3D SUBSURFACE MODELING OF THE CONTIGUOUS UNITED STATES: A CASE STUDY FROM THE BLACK HILLS REGION, SOUTH DAKOTA, USA

Three-dimensional subsurface geologic models are crucial for a broad range of applications including natural resource evaluation, geohazard assessment, and environmental remediation; yet publicly available subsurface data is often incomplete or challenging to obtain and synthesize into a useful product. Constructing a seamless three-dimensional body of subsurface geological data spanning the contiguous United States is a strategic goal of the USGS National Cooperative Geologic Mapping Program (NCGMP), which aims to disseminate non-proprietary, consistent, and versionable subsurface geologic data at regional and national scales to the public.

In this case study, we present a methodology for efficient construction of consistent, seamless volumetric 3D subsurface geologic models and associated surfaces expandable to the regional scale. Our workflow involves: (1) Data compilation from a disparate group of sources, and storage of aforementioned data in a consistent, flexible format (2) Iterative mesh construction, validation, and editing to produce surfaces of the top of the Precambrian, select Phanerozoic stratigraphy, and major structural features to the highest confidence possible (3) Generating geologically reasonable and consistent interlocking faulted volumes of subsurface geology with queryable geologic name, spatial extent, uncertainty, and attributed source data and metadata (4) Export of synthesized data into common, readily available format for distribution. This methodology was tested utilizing data from the Black Hills region of South Dakota, of which seven units have been modeled using publicly available raster datasets, surficial mapping, geologic cross sections, structure-contour and isopach maps, downhole data from water and hydrocarbon wells, polygons derived from geophysical data, and existing geological models. Of note is the easily modifiable nature of this methodology, allowing for continual improvement and upgradability of this digital infrastructure on the local or regional scale once a national framework is established. Expansion of the case study model to regional scales will require consensus on regional stratigraphic correlation from state geological survey partners within NCGMP, as well as data inputs from a wide variety of sources.