关键词: 大地电磁, 辅助场, 二维正演, 有限元直接迭代算法, 二次插值, 网格

Abstract: This paper focuses on the calculation of the auxiliary field-another important factor related the precision of MT 2D forward modeling.The purpose is to raise the precision of forward calculation and provide MT inversion with more sufficient supports.It suggests constructing a quadratic interpolation shape function in the surface grid for calculation of the auxiliary field,when employing the current available method of linear interpolation for calculating the principal field.In this work,the Direct Iteration Finite Element Algorithm(DIFE)is utilized to solve the calculations of interpolation nodes.This new algorithm is tested through model calculation.Comparing with the 2D forward modeling of the auxiliary field by the linear interpolation,the quadratic interpolation does not increase much amount of computation,but raises the precision of MD forward calculation.We also discussed the reason from theory,and on the basis of the result,the current design criterion for 2D forward grid has been improved.The element size can be enlarged to one skin effect depth.So the new algorithm has special meaning to the study of complex topography forward problem.We will further discuss this question and compare the results.Besides,the calculation of auxiliary field in 3D forward modeling is also very interesting.

Key words: magnetotelluric, auxiliary field, 2D forward modeling, DIFE, quadratic interpolation, grid