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网式大地电磁(Network-MT,N-MT)法采用长数公里至数十公里的电话线为电极线测量电场,很难形成两条笔直且相互垂直的电极线,因此阻抗张量的计算不如大地电磁法中直接.本文依据阻抗张量的旋转规则提出了一种计算N-MT阻抗张量的简便算法.依据该算法计算了中国东北地区5个N-MT测站的阻抗张量,获得了基于阻抗张量的视电阻率、相位曲线和最佳主轴方位角分布图像,为我国东北N-MT资料的进一步处理和解释提供了基础性数据.此外,本文对比分析了朝阳测站中6条N-MT观测电极线上阻抗张量旋转值与观测值之间的差异,重点讨论了产生这种系统性偏差的各种因素,提出其主要因素可能来自“电场等效各向异性”效应,即测站附近的地壳内部存在与观测电极线尺度相比拟的横向非均匀构造,而测站各电极线沿不同方向跨越不均匀构造,此时各电极线上的电场分量不遵循同一电场矢量的分解准则,导致地表观测三角形内阻抗分量不满足统一的阻抗张量旋转规则.
Network-MT (N-MT) method uses a telephone line of several kilometers to several tens of kilometers to measure the electric field for the electrode line, and it is difficult to form two straight and perpendicular electrode lines. Therefore, the calculation of the impedance tensor It is not as direct as that in the earth magnetism method.This paper presents a simple and convenient algorithm for calculating the impedance tensor of N-MT according to the rotation rule of impedance tensor.According to this algorithm, the impedance tensor of five N-MT stations in northeast China is calculated, Obtained apparent resistivity, phase curve and azimuth angle distribution image based on impedance tensor, which provided basic data for further processing and interpretation of N-MT data in Northeast China.In addition, The differences between the values of the impedance tensors of the six N-MT observational electrodes and the observed values are mainly discussed, and the main factors that cause this systematic deviation are discussed. The main factors may come from the “equivalent field anisotropy ”Effect, that is, there is lateral heterogeneous structure in the crust near the station compared with the standard of the observed electrode line, and the electrode lines of the station cross unevenly in different directions, and the electric field components of each electrode line do not follow same The decomposition criterion of the electric field vector leads to the fact that the internal impedance component of the observed triangle in the earth does not satisfy the uniform rotation rule of the impedance tensor.