A regional groundwater model is being developed in the Elkhorn and Loup River Basins of Nebraska to evaluate the impact of groundwater irrigation on streamflow and assess the impact of specific management scenarios on ground- and surface-water resources. Early in the development of the Elkhorn-Loup model (ELM), the refinement of the existing hydrogeologic framework was identified as a critical need. within the ELM region, available test hole Information includes lithologic and geophysical logs, but these test holes are often separated from one another by 30 to 40 kilometers. Groundwater managers and the Natural Resource districts were intimately involved in the planning and the implementation of the geophysical data collection. in lieu of drilling additional test holes at great expense, ground-based geophysical methods including time-domain electromagnetics (TDEM) and audiomagnetotellurics (AMT) have been utilized as ‘virtual’ boreholes at specific locations to improve the interpreted base of the principal aquifer and identify the regional confining unit where sparse data exist. 72 TDEM soundings and 132 AMT soundings were acquired from 2008 through 2010. in 2008 and 2009 the soundings were intentionally located near existing or planned test holes in order to satisfy the participating Natural Resources Districts that the geophysical sounding data reflected the character of the lithologic and geophysical logs acquired in each test hole and, in a sense, to calibrate the sounding data to the test hole log data. in 2010 the TDEM sounding locations were scattered throughout the ELM area and the AMT data were acquired along one 22 km long profile to delineate subsurface paleochannel features. the use of ground-based geophysics has filled a critical data need over a very large area at a substantially lower cost than drilling test holes. Additional virtual borehole TDEM soundings are scheduled for the summer of 2011.


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