Relative hydraulic conductivity and effective saturation from Earth’s field nuclear magnetic resonance – a method for assessing the vadose zone

Abstract

ABSTRACTBeside the water content, petrophysical nuclear magnetic resonance (NMR) techniques in the lab and in boreholes as well as in the field, provide estimates of the hydraulic conductivity of water saturated sediments and rocks. In the vadose zone, the hydraulic conductivity is a function of the water saturation. Regarding the characterization of the vadose zone, the magnetic resonance sounding (MRS) method is expected to have great potential. However, so far, the petrophysical relationship of the hydraulic properties under partial saturation conditions and the NMR parameters in the Earth’s magnetic field is not fully understood. In this study, laboratory NMR experiments in the Earth’s field (EFNMR) are performed in comparison to conventional high field NMR (HFNMR). Sand‐filled columns were used to generate partially saturated conditions by simulating capillary fringes (grain sizes from fine to coarse). We investigate the ability of both NMR techniques to determine the residual water content and the dependency of the NMR relaxation times on the water saturation degree. We note that EFNMR measurements tend to underestimate the residual water content due to long measurement dead times. Furthermore, it shows that the HFNMR relaxation time , as a function of the saturation, behaves according to the Brooks‐Corey model that describes the water retention function and thus allows for the prediction of the relative hydraulic conductivity ,. The EFNMR relaxation time as a function of the saturation degree differs from the Brooks‐Corey expectation due to the influence of the dephasing relaxation rate that is, in general, responsible for the difference of and . We assume that the dephasing relaxation rate itself, when induced by internal magnetic field gradients, depends on the water saturation. We introduce a model that accounts for this dependency with a weighting factor for the dephasing relaxation rate, given as a power law of the saturation degree. The model enables the description of as a function of the water saturation and thus provides the estimation of from . We compare the NMR based predictions with the functions estimated from gravity induced outflow experiments at the columns. The results are in agreement within half a decade for every sand sample of the study. In principle, the suggested approach can be applied for estimating in situ by MRS measurements in the vadose zone. We discuss the potential and limitations of this approach for MRS.