Three Dimensional Modeling of Magnetotelluric Data Using Finite Element Method

The magnetotelluric response of a resistivity model is usually obtained by solving boundary value problem of the Helmholtz type equation concerning electric field， which introduced from the Maxwell's equation. In this case， electric field is not continuous at the boundary between different resistivity mediums. This condition raises error in the numerical modelling using the finite element method or the finite difference method. To overcome the condition, I formulated the finite element equation using secondary field components resulting from the anomalies. The primary field is calculated analytically for semi-infinite uniform or horizontally layered earth. I use the finite element method composed of 8 nodes isoparametric hexahedral elements to calculate the secondary field excited by electric charges appeared at the anomalies having deferent resistivity from the host medium. Current sources at the resistivity anomaly depends on the resistivity contrast and the primary electric field, which insert at the right hand side of the finite element equation. I use boundary condition which take account for asymptotic behavior of the secondary field far away from the resistivity boundary. I applied this modelling method to simple three dimensional prism model and compared the result with the responses of the integral equation modeling algorithm (Ting and Hohmann, 1981) and the hybrid FEM (Gupta，et al.1987).