THE NATIONAL SUBSURFACE MODEL: A NEW MAPPING PROGRAM COMPILING 3D GEOLOGY FOR THE NATION

Resource assessments and process-model applications require seamless digital depiction of the Nation's 3D geology at regional- to national-scale, yet subsurface data currently exist with incomplete areal coverage in disparate formats and publications. The USGS National Cooperative Mapping Program has begun development of the National Subsurface Model—a digital infrastructure for the intake, curation, and dissemination of 3D geomodels and model-relevant subsurface geologic data. Creation of a seamless body of digital 3D geological information requires methods of compiling, storing, and serving data that are scalable, versionable, persistent, easily accessed and distributed, non-proprietary, and utilize a flexible but consistent data structure.

The presented data model uses a 2.5D approach wherein subsurface data are stored and served as a continuous fishnet of vector polygons emulating raster cells. Multiple feature-level attributes are attached to each cell or polygon centroid allowing for attribution of geo-surface name, data type, elevation, thickness, spatial uncertainty, and keys to associated tables with feature-level metadata attributing original published sources and authors. This vector cell approach is versionable and allows for rapid updating of depth, thickness, and unit characteristics attributes, along with querying, subsetting, and ready export to standard raster formats. To date, work has involved compiling the elevations of two geologic horizons that can be mapped across most of North America: (1) the generally unconformable surface that separates layered Phanerozoic rocks from highly deformed or crystalline rocks; and (2) the base of the Cenozoic section. Augmenting the elevation grids are overlay polygons that carry additional attribute information defining the nature of the surface mapped in specific regions and compilation methodology. These two mapped surfaces represent the beginning of a seamless, digital subsurface model that provides users web-searchable, viewable, and downloadable national-scale subsurface data. Progress on seamless subsurface mapping at multiple scales and from multiple sources will require consensus on regional- to National-scale stratigraphic unit correlation and require data inputs from multiple mapping partners.