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

该项目的最终科学目标是澄清地球内部深处水和其他挥发物的分布。在所有行星中，地球是独一无二的，其表面拥有海洋，这对于生命的起源和支持至关重要。据推测，410至660公里深处的地幔岩石储存的水量约为世界海洋的10倍，并且全球水循环从大气层一直延伸到中地幔。 例如，地幔可以充当水库，缓冲海洋免受表面地质过程可能引起的变化的影响。在这个项目中，我们正在使用新颖的电磁地球物理方法对地球表面深处地幔的电导率变化进行成像。 由于地幔岩石的电导率对即使是少量的水也高度敏感，这些图像将使我们能够限制地球深处水的分布，并提高我们对地球深层水循环的理解。 这项研究的结果有可能影响我们对海洋演化的理解，或许最终影响我们对生命本身的理解。为了提高上地幔和过渡区电导率的分辨率，我们将利用周期性海洋潮汐流穿过地球主磁场所感应的大规模电流。我们使用过去 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（以及陆地地磁数据）与 3D 潮汐模拟相结合，以反演地幔电结构。海底数据库由电缆和点电场和磁场测量组成，这些测量主要集中在太平洋盆地，因此是我们研究的重点。这项工作成功的关键是，由于现代卫星测高和复杂的数据同化方案，我们对公海潮汐流的了解最近有了很大的进步。将潮汐强迫函数模型与 3D 电导率模型相结合，然后通过测量对其进行约束，是实现科学目标的技术方法。该项目得到了地球物理学和海洋地质与地球物理学计划的支持。

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.