Magnetotelluric Forward Modelling For Three-dimensional Structures With Arbitrary Anisotropy

Magnetotelluric sounding method(MT)is a kind of passive source electrical prospecting method,which observes natural changes in electromagnetic field.As a result,it is equipped with high portablility and works with high efficiency,and is widely used in the fields such as oil and gas exploration,mineral exploration,geothermal exploration and deep geophysical investigation,environmental geophysics,marine geophysics and so on.In recent years,with the development of information technology and equipment,this method has been at the forefront of electric field study.Previous MT theoretical research and practical application was mainly based on isotropic media.However,according to lots of theoretical and practical studies,there is a general distribution of anisotropy in the earth caused by both macroscopic and microcosmic factors.To make a better analysis and investigation of the real situation and obtain useful conclusion to guide geological work and other exploration and development work,anisotropic research should be made in magnetotelluric theories.The introduction of anisotropy changes the scalar resistivity(or scalar conductivity)of isotropic model into a tensor resistivity(or tensor conductivity).With a definition of three axial resistivities in anisotropic axial coordinate system and three Euler's angles used in the rotating tranformation of the obeserving coordinate system into the anisotropic axial coordinate system,arbitrarily anisotropic tensor model parameters can be easily calculated.On the basis of three-dimensional anisotropic medium,we derive the staggered-grid finite difference equations of MT forward problem and elaborate the configuration of boundary conditions.To solve the corresponding large sparse system of linear algebraic equations,an iterative preconditioned biconjugate gradient stabilized method is used and magnetic field divergence correction is also introduced to fasten the convergence process at each iteration step.Through the computation of a few isotropic and anisotropic models,our algorithm is tested to be effective and accurate compared with the results of predecessors.Moreover,with a simple three-dimensional model,many groups of anisotropic parameters are used to compute MT response,showing clearly anisotropic effects.It turns out that anisotropic results are considerably different from isotropic ones.Therefore,it is necessary to carry out research in MT anisotropic theories.