Monitoring bedrock vadose zone water storage dynamics with time-lapse borehole nuclear magnetic resonance well logging

Abstract

Bedrock vadose zone water storage dynamics are a critical component of the hydrologic cycle in many catchments, but direct observations of these dynamics are rare. Nuclear magnetic resonance (NMR) methods are sensitive to volumetric water content and to pore chemistry and structure, making NMR a candidate for directly observing bedrock vadose zone water storage dynamics and the material properties associated with them. However, applications of NMR to study water storage in bedrock vadose zones are rare. Here we present the first use of time-lapse borehole nuclear magnetic resonance well logging to monitor and characterize seasonal water content changes in the deeply weathered bedrock vadose zone at two sites in Northern California. The work is presented in two chapters, each of which focuses on one of the two aspects of the NMR measurement: water content and relaxation times. In the first chapter, we evaluate the ability of borehole NMR to quantify water content changes in weathered bedrock. We show strong agreement between estimates of dynamic water storage derived from NMR and independent estimates from neutron logging and mass balance calculations. The agreement between NMR and neutron estimates of dynamic storage suggests that all seasonally exchanged bedrock water is hosted in fractures, and not the matrix, at these sites. The depths of dynamic storage we observe are up to 9 m and likely reflect the depth extent of root-water uptake. In the second chapter, we document the relationship between bedrock weathering and NMR relaxation times. We find that the sum of echoes (SE) is a useful approach for characterizing NMR relaxation times in the vadose zone, and we use the functional dependence of SE on water content to show that enhanced bedrock weathering extent is associated with faster relaxation times. We find evidence that NMR relaxation times can be sensitive to changes in pore pressure associated with recharge events. The work presented here establishes borehole NMR well logging as a viable method for in situ vadose zone monitoring and characterization.