Theoretical And Inversion Research Of Magnetotelluric Fields In Two-dimensional Anisotropic Media

Magnetotelluric method(MT),the important part of electrical exploration,with many advantages,such as using natural fields,light equipment and the huge prospecting depth,has played one of the most important roles in geophysical exploration.Lots of studies on MT have been made about theory.Also,with the facts that numbers of applications have been made successfully both at home and abroad in the past half century,MT has gained the world's approval.However,the majority of these research results currently are based on assuming the geoelectrical media underground being isotropic.In decades,more and more evidence has proved that the anisotropy phenomenon,caused by kinds of factors,is prevalent in the crust and upper mantle.It's easy to get wrong and contradictory results while processing and interpreting magnetotelluric measured data by simply using the electrical isotropic theory.Due to the complexity of anisotropy processing and too many parameters needing to be considered,scholars didn't acquire remarkable achievements until computer technology made great advancements,making it possible to fit numerical simulation technology in forward modeling,although they realized the phenomenon that resistivity may vary with the different directions at an early time.Meanwhile,they had no choice but to attempt to study on the analytic expressions for some simple models because of the poor computational capabilities.Nowadays,it's quite easy to forward magnetotelluric fields in two-dimensional or three-dimensional electrical anisotropic media.But widely recognized algorithms of inversion of magnetotelluric fields in two-dimensional or three-dimensional arbitrary anisotropic media haven't been published,which are still hot spots being explored by all the scholars around the world.The main nodus is that it's difficult to solve the coupled system of partial differential equations composed of the components Ex and Hx in the electromagnetic field with the limited circumstances.In this paper,a kind of algorithm of the finite-difference modeling of magnetotel-luric fields in 2-D generally anisotropic block structures is introduced in detail,which has been widely accepted by lots of scholars in the geophysics field,as the forward method of modeling in arbitrary anisotropic media.Then we consider the specified conditions,where assuming the principal axis anisotropy theoretical conductivity instead of arbitrary anisotropy conductivity,to acquire the decupled system of partial differential equations composed of the components E_x and H_x in the electromagnetic field.With the features that the values of Ex in E-mode only have relationships with ?xx,and the values of Hx in H-mode only have relationships with ?yy and ?zz,we make the inversions of 5 different kinds of theoretical models including high-resistivity abnormal structure,low-resistivity abnormal structure,single abnormal structure and dual abnormal structures,using the least-squares method by writing Matlab programs.The results show the good effects including the resistivity recovery and the delineation of abnormal body position in x directions under E-mode,also iny directions under H-mode,but unobvious effects in z directions under H-mode.The positions and the resistivities can't be delineated easily from the results in z directions cause they are close to the background.