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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。
The problem of local distortion has been troubling the interpretation of the electromagnetic data of the earth for decades. The development of the three-dimensional numerical simulation of magnetotelluric provides a technical basis for analyzing the characteristics of local distortion and obtaining reliable inversion imaging results. In this paper, three-dimensional numerical simulation method is used to simulate the typical three-dimensional local distortion model. The 3D numerical simulation results show that the polarization mode apparent resistivity curve (TM mode corresponding to the two-dimensional case) of the vertical electrical interface of the electric field component moves upwards and then downwards while passing through the interface of the low resistivity anomalous body. When passing through the interface of high resistivity and anomalous body, the curve will move up and down first, which is related to the secondary electric field generated by accumulating the surface charge at the interface of the electric part. The horizontal variation of apparent resistivity and phase in the XY and YX modes in the three-dimensional model near the three-dimensional anomaly is approximately vertically symmetric, which is in agreement with the change of apparent resistivity when the electric field perpendicularly crosses the electrical interface. When the line is spread along the X direction and the Y direction respectively, the XY and YX modes in the three-dimensional case correspond to the TM mode in the two-dimensional case respectively. The effect of low resistivity and small anomaly on the distortion of regional tectonic response is more serious than that of the high resistivity and small anomaly. Two kinds of polarization modes that respond to two-dimensional region with low resistivity and small anomalies have very obvious distortions depending on the resistivity and phase. In contrast, the distortion of the TE mode is greater than the TM mode, so we are doing two-dimensional inversion When interpreting, you can give priority to fitting TM mode data. Although the three-dimensional distortion response at the measuring point above the center of the small abnormal body is quite different from the response of the corresponding real two-dimensional region, it can be equivalent to the response of some two-dimensional model. The pseudo-two-dimensional response caused by the local distortion is actually Field data interpretation is the need to pay attention.