Three-Dimensional Delineation of the Pierre Shale Bedrock from Electrical Resistivity Surveys Conducted in Two Areas of Interest Within and Near Ellsworth Air Force Base, South Dakota, 2022

In 2022, the USGS, in cooperation with the U.S. Air Force Civil Engineer Center (USAF/CEC), performed geophysical surveys at two areas of interest within and near Ellsworth Air Force Base (EAFB) to delineate the Cretaceous Pierre Shale bedrock. Electrical resistivity tomography (ERT) data were collected in two grid patterns using the dipole-dipole array within and near EAFB and site information—including latitude, longitude, and elevation—were surveyed along geophysical transects comprising the grids using real-time kinematic (RTK) techniques. The purpose of collecting ERT data in grids rather than along individual transects was to create three-dimensional (3D) surfaces of the bedrock, which improve site characterization by providing higher resolution images of the bedrock compared to transects that provide only two-dimensional (2D) information. ERT data were processed using EarthImager2D software from Advanced Geosciences, Inc. (https://www.agiusa.com/agi-earthimager-2d) to remove noisy measurements and produce subsurface electrical resistivity profiles. Monitoring wells and soil borings with depth to bedrock information from the South Dakota Department of Agriculture and Natural Resources and Ellsworth Air Force Base were combined with subsurface electrical resistivity profiles to delineate the bedrock surface elevation along each transect in the grids. After bedrock delineation was completed, interpolation techniques were used to construct three-dimensional bedrock elevation surfaces for each grid.

Unprocessed and processed geophysical are provided in zipped folders (.zip) as machine- and user-readable files with accompanying text files that explains the content of each file. Site information from RTK surveys, bedrock delineation results along each transect in the two grids, and monitoring wells or soil borings used to aid delineation are included as comma-separated values (.csv) files.

2022年，美国地质调查局（United States Geological Survey, USGS）联合美国空军土木工程中心（U.S. Air Force Civil Engineer Center, USAF/CEC），在埃尔斯沃思空军基地（Ellsworth Air Force Base, EAFB）及其周边的两处重点区域开展地球物理勘探工作，以厘清白垩纪皮埃尔页岩的基岩分布范围。本次勘探采用偶极-偶极排列（dipole-dipole array）的布设方式，在埃尔斯沃思空军基地及其周边区域以两种网格模式采集了电阻率层析成像（Electrical Resistivity Tomography, ERT）数据；同时依托实时动态差分（Real-time Kinematic, RTK）技术，沿构成网格的地球物理测线采集了包含纬度、经度与高程的站点信息。相较于仅能提供二维（2D）信息的单条测线，采用网格布设采集ERT数据的核心目标是构建基岩的三维（3D）表面模型，通过提供更高分辨率的基岩成像结果，优化场地特征刻画效果。本次研究使用先进地球科学公司（Advanced Geosciences, Inc.）开发的EarthImager2D软件（https://www.agiusa.com/agi-earthimager-2d）对ERT数据进行处理，以剔除噪声测量值并生成地下电阻率剖面。将南达科他州农业与自然资源部、埃尔斯沃思空军基地提供的监测井与土壤钻孔数据（包含基岩埋深信息）与地下电阻率剖面相结合，以此厘定网格内各测线处的基岩表面高程。完成基岩厘定工作后，研究人员借助插值技术为每个网格构建了三维基岩高程表面模型。未经过处理与已处理的地球物理数据以压缩文件夹（.zip）形式提供，包含机器可读与人类可读的文件，并附带说明各文件内容的文本文件。RTK勘探获取的站点信息、两个网格内各测线的基岩厘定结果，以及用于辅助厘定的监测井与土壤钻孔数据，均以逗号分隔值（comma-separated values, .csv）格式文件提供。