Modeled Thickness of the Overburden Geomodel Unit (obthk_f)

The obthk_f grid represents the modeled thickness of the Overburden geomodel unit at a 500 foot resolution. It is one grid of a geomodel that consists of eleven grids and a spatial extent polygon shapefile. As part of a U.S. Geological Survey Groundwater Resources Program study, a three-dimensional geomodel was constructed for approximately 53,000 mi2 of the Columbia Plateau in Washington, Oregon, and Idaho. This geomodel was constructed to define the general aquifer system geometry for use in a regional numerical groundwater flow model. Simplifications and assumptions consistent with this ultimate goal and the uncertainty in the available data were made. The geomodel units consist of Miocene age Columbia River Basalt Group strata and younger alluvial overburden covering approximately 44,000 mi2. Data were compiled from numerous databases and detailed studies completed during the past three decades. These data include stratigraphic interpretations made on more than 13,000 wells and a contiguous compilation of surficial geology and structural features spanning the study area. These data were simplified and used to construct piecewise-smooth trend surfaces that represent upper and lower subsurface geomodel unit boundaries in this complex folded and faulted terrain. The smoothness of the surfaces implicitly represents uncertainty in prediction of each surface resulting from data gaps, errors in borehole interpretations, errors in mapped contact locations, and uncertainty in the shape of the paleo-surface upon which flood basalts were emplaced. Using a rule-based algorithm, surfaces were then recombined to construct a fully three-dimensional model with a 500-foot grid resolution that is consistent with the data and for which error estimates may be made. The modeling process yielded improved estimates of unit volumes, refinement of location of large structural features, and identification of features that may be important for ongoing groundwater studies.obthk_f网格代表了覆盖层地质模型单元（Overburden geomodel unit）的模拟厚度，空间分辨率为500英尺。该网格是由11个网格及空间范围多边形形状文件（spatial extent polygon shapefile）组成的地质模型（geomodel）的其中一个网格。作为美国地质调查局（U.S. Geological Survey）地下水资源计划研究的一部分，科研团队为华盛顿州、俄勒冈州及爱达荷州境内约53000平方英里的哥伦比亚高原构建了三维地质模型（geomodel）。构建该地质模型的目的是明确区域数值地下水流模型（numerical groundwater flow model）所需的含水层系统整体几何形态。建模过程中结合该最终目标与现有数据的不确定性，进行了合理简化与假设设定。该地质模型的单元包括中新世时期的哥伦比亚河玄武岩群（Columbia River Basalt Group）地层，以及覆盖约44000平方英里的较年轻冲积覆盖层（alluvial overburden）。数据来源于过去三十年间完成的众多数据库与详细研究成果。这些数据包含对13000余口钻井的地层解译（stratigraphic interpretations）结果，以及覆盖研究区的连续地表地质（surficial geology）与构造特征（structural features）汇编资料。科研人员对这些数据进行简化后，用于构建分段平滑趋势面（piecewise-smooth trend surfaces），以此表征该复杂褶皱断裂地形下地质模型单元的地下上下边界。这些趋势面的平滑程度隐性反映了各界面预测的不确定性，该不确定性源于数据缺口、钻孔解译（borehole interpretations）误差、制图接触位置误差，以及溢流玄武岩（flood basalts）就位时所处古地表（paleo-surface）形态的不确定性。随后科研人员采用基于规则的算法（rule-based algorithm），将这些趋势面重新组合，构建出符合数据特征且可开展误差估算（error estimates）的、分辨率为500英尺的完整三维地质模型。本次建模过程优化了地质单元体积的估算结果，细化了大型构造特征的定位精度，并识别出对后续地下水资源研究具有重要意义的各类地质特征。