Base of aquifer contours enhanced with airborne electromagnetic (AEM) surveys of the principal aquifer for portions of the North Platte, South Platte, and Twin Platte Natural Resources Districts, western Nebraska

The U.S. Geological Survey and its partners have collaborated to complete airborne geophysical surveys for areas of the North and South Platte River valleys and Lodgepole Creek in western Nebraska. The objective of the surveys was to map the aquifers and bedrock topography of selected areas to help improve the understanding of groundwater-surface-water relationships to be used in water management decisions. Frequency-domain (2008 and 2009) and time-domain (2010) helicopter electromagnetic surveys were completed, using a unique survey flight line design, to collect resistivity data that can be related to lithologic information for refinement of groundwater model inputs. To make the geophysical data useful for multidimensional groundwater models, numerical inversion is necessary to convert the measured data into a depth-dependent subsurface resistivity model. This inversion model, in conjunction with sensitivity analysis, geological ground truth (boreholes), and geological interpretation, is used to characterize hydrogeologic features. The two- and three- dimensional interpretation provides the groundwater modeler with a high-resolution hydrogeologic framework and a quantitative estimate of framework uncertainty. This method of creating hydrogeologic frameworks improved the understanding of the actual flow path orientation by redefining the location of the paleochannels and associated bedrock highs. The improved models represent the hydrogeology at a level of accuracy not achievable using previous data sets.美国地质调查局（U.S. Geological Survey）及其合作伙伴已合作完成内布拉斯加州西部北普拉特河谷、南普拉特河谷以及洛奇普莱克溪区域的航空地球物理勘测。本次勘测的目标为绘制选定区域的含水层与基岩地形图，以加深对地下水-地表水相互关系的理解，为水资源管理决策提供支撑。本次勘测采用独特的飞行航线设计，完成了频域（2008年、2009年）与时域（2010年）直升机电磁勘测，采集可与岩性信息关联的电阻率数据，用于优化地下水模型的输入参数。为使该地球物理数据可应用于多维地下水模型，需通过数值反演将实测数据转换为深度相关的地下电阻率模型。该反演模型结合敏感性分析、地质实地验证数据（钻孔）与地质解译结果，可用于刻画水文地质特征。二维与三维解译可为地下水建模人员提供高分辨率水文地质框架，以及该框架不确定性的定量评估结果。该水文地质框架构建方法通过重新划定古河道及相关基岩高地的位置，优化了对实际水流路径走向的认知；经优化后的模型在水文地质刻画精度上远超过往数据集。