagnetic radiation plays an important role.
5) Coal and rock mass generate free charge through stepanov effect under the action of force, and the movement of cracks propagates along or across grains, making the crack front become a charged dangling bond, and local stress concentration will increase the kinetic energy of bound electrons and escape to become free electrons.
6) Under the action of non-equilibrium stress, surface defects (grain boundaries), line defects and point defects (vacancies and interstitial atoms) in coal and rock mass produce charge separation phenomenon.
7) Pore fluid (water, electrolyte solution, gas, etc. ) Electrokinetic phenomenon and electric field are generated on the pore surface of coal and rock through electrokinetic effect.
8) Heterogeneity and unbalanced stress of coal and rock materials make coal and rock bear different loads, causing activities such as charge migration and diffusion.
1.3.2 electromagnetic radiation mechanism of coal and rock deformation and fracture
Electromagnetic anomalies before (1) earthquake [65 ~ 80]
The study of electromagnetic anomalies before earthquakes was discovered earlier in the Soviet Union. Such as 1948 Ashbalde earthquake in Central Asia in the Soviet Union and 1966 Tashkent earthquake, light phenomena were found, and electromagnetic anomalies such as atmospheric potential changes were recorded. Mikhalkov and others recorded the disturbance of natural electric field 3-4 hours before the 1924 earthquake. The critical frequency variation of ionosphere over the 1977 Tawak M _ S5.2 earthquake was measured. 1978, m.a. Sadofsky measured the abnormal distortion of natural electromagnetic radiation with instruments.
The research on electromagnetic precursors of earthquakes in China made rapid progress in the late 1970s, and most of them were conducted in local seismological institutions and scientific research units. Although various methods have been adopted, the results are similar to the understanding, and more observation data have been accumulated at present. It has been found out that before the 1966 Xingtai earthquake of magnitude 7.2 in Hebei, the 1969 Bohai earthquake of magnitude 7.4, and the 1976 Tangshan earthquake, electrical equipment such as radios, alarm clocks, and teletypewriters were suddenly severely disturbed. Some monitoring radars, radio telescopes, satellite ground stations and seismic stations all recorded the changes of abnormal electromagnetic phenomena before the earthquake, such as geoelectric field and spontaneous potential. In addition, electromagnetic radiation can also be observed in industrial blasting, nuclear explosion and coal mine roof collapse.
Early research focused on the observation and analysis of photoelectric phenomena in the field when earthquakes occurred through instruments; In the future research, the mechanism of electromagnetic radiation before the earthquake and its influencing factors are studied by experimental means in order to achieve a theoretical breakthrough. The theoretical study of earthquake electromagnetic radiation mainly focuses on two major issues: first, the formation and radiation mechanism of various electromagnetic radiation sources in the process of earthquake preparation, and exploring various possible electromechanical conversion effects in the process of earthquake preparation; The theoretical estimation results show that the radiation spectrum of the source is very wide, which may be 0. 1 Hz ~ 1000 kHz. The second is the spatial coupling and propagation mechanism between wave field and surrounding medium.
Some gratifying achievements have also been made in the study of electromagnetic radiation mechanism and earthquake prediction before earthquakes. Xiong Hao believes that the electromechanical conversion mechanism in the process of earthquake preparation includes: triboelectric effect, piezoelectric effect, stepanov effect and electrokinetic effect. A possible physical mechanism of electromagnetic radiation before earthquakes is electromechanical conversion effect, which has been confirmed in the experimental study of electromagnetic radiation from rock fracture [56]; One is generated by charge accumulation and discharge. According to the characteristics of rock fracture in the process of earthquake preparation and occurrence, Dong Jiping and others put forward a finite moving source model of electromagnetic radiation, and derived the formulas for calculating the vector magnetic potential of rectangular and disc-shaped fractures in homogeneous media. Through the discussion of some physical parameters and related problems in the formula, it is shown that the radiation of electromagnetic excitation sources with different shapes and different rupture modes is an important reason for the uncertainty of seismic electromagnetic observation.
At present, it is generally believed that the short-term and imminent earthquake prediction can be made effectively by using the electromagnetic radiation phenomenon before the earthquake.
(2) Experimental research progress on electromagnetic radiation of coal and rock deformation and fracture [8 1 ~ 98]
Because the study of various physical effects in the process of rock fracture is not only of great value to understand the mechanism of solid fracture, but also of practical significance to understand the related phenomena during earthquakes, in order to better understand the mechanism of earthquake electromagnetic radiation and the prediction of other geo-dynamic disasters, people have carried out a series of laboratory studies on electromagnetic radiation of rock fracture. In recent ten years, the experimental research on electromagnetic anomalies and electromagnetic radiation of coal and rock deformation and fracture before earthquakes has made rapid development.
Guo et al. [8 1 ~ 82] recorded electron emission for the first time in granite uniaxial compression fracture experiment with G-M counter and plastic scintillator detector, and the electron energy was 0.05 MeV and 0.35 MeV respectively. According to the research results, the visible light and other electromagnetic radiation in rock cracks are explained theoretically. Qian Shuqing and others [83] also used uniaxial compression until the rock broke, and placed antennas with different frequency bands in different directions at a distance of 2 m from the rock sample. The electromagnetic radiation signal of the whole process of rock sample deformation and fracture was recorded synchronously with 14 tape drive. The experimental results show that: ① four frequency bands (VLF: 20 ~10 khz; IF: 530 kHz, 650 kHz, 1.65 MHz, 2.3 MHz; High frequency: 5 MHz, 16 MHz; VHF: 95 MHz), but the time of different frequencies is sometimes out of sync. Generally, the ultra-low frequency VLF electromagnetic radiation signal appears first, and it takes about 200 ms for other frequency bands to appear. ② The electromagnetic radiation intensity is the largest at the moment of main rupture, but the signal intensity in different directions is different. This also shows that, on the one hand, the frequency of electromagnetic radiation produced by rocks in the process of fracture is not single, and its spectrum is very wide; On the other hand, due to the inhomogeneity of the medium, the spatial and temporal distribution of electromagnetic radiation intensity in the propagation process is also uneven. Qian Shuqing et al. [84] also made experimental research on electromagnetic radiation signals and acoustic emission signals of rock samples subjected to shear fracture and sliding friction under biaxial pressure. It is found that the signals received by different antennas of rock samples are not synchronous and have different amplitudes, and the antenna located at the crack has the largest amplitude. Therefore, it is considered that the electrical signal is caused by piezoelectric effect and electrostatic charge on the new surface of the crack, while the magnetic signal is caused by the ionization of surrounding gas excited by electrons emitted when the rock is broken. Qian Shuqing [85] measured the electromagnetic signals produced by concrete specimens in the process of compression fracture, and thought that electromagnetic radiation signals were produced when the specimens were destroyed to 80%. Wang Chilun et al. [57] used BTI600A superconducting quantum interference quadratic gradiometer (accuracy can reach 10 ~ 14t), PARC4400 signal processing system and modified unpowered rock crusher to observe the electromagnetic radiation during uniaxial compression and fracture of feldspathic sandstone and quartzite, and recorded the electromagnetic pulse and frequency spectrum, and obtained the signal magnetic field intensity of 60. Wu Xiaoping et al. [86] made an experimental study on compressive electrification of granite. Cao Huixin et al. [87] made an experimental study on the electromagnetic signal and acoustic signal generated when gabbro, marble and other rocks were fractured under uniaxial pressure. The main results are as follows: ① The radiation intensity of electromagnetic signal in the process of rock fracture is related to the lithology, water content, fracture degree, loading rate and fracture state of rock; ② Electromagnetic radiation signals are more developed and last longer than acoustic emission signals; ③ In the experiment, the main spectrum of electromagnetic signal with copper antenna as receiving sensor is below 6 kHz, the main spectrum of electromagnetic signal with inductance coil as sensor is around 10 kHz, and the main spectrum of acoustic emission is below 3 kHz.
American scholar Nitson [88] reported the research results of rock piezoelectric effect in the laboratory. The results show that the deformation and fracture of isochronous piezoelectric materials will produce electromagnetic radiation in radio frequency band. Stepanov, a scholar in the former Soviet Union, found that materials without piezoelectric materials will also generate charges when loaded, and the charges will be generated in the uneven deformation area on the crack surface of the sample.
Vostretsov et al. [89] used a numerical computer measurement system to measure the electromagnetic radiation signal during rock deformation. Cress et al. [59] think that there is electrostatic charge distribution on the surface of new debris when rock is broken, and the strong electric field generated by its own vibration and rotation and charge separation on the fracture surface is the main reason for generating low-frequency and high-frequency electromagnetic radiation. Kurlenya et al. [6 1] respectively studied the fracture stage based on electromagnetic radiation research, the joint detection of electromagnetic radiation and acoustic emission related to rock fracture, the emission spectrum change during rock fracture and the detection method of electromagnetic radiation signal during rock fracture. Afanasenko et al. [44] studied natural and industrial electromagnetic fields to predict the outburst danger in granite mining. Poturayev et al. [92] jointly detected the electromagnetic radiation and acoustic emission signals of coal, gravel, granite and marble samples during deformation and fracture under uniaxial compression. The results show that the local increase of the pulse number of electromagnetic radiation and acoustic emission signals is related to the critical strength of the sample.
Because the process of rock deformation and fracture is the accumulation of elastic strain energy and the release of various energy forms (including strain energy, electromagnetic radiation energy, heat energy, etc.). ), researchers generally use the method of comparative study with acoustic emission (AE) when studying electromagnetic radiation (EME). Robsman et al. [90] made an experimental study on the pulse number of electromagnetic radiation and acoustic emission produced by rock fracture, and established a general model of pulse number, amplitude and spectrum change. The experimental results of electromagnetic radiation and acoustic emission of concrete by Sobolev et al. [9 1] show that electromagnetic radiation and short acoustic emission occur at the same time, while long acoustic emission is not accompanied by electromagnetic radiation.
Perriemann [65] put forward and tested five mechanisms of electromagnetic radiation generated by sound waves through the medium from the microscopic point of view: ① electromagnetic radiation is caused by uneven embedding of surface charges on the cracks of ionic crystals; (2) The crack vibration of the capacitor produces electromagnetic radiation; ③ The vibration of charge dislocations in the crystal produces electromagnetic radiation; (4) The vibration of floating charges generated by trace metals generates electromagnetic radiation; ⑤ When the electrosphere is compressed and expanded, the capacity of the paleosphere changes, and radiation can generate electromagnetic radiation. Frid [93] has carried out a series of studies on this: the influence of stress state, gas content, water content and porosity of coal and rock mass on electromagnetic radiation produced by mining face has been studied; The electromagnetic radiation characteristics of different mines are studied. It is considered that coal rock and gas outburst can be predicted by monitoring the electromagnetic radiation produced by coal rock fracture. Frid [93] thinks that the electromagnetic radiation method can be used to study the water injection effect of coal seam with rock burst and coal explosion danger. Giannakopoulou, Vaia et al. [94] measured the low-frequency electromagnetic radiation signal of borehole in situ, and studied its dynamic deformation and failure characteristics. Poturayev et al. [92] conducted experimental research on coal, clay rock, sandstone, granite, quartzite and limestone by using electromagnetic radiation and acoustic emission methods, and recorded the electromagnetic radiation and acoustic emission during rock deformation and crack propagation. The results show that it is possible to monitor the stress state of coal seams prone to outburst in adjacent working faces by using the combined characteristics of electromagnetic radiation and acoustic emission.
In the aspect of coal and rock deformation and fracture, He Xueqiu and Liu Mingju proved for the first time in China that electromagnetic radiation is generated during coal deformation and fracture, and pore gas affects the generation of electromagnetic radiation; On this basis, the energy dissipation in the process of coal and gas outburst is analyzed, and the principle of predicting coal and gas outburst by electromagnetic radiation method is analyzed. Using borehole electromagnetic radiation receiving system to measure electromagnetic radiation in coal and rock in working face and evaluate outburst danger, electromagnetic radiation method is a non-contact dynamic phenomenon prediction method with broad prospects. He Xueqiu and Wang Enyuan studied the frequency spectrum and law of electromagnetic radiation of coal and rock mass containing gas, and found that the time series of electromagnetic radiation signals generated in the process of coal and rock mass fracture conforms to Hurst statistical law, indicating that the electromagnetic radiation signals gradually increase with the deformation and fracture process of coal and rock mass, which is of great significance to predict the dynamic phenomenon of coal and rock mass disasters. Dou and He Xueqiu's experiments and test results show that electromagnetic radiation will be produced in the process of deformation and failure of coal and rock mass. Before the impact failure of coal and rock, the electromagnetic radiation intensity is generally below a certain value; In the process of impact failure, the intensity of electromagnetic radiation suddenly increases, and according to this law, the danger of rock burst can be evaluated and predicted.
(3) Generation mechanism of electromagnetic radiation of coal and rock deformation and fracture [39,465,438+0,64,99,654,38+000]
The study of electromagnetic radiation in the process of deformation and fracture of coal and rock mass is closely related to earthquake research and rock fracture research.
Guo et al. [99] theoretically explained the electron emission mechanism of rock compression fracture by using the model of compressed atom in solid. It is considered that when rock is compressed, the kinetic energy of atoms increases sharply under the Pauli repulsion of adjacent atoms, and electrons will be ionized to form electron emission by overcoming the Coulomb attraction of the nucleus and the Pauli potential wall constraint of adjacent atoms. Zhu et al. put forward the hypothesis that the electromagnetic radiation of rock fracture is generated by the accelerated expansion of crack tip charge, and established the mathematical model of electromagnetic radiation. Guo et al. calculated the change of molecular orbital energy level caused by the change of molecular configuration during the fracture of granite rock under uniaxial pressure by using the full micro-overlap method (CNDO method) in quantum chemical calculation, and put forward a theoretical explanation of the fracture mechanism of silicon-containing oxygen tetrahedron rock.
In the aspect of electromagnetic radiation mechanism of loaded coal and rock mass, predecessors have done in-depth research and achieved gratifying results, and put forward many electromagnetic radiation mechanisms.
1)Nitson's piezoelectric effect mechanism, which holds that piezoelectric effect is the cause of electromagnetic radiation, but it does not conform to the experimental phenomenon that other rocks without piezoelectric materials also produce electromagnetic radiation;
2) the mechanism of electromechanical effect (including piezoelectric effect, stepanov effect, triboelectrification and the destruction and fracture of electric double layer) and electrokinetic effect that produce electromagnetic radiation;
3) The mechanism of electromagnetic radiation produced by the relaxation of dislocations of separated charges and moving charges on the crack wall;
4) Wang Enyuan thinks that one kind of electromagnetic radiation produced by deformation and fracture of coal and rock is Coulomb field produced by accumulated charges on the surface, and the other is electromagnetic radiation produced by variable-speed motion of charged particles, that is, pulse wave;
5) He Xueqiu and Liu Mingju believe that the stress-induced dipole transient, the variable-speed movement of the separated charge at the crack edge with the crack propagation and the relaxation of the separated charge at the crack wall produce electromagnetic radiation mechanism;
6) Nie Baisheng's stress-induced electric dipole transient, the variable-speed motion of separated charges caused by crack propagation and friction, the energy dissipation of RC circuit oscillating on the crack wall, the relaxation of separated charges, and the bremsstrahlung generated by the collision of high-speed particles with the crack wall.
(4) numerical research progress of electromagnetic radiation of coal and rock deformation and fracture [10 1 ~ 132]
Since acoustic emission technology was used to monitor and predict the stability of mines and tunnels in 1950s, it has been widely used in in-situ stress testing, rock mass stability monitoring, rock fracture mechanism, earthquake sequence characteristics and so on for decades [103 ~ 106]. However, its theoretical research is not yet mature, and it is one of the few disciplines whose theoretical research lags behind engineering practice. Therefore, many scholars at home and abroad have carried out a series of research, including experimental research, mechanism discussion and numerical simulation.
In the aspect of numerical simulation, Dou and He Xueqiu studied the influence of mechanical properties of coal and rock mass around roadway on stress distribution by numerical simulation. The results show that the bulk modulus, cohesion and internal friction angle of coal and rock around the roadway have great influence on the stress distribution. According to this result, the stress distribution can be changed by pressure relief blasting, and the risk of rockburst can be reduced or eliminated [165438] Fu et al. [1 16] Based on the basic principles of statistical meso-damage mechanics and rock acoustic emission, the basic idea and framework of numerical simulation of rock acoustic emission law are put forward, and several key problems in concrete implementation are expounded by taking the finite element method as an example. Tang Chunan and others use the newly developed RFPA2D system to analyze the failure process of materials. Taking the failure process analysis of heterogeneous brittle materials such as rocks and concrete as an example, this paper expounds the opportunities brought by numerical simulation method to the development of brittle material failure theory, and briefly describes the application prospect of RFPA2D in the study of coal seam movement, underground engineering stability, earthquake preparation mechanism and composite material failure. On the basis of damage mechanics theory and random strength theory, and assuming that the physical and mechanical properties of rock particles obey Weibull distribution, the finite element method is used to simulate acoustic emission. Numerical calculation shows that this numerical method can successfully simulate the acoustic emission phenomenon in the whole process of rock stress by computer. Ji et al. studied the acoustic emission mode and law of concrete damage.
Because the electromagnetic field theory is very mature, with the rapid development of computer technology, the numerical simulation of electromagnetic radiation propagation has achieved remarkable results. Sun Hongxing [130] Based on Maxwell's equation of electromagnetic wave propagation, the theory of attenuation characteristics of high-frequency pulse electromagnetic wave of geological radar in underground lossy media is introduced and analyzed. Dou Weiping et al. [13 1] realized the transformation from near field to far field by using the principle of love field equivalence, and developed the software system of finite difference time domain method based on MATLAB platform. Wang Changqing et al. [132] based on the classical Maxwell equation, simulated the propagation of plane electromagnetic waves excited by electric and magnetic step electromagnetic pulses in two-dimensional and three-dimensional space, and compared them with the examples in Harmuth. The results show that they are in good agreement under the limit conditions, which shows that the applicability of Maxwell equation is undeniable. Henderson and Zhao studied the electric field of oscillating electric dipole in layered media by numerical simulation. The results show that: ① under the typical layered medium structure, the electric field information generated by this signal source cannot be observed on the surface; ② When faults exist, the surface dipole electric field can be increased by 2 ~ 3 orders of magnitude. However, the numerical simulation of these electromagnetic fields is based on the known position, direction, intensity and frequency of electromagnetic radiation sources. However, the electromagnetic radiation in the process of coal and rock fracture is complex, and its radiation source, intensity and frequency are uncertain, so the numerical simulation research on it has not been reported.