Pseudo-Three-Dimensional Magnetotelluric Inversion

Introduced by Tikhonov and Cagniard in 1950s, Magnetotelluric (MT) is a geophysical technique that uses measurements of these naturally-occurring time-varying electromagnetic fields on the Earth's surface to infer the electrical resistivity structure of the subsurface. It is a necessary method in deep-Earth studies. MT method has been widely used at home and abroad in geothermal prospecting, mining exploration, studies of crust and upper-mantle, marine geophysics, environmental geophysics and geological engineering, etc. Especially in petroleum and gas explorations which are connected with the national economy and the people's livelihood, MT plays a crucial role. Interpretation methods for MT largely involve the use of one-dimensional and two-dimensional modeling and inversion algorithms currently. However, the real geoelectric structure is very complex, which is difficult to satisfy the hypothesis for the underground structure to be 1D or 2D. We couldn't get reliable geoelectric model through one-dimensional or two-dimensional interpretation for real datasets. It is urgent to develop the technique of three-dimensional MT inversion algorithm to obtain reliable geoelectric model for petroleum and gas explorations and predictions. The work of this dissertation is on these topics. This paper consists mainly of five parts as follow.The first part reviews the study status of three-dimensional MT inversion briefly, introduces the developed methods of three-dimensional MT inversion, such as conjugate gradient maximum likelihood inversion, nonlinear conjugate gradient inversion, quasi-linear approximation inversion, rapid relaxation inversion, Bayesian statistics inversion and artificial neural network inversion, etc.The second part gives the basic theoretical formulas of 3D MT staggered-grid finite difference method. The validity and feasibility are proved by comparing the 3D forward modeling results with analytic solution of TM mode to abutting quarter-spaces, and with 2D forward modeling results to 2D prism using 2D finite element method.The third part discusses the feasibility of interpreting 3D MT data using 2D inversion. We give the two-dimensional reduced basis OCCAM'S inversion (REBOCC) results of 3D/1D and 3D/2D, and compare the magnetotelluric apparent resistivity and phase responses between 3D prism conductor and 2D prism conductor geological models, find that the responses are very similar between 3D YX-mode and 2D TM-mode. The location of conductor could be determined...