Inversion And Application Of The Self-potential Anomaly Caused By Coal Fires

Coal fires are an ecological disaster faced by all major coal-producing countries in the world,posing a serious threat to national energy security and sustainable economic development.China is the country with the most serious coal fires in the world,which seriously destroy the ecological environment,threaten the safe production of coal mines and cause huge economic losses.The detection of coal fires is of great significance to the efficiency and economy of fire control projects.However,due to the unclear formation and development of underground coal fires,the state of the combustion system,and numerous influencing factors,the accurate detection of coal fires has become a worldwide problem.In this paper,we focus on the self-potential method,a common geophysical method for detecting coal fires.A combined research method which includes theoretical analysis,sandboxing experiments and numerical simulation is adopted.The formation mechanism of self-potential anomaly in coal fire areas is analyzed theoretically.The sandbox experiment is also carried out to verify the source of the anomaly.According to the basic theory of geophysics,the constitutive equation of the correlation between the surface self-potential of the fire area and the source current density of the 3D underground space is constructed,and the 3D compact constrained inversion method of the source current density is proposed.The algorithm and the corresponding programming tools are used to develop the inversion code.The numerical calculation program is used to establish the forward and inversion model,and the reliability and accuracy of the 3D inversion algorithm are tested.The main results and conclusions reached are as follows:The sandbox experiment system is constructed.The 3D inversion code is run to achieve accurate targeted location of the fire source.The self-potential anomaly in coal fire areas mainly comes from two aspects: one is the thermal potential caused by the temperature gradient between the burning zone and the ground surface;the other is the redox potential caused by the combustion of coal(violent redox reaction).Among them,the thermal potential dominates and increases with the combustion center temperature.The redox potential amplitude is small,and it does not contribute much to the overall self-potential anomaly.A 3D inversion interpretation algorithm for self-potential data is proposed.The effectiveness of the inversion algorithm is verified by a numerical simulation program,which accurately restores the true position of the anomaly,indicating the proposed 3D inversion algorithm can effectively invert the source current density anomaly of different magnitudes.A sandbox experimental system with self-potential anomaly is constructed.The electrical heater is used to simulate the heat source.Five sets of experiments are carried out for different occurrence conditions(different depths and distribution directions).Both dipole and polar self-potential anomaly is obtained respectively.The inversion interpretation algorithm performs iterative compact constrained inversion on the selfpotential data collected in each experiment.A very compact source current density is obtained,and the inversion results well restore the true position of the heat source.The self-potential anomaly experiment and 3D inversion of real coal fire are carried out.The resistivity and self-potential data in the sandbox are collected.It is found that the self-potential anomaly in the fire zone is attenuated as the combustion process in the fire zone weakened.In the inversion process,the position of the source current density is consistent with the true position of the coal fire.As the coal fire is extinguished,the source current density is also attenuated.Comprehensive geophysical exploration was carried out in Xinjiang.The distribution of source current density in the fire zone was obtained by inverting the selfpotential data.The inverted source current density center is consistent with the magnetic,self-potential curve as well as the resistivity anomaly.The inverted result is also verified by the post-drilling temperature measurement data.The research results in this paper provide a theoretical support for the correct understanding of the causes of self-potential anomaly in coal fire areas.The proposed inversion method of self-potential anomaly has important practical significance for the precise positioning and efficient treatment of coal fires.This dissertation contains 32 figures,1 table and 130 references.