Influence of two-dimensional magnetotelluric anistropic parameters on apparent resistivities
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摘要: 大地电磁法是广泛应用于深部地质结构探测、油气和矿产资源勘查等领域的一种地球物理方法.电性各向异性对电磁观测数据有很大影响,但介质各向异性参数对不同模式视电阻率的影响还较少有较为系统的研究.本文基于Maxwell方程组,推导了二维大地电磁场在任意各向异性介质中电场和磁场相互耦合的变分方程,结合有限单元法及并行计算编写了二维大地电磁任意各向异性正演程序,采用三角形网格剖分.验证程序正确性后,以倾斜板状体作为模型来研究三个主轴电阻率及三个旋转欧拉角和四种模式的视电阻率之间的关系.结果表明,主轴各向异性时,xy模式视电阻率几乎只受x方向电阻率影响,yx模式视电阻率主要受y方向电阻率影响,但同时也受z方向电阻率一定影响;三个欧拉角中只有倾角不为零时,yx模式视电阻率受倾角大小的影响较大,xy模式视电阻率几乎不受倾角的影响;只有走向角不为零时,四种模式的视电阻率同时受x、y两个主轴电阻率和走向角的大小的影响.Abstract: Magnetotelluric method is a geophysical method widely used in deep geological structure detection, oil and gas and mineral resources investigation. At present, methods of investigation and survey for multiple structures of rocks and minerals inside the earth are gradually improved and the research on the anisotropy of underground medium is continuing to study ulteriorly because electrical anisotropy has great influence on electromagnetic observation, but the effect of anisotropy on the apparent resistivities of different modes is seldom studied systematically. In this paper, based on Maxwell equations, the variation equations of coupled electric and magnetic fields together are derived in magnetotelluric field of the 2D arbitrary anisotropic media, and we develop the two-dimensional arbitrary anisotropic forward modeling code using finite element method with triangulated mesh. The paper verifies the validity of the code compared to the previous results. And in order to study the relationships between three principal resistivities and three Euler rotation angles and the four modes of apparent resistivities, we compare and analyze the responses of three models by using the inclined plate as the study object. As the results show, firstly, when the Euler angles are all zero meaning the principal anisotropy, the apparent resistivities in xy and yx modes mainly reflect the existence of anomalies. And the apparent resistivity in xy model mainly reflects the principal resistivity in x direction, and the apparent resistivity in yx mode mainly reflects the resistivity in y direction which is also influenced slightly by the principal resistivity in z direction. Secondly, when the dip angle is not zero only, the apparent resistivity in yx mode is greatly affected by the size of dip angle, while the apparent resistivity of xy mode is almost unaffected by dip angle. And what's more, when the strike angle is not zero only, four kinds of apparent resistivity are affected by the strike angle and the resistivities in x and y direction at the same time.
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Key words:
- 2D magnetotelluric /
- Forward modeling /
- Anisotropy parameters /
- Finite-element method
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