Ocean Wave Induced Noise in the Vertical and Horizontal E-field Components

Understanding the electromagnetic noise that reaches the bottom of the sea is of fundamental interest in a range of contexts. In magnetotellurics the natural electromagnetic field variations is used to detect resistive or conductive bodies in the subsurface. Effects of air-sea boundary give different noise spectra in the different E-field components, and it is of fundamental and technological interest to compare the two. Only recently has it been possible to measure the relatively weak vertical component with sufficient accuracy. We compare the vertical and horizontal components, in particular their relation to the theory of ocean wave generated fields. Electric field variations due to ocean waves have been studied widely. Here we generalize Weavers solution for an infinitely deep sea, to a 3-layer shallow sea model. This solution differs qualitatively from the Weaver solution and the solution quoted by Cox through the fact that there are no horizontal field components in our solution. This prediction is well confirmed by our measurements in the ocean wave range of frequencies. It is observed that the weaker vertical E-field component carries a clear effect of ocean waves, while the stronger horizontal components do not, and appears to be dominated by magnetotelluric noise.