This report is a digital data release for a helicopter electromagnetic and magnetic survey that was conducted during March 2007 in three 93-square-kilometer (36-square-mile) areas of eastern Nebraska as part of a joint State of Nebraska and U.S. Geological Survey study. The objective of the survey is to improve the understanding of the relationship between surface-water and ground-water systems critical to developing water resource management programs. The electromagnetic equipment consisted of six different coil-pair orientations that measured electrical resistivity at separate frequencies from about 400 hertz to about 115,000 hertz. The electromagnetic data were converted to electrical resistivity geo-referenced grids and maps, each representing different approximate depths of investigation for each area. The range of subsurface investigation is comparable to the depth of shallow aquifers. The three areas selected for the study, Ashland, Firth, and Oakland, have glacial terrains and bedrock that typify different hydrogeologic settings for surface water and ground water in eastern Nebraska. The geophysical and hydrologic information from U.S. Geological Survey studies are being used by resource managers to develop ground-water resource plans for the area.
The Sand Hills of western Nebraska are part of an extensive terrain of dunes distributed across the Great Plains. These dune complexes provide unique ecosystems, recharge to underlying aquifers including the Ogallala aquifer, and are very sensitive to short and longer term climate changes. Understanding the hydrogeologic framework of these dune systems and their underlying aquifers is essential in management of groundwater resources and ecosystems. The hydrogeologic framework is a critical component in understanding implications of climate change for one of the most productive agricultural regions of the United States A frequency domain airborne electromagnetic (AEM) resistivity survey performed in 2010 in the area of Crescent Lakes National Wildlife Refuge (CLNWR) was extended several 10s of kilometers to the north and east with a time domain AEM survey conducted in early 2012. Previous hydrologic studies in the area have demonstrated that a dune dam has blocked the now partially buried Blue Creek. The dune dam is defined by a resistive zone that has a depth of 45 meters suggesting that it influences modern ground-water flow paths and surface-water features. In addition shallow lakes in the immediate area have variable salinity. This difference in salinity appears to be related to the lake interaction with the regional groundwater system. The AEM resistivity depth sections show the subsurface distribution of dune sands and saline waters that act as a natural tracer for groundwater flow. The flow path of the saline waters follow the same patterns as classical groundwater models proposed nearly two decades ago. Some saline waters are perched above more resistive zones that may contain fresher waters. Other conductive zones suggest downward flow from saline lakes. In general flow paths are consistent with previously suggested paleo-channels of the ancestral Blue Creek. Ground-water flow paths controlled by buried topography interpreted from new geophysical surveys are important in predicting how possible deep and shallow groundwater are interconnected.
The Crustal Imaging and Characterization Team (CICT) of the U.S. Geological Survey (USGS) entered into an agreement with the Bureau of Land Management (BLM) to conduct geophysical surveys at the BLM Exell Helium Plant near Masterson, TX in preparation for the transfer of ownership for disposal of the site. The Exell Helium Plant was constructed in the 1940?s to produce high-grade helium for the country and was closed in the late 1990?s. The overall project concentrated on research-oriented goals to derive better estimates of potential contamination volumes and lateral extents to aid in remediation through the use of a broadband EM induction system, dc resistivity, induced polarization, and the subsequent integration with advancing GPS technologies. The site-specific tasks included (1) delineation and volume estimation of potential contamination in two evaporation ponds, two oxidation ponds, a production blowdown pit and flare, and a former oil/water separator pit, (2) characterization of several historical spill areas, (3) characterization and volume calculation of a historical landfill, (4) location and characterization of pipelines for potential leaks and contamination as well as of a chromium sludge pit, and (5) a site-wide reconnaissance survey for the location of undocumented burial pits. Results indicate that no further waste is located within the 300-acre property and that hydrocarbons discharged to the evaporation ponds, oxidation ponds, and the blowdown pit have largely biodegraded and have not traveled far through the geologic section.
In June 2010, the U.S. Geological Survey conducted airborne electromagnetic and magnetic surveys of the Yukon Flats and Fort Wainwright study areas in central Alaska. These data were collected to estimate the three-dimensional distribution of permafrost at the time of the survey. These data were also collected to evaluate the effectiveness of these geophysical methods at mapping permafrost geometry and to better define the physical properties of the subsurface in discontinuous permafrost areas. This report releases digital data associated with these surveys. Inverted resistivity depth sections are also provided in this data release, and data processing and inversion methods are discussed.
An Airborne Electromagnetic (AEM) survey was conducted over the glacial deposits of eastern Nebraska for mapping groundwater resources. The area of the AEM survey is covered with 100-150 meter thick Kansan and Illinoisan Age glacial deposits deposited unconformably upon the Cretaceous Carlile, Greenhorn-Graneros, and Dakota formations. 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 formation, sandstone deposits may serve as a possible aquifer, however, to this date they have not be 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 Cretaceous Carlile formation, composed of marine shales, is from 3-10 ohm. Resistivity for shale within the Dakota formation is 3-20 ohm-m and Dakota sandstones are 25-40 ohm-m. The sands and gravels within the 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 gravels (<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 boreholes 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, which may serve as an alternate groundwater resource in the area.
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