Evaluation of Electromagnetic Induction as a Reconnaissance Technique to Characterize Unsaturated Flow in an Arid Setting

The use of apparent electrical conductivity (EC a ) measured with electromagnetic (EM) induction was examined as a reconnaissance tool for characterizing unsaturated flow in a semiarid region in the Chihuahuan Desert of Texas. Aboveground conductivity meters (EM31 and EM38) were used to measure EC a along transects in various geomorphic settings. Eight boreholes were drilled at different locations along the transects, and a downhole conductivity meter (EM39) was used to measure EC a . Samples were collected for analysis of clay, water, and chloride content to evaluate factors affecting spatial variability in EC a . Variations in EC a measured with the aboveground EM31 meter were affected by variations in clay content in a playa/interplaya setting, water content in a fissure, and chloride content adjacent to a drainage system. These factors affecting EC a were confirmed by comparing EC a measured with the downhole EM39 meter and clay, water, and chloride content of soil samples from boreholes. The hydrologic significance of parameters controlling EC a was evaluated. Variations in clay content are not hydrologically significant in this basin. High correlations between EC a and water content are difficult to interpret because in some areas water content variations simply reflect variations in clay content, as in the playa/interplaya setting, whereas in other areas higher water contents reflect higher water flux, as in the fissure. In some areas water content was below threshold values; therefore, EC a did not respond to water content or salinity in these areas. Although EM induction alone cannot distinguish causes of EC a changes, it provides a valuable tool for delineating variations in EC a that can be used to guide borehole locations and to provide valuable information for interpolating and extrapolating from point estimates provided by borehole data.