Measuring Soil Moisture with a Slim-line Logging NMR Tool in Field

Soil texture and soil moisture transport are key factors in soil hydrology, crop planning and optimization, and the modeling of soil-vegetation-atmosphere processes, regarding both CO₂, the moisture and the energy balances. Nuclear magnetic resonance (NMR) can quantitatively detect water protons and thus it is a direct method to measure soil moisture and pore size. The design and use of a first-generation Slim-Line Logging (SLL) NMR tool for measuring water content in the vadose zone at shallow penetration depths had been reported earlier [1]. Then, a second-generation SLL NMR tool has been developed that reaches a penetration depth of 20 mm into the borehole wall without compromising sensitivity [2].

In this case, natural soil in a field site was studied with the second-generation SLL NMR tool to demonstrate that the sensor can follow the dynamics of imbibition and migration of a moisture front. Vadose zone soil moisture, and T₂ profiles were recorded up to 80 cm depth in a vertical borehole drilled into grassland sandy silt soil. The SLL tool was fixed at certain position to monitor the content variations of bound water, mobile water and total water. In addition, infiltration studies with flow rates of 50 L/h/m² were performed to assess soil water content at maximum saturation and the timescale of imbibition and subsequent desaturation. For both processes, exponential behavior was observed, in good agreement with [3]. In the next step, T₂-T₂ experiments will be carried out in the lab to quantitatively estimate water transport between different pore sizes.


References:
1. O. Sucre, A. Pohlmeier, A. Miniere and B. Blümich, Journal of Hydrology, 406 (2011) 30-38.
2. J. Perlo, E. Danieli, J. Perlo, B. Blümich and F. Casanova, Journal of Magnetic Resonance, 233 (2013) 74-79.
3. T.J. Marshall, J.W. Holmes and C.W. Rose, Soil Physics, 3rd edition, Cambridge University Press, 1996.