AEM MAPPING OF GROUNDWATER RESOURCES WITHIN THE GLACIAL DEPOSITS AND CRETACEOUS DAKOTA FORMATION OF EASTERN NEBRASKA

An Airborne Electromagnetic (AEM) survey was conducted over the glacial deposits of eastern Nebraska for mapping groundwater resources. The AEM survey area is covered with 100-150 meter thick pre-Illinoisan Age glacial material deposited unconformably on Cretaceous Age bedrock, including the Carlile Shale, Greenhorn Limestone, Graneros Shale and the Dakota Group. The hydrogeology of the area is dominated by a thick sequence of the clay-rich Clarkson Till. Thin layers of sands and sand and gravel within the glacial deposits provide the only substantial aquifer in the area. Within the Dakota Group, sandstone deposits may serve as a possible aquifer, however, to this date they have not been utilized for production wells within the survey area. Petrophysical data indicate that the clay-rich till of the glacial deposits are from 5- 15 ohm-m, the fine sands and silts are 15-25 ohm-m and the sands and gravels are 25-50 ohm-m. Resistivity within the Carlile Shale ranges from 3-10 ohm. Resistivity for shale within the Dakota Group is 3-20 ohm-m and Dakota sandstones are 25-40 ohm-m. The sands and gravels within the 100-150 meter thick glacial deposits are detected by the AEM as are the Dakota sandstones up to 300 meters in depth. Thin layers of sand and gravel (<10 meters) at a depth of 100 meters sitting on top of the Dakota sandstones are not differentiated by the AEM and provide a single resistive target. The AEM survey provided the detail to connect the limited borehole data to indicate that the glacial aquifer is limited to the thin layers of sand and gravel within the Clarkson Till. The AEM also mapped a potential deeper aquifer within the sandstones of the Cretaceous Dakota Group, which may serve as an alternate groundwater resource in the area.