Characterization of Wetting Front Geometry and Fluid Migration in the Vadose Zone Using Surface Time-Lapse Seismic First-Arrival Tomography

Characterizing and quantifying 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 (e.g. soil type and saturation). Quantifying parameters is 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 at many scales. Many in-situ methods for calculating K (i.e. constant head permeameter method) use surface measurements to quantify subsurface fluid flow. However, the geometry and distribution of the fluid migrating through the subsurface is not determinable from surface measurements, and therefore these techniques 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 the vadose zone is developed using time-lapse seismic first-arrival tomography (TLSFT). The TLSFT method is based on the concept that 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 successfully images a migrating wetting front through the vadose zone at different time steps to visually characterize the geometry and distribution of water. Calculated K values using TLSFT infiltration data with an empirical formula are within one to two orders of magnitude of calculated K from an Amoozemeter experiment and known K of the Sequatchie soil series at the B-4 plot. TLSFT calculated ranges of K values are reasonable when considering the spatial and temporal extent of K at a single site.