Advancement and validation of surface nuclear magnetic resonance spin-echo measurements of T2

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 [Formula: see text], 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 [Formula: see text]. 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 [Formula: see text] 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 [Formula: see text]-values estimated by surface NMR to [Formula: see text]-values determined from the logging data. The surface and logging results showed very close agreement at depths where [Formula: see text] is long, but echoes cannot be detected from depths where [Formula: see text] is shorter than the minimum echo time. As anticipated, we found that [Formula: see text] derived from spin echoes was generally much longer than [Formula: see text], 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 [Formula: see text] exhibited greater variation and sensitivity to pore size than [Formula: see text] in coarse-grained materials, while [Formula: see text] provided greater sensitivity in fine-grained materials where no echo signal was detected. Given these complementary advantages of [Formula: see text] and [Formula: see text] measurement, we advocate adoption of a framework combining spin-echo and free induction decay data to improve characterization of groundwater aquifers.