| Mine water disaster is one of the main disasters in the process of coal mine production and construction.Mine transient electromagnetic method(TEM)is a kind of good geophysical method which is widely used to detect coal mine hidden water.The basic theory of mine TEM is lagging behind,especially the distribution law of whole-space transient electromagnetic field and the whole-space TEM response law of typical water-rich area.In this study,the three-dimensional finite-difference time-domain(FDTD)method based on Yee grid is used to study the diffusion law of whole-space transient electromagnetic field and mine TEM responses of water-rich area in coal mine,and the results are used to detect water-rich area in coal mine.The main research contents and achievements are as follows:(1)In this study,aiming at the multi-directional multi-angle detection problem,the method of replacing the actual transmitting loop with multiple sets of transmitting coils is used,to achieve the loading of any direction in the right-angled grid.The electromagnetic field equations and the treatment method of the underground roadway are studied,and the effect of the treatment is verified.For the boundary effect problem,the convolutional perfectly matched layer absorbing boundary(CPML)is used to replace the traditional Dirichlet boundary condition,which improves the precision of the late solution.The effect of the algorithm is verified by using the homogeneous whole space and the layered whole space model.The results show that the FDTD method used in this study has high reliability and precision.(2)The influence of the length of transmitting loop on the whole-space TEM responses is studied,and the distribution law of the whole-space transient electromagnetic field excited by small loop is studied.The distribution law of whole-space transient electromagnetic field under the vertical excitation,horizontal excitation and excitation direction of 45° with interface is studied by using layered medium model.Meanwhile,the influence of roadway on distinguishing anomalous body of different azimuths in the whole space is studied.The results show that the smaller the length of transmitting loop,the closer the apparent resistivity of TEM responses is to the model resistivity.The distribution of the transient electromagnetic field in the whole space is different from that of the ground half space,and the distribution of transient electromagnetic field in the whole space is also affected by the excitation angle,besides by the electrical parameters of the formation.For the low-resistivity body located in one side of the roadway,due to the effect of the whole space,a low resistivity anomaly will be generated when the transmitting loop is located on the other side of the roadway,but the response is weaker.The results provide a reference for the identification of the anomalous body's location of different lateral sides.(3)Based on stratigraphic data related to coal measures located at North and Mid-eastern China,the three-dimensional whole-space geophysical model of typical water-rich area,including water-bearing fault,water-bearing collapse column and goaf water,is established.Combined with the actual excitation and observation mode,the contour distribution law of the whole-space TEM apparent resistivity of the water-rich area,including water-bearing fault,water-bearing collapse column and goaf water is studied.The results show that,the low resistivity anomaly areas of the apparent resistivity contours caused by the water-bearing faults are consistent with the location and morphology of the faults,and the distribution of apparent resistivity can reflect the change of fault dip.The low resistivity anomaly areas of the water-bearing collapse column in detecting direction towards floor is larger and the resistance is lower.The closer the collapse column roof is to the coal seam floor,the lower the apparent resistivity is.The low resistivity anomaly areas of the water-bearing collapse column in the driving face is approximately elliptical and the apparent resistivity response in detecting direction towards floor is low relatively obvious.The low resistivity anomaly areas of goaf water is basically consistent with its position.When the goaf water is located in one side of the roadway head,due to the effect of the whole space,a low resistivity anomaly area of a relatively small extent is generated on the other side,but the value of the contours is relatively high.(4)The whole-space TEM response law of the water-rich area in coal mine provides guidance for the processing and interpretation of the mine TEM detection data.In this study,the mine TEM is used to detect the hidden water-rich area in the underground coal mine.By comparing the actual detection results with the numerical simulation results,and combining with the integrated analysis of the actual mine hydrogeological data,the location and development range of the water-rich area are preliminarily determined.The results of the interpretation are validated by the late drilling data. |
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