Examining the information content of time-lapse crosshole GPR data collected under different infiltration conditions to estimate unsaturated soil hydraulic properties

• Published in: Advances in water resources 2013-04, Vol.54, p.38-56
• Main Authors: Scholer, M., Irving, J., Looms, M.C., Nielsen, L., Holliger, K.
• Format: Article
• Language: English
• Subjects: Crosshole ground-penetrating radar; Earth sciences; Earth, ocean, space; Estimates; Exact sciences and technology; Fluid flow; Ground penetrating radar; Hydraulics; Hydrology; Hydrology. Hydrogeology; Infiltration; Information content; Inversions; Markov-chain-Monte-Carlo; Mathematical models; Soil hydraulic properties; Stochastic inversion; Vadose zone
• ISSN: 0309-1708; 1872-9657
• DOI: 10.1016/j.advwatres.2012.12.011

► Estimate hydraulic properties from GPR data collected under 3 infiltration conditions. ► Consider value of adding different amounts of time-lapse GPR data into the inversion. ► Forced infiltration is found to offer the greatest parameter refinement. ► Considering greater amounts of time-lapse data refines the hydraulic parameters. ► Inconsistencies observed with regard to the field data point to model errors. Time-lapse geophysical data acquired during transient hydrological experiments are being increasingly employed to estimate subsurface hydraulic properties at the field scale. In particular, crosshole ground-penetrating radar (GPR) data, collected while water infiltrates into the subsurface either by natural or artificial means, have been demonstrated in a number of studies to contain valuable information concerning the hydraulic properties of the unsaturated zone. Previous work in this domain has considered a variety of infiltration conditions and different amounts of time-lapse GPR data in the estimation procedure. However, the particular benefits and drawbacks of these different strategies as well as the impact of a variety of key and common assumptions remain unclear. Using a Bayesian Markov-chain-Monte-Carlo stochastic inversion methodology, we examine in this paper the information content of time-lapse zero-offset-profile (ZOP) GPR traveltime data, collected under three different infiltration conditions, for the estimation of van Genuchten–Mualem (VGM) parameters in a layered subsurface medium. Specifically, we systematically analyze synthetic and field GPR data acquired under natural loading and two rates of forced infiltration, and we consider the value of incorporating different amounts of time-lapse measurements into the estimation procedure. Our results confirm that, for all infiltration scenarios considered, the ZOP GPR traveltime data contain important information about subsurface hydraulic properties as a function of depth, with forced infiltration offering the greatest potential for VGM parameter refinement because of the higher stressing of the hydrological system. Considering greater amounts of time-lapse data in the inversion procedure is also found to help refine VGM parameter estimates. Quite importantly, however, inconsistencies observed in the field results point to the strong possibility that posterior uncertainties are being influenced by model structural errors, which in turn underlines the fundamental importance of a systema