Effect of Internal Magnetic-Field Gradients on Nuclear-Magnetic-Resonance Measurements and Nuclear-Magnetic-Resonance-Based Pore-Network Characterization

2018 ◽  
Vol 21 (03) ◽  
pp. 609-625 ◽  
Author(s):  
Saurabh Tandon ◽  
Zoya Heidari
Keyword(s):  
Magnetic Field ◽  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Pore Network ◽  
Internal Magnetic Field ◽  
Field Gradients ◽  
Magnetic Field Gradients ◽  
Network Characterization ◽  
Nuclear Magnetic

Nuclear magnetic resonance secular relaxation measurements as a method of extracting internal magnetic field gradients and pore sizes

2016 ◽  
Vol 4 (4) ◽  
pp. T557-T565 ◽  
Author(s):  
Andrew Johnson ◽  
Hugh Daigle
Keyword(s):  
Magnetic Field ◽  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Internal Magnetic Field ◽  
Relaxation Rates ◽  
Bulk Fluid ◽  
Pore Sizes ◽  
Field Gradients ◽  
Magnetic Field Gradients ◽  
Nuclear Magnetic

Nuclear magnetic resonance (NMR) has been used as a common and powerful tool for petrophysical investigation of fluid-bearing porous media. A major complication of NMR analysis occurs, however, when diffusion of fluid protons through magnetic field heterogeneities becomes nonnegligible. A quantity called the secular relaxation rate ([Formula: see text]) has been defined as the difference in transverse and longitudinal relaxation rates ([Formula: see text]-[Formula: see text]) and can be shown to isolate the effects of diffusion as a function of pore system parameters. We have developed results that extract internal magnetic field gradient strengths based on changes in [Formula: see text] as a function of the NMR interecho spacing. We also indicated that an optimization algorithm can be used to invert for volumetrically weighted mean pore sizes. The benefit of these types of analyses is to provide simple methodologies for inferring the average strengths of internal magnetic field gradients and pore sizes from NMR measurements without the need for independent measurements of pore size, such as from mercury injection porosimetry. In addition, secular relaxation analysis removes complicating effects provided by bulk fluid and other nondiffusion relaxation mechanisms.

Nuclear magnetic resonance microscopy

1989 ◽  
Vol 47 ◽  
pp. 828-829
Author(s):  
Paul C. Lauterbur
Keyword(s):  
Magnetic Field ◽  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Magnetic Fields ◽  
Signal To Noise ◽  
Field Gradients ◽  
Useful Signal ◽  
Magnetic Field Gradients ◽  
Observation Times ◽  
Nuclear Magnetic

Nuclear magnetic resonance imaging can reach microscopic resolution, as was noted many years ago, but the first serious attempt to explore the limits of the possibilities was made by Hedges. Resolution is ultimately limited under most circumstances by the signal-to-noise ratio, which is greater for small radio receiver coils, high magnetic fields and long observation times. The strongest signals in biological applications are obtained from water protons; for the usual magnetic fields used in NMR experiments (2-14 tesla), receiver coils of one to several millimeters in diameter, and observation times of a number of minutes, the volume resolution will be limited to a few hundred or thousand cubic micrometers. The proportions of voxels may be freely chosen within wide limits by varying the details of the imaging procedure. For isotropic resolution, therefore, objects of the order of (10μm) may be distinguished.Because the spatial coordinates are encoded by magnetic field gradients, the NMR resonance frequency differences, which determine the potential spatial resolution, may be made very large. As noted above, however, the corresponding volumes may become too small to give useful signal-to-noise ratios. In the presence of magnetic field gradients there will also be a loss of signal strength and resolution because molecular diffusion causes the coherence of the NMR signal to decay more rapidly than it otherwise would. This phenomenon is especially important in microscopic imaging.

Determining fractal dimension from nuclear magnetic resonance data in rocks with internal magnetic field gradients

Geophysics ◽  
2014 ◽  
Vol 79 (6) ◽  
pp. D425-D431 ◽  
Author(s):  
Hugh Daigle ◽  
Andrew Johnson ◽  
Brittney Thomas
Keyword(s):  
Magnetic Field ◽  
Nuclear Magnetic Resonance ◽  
Fractal Dimension ◽  
Magnetic Resonance ◽  
Pore Size ◽  
Internal Magnetic Field ◽  
Pore System ◽  
Fractal Scaling ◽  
Field Gradients ◽  
Magnetic Field Gradients

Pore size distributions in rocks may be represented by fractal scaling, and fractal descriptions of pore systems may be used for prediction of petrophysical properties such as permeability, tortuosity, diffusivity, and electrical conductivity. Transverse relaxation time ([Formula: see text]) distributions determined by nuclear magnetic resonance (NMR) measurements may be used to determine the fractal scaling of the pore system, but the analysis is complicated when internal magnetic field gradients at the pore scale are sufficiently large. Through computations in ideal porous media and laboratory measurements of glass beads and sediment samples, we found that the effect of internal magnetic field gradients was most pronounced in rocks with larger pores and a high magnetic susceptibility contrast between the pore fluid and mineral grains. We quantified this behavior in terms of pore size and Carr-Purcell-Meiboom-Gill (CPMG) half-echo spacing through scaling arguments. We additionally found that the effects of internal field gradients may be mitigated in the laboratory by performing [Formula: see text] measurements with different CPMG half-echo spacings and fitting the apparent fractal dimensions determined by the NMR measurements with a model to determine the true pore system fractal dimension.

PFG NMR and internal magnetic field gradients in plant-based materials

2002 ◽  
Vol 20 (7) ◽  
pp. 567-573 ◽  
Author(s):  
Nikolaus Nestle ◽  
Asal Qadan ◽  
Petrik Galvosas ◽  
Wolfgang Süss ◽  
Jörg Kärger
Keyword(s):  
Magnetic Field ◽  
Internal Magnetic Field ◽  
Field Gradients ◽  
Magnetic Field Gradients ◽  
Pfg Nmr
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