大地电磁场源效应与考虑地球曲率的二三维正反演研究

Based on various assumptions MT source model, such as the plane wave, high altitude line current source, high surface current source and outer space ring current source, etc., the magnetotelluric response will be simulated to have an analysis of the field source effect characteristics of magnetotelluric sounding. We will try to establish the influence degree of the effect of magnetotelluric sounding field source and discriminant rules at first. Then, by setting up a layer of one-dimensional ball shape model, the two-dimensional cylindrical model and three-dimensional spherical model for magnetotelluric sounding, we will complete the numerical simulation of the electromagnetic wave equation under different coordinate system, and accomplish the analysis of the effect mechanism of earth curvature of magnetotelluric sounding and the condition and pattern of the influence. Third, through the research of magnetotelluric sounding inversion algorithm of two-dimensional cylindrical coordinates and three-dimensional spherical coordinates system, we will develop a 2Dand 3D inversion computing platform to consider the effect of electromagnetic field source and the influence of curvature of the earth. At last, using the Geomagnetic Depth Sounding (GDS) stability and low frequency magnetic field to detect the characteristics of the deep, to overcome the drift problem of long period the earth electric field, a joint inversion of magnetotelluric and GDS will be used to establish a practical inversion software system, which will be helpful for integrated interpretation method to support a large-scale cross-regional deep electrical structure detection theory and method including considering the effects of the electromagnetic field source effect and the curvature of the earth.

通过多种假设场源模型（如平面波、高空线电流源、高空面电流源和高空环电流源等）的分析，实现大地电磁测深场源响应模拟，研究大地电磁测深场源效应的机理与特征，分析场源效应对大地电磁测深数据的影响程度和范围。通过设立大地电磁测深一维球体层状模型、二维柱状模型和三维球状模型，研究不同坐标系的大地电磁场二三维正演数值模拟方法，模拟地球曲率对大地电磁场的影响机理，分析地球曲率影响大地电磁测深的条件和形式。开展二维柱坐标和三维球坐标体系下的大地电磁测深反演算法研究，实现考虑大地电磁场源效应与地球曲率影响的二三维反演计算平台。同时，利用地磁测深的低频磁场稳定和探测深度大的特点，研究大地电磁和地磁测深联合反演方案，既有利于克服长周期大地电场的漂移问题，又能够辅助深部电性结构反演解释。通过建立考虑大地电磁场源效应和地球曲率影响的正反演综合解释方法体系，为大尺度跨区域深部电性结构探测提供理论和方法技术支撑。

针对地球介质在高纬度地区和大尺度范围条件下会存在不满足传统大地电磁测深理论的平面波垂直入射和地球水平介质假设的情况，对在高纬度区域和赤道附近可能出现的线电流源、片电流源以及波数场源的大地电磁响应特征进行分析和总结，在此基础上提出了频域截断校正法、归一化校正法、测点平均校正法、考虑场源效应的大地电磁反演、时间域延长校正法、时间域剔除法校正法等几种适用于场源效应校正的方法。同时基于有限差分理论和有限内存拟牛顿法(L-BFGS)开发了球、柱坐标系下大地电磁二三维正演算法和考虑地球曲率的大地电磁二三维反演程序，并对比分析了传统的直角坐标系和球坐标系下二三维正反演的差异情况。为了提高大地电磁场正反演模拟的精度和效率以适应复杂地质构造情况，研究了基于二次场方法的并行三维大地电磁正反演、基于矢量有限单元方法的三维大地电磁正反演和基于自适应有限元正演的大地电磁三维反演算法。全面收集整理了青藏高原东缘长剖面大地电磁资料，总结了青藏高原东缘地壳上地幔大地电磁探测工作程度，系统地对青藏高原东缘地壳上地幔电性结构、与扬子西缘接触关系、汶川地震和芦山地震的电性孕震环境及弱物质流通道等方面进行了梳理和分析，并选取长剖面开展考虑地球曲率的反演解释，得到研究区可靠的电性结构。