Finite Element 2D Magnetotelluric Forward Based On Unstructured Grid

Magnetotelluric electromagnetic is a kind of exploration method which USES the earth's natural electromagnetic field as the field source to probe the electrical structure of the earth.The main principle is that the electromagnetic waves of different frequencies have different propagation depth in the medium,the geomagnetic field is measured at different frequencies on the surface of the earth,and the electrical structure of the underground is inferred through data analysis.The electromagnetic has many obvious advantages,such as: high lateral resolution and is not affected by the high resistance layer shielding,great response to he low resistance layer.It is widely used in energy,mineral,hydrological detection and other fields,the mainest is its detection depth,low cost and thus are often used in the deep structure detection.The main purpose of this paper is to calculate the electromagnetic response of different models by numerical simulation,and analyze the effect of different models on the calculation results.Based on electromagnetic theory.This paper starts from maxwell's equationsand deduced the corresponding problems according to the boundary conditions which the magnetotelluric met based on two-dimensional electrical structure,using the finite element method in numerical simulation combined with unstructured grid subdivision for the forward solution,Meanwhile,the combination of LDLT(Cholesky method)and CSR(Compressed Sparse Row)storage technology is used to compress the matrix.finally through the establishment of a variety of geoelectric model,solving the magnetotelluric response under different models,From the result of homogeneous half space,layered model and the two-dimensional spherical model,the error in the high frequency(shallow)is relatively small,the error in the low frequency(deep)less than five percent,the overall effect has reached the computing requirements.The forward results show that the finite element based on unstructured meshes well adapt to the complex geoelectric model.