Shallow marine electromagnetic measurements for defence applications

Abstract

Sea trials involving the use of marine electromagnetic sensors from DSTO and Flinders University were carried out in Jervis Bay, NSW (Nov 2001 and May 2002) to measure the amplitude of towed controlled source dipole electric fields over various horizontal ranges. The source simulates corrosion-induced ship electromagnetic fields. Three instruments were deployed in 18 m of water, approximately 100 m into the bay to measure two- or three-component electric fields and three-component magnetic fields sampled at 40 Hz (Flinders) and 1800 Hz (DSTO). The towed array of dipole sources produced a variety of signal waveforms of varying frequency and amplitude, and was tracked using either a GPS unit located a few metres behind the towing vessel, or two GPS units, attached to the towing vessel and the rear of the array. This paper primarily reports on aspects of the towed field propagation through shallow water. Measurements were used to confirm theoretical predictions of electric field characteristics based on numerical modelling of a linear array of current sources and sinks in a layered half-space. Additionally, a large magnetic storm was also recorded in the May experiment, to provide magnetotelluric estimates of seabed conductivity. We show that instruments used in the experiment have both defence applications in electromagnetic field detection, and can also provide shallow geological structure for exploration of the continental shelf.