GEOPHYSICAL MAPPING OF GLACIAL SEDIMENTS TO THE HYDROGEOLOGIC FRAMEWORK OF EASTERN NEBRASKA: METHODS FOR OTHER GLACIATED TERRAINS

Pre-Wisconsin glaciation of eastern Nebraska resulted in extensive deposits of till and other sediments that are obscured by a cover of younger loess. Aquifers within these glacial sediments are limited in extent and connection to surface water is poorly known. This has created a demand for innovative approaches to data collection for development of hydrogeologic framework models. Airborne Electromagnetic (AEM) surveys were used to map aquifers in these glacial terrains. Tool selection was based on extensive testing of various geophysical methods near Oakland, Nebraska, along with AEM surveys in five other areas of eastern Nebraska in 2007 and 2009. Surveys of highly resistive areas within the glacial sediments correlate to and expand on known sand and gravel aquifers. To date, 680 km² have been surveyed using AEM. Surveyed sites were evaluated for spatial distribution of clay content and thickness of till. The Ashland survey shows areas of highly resistive material interpreted as sand and gravel units of alluvial or glacial origin. The Firth survey shows two distinct bands of sand and gravel forming shallow alluvial features in the till. The Oakland area contains deep paleochannels that were not mapped by the AEM survey but which did detect shallow alluvial sediments. Deep paleochannel fills are important sources of water and known to underlie the surveyed areas, but were not detected using the frequency-domain AEM system. Results from the Ashland, Firth, and Oakland sites led to additional AEM surveys. The Oakland area contains the thickest glacial till mapped with AEM and was selected as a test area for the effectiveness of other ground-based geophysical methods. Results of the additional ground surveys suggest that airborne time-domain systems are capable of mapping the deeper paleochannels in the Oakland area. AEM surveys within limits of the physical system can define areas of shallow, localized sand and gravel aquifers and buried aquifers in glacial terrains.