Time-lapse surface-to-surface GPR measurements to monitor a controlled infiltration experiment

The knowledge of moisture content changes in shallow soil layers has important environmental implications and is fundamental in fields of application such as soil science. In fact, the exchange of energy and water with the atmosphere, the mechanisms of flood generation as well as the infiltration of water and contaminant into the subsurface are primarily controlled by the presence of water in the pores of shallow soils. At the same time, the estimation of moisture content in the shallow subsurface is a difficult task. Direct measurements of water content require the recovery of soil samples for laboratory analyses: sampling is invasive and often destructive. In addition, these data are generally insufficient to yield a good spatial coverage for basin-scale investigations. In-situ assessment of soil-moisture contents, possibly at the scale of interest for distributed catchment-scale models, is therefore necessary. The goal of this paper is to assess the information contained in surface-tosurface GPR surveys for moisture content estimation under dynamic conditions. GPR data are compared against and integrated with TDR (Time Domain Reflectometry) data. TDR and surface-to-surface GPR data act at different spatial scales and two different frequency ranges. TDR, in particular, is widely used to estimate soil water content, e.g. converting bulk dielectric constant into volumetric water content values. GPR used in surface-to-surface configuration has been used increasingly to quickly image soil moisture content over large areas. Direct GPR wave velocity is measured in the ground. However, in the presence of shallow and thin low-velocity soil layers, such as the one generated by an infiltrating water front, dispersive, guided GPR waves are generated and the direct ground wave is not identifiable as a simple arrival. Under such conditions, the dispersion relation of guided waves can be estimated from field data and then inverted to obtain the properties of the guiding layers. In this paper, we analyze the GPR and TDR data collected at an experimental site of the University of Turin, during a controlled infiltration experiment.