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 re-scaled neutron intensities observed below the terrain surface with intensities measured above a water body. An experimental set-up with a CRNS sensor deployed at different depths up to 10 meters below the surface in a groundwater observation well combined with particle transport simulations revealed the response of downhole thermal neutron intensities to changes in 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 decimeters, which depends on soil moisture and soil bulk density, exceeds the one 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.