Quantitative Hydrogeological Framework Interpretations Using Helicopter Electromagnetic and Ground Capacitive-Coupled Resistivity Surveys.

Increasingly complex groundwater management issues require more accurate and detailed hydrogeologic frameworks for groundwater models used in resource management. These complex issues have created the demand for innovative methods. In complicated terrains, groundwater modelers benefit from high-resolution geologic and soil maps and their related hydrogeologic-parameter estimates. The USGS and its partners have collaborated to use airborne geophysical and ground-based 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 irrigation canals and their interconnection to the aquifers and to map bedrock topography of the area to help improve the understanding of groundwater-surface water relations to be used in water management decisions. Frequency-domain helicopter electromagnetic (HEM) and ground-based capacitively-coupled resistivity surveys collected resistivity data that was used to interpret lithologic information for 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.  The HEM images ~70 m into the earth while the capacitively-coupled resistivity images the near-surface (0.5 to 10 m).  These inverted models, in conjunction with sensitivity analysis, geological ground truthing (boreholes), and geological interpretation, are 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. These methods of creating hydrogeologic frameworks improve 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.