CHARACTERIZATION OF WETTING FRONT GEOMETRY AND FLUID MIGRATION IN THE VADOSE ZONE USING SURFACE TIME-LAPSE SEISMIC FIRST-ARRIVAL TOMOGRAPHY

Characterizing vadose zone parameters and processes are critical for assessing environmental, agricultural, and engineering problems. The shallow subsurface is essential to the geologic and hydrologic cycles because it supports agriculture and ecosystems, influences water resources, and acts as a repository for contaminants. Fluid migration in the vadose zone is dependent on a number of soil characteristics. The quantification of parameters are often the primary goal of hydrological fluid flow investigations, however the values calculated can be misrepresentative of the subsurface due to anisotropic features. Hydraulic conductivity (K), the most common quantitative parameter used to describe fluid flow through a porous medium, is complicated in the vadose zone due to spatial and temporal variations. Many in-situ methods for calculating K (i.e. constant head permeameter method) use surface measurements to quantify subsurface fluid flow. The geometry and distribution of the fluid migrating through the subsurface is not determinable from surface measurements, therefore they may not provide a comprehensive understanding of field-scale fluid flow. In this study, a more robust, non-invasive method to image a migrating wetting front in in the vadose zone uses time-lapse seismic first-arrival tomography (TLSFT). Variations in seismic compressional wave (P-wave) arrival times are used as a proxy for the relative saturation changes due to an advancing wetting front generated from surface infiltration. Two constant flux applicator infiltration experiments were conducted at the East Tennessee Research and Education Center B-4 plot, while simultaneously collecting TLSFT data to image a migrating wetting front. The TLSFT infiltration method is successful in imaging an isolated wetting front migrating through the vadose zone at different time-steps to visually characterize the wetting front geometry and distribution. An additional infiltration experiment using an Amoozemeter, a standard instrument and methodology used to calculate K, was also conducted for quantitative comparison to the TLSFT infiltration method. Calculated K values from the TLSFT method are within one order of magnitude of the calculated K from the Amoozemeter experiment and known K values for the B-4 plot soil type.