Characterization and Application of Underwater Extremely Low Frequency Magnetic Fields

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.

Keywords

• underwater extremely low frequency magnetic field
• shaft-rate magnetic field
• stern shaft
• time-harmonic electric dipole
• propagation and attenuation characteristics