A Coupled Hydrogeological-Petrophysical Analysis of Geophysical Variations in the Vadose Zone

This study compares fluid content profiles derived from high-resolution geophysical data acquired before and after a controlled light non-aqueous phase liquid release experiment with those from hydrogeological models (Van Genuchten and Brooks-Corey) based on laboratory saturation-capillary pressure data for the porous medium. Standard petrophysical relationships (e.g., Archie\u27s equation, Topp\u27s equation and Complex Refractive Index Method) were used to convert in situ geophysical properties to fluid contents and saturations. The total fluid content profiles estimated from neutron probe data using a linear relationship is a good overall match to the hydrogeological prediction for both uncontaminated and contaminated conditions. In contrast, the water saturation and content profiles estimated from the vertical resistivity probe and time domain reflectometry data, respectively, differ appreciably from the hydrogeological model predictions of water content. Further, these differences are greater for the three-phase contaminated case. The results of this study suggest that care should be exercised when estimating fluid content from geoelectrical properties using standard petrophysical relationships. The inference of water content from geoelectrical probes therefore requires more study necessitating the development of petrophysical relationships that are applicable over a wider range of vadose zone conditions