| The risk of major rock mass disasters lies in the development and utilization of the entire underground space,and accurate early warning of rock mass structure instability can provide a basis for on-site measures to avoid disasters.Large-scale and regional monitoring and early warning technology with high degree of automation and less human interference is the development direction of underground engineering disaster monitoring and early warning.As a real-time,non-contact,and strong precursory geophysical monitoring method,electromagnetic radiation technology has played an important role in rock mass structural stability and stress state assessment,dynamic disaster monitoring and early warning.However,the current electromagnetic radiation technology can only be used for time series early warning of disasters,and cannot realize the localization of disaster areas,which restricts the development and application of this technology.In response to this issue,this paper focuses on the near-field low-frequency electromagnetic localization method of rock fractures.The research is carried out by means of experimental testing,theoretical analysis,and numerical simulation.The law of low-frequency electromagnetic radiation and the near-field vector characteristics of electromagnetic field generated from rock fracture are studied.The electromagnetic field source model of rock fracture is constructed.A near-field localization method for rock fractures based on electromagnetic vector characteristics is established and verified numerically and experimentally.The main results obtained in this paper are as follows:(1)The law of low frequency electromagnetic radiation induced by rock fracture is obtained.Under different loading methods such as tensile(Brazilian test),shear,and uniaxial compression,the dominant frequency of the electromagnetic signal generated during the main fracture is in the 1～10 kHz frequency band.As the crack length increases,the intensity of the electromagnetic signal increases,while the dominant frequency decreases.The main observations at the laboratory scale are transient and near-field electromagnetic signals induced by rock fracture.(2)The vector properties of near-field electromagnetic fields generated by rock fracture are revealed.A new electromagnetic vector sensor suitable for near-field electromagnetic monitoring of rock fracture is developed,and the fracture process of Brazilian test is monitored based on a centrosymmetric sensor array.The waveforms of the three-component signals monitored by the electromagnetic vector sensor are obviously different,but the dominant frequency is the same in the frequency domain,and the amplitude-frequency distribution is similar.The three-component signal can be regarded as the components of the electromagnetic field vector corresponding to the main rupture in three directions.The electromagnetic vector change process of a single measuring point is very complex,but in an extremely limited time,the direction trend in a single channel is relatively consistent.However,there is a certain difference between the vector composite signals of different measuring points.The vector electromagnetic field caused by the crack is distributed non-uniformly around the specimen.(3)The equivalent electromagnetic field source model of rock fracture is constructed.Based on the analysis of the oscillation process of the electric dipole on the crack surface,the electromagnetic signal generated by the rock fracture is regarded as the superposition of the electromagnetic effects generated by the forced and damped oscillations of multiple dipoles in different directions and frequencies.Numerical simulations confirm the equivalence of the vector synthesis of field sources and the synthesis of electromagnetic field components.Therefore,the field source is equivalent to a charge aggregate,and an equivalent electromagnetic field source model induced by random multiple cracks is established in the near-field range.(4)The extraction method of localization signal and the effective localization index are proposed.There is an obvious correlation between the electromagnetic signal generated by rock fracture and the stress drop.Therefore,based on the start and duration of the stress drop,a single waveform of the electromagnetic radiation event corresponding to the stress drop is extracted as the localization signal.And the validity is judged by the sample entropy difference between the signal and the noise.Based on the vector composite signal and frequency domain features,the peak index and peak matching index,as well as the dominant frequency amplitude index and the characteristic frequency index are extracted as electromagnetic localization indexes,respectively.The calculated direction angles are consistent with the theoretical and experimental results.(5)A near-field electromagnetic localization model for rock fracture is constructed.For any point in space,the equivalent electromagnetic field source must exist on the unique definite plane passing through the point and perpendicular to the direction of the magnetic induction intensity of the point.The magnetic induction intensity vector synthesized by several groups of electromagnetic sensors can determine the plane where the equivalent electromagnetic field source is located,and then the space region where the field source is located can be determined by intersecting the planes.(6)A near-field electromagnetic localization method for rock fracture is established,and numerical analysis and laboratory verification are carried out.According to the time-frequency characteristics of the signals used for localization,different index combinations can be used and substituted into the nonlinear positioning equation system.The system of equations is established based on the near-field electromagnetic localization model,which is converted into a nonlinear least squares problem and solved by an iterative method.The feasibility of the above method is verified by numerical analysis and controlled source experiments,and then the fracture localization in the process of rock Brazilian test and uniaxial compression is carried out,and the results verify the effectiveness of the above localization method.The above research results can lay a theoretical and experimental foundation for the electromagnetic monitoring and early warning of rock dynamic disasters.They help to improve the reliability of dynamic disaster monitoring and early warning and are of great significance for continuous innovation in the field of electromagnetic radiation monitoring. |
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