Study On Characteristics And Generation Mechanism Of Low-Frequency Magnetic Signals In Coal Failure Under Static Load

A large number of casualties are caused when the rock burst or coal and gas outburst accidents occur.And the production of coal mine is seriously restricited by dynamic disasters.Now traditional methods such as borehole parameter methods are always used to monitor and forecast the dynamic disasters,while the traditional methods have the disadvantages of time-consuming,force-consuming,discontinuous and big human influence.Nowadays,Geophysical methods which can execute the continuous dynamic monitoring of the disasters are highly favored by miners such as:acoustic emission,microseismic,electrical signal,magnetic signal,et al.Among them,magnetic signal which can achieve the goal of no contact testing is one of the research hotspots.Many researchers have indicated the characteristics and influencing factors of magnetic signals during coal failure by laboratory test and field test and proposed the possible generation mechanism of the signals.However,neither large-scale application nor widely recognized equipment exists thus far.Then,it is necessary to further study the related basic theories of the magnetic signals.Low-frequency magnetic signals with the advantages of fewer interfering factors and longer propagation range are more conducive to improve the accuracy of monitoring and early warning technology for coal or rock dynamic disasters.Thus,low-frequency magnetic signals are the core of this study.Furthermore,the original stress equilibrium state is broken because of mining activities.In working face,load from the produced coal transfers to the coal in front of the working face.And then,stress concentration zone is formed.When the stress of coal in the stress concentration zone exceeds the shear strength,the compressive strength and the tensile strength,damage occurs within the coal body.If the internal friction of coal is not enough to prevent the liberation of gas expansion energy and elastic energy in the coal,coal and gas outburst will happen.Based on the above analysis,it is known that coal and gas outburst always happen in stress concentration zone.While the forming process of new stress equilibrium state and the load of coal in concentration zone are all slow.Process of coal fracturing in concentration zone is like a static load damage process.Thus,study of the characteristics of low-frequency magnetic signals emitted from coal during static load damage is significant to promote the development of the monitoring and early warning technique for dynamic disasters in coal mine.Taking many factors like a strong equipmental magnetic noise and a weak desired signal into consideration,self-made laboratory equipment with low magnetic noise is used to perform the study of coal failure under static load.Based on the previous studies,low magnetic noise experimental system was established and was used to study the low frequency magnetic signals from coal failure.Starting with the basic theory,characteristics and conditions of crack propagation,stress evolution rulse were theory analyzed.Then generating mechanism of the signals was propsed in theory.Combining with correlated characteristics between magnetic and micro-seismic signals,the low magnetic signal is divided into three phases and the generation mechanism of abnormal magnetic signals in these three phases is proposed.According to the experimental results characteristics of low-frequency magnetic signals emitted from shaped coal samples,raw coal samples and rock samples in time and frequency domin were analyzed.Influencing factors and their influence ways on magnetic signals were also analyzed.In order to prove the experimental results,combining with magnetic-mechanical coupling model,numerical simulation software RFPA^2D was used to study the characteristics of magnetic signals.Relations between magnetic pulses,magnetic energy and properties of bedding,quantity of bedding,loading speed,and intensity of the sample were also studies.Finally,taking NO.10coal mine as an example,characteristics and generation mechanism of low-frequency magnetic signals tested in coal mine were studied.In general,taking coal sample as the study object,.low-frequency magnetic signal as an aim,this paper studied the characteristics,influencing factors and generation mechanism of the signal.Many research methods were used such as:theoretical analysis,laboratory study,numerical simulation and field test.Various experimental methods support each other and verify each other.The research results are of great guiding significance in riching the correlation theory,improving the monitoring and early warning technology of dynamic disasters in coal mine.And the results are of great engineering significance in promoting the magnetic measurement technology in monitoring and warning dynamic disasters in coal mine.The main results of this paper are as follows:?1?Experimental study on generation mechanism,similarities and differences of low frequency magnetic signal emitted from raw coal and shaped coal sample under static load failureComparative analysis was performed between low-frequency magnetic signals and micro-seismic signals.Results show that:?1?Low-frequency magnetic signals and microseismic signals are synchronously generated when break failure occurs,however the main frequency band of magnetic signals and microseismic signals emitted form shaped coal sample is inconsistent and emitted form raw coal sample is consistent;?2?For the signals from same burst,Low-frequency magnetic signals occur earlier than the microseismic signals but end later than the microseismic signals and last longer;?3?The abnormal magnetic signal is divided into three phases.The generation mechanism of abnormal magnetic signals in these three phases is proposed,which are weak damage caused by compaction,macroscopic plastic failure,macroscopic brittle failure and damage caused by residual elastic energy release.Macroscopic plastic fracture is the main generation mechanism of magnetic signal from shaped coal sample,and macroscopic brittle fracture is the main generation mechanism of magnetic signal from raw coal sample.?2?Revealing the characteristics and abnormal strength of low-frequency magnetic signals emitted from shaped and raw coal samples in laboratory studyThe results show that:?1?Low-frequency magnetic signals of shaped coal is weak.the intensity of the signals is 9.5¹29.9 nT,and the main frequency band is0.1²0 Hz;?2?Low-frequency magnetic signals of raw coal is strong.the intensity of the signals is 11.63³15.7 nT,and the main frequency band is 0.1⁴0 Hz;?3?Low-frequency magnetic signals from coal with horizontal bedding is stronger than that from coal with vertical bedding,and is 1.17 times stronger than that from coal with vertical bedding;?4?Magnetic pulses and energy accumulated in the whole process of sample failure can be divided into four stages,which are zero stage,slow increase,rapid increase and smooth change,respectively;?3?Influencing factors of low-frequency magnetic signals emitted from coal samples and their correlation with the magnetic signalInfluencing factors of the signals were analyzed by experiment.Results show that:?1?For the shaped coal,the larger the granularity,the greater the probability of testing the signal.Molding pressure has positive correlation with the maximum amplitude and energy of magnetic signals;?2?The intensity of the sample and loading speed also have positive correlation with the maximum amplitude and energy of magnetic signals;?3?Magnetic field perpendicular to crack surface is strongest,while magnetic field paralleled to the crack surface is weakest.?4?Establishing the magnetic-mechanical coupling model and Numerical simulation on failure characteristics of coal sample and change characteristics of magnetic signalsBased on the magnetic-mechanical coupling model and RFPA^2D simulation software,the numerical simulation study of magnetic signals was perforemed.Results show that:?1?Coal samples without beddings are in the form of single slope shear failure.Samples with vertical beddings are in the form of splitting failure,and its cracks growth along the bedding plane.And a divergent crack can be seen in the coal sample with horizontal beddings;?2?The number of beddings is negatively correlated with the sample strength.For coal with horizontal beddings,the number of beddings shows a positive correlation with the deformation,while for coal with vertical beddings,the number of beddings has a negative correlation with the deformation;?3?The intensity of the sample and loading speed both show a linear positive correlation with magnetic pulses and energy;?4?Based on the curve of magnetic pulses and energy,the entire loading process can divided into four stages,and the change trend is“zero stage-slow increase-rapid increase-smooth change”.The simulation results are consistent with the experimental results.?5?Influence of bedding nature and bedding number on characteristics of magnetic signals emitted from coal under static load failureSimulation results show that:?1?Deformation of samples with horizontal beddings is bigger than that of samples with vertical beddings when the number of beddings is same;?2?Magnetic pulses and energy of coal samples with horizontal beddings is all bigger than those of samples with vertical beddings.This result is consistent with the experimental results;?3?Bedding numbers of sample with horizontal bedding has a linear positive correlation with accumulated pulses and enrgy,and increasing rate of accumulated energy is higher than that of the accumulated energy pulses;?4?Bedding numbers of samples with vertical bedding has a liner negative correlation with accumulated magnetic energy,while no obvious correlation can be see between magnetic pulses and bedding numbers.?6?The correction between microseismic and magnetic signals from field test and the generation mechanism of magnetic signalsField test results show that:?1?The magnetic field intensity is the strongest in the direction perpendicular to the coal wall of roadway,approximately between 80 and1500 nT.The magnetic field intensity in the direction point to the driving face is relatively weak,approximately 20 to 500 nT.The magnetic field strength in the vertical direction is 100 to 1000 nT.Abnormal magnetic signal waveform has long-time pulse characteristics.?2?The magnetic field signal and the microseismic signal are synchronously generated in the time domain,but the duration of the magnetic field signal is much greater than the duration of the microseismic signal.?3?The abnormal magnetic field in the field is the result of tensile and shear damage caused by the elastic waves generated by the blasting and the transverse tensile failure of the coal wall.