MODELING BEDROCK ELEVATION OF PENNSYLVANIA USING AN ADAPTIVE GIS METHODOLOGY

The Pennsylvania Geological Survey has embarked on a new endeavor to create a 3D geologic model of Pennsylvania in support of the US GeoFramework Initiative. The Survey is creating digital elevation models of major geologic bounding surfaces. As a first step, the Survey developed a digital surface that represents the bedrock elevation beneath unconsolidated sediments. Over the years, geologists have used subsurface data from water wells, geotechnical borings, and seismic surveys to map the bedrock elevation. The historical process of generating a bedrock elevation map, which involves contouring the data by hand, is a laborious process and subject to radical changes in interpretation whenever new data are collected. Using digital mapping techniques, geostatistical analysis, and GIS workflow models, the Survey’s newly developed method to generate a bedrock elevation surface has shifted away from time-consuming manual efforts and towards automated computer processing, which allows for the rapid update of this surface as new data are collected.

The Survey created a 100-meter resolution digital raster depicting bedrock elevation beneath unconsolidated sediments of Pennsylvania using this newly developed GIS-based methodology. A topographic position index (TPI) raster was produced with five classifications: ridge, upper slope, middle/flat slope, lower slope, and valley. A linear regression relationship between the TPI product and the square root of sediment thickness was established for the five TPI classes in each of the 23 physiographic sections within Pennsylvania. This statistical relationship was used to create a surrogate model for depth-to-bedrock to predict sediment thickness in areas of low data density. A statewide sediment thickness model was generated by merging the empirical data points (extracted from well logs) with the surrogate model. The bedrock elevation raster was calculated by subtracting the sediment thickness model from a surface topography digital elevation raster. The Survey plans to refine and update the model on a periodic basis as new sources of depth-to-bedrock data become available.