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Article
SYMPOSIUM PAPERS

Stray Field Nuclear Magnetic Resonance of Soil Water

Development of a New, Large Probe and Preliminary Results

^a University of London, Queen Mary and Westfield College, Chemistry Dep., London E1 4NS, UK
^b Russian Academy of Science, Institute of Chemical Physics, Moscow, 117977, Russia

* Corresponding author (a.r.preston{at}qmul.ac.uk)

Received for publication June 2, 2000. Development, characterization, and preliminary results of a recent technique capable of local measurements of pore-size distribution by a spatially resolved low resolution nuclear magnetic resonance (NMR) technique are described. Potential environmental uses include studying the change in pore-size distribution caused by surface compaction, which influences surface runoff, and obtaining information on the physical state of non-aqueous compounds in porous materials, which should aid the selection of appropriate soil remediation methods. Stray field (STRAFI) imaging is an NMR technique that allows distortion-free imaging of materials with short NMR relaxation times. The sample is placed in the strong axial fringe field gradient of a superconducting NMR magnet. We report on a new, unique, large 5-cm-diameter STRAFI probe, and its use for three preliminary test cases: water in ceramics of known pore size, paraffin wax and oil in sandstone rock, and water in soil at different matric potentials. The imaging is confined to one dimension with a spatial resolution of the order of 100 µm for protons. The optimum position for imaging occurs at 2.62 T and a gradient of 12.1 T/m. Water relaxation decay curves can be measured at any position in the 8-cm-long sample. These curves are decomposed into a series of terms each corresponding to a different pore size. Preliminary results show continuum fits to decay curves for a soil drained to three different matric potentials. Such information will be useful for interpreting water retention curves and will lead to understanding of the behavior of fluids in the vadose zone.

Abbreviations: MRI, magnetic resonance imaging • NMR, nuclear magnetic resonance • RF, radio frequency • STRAFI, stray field

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