Base of aquifer contour boundary of the principal aquifer for portions of the North Platte, South Platte, and Twin Platte Natural Resources Districts, western Nebraska

The U.S. Geological Survey and its partners have collaborated to complete airborne geophysical surveys for
areas of the North and South Platte River valleys and Lodgepole Creek in western Nebraska. The objective
of the surveys was to map the aquifers and bedrock topography of selected areas to help improve the
understanding of groundwater-surface-water relationships to be used in water management decisions.
Frequency-domain (2008 and 2009) and time-domain (2010) helicopter electromagnetic surveys were
completed, using a unique survey flight line design, to collect resistivity data that can be related to lithologic
information for refinement of 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 inversion model, in conjunction with sensitivity
analysis, geological ground truth (boreholes), and geological interpretation, is used to characterize
hydrogeologic features. The two- and three- dimensional interpretation 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 hydrogeology at a level of accuracy not achievable using
previous data sets.