Two-dimensional MT Forward Modeling Study For Anisotropic Media

Magnetotelluric(MT),using the natural electromagnetic field as its source,has deep prospecting and is sensitivities to the electrical characteristic of reservoirs.Com-pared with other electrical and magnetic geophysical methods,on the one hand,MT cost low and has wide frequency band and deep exploration depth.On the other hand,MT observation is not affected by the high resistance layer shielding and more sensi-tive to low resistance layers.MT is widely used in the geophysical study,for example,the structure of high conductive layers of crust and upper mantle,the fluid cycle in the subduction zone.MT forward numerical simulation has always been one of the key and difficult points in the geophysical methods.And MT forward numerical simulation is also the foundation for the further MT inversion and MT interpretation.As a matter of fact,it turns out that we have to consider the common existence of anisotropic medium and the complex dynamics of terrain factors.Both the two factors,further increased the difficulty of the MT forward numerical simulation.In view of this phenomenon,this paper mainly studies the two dimensional MT numerical simulation about the arbitrary anisotropic medium,in order to achieve one forward modeling algorithm for any terrain and any arbitrary anisotropic medium.By the way,this paper also try to improve the stability and accuracy of MT forward numerical simulation.Up till the present moment,the forward numerical simulation methods for the two dimensional MT under anisotropic medium usually choose two kinds of methods.One is finite difference method and the other is the finite element method.Finite difference method mainly uses rectangular grid as its basic unit.And finite element method is gradually developed to use irregular grid,but its interpolation basis function is mainly linear function.Finite difference method for two dimensional MT numerical simulation under ar-bitrary anisotropic medium is relatively perfect.However,the existing finite difference forward numerical simulation work did not compare their simulation results with the an-alytical solution results of anisotropic medium model.So,this paper embarks from the maxwell's equations,based on the finite difference theory,completed finite difference forward modeling under two-dimensional MT numerical simulation under anisotropic medium.Meanwhile,we compared the simulation results with those of analytical so-lutions for an anisotropic medium fault model,to verify the correctness of the finite difference algorithm.Finite difference,using rectangular element in method as the basic unit,is diffi-cult to complete the MT numerical simulation of complex terrain.Domestic and for-eign scholars actively study the finite element method for two-dimensional MT under anisotropic medium.Finite element method(fem)is to some extent perfect.But the existing finite element algorithm mainly adopts linear interpolation function.The cal-culation precision of the linear interpolation basis function is limited,especially for area where electromagnetic field change dramatically.Quadratic interpolation basis function can more accurately depict unit changes relatively.Furthermore,the field value of the first order partial derivative of the linear interpolation function is constant,resulting in large error in solving the auxiliary field.For flexible accurately fitting undulating terrain and underground irregular structure,improving the accuracy of numerical simu-lation,this paper deduces the second-order finite element formula for two-dimensional MT under arbitrary anisotropic medium,using unstructured grid,and the grid size can be adjusted according to the specific model.The finite element algorithm we deduced is consistent with the analytical solution under anisotropic medium,and consistent with the previous results of the finite difference.Based on algorithm we completed,this paper firstly analyzes the influence of the different anisotropic parameters on the MT observation.Analyzing the apparent resis-tivity and phase curve for dipping anisotropic,horizontal anisotropic and axis anisotropic model,we got some regular conclusions.Secondly,this paper discussed MT observa-tion for the ocean coast anisotropic model.And this paper try to give out an empirical equation about the characteristic distance and characteristic frequency.