Mapping of electromagnetic noise in a magnetic resonance sounding context

Abstract

The applicability of magnetic resonance sounding in mapping the water content and the hydrological properties of the sub-surface in industrialized areas is severely limited by electromagnetic noise. Efficient ways of mitigating the noise must be developed before the technique can become a ubiquitous tool. In this paper we demonstrate an instrument developed for efficient mapping of noise at a given site prior to a magnetic resonance sounding. The instrument consists of two small induction coils connected to a digital oscilloscope controlled by a PC. Using the instrument, measurements of the electromagnetic noise are easily performed at several places within the site. Signal processing of the measurements provide a quantified understanding of the contributions from different noise sources, primarily powerline harmonics and impulsive noise. Further the spatial distributions of the noise components are also obtained. Based on this knowledge the optimum spot for a magnetic resonance sounding with the least distortion by noise can be identified. The instrument is now in routine use at the Hydrogeophysics group at Aarhus University.