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SPECIAL SECTION: Hydrogeophysics |
IGP
Corresponding author: jeanroy_igp{at}videotron.ca
During the last 10 years, applied geophysics techniques have made significant progress in the exploration, quantification, and management of groundwater (GW) through wider applications of classic techniques and their integration. These include resistivity, including resistivity imaging and vertical electric sounding (VES); induced polarization (IP); spontaneous polarization (SP); time- and frequency-domain electromagnetics (TDEM, FDEM); ground-penetrating radar (GPR); very low-frequency EM (VLF); seismic; magnetic; gravity; and gamma-ray spectrometry. During that ten-year interval, however, one technique that stands out as truly new and highly relevant for GW is magnetic resonance sounding (MRS), a field application of nuclear magnetic resonance (NMR).
Functionally, MRS fits between two established techniques: atomic absorption spectrometry (AAS) and time-domain electromagnetics (TDEM). AAS is used in laboratories on carefully prepared samples and has no in-situ depth of penetration, but it performs well in element discrimination and determination of their concentration. TDEM has good depth of penetration (i.e., in suitable cases, it can measure in-situ ground conductivity as a function of depth down to several hundred meters) but it can not discriminate between elements. MRS shares some of these characteristics: it has excellent element selectivity but for one element only—hydrogen, a major component of the water molecule. Also, MRS has moderate depth of penetration, in particular over resistive terrain (i.e., up to 150 m), while quantifying water content and pore size as a function of depth.
NMR in a nutshell
NMR is one of the numerous processes of interaction between EM fields and matter. Most familiar ones occur at the level of electrons; NMR works at the nuclei level by exploiting two nuclear properties: (1) a net angular momentum 
and (2) a net magnetic moment µ. Only 
42 isotopes (30 elements involved) are abundant enough and have both of these properties in exploitable magnitude. The gyromagnetic ratio 
= µ/ is an atomic constant that uniquely characterizes each of these isotopes. Here, we are only concerned with hydrogen nuclei (1H+) with = 2.675 x 108 rad•s–1T–1. At equilibrium, the net magnetic moment of the volume investigated for a given isotope is aligned with the ambient (static) magnetic field Bs. We can put it out of this alignment by (1) momentarily changing Bs or (2) exciting the volume at the resonance, Larmor frequency fL = Bs/2
. After excitation, because of their angular momentum, the excited nuclei will not immediately return to equilibrium but will precess around their ambient magnetic field at frequency fL during a relaxation time characterized by decay time constant Td. There is a far-ranging analogy between the precession of nuclei in a static magnetic field and the precession of a top in a gravity field. Through discrete differences in energy levels, a quantum perspective is also useful for several aspects. Two types of rotations are relevant: precession of the nuclei around Bs and nutation around the excitation field Bfl. The various NMR decay time constants (T1, T2, and T2*) are significant to petrophysical studies. In ground geophysics, we exploit the NMR concept with magnetometers and for MRS. In borehole geophysics, NMR logging tools provide highly relevant diagnostic information for petroleum exploration; due to cost factors, NMR logging is not yet generalized for GW projects.
MRS implementation
For MRS work, we designate the Earth's magnetic field, Be, as static (i.e., Bs = Be). The practical implementation uses a large loop laid on the ground generally in a manner quite similar to a single loop time-domain EM setup (Figure 1, bottom part), but additional loop shapes are also used (Figure 2). The MRS instrument (Figure 1, upper part) energizes this loop during the excitation step and uses the same loop during the detection step. A laptop PC provides control, monitoring, data recording, processing, and inversion. In this implementation, each module is < 20 kg so the entire system can be backpack-transported.
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A sounding acquired in a park in the Netherlands at the margin of a dunes area nicely illustrates the approach (Figure 3). In this data summary (left panel), three quantities are displayed for each Q value used: the initial value (E0 – *) of the NMR signal, the average noise level (•)—both in nV using the left y-axis—and the signal decay time constant (T2* – 
) in ms—using the right y-axis. The sounding parameter, Q for MRS, is the variable that allows depth discrimination. For example, in a Schlumberger VES, the sounding parameter is the operator-controlled interelectrode distance.
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MRS) and signal decay rates (e.g., T2*) as inverted parameters over discrete depth intervals (Figure 3, right panels). Below the water table, MRS is an estimate (
MRS) of the effective porosity, while the signal decay rate is related to the water-bearing pore size. A more complex, two-pulses excitation scheme is also used (Legtchenko et al., 2003), from which an estimate of T1 (e.g., T1*) is made. The left panel of Figure 3 shows one peak without major inflexions so that only one main aquifer is resolved under these conditions which fit with the known hydrogeological information over the depth range considered. Often, because of mixed grain size or presence of fine sediment, the transition near the water table is gradual rather than abrupt. At the Waalwijk-1 site (Figure 3), the estimated depth to the water table is 8 m. The inversion strategy and parameters also contribute to how smooth the transition is between vadoze zone and saturated formations. MRS and GW investigation
Raw or inverted MRS data typically provide water content and the NMR decay time constant as a function of depth. Information acquired through MRS surveys allows, under suitable conditions, not only detection and positive identification of water-bearing layers but also the determination of their vertical geometry (depth and thickness), their free-water content (the amount of water free to move under realistic hydraulic gradients) and an estimate of key parameters such as hydraulic conductivity (K) and transmissivity (T). For a given lithology/mineralogy, the longer the NMR decay rate, the coarser the water-bearing pore size below the water table. This important observation was first explained in 1962 by Korringa et al. in their "KST" model and later confirmed through empirical observations (Kenyon et al., 1989). In fact, through spectra analysis of the decay rate, the relationship between NMR decay rate and pore size allows the determination of pore-size distribution. In-situ pore-size estimation by NMR is made possible by the fact that the smaller the pore, the faster the relaxation of the precessing 1H+ nuclei through repeated contacts with the solid grain surface.
Because of the close link between pore size, throat size, hydraulic permeability, and hydraulic conductivity, NMR logs can reliably supply flow properties. MRS, which is less advanced than its borehole-logging counterpart, is less reliable in environments where magnetic minerals are present. Also, in most cases, MRS supplies an average decay rate instead of a decay rate spectrum. Above the water table, in particular at depths below the reach of GPR, MRS can supply information that is difficult to acquire noninvasively, such as water content and water film thickness or water drop size (Roy and Lubczynski, 2005). However, exploitation of MRS in the vadoze zone still needs calibration. GW resources assessment needs data about four dominant characteristics: recharge, aquifer storage, flow property estimation, and GW quality. These characteristics are usually quantified using a combination of techniques including pump and recovery tests, other hydrogeological methods, and numerical model methods including 1D and distributed models. MRS is likely to play an increasingly significant role in such resource mapping and quantification strategies.
MRS capabilities and limitations
A decade of tests and evaluations leads to the conclusion that MRS is a highly appropriate method for many GW applications due to (1) its inherent selectivity to 1H+ and therefore near-surface GW; (2) its performance as a noninvasive sounding tool (i.e., information as a function of depth); and (3) the relevance of its inverted parameters to aquifer and aquitard characterizations, MRS and Td. MRS is most commonly used in a sounding mode; i.e., 1D, water quantity (MRS) and signal decay time (Td) are estimated as functions of depth for both the vadoze and saturated zones. The hydrogeological significance of MRS needs careful consideration. K and T calibrations exploiting relaxation time Td have progressed significantly and lithology-dependent factors have already been evaluated (Vouillamoz, 2003). Signal decay spectral analysis, currently limited to MRS data sets with high SNR, allows the water content to be resolved into components of pore size such as fine, medium, and coarse (Figure 4). The excitation moment Q displayed along the y-axis to stress the relationship (sounding parameter) between Q and depth is an alternate way of displaying a summary of the MRS data set.
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MRS and Td. Two aspects of aquifer storage quantification through MRS are summarized in Figure 5.
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In MRS, an equivalence limitation exists which is analogous to the one in resistivity work where only a layer's conductance can be estimated instead of both its resistivity and thickness for some specific structures. A series of MRS responses for aquifers of the same porosity*thickness product (2 m of water) is shown on the right of Figure 5. MRS soundings easily resolve an 80-m aquifer but will only determine the porosity*thickness product when the thickness is reduced to less than 10 m under the modeled conditions.
Contrary to most noninvasive geophysical techniques, MRS can discriminate to some extent the "type" of water detected mostly through decay rate analysis: i.e., bound water versus free water and eventually retained water versus water released by gravity. Currently, with some exceptions for carbonates, instrumental characteristics limit MRS response mostly to free water (i.e., clay-bound water is not detected with MRS contrary to the case with NMR logs). In this MRS context, Figure 6 summarizes water "types" both in saturated and unsaturated zones. One of the most difficult properties to discriminate, from a geophysical perspective, is the separation between the specific yield and the specific retention of the vadoze zone or the dewatered cone during the GW pumping process. Specifically, determining the fraction of groundwater which can flow by gravity in contrast to the fraction retained on the pores' walls. This ratio is highly dependent on the distribution of the rock's grain size and surface properties. MRS has the potential to contribute to the determination of these two critical characteristics.
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Research and development directions involve SNR improvement, 2D and 3D capability, and a widening of the NMR signal aperture-window. Important progress is being made with respect to the hydrogeological control and calibration of the technique (Figure 7). After suitable development along several R&D directions, one can expect better ground penetration, higher GW selectivity, and higher relevance of inverted parameters than GPR, possibly with less spatial resolution. However, it is most likely that the optimal use of the MRS technique will be tightly integrated with other geophysical techniques to supply the most relevant information in a rapid and cost-effective way.
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Suggested reading
"Applications of the integrated NMR-TDEM method in groundwater exploration in Israel" by Goldman et al. (Journal of Applied Geophysics, 1994). "Pore-size distribution and NMR in microporous cherty sandstones" by Kenyon et al. (SPWLA 13th Annual Logging Symposium, 1989). "Theory of spin pumping and relaxation in systems with a low concentration of electron spin resonance centers" by Korringa et al. (The Physical Review, 1962). "A review of the basic principles for proton magnetic resonance sounding measurements" by Legchenko and Valla (Journal of Applied Geophysics, 2002). "A complex geophysical approach to the problem of groundwater investigation" by Legchenko et al. (2003 SAGEEP Proceedings). "Magnetic resonance sounding applied to aquifer characterization" by Legchenko et al. (Ground Water, 2004). "MRS contribution to hydrogeological system parametrization" by Lubczynski and Roy (Near Surface Geophysics, 2005). Hydrological Verification of Magnetic Resonance Soundings, Maun Area, Botswana by Mangisi (master's thesis, ITC, 2004). Magnetic Resonance Sounding–A Reality in Applied Hydrogeophysics edited by Plata et al. (Boletin Geologico y Minero, Special Issue, 2007). "The case of an MRS-elusive second aquifer" by Roy and Lubczynski (Proceedings of the 2nd MRS International Workshop, 2005). "MRS multi-exponential decay analysis: aquifer pore-size distribution and vadose zone characterization" by Roy and Lubczynski (Near Surface Geophysics, 2005). Principles of Magnetic Resonance, 3rd edition, by Slichter (Springer-Verlag, 1996). "La charactérization des acquifères pau une méthod non-invasive: les sondages par résonance magnétique protonique" by Vouillamoz (Thèse de l'Université–de Paris, 2003). Resonance Sounding–a Reality in Applied Hydrogeophysics edited by Yaramanci et al. (in issues 3–4 of Journal of Applied Geophysics, 2008).
Acknowledgments:
The support of ITC and the collaboration of BRGM, CSIR-Envirotek, DWA-B, DWA-N, Ecole Polytechnique, GSD, GSN, IGM, IRIS Instruments, MBG, UQAC, WCS, and WRC is gratefully acknowledged. In particular, I want to acknowledge 10+ years of collaboration with my former hydrogeologist colleague at ITC, M. W. Lubczynski. My thanks to Rick Miller and an anonymous reviewer for help in improving the clarity of the text.
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Z showing lithology dependence (broadly classified as granites, sands, and chalk). (right) MRS-T calibration after integration of lithology-dependent factor (Vouillamoz, 2003).