海洋目标及环境极低频电磁特征与传播机理研究

Nowadays, security issues such as foreign military investigations, port invasions, illegal immigration and plundering of marine resources are becoming increasingly prominent. Underwater targets are major threats to maritime security. In complex marine environment, how to quickly and efficiently monitor and identify underwater targets is of great significance for marine safety..During the underwater targets’ navigation, the corrosion current and cathodic protection current are modulated by the propeller, generating extremely low frequency (ELF) electromagnetic signal, which is equivalent to horizontal electric dipole source. It is an important physical property for identifying, locating and tracking underwater targets. The ELF electromagnetic wave is mainly diffused in seawater, and the wave field is dominant in the air. The propagation characteristics across the medium are still unclear. Large-scale spatiotemporal motion of seawater can generate complex electromagnetic noise fields by cutting the geomagnetic field, resulting in great interference to ELF signal detection and feature extraction..This project employs the horizontal electric dipole buried in seawater as the radiation source. The ELF electromagnetic wave cross-medium and rough surface propagation model is established. The propagation characteristics of ELF electromagnetic signals are studied. The electromagnetic response model of large-scale space-time motion of ocean medium is established to study the generation, propagation and distribution characteristics of ocean electromagnetic noise. The works lay the theoretical foundation for non-sound detection of underwater targets.

当今，外来军事侦查、港口入侵、非法入境和海洋资源掠夺等安全问题日益突出，水下目标是我国海洋安全的重大威胁。在复杂的海洋环境下，如何快速、高效地监测、识别水下入侵目标体，对于我国海洋安全具有重要意义。.水下目标在航行过程中，腐蚀电流及阴极保护电流受到螺旋桨调制能够产生极低频（ELF）电磁信号，等效为水平电偶极子源，是识别、定位、追踪水下目标的重要物理属性。ELF电磁波在海水中以扩散场为主，在空气中以波动场为主，跨介质传播特性尚不清楚。海水介质在地磁场中的大尺度时空运动能够产生复杂的电磁噪声，对ELF信号检测和特征提取造成极大的干扰。.本项目以海水中的水平电偶极子为辐射源，建立ELF电磁波跨粗糙分界面传播模型，研究ELF电磁信号跨介质传播特性；建立海洋介质大尺度时空运动电磁响应模型，研究海洋电磁噪声产生机理、传播规律和分布特征，为水下目标非声探测奠定理论基础。

当今，外来军事侦查、港口入侵和等安全问题日益突出。在复杂的海洋环境下，快速、高效地监测、识别水下入侵目标体对于海洋安全具有重要意义。. 水下目标在航行过程中，腐蚀电流及阴极保护电流受到螺旋桨调制产生极低频信号。本项目以水平电偶极子为辐射源，建立了有限海深/无限海深条件下极低频电磁波跨介质传播模型。按照等效表面阻抗理论，粗糙分界面引起的增量阻抗很小，可忽略对极低频电磁波传播的影响。在分界面上，磁场按指数衰减，由于多径干涉相消效应的影响，衰减曲线存在拐点。. 本项目建立了基于Stokes波浪方程的海浪运动电磁响应模型。该海浪磁场模型考虑海水深度的影响，当海水深度较浅时，较Weaver模型更加精确。海浪磁场强度与海浪振幅、地磁场成正比。随着垂直距离增加，海浪磁场幅度按指数衰减。当海水深度＞150m时，该海浪磁场模型与Weaver模型的预测结果一致。海浪磁场频谱与海水运动速度相关，在频域中呈一定带宽的分布状态，与水下目标极低频信号的频率特征不同。. 本项目建立了基于球谐函数的海流速度模型、地磁场模型及海流磁场模型。海流磁场的分布与海流速度、地磁场分布等相关。大尺度的海洋活动产生准静态磁场，但幅度远小于地磁场的幅度。海流磁场的周期长，与水下目标极低频信号的频率范围不同。海流磁场在空间上衰减较慢，目标信号衰减较快，因此可以利用不同空间位置处背景噪声的相关性实现噪声抑制，提升目标检测能力。. 本项目取得的理论成果在复杂海洋噪声中提取微弱目标信号具有重要的科学意义和重大的应用前景，可为海洋可控源电磁勘探、海洋背景磁场测量、水下磁通信等领域的发展提供理论支撑。

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