Simultaneous inversion for mantle conductivity and source field: towards a 3-d global reference model

尽管地幔电导率主要是随着深度的增加而径向增加，但越来越多的证据表明，上地幔和中地幔存在显著的横向非均质性，反映了组成、物理状态、温度和挥发分含量的变化。传导性是对地震密度和弹性参数的补充；通过共同配准和联合解释这些参数，可以更好地约束地球内部的组成和物理状态。该项目的目标是开发这些3D变化的显著改进的图像，特别是在过渡带和上地幔。该项目将数据分析的新方法与数值模拟和地球物理逆方法相结合。将经验正交函数方法应用于地磁观测站数据，以提取地球传导地幔在日变化和较长周期内的外部激发的主要模式。然后，通过同时调整外部源结构和三维地球电导率，对对应于最对称的大尺度源的这些模式进行拟合。这种联合反演的一个关键约束是物理上合理的外部震源空间结构的先验知识。这种结构是通过使用参数模型并通过根据源几何知识和电离层一般环流模型的输出而形成的多变量空间协方差来实施的。联合注册的电学和弹性模型将有助于地球动力学家在三维层析图像中分离更好的温度和成分效应，并有助于矿物物理学家建立与地球深部物质有关的状态方程。这项研究还将改进外部源场的特征，它与三维电导率模型一起，将对处理卫星磁数据的人非常有用，例如，为主场或地壳场的建模提供改进的外部/感应场校正。目前和计划中利用磁性卫星数据探测地幔传导性的工作主要取决于改进的外部来源时空模拟，并最终将从这一项目的结果中受益匪浅。该项目通过为职业生涯早期的女科学家提供博士后支持，促进了下一代地球科学研究人员的多样性和培训。目前正在制作一个全球三维电导率参考模型，供广大地球科学研究人员利用，例如提供边界条件和大尺度背景，以便对区域大地电磁和地磁感应研究的数据进行建模和反演，包括地球示波器骨干和可移动大地电磁测深阵列。

Although mantle electrical conductivity primarily increases radially with depth, a growing body of evidence indicates there is significant lateral heterogeneity in the upper and mid-mantle, reflecting variations in composition, physical state, temperature, and volatile content. Conductivity complements seismically-derived density and elastic parameters; through co-registration and joint interpretation of these parameters, one can obtain better constraints on the composition and physical state of Earth's interior. The goal of this project is to develop significantly improved images of these 3D variations, especially in the transition zone and upper mantle. The project combines new approaches to data analysis with numerical modeling and geophysical inverse methods. An empirical orthogonal function approach is being applied to geomagnetic observatory data to extract the dominant modes of external excitation of the Earth's conducting mantle at daily variation and longer periods. These modes, which correspond to the most symmetric large scale sources, are then fit by simultaneously adjusting external source structure and 3-d Earth conductivity. A key constraint in this joint inversion is a priori knowledge of physically reasonable external source spatial structure. Such structure is enforced through use of parametric models, and through multivariate spatial covariances developed from knowledge of source geometries and outputs of ionospheric general circulation models. Co-registered electrical and elastic models will be useful to geodynamicists, to separate better temperature and compositional effects in 3-d tomographic images, and by mineral physicists developing equations of state relevant to deep Earth materials. The research will also result in an improved characterization of external source fields, which, together with the 3D conductivity model, will be very useful to those working with satellite magnetic data, e.g., to provide improved external/induced field corrections for modeling of the main or crustal fields. Current and planned efforts to use magnetic satellite data to probe mantle conductivity in particular depend critically on improved spatio-temporal modeling of external sources, and will ultimately benefit significantly from the results of this project. The project contributes to diversity and training of the next generation of Earth science researchers by providing post-doc support for an early-career female scientist. A global 3-d conductivity reference model is being produced that will be available as a resource to a broad community of Earth Science researchers, e.g., to provide boundary conditions, as well as large scale context, for modeling and inversion of data from regional magnetotelluric and geomagnetic induction studies, including the EarthScope backbone and transportable MT arrays.