A change in perspective: downhole cosmic-ray neutron sensing for the estimation of soil moisture

Abstract

Abstract. Above-ground cosmic-ray neutron sensing (CRNS) allows for the non-invasive estimation of the field-scale soil moisture content in the upper decimetres of the soil. However, large parts of the deeper vadose zone remain outside of its observational window. Retrieving soil moisture information from these deeper layers requires extrapolation, modelling or other methods, all of which come with methodological challenges. Against this background, we investigate CRNS for downhole soil moisture measurements in deeper layers of the vadose zone. To render calibration with in situ soil moisture measurements unnecessary, we rescaled neutron intensities observed below the terrain surface with intensities measured above a waterbody. An experimental set-up with a CRNS sensor deployed at different depths of up to 10 m below the surface in a groundwater observation well combined with particle transport simulations revealed the response of downhole thermal neutron intensities to changes in the soil moisture content at the depth of the downhole neutron detector as well as in the layers above it. The simulation results suggest that the sensitive measurement radius of several decimetres, which depends on soil moisture and soil bulk density, exceeds that of a standard active neutron probe (which is only about 30 cm). We derived transfer functions to estimate downhole neutron signals from soil moisture information, and we describe approaches for using these transfer functions in an inverse way to derive soil moisture from the observed neutron signals. The in situ neutron and soil moisture observations confirm the applicability of these functions and prove the concept of passive downhole soil moisture estimation, even at larger depths, using cosmic-ray neutron sensing.

Buoy‐Based Detection of Low‐Energy Cosmic‐Ray Neutrons to Monitor the Influence of Atmospheric, Geomagnetic, and Heliospheric Effects

“…The largest difference between epithermal and thermal neutrons by applying the same correction occurs in respect to variations in absolute air humidity. We found that the linear regression slope, 0.0023, is less than half of that of epithermal neutrons and very close to the value recently found by Rasche et al (2023). The difference between the thermal and the epithermal neutron response to air humidity is likely linked to the generally higher production rate of thermal neutrons by epithermal neutron moderation compared to the thermal neutron absorption rate, which leads to a weaker response of thermal neutrons to variations in environmental hydrogen (Weimar et al, 2020).…”

“…The observation data in Figure 4 demonstrate that the thermal neutrons response to air humidity is much smaller compared to epithermal neutrons. Using the recently published correction factor, α = 0.0021 m 3 /g (Rasche et al, 2023), which is very close the empirical observation from the buoy, the new correlation becomes R 2 = 0.01 for thermal neutrons and thereby confirms the insignificance of the temperature effect.…”

“…For instance, Andreasen et al (2017) and Rasche et al (2021) did not correct thermal neutrons for variations in air humidity, arguing that the traditional correction functions have been derived for epithermal neutrons only. From dedicated simulations, Rasche et al (2023) found a new value for thermal neutron correction, α = 0.0021 m 3 /g. In contrast, based on empirical findings, Jakobi et al (2018Jakobi et al ( , 2022 correct thermal neutron intensities for air pressure and absolute humidity but not for variations in incoming radiation.…”

“…Consequently, the observations in this study indicate that epithermal and thermal neutron intensities need to be corrected for all three atmospheric variables. With respect to existing correction approaches, it is evident that the correction factor for air humidity should be different for epithermal and thermal neutrons, using α = 0.0054 m 3 /g (Rosolem et al, 2013) and α = 0.0021 m 3 /g (Rasche et al, 2023), respectively.…”

“…CRNS detectors, however, are sensitive to the surrounding environment up to radii of 300 m (Desilets & Zreda, 2013;Köhli et al, 2015). Hence, Franz et al (2013) suggested short measurements on a lake to calibrate the pure-water limit of the sensor response, which was conducted using rafts for a few days by McJannet et al (2014), Andreasen et al (2017) and Rasche et al (2023). The first long-term experiment of CRNS detectors on a lake was proposed and conducted in 2014 and later reported by Schrön et al (2016) and Schrön (2017).…”