| The geological conditions of coal mines in China are complicated.With the continuous increase of the intensity and depth of coal mining,coal and rock dynamic disaster is becoming more and more serious.Electromagnetic radiation(EMR)of coal and rock fracture,as a non-contact,real-time,dynamic and continuous geophysical monitoring method,has been widely used in the field of monitoring and early-warning of coal and rock dynamic disaster in mines.However,most application cases of EMR method are the time-scale monitoring at present,lacking of spatial position information of hazardous area.Therefore,it is necessary to extend EMR technology from time-series parameter monitoring to spatial disaster localization,and achieve the dangerous area localization in the breeding stage of coal and rock dynamic disaster.This has great significance to the effectively improve the pertinence of taking danger-relief measures.and reduce the possibility of dynamic disaster.Based on this,this dissertation focused on the EMR localization of coal and rock fracture,and carried out comprehensive study on EMR from the three aspects,i.e.stress-acoustic-electric correlation of coal and rock failure,time-frequency characteristics of EMR waveform,and EMR directionality.Combined with the classical electromagnetic theory,the EMR localization model of coal and rock failure was constructed,and its feasibility and effectiveness were comprehensively verified by numerical analysis and laboratory experimental tests.Through systematic study,the following main conclusions can be draw:(1)The load-acoustic-electric full-waveform synchronous acquisition system of coal and rock fracture under loading was constructed,and the experimental study on the correlations between stress drop,acoustic emission(AE)and EMR of coal and rock fracture was carried out using this system.The results show that,EMR signal only occurs with stress drop and large intensity AE.EMR event number is much smaller than AE event number.Specifically,AE event numbers of coal and rock samples are 122.6 and 110.5 times of EMR event numbers,respectively.Compared with AE,EMR has better correlation with stress drop.Compared with the coal samples,the rock samples have higher correlations between AE,EMR and stress drop.Particularly,the correlation coefficients between EMR and AE,EMR and stress drop,and AE and stress drop of the rock samples are 0.697,0.889 and 0.782,respectively,while those of the coal samples are 0.538,0.889 and 0.529,respectively.(2)The time-frequency characteristics of EMR waveform of coal and rock fracture were studied by using empirical wavelet transform(EWT)and Hilbert transform(HT).The results show that,the different frequency components of the EMR signal of coal and rock fracture can be effectively decomposed by EWT method.Based on EWT,the low-,band-and high-pass orthogonal Meyer wavelet filters with narrow transition bands can be constructed arbitrarily.Using these filters,the phase-free distortion filtering can be achieved for EMR signal of coal and rock fracture.The frequency of the EMR signal of the main fracture of coal and rock splitting and uniaxial compression is relatively low.In detail,as for Fourier spectrum,EMR measured by three-axis electromagnetic antenna mainly concentrates in the frequency band lower than 0.7～3.5 kHz,the main-frequency is mainly distributed in 0.12～1.4 kHz.As for Hilbert spectrum,high amplitude frequency point of EMR signal is mainly distributed in 0.25～2.2 kHz.Therefore,the EMR signal belongs the very low frequency band and below.(3)The directionalities of three-axis electromagnetic antenna and EMR signal of coal and rock splitting fracture were experimentally studied.The results show that,the antenna has significant directivity.The measured signal intensity of antenna is proportional to the cosine of the angle between the directions of the antenna axis and the actual electromagnetic induction intensity.Thus,the vector synthesis condition is satisfied for the three axes(i.e.x,y,and z)of antenna.The direction of electromagnetic induction intensity of the electromagnetic field generated by coal and rock splitting fracture tends to be parallel to the splitting crack plane.(4)Theoretical analysis was conducted for the mechanism of coal and rock fracture under loading,charge separation mechanism,and EMR mechanism.The results show that,the charge separation in the process of coal and rock fracture is the result of a combination of various mechanisms.The crack propagation directly or indirectly leads to a large proportion of charge separation,and the separation charge mainly exists with form of electric dipole(electrical layer).EMR of coal and rock fracture is the superposition result of various EMR mechanisms.The separation charge motion is the main direct cause of EMR of coal and rock fracture.As for the actual EMR monitoring of coal and rock fracture in the laboratory scale and the coal mine site,the observation distances are all much smaller than the EMR wavelength,and the observation points are located in the induction field of the EMR generated by the separated charges.(5)Numerical analysis was conducted for the electromagnetic fields of the spatial single and multiple moving charges.The results show that,as for the spatial multiple low-speed moving charges,when the observation distance is much larger than the distances between charges,these moving charges can be regarded as an equivalent source.The vector form of this equivalent source is an algebraic addition of product of each moving charge and its velocity.As for the generated magnetic field,this equivalent source is equivalent to the multiple moving charges,and the direction of electromagnetic induction intensity is perpendicular to the vector of this equivalent source.In the actual monitoring process of EMR of coal and rock fracture,an EMR event can be equivalent to be emitted by a time-varying equivalent source,and the EMR field source model of coal and rock fracture can be summarized as a model of EMR induced field based on the moving charge.(6)Based on the distribution characteristics of EMR induction field induced by moving charge,the EMR localization model for coal and rock fracture is constructed from the aspect of electromagnetic induction intensity.The EMR localization system was built,and based on MATLAB software,the EMR localization software was designed and compiled.The feasibility and effectiveness of the EMR localization model were comprehensively verified from numerical analysis and laboratory experimental tests.The localization results are ideal,therefore,the EMR localization model constructed in this dissertation is feasible and effective.The study results of this dissertation have important theoretical and practical significance for further revealing the EMR mechanism of coal and rock fracture,as well as for the research and application of EMR localization of coal and rock fracture in the laboratory and mine site. |
PDF Full Text Request