Instrumentation for detection and generation of ELF emissions in seawater

Abstract

A portable underwater three-axis magnetic and electric field sensor system with remotely controlled data acquisition has been developed for operation in shallow seawater at ELF. Electric field sensors consist of silver/silver chloride electrodes and magnetic sensors are based on multi-turn induction coils designed and wound to minimise stray capacitances, thus resulting in higher coil resonant frequencies. A Helmoltz coil system was designed for calibration of the magnetic sensors. The three electric and three magnetic field components are sampled simultaneously, multiplexed and transmitted along 1 km of cable to the control station for recording and analysis. This instrumentation enables environmental ELF electric and magnetic fields to be recorded with sensors placed on the seabed to minimise motion-induced noise. The vertical magnetic fields recorded in shallow seawater within and outside the Sydney metropolitan region are presented. Experiments have also been performed with a moving horizontal electric dipole representing a controlled source of known dipole strength and frequency. The dipole is attached to a vessel that is tracked using differential GPS so that the relative geometry between source and sensors is known as a function of time. Comparison of these controlled source emissions with forward modelling over different seabeds gives good agreement for realistic ranges of seabed electrical conductivity.