PRELIMINARY RESULTS FROM HELICOPTER ELECTROMAGNETIC SURVEYS OVER A PALEOVALLEY AQUIFER IN EASTERN NEBRASKA

Innovative approaches to water resources data collection are evolving in response to demands for the development of accurate hydrogeologic frameworks. Helicopter electromagnetic (HEM) surveys, flown in five selected areas of eastern Nebraska in 2007 and 2009, mapped high-resistivity areas that correlate to and expand on known sand and gravel aquifers. To date, a total area of 680 km² (263 mi²) has been surveyed using HEM. General flight line spacing was 270 m (890 ft). The primary purpose of the survey in 2007 was to test the applicability of HEM technology in identifying sand and gravel units underlying or within glacial deposits. Results from the 109 km² (42 mi²) Firth pilot-study area indicated that expanding the study area to the west might allow better delineation of the interconnectivity of sand units with surface-water systems. Apparent resistivity maps from the Firth area showed two elongated bands of high-resistivity material that were interpreted as sand and gravel forming shallowest portions and sides of a paleovalley aquifer. Evaluation of the depths of signal detection, which reached to 45 m (150 ft) in the deepest part of the sand and gravel unit, and the horizontal and vertical resolution were based on inverse modeling of vertical sections and resolution imaging, and not from the apparent resistivity maps. The data provide the aquifer’s general trend and areal extent, though not its full depth.

At Firth the 2009 survey extended the 2007 survey westward, but used the same flight line spacing and orientation as in 2007 and covered approximately 285 km² (110 mi²). The 2009 survey was to delineate the western end of the paleovalley aquifer. The apparent resistivity data collected in overlapping flight blocks of the 2007 and 2009 surveys were congruent. Geologists expected the paleovalley aquifer to appear as in the first survey. However, preliminary data from the second block show highly resistive material is more widespread than in the original flight block.

In addition to the paleovalley area, a separate small survey block of 78 km² (30 mi²) was completed in the Swedeburg area. This survey is located on the western edge of the Todd Valley. The apparent resistivity maps from each survey are congruent and define areas of possible shallow and localized sand and gravel aquifers.