Research And Applications Of 2-D MT Forward Modeling And Inversion With Topography

Since 1970s, most researchers have concentrated on numerical modeling and inversion of geophysical problem with complex geometry. So far, effort on many problems is still undergoing and some significant progress already has been made. Numerical modeling and inversion under complex situation are challenging and under development. The complex geometry brings significant impact in the magnetotelluric data, measuring at the surface. Without considering the complex geometry, sometimes it gives biased interpretation, likewise biased inverted structure and lead to difficulty in interpretation of the field data, therefore, in the fieldwork with complex fluctuation, the geometry impact should be taken into account seriously. It is import to theoretically modeling the problem with complex geometry and its impact on the modeling results, which in turn assists the interpretation of real measuring data. For 2-dimensional (2D) problem with complex geometry, no analytical solution exists, and numerical modeling is needed to solve it. The only way to eliminate the geometry effect is to fully consider the problem with geometry modeling. It is important to carry out the research of 2D forward modeling and inversion with topography either theoretically or pratically.Firstly, based on the electromagnetic propagation equations, boundary problem and variational problem, finite element method are used to modeling 2D MT problem with complex geometry. The work has been done as follows:(1) Correction scheme according the distribution of electromagnetic fields are proposed to the traditional grid generation; (2) Brief discussion are forwarded in the solver of large sparse matrix with complex entries; (3) 2D Modeling results are compared with 1D results for validity and modeling analysis are carried out for COMMEMI 2D-0 model; (4) Some important results are made after the modeling of the impact of geometries (e.g., ridges and valleys), which give theoretical help in practice.Secondly, the 2D inversion was carried out with least square method with smooth constraint, in which the calculation of sensitivity matrix and iterative solver of the inversion equations and the choice of the tradeoff parameter are analyzed. From the results, the inversion of TM and TE reflects the true geo-electrical structure and TE mode is much more sensitive to the low resistivity structure than TM mode. The hybrid inversion of both polarization data gives better interpretation for the geo-electrical structure.Lastly, the inversion was implemented for V5-2000 field data and discussed from the qualitative analysis interpretation and quantitative interpretation of inversion.