Collaborative Research: Imaging Electrical Conductivity in the Upper Mantle with Ocean Tidal Source Fields

这个项目的最终科学目标是澄清地球深处水和其他挥发物的分布。在所有行星中，独一无二的是，地球表面有海洋，这是生命起源和生存的关键。据推测，410至660公里深处的地幔岩石储水量大约是世界海洋储水量的10倍，而且全球水循环从大气一直延伸到地幔中部。例如，地球的地幔可以作为水的储藏库，缓冲海洋可能由地表地质过程引起的变化。在这个项目中，我们正在使用新的电磁地球物理方法来成像地球表面深处地幔的电导率变化。因为地幔岩石的导电性对即使是少量的水也非常敏感，这些图像将使我们能够限制地球深部水的分布，并提高我们对地球深部水循环的理解。这项研究的结果有可能影响我们对海洋进化的理解，甚至可能最终影响生命本身。为了提高上地幔和过渡带电导率的分辨率，我们将利用周期性海洋潮汐流通过地球主磁场引起的大尺度电流。我们利用过去30年收集的大量海底数据来估计半日和日波段的潮汐电磁场，然后将这些数据（连同地球地磁数据）与三维潮汐模拟相结合，反演地幔电性结构。海底数据库包括电缆和点电场和磁场测量数据，这些数据主要集中在太平洋盆地，因此这是我们研究的重点。这项工作取得成功的关键是，由于现代卫星测高和复杂的数据同化方案，我们最近对公海潮流的认识有了很大的提高。将潮汐强迫函数模型与三维电导率模型相结合，然后用测量结果约束它们，是实现科学目标的技术途径。该项目由地球物理和海洋地质与地球物理项目支持。

The ultimate science objective of this project is clarification of the distribution of water and other volatiles in the deep interior of the Earth. Uniquely among the planets, Earth has oceans at its surface that are key to the origin and support of life. It has been postulated that mantle rocks at depths between 410 and 660 km store about 10 times as much water as is in the world oceans, and that the global water cycle extends from the atmosphere down to the mid-mantle. For example, Earth's mantle may serve as a reservoir for water, buffering the oceans from variations that might be induced by surface geological processes. In this project we are using novel electromagnetic geophysical methods to image electrical conductivity variations in the mantle deep beneath Earth's surface. Because the conductivity of mantle rocks is highly sensitive to even small amounts of water, these images will allow us to constrain the distribution of water in the deep Earth, and improve our understanding of deep Earth water cycles. Results of this research have the potential to impact our understanding of the evolution of the oceans, and perhaps ultimately life itself. To improve resolution of upper mantle and transition zone electrical conductivity we will exploit large scale electric currents induced by the flow of periodic ocean tidal currents through the Earth's main magnetic field. We are using an extensive base of seafloor data collected over the past 30 years to estimate the tidal electromagnetic field in the semidiurnal and diurnal bands, and then combining these (along with terrestrial geomagnetic data) with a 3D tidal simulation to invert for mantle electrical structure. The seafloor database consists of both cable and point electric and magnetic field measurements that are heavily concentrated in the Pacific basin, which is thus the focus of our study. Key to the success of the effort is the recent great improvement in our knowledge of open-ocean tidal currents that has resulted from modern satellite altimetry, and sophisticated data assimilation schemes. Combination of models of the tidal forcing function with 3D electrical conductivity models, and then constraining them with measurements, is the technical approach that will lead to the science objective.This project is supported by the Geophysics and Marine Geology & Geophysics Programs.