Impedance Tensor of Network‐MT and the Influencing Factors

Abstract

AbstractIn Network‐MT(N‐MT) method, several to tens 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 by impedance tensor rotation and observed results with six electrode lines at 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.