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水和其他挥发物在地球深处的分布。在行星中独一无二的是，地球表面有海洋，这些海洋是生命起源和维持的关键。据推测，在410至660公里深处的地幔岩石储存的水大约是世界海洋中的10倍，全球水循环从大气向下延伸到地幔中段。例如，地幔可以作为水的储存库，缓冲海洋，使其不受地表地质过程可能引起的变化的影响。在这个项目中，我们正在使用新的电磁地球物理方法来成像地球表面深处地幔中的电导率变化。由于地幔岩石的导电性对少量水高度敏感，这些图像将使我们能够限制地球深处的水的分布，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进化的理解，也许最终会影响生命本身。为了提高上地幔的分辨率和过渡区的电导率，我们将利用周期性海洋潮流流过地球主磁场所产生的大范围电流。我们正在使用过去30年收集的大量海底数据来估计半日和全天频带的潮汐电磁场，然后将这些数据(以及陆地地磁数据)与三维潮汐模拟结合起来，以反演地幔的电结构。海底数据库包括大量集中在太平洋海盆的电缆和点电场和磁场测量，因此这是我们研究的重点。这一努力取得成功的关键是，由于现代卫星测高和复杂的数据同化计划，我们对公海潮流的认识最近有了很大的改善。将潮汐强迫函数模型与三维电导率模型相结合，然后用测量来约束它们，是实现这一科学目标的技术途径。该项目得到了地球物理和海洋地质与地球物理计划的支持。

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.