四叉树网格线源频率域电磁法二维正演研究  

2D forward modeling for line sources frequency domain electromagnetic method using quadtree grid

作　　者：王培杰 陈小斌 张赟昀 WANG PeiJie;CHEN XiaoBin;ZHANG YunYun(Institute of Geology,China Earthquake Administration,Beijing 100029,China;National Institute of Natural Hazards,Ministry of Emergency Management of China,Beijing 100085,China)

机构地区：[1]中国地震局地质研究所,北京100029 [2]应急管理部国家自然灾害防治研究院,北京100085

出　　处：《地球物理学报》2024年第5期1999-2014,共16页Chinese Journal of Geophysics

基　　金：国家自然科学基金地震联合基金项目(U1839205);应急管理部国家自然灾害防治研究院基本科研业务专项(ZDJ2019-26)联合资助。

摘　　要：可控源电磁法正演面临的问题,主要表现在源的奇异性和边界条件的处理困难.本文首先利用面积分将源的模拟精度转化为网格的离散精度,然后利用四叉树网格局部任意加密的特性,有效解决了奇异源模拟精度不高的问题.由于大地中的电磁场迅速衰减,在距源很远的侧边界,地下的电磁场主要是由空气中的电磁场感应产生的,而非从源经由大地传播而来,因此在地中使用一维平面波侧边界条件比一阶吸收侧边界条件更为合理,精度更高.通过算例分析,我们还发现空气侧边界的场强叠加了大地介质感应的二次场而显著改变,因此空气侧边界使用基于一次场的一阶吸收边界条件,是导致线源正演边界处计算精度不高的主因.本文中,我们使用四叉树网格将边界发散式向外扩展,以较少的网格覆盖更大的边界区域,来确保核心区的计算精度.研究还发现,线源全区视电阻率能较好地反映地下电性结构的变化,且趋势形态与大地电磁卡尼亚视电阻率相似,使用成熟的大地电磁反演技术对线源全区视电阻率进行反演,可以得到地下电性结构较好的近似模型.特别是倾子计算的全区视电阻率也能反映地下结构的变化,这为无电场观测的地下结构探测提供了可能.但二者之间仍存在差别,线源全区视电阻率还具有一定的几何测深能力,需开发基于线源全区视电阻率正演的二维/三维反演技术.The forward modeling of control source electromagnetic methodspresents challenges arising from source singularities and difficulties in handling boundary conditions.This paper addresses the issue of low simulation accuracy for singular sources by transforming the simulation accuracy of the source into that of the gird using surface integration and locally refining the quadtree grid arbitrarily.The study reveals that the electromagnetic field in the subsurface is induced by the electromagnetic field in the air at the lateral boundary far from the source,rather than being directly transmitted from the source through the earth.Consequently,utilizing 1D plane-wave side boundary conditions is considered more reasonable and accurate than employing first-order absorbing side boundary conditions.Through case analysis,we also found that the field at the air side boundary is significantly altered by the secondary field induced by the earth medium.Unreasonable first-order absorbing air boundary conditions,based on primary fields,are identified as the main cause of low modeling accuracy at the boundary.However,expanding the grid outward to cover a larger boundary region can ensure computational accuracy in the core region.Additionally,the study finds that the full-zone apparent resistivity of the line source can effectively reflect the underground electrical structure and can be utilized to obtain approximate models of underground electrical structures by substituting Cagniard apparent resistivity in magnetotelluric inversion techniques.Particularly,the full-zone apparent resistivity of the tipper can also reflect the variation of underground structures,providing possibilities for underground structure detection without electric field observations.However,it emphasizes a clear distinction between the two,highlighting that full-zone apparent resistivity also possesses certain geometrical sounding capabilities.The article concludes by emphasizing the urgent need for the development of full-zone apparent resistivity 2D/3D inver

关 键 词：线源频率域电磁法 正演 四叉树网格 有限差分法 全区视电阻率 

分 类 号：P631[天文地球—地质矿产勘探]