Study On Transient Electromagnetic Detection Of Mined-out Area And Dam Leakage

Transient electromagnetic method,with its advantages of large depth of detection and high horizontal detection resolution,has achieved good results in the search for water-bearing geological structures.In this thesis,a simplified three-dimensional complex model of Mined-out Area is designed for the transient electromagnetic survey of complex formations in water-cut Mined-out Area.Aiming at the leakage electromagnetic transient detection of dams,a trapezoidal dam model with two leakage passages is designed.The characteristics of the transient electromagnetic response of the two models at the surface and the cross-section were analyzed using the large loop source and the rectangular small loop source,respectively.The vertical electrical resistivity of the model was used to analyze the vertical electrical characteristics of the model.In addition,inversion processing of measured data is performed to verify the practical application effect of the transient electromagnetic method.In response to the above problems,the following studies were mainly conducted in this thesis.Firstly,the three-dimensional forward modeling of ground exploration geological problem model is realized by non-uniformly split time-domain finite difference method,three-dimensional modeling of transient electromagnetic field response in Mined-out Area and seepage channel is realized,and response characteristics of common water-bearing structures are achieved.Analyze and complete the forward simulation program to simulate the transient electromagnetic response of a complex formation medium.Through analysis,it is found that the electromagnetic field contours are concentrated in the low-resistance medium,whether on the ground or the surface,and the low-resistance medium attenuates the magnetic field strength slowly at the projection position on the ground,and the high-impedance anomalies are exactly the opposite;the contours of the vertical component are distributed.The position of the anomalous body outlined in the contour map of the electromagnetic field component contour is changed in the position,the range,and the outline of the anomaly set by the model.Secondly,using the large loop source and rectangular loop source detection methods to analyze the apparent resistivity characteristics of ground transient electromagnetic responses of the above two types of water-bearing geological structure models,and use the method of calculating vertical resistivity of vertical magnetic field components at the center point.From the longitudinal perspective,the change in apparent resistivity in the pseudosections matches the longitudinal distribution of the model resistivity,and the apparent resistivity profile corresponding to the Mined-out Area and embankment seepage channel model is obtained,which can reflect the detection target and It can better fit the size of the model,and use this calculation method to explore the feasibility of ground transient electromagnetic method to detect complex strata.Finally,according to the conclusion,the transient electromagnetic measurement data of coal mine Mined-out Area and hydropower station cofferdams are processed.The apparent resistivity pseudosections were plotted and inversion analysis was performed using inversion software.From the inversion results,the approximate location of the anomaly can be inferred more accurately.The subsequent actual geological data also proved the correctness of the inference conclusion.This thesis verifies the feasibility of the transient electromagnetic exploration of the strata and dike leakage in complex Mined-out Area.The dissertation mainly carried out related researches based on model tests and exploration examples.