Stray Field Nuclear Magnetic Resonance of Soil Water ...

KINCHESH ET AL.: A NEW STRAY FIELD NMR PROBE 497APPLICATIONStimes. The diffusion effect is enhanced by the large sizeSandstoneof the gradient in STRAFI experiments.Two sandstone samples were prepared, each 15 mmthick by 30 mm square. They were placed with one faceCeramicsjust immersed in mineral oil for one sample and in An aim of the work is to use STRAFI to investigatemolten paraffin wax for the other. Capillary absorption pore-size distributions in soils. An initial feasibility testof the liquids was followed visually by observing the was to apply the technique to a series of highly porousside faces of the blocks. The latter were removed from ceramics, each with a narrow pore-size distribution.contact with the liquids when the wetting front was Discs of Coralith (Fairey Industrial Ceramics, Staffs,about halfway up the faces. The paraffin was allowed UK), made of alumina particles bonded by glass withto solidify.mean pore sizes of 300, 30, 20, 3, and 1 m, were used.One-dimensional STRAFI profiles were then acquired(Further details on these materials together with waterof each block using quadrature echo trains, x retention curves are given in Whalley et al., 2001.) Each( y echo) ne , where x and y are square RF disc was 8 mm thick and 31 mm in diameter. They werepulses of duration 20 s, and x and y denote the relative saturated with distilled water by placing them in a waterphases,which differ by 90 degrees. One complex point filled desiccator connected to a vacuum line for twoper echo was recorded for each of ne 32 echoes. The hours. The saturated discs were separated from eachtime, , between the first two pulse centers was 35 s, other by acetate sheets, and stacked into a cylindricalgiving a time between successive echoes, t e ,of70s. phantom that was then wrapped in Parafilm sealingFor the image (Fig. 2a), 140 slices were acquired. In tissue (Gallenkamp, Loughborough, UK) to preventFig. 2b, markedly different behavior for the two samples evaporation during measurement. The STRAFI systemis seen by comparing signal versus echo number. The was then used to address each disc in turn, and T 1 valuessignal for the solid paraffin steadily increases initially were determined for each disc using the progressivewith echo number indicating that a tip angle consider- saturation method with 40 different recovery timesably less than 90 degrees was used. The level after a spaced logarithmically.few echoes shows very little decay, however, unlike the A single exponential was fitted to the relaxation curveoil signal that decays rapidly. This rapid decay is caused of each disc using the Varian VNMR software. Figure 3by the much faster diffusion in the liquid as opposed to shows the linear relationship obtained by plotting thein the solid as well as by differences in the relaxation reciprocal of T 1 against that of pore diameter. This isas expected but the quality of the fit is a good illustrationof the power of NMR to determine pore size. With theSTRAFI method the spatial variation may be studied.The relaxation of water in a pore is an average valuebecause of the fast exchange of hydrogen between sites.The average is a weighted one; so that as the proportionof free to bound water varies, the average relaxationtime will vary. Hence follows the relation between relaxationtime and pore size. It is expected that in Fig. 3,the intercept should equal the relaxation rate of bulkwater, and the slope should be the relaxivity. The valuesFig. 3. Dependence of the longitudinal relaxation time, T 1 , as deter-mined by stray field (STRAFI) nuclear magnetic resonance (NMR)on the nominal pore diameter for a series of water-saturated ceramicdiscs in a single phantom, each with a narrow pore-sizedistribution and high porosity (ca. 35%). The T 1 recovery curvewas fitted with a single exponential. The line is a weighted leastsquares fit to the data.Fig. 2. Stray field (STRAFI) profiles of two sandstone blocks intowhich molten paraffin wax and mineral oil have diffused from theirright 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 theechoes along the echo train for paraffin and mineral oil. Pointsplotted are the averages over six adjacent slices at positions indicatedin the profile by short bars.