Date of this Version
Three-Dimensional Geological Mapping: WORKSHOP EXTENDED ABSTRACTS, Minneapolis, Minnesota – October 8, 2011
The U.S. Geological Survey and its partners have collaborated to create 3D geologic maps for areas of the North and South Platte River valleys, including Lodgepole Creek, in western Nebraska using airborne electromagnetic surveys. The objective of the surveys is to map the 3D configuration of aquifers and bedrock topography created by the paleochannels of the ancestral Platte River. The ultimate goal is to gain a new understanding of groundwater–surface-water relationships to improve water management decisions through the use of groundwater management models. This goal was not achievable using traditional mapping methodologies, including surface geologic maps and borehole drilling logs. Airborne electromagnetic surveys provided nearly continuous information (data is collected every 3 to 20 meters along a flight path) of the subsurface electrical-resistivity variations (immediately below the sensor) at a depth range of 2 to 300 m below ground surface. To make the geophysical data useful for 3D geologic mapping, numerical inversion is necessary to convert the measured data into a depth-dependent subsurface electrical-resistivity model. The electrical-resistivity model, combined with sensitivity analysis, geological ground truth (boreholes), and geologic interpretation, is used to characterize geologic features. The 3D map provides the groundwater modeler with a high-resolution geologic framework and a quantitative estimate of framework uncertainty. This method of creating geologic frameworks improves the understanding of the actual flow-path orientation by redefining the location of the paleochannels and associated base of aquifer highs. The improved models depict the hydrogeology at a level of accuracy not achievable using previous data sets.