3D Numerical Simulation Of Down-hole Transient Electromagnetic Method Using Time Domain Finite Difference Method

As one of the important methods used to geophysical exploration and geological prospecting, Down-hole Transient Electromagnetic Method is a geophysical survey method of mineral resource which measures the transient electromagnetic field in a borehole by placing the receiving coils in the borehole. The advantages of Down-hole TEM are that it does not influence of overburden and electromagnetic, high vertical resolution and has better resolution in a certain range around the Bore-hole. Therefore, it was widely used in the investigation of deep geological structures and searching buried ore bodies. However,the Down-hole TEM forward theory research has development slow and become urgent problems to be resaved. In this article, we use numerical simulation method and discuss the down-hole TEM responses of overburden influence in uniform medium and typical geologic-geophysical models.In this paper, we firstly start from Maxwell equations that Down-hole TEM satisfies.We are use stable convergence Dufort-Frankel method, derivation difference equation of Total field and Secondary field based on time domain finite difference algorithm and used a non-uniform meshing technology on the computational domain. Second, we were analyzed the boundary conditions important. Based on the PML absorbing boundary conditions and modified the complex frequency offset PML absorbing boundary condition application conditions to satisfy the low-frequency electromagnetic field propagation.Compared the Dirichlet boundary conditions and the Remedial CFS-PML absorbing boundary conditions, we can obtain that the Remedial CFS-PML absorbing boundary conditions has better absorption for the low-frequency electromagnetic waves than the Dirichlet boundary conditions.With that, this paper were establish typical geologic-geophysical models and deep analysis of the Down-hole TEM anomalies response characteristics from the Total field and Secondary field. Final, analysis summary the main factors that affect the Down-hole TEMabnormal response. The analyses results indicate that: induction potential gradually becomes smaller with the observation time delay in uniform half space; the inclined conductive response relate the source position and observation time. Then, the abnormal field can reflect the position of anomaly space parameters. The results show that the abnormal response of down-hole TEM become weaker with time delay but the abnormal response characteristics remain unchanged in different dill-hole, and the abnormal response difference in different source direct is obviously. According to the measure results can be judged abnormal body space location information. The fault response relate to depth of the target body, the coupling relationship between the target body and the target body resistivity difference. In the section of the fault, the Down-hole TEM anomalies response is maximum. Influenced factors of overburden were analyzed by using the parameter EA.Finally, summarizes the impact of the Down-hole TEM anomalies response factors. We research the work and provide the basis for reference for the down-hole TEM abnormal response characteristics and explanation information.