Typical Geoelectric Anomaly Of 3D Down-hole Transient Electromagnetic Response Research

The down-hole transient electromagnetic method(the Down hole,TEM)referred to a deep geophysical prospecting method that the receiver sensor was put in the well-drilling to observe the response of TEM.Due to the receiver sensor under the ground,it directly blocked part of ground electromagnetic noise,compared with the ground transient electromagnetic method with the advantages of the secondary field interference and less susceptible to the surface of the low resistance layer,probing the deep target closely,and high longitudinal resolution and so on.Thus it obtained the stronger ground detection ability than the transient electromagnetic method.The down-well transient electromagnetic method could be used to determine the buried depth of abnormal body position,combination and track the extension direction of abnormal body with good \ "side view\" ability.Therefore it was widely used in deep regional geological survey and searching for the well concealed blind ore.But the theoretical research of down-well transient electromagnetic method was relatively lagging behind.So based on the summarization of predecessors' researches,the article deeply explored the typical geological and geophysical model of transient electromagnetic response under the complex cases through the method of numerical simulation.Using the classical finite difference time domain algorithm of Wang and Hohmann,from the point of Maxwell's equations,the article took the stable and convergent Dufort-Frankel finite difference discrete method to disperse the transient electromagnetic resultant field and secondary field diffusion equations,using non-uniform grid subdivision technology for subdivision model.In the study of absorbing boundary conditions,on the basis of perfectly matched layer absorbing boundary condition,in order to meet the characteristics of low frequency transmission,the CFS-PML boundary condition was modified.Comparing with the conventional contrast application effect of the first kind of Dirichlet boundary condition,the results showed CFS-PML boundary condition couldbetter meet the spreading requirements of the low frequency electromagnetic field.By absorbing experience of predecessors' research summary,combining the actual case,the part orebody was often composed of multiple conductors together to build complex geological and geophysical model,which thoroughly analyzed the vertical contact zone,anticline,syncline,horst,graben,inclined plate,and the abnormal response characteristic of a cube model.Under the down-well observation system,the diffusion rules of Tem field changing with time and depth were studied in detail.Results showed that the abnormal response was mainly influenced by the source and target body coupling relationship,size,buried depth,occurrence conditions,electrical properties,structural factors and so on.Finally this paper summarizd the factors of affecting down-well transient electromagnetic response,and provided the referrences for the future of the down-well TEM response analysis and data interpretation.