On the magnetic field induced by swell in inhomogeneous seawater

Abstract

The magnetic field induced by ocean swell moving in the geomagnetic field is an important electromagnetic noise in the ocean environment for magnetic anomaly detection, and it is calculated in this work accounting into the fact that the conductivities of the ocean are inhomogeneous vertically, which is ignored in the former works, and the results are verified by comparing with those obtained by the former models. The swell-induced magnetic fields at 2 locations in the ocean are calculated and analyzed based on the statistical seawater conductivity data sets and the International Geomagnetic Reference Field. Taken the model developed here as a correct reference, the error of the former model is estimated firstly, and it reveals that the error is lower in shallow water and decreases with the wave period. It is further concluded that both the amplitude and vertical distribution range of the induced magnetic field increase with the wave period. The spectra of swell-induced magnetic field calculated with ocean wave spectrum under different wind scales are evaluated, and the impact ranges of ocean wave magnetic noise are evaluated at the sensitivity of a marine magnetometer. In the end, after examining the variation of the induced magnetic field at sea surface due to the change of geomagnetic field and seawater conductivity, it reveals that the annual variation presents a clear periodic change pattern which mainly depends on the annual variation of the conductivity near the sea surface, and the amplitude of the induced magnetic field changes in strong positive correlation with the intensity of geomagnetic field. The studies in this work are significant for applications such as magnetic anomaly detection of marine targets.