An examination of NMR logging to determine soil water content for agricultural investigations

Abstract:

Nuclear magnetic resonance (NMR) logging is a geophysical method that is directly sensitive to hydrogen protons in water and in oil and is an established method for characterizing the porosity of petroleum reservoirs. The more recent development of slim-line NMR logging tools for near-surface geophysical investigations has aided in the enhanced use of NMR for groundwater applications to estimate the porosity of aquifers. However, while NMR has now been widely used to characterize saturated systems, it’s use in partially saturated systems is still being developed. Here we examine the use of NMR measurements to estimate soil water content under different field conditions at a range of agricultural sites. The application of NMR for agricultural studies could provide an understanding of soil water content that is critical for designing efficient irrigation practices.

NMR measurements were collected using a small-diameter NMR probe at three sites in northwestern Illinois and one site in central California. Data were collected at in situ water content conditions at 15 locations across the three Illinois sites in May 2016. At these sites, in addition to collecting NMR data at field conditions, NMR data were collected at six of the locations after infiltrating with 20 to 50 gallons of water. Data were collected at in situ water conditions in 19 PVC-cased boreholes at an irrigated field site in central California in June and September 2016. To verify the NMR water content estimates, gravimetric water content was determined in the laboratory on soil samples collected at each location and, at the California locations, water content was also measured using time domain reflectometry (TDR). Water contents ranged from 0.2 to 41.5% across all location. At high water contents the NMR data matched well with both TDR and gravimetric water content; at low water contents, the NMR data in PVC-cased boreholes are relatively lower than TDR water content. The results of the two NMR field studies show that the borehole NMR measurements could provide valuable information about groundwater evaluations in the agricultural conditions.