网式大地电磁阻抗张量及其影响因素分析

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.

Impedance tensor of Network-MT and the influencing factors

In Network-MT(N-MT) method, several to decades kilometers telephone lines are used as the electrode line to measure the electric field. Two straight and mutually perpendicular electrode lines are hardly obtained, thus the impedance tensor of N-MT can not be calculated directly as in MT method. In this article, a simple method to calculate N-MT impedance tensor is presented based on the rotation rule of impedance tensor. Impedance tensors of five N-MT stations in NE China are calculated with this method, and the apparent resistivity, impedance phases, and optimal strike directions are derived subsequently. Furthermore, the differences between calculated results rotated by impedance tensor and observed results with six electrode lines in Chaoyang station are studied. The factors inducing the differences are analysed carefully and “Equivalent Anisotropy Effect of Electric Field" is presented as the main factors. This effect means that there are some heterogeneous structures in the crust whose scale is analogous with the length of electrode lines, and different electrode lines traverse the heterogeneous structures along different direction, thus the electric field observed in different electrode lines is not the component of the same electric field vector, then the impedance components of the observation triangle on surface do not satisfy the rotation rule of impedance tensor.