Quantitative Hydrogeological Framework Interpretations from Modeling Helicopter Electromagnetic Survey Data, Nebraska Panhandle

The need for allocation and management of water resources within the state of Nebraska has created a demand for innovative approaches to data collection for development of hydrogeologic frameworks to be used for 2D and 3D groundwater models. In 2008, the USGS in cooperation with the North Platte Natural Resources District, the South Platte Natural Resources District, and the University of Nebraska Conservation and Survey Division began using frequency domain helicopter electromagnetic (HEM) surveys to map selected sections of the Nebraska Panhandle. The surveys took place in selected sections of the North Platte River valley, Lodgepole Creek, and portions of the adjacent tablelands. The objective of the surveys is to map the aquifers of the area to improve understanding of the groundwater-surface water relationships and develop better hydrogeologic frameworks used in making more accurate 3D groundwater models of the area. For the HEM method to have an impact in a groundwater model at the basin scale, hydrostratigraphic units need to have detectable physical property (electrical resistivity) contrasts. When these contrasts exist within the study area and they are detectable from an airborne platform, large areas can be surveyed to rapidly generate 2D and 3D maps and models of 3D hydrogeologic features. To make the geophysical data useful to multidimensional groundwater models, numerical inversion is necessary to produce a depth-dependent physical property data set reflecting hydrogeologic features. These maps and depth images of electrical resistivity in themselves are not useful for the hydrogeologist. They need to be turned into maps and depth images of the hydrostratigraphic units and hydrogeologic features. Through a process of numerical imaging, inversion, sensitivity analysis, geological ground truthing (boreholes), geological interpretation, hydrogeologic features are characterized. Resistivity depth sections produced from this process are used to pick contacts between hydrostratigraphic units. This provides a 3D image of the hydrostratigraphic units interpreted from the electrical resistivity derived from the HEM tied to statistical confidences on the picked contacts. The interpreted 2D and 3D data provides the groundwater modeler with a high-resolution hydrogeologic framework and a solid understanding of the uncertainty in the information it provides. This interpretation facilitates more informed modeling decisions, more accurate groundwater models, and development of more effective water-resources management strategies.