MRS as a Groundwater Investigation Tool

Abstract

During the past 75 years, indirect measurements, e.g. surface geophysical techniques, contributed to groundwater investigations. Such non‐invasive techniques permit lowering costs and delays in acquiring subsurface information while preventing invasion or interconnection between hydrostratigraphic units. However, at results exploitation step, some ambiguity is noticed. During the successive developments of the different indirect techniques, two goals have been targeted: (1) lower this level of ambiguity and (2) supply results directly useful for groundwater resource characterization. MRS (Magnetic Resonance Sounding) is a significant step in this direction. MRS is NMR‐based using the Earth's magnetic field as static field and provides its own excitation field which, contrary to usual laboratory case, is not an RF field but an audio‐frequency field i.e. the hydrogen nuclei (1H+) Larmor frequency. The excitation field moment is used as sounding parameter i.e. as the controlled parameter for depth discrimination. MRS supplies, as a function of depth, the in‐situ water content and the NMR signal decay rate. This decay rate is linked to pore‐size and therefore to the investigated medium water flow properties. Below the water table, the MRS‐supplied water content is roughly equivalent to effective porosity under suitable conditions; bound water is usually not quantified with current generation MRS tools. As any NMR technique, MRS is sensitive to the static field homogeneity which is the topic of another contribution. MRS ‘delivers the goods’ on a commercial basis since 1996 in most places. It performs non‐invasively down to about 150 m in a resistive medium; this bottom limit is reduced as the medium gets more electrically conductive. The support of ITC and the collaboration of the following institutions and their personnel are greatly appreciated: AGEOS, BRGM, CSIR‐Envirotek, DWA‐B, DWA‐N, École Polytechnique, GSD GSN, IGM, INRS‐ETE, IRD, IRIS, MBG, QUALITAS, UQAC, UQAT, WCS, WRC.