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

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Abstract
The use of apparent electrical conductivity (ECa) 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 ECa 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 ECa. Samples were collected for analysis of clay, water, and chloride content to evaluate factors affecting spatial variability in ECa. Variations in ECa 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 ECa were confirmed by comparing ECa measured with the downhole EM39 meter and clay, water, and chloride content of soil samples from boreholes. The hydrologic significance of parameters controlling ECa was evaluated. Variations in clay content are not hydrologically significant in this basin. High correlations between ECa 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, ECa did not respond to water content or salinity in these areas. Although EM induction alone cannot distinguish causes of ECa changes, it provides a valuable tool for delineating variations in ECa that can be used to guide borehole locations and to provide valuable information for interpolating and extrapolating from point estimates provided by borehole data.