Abstract
Weak electric currents are induced in moving seawater by cutting the geomagnetic fields. These electric currents can produce measurable electromagnetic fields that may be used for some purposes such as monitoring of ocean internal waves. This article is aimed at presenting the procedure to calculate the electromagnetic fields owing to the wake raised by an undersea moving slender body. A pair of Havelock point sources are introduced to model the moving body, which generate the similar wake at places far from the body. The ocean is taken to be of finite-depth with density stratification due to thermocline. Three distinct forms of water-flow wake can be identified, the free-surface Kelvin wake, the internal interfacial wake, and the localized volume wake. The electric currents evoked by the motional wake may produce observable electromagnetic fields, which may be solved using rigorous electromagnetic field theory. At the sea level, the magnitudes of the induced electric field and magnetic field are on the order of a few microvolts per meter and one nano-Tesla, respectively, which are appreciable in terms of nowadays marine electric and magnetic sensors.
Highlights
As a kind of motion of seawater, the wakes of ships or submarines can produce electromagnetic fields, and the magnitudes of induced magnetic fields at the sea level are reported on the order of one nano-Tesla[5,6,7,8,9,10,11,12,13]
Excitation of the internal interfacial wave requires some conditions determined by the Froude numbers, which determine the behavior of the internal wave as a long-range or short-range traveling wave
According to the three forms of wake, the electromagnetic fields induced by the wakes may be described as the three forms, too
Summary
As a kind of motion of seawater, the wakes of ships or submarines can produce electromagnetic fields, and the magnitudes of induced magnetic fields at the sea level are reported on the order of one nano-Tesla (nT)[5,6,7,8,9,10,11,12,13]. The most prominent form is the air-seawater surface wave or the free-surface Kelvin wake, which is a quasi-sinusoidal oscillation and deemed to be a kind of infra gravity waves. It may be regarded as a unique accompanying signature of a moving body, a phenomenon of long distance effect is dominant. It seems that only the magnetic anomaly fields induced by the free-surface Kelvin wake has drawn enough attention so far It seemed that the first simulation results for a ship or a submerged vehicle in an infinite-depth ocean were provided by Madurasinghe et al.[7,8]. As a more practical model, the case for a finite-depth ocean with density stratification (air-seawater1-seawater2-sediment) has not been considered, which motivates the preset study
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