中文摘要：

文中提出一种计算大地电磁二维正演辅助场的新方法。在主场满足线性插值基函数的前提下，通过构建二次插值基函数来提高辅助场计算中数值求导的精度。通过与线性插值函数法的计算结果进行对比，证明该算法在基本不增加计算量的同时可以大幅度提高大地电磁二维正演精度。文中还研究了网格对辅助场计算的影响，结果表明二次插值法较线性插值法对网格的要求更为宽松。此外还以均匀半空间模型为例讨论了地表网格纵向间距与模型之间的关系，这对正演计算中的网格设计有参考意义。

英文摘要：

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.