Project Objectives:This project has been designed to analyze the resolution of two different geophysical imaging techniques (electrical resistivity tomography and cross-borehole ground penetrating radar) for monitoring subsurface flow and transport processes within the vadose zone. This is to be accomplished through a coupled approach involving large scale unsaturated flow modeling, petrophysical conversion of the resulting 2 and 3 Dimensional water content and solute concentration fields to geophysical property models and generation of synthetic geophysical data, followed by the inversion of the synthetic geophysical data. The resolution, strengths, and limitations of the geophysical techniques will then be ascertained through an analysis involving comparisons between the original hydrologic modeling results and inverted geophysical images. Increasing levels of complexity will be added to the models as the project progresses through the addition of heterogeneity in the original hydrologic property model, and by adding uncertainty to the petrophysical relationship that couples the geophysical model to the hydrologic modeling results.
Progress:To date much of the focus has been on the unsaturated flow modeling, including defining the hydrologic properties and their distributions within the models. Most of the work to date has involved running 2D and 3D simulations with straightforward homogenous and layered property distributions. We are beginning to work on developing more complex hydrogeologic property models, particularly for the 3-D modeling which requires using parallel computing resources at Sandia National Labs. A simple six-layer model has been designed to crudely simulate subsurface conditions at the Sandia-Tech Vadose Zone facility where two previous EMSP funded experiments were completed (EMSP Projects 55332 and 70267; Brainard et al., 2003. The hydrologic properties that have and are being incorporated into the model have used both laboratory measurements (Baker, 2002) as well as a pedo-transfer function approach (Chang, 2003). Because the latter correlates grain-size distribution to hydraulic properties, we have recently begun a detailed grain-size analysis of continuous core collected at the site using a laser device that is available at the Sandia National Laboratory facilities in Carlsbad, New Mexico. This will also provide us additional information in the spatial variability at the site, at least in the vertical direction. In hydrologic modeling activities we have focused on developing basic one-, two-, and three-dimensional (1D, 2D, and 3D) models. The 1D and 2D simulations have been conducted with HYDRUS-2D (Simunek J., and van Genuchten ,1999) and iTOUGH2 (Finsterely, 1997), and have allowed for efficient examination of optimal grid resolution as well as examination of initial and boundary condition effects on simulated water content fields. These simulations have also provided a foundation for development of more complex 3-D heterogeneous hydraulic property fields by allowing us to 1) constr...
