Characterization of unsaturated porous media by high-field and low-field NMR relaxometry

A comparison study of nuclear magnetic resonance relaxometry at high and low magnetic field (7 and 0.1 T) has been initiated for investigating the influence of the magnetic field strength, variable clay content, and different degrees of saturation on the relaxometric properties of four ideal porous media. The samples consisted of medium sand with increasing fractions of kaolin clay ranging from 0 to 15%. Six different volumetric water contents between saturation and $\theta$ = 0.05 were used. Changes in water content of the samples were achieved by slow evaporation. T2 relaxation curves were monitored by the Carr-Purcell-Meiboom-Gill sequence and were further analyzed by inverse Laplace transformation, yielding T2 distribution functions. Sand shows a slight continuous shift with decreasing water content of a bimodal distribution function of T2 to faster relaxation at high and low magnetic field. Sand-clay mixtures show broad, bimodal distribution functions for both magnetic field intensities which shift slightly with decreasing water content. Signal amplitude behavior with variation of saturation degree was also monitored. An expected proportionality of the total signal amplitude with water content was observed for all samples at 0.1 T, whereas at 7 T deviations occurred for samples with a clay content higher than 5%, which are assigned to loss of signal in the first echo periods. The relaxivity in unsaturated clay-based porous media is mostly surface dominated, as the weak and comparable dependence of 1/T2 on TE at both field strengths shows. Nevertheless, for a reliable determination of water content in mixed systems with varying texture and saturation the employment of multiecho sequences at low magnetic field strength are preferable.