Description:
<jats:p>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.
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