Research On Three-dimensional Magnetotelluric Modeling In General Anisotropic Media

Magnetotelluric (MT) method is usually used for studying the Earth’s interior conductivity structure by recoding and analyzing the natural electromagnetic fields. It has been widely used in mineral prospecting and exploration, studying the crust and upper mantle electrical structure, marine geophysics and oil exploration.Due to the lack of anisotropic media electromagnetic numerical simulation methods and software, in the past, we often assumed subsurface resistivity is isotropic when interpretating the MT data. However, with the discovery of a variety of geological phenomena in the production practice, it is difficult to use an isotropic resistivity model to interpret the observed data.Numerical simulation is an effective way to study electromagnetic fields in anisotropic media. The Earth’s interior is usually a three-dimensional electrical structure. The results will be confusing or even wrong if the three-dimensional magnetotelluric data was processed by one-dimensional or two-dimensional inversion method. Nowadays, research on MT field in general anisotropic media is still in the first stage, and it is necessary to develop a fast and effective algorithm for three-dimensional MT forward modeling in general anisotropic media.By using the Ohm’s law and the basic concepts of the conductivity tensor, the thesis introduces the basic theory of 1D MT modeling in general anisotropic media. Through analyzing the forward responses of the four basic anisotropic models (vertical anisotropy, azimuthal anisotropy, dipping anisotropy and horizontal anisotropy), we give a preliminary understanding of the effect of resistivity anisotropy. Then, we use the finite element method to model the 3D MT fields in general anisotropic media. The boundary value problem is solved by the weighted residual method and using hexahedral mesh. The weighted residual equation is then transformed into a large linear equations. The formed stiffness matrix is a symmetric, positive definite, sparse complex matrix and stored by compressed row format to save memory. The linear equations is solved by using preconditioned conjugate gradient method to get accuracy results. Finally, we write the Fortran code for 3D MT forward modeling in general anisotropic media.The COMMEMI 3D2-Model is used to verify the accuracy and effectiveness of our algorithm compared with the results given by the 3D MT modeling algorithm in isotropic media by using staggered-grid finite-difference method. A three-dimensional anisotropic model is used for calculating the forward responses, and a better understanding of the influence of resistivity anisotropy is achieved.