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The Whole Range Surveying and Inversion of 2D Frequency-Domain Controled Source Electromagnetic Method

Author: GaoYuan
Tutor: LiuJianXin
School: Central South University
Course: Earth Exploration and Information Technology
Keywords: fCSEM whole range survey and inversion finite element method least squares method terrain
CLC: P631.325
Type: Master's thesis
Year: 2011
Downloads: 175
Quote: 1
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Abstract


At present, Frequency-domain Controled Source Electromagnetic Method(fCSEM), like CSAMT, is surveying in the area where is far away from the source and then calculating the apparent resistivity to inverse using MT method or inverse directly. However, using the inversion method of MT exists error of course. And direct invresion is very hard to obtain the real result of the area near the source because of only having the survey date far away from the source. Also, this direct invrsion method is not very quick because of having many grids. Nowadays, academician He ji-shan put forward another fCSEM,the Wide Field Electromagnetic Sounding Method(WFEM). It can survey the whole range and calculat the wide filed apparent resistivity without field source effect. Inspired by WFEM, the idea of whole range surveying and inversion is put forward in orde to achieve the exact and quick inversion of 2D or 3D fCSEM. It surveys the ground electric field or magnetic field in the area around the source whether far or near. Without calculating the apparent resistivity, we directly use the electric field or magnetic field to inverse.In order to prove the feasibility of whole range surveying and inversion, this work is studying the 2D problem. The earth structure is two dimension. The medium undground is isotropy. The source is a line current and its direction parallel with the tend towards of earth structure. When solving the forward problem, we use finite element method with triangular linear interpolation which can calculate the response of earth with terrain. When solving the inverse problem, it bases on the least squares method and adds a smooth condition. And we make the roughness factor like the damping factor in the inverse process. What’s more, how to calculate the Jacbian is very important. In here, we use an auxiliary field to calculate the approximate Jacbian. It can obtain the Jacbian quickly and accurately by this metod.First, we study the affect of terrain to the electromagnetic response and show the importance of 2D inversion when the earth structure is two dimension. Second, by inversing some earth models, we prove the feasibility of whole range surveying and inversion; it achieves the 2D quick and exact inversion of fCSEM. Also, by linear interpolation of the two adjacent measuring points’apparent resistivity when the frequency is the highest, we get a good initial inverse model.

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CLC: > Astronomy,Earth Sciences > Geology > Geology, mineral prospecting and exploration > Geophysical exploration > Electrical prospecting > Method > Electromagnetic Method
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