回线源瞬变电磁法的三维正演研究  

Study on three-dimensional forward of transient electromagnetic method excited by loop source

作　　者：李建慧[1,2] 朱自强[2] 鲁光银[2] 曾思红[1] 

机构地区：[1]中国地质大学(武汉)地球物理与空间信息学院,武汉430074 [2]中南大学地球科学与信息物理学院,长沙410083

出　　处：《地球物理学进展》2013年第2期754-765,共12页Progress in Geophysics

基　　金：国家自然科学基金项目资助(41174061);中国博士后基金(2012M521498);中央高校基本科研业务费专项资金联合资助

摘　　要：本文利用矢量有限元法对回线源激发的瞬变电磁场进行了三维数值模拟.根据瞬变电磁场扩散的"烟圈"原理,H型和K型地电模型可近似看作埋置于均匀半空间中的水平板状体地电模型,称为准H型或准K型地电模型.在准H型地电模型中,采用了3种不同的网格剖分方案;从方案1至方案3,水平低阻板状体范围不断扩大,逐渐接近于H型地电模型.中心回线瞬变电磁法的数值模拟结果表明:在12.8ms以内,不同网格剖分方案矢量有限元法解与一维解析解相对误差均在5%以内;在19.2ms时,三种网格剖分方案的相对误差依次为12%、5%和2%左右.再利用准H型和准K型地电模型对大回线源激发瞬变电磁场的数值结果进行了验证,结果表明:两种地电模型矢量有限元法解均满足精度要求,但准K型地电模型数值精度更高.最后对直立低阻薄板地电模型进行了数值模拟,取得了与物理模拟一致的感应电动势曲线形状.Vector finite element method （VFEM） was applied to simulating transient electromagnetic field excited by loop source in this paper. Based on the ＇smoke ring＇ theory of the transient electromagnetic field dispersion, the Htype or K-type geoelectric model is viewed approximately as a horizontal low or high resistivity sheet in the homogeneous half-space, which is called quasi-H-type or quasi-K-type geoelectric model. Three schemes were adopted for meshing quasi-H-type geoelectric model. From Scheme 1 to Scheme 3, the length and width of horizontal low resistivity sheet become larger, and these geoelectric models are close to the H-type geoelectric model gradually. The numerical simulation results of central loop transient electromagnetic method （TEM） shows that the relative errors of the VFEM solutions using various meshing schemes and 1D analytic solutions are within 5% in 12.8 ms, and when it is 19. 2 ms, the relative errors for three meshing schemes are 12%, 5% and 2% or so, respectively. For large loop source TEM, the accuracy of VFEM is tested by comparing 1D analytic solutions of quasi-H-type and quasi-K-type geoelectric models. The results show that the accuracy of these geoelectric models meets the accuracy requirement, but the quasi-K-type geoelectric model has higher accuracy. At last, the plots shape of EMF computed by VFEM coincides with physical simulation plots for upright low resistivity sheet geoelectric model.

关 键 词：回线源 瞬变电磁法 “烟圈”原理 准H型地电模型 正演 

分 类 号：P318[天文地球—固体地球物理学]