Surface NMR instrumentation and methods for detecting and characterizing water in the vadose zone

Abstract

ABSTRACTA commercially available surface NMR instrument was modified to address the challenges of using earth’s field surface NMR to detect and characterize water in the unsaturated (or vadose) zone. The modified instrument incorporates faster switching electronics to achieve an instrument dead time of 2.8 ms, and higher output power electronics to enable a maximum coil voltage of 8000 volts and coil current of 800 amps. The instrument was used to collect and interpret surface NMR data at several active vadose zone investigation sites in the western US. A 6‐week surface NMR experiment was conducted at a managed aquifer storage and recovery facility in Arizona, to explore the measurement capabilities and limitations of the instrument, during a managed infiltration event. The resulting time lapse surface NMR data were used to map zones of held water prior to the flood event, image the influx of water through the top 15 metres of the subsurface during and after the event, quantify the spatial and temporal distribution of infiltrating water throughout the event, and characterize the distribution of water in different relative pore sizes throughout the event. Data obtained at pseudo‐static vadose zone investigation sites indicate that the surface NMR instrument can detect and image some forms of water held in unconsolidated vadose zone formations, at depths up to 30 metres. Complementary NMR logging data indicate that the surface NMR instrument does not detect all of the water held in these pseudo‐static formations, but that the non‐invasive surface NMR data may yield valuable information nonetheless.