Motion induced electromagnetic fields in the ocean: Exploratory data analysis and signal processing

Abstract

We will in this thesis analyse data from antennas located at the seafloor measuring the vertical component of the natural electric field. The internal source to electromagnetic fields in the ocean is saltwater crossing the geomagnetic field, and the main contributor to the motion induced vertical electric field is the water velocity in the East-West direction weighted by the North component of the geomagnetic field. The motivation is to study the motion induced signal which is present in the frequency range 0.1-10 Hz. This is a frequency range of interest when using electromagnetic methods in marine hydorcarbon exploration.

To analyse the electric field data we have implemented and applied the multitaper estimator for spectrum estimation. The multitaper estimator also provide for a test for periodic (sinusoidal) components, which we have implemented and applied. To further analyse the statistics of the motion induced electric field, we have applied both conventional estimators to estimate the statistical properties and the kernel smoothing estimator to estimate the distribution of the data.

The electric field data contained a prominent oscillation visible in the time series, and the spectrum estimates of the recorded data show a prominent peak about 0.15 Hz and with features just above 0.1 Hz and at 0.24 Hz. These features corresponds to the observed periods of the surface waves during the recordings. While the frequency of the prominent peak is rather stable, its level changes more than 10 dB during the recording (30 minutes). Theory and other experiments shows that the surface waves causes pressure fluctuation in the ocean, causing both disturbance in the seafloor and the seawater, which induce electric fields. This mechanism is the most likely source to the fluctuations we see in the measured data.