IMPROVED HYDROGEOLOGIC FRAMEWORKS USING HELIBOURNE ELECTROMAGNETIC SURVEYS FOR THE NORTH PLATTE VALLEY, WESTERN NEBRASKA OPTIMIZATION MODEL

<>Improved Hydrogeologic Frameworks Using Helibourne Electromagnetic Surveys for the North Platte Valley, western Nebraska Optimization Model The need for more accurate hydrogeologic frameworks for groundwater modeling has created the demand for innovative approaches to data collection. Traditional methods, such as test-hole drilling, aquifer tests, and surface geophysics, can provide information on a local or point scale. Interpolation between known points is then needed to produce a hydrogeologic framework. The limited amount of data and the distance between known data points can produce a hydrogeologic framework with substantial uncertainty. In complicated terrains, groundwater modelers benefit from continuous high-resolution geologic maps and their related hydrogeologic-parameter estimates. The USGS Crustal Imaging and Characterization Team (CICT), the Nebraska Water Science Center (NEWSC), the University of Nebraska, and local Natural Resources Districts have collaborated on several projects using airborne geophysical surveys to provide near-continuous coverage of large areas of the North Platte River valley in western Nebraska. The objective of the surveys was to map the aquifers and bedrock topography of the area to help improve the understanding of groundwater-surface water relations to be used in water management decisions. Frequency-domain helibourne electromagnetic (HEM) surveys were completed, using a unique survey flight line design, to collect resistivity data that can be related to lithologic information needed for groundwater model optimization. The geology of the area consists of Quaternary eolian sand and alluvial deposits overlying Tertiary Broadwater fluvial deposits, the Tertiary White River Group, and Cretaceous undifferentiated deposits that tend to be confining units in the project area. Presently, only the HEM results for the changes in bedrock topography are included in the groundwater model. When compared to previous work, these results show substantial revisions in the configuration of the aquifers and contribute to improved model geometry and calibration.