Evaluating water distribution in unsaturated zone using nuclear magnetic resonance

Abstract:

Probing water distribution and transportation has always been a crucial but challenging task in characterizing hydraulic properties in the critical zone. Nuclear magnetic resonance (NMR), the only geophysical method that has direct sensitivity to water, has emerged as a novel tool for hydrogeological investigation in both borehole and surface measurements. Recent interest in studying water storage under unsaturated conditions is surging, however, the application of NMR in unsaturated critical zones has been limited to due to the poorly developed understanding of NMR responses in unsaturated conditions. , it is essential to test the applicability of NMR methods in unsaturated environment. The purpose of this study is to quantify spatial variability of water in an unsaturated zone using integrated NMR methods. We acquired three forms of NMR measurements including surface NMR (sNMR), borehole NMR (bNMR), and laboratory NMR in a merokarst (carbonate aquifers interbedded with shale) watershed of Konza Prairie, Kansas. The bNMR data suggests a significant amount of water exhibit in both limestone and shale layers during the drying season in Konza. By comparing sNMR with bNMR data, it is evident that sNMR underestimates the volumetric water content due to lack of signals from water in micropores in fast relaxation. On the contrary, bNMR data agrees well with laboratory NMR in both water content and T2 distribution. To benefit from both the penetration provided by sNMR and accurate water distribution provided by bNMR and laboratory NMR, we extracted signals lower than 10ms from laboratory NMR and appended them to the sNMR data. As an outcome, the volumetric water contents of shale layers and carbonate layers both increases. Our results indicate that a combination of different forms of NMR measurement is necessary to overcome the instrumental limitations and appropriately evaluate the unsaturated zone in critical zones.