QUANTITATIVE HYDROGEOLOGICAL FRAMEWORK INTERPRETATIONS USING HELIBOURNE ELECTROMAGNETIC SURVEYS FOR THE NORTH PLATTE VALLEY, WESTERN NEBRASKA GROUNDWATER MODEL

Increasingly complex groundwater management requires more accurate hydrogeologic frameworks for groundwater models used in resource management. These complex issues have created the demand for innovative approaches to data collection. In complicated terrains, groundwater modelers benefit from continuous high-resolution geologic maps and their related hydrogeologic-parameter estimates. The USGS and its partners have collaborated to use airborne geophysical surveys for near-continuous coverage of 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 to refine groundwater model inputs. To make the geophysical data useful to multidimensional groundwater models, numerical inversion is necessary to convert the measured data into a depth-dependent subsurface resistivity model. This inverted model, in conjunction with sensitivity analysis, geological ground truthing (boreholes), and geological interpretation, is used to characterize hydrogeologic features. The interpreted two and three dimensional data provides the groundwater modeler with a high-resolution hydrogeologic framework and a quantitative estimate of framework uncertainty. This method of creating hydrogeologic frameworks improved the understanding of the actual flow path orientation by redefining the location of the paleochannels and associated bedrock highs. The improved models represent the actual hydrogeology at a level of accuracy not achievable using previous data sets. This allows the groundwater model to be used in as a management tool.