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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

View larger version (33K): [in a new window]   Fig. 1. One-dimensional 1H stray field (STRAFI) image of the resolution test phantom, made from alternating Perspex and glass discs each of 50 mm diameter. Thickness of each glass spacer is given above the appropriate minimum in the profile. The slice separation (step size) is 90 µm. The intensity profile is the sum of the first five echoes of the STRAFI echo train, with one point per echo.

View larger version (28K): [in a new window]   Fig. 2. Stray field (STRAFI) profiles of two sandstone blocks into which molten paraffin wax and mineral oil have diffused from their right faces. Markers were placed in contact with the left faces. (a) Intensity distributions recorded from points at the peak of the first, second, and third echoes of the echo train. (b) Amplitude of the echoes along the echo train for paraffin and mineral oil. Points plotted are the averages over six adjacent slices at positions indicated in the profile by short bars.

View larger version (13K): [in a new window]   Fig. 3. Dependence of the longitudinal relaxation time, T1, as determined by stray field (STRAFI) nuclear magnetic resonance (NMR) on the nominal pore diameter for a series of water-saturated ceramic discs in a single phantom, each with a narrow pore-size distribution and high porosity (ca. 35%). The T1 recovery curve was fitted with a single exponential. The line is a weighted least squares fit to the data.

View larger version (20K): [in a new window]   Fig. 4. Spatial dependence of the amplitudes of two exponential decay terms needed to fit peak amplitudes in echo trains from a series of positions along a cylinder made up from two half cylinders in the form of a bi-wedge. The mean pore sizes are 1 and 20 µm and are shown above the graph.

View larger version (18K): [in a new window]   Fig. 5. Diagram of the apparatus developed to permit variation of the matric potential of a soil sample within the stray field (STRAFI) coil. The potential can be altered by raising or lowering the reservoir. It is important that the sintered plate is saturated with water.

View larger version (19K): [in a new window]   Fig. 6. Distributions of exponential decay terms produced by CONTIN analysis of the peak amplitude of successive echoes from a long train of pulses applied to a loamy sand in the variable matric potential holder. Three distributions produced by successively draining the soil to 20 hPa (=20 cm), 40, and 60 hPa are shown. Fits were made to 54 logarithmically spaced decay terms.