CSAMT 3D Modeling For Anisotropic Media

Controlled source audio-frequency magnetic telluric(CSAMT)is a kind of frequency sounding method which uses artificial source to transmit electromagnetic signals of different frequencies.By receiving signals in far zone,we can get the apparent resistivity of the observation region.The resolution and signal-to-noise ratio of CSAMT are higher and its observation time is shorter,compared to MT.So CSAMT is often applied to field exploration,especially in geothermal prospecting in recent years.With the fast development of CSAMT,the forward and inversion problems of 3D CSAMT has become a new study point.At present most researches about 3D CSAMT forward are based on isotropic medium.But underground medium is very complex and anisotropy in fact,due to the influence of underground sedimentary and rock composition.Research on isotropy is hard to reflect the real subsurface structure.So research on anisotropy CSAMT has more practical significance.In addition,there are a series of additional effects,which cause changes of Cagniard resistivity and increase the difficulty of exploration,such as near source effect,overprint effect,shadow effect and static effect,due to the artificial source.Therefore it's important to analyze the influences between anisotropy and source effect.In this work,we studied finite difference algorithm of CSAMT in an-isotropic medium and simulated the source effect and static effect which are common in CSAMT exploration.In this paper,firstly it introduces the theory of CSAMT,the special circumstances of source effects and static effect and tells how to deal with the conductivity in an-isotropic medium.Then governing equations of CSAMT forward are derived in detail from the Max well's equations according to the former researches on CSAMT and an-isotropic medium forward.Large-scale sparse equations are constructed based on staggered-grid finite difference method combining with multi-component of anisotropy.After Krylov numerical iteration method preprocessing,large sparse system of linear equations are solved by QMR algorithm with divergence correction.We implemented a CSAMT three dimensional forward in an-isotropic medium program using FORTRAN programming language and verified the correctness of our code with a stratified model.Then several models are studied through numerical simulation using the forward program.According to the simulations,we studied the relation between and among an-isotropic medium,source effect and static effect by analyzing the apparent resistivity and phase curve graphs changing with frequencies.