Estimating water content and soil texture using nuclear magnetic resonance

Abstract:

Nuclear magnetic resonance (NMR) is a geophysical method that rapidly gaining popularity and, in hydrogeophysics, is used to evaluate groundwater resources with measurements collected on fully saturated systems. Two NMR measures, the initial signal magnitude and the relaxation time distribution, are determined from the raw NMR signal and used to estimate the porosity and the pore size distribution of the measured volume. While the theory linking the relaxation time distribution to the pore size distribution in saturated media is well established, in unsaturated systems the theory breaks down. Here we examine the use of NMR measurements to estimate water content and soil texture, which is related to the pore-size distribution, in unsaturated, agricultural settings. Such information is important for understanding infiltration patterns and tracking the fate and transport of nutrients and contaminants. NMR measurements were collected in the field, using a non-invasive surface-based instrument and a downhole tool, at 15 locations across 3 field sites located in northwestern Illinois. Field measurements were collected at in situ water content conditions. Soil samples for laboratory analysis were collected at each measurement location. The soil samples were used to determine the in situ gravimetric water content, the soil texture, and saturated laboratory NMR response. The soil samples represent a large range of textures, from sand to clay, and in situ NMR determined water content ranged from 5 to over 30%. Good agreement was found between the NMR estimated water content in the field and the gravimetric water content determined in the lab. Comparing the field NMR measurements to the laboratory NMR measurements on saturated samples, we see consistent NMR responses for samples with high amounts of sand, and consistent NMR responses for samples with high amounts of clay. Our results suggest that even in unsaturated field conditions, NMR can be used qualitatively to assess soil texture.