Measuring of water concentration in rock and soil with muon radiography

Dry rock and soil have the capability to absorb and contain water because of their porosity. Measuring water concentration in different materials is important for many applications. For example, in heap leaching the leaching liquid distribution directly affects the metal recovery. The water concentration is the key parameter affecting the slope stability, and its measurement is therefore of a great importance for landslide prevention and tailing dams safety. Measurement of this parameter is however a significant challenge, and is done mostly by measuring rainfall in the field and/or analyzing soil samples in the laboratory. These measurements are model-dependent and require significant extrapolation of the results. 

A direct measurement of the water concentration in-situ is offered by muon radiography - through measuring density distribution based on cosmic-ray muon flux monitoring. By measuring the flux of muons from different directions we can reconstruct density maps of any object they traverse, including large volumes of rock. By monitoring the muon flux, our sensors are sensitive to the density changes in any material above it.  From these density changes we can infer the amount of water contained within the rock or soil.
         
We performed the first in-situ, fully volumetric monitoring of the water concentration in an active leaching heap in Chile.  Here we discuss the results of these pioneering measurements and the potential for muon imaging of the water concentration for different applications.