Study On Response And Perspective Technology Of Transient Electromagnetic Field In Mine Water Bearing Structure

Mine transient electromagnetic method is one of the important means of investigation,and it is possible to construct a multi-method,multi-stage and up-and-down three-dimensional comprehensive early warning model for coal mine water inrush hazard prediction.The method has the advantages of high efficiency and sensitive to the aquifer structure.However,compared with the ground transient electromagnetic method,the method has not yet fully understood in theory.There are still many problems to be solved,such as calculating the apparent resistivity is too small,geoelectric anomaly space positioning and orientation detection problem,effective detection depth and time-depth conversion and other issues.In this paper,aiming at the problem of geodetic spatial location and orientation detection,the numerical simulation method is used to study the response characteristics of the geoelectric anomalies in the whole space.Based on orthogonal hexahedron division,the unconditional stable Du Fort-Frankel difference scheme of finite volume method(generalized difference method)is deduced from the electric field diffusion equation in the three-dimensional computational space.It avoids the problem of calculating the derivative of conductivity in the difference scheme constructed from the magnetic field diffusion equation.The analytic solution of the uniform whole-space magnetic dipole source transient electric field is used as the initial condition.Under the condition of large solution area,the non-uniform grid space discretization method will not take too much memory and time and does not bring too much truncation error when applying the first type boundary condition on the truncated boundary.Using C++ language programming,the efficiency of the program implementation has been improved.The maximum error between the numerical solution and analytic solution of the uniform whole-space model is not more than 11%.The spatiotemporal characteristics of the transient and electrical components of the whole space are studied by using the analytic method of the whole space transient electric field.The results show that the diffusion of the transient electromagnetic field in the whole space is obviously different from that of the half-space transient electromagnetic field,and the electric field component diffuses out like water wave.The numerical results show that the influence of the high resistivity coal seam on the diffusion of the transient electromagnetic field is very small,and the low resistivity coal seam has a great effect on the distortion of the transient electromagnetic field,which is not conducive to the transient electromagnetic method.The size of the anomalous body,the spatial form,the depth of the buried,the resistivity directly affect the response value in the receiving device.The influence of the geometric dimension of the anomaly on the response value of a single measuring point is non-linear,and the effect is gradually smaller after increasing to a certain scale.The depth of the low impedance anomaly has a great influence on the response value of the same device.And the eddy current field in the shallow low resistivity anomalous body is very strong,which is directly induced by the primary field shutdown,and is much larger than that of the eddy current in the earth.The variation of the spatial shape of the anomalous body can be reflected on the measuring line,and there is a certain difference between the response characteristics of the tilt anomaly body and the half space and the induced electromotive force on the anti-tilt side is greater than the other side.Two kinds of transient electromagnetic face detection methods both can reflect the front of the low resistivity anomalies.The short exploration line can be used to determine the direction of the anomaly according to the direction of the slope and fan observation is more accurate in locating the anomaly.In theory,the effect of fan observation is better.The results show that the transient electromagnetic method can effectively reflect the position of low resistivity anomalies and The new detection method is helpful to solve the problem of the difficulty of locating the abnormal space and the time depth conversion in the current construction mode.The three methods of the curve contrast method,the abnormal body response coefficient back projection imaging method and the geometric imaging method can explain the position of the abnormal body to a certain extent.The combination of the three methods can improve the accuracy of interpretation.It is necessary to further study the inversion interpretation of mine transient electromagnetic perspective.