层状介质磁偶源瞬变电磁法正演及视电阻率计算

Forward Simulation and Apparent Resistivity Calculation of Transient Electromagnetic Method Using Magnetic Dipole Source on the Layered Medium

摘要: 瞬变电磁法正演是进行多维瞬变电磁勘探理论研究的基础，通过正演计算可获得不同地电断面各场量瞬变耦合曲线，而中晚期场量均随着时间的增加迅速减小，将场量换算成视电阻率曲线，可获取地电断面的重要特征。本文首先导出层状介质磁偶源瞬变电磁法垂直磁场的计算公式及数值计算方法，建立典型的层状介质模型进行正演模拟，获得了磁场分量的衰减曲线，并将场量衰减曲线换算成视电阻率。计算结果表明视电阻率曲线直观的反映了地电断面特征，瞬变电磁法对低阻介质层的异常反应明显，同时场量误差分析表明正演结果可靠。

Abstract: The forward of transient electromagnetic method (TEM) is the basis of the theoretical study of multi-dimen- sional transient electromagnetic prospecting. Transient coupling curves of each field component of different geoelectric sections can be obtained through forward calculation, and all the field components decrease rapidly as time increases in the middle and late time. Important characteristics of the geoelectric sections can be acquired by converting the field components to apparent resistivity curves. The calculation formula is derived from Hankel transform and G-S transform, and the numerical algorithm is proposed for the vertical magnetic field of transient electromagnetic method with mag- netic dipole source in the layered medium. Then the magnetic field components, calculated for several typical layered medium models, are converted to apparent resistivity values according to the definition of the apparent resistivity. The relative errors of the field components between the forward results and theoretical values are all less than 0.1%. The calculation results show that the apparent resistivity curves directly reflect the characteristics of geoelectric sections. And the anomaly of the low resistivity layer is more obvious than that of high resistivity layer. Furthermore, the negli- gible errors show that the forward results are reliable. It is valuable for theoretical research and practical application of transient electromagnetic method.