ABSTRACT

Obtaining reliable estimates of hydrogeologic properties from nuclear magnetic resonance (NMR) measurements requires the ability to measure NMR relaxation parameters that are most sensitive to pore-scale geometry. Conventional surface NMR measurements of the free induction decay yield accurate estimates of the relaxation time parameter T2*, but it has been shown that this parameter can exhibit limited sensitivity to pore size and permeability. We evaluated an improved surface-NMR scheme that uses spin-echo signals to estimate the more robust and readily usable relaxation parameter T2. The acquisition methodology builds upon previous spin-echo schemes and incorporates robust phase-cycling procedures, which remove responses that can potentially interfere with the echo signals. A new two-stage linear inversion was used to derive quantitative estimates of T2 with depth. The method was evaluated in two field experiments at sites in the central and western United States. At one site, NMR logging measurements in a nearby borehole provided the first opportunity to compare T2-values estimated by surface NMR to T2-values determined from the logging data. The surface and logging results showed very close agreement at depths where T2 is long, but echoes cannot be detected from depths where T2 is shorter than the minimum echo time. As anticipated, we found that T2 derived from spin echoes was generally much longer than T2*, derived from the free induction decay. We explain the observed differences by considering the magnitude of inhomogeneity in the background magnetic field. We note that T2 exhibited greater variation and sensitivity to pore size than T2* in coarse-grained materials, while T2* provided greater sensitivity in fine-grained materials where no echo signal was detected. Given these complementary advantages of T2 and T2* measurement, we advocate adoption of a framework combining spin-echo and free induction decay data to improve characterization of groundwater aquifers.

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