Characterization and Application of Underwater Extremely Low Frequency Magnetic Fields
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摘要: 极低频水下磁场主要源于舰船桨轴旋转等因素, 在海洋环境中具有线谱特征明显、传播距离远、与船舶运动特征关联性强的特点, 可作为非声探测的特征信源。研究极低频水下磁场模型、传播与空间分布特性, 对推动极低频磁场探测应用具有重要价值。轴频磁场是极低频水下磁场的重要组成部分, 文中重点分析了轴频磁场产生机理, 建立了腐蚀电流内调制轴频磁场时谐偶极子模型, 仿真计算了轴频磁场在海洋环境中的传播特性和空间分布特性, 初步给出了轴频磁场衰减规律, 通过可控模拟源海上试验对建立的偶极子模型进行了验证。研究结果表明: 水平时谐电偶极子产生的轴频磁场在海底平面上横向分量能量最大, 垂直分量次之, 纵向分量最小, 同时轴频磁场具有一定的指向性分布, 综合利用水平分量和垂直分量进行探测可有效弥补单一分量探测盲区; 浅水环境中水平时谐电偶极子产生的轴频磁场存在多路径传播特性, 在近区以直达波和反射波为主, 在远区则以海面直达波为主; 轴频磁场频率特性与船舶的转速、桨叶数相关联, 可作为目标辨识的有效特征。上述结论可为极低频磁场探测识别提供支撑。Abstract: Underwater extremely low frequency magnetic field mainly comes from the rotation of the ship’s stern shaft and has obvious line spectrum characteristics, long propagation distance, and strong correlation with the ship’s motion in marine environments, which can be used as a feature source of non-acoustic detection. The study of underwater extremely low frequency magnetic field models, propagation, and spatial distribution properties is of great value in promoting applications of extremely low frequency magnetic field detection. Shaft-rate magnetic field is an important component of underwater extremely low frequency magnetic field. In this paper, the generation mechanism of the shaft-rate magnetic field was analyzed, and the time-harmonic dipole model of the shaft-rate magnetic field generated by the corrosion current under the effect of internal modulation was established. The propagation and spatial distribution characteristics of the shaft-rate magnetic field in the marine environment were simulated, and the decay law of the shaft-rate magnetic field was initially given. The dipole model was validated by the controlled simulation source sea test. The results show that the shaft-rate magnetic field generated by the horizontal time-harmonic electric dipole has the largest energy in the horizontal component, followed by the vertical component, and the energy in the longitudinal component is the smallest. At the same time, the shaft-rate magnetic field has a certain directional distribution, and the detection utilizing the horizontal and vertical components can effectively make up the detection blind area by a single component. The multi-path propagation characteristics of the shaft-rate magnetic field generated by the horizontal time-harmonic electric dipole in the shallow water environment exist, and they are dominated by direct and reflected waves in the near region and by direct waves from the sea surface in the far region. The frequency characteristics of the shaft-rate magnetic field are correlated with the ship’s speed and number of blades, which can be used as an effective feature for target identification. The above conclusions can provide support for the identification of extremely low frequency magnetic field detection.
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图 1 空气-海水-海床3层水平电偶极子模型示意图
Figure 1. Air-seawater-seabed three-layer horizontal electric dipole model
图 2 水平时谐电偶极子与倾斜电偶极子产生磁场在海底平面衰减对比曲线
Figure 2. Attenuation curves of magnetic field generated by horizontal time-harmonic electric dipole and tilted electric dipole in seafloor plane
图 3 轴频磁场在海底平面指向性分布
Figure 3. Directivity distribution of shaft-rate magnetic field in seafloor plane
图 4 轴频磁场水平分量和垂直分量在海底平面分布
Figure 4. Distribution of horizontal and vertical components of shaft-rate magnetic field in seafloor plane
图 5 海床平面上轴频磁场横向分量幅度随纵向距离变化曲线
Figure 5. Variation of horizontal component amplitude of shaft-rate magnetic field with longitudinal distance in seafloor plane
图 6 海床平面上轴频磁场横向分量相位随纵向距离变化曲线
Figure 6. Variation of horizontal component phase of shaft-rate magnetic field with longitudinal distance in seafloor plane
图 7 轴频磁场在海底平面分布图
Figure 7. Distribution of shaft-rate magnetic field in seafloor plane
图 8 轴频磁场在不同深度上的平面分布
Figure 8. Plane distribution of shaft-rate magnetic fields at different depths
图 10 轴频磁场空间分布垂直切片图
Figure 10. Vertical slice of spatial distribution of shaft-rate magnetic field
图 11 信号源产生的极低频磁场实测结果与建模数值计算结果对比图
Figure 11. Comparison of measured values of extremely low frequency magnetic field generated by signal source with numerical calculation of modeling
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