| The transient electromagnetic method is a kind of geophysical exploration methods with electrical signal, which is developed rapidly and used widely. Due to the transient electromagnetic method has an obvious respond on low resistance geological body, it is the preferred method of geophysical survey in the detection of coal mine water hazards, which has broad application prospects and the huge development potential.With the support of the coal resources development, coal goaf has become a disadvantage to affect the normal production and construction of human life. It is a necessary means in coal mine and people in mined-out area for production and construction, which used the transient electromagnetic method to detect the coal mine water-filling-goaf. However, according to the traditional experience, people have always think that the low resistance response characteristics on the water-filling-goaf is the evidence to discriminate the normal body and the abnormal body. Though the analysis of several common electric law theory about the transient electromagnetic method to detect the coal mine water-filling-goaf, the paper drawed the conclusion that contrary to traditional experience on the base of the transient electromagnetic Id inversion study, which has theoretic guidance significance in the transient electromagnetic method to detect the coal mine water-filling-goaf.First of all, on the basis of the molecular polarization dielectric theory, the paper analyzes the micro polarization characteristics of the water-filling-goaf and surrounding rock in the induction electromagnetic field, and argues that the inducing-voltage of the low resistance of mined-out area with water-filling is less than the high resistance of the surrounding rock, which is also the secondary voltage field of the transient electric magnetic field. Secondly, on the bases the induced polarization theory in the electrical method, the paper discussed the memberaneeffect of the water-filling-goaf, considered that water-filling-goaf is equivalent to a closed space if the coal-mining has not caused roof falls and associated fissure, and equivalent to a storage power but a big battery, so the water-filling-goaf appears low voltage than the surrounding rock in the electromagnetic field. It concludes that the apparent resistivity of the water-filling-goaf in curve chart has a relatively high resistance reaction.The paper also established Id forward modeling about the water-filling-goaf on the dipole source excitation condition, through the analysis of the transient electromagnetic response characteristic curve, argued that whether even half space or medium level of layered media the induced-voltage of the water-filling-goaf which is low resistance is less than the surrounding rock which is high resistance and the apparent resistivity of the water-filling-goaf is higher than the surrounding rock. At last, according to the example that the application of the transient electromagnetic method to detect a water-filling-goaf which is about the No.3coal seam of mined-out area in southeast of Shanxi province, through the analysis of the geological features of the No.3coal seam and the room and the characteristics of the pillar type the coal mining method, the paper explained that the apparent resistivity on the Id inversion curve and the apparent resistivity plane equivalence value maps which appears relatively high resistance feature is the evidence to enclose the scope of the area of the water-filling-goaf in the No.3coal seam.In conclusion, in detecting the mined-out area or the water-filling-goaf by transient electromagnetic method, we should combine the corresponding geological conditions with the coal mining methods, and come to a correct transient electromagnetic explain basis in a comprehensive analysis of the transient electromagnetic response characteristics to provide a reasonable judgment for guidance on safe production and construction in people’s work. |
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