Advancements and Applications of Low-Field NMR for Characterizing and Monitoring Hydrogeologic Properties in the Saturated and Unsaturated Zones

Abstract:

Geophysical NMR measurements allow direct quantification of fluid content and estimation of critical flow and storage parameters, including bound/mobile porosity, pore size, and permeability. Following widespread adoption of NMR logging in the oilfield, novel low-field NMR technologies have been developed for measuring groundwater and hydrogeologic properties in the near-surface. Adaptation and optimization of these technologies has been required to meet the unique objectives of near-surface investigations. Here we present recent technical enhancements to low-field NMR instrumentation and methodologies along with field results from hydrogeologic characterization and monitoring applications. Improvements to circuit design have led to a significant reduction of the NMR echo-time, yielding more accurate detection and quantification of signals with short T₂ (i.e. associated with water bound in clays and silts); the shorter echo-time also suppresses artifacts associated with magnetic mineralogy. Dynamic acquisition schemes including multi-frequency and multi-wait-time scanning have further increased the signal-to-noise-ratio (SNR), particularly for short T₂ signals. Taken collectively, these improvements yield a factor of 4 increase in SNR, allowing faster measurement times and significant reduction of the variance and uncertainty in estimated hydrogeologic parameters. These specific advancements have been implemented both in downhole NMR logging tools as well as non-invasive soil moisture sensors, and are presented here with field applications including (1) quantification of porosity and hydraulic conductivity (K) in unconsolidated aquifers, (2) monitoring of changes in subsurface biogeochemistry, and (3) quantification and monitoring of changes in shallow soil moisture.