Three-dimensional Magnetotelluric Forward Modelling Based On An EFGM-FEM Coupling Method

As an important technique for interpreting magnetotelluric(MT)data,threedimensional(3D)forward modeling has made great progress in the past few decades.The mature methods currently formed include the finite difference(FD),finite element(FE)and integral equation(IE).However,these numerical simulation algorithms rely greatly on the preliminarily defined meshes,which largely influence the simulation accuracy.Also,the mesh reconstruction is difficult.To overcome the dependence of the above algorithms on mesh quality,this paper proposes a 3D MT forward modeling technique based on the meshfree method.The meshfree method dose not require complex mesh generation,but discrete the model by nodes,whose spacing and density can be freely adjusted according to requirements,leading to the more flexible subdivision form and adaptive analysis.Meanwhile,the meshfree method is more accuracy than the finite element under the same condition.The main research contents of this paper are as follows:1)A model discretization method based on non-conformal meshes is established.Discrete nodes are used in the central calculation area to accurately describe the complex geoelectric model,and the meshfree method is chosen to simulate the electromagnetic field in this area;while regular hexahedral meshes are used in the expanded area,and the FE method is used for simulation to avoid the calculation error of the meshfree method due to too few nodes.Moreover,it can alleviate the shortcomings of low computational efficiency of the meshfree method to a certain extent;2)This paper firstly deduces the control equation of 3D MT forward modeling based on the weighted residuals approach from Maxwell equation,and then uses the meshfree and FE method to construct the element matrix in different regions respectively.Finally,the two methods are combined to be solved according to the coupling conditions.This paper realizes the 3D MT forward calculation based on the global weak radial point interpolation method(RPIM)meshfree algorithm,and discusses the shape function construction method based on RPIM in detail.Considering the discontinuity of the electromagnetic field at the abrupt interface,a visual criterion is introduced to select the support domains of the points on both sides of the interface;3)A 3D layered model are used to verify the effectiveness of the method in this paper.The results show that under the same node scale,the meshfree method is more accuracy than the FE algorithm.To further verify the universality of the meshfree-FE coupling method,the model with single anomaly body,multi-abnormal body and undulating terrain are simulated with good results.The electric field characteristics analysis near the abrupt interface proves the effectiveness of visual criterion,and the discontinuity of the electric field is well characterized;To sum up,this paper proposes a new 3D MT forward modeling technology based on the meshfree-FE coupling method.Numerical experiments show the high computation accuracy and stability of our method,proving to be a potential MT forward modeling technique.