THREE-DIMENSIONAL GEOLOGY AND HYDROSTRATIGRAPHY OF THE PLATTE RIVER VALLEY NEAR ASHLAND, NEBRASKA USING HELICOPTER ELECTROMAGNETIC (HEM) MAPPING

This study used the results of a Helicopter Electromagnetic (HEM) geophysical survey to map the three-dimensional geology and hydrostratigraphy of a stretch of the Platte River valley and adjacent glaciated uplands in eastern Nebraska. Subsurface stratigraphy was interpreted using earth resistivity models from the HEM survey in conjunction with subsurface data, including Nebraska Geological Survey test holes, deep oil and gas well logs, and registered well logs. Thick (24-30 meter) surficial deposits of highly conductive materials (clay and silt rich glacial till and loess) limited the effectiveness of the HEM results for mapping in the glaciated uplands. However, sand and gravel deposits in recent and abandoned alluvial deposits of the Platte River, even when covered by 3-5 meters of fine-grained sediments including overbank fines and loess, could be mapped in detail using the HEM survey results. In the Platte River Valley the HEM survey penetrated up to 80 meters of alluvial sediments, the Cretaceous Dakota siltstones and sandstones, and the underlying Pennsylvanian shales and limestones. The HEM results allow us to determine the connectivity of important hydrostratigraphic units in the subsurface that could not have been determined at this detail through drilling alone. Integrated analysis of borehole and HEM data were used to develop three-dimensional hydrostratigraphic models that reveal stream-aquifer connections and aquifer heterogeneities at a scale not previously possible. These models were then used to improve estimates of hydrostratigraphic unit volumes, saturated thicknesses, and drainable groundwater volumes.