DEVELOPMENT OF BEDROCK SURFACE ELEVATION AND DEPTH TO BEDROCK MAPS FOR THE COVENTRY QUADRANGLE, CONNECTICUT, USING WATER WELLS AND LIDAR DATA

The Coventry Quadrangle was used as a pilot project area for showcasing the utility of water well data in providing basic information for subsurface bedrock elevation mapping, one of the principal information needs of environmental applications.

The primary products for the quadrangle are the Bedrock Surface Elevation Map, Depth to Bedrock map, and Bedrock Elevation Confidence Isopleths map for the Coventry Quadrangle. These maps are part of a multi-layer product of the Digital Compilation of Surficial Geologic Map and Bedrock Contours for the Coventry Quadrangle, Eastern Connecticut. Mapping and analyses were supported by the National Cooperative Geologic Mapping Program, U.S. Geological Survey in cooperation with the Connecticut Geological and Natural History Survey, Department of Environmental Protection.

Bedrock elevations and depth to bedrock isopleths for the USGS Coventry Quadrangle in eastern Connecticut were mapped using cokriging estimation. Kriging works by developing statistical measures of spatial autocorrelation of the variable to be mapped (the variate). Kriging is used to estimate the variate's value where samples were not available. Cokriging works by developing statistical measures of spatial correlation between one or more ancillary variables (the covariates) with the variate to provide additional information. For mapping bedrock elevations, we determined depth to bedrock at approximately 3200 locations by examining private well completion reports, which include a boring log indicating minimum depth to rock, which is the variate. We used Light Detection And Ranging (LIDAR) data as a surface elevation covariate. Cokriging provided bedrock elevation point estimates, with a variance estimate, which yielded 20' interval bedrock contours. Subtracting the bedrock surface from the topographic surface yielded a depth to bedrock map. A bedrock elevation confidence map was produced from the variance estimates. These three products constitute the highest resolution bedrock elevation maps to date. They are also the only maps of their kind with an associated map of statistical uncertainty to support the choice of a 20' contour interval.