Numerical Simulation Of The Transient Electromagnetic Response Of Three-dimensional Thin Sheets In Near Region Of A Grounded Line Source Based On Integral Equation

The Long-offset transient electromagnetic (LOTEM) measures in the far zone, whose application is restricted by weak signal. While the grounded line source of transient electromagnetic has many advantages of signal-to-noise ratio, high detection sensitivity, and portable devices etc. in the near field, which are more and more popular in geophysical exploration. However, its forward calculation is not mature enough, hindered the development of the method. This paper tries to calculate the TEM response using integral equation based on three-dimensional thin sheets for developing a fast and stable algorithm which can be integrated in practical software.This paper introduces briefly the advantages of line source near the area of the method by the calculation of one-dimensional grounded source, and mainly studies the numerical simulation of electromagnetic field of grounded source in near zone based on3D Sheet integral equation in the layered medium, analyses the response characteristics of different combination of geological thin sheet body as well. The integral equation of thin sheet derives from Maxwell equation. At the same time, this paper offers readers the solution of the above method. By using Schelkunoff potentials to present electromagnetic field, Electric Green’s tensors and magnetic Green’s tensor of plate and grounded source are derived. Therefore, we obtain Hankel integral form of the electromagnetic field. This is because electromagnetic component of grounded source can be obtained by the Fast Hankel transform, and then transformed into time domain to get the derivative of induced magnetic field corresponding with time and the voltage between the electrodes. Instability caused by conductivity differences between the thin sheet and background field limits the solution of the equation. To separate Green’s tensor, we divide the induction plate current density into curve-free and divergence-free current. The numerical results are stable only if the equation linearly integrates along the thin plate, which eliminates the distortion term.The research results show that by the use of discretizing the grounded line source into infinite amount of dipoles, integrating along the wire, and superimposing the dipole field, the method calculates transient electromagnetic response in near zone with high accuracy. The algorithm of3D model based on the thin sheet is not only feasible, but also efficient as well as effective. The extreme of electromagnetic response corresponds with the boundary. The measured channel of the maximum dBz/dt corresponds with the depth of thin plate. The voltage response of the middle region plate is the maximum and the boundary symbol reverse. The voltage increases with the time. Lateral resolution of the electric field is higher than that of magnetic field.37pictures,2tables,69references.