The Study On The Deep Electric Conductivity Structure Along The Profile From Sonid Zuoqi, Inner Mongolia To Dandong, Liaoning Province

The subduction zone in the Western Pacific plate is the most typical, having the largest length of the subducted slab in the world, where intensive interaction occurs between the Pacific Plate, the Philippines sea plate and the Eurasian Plate. It poses great impact on the plate margins as well as dynamic processes, stress fields and tectonic motions in the Eurasian continent. One of the signatures of this slab is the abnormal high-velocity zone at the mantle transition zone (410-660km in depth) beneath Northeast China revealed by seismic tomography, where it looks like a horizontal"tongue", and seems to be stagnant. Consequently, this area becomes a focused target in the research of mantle geodynamics and global tectonics.Northeast China and North China lie in the back-arc area with earthquakes and Quaternary volcanoes. Their geological structures, basin formation and evolution, and distribution of solid mineral and oil-gas resources are closely related to the West Pacific subduction zone. Therefore,the study on the deep electric conductivity structure of the Northeast in china is very important for understanding of the dynamics of these regions.In order to identify the structural features of the lithosphere-mantle transition zone and the deep coupling between the tectonic blocks at depth in Northeast and North China, a magnetotelluric (MT) profile is designed with the length about 1150km along Sunid Zuoqi to Dandong for long-period and ultralong-period MT sounding.This thesis focuses on the deep electrical structure of the profile from Sunid Zuoqi to Dandong by using normal MT data and long-period MT data. The advanced data processing and analysis technology of MT are employed and the qualitative and quantitative data from field surveys are analyzed to acquire the reliable electrical structure in crust and upper mantle. Through the data inversion using the 2D Nonlinear Conjugate Gradient (NLCG) method, a detailed electric model is yielded. At last, the interpretation of the electrical model is performed considering the geology and other geophysical data. The main contents and results are as follows:1. On foundation and application of MT theoryThe MT theory is briefly presented and the equations of apparent resistivity and phase are deduced from Maxwell equations based on the two hypothesis in MT. Also the main features of 2D electromagnetic sounding are introduced and the discussion of 2D electrical structure is conducted. The TM and TE mode are finally discussed.2. Overview of data processing and inversion of MT dataThe flow of data processing in MT is introduced including spectral analysis to the electric and magnetic fields in the time domain, yielding the corresponding spectral information. Then we compute the impedance tensor elements in survey direction, determine the electrical main-spindle of underground medium and its apparent resistivity.The advanced method of data processing has been discussed including Robust remote reference technology in estimating impedance tensors, and Swift, Bahr and GB impedance tensor decomposition technology. The comparison of 2D normal inversion methods (OCCAM, RRI and NLCG) is made at the same time.3. Acquisition and analysis of data from the Sunid Zuoqi–Dandong profileThe survey plan is carried out through field survey and consideration of other information. And acquisition of the field data follows strictly the standard codes for field measurement. The original data in the time series of 48 MT sites including 12 long-period sites and 36 broadband sites on the profile is processed by the SSMT2000 software with Robust remote reference function and MTEDITOR software to edit power spectrum. Finally we obtain the date of apparent resistivity and impedance phase. The primary lateral changes of the electrical conductivity in the target zone are analysed based on the resistivity and phase date. The dimensional feature of the structure is determined by skewness of the target with the Swift method. Meanwhile, We analyze the azimuth of the main axis, and combining the fault's strike of the region, It is concluded that the strike of the regional electric structure is roughly perpendicular to the survey line. So the data is rotated directly to the survey orientation. In this case the XY mode corresponds to the TE mode, while the YX mode corresponds to the TM mode. The 2D NLCG method is chosen for inversion and interpretation.4. Choice of parameters in 2D inversionIn this thesis, three issues on 2D NLCG inversion are discussed separately: varied polarization modes of data inversion, choice of the regularization factors and the t value of floor errors. The analysis of regularization factor L's curves lead to the best factor through inversions of different regularization factors, and We choose the best threshold value of errors in the same way.The 2D inversions by using of TE mode, TM mode,TE and TM joint mode are conducted, respectively. It was found that the TM mode is sensitive to the shallow structure, while the TE mode is sensitive to the deep structure. And TM mode is stable to the 3D low resistivity target, while the TE mode is stable to the 3D high-resistivity target. Finally,the TM mode to fulfill the inversion of the profile was choosed.5. Inversion of the Sunid Zuoqi -Dandong profile with topographyThe 2D initial model with topography is produced using Automatic Generating Technique for the grid of central measurement points and TM mode. The regularization factor is set to be 50, and the threshold value of errors in TM mode is 2%. The ultimate model of the target zone is obtained after 116 iteration. And the RMS value is 4.41.6. Interpretation of the Sunid Zuoqi–Dandong profileBased on the final inversion model of the target profile, I primarily determined the location of main faults and boundaries and their extension to depth. At the same time, the location of the low-resistivity layer in crust and the depth of the high-resistivity layer in upper mantle are inferred from the result. The distribution of low resistivity layers in crust and the relief of high resistivity layers in upper mantle may reflect the property and evolution history of tectonics. The deep electrical structure derived from this work may provide evidence to elucidate the influence of the subduction zone in the Western Pacific plate on Northeast and North China.