Nuclear-Magnetic-Resonance Free Induction Decay in a Two-Spin System

1967 ◽  
Vol 158 (2) ◽  
pp. 246-252 ◽  
Author(s):  
Moses Lee ◽  
D. Tse ◽  
W. I. Goldburg ◽  
I. J. Lowe
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Spin System ◽  
Free Induction Decay ◽  
Free Induction ◽  
Nuclear Magnetic

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

Geophysics ◽  
2014 ◽  
Vol 79 (2) ◽  
pp. EN15-EN23 ◽  
Author(s):  
Elliot Grunewald ◽  
Rosemary Knight ◽  
David Walsh
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Pore Size ◽  
Free Induction Decay ◽  
Field Experiments ◽  
Spin Echo ◽  
Coarse Grained ◽  
Background Magnetic Field ◽  
Free Induction ◽  
Nuclear Magnetic

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.

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