Magnetic Resonance Sounding: New Method for Ground Water Assessment

Abstract

The advantage of magnetic resonance sounding (MRS) as compared to other classical geophysical methods is in its water selective approach and reduced ambiguity in determination of subsurface free water content and hydraulic properties of the media due to the nuclear magnetic resonance (NMR) principle applied. Two case examples are used to explain how hydrogeological parameters are obtained from an MRS survey. The first case example in Delft (the Netherlands) is a multiaquifer system characterized by large signal to noise ratio (S/N = 73), with a 24 m thick, shallow sand aquifer, confined by a 15 m thick clay layer. For the shallow aquifer, a very good match between MRS and borehole data was obtained with regard to effective porosity n(e) approximately 28% and specific drainage S(d) approximately 20%. The MRS interpretation at the level deeper than 39 m was disturbed by signal attenuation in the low resistivity (approximately 10 omega(m)) media. The second case of Serowe (Botswana) shows a fractured sandstone aquifer where hydrogeological parameters are well defined at depth > 74 m below ground surface despite quite a low S/N = 0.9 ratio, thanks to the negligible signal attenuation in the resistive environment. Finally, capabilities and limitations of the MRS technology are reviewed and discussed. MRS can contribute to subsurface hydrostratigraphy description, hydrogeological system parameterization, and improvement of well siting. The main limitations are survey dependence upon the value of the S/N ratio, signal attenuation in electrically conductive environments, nonuniformity of magnetic field, and some instrumental limitations. At locations sufficiently resistive to disregard the signal attenuation problems, the MRS S/N ratio determines how successfully MRS data can be acquired. Both signal and noise vary spatially; therefore, world scale maps providing guidelines on spatial variability of signal and noise are presented and their importance with respect to the MRS survey results is discussed. The noise varies also temporally; therefore, its diurnal and seasonal variability impact upon the MRS survey is covered as well.

Application of surface geophysical methods, with emphasis on magnetic resonance soundings, to characterize the hydrostratigraphy of the Brazos River alluvium aquifer, College Station, Texas, July 2006: A pilot study

“…This signal is then recorded at the surface by the receiver after a turn-off time (Legchenko and others, 2002). During this turn-off time, three parameters of the MRS signal are measured (Lubczynski and Roy, 2004): amplitude (E o ), relaxation time (T1*), and phase (φ). The amplitude of the magnetic field produced during this time is proportional to the water content, whereas the phase is influenced by the electrical conductivity of the hydrostratigraphic layer and affects the attenuation of the MRS signal (Lubczynski and Roy, 2004).…”

“…During this turn-off time, three parameters of the MRS signal are measured (Lubczynski and Roy, 2004): amplitude (E o ), relaxation time (T1*), and phase (φ). The amplitude of the magnetic field produced during this time is proportional to the water content, whereas the phase is influenced by the electrical conductivity of the hydrostratigraphic layer and affects the attenuation of the MRS signal (Lubczynski and Roy, 2004). Relaxation time is directly related to the mean pore size (and therefore hydraulic conductivity) in the formation (table 2) and provides a general estimation of free water or bound water (Legchenko and others, 2002).…”

“…MRS data can help delineate the subsurface hydrostratigraphy and identify the geometric boundaries of the hydrostratigraphic units by indicating changes in the free water content, transmissivity, saturated thickness, and hydraulic conductivity (Lubczynski and Roy, 2004). Typically, this is only possible if there is a high signal-to-noise ratio ( fig.…”

“…5D). If the signal-tonoise ratio is too low, it might not be possible to distinguish hydrostratigraphic boundaries at depth (Lubczynski and Roy, 2004). The aquifer geometry in this application encompasses the lateral extent of porous and permeable materials.…”

Integration of hydrogeophysics and remote sensing with coupled hydrological models

“…The magnetic field and noise measurements were carried out to confirm suitability of the selected site for the MRS survey and to optimize that survey (Lubczynski and Roy, 2004) whereas the geoelectrical profiles to support the MRS inversion process. The assessment of natural magnetic field was executed to determine the local scale magnetic inhomogeneity and to determine Larmor frequency (f l ) necessary for the MRS survey.…”