Soil hydraulic interpretation of nuclear magnetic resonance measurements based on circular and triangular capillary models

Abstract

Geophysical nuclear magnetic resonance (NMR) applications are used to estimate pore size distributions (PSDs) of rocks and sediments. This is commonly realized by empirical calibration using information about the surface-to-volume ratio of the material. Recent research has developed joint inversion concepts for NMR relaxation data that provides the PSD with a minimum of information. The application requires the NMR signal of a sample at saturation and at least one at partial saturation and at known suction. The new inversion concept physically simulates the desaturation process as part of the forward operator. The cross-section of the model capillaries in the underlying bundle can be either circular or triangular. Our study investigates the performance of the NMR joint inversion to predict water retention function (WRF) and capillary-based hydraulic conductivity (Kcap) as functions of saturation for different sands. The angularity of the pores has no significant impact on the estimated WRF but affects the Kcap estimation significantly. Our study shows that the WRF is predicted reliably for sand samples under fast diffusion conditions. The Kcap estimations are also plausible but tend to systematic overestimation, for which we identified the tortuosity being the main reason. Because NMR relaxation data generally do not provide tortuosity information, a plausible tortuosity model remains an issue of classical calibration. Further development of the approach will thus consider tortuosity measurements (e.g., by electrical resistivity measurements and/or gradient NMR) and will consider the relaxation mechanisms outside fast diffusion conditions to enhance its applicability for coarse soils.