羌塘地块地壳电性各向异性特征研究

发布时间：2018-04-26 16:31

本文选题：电性各向异性 + 大地电磁　；参考：《中国地质大学》2016年博士论文

【摘要】：电性各向异性,即介质电导率随方位变化的现象,已被证实广泛存在于地球地壳和上地幔中,对观测电磁场有不可忽视的影响。大地电磁方法通过测量地球表面天然电磁场的变化量,能探测地球深部电性结构,可用于研究地下介质电性各向异性特征。本文首先使用基于非结构网格的自适应有限元法来求解对角各向异性情况下大地电磁正演控制方程,非结构三角网格的应用既有利于起伏地形和复杂地质构造的建模,又便于模拟包含多种尺寸结构的地电模型；网格的自动细化则可避免人为网格划分导致的计算误差并提高计算效率。对经典四层层状模型和二维各向异性块体模型进行了正演模拟试算,并分别与一维准解析解、二维有限差分数值解结果进行对比,验证了正演算法的准确性。接着构建了包含模型结构惩罚项和各向异性惩罚项的反演目标函数,反演算法则采取了经典模型空间Occam策略,使得模型的搜索过程非常稳定,能在较少的迭代次数内获得合理的反演结果。反演试验对象为海底起伏各向异性复杂模型,反演结果准确反映了各向异性体的存在,并还原了真实模型的主要电性特征。同时探讨了相关反演参数对反演的影响,发现反演并不依赖初始模型和初始正则化参数的选取,但结构惩罚项和各向异性惩罚项显著影响了反演过程和结果。探讨了羌塘地块的地质构造概况和主要地质和地球物理特征,分析了羌塘地块的天然气水合物发育和储存条件。对天然气水合物的音频大地电磁各向异性响应特征进行了详细讨论,证明了音频大地电磁方法能准确显示天然气水合物各向异性体空间位置和分布情况,并对天然气水合物的饱和度变化以及多个赋存岩层的存在都有所反映。结合相关测井资料和已探明的天然气水合物储层相关物性参数,对青藏高原北部祁连山木里地区的音频大地电磁实测资料进行了带先验信息约束的各向异性反演解释。反演结果反映了天然气水合物储层的各向异性特征,并说明了天然气水合物储存的有利条件。最后对羌塘地块三条南北走向的大地电磁测线进行了各向异性处理解释。反演结果表明羌塘地块地下介质电性分层明显,上地壳存在延展广的高阻层,表明印度岩石圈的俯冲前缘应该并没有越过班公湖-怒江缝合带。中下地壳普遍发育高导层,其中南、北羌塘地块下方均发育了位置相对固定的高导体,彼此被羌中隆起区的相对高阻区域分隔。南、北羌塘地块下方高导体沿东西构造走向电阻率相较于沿南北测线走向电阻率小很多。中下地壳高导体与部分熔融有关,高导体区域的各向异性特征佐证了物质向东运移的可能性。
[Abstract]:The phenomenon of electrical anisotropy, that is, the variation of dielectric conductivity with azimuth, has been proved to exist widely in the earth's crust and upper mantle, which can not be ignored in the observation of electromagnetic fields. The magnetotelluric method can be used to study the electrical anisotropy of underground media by measuring the variation of natural electromagnetic field on the surface of the earth. In this paper, an adaptive finite element method based on unstructured meshes is first used to solve the magnetotelluric forward control equations under diagonal anisotropy. The application of unstructured triangular meshes is beneficial to the modeling of undulating terrain and complex geological structures. It is also convenient to simulate the geoelectric model with various sizes, and the automatic mesh refinement can avoid the calculation error caused by artificial meshing and improve the computational efficiency. The forward simulation of the classical four layer model and the two dimensional anisotropic block model is carried out, and the results are compared with the results of one dimensional quasi-analytical solution and two dimensional finite difference numerical solution, respectively. The accuracy of the forward algorithm is verified. Then the inversion objective function including structural penalty term and anisotropic penalty term is constructed, and the classical model space Occam strategy is adopted in the inversion algorithm, which makes the searching process of the model very stable. Reasonable inversion results can be obtained in fewer iterations. The object of inversion experiment is the complex model of seafloor undulating anisotropy. The inversion results accurately reflect the existence of anisotropic bodies and restore the main electrical characteristics of real models. It is found that the inversion does not depend on the selection of initial model and initial regularization parameters, but the structural penalty term and anisotropic penalty term significantly affect the inversion process and results. The geological structure and main geological and geophysical characteristics of Qiangtang block are discussed, and the development and storage conditions of natural gas hydrate in Qiangtang block are analyzed. The characteristics of the acoustic magnetotelluric anisotropy response of gas hydrate are discussed in detail. It is proved that the audio magnetotelluric method can accurately display the spatial position and distribution of the anisotropic body of natural gas hydrate. It also reflects the change of gas hydrate saturation and the existence of several rock formations. Based on the correlation logging data and the proven physical parameters of gas hydrate reservoirs, the anisotropic inversion interpretation with prior information constraints on the audio frequency magnetotelluric field data in Muli area of Qilian Mountains in the northern Qinghai-Tibet Plateau is carried out. The inversion results reflect the anisotropic characteristics of gas hydrate reservoirs and illustrate the favorable conditions for gas hydrate storage. Finally, the anisotropic interpretation of three magnetotelluric lines with north and south strike in Qiangtang block is given. The inversion results show that the electrical stratification of the underground media in the Qiangtang block is obvious, and the upper crust has a high resistivity layer with wide extension, indicating that the subduction front of the Indian lithosphere should not have crossed the Bangonghu-Nujiang suture zone. The upper and lower crust generally developed high conductance layer, in which the south and north Qiangtang blocks developed relatively fixed high conductors, which were separated by the relatively high resistivity area of the middle and middle Qiang uplift area. The resistivity of the high conductors along the east-west strike of the Qiangtang block in the south and north is much smaller than that along the north-south line of the Qiangtang block. The high conductors in the middle and lower crust are related to partial melting, and the anisotropic characteristics of the high conductor regions confirm the possibility of eastward migration of matter.
【学位授予单位】：中国地质大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：P631.3

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