Analysis Of Weak Magnetic Signal Induced By Sea Waves And Ship Wakes

The seawater is a weak conductive medium. When the seawater moves through the geomagnetic field, it could generate time-changing electric currents in the medium. The electric currents would produce measurable electromagnetic fields in the whole space. This thesis is concerned with simulations and analyses to the weak magnetic fields induced by three different motions of seawater that cut the geomagnetic field: the sea waves driven by winds, the wakes due to moving ship and moving submarine. The examinations to the induced magnetic fields will deepen our knowledge about the oceanic magnetic environment. It is also helpful in providing a reliable theoretical foundation for aero magnetic anomaly detection of underwater moving target and electromagnetic exploration of seabed resources.In order to perform analysis to the solution of the weak magnetic fields induced by seawater moving through the geomagnetic, a two-step procedure will be followed:First, we need to calculate the velocity fields of moving seawater as accurate as possible. So, modeling of the velocity fields of seawater for wind waves and ship wakes constitutes an essential part of the work. In this part, by combining the physical properties of seawater and our problem at hand, we take the seawater as an ideal fluid, namely an incompressible and non- viscous fluid, so that the velocity satisfies ?? V?0. The motion of seawater of wind waves and ship wakes can also be taken to be irrotational, so that the velocity also satisfies ?? V?0 and can be characterized by a potential function ? such that V? ???, which leads to the Laplace equatio n 2??=0. To obtain the unique solution of the velocity potential for wind waves and ship wakes, we adopt the classical Pierson-Moscowitz spectrum to generate the surface waves to match the velocity potential for wind waves, and use the ship spectrum to generate the surface Kelvin wakes to match the velocity potential for ship wakes, respectively.Secondly, after the velocity field is resolved, the electric current distribution in the seawater medium can be found. Then we can solve the Maxwell equation to obtain the induced electromagnetic field under a reasonable stratified oceanic configuration. Unique solutions can be extracted by enforcing the boundary conditions, i.e., the tangential components of both the electric and magnetic fields must be continuous.Finally, based on the deduced accurate formula, a great deal of simulations are carried out. Space-time distributions of the induced magnetic fields by the wind waves and ship wakes, as well their frequency spectrum properties, are analyzed.