瞬变电磁场的直接时域数值分析

DIRECT TIME-DOMAIN NUMERICAL ANALYSIS OF TRANSIENT ELECTROMAGNETIC FIELDS

为了深入了解瞬变电磁场的勘探原理 ,直接在时间域对负阶跃脉冲激发的二维瞬态场进行了数值分析 .采用的方法是从反映电磁场基本规律的麦克斯韦方程组出发 ,导出时域电场的齐次扩散方程 ,对所研究的空间区域作差分离散 ,源作为初始条件加入 ,利用准静态近似处理空中边界 ,然后进行时间的逐步递推 ,由此展现瞬变电磁场在地下扩散随时间发展的全过程 .通过模拟计算不同时刻瞬态电场在地下的分布形态及地面上感生电动势相应的变化 ,揭示了低阻异常体对感应涡流的聚集作用 ,低阻覆盖层对瞬变场扩散的减速作用 ,及瞬变场的延时效应 .因此 ,瞬变电磁法对低阻体是敏感的 ,有上覆低阻层时探测同样的深度需要较长的时间 ,而延时效应瞬变场的晚期时段可反映埋藏较浅的异常体 .

In order to understand exploration principle, a numerical analysis is made in time domain for two dimensional transient field excited by a negative step pulse. The method used in this paper is to derive the homogeneous diffusion equation of time domain electric field from Maxwell's equations and to do the difference discretion in the investigated region. The source is joined as initial condition and the boundary in the air is dealt with by quasi static approximation, then the time stepping is carried out. Hence could be shown the entire process of transient electromagnetic fields diffusing with time in the earth. The distribution patterns of transient electromagnetic fields at different instants in the earth and the corresponding changes of electromotive force on the earth are calculated, which could enunciate the conductor's concentrating effect on induced eddy current, a decrease in diffusing velocity for conductive overburden and delay effect of transient fields. Therefore, transient electromagnetic method is sensitive to conductors, and a longer time is needed to detecting the same depth where conductive overburden exists. Because of delay effect the bodies embedded in upper subsurface could be detected in late time. [