Three-dimensional monitoring of Vadose Zone infiltration using electrical resistivity tomography and cross-borehole ground-penetrating radar

We discuss results from field experiments conducted at the Sandia-Tech Vadose Zone experimental facility on the New Mexico Tech campus at Socorro, New Mexico, as part of a project to develop a joint hydrological-geophysical method to characterize fluid flow properties of the vadose zone. The site contains dense arrays of tensiometers, access tubes for neutron moisture and ground-penetrating radar probes, and arrays of surface and subsurface electrical resistivity tomography electrodes installed in shallow clays, sands and gravels. We collected electrical resistivity tomography (ERT), cross-borehole ground-penetrating radar (XBGPR) and neutron data before and during a controlled infiltration of water at the site. Using local, empirical relations, we estimated subsurface moisture contents from images generated with the XBGPR and ERT data. The XBGPR images provided an excellent comparison to the neutron-derived moisture contents along a plane through the center of the experimental site. The ERT results were limited in terms of resolution by the coarse electrode spacing and inversion mesh used at the site, but provided a full three-dimensional picture of the wetting front as it progressed.