COMBINED USE OF FREQUENCY-DOMAIN ELECTROMAGNETIC ANDELECTRICAL RESISTIVITY SURVEYS TO DELINEATE THEFRESHWATER/SALTWATER INTERFACE NEAR SALINE LAKES IN THENEBRASKA SAND HILLS, NEBRASKA, USA

摘要

We investigate the use of frequency-domain electromagnetic (FDEM) and electrical resistivity (ER) surveys for rapid and detailed characterization of the direction of lake-aquifer fluxes and the configuration of salt plumes generated from saline lakes. This methodology was developed and applied at several lakes in the Nebraska Sand Hills, Nebraska, in an area with both freshwater and saline lakes hydraulically connected to the freshwater surficial aquifer. The FDEM survey was conducted by mounting the instrument on a fiberglass cart towed by an all-terrain vehicle. The towed FDEM surveys covered about 25 km per day and served as a reconnaissance method for choosing locations for the more quantitative and detailed ER surveys. Around the saline lakes, areas with high electrical conductivity are consistent with the regional direction of ground-water flow. Lower electrical conductivity was measured around the freshwater lakes with anomalies correlating to a paleovalley axis inferred from previous studies. The efficacy of this geophysical approach is attributed to: (1) significant contrast in electrical conductivity between freshwater and saltwater, (2) near-surface location of the freshwater/saltwater interface, (3) minimal cultural interference, and (4) relative homogeneity of the aquifer materials.