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Research On Inversion Method Of Magnetotelluric Static-Shifted Data

Posted on:2017-04-13Degree:DoctorType:Dissertation
Country:ChinaCandidate:J ZhouFull Text:PDF
GTID:1220330488963378Subject:Solid Earth Physics
Abstract/Summary:PDF Full Text Request
In Magnetotelluric sounding(MT), the range of observation signal spectrum is wide, its exploration depth is from dozens of meters to hundreds of kilometers, and is widely used in all fields, including deep geological structure, oil and gas resources survey, deep mineral exploration and engineering survey, which becomes the most widely electromagnetic prospecting method in recent years. With the development of large data sets of three-dimensional MT detection work, this marks that the high-precision and three-dimensional data processing technology are carried on the agenda of magnetotelluric exploration. At the same time, with the rapid development of computer and other related disciplines, MT data processing and interpretation technology that represented by three dimensional forward and inversion technology have also got rapid development. However, practical work in the technical requirements of processing and interpretation not only embodies in three-dimensional, also puts forward higher requirements for large data sets inversion calculation efficiency and accuracy of interpretation model, which becomes the technology bottleneck restricting magnetotelluric exploration and actual effect. Among them, in complex geological and geophysical models, static shift that caused by the surface uneven electrical body has become one of the main technical problems in the MT data processing and interpretation. For the problem of magnetotelluric static shift, and the technical defects of current mainstream correction methods represented by spatial filtering method, we put forward to use the characteristics of impedance and phase that is smaller influenced by the static shift. Based on the technology of three-dimensional forward and inversion, the study of direct inversion method considering MT static shift, we can provide a more reasonable and objective technological means for processing static shift problem. Above mentioned study, for improving the application effect of MT method in practical exploration work, it has positive significance.The forward problem is the core content of the theoretical study of magnetotelluric method. Based on Maxwell’s equations as control equations, this paper analyzes the boundary conditions of three-dimensional MT problems, and completes the study of three-dimensional magnetotelluric boundary value problems. Carried out a research on the principle of Galerkin method, combining with three-dimensional magnetotelluric boundary value problem, we complete the derivation of three-dimensional magnetotelluric weighted margin equation. According to the characteristics of existing mesh generation method, hexahedral element subdivision is chosen as the model of discretization method, realize the mesh generation of the study area, the introduction of vector and the shape function in combination with Galerkin equation of the three-dimensional magnetotelluric, thus complete the unit vector finite element analysis, and get the stiffness matrix in the unit. Because the three-dimensional MT model after discretization has a large number of edges(or nodes), so in procession of the whole region stiffness matrix accumulation, saving computation and storage space must be considered. Based on the above technical considerations, the paper analyzes the superposition law of element stiffness matrix, completes stiffness matrix under the compression condition using line superposition calculation, and imposing by line standard compressed storage(CSR). On the basis of the overall stiffness matrix assembly, realize BICGSTAB based on incomplete LU decomposition pre conditions. After the above system of linear equations solver completes home value solving, using the finite difference method realizes the calculation of the auxiliary field. On this basis, the calculation formula of magnetotelluric response function under the condition of three dimensional medium is deduced, including the impedance tensor, apparent resistivity and impedance phase, tipper amplitude and phase, and finally complete the forward modeling program of three-dimensional magnetotelluric vector finite element. Using three-dimensional forward modeling program calculates one-dimensional horizontal layered model and three dimensional location model of MT research. Comparing simulation results of the above models with the analytical solution or simulation results that has been accepted and published, it shows that in this paper, the research of the three-dimensional MT vector finite element forward modeling calculation result is correct, and has a good precision.Research on magnetotelluric static shift principle can help understand change rules and characteristics of magnetotelluric response function under the influence of the static shift effect. From the relationship between MT electric and magnetic fields and the transfer function(impedance tensor) can be seen, the study of static shift mechanism under the condition of 2d and 3d MT, for understanding the impact characteristics of MT static shift effect on the observation data, which is a great significance. On this basis, for static shift distortion of the theoretical model, we do the 2d and 3d numerical simulation, and deeply analyze on response characteristics of the magnetotelluric apparent resistivity and impedance phase in static effect. The MT static shift theory research and numerical simulation results show that when the regional electrical structure characteristics of MT data dimension reduced to a certain extent, the influence of the magnetotelluric impedance phase has the characteristics of static shift influence. It is put forward based on the MT static shift impedance phase estimation and inverse that establishes a theoretical foundation.Considering the static shift of magnetotelluric inversion method research, including two parts content of static shift estimation method and shift estimation value in the inversion application strategy, both two can connect with model constraint by high dimensional inversion. In this thesis, first of all, using this feature that the impedance phase is smaller influenced by static shift, builds estimated objective function. According to the impact characteristics of static shift for MT inversion(mainly shows in the vertical electrical migration of inversion interpretation that has strong multiple solution, and other regions inversion are relatively reliable), this is put forward in the shift estimation model based on the conventional inversion model. Based on the technology of three-dimensional magnetotelluric forward, adopting the method of one dimensional linear search can realize fast estimation method of magnetotelluric static shift. Shift estimation value is applied into initial model correction of the inversion(usually obtained by lower dimensional inversion), under the constraints of the model using the measured data in two or three dimensional magnetotelluric inversion, can guarantee the inversion model of deep structure, on the basis of objective and reliable implementation of the model, the structure of the shallow quick-correction, eventually for magnetotelluric data explanation provides more reliable inversion interpretation model. Using the research results of theoretical model and actual work checks calculation for AMT data, calculation results show that static shift estimation results from static shift speed calculating of the proposed method are more objective and accurate. It is applied in inversion model constraint, based on the above model, for the original data inversion, which can be a good approximation to the real electrical structure of the model.
Keywords/Search Tags:Magnetotelluric Sounding, Static Shift, Three-dimensional Modeling, Edge-based Finite Element Method, Inversion Method
PDF Full Text Request
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