| China's energy structure determines its dependence on coal resource,and along with the coal mine mining to the deep areas,the number of coal goaf area is increasing.And water inrush disasters occurred easily in the goaf and water logged zones,seriously affects people's work and lives.In order to avoid the frequent occurrence of coal mine accidents,it is of great significant to find a way that can detect the accurate position and the water rich property of this kind of complicated goaf.At present,the geophysical methods of exploring coal mine disaster water source mainly are the direct current resistivity method and the mine transient electromagnetic method.The mine transient electromagnetic method is widely used in the exploration of coal mine disaster water source,with the characteristics of convenient construction,high working efficiency and high sensitivity to low resistivity.Through transient electromagnetic physical simulation experiments and the forward numerical simulation of different geoelectric models of single layer wateraccumulated goaf,high resistance goaf and multi-layered goaf,the transient electromagnetic response and reflection characteristics of cross section of goaf under different conditions are analyzed.it can be known from the results of experiments and numerical simulations that for the single layer water-accumulated goaf in high resistance surrounding rock,the signal of transient electromagnetic method is obvious,while,for the high resistance single layer goaf that is filled with air,the signal is not sensitive.In addition,there are still some errors in the location of the mined out area.For multi-layered goaf,the thickness of the middle layer between the two coal seams is proportional to the induced electromotive force.And by numerical simulation,the transient electromagnetic characteristics of upper and lower coal seams were studied under conditions: coal seam with water and without water.The analysis of response result of multi-layer complex goaf shows that the upper and low resistance goaf has adverse effects on the resolution of the transient electromagnetic method and weakens the detection ability of the transient electromagnetic method.In order to overcome this disadvantage,it is necessary to improve the transient electromagnetic detection method to detect the geological anomaly behind the low resistivity shielding layer.An algorithm is proposed to eliminate the influence of the complex goaf on the transient electromagnetic precision in data processing.Given the shielding effect of the complex goaf in this paper,the method of improved damping least square method is used to fit the transient electromagnetic data.The contents of the study are as follows: Firstly,use the resistivity profile obtained by“smoke ring” image inversion as the initial model of damped least squares inversion,and then the measured data were transformed into the late apparent resistivity.The relative error of apparent resistivity is used as the convergence criterion,and the iterative equations are derived.Then complete the transient electromagnetic data fitting by setting upper and lower bound parameters.Adopting the proposed method for modifying damping factor to complete the theoretical data fitting of damped least squares under the condition that the convergence is good and the iteration parameter reaches the maximum step size,which can improve the computing speed and accuracy of inversion algorithm.Finally,the above forward and inversion theory study of the complex goaf combined with the complex goaf detection example proves that the above conclusions are correct and demonstrates the validity and correctness of the damped least square method,which can indicate that the transient electromagnetic method has a good effect on the detection of complex goaf. |
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