| The mine transient electromagnetic method(MTEM)plays an important role in water hazard detection in coal mines,but its basic theory research still lags behind the method research.Based on the mine multi-turn small loop source TEM method,the analytical formulas of half-space and whole-space TEM field were derived.In addition,the FE equations of TEM field of the mine multi-turn small loops source were derived.The model used the tetrahedral mesh strategy and the vector basis functions of the lattice edges were derived.The Galerkin method was used to solve the governing equation of the FE method.Combined with MUMPS parallel solver,LU decomposition of the coefficient matrix of the FE equation was realized.The laws of TEM response in half-space and whole-space were analyzed.For surface TEM detection,the existence of water conductive fractures in mine would make TEM response decays slower than that of only aquifers.Single detection has low accuracy in identifying the wide distributed overlying aquifers,gob water,and water conductive fractures.TEM detection at different locations is needed in order to determine the whole area of water distribution.The "geometric gravity center"(GGC)effect of TEM response was proposed.The intensity of the TEM response of the anomalous body at the beginning depends on the distance between the anomalous body and the multi-turn small loop source.Moreover,the concept of "electromagnetic field projected area"(EMFPA)corresponding to the TEM field was proposed.The attenuation law of induced electromagnetic field of the anomalous body mainly depends on its electrical property,size,and EMFPA.The influence of roadway on the TEM field is very small,and the influence of metal equipment in the roadway on the TEM field mainly comes from the excavator.In addition,the field superposition characteristics of the TEM fields was used to subtract the total field response from the response of the excavator.Based on the forward simulation results,coal mine surface TEM detection and mine roadway advanced detection were carried out and ensured the mining safety. |
