Abstract: In this paper we analyze the transient responses directly in frequency-domain using time-domain difference (FDTD). Since two-dimensional structure is convenient to be shown by figures, we use line source 2D geo-electrical models. By computing “smoke ring” diffusion in the earth and vertical electromotive force on the earth about uniform two-layer and uniform two-layer with a conductor indicates that transient fields have delay phenomenon. It means that once an anomalous body causes transient responses, this response might delay a long time. In engineering explorations, we have used later time delay to prospecting sallower bodies, such as abandoned coal miners located at 20m～150m depth range and have had good results. The numerical resolutions of this study support the previous practices theoretically.

Key words: Transient electromagnetic field, Time-domain finite difference, Diffusion, Delay

摘要： 直接从时间域出发,应用时域有限差分方法(FDTD)分析地下和地面的瞬变响应。由于二维情况便于图形表示,采取了线源二维地电模型。通过对二层均匀地电结构和含二维低阻体时地面垂直感生电动势曲线的计算,说明瞬变场响应有滞后现象,即异常地质体一旦引起瞬变响应,就会往后延时,并使响应延续较长时间。在实际的工程勘察中,曾用3～5ms以后的延迟时间探测过较浅的异常体,如20m至150m深度范围内的地下老窑采空区,并取得了较好的探测效果,上述数字模拟结果为以往的实践提供了理论依据。

关键词: 瞬变电磁场, 时域有限差分, 扩散, 滞后