Vertical radar profiles for the characterization of deep vadose zones

The deep vadose zone, down to a few tens of meters below the soil surface, is difficult to investigate and characterize, especially from a hydraulic point of view. We present a method well suited for this purpose, that makes use of a radar transmitting antenna at the surface and a radar receiving antenna deployed in a plastic-cased borehole. The technique is often referred to as vertical radar profiling (VRP). Vertical radar profiling yields information on (i) the distribution of radar velocity as a function of depth, from which moisture content distributions can be inferred, and (ii) the presence of reflecting horizons within the formation, often associated with lithological contacts. High-resolution, time-lapse VRPs were acquired from October 2002 to December 2003 at a contaminated site in Trecate, Northern Italy, from several existing boreholes originally installed for monitoring and remediation, with the aim of characterizing the dynamic hydrologic behavior of the deep vadose zone. We discuss data acquisition, processing, and inversion of these VRP data to derive moisture content profiles as a function of time. The VRP-derived moisture content profiles were used to calibrate a dynamic Richards' equation model via a Monte Carlo inversion approach. In this way, it was possible to identify the value of subsurface hydraulic parameters, in particular hydraulic conductivity, of the main lithological units, and the parameters' uncertainty. The location of lithology contacts was derived via analysis of up-going radar reflection events contained in the same VRP data.