The challenge of measuring rock moisture–a laboratory experiment using eight types of sensors

Abstract

Surface geomorphological processes and the decay of heritage buildings are amplified by rock weathering, which mostly either requires water or is augmented by its presence. Measuring of water content in rock is, however, challenging and different scientific aims require different approaches. To find the most suitable rock moisture investigation methods, we conducted an experiment using eight types of moisture measurement (1D resistivity, 2D resistivity, TDR, borehole humidity, microwave reflectance, capacitance, IR thermography, and uranine-probes) under controlled conditions in a sandstone block that was subject to a slow wetting and drying cycle and to a series of freeze-thaw cycles. Critical evaluation of methods shows that measurement of dielectric properties as a proxy of rock water content can be recommended for most research aims whether for long-term monitoring, non-destructive measurement of surface moisture patterns, or for applications in deeper areas of rock. Moreover, observation of moisture dynamics in deeper subsurface requires either drilling inside the rock or the use of ERT. To determine the location of the subsurface evaporation front, uranine-probes and borehole humidity sensors are recommended. Lastly, the specific nature of freeze-thaw environments shows that the use of 1D resistivity and TDR can outperform other methods tested, with TDR being more reliable than resistivity but lacking in defined depth moisture fluctuations. To challenge the disadvantages of tested methods, a combination of techniques should always be considered.