Magnetic And Electrical Signature Of Coal Fires And Comprehensive Detection Methodology

Coal fires are a global catastrophe and occur in all major coal producing countries.China is the largest producer and consumer of coal in the world.Meanwhile,China suffers the most serious coal fires in the world,especially in Xinjiang Uygur Autonomous Region,Inner Mongolia Autonomous Region,Shanxi Province and Ningxia Province.The main characteristics of coal fires are high temperature,large areas and long duration.Coal fires will not only directly burn out non-renewable coal resources,which is a great loss for the state economy,they also threaten the ecological environment as well as the health of the local residents and the safety of their properties.Coal fires have become a major issue threatening the security of state energy strategy and harmonious development of ecological environment.Related research and engineering implementation are imminent.The ultimate goal of coal fire research is to extinguish the burning coal seam.The detection of coal fires is of paramount importance prior to develop any strategies to control or extinguish them,which will improve the effectiveness of fire fighting engineering such as water injection,grouting,excavation and surface sealing.Thus,the detection of coal fires is the emphasis in the scope of coal fires research.However,coal fires are a complicated problem involving interdisciplinary fields such as geology,mine safety,geophysics,ecological environment and engineering thermophysics.In addition,their origin and development history is unclear,the combustion system is unknown and the influencing factors are diverse.The accurate detection of coal fires has always been a worldwide problem.This Dissertation focuses on the geophysical investigation of coal fires.A comprehensive research method consists of theoretical analysis,experimental test,numerical simulation,sandbox experiment and field detection is adopted to research the mechanism of magnetic and electrical anomalies of coal fires,the characteristics of magnetic and electrical anomalies in different stages of coal fire development,and the multiple geophysical techniques and data fusion methods in the detection of coal fires.The main results and conclusions are as follows:Based on the geomagnetism theory and the experimental test,the mechanism of magnetic anomaly caused by coal fires is expounded.The contribution of rock magnetic susceptibility,thermoremanent magnetization and the transformation of iron mineral composition to magnetic anomaly under high temperature is comprehensively analyzed.Geomagnetic models of coal fires are established by means of numerical method and forward modeling is carried out.The characteristics of magnetic anomaly response in different stages of coal fires are analyzed.The variation law of the magnetic anomaly of the coal fires with differnent temperature,depth and burning duration is obtained.Magnetic investigation is carried out in the 5th fire area of the Heshituoluogai State Key Coal Fire in Xinjiang.The magnetic data are subjected to a variety of data filtering techniques(such as diurnal fluctuation,pole reduction and upward continuation),which successfully eliminate the near-surface effect.The fire areas are clearly distinguished in the north and south part,and their distribution coincides with the result from surface survey.The drawback that magnetic method is not sensitive to high temperature fire area is pointed out.Meanwhile,the remedy,i.e.,joint interpretation,is also proposed.Combined with the actual geological of coal fire area and geophysical theory,the origin of self-potential anomalies dominated by thermoelectric potential is expounded.The experiment system of self-potential anomaly in coal fire area is designed.The generation process of the thermoelectric potential and redox potential is physically simulated.The source and influencing factors of the self-potential signal in the coal fire area are clarified.Self-potential investigation is carried out in the 5th fire area of the Heshituoluogai State Key Coal Fire in Xinjiang.The self-potential data is optimized using upward continuation.The three self-potential anomaly areas are delineated which distribute within the scope determined by surface survey.Meanwhile,the defect that self-potential method can not completely reflect the distribution of coal fires either is also indicated.The comprehensive index---F is proposed to characterize the fire area based on the magnetic anomaly and the self-potential anomaly data fusion is proposed.This index overcomes the defects of both magnetic method and self-potential method when they are used to detect coal fires alone.Based on the petrophysics,the change law of rock resistivity with temperature and its influencing factors were analyzed through experiments.The change of rock resistivity under high temperature is the result of the coupling effect of the increase of charge carrier concentration and the development of thermal fracturing.The geoelectric models of coal fires are established by means of numerical method and the forward simulation and inversion are carried out.The abnormal resistivity characteristics of coal fires at different stages of development are obtained.The abnormal resistivity shape of coal fires and its dynamic development process are mastered.Electrical resistivity tomography(ERT)is conducted in the Anjialing Open Pit Mine in Shanxi Province.Low resistivity anomalies related to coal fires are clearly observed in the resistivity profiles.The results validate the effectiveness of ERT method to detect coal fires.The mechanism of the formation of induced polarization anomaly caused by coal fires is illuminated.coal combustion can lead to greater induced polarization anomaly because the pyrolysis of coal results in a higher carbon content,and thereby enhances its phenomenon.The enhancement of the electron polarization ability and the charge carrier exchange capacity of coal due to high temperature is believed to be the reason.The sandbox experiment regarding the induced polarization signature of coal fires is designed and implemented.Coal fires cause a stronger induced polarization signal than the raw coal itself.The secondary voltage data are inversed iteratively by selfdeveloped inversion program.After integrating and standardizing all the inverted results,the actual geometric distribution of coal fires is obtained.Chargeability,resistivity and self-potential data are collected in the Lewis Coal Fire in the United States.High chargeability anomaly and low resistivity anomaly are observed at the position where is verified by the surface survey,indicating that the induced polarization method can be used for coal fire detection.The inverted chargeability and resistivity data are converged to get a standard burning front index(IB).The distribution of this index eliminates other disturbances in the resistivity and chargeability tomograms.It also coincides with surface temperature data and self-potential data.The results of this dissertation provide theoretical support for correctly understanding the mechanism of magnetic and electrical anomalies of coal fires as well as the magnetic and electrical characteristics at different stages.The proposed fusion method of multiple geophysical data is of great significance for the accurate location of coal fires as well as the effective fire fighting engineering.This dissertation contains 62 figures,11 tables and 172 references.