Comparison Of Magnetotelluric TE And TM Polarization Modes

In the magnetotelluric (MT) survey, the inhomogeneous mediums underground make polarization apparent resistivity curve with TE and TM modes of measured data different. Therefore, different modes can get different interpretation results which will bring difficulty to understand and judge geologic configuration. Our long work showed that polarization apparent resistivity curve with TE and TM modes had obvious difference, however, in specific geologic configuration, there were some law in their difference which researching and understanding would play an important role on data processing and interpretation.Some MT basic theories are introduced firstly in the paper, which contain lots of important formulas and principles and discussion of finite-element forward method and some inversion methods in detail. As to forward, emphasis is on analyzing basic equations, boundary conditions and finite-element solutions for magnetotelluric field in two dimension of finite-element forward method. For inversion, principles of OCCAM inversion, RRI inversion and NLCG inversion are described, and these three methods are compared which help determine the forward and inversion methods using for the models in the paper. Finite-element forward is for all models in the paper. And OCCAM and NLCG are respectively for one dimensional and two dimensional inversions. But initial model of two-dimensional inversion is one dimensional inversion model.The paper put emphasis on carrying forward and inversion and finding changing laws of polarization apparent resistivity curve with TE and TM modes by constructing theoretic models. Common geologic structure models such as horst, graben, fault, static shift, Shallow low-resistance body etc. which are constructed are used during forward and inversion and analyzed their results to find which model can gain best fitting result. And comparison the changing patterns of polarization apparent resistivity curve with TE and TM modes serves to show their own characters between of two kinds of polarization apparent resistivity curves in different models. Then following conclusions are gained that are TE mode has good vertical resolution without enough horizontal resolution whereas TM mode has horizontal resolution with bad vertical resolution; For horst structure, apparent resistivity curve of TM mode always above apparent resistivity curve of TE mode always in horst area, apparent resistivity curve of TM mode always under apparent resistivity curve of TE mode always in graben area, and for horst and graben models, TE mode has more sensitive than TM mode which can reflect real model better; And in the fault area, apparent resistivity curve of TE mode gradually shift down to below apparent resistivity curve of TM mode and come into being"trumpet-shaped". There is local inhomogeneous two and three dimensional bodies with electric property that generate static effects which has great effect on apparent resistivity curve of TM mode, but has little effect on apparent resistivity curve of TE mode. And apparent phase curve of TM mode suffers little or no effect from static effects whereas apparent phase curve of TE mode has obvious static effects with low degree effect. In geologic work, we find static effects has great effect on apparent resistivity curve of TE mode that show actual geologic configuration is three dimensional but not two dimensional. TE mode can reflect shadow low resistance body further better than TM mode.We use actual MT data to demonstrate above our conclusions at last in the paper, and prove that these conclusions can provide good reference for actual data interpretation and direct significance for acquiring real geologic configuration.