| 大地电磁局部畸变异常特征三维数值模拟研究 |
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| 引用本文： | 蔡军涛,赵国泽,徐锡伟.大地电磁局部畸变异常特征三维数值模拟研究[J].CT理论与应用研究,2017,26(4):391-402. |
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| 作者姓名： | 蔡军涛 赵国泽 徐锡伟 |
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| 作者单位： | 1. 中国地震局地质研究所活动构造与火山重点实验室, 北京, 100029; |
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| 摘 要： | 局部畸变问题曾经困扰大地电磁资料反演解释几十年,大地电磁三维数值模拟技术的发展为剖析局部畸变特点和得到可靠的反演成像结果提供了技术基础。本文采用三维数值模拟成像方法对典型三维局部畸变模型进行模拟分析。三维数值模拟结果显示:电场分量垂直电性分界面的极化模式视电阻率曲线(对应二维情况下TM模式)在穿越低阻异常体界面时,曲线会先上移后下移,而在穿越高阻异常体界面时,曲线会先下移后上移,这与电性分界面处积累面电荷产生的二次电场有关。三维模型中XY模式、YX模式视电阻率和相位在三维异常体附近的水平变化是呈现近似垂向对称的,该现象与电场垂直跨越电性界面时视电阻率的变化规律是吻合的,当测线分别沿X方向和Y方向展布时,三维情况下的XY和YX模式分别对应二维情况下的TM模式。低阻小异常体对区域构造响应的畸变影响比高阻小异常体要严重。低阻小异常体对二维区域响应的两种极化模式视电阻率和相位都有非常明显的畸变影响,相比较而言对TE模式的畸变要大于TM模式,因此我们在做二维反演解释时,可优先考虑拟合TM模式数据。位于小异常体中心上方测点的三维畸变响应虽然与对应真实二维区域响应的差异比较大,但可以等效于某种二维模型响应,这种由局部畸变造成的假二维响应在实际野外数据的解释中是需要注意的。
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| 关 键 词： | 大地电磁 三维 局部畸变 数值模拟 |
| 收稿时间： | 2016-12-01 |
The Study of Local Distortion in Magnetotelluric Using Three-dimensional Modeling |
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| CAI Jun-tao,ZHAO Guo-ze,XU Xi-wei.The Study of Local Distortion in Magnetotelluric Using Three-dimensional Modeling[J].Computerized Tomography Theory and Applications,2017,26(4):391-402. |
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| Authors: | CAI Jun-tao ZHAO Guo-ze XU Xi-wei |
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| Institution: | 1. Key Laboratory of Active Tectonics and Volcano, Institute of Geology, China Earthquake Administration, Beijing, China, 100029;2. Beijing Research Institute of Uranium Geology, Beijing, 100029;3. State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing, China, 100029 |
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| Abstract: | Local distortion by small-scale structures has impeded the interpretations of the magnetotelluric method (MT) for several years. The developing of three-dimensional (3D) MT modeling makes it is possible for the study of the local diction and reliable imaging of the subsurface electrical conductivity structure. In this paper, several general 3D local distortion models are designed and model responses are computed by 3D modeling. The apparent resistivity curve of polarization mode with electric field perpendicular to the electrical interface, which is corresponding to TM mode in three-dimensional (2D), will upward shift at first and then downward shift when it across small conductive surface body. While crossing resistive body, the curve will downward shift at first and then upward shift. This phenomenon is associated with secondary electric field caused by the accumulative surface electric charges at the boundaries of the anomaly. In 3D model, the variation in horizontal slice of apparent resistivity and phase for YX mode and XY mode are approximate vertically symmetrical, which is consistent with the case that curves of polarization mode with electric field perpendicular to the electrical interface. When the survey line along the east-west direction, the YX mode of the 3D response corresponds to the TM mode in the 2D case. While the survey line along the south-north direction, the XY mode of the 3D response corresponds to the TM mode in the 2D case. The distortion on regional response caused by small conductive inhomogeneous are more severe than that of resistive inhomogeneous. The apparent resistivity and phase of both two polarization modes are obviously affected by local distortion of small conductive anomaly, while the distortion on TM mode response is slightly lesser than that on TE mode caused by electrical anomaly. Therefore, when two-dimensional inversion is used for MT data, it is preferable to fit the TM model data. At the position located directly over the small inhomogeneous, the difference between 3D distortion response and 2D corresponding regional response is much large. But this 3D distortion response can be equivalent to a 2D response. It is more important to pay more attention to this false 2D response caused by local distortion in MT interpretation. |
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| Keywords: | magnetotelluric three-dimensional local distortion numerical modeling |
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