| 带地形的大地电磁各向异性二维模拟及实例对比分析 |
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| 引用本文： | 霍光谱,胡祥云,黄一凡,韩波.带地形的大地电磁各向异性二维模拟及实例对比分析[J].地球物理学报,2015,58(12):4696-4708. |
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| 作者姓名： | 霍光谱 胡祥云 黄一凡 韩波 |
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| 作者单位： | 1. 河南省地质调查院, 郑州 450000;2. 中国地质大学地球物理与空间信息学院, 地球内部多尺度成像湖北省重点实验室, 武汉 430074 |
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| 基金项目： | 国家深部探测专项第3项目(SinoProbe-03),"十二五"国家科技支撑计划课题(2011BAB04B01),国家重点基础研究发展计划(2013CB733203)和国家自然科学基金(41274077、41474055)联合资助. |
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| 摘 要： | 带地形的大地电磁二维正演数值模拟多数基于电性各向同性理论,由于地球内部电性各向异性现象的普遍存在,基于电性各向异性理论研究地形起伏情况下大地电磁二维正演数值模拟就显得非常迫切.本文首先由麦克斯韦方程出发,引入张量电导率,求得一组关于平行走向的电场分量Ex和磁场分量Hx的二阶偏微分方程,使用有限差分法求解出Ex和Hx的近似解,并以此求得其他场分量;其次,引入地形因素,改变变量在网格节点中的排列方式,选择交错排列方式从而给有限差分系数矩阵的最大带宽分配合理的存储空间;最后,使用Weaver的方法解决TM模式下,在地-空分界面垂直于构造走向的一些区域存在不同电导率的问题.通过对带地形的二维电性各向异性结构做正演模拟,研究地形因素对大地电磁响应的影响;以电性各向异性理论为基础,将地形因素引入对实测大地电磁资料的处理中,通过做二维正演拟合和未引入地形因素的结果做对比,说明电性各向异性现象的普遍存在,认识地形因素对观测大地电磁场的影响,为今后分析解释实测大地电磁资料包含地形因素和电性各向异性情况提供理论基础和技术指导.
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| 关 键 词： | 地形 电性各向异性 大地电磁 有限差分 张量电导率 |
| 收稿时间： | 2015-05-15 |
MT modeling for two-dimensional anisotropic conductivity structure with topography and examples of comparative analyses |
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| HUO Guang-Pu,HU Xiang-Yun,HUANG Yi-Fan,HAN Bo.MT modeling for two-dimensional anisotropic conductivity structure with topography and examples of comparative analyses[J].Chinese Journal of Geophysics,2015,58(12):4696-4708. |
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| Authors: | HUO Guang-Pu HU Xiang-Yun HUANG Yi-Fan HAN Bo |
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| Institution: | 1. Henan Institute of Geological Survey, Zhengzhou 450000, China;2. Hubei Subsurface Multi-scale Imaging Key Laboratory, Institute of Geophysics and Geomatics, China University of Geosciences, Wuhan 430074, China |
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| Abstract: | The magnetotelluric (MT) method is a technique for probing electrical conductivity structure of the Earth, from the near-surface down to upper mantle. MT observations can be significantly influenced by topography. Most of existing forward modeling algorithms for 2-D MT with topography are based on the electrical isotropic theory. However, it has been well established that the electrical anisotropy is widely present in the earth interior. Since the electrical anisotropy in crust and upper mantle is the connection between electric structure and the geological structure, it is vital to account for anisotropic effects while modeling 2-D MT fields with topography.#br#We present a 2-D MT modeling approach for anisotropic media with topography. The solution is based on a finite-difference (FD) discretization of the second-order Maxwell's equation in terms of electric fields parallel to strike for TE mode and magnetic fields parallel to strike for TM mode. The topography effect is accounted by changing the way in which the sampling electromagnetic fields are arranged, i.e. the sampling fields are in a staggered-order to make sure that the maximum bandwidth of the FD coefficient matrix can be assigned moderate memory spaces. The Weaver's approach is used to deal with the problem that the conductivity of some area on the air-earth interface may vary in the direction perpendicular to strike. Once the primary field is solved, the dual field can be obtained very easily by applying a discrete differential operator to the primary field.#br#Two synthetic models for modeling the horst structure and graben structure, respectively, are used to evaluate the effects of topography. The responses of models with topography are compared with that of models without topography. It is found that most significant differences occur in the regions with sharp topography boundaries, such as the boundary between the foot of a mountain and the mountainslope and the boundary between the mountain slope and the mountaintop, while the minor differences appear in flat regions. The topography has much greater impact on the TM responses than TE responses, either for the horst model or the graben model. The graben structure can have more effects than the horst structure. The sharper the slope, the greater the influence is.#br#We demonstrate the validity of the algorithm for 2-D MT anisotropic forward modeling with topography by numerical tests on both synthetic models and real datasets. The effects of topography are assessed by analyzing the modeling results of the horst model and the graben model. Finally, the impacts of the topography on MT responses for both large scale models and sharp slope models are evaluated by fitting the observed MT data through forward modeling. |
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| Keywords: | Topography Electrical anisotropy Magnetotelluric Finite difference Tensor conductivity |
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