A Laboratory Study to Determine the Effect of Partially Saturated Conditions on Relaxation

Nuclear magnetic resonance, NMR, is a geophysical method that is sensitive to the quantity as well as the physical and chemical environments of hydrogen in porous media. In near-surface geophysics, NMR is used to determine water content and estimate pore size distribution in the top 100 m of the surface. The interpretation of NMR data in near-surface geophysics generally assumes fully saturated conditions exist in the measured volume; however, little is known about the effect partially saturated conditions have on the interpretation NMR data. In this laboratory study, we examine the effect of partially saturated conditions on measured NMR parameters. The NMR experiment consists of measuring a multi-exponential decay of magnetization after an oscillating magnetic field is applied to a sample. The multi-exponential decay is characterized by the initial amplitude, M0, and the mean log decay time, T2ml. The initial amplitude of this decay, M0, is proportional to the total water content present in a sample. In fully saturated conditions, the T2ml value is related to the surface-area-to-volume ratio. To understand the effect of partially saturated conditions on the interpretation of NMR data, we have collected NMR measurements on samples at different saturations during periods of wetting and drying and determined M0 and T2ml. We observe that T2ml increases with saturation, and M0 is linearly proportional to water content. In addition, the T2ml values at similar water contents were are shorter under drying than wetting conditions. We infer that the observed differences in the T2ml values are due to changes in the geometric distribution of water within the pore space of the sample. The results from this study will help to improve the interpretation of NMR measurements in partially saturated porous media.