Study On The Electrical Anisotropic Structure Of The Crust In Qiangtang Terrane

Electrical anisotropy, the property of current density in a medium being directionally dependent, which widely exists in the crust and upper mantle, has shown great influence on the observation of the electromagnetic field in earth. Magnetotellurics sounding (MT) is an electromagnetic geophysical method for inferring the earth’s subsurface electrical resistivity from measurements of natural geomagnetic and geoelectric field variation at the Earth’s surface. MT is a very effective way to detect the deep large-scale electrical structure and can show the evidence of electrical anisotropy. In this thesis the MT governing equation of electromagnetic field in 2D medium with triaxial anisotropy is firstly derived, and is then solved with an adaptive finite element method using unstructured grids. The unstructured triangle grids can easily simulate general models with not only bathymetry and topography, but also large-range scales and complex structures. The adaptive refinement can avoid the calculation errors caused by the improper man-made grid generation, and greatly improve the computational time. To ensure numerical accuracy, this adaptive refinement applying a posteriori error estimator based on dual error estimate weighting is performed iteratively to refine the grid where the element solution accuracy is not sufficient. The adaptive finite element method for forward modeling is validated by two models. One is a four-layer 1D model and the result is compared with the standard quasi-analytic solution, and the other is a 2D anisotropic block model and the result is compared with the standard finite difference solution. Both the results approximate the standard solutions and the relative errors are considerately acceptable. The anisotropic inversion strategy employs the classical model space Occam method, so that the parameters search process can be very stable and the reasonable inversion results can be achieved with few iteration steps. A complex 2d anisotropic seafloor model with bathymetry is designed to test the inversion method. The inversion results of this synthetic model successfully recover the main electrical features including anisotropy, and the forward response of the final inversion model matches with the synthetic observed data. The results of these synthetic models above have demonstrated the validity of the algorithms of the MT forward modeling and inversion with electrical anisotropy. The influences of different parameters on this inversion method is also discussed. It’s found that the inversion doesn’t rely on the values of the initial model and regularization parameter, but the inversion processing and results are sensitive to the structure penalty and the anisotropy penalty.Then the geological structure and the main geological and geophysical characteristics of the Qiangtang block are discussed. The developments of natural gas hydrate(NGH) and storage conditions in Qiangtang are analyzed. We also make a detailed discussion on the Audio Magnetotellurics(AMT) response characteristics of the anisotropic NGH reservoir, which proves that the AMT method can accurately display the anisotropic spatial location and distribution of NGH reservoir, and it also reflects the NGH saturation change and the exist of the multiple occurrence of rocks. Combined with the existing logging data and the physical parameters of the discovered gas hydrate reservoir, the priori information constraint inversion is conducted. Then the anisotropic inversion for the AMT measured data of Muli district in Qilian mountain, northern Tibetan Plateau is also presented. The inversion results reflect the anisotropic characteristics of the gas hydrate reservoir, and explain the advantages of natural gas hydrate storage.Finally, the anisotropic inversion for the three south-north trending MT survey lines in Qiangtang block is conducted. The inversion results show that the underground dielectric stratification is obvious, and the high resistivity layer with wide extension exists in the upper crust, which indicates that the subducted leading edge of the India’s lithosphere has not crossed the Bangong-Nujiang suture zone. The high conductive layer is developed widespread in the lower crust, and the high conductors with relatively fixed position are developed below the North and South Qiangtang block, which is separated from each other by a relatively high resistance region in central Qiangtang uplift area. The resistivity of high conductor which along the east-west structural trend is smaller than that which along the south-north structural trend; the high conductor in the middle-lower crust is related to the partial melting, in addition, its anisotropic characteristics indicate the possibility of substance’s eastward migration.