Monitoring soil moisture in the deeper vadose zone: A new approach using groundwater observation wells and cosmic ray neutrons

Monitoring soil moisture at depths greater than one meter is generally challenging and often highly invasive as it requires opening large soil pits. As a result, this deeper vadose zone is often not monitored at all. On top of that, conventional soil moisture sensors usually have only a small measurement volume. On the other hand, soil moisture estimates derived from above-ground Cosmic-Ray Neutron Sensing (CRNS) are a representative average over an area of several hectares but only of the upper half meter of the soil. To this day, it is commonly believed that cosmic radiation cannot be used to monitor soil water content below this depth. As a consequence, large parts of the root-zone and deeper unsaturated zone have remained outside the observational window of the method. The estimation of soil moisture in greater depths typically requires additional invasive measurements, other active geophysical methods, or mathematical models which extrapolate surface soil moisture observations.

Against this background, we investigated the possibility of using passive detection of cosmogenic neutrons in existing monitoring infrastructure (e.g. groundwater wells). We hypothesized that this method provides a larger measurement volume than traditional techniques based on active neutron probes while requiring less safety restrictions.

Our neutron transport simulations demonstrated that this downhole-CRNS technique would be sensitive enough to detect changes of water content in depths down to 5 meters and above, depending on the temporal resolution of measurements. The simulations also revealed a large measurement radius of several tens of cm depending on the soil moisture content and soil bulk density.

From the theoretical results we derived a functional relationship between soil moisture and detectable neutrons and tested it in a groundwater observation well. Additional installations of supporting soil moisture sensors have been used to validate the model predictions as well as the neutron signals monitored by the CRNS detector. The study demonstrated the general applicability of downhole Cosmic-Ray Neutron Sensing for the estimation of soil moisture in greater depths and at temporal resolution of two days.