| The ground-hole transient electromagnetic method utilizes the law of electromagnetic induction to observe the secondary eddy current field.The observation stages are divided into early,transitional and late stages in terms of time.mainly by measuring the pure secondary field signals of which varying with time,the geological information of different depth is obtained.The ground-hole transient electromagnetic method is often used in the detailed exploration of metal deposits,which has the advantages of strong anti-interference ability,basically not affected by the topographic fluctuation,and has the advantages of good coupling,strong abnormal response,simple shape,strong resolution and so on.In this paper,the response characteristics and distribution law of typical abnormal bodies are studied by means of ground-hole transient electromagnetic method.Firstly,Maxwell's equations were deduced as discrete format by the finite difference method,then the time variation law of magnetic field component was obtained by introducing a vertical magnetic dipole source,in which the non-uniform meshing technique was used to divide the established model,and the modified Liao's absorption boundary condition was applied to perform high-order correction and processing of the boundary and corner points.Finally,a three-dimensional finite difference algorithm is formed.Typical geoelectrical anomaly models of ground-hole transient electromagnetics method were established,and the forward modeling of several different geoelectric models such as homogeneous half space,G type,D type,K type and H type,as well as the geological models of 0 °,45 °,90 °,135 ° inclined low resistivity plate bodies and nappe structures,etc.,were carried out.Then the secondary field response characteristics of the corresponding anomalous body were analyzed,and the response curves and contour map at different times were compared.Finally,the transient electromagnetic response characteristics of a typical abnormal body were summarized. |
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