Development of and tests with the NMR technique to detect water bearing fractures

Abstract

The theory and application of Nuclear Magnetic Resonance Soundings also referred to as Magnetic Resonance Soundings (MRS) for the exploration of groundwater in porous aquifers has been well developed and tested. To date the method is usually applied on surface to assess the groundwater potential of thick porous water saturated geological formations. In hard rock aquifers, ground water is normally encountered in fractures or fracture zones. In this paper the authors describe the development of theoretical aspects of the technique for the detection of thin water bearing fractures, both from the surface as well as in underground mines. The theory has been extended to general geometries to describe the detection of discrete water saturated fractures and to investigate the application under mining conditions. This theory has been applied to synthetic models for underground conditions under different geometries, as well as for the usual surface based groundwater exploration situation. This paper describes the results of these simulations and presents field data from both surface and underground measurements collected with a new NMR instrument developed for shallow investigations. Signal amplitudes of <100 nV are normally expected, but noise levels are often a few orders of magnitude higher. The impact of electromagnetic noise on the measurements is discussed and it is shown that this remains one of the main obstacles for the MRS method.