Study On 3D Finite-Difference Numerical Simulation Of Downhole Transient Electromagnetic Field

Down-hole TEM has strong sensitivity in the detection of deep concealed ore bodies and well-guided geological bodies around the borehole,and has been widely used in the fields of hydrology and engineering geology survey,concealed mineral exploration and archeology.The working mode received in the ground excitation hole enables it to obtain strong useful signals,and the anomalous characteristics are obvious.The application of Down-hole TEM has achieved significant results so far,but there are still some theoretical problems that need to be addressed.In-depth research.This paper focuses on the FiniteDifference Time-Domain of the Down-hole TEM,which aims to obtain the Down-hole TEM response characteristics of typical anomalous bodies through three-dimensional forward modeling,Provided references for data processing and interpretation in practical work.The Laplace transform and Fourier transform were applied to convert the frequency domain integral to the time domain,respectively,and obtained the analytical solution of the time-domain electromagnetic field excited by the ground surface and received underground.The center-difference discrete method is used to derive the three-dimensional FiniteDifference Time-Domain equation of Down-hole TEM field based on Maxwell equations.Based on the numerical stability conditions of the Finite-Difference Time-Domain of uniform half-space,a gradient non-uniform meshing method and the absorbing boundary conditions of modified Liao are introduced.a three-dimensional Finite-Difference TimeDomain algorithm for Down-hole TEM fields was developed.The diffusion process of Down-hole TEM field is theoretically analyzed,and the graphical simulation methods such as attenuation curve,three-dimensional slice and plane contour are used to analyze the numerical simulation results.In this process,a uniform halfspace,horizontal layer,the models of two observation methods "localized heterogeneous" and "local heterogeneous" are established.Model,and numerical simulations were performed.The results show that the magnitude and decay rate of the uniform half-space TEM response Closely related to the resistance of the anomalous body and surrounding rock,anomalous body size,spatial location of the excitation field source,and overlay and so on.