Use of Towed Electromagnetic Conductivity Surveys for Reconnaissance Geophysics: An Example from Saline Lakes In the Nebraska Sand Hills

Developing an understanding of the hydrogeology of saline lakes requires knowledge of study area hydrostratigraphy and the location of subsurface structures that control ground- water flow and salinity. Electrical resistivity tomography (ERT) is a quantitative geoelectrical method useful for developing hydrostratigraphic models and delineating the distribution of saline ground water. However, ERT methods require the emplacement of numerous electrodes along the survey line, limiting productivity to a few kilometers (km) per day. One way to optimize ERT survey location and design is to perform reconnaissance surveys around saline lakes using non-contact geophysical methods capable of identifying geoelectrical trends and targets for follow-up ERT testing. For this study, we tested the use of a towed multi-frequency electromagnetic (EM) conductivity instrument (capable of penetrating more than10 meters in this environment) to conduct reconnaissance geophysical surveys around saline lakes.

In May 2008, EM surveys were conducted around three saline lakes near the Crescent Lake National Wildlife Refuge in the Sand Hills of western Nebraska. Over 25 km of data were collected per day by mounting the GPS-enabled EM instrument to a fiberglass cart and towing with an all-terrain vehicle (ATV). Data were collected at 15 frequencies from 3-63 kHz. Field-plotting of the geo-referenced data revealed trends in EM conductivity distribution around the lakes. The lowest conductivities (<10 mS/m) were found along the northern and western shores, with the highest conductivities (>100-500 mS/m) focused on the eastern and southern shores, consistent with regional and local ground-water flow directions. In addition, the EM data delineated numerous specific targets for follow-up ERT testing, and comparison of high- and low-frequency EM data enabled prediction of ground water salinity trends with depth. This study showed that use of towed EM conductivity methods is an effective way to perform rapid geophysical reconnaissance of saline lake environments.