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 (K cap) as functions of saturation for different sands. The angularity of the pores has no significant impact on the estimated WRF but affects the K cap estimation significantly. Our study shows that the WRF is predicted reliably for sand samples under fast diffusion conditions. The K cap 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. 1 INTRODUCTION Quantifying the soil hydraulic functions is challenging. The water retention function (WRF) of soil samples can be

A critical mini-review on the low-field nuclear magnetic resonance investigation of pore coupling effects in near-surface environments

“…However, to address some of these research questions, e.g., for the study of physico-chemical processes in the soil (e.g., wetting or swelling), an accurate measurement of water distribution within different pore environments is crucial and only possible if eventual distortions of the NMR signals, related to pore coupling effects, are accounted for. This is also relevant, and must still be examined, e.g., for the estimation of water retention curves from NMR measurements in the saturated material (e.g., Jaeger et al, 2009;Costabel and Hiller, 2021). Furthermore, the complexity and heterogeneity of top-soils may lead to large differences in surface relaxivity values for neighboring pore environments (e.g., Jaeger et al, 2009;Benavides et al, 2017); the effect such ρ distribution will have on pore coupling remains to be investigated.…”

“…Furthermore, the complexity and heterogeneity of top-soils may lead to large differences in surface relaxivity values for neighboring pore environments (e.g., Jaeger et al, 2009;Benavides et al, 2017); the effect such ρ distribution will have on pore coupling remains to be investigated. Likewise unexplored, remains whether a potential transition from the slow into the fast diffusion regime, observed in analytical models with a decrease in saturation level (Costabel, 2011), would translate to a reduction on pore coupling effects. Whether pore coupling strength can be used as an estimator of the degree of connectivity between the pores (Carneiro et al, 2014;Tian, 2021) to evaluate hydraulic conductivity and estimate permeability, is, similarly, an interesting open question, with potential applications to determine, e.g., soil contaminants' flow speed and extent.…”