Analysis of the Magnetic Signature of Surface Waves Measured in a Laboratory Experiment

Abstract

A magnetic signature is created by secondary magnetic field fluctuations caused by the phenomenon of seawater moving in Earth’s magnetic field. A laboratory experiment was conducted at the SUrge STructure Atmosphere INteraction (SUSTAIN) facility to measure the magnetic signature of surface waves using a differential method: a pair of magnetometers, separated horizontally by one-half wavelength, were placed at several locations on the outer tank walls. This technique significantly reduced the extraneous magnetic distortions that were detected simultaneously by both sensors and additionally doubled the magnetic signal of surface waves. Accelerometer measurements and local gradients were used to identify magnetic noise produced from tank vibrations. Wave parameters of 4 m long waves with a 0.56 Hz frequency and a 0.1-m amplitude were used in this experiment. Freshwater and saltwater experiments were completed to determine the magnetic difference generated by the difference in conductivity. Tests with an empty tank were conducted to identify the noise of the facility. When the magnetic signal was put through spectral analysis, it showed the primary peak at the wave frequency (0.56 Hz) and less pronounced higher frequency harmonics, which are caused by the non-linearity of shallow water surface waves. The magnetic noise induced by the wavemaker and related vibrations peaked around 0.3 Hz, which was removed using filtering techniques. These results indicate that the magnetic signature produced by surface waves was an order of magnitude larger than in traditional model predictions. The discrepancy may be due to the magnetic permeability difference between water and air that is not considered in the traditional model.

Electromagnetic Fields Induced by Three-Dimensional Short Ocean Waves in a Horizontally Layered Seabed

“…This is because the vertical geomagnetic field has one more contribution in 𝐵 𝑧 than in both 𝐵 𝑥 and 𝐵 𝑦 , as seen in ( 11) and (15). In sharp contrast to the magnetic flux density, the vertical electric field is always much smaller than the horizontal electric field, which is a consequence of cancelling in two horizontal velocities, the values of which are very close, as seen in ( 5), (14), and (16).…”

“…To study this problem, an accurate model that simulates the electromagnetic fields of short-period ocean waves is needed [14]. Reference [15] was the first to obtain an exact solution by directly solving Maxwell's equations using the boundary conditions of a conducting half-space.…”