23 Nov 2022
23 Nov 2022
Status: this preprint is currently under review for the journal HESS.

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

Daniel Rasche1,, Jannis Weimar2,, Martin Schrön3, Markus Köhli2,4, Markus Morgner1, Andreas Güntner1,5, and Theresa Blume1 Daniel Rasche et al.
  • 1GFZ German Research Centre for Geosciences, Section Hydrology, 14473, Potsdam, Germany
  • 2Physikalisches Institut, Heidelberg University, Im Neuenheimer Feld 226, 69120 Heidelberg, Germany
  • 3UFZ – Helmholtz Centre for Environmental Research GmbH, Dep. Monitoring and Exploration Technologies, Permoserstr. 15, 04318, Leipzig, Germany
  • 4Physikalisches Institut, University of Bonn, Nussallee 12, 53115 Bonn, Germany
  • 5University of Potsdam, Institute of Environmental Sciences and Geography, 14476, Potsdam, Germany
  • Daniel Rasche and Jannis Weimar contributed equally to this work.

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.

Daniel Rasche et al.

Status: open (until 18 Jan 2023)

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Daniel Rasche et al.

Daniel Rasche et al.


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Short summary
We introduce passive downhole cosmic-ray neutron sensing (d-CRNS) as an approach for the non-invasive estimation of soil moisture in deeper layers of the unsaturated zone which exceed the observational window of above-ground CRNS applications. Neutron transport simulations are used to derive mathematical descriptions and transfer functions while experimental measurements in an existing groundwater observation well illustrate the feasibility and applicability of the approach.