海洋潮汐电磁信号全球与区域尺度上的三维正演模拟研究

The motion of seawater across the earth’s magnetic field generates electric currents within the ocean hence induces magnetic fields. Among other types of electromagnetic fields related to ocean motion, the oceanic tidal electromagnetic signal is of particular interest to geophysicist since it can indicate the electrical properties of lithosphere and upper mantle. The signal is becoming a strong supplementary to traditional marine magnetotellurics, not only because the electromagnetic fields could be observed by satellite, on-shore and ocean bottom stations, but also due to the high sensitivity of toroidal magnetic mode to the subterraneous structure. Current studies, which mostly focus on satellite observations and 1-D layered global model, have not considered the 3-D effect of deep electrical structure, and seldom discuss high-resolution regional cases of ocean bottom records in detail. This program aims to develop the 3-D forward modelling method of oceanic tidal electromagnetic fields by finite difference method in spherical coordinate, in global and regional model respectively. The sensitivity test to electrical structure of oceanic lithosphere, upper mantle and subduction zone, especially 3-D resistivity anomalies, would be designed base on synthetic models. At last, the numerical simulations of M2 electromagnetic field records of several ocean bottom station in northwestern Pacific basin would be implemented. With great progress in satellite and deep ocean geo-electromagnetic observation techniques， 3-D numerical simulation of oceanic tidal electromagnetic field would be important to analysis and explain the massive electromagnetic data. What's more, an efficient and stable forward modeling method is the crucial procedure of joint inversion with magnetotellurics to constrain the electric structure of oceanic lithosphere and upper mantle.

海洋潮汐电磁场是当今地球电磁学国际前沿科研方向，这种由海洋潮汐运动引发的电磁信号携带了周围介质的电性参数信息，能被卫星和海底及海岸台站等多种观测手段所记录。海洋潮汐电磁信号具有环向模式，在海洋深部结构探测上是传统海洋大地电磁法的有力补充。现有工作大多基于一维层状地球模型研究卫星观测资料，忽略海洋岩石圈及上地幔三维精细结构的影响，对海底台站记录及高精度区域模拟的关注度较低。本项目计划利用球坐标系下的有限差分法分别在全球和区域尺度上完成对海洋潮汐电磁信号的三维正演模拟计算，并结合数值模拟结果探讨该信号对海洋岩石圈及上地幔电性结构的敏感度，最终完成对太平洋盆地海底台站实测M2潮电磁信号的模拟。我国卫星与海底电磁观测技术发展极为迅速，精确高效的三维正演算法有助于解释观测资料，也是后期利用此信号反演海洋岩石圈及上地幔复杂电性结构的关键步骤。

海洋潮汐电磁信号由海洋潮汐运动引起，能被卫星以及海底电磁台站所记录，并为地球深部电性结构提供新约束。海底电磁数据包含Toroidal模式磁场，对海洋岩石圈结构非常敏感，但前人研究多集中在卫星数据和一维地球模型，且缺乏对物理机制和信号敏感度的详细探讨。本项目在这一背景下利用三维交错网格有限差分法实现了该信号全球和区域尺度上的正演。项目组从半解析解和三维数值模拟两方面探究了海洋潮汐电磁信号的卫星观测和海底观测对海洋岩石圈和其下软流圈电性结构的响应，结果表明海底水平磁场的振幅对岩石圈横向电阻非常敏感，其相位包含了软流圈电阻率信息，而卫星磁场数据主要受到软流圈电阻率的影响，尤其是在印度洋海域上方。此外，项目组还发现了潮汐运动类型和电磁信号模式之间的关系：梯度场类型的海水运动产生Poloidal型磁场，而漩涡流产生Toroidal型磁场。最终，项目组拟合了北东太平洋(41°N, 165°W)的十二个海底电磁台的实测信号，最佳模型表明研究区域岩石圈的横向电阻约为7.0\times{10}^8\ \Omega m^2。项目在三维正演方法、信号物理机制以及应用前景方面上的研究为海洋潮汐电磁实测数据的三维反演打下解释结实基础。

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