The effects of temperature on shallow borehole NMR measurements in permafrost

Abstract

High fidelity observations of the amount and state of water within permafrost help constrain the seasonal behavior of soil moisture and the effects of soil moisture on the surface energy balance. This work emphasizes the necessity for temperature-specific calibrations of low-frequency borehole NMR measurements. Constraining the effects of temperature on NMR signatures will allow for more reliable NMR inspection of hydrogeochemical parameters in permafrost ecosystems. We find that calibration at typical laboratory temperatures (20 °C) and subsequent measurement at typical permafrost active layer temperatures (∼0 °C) can result in an 18% bias in reported NMR water content values, and therefore temperature compensation is required under most scenarios. This is particularly important for active layer conditions that may include steep vertical temperature gradients. Similarly, seasonal time-lapse measurements of permafrost active layer may encounter substantial soil temperature variations which would also require temperature compensation on the observed NMR water content estimate.