Joint inversion of Wenner and dipole–dipole data to study a gasoline-contaminated soil

Abstract

The goal of this work was to study a contaminated soil due to a gasoline spill produced by fissures in a concrete purge chamber located along a gas transmission line. A monitoring well drilled 16 m down gradient from the purge chamber revealed the presence of a gasoline layer of 0.5 m thick at 1.5 m depth, floating on top of the water table. A second well, drilled 30 m away from the first well, and in the same direction, did not show any evidence of contamination. To investigate this problem, a geoelectrical survey was conducted, combining dipole–dipole and Wenner arrays. First, four dipole–dipole profiles in a direction perpendicular to the longitudinal axis joining the wells were carried out. The electrical tomographies obtained from the 2D inversion of the data showed that the contaminated region was characterized by a resistive plume located at a depth between 1 to 2 m and had lateral extent of about 6–8 m. The longitudinal extension was less than 20 m, since the last profile located 30 m farther from the chamber did not show this kind of anomaly. To better determine the longitudinal extension, we performed a dipole–dipole profile along a line in this direction. The inverse model confirmed that the extension of the contaminated section was about 16 m. To complete the study of the deeper layer, we carried out Wenner soundings. The results of the inversion process indicated that to a depth of 20 m the soil was very conductive, because of the presence of clays as the main constituents, which confine the contaminant within this impermeable surrounding. To improve the inverse model, we performed a joint inversion of dipole–dipole and Wenner data. Analysis of the depth of penetration showed that it increased to 25 m and comparing the resulting model with the ones obtained from each array separately, we concluded that the joint inversion improves the depth obtained by the survey, while maintaining the shallow lateral resolution.