SPATIAL ERRORS FOR WELL LOGS USED IN GEOLOGIC MAPPING: THE CONSEQUENCES OF BEING WRONG
PHILLIPS, Andrew C.1, GRIMLEY, David A.2 and KEEFER, Donald A.1, (1)Illinois State Geological Survey, Prairie Research Institute, University of Illinois, 615 E. Peabody Dr, Champaign, IL 61820, (2)Illinois State Geological Survey, University of Illinois, 615 E. Peabody Dr, Champaign, IL 61820, phillips@isgs.illinois.edu
In Illinois, as in many states, the main source of subsurface information on Quaternary deposits is water-well logs. Of available data, these data are the most poorly constrained in location, elevation, and, in fact, geology, yet they are crucial for mapping stratigraphic continuity and constructing the topography of buried surfaces. The first step in mapping is to verify the horizontal locations of wells using the best available location information, including permits, aerial photography and topographic maps. This process results in location changes of 10-2500 ft or more. However, because the precision and quality of the supporting information varies, the error of the corrected location may range from 0-1000 ft. After updating locations, land surface elevations of the wells are corrected based on the best available topographic data. The error of this new elevation value depends on the updated horizontal location error and the local relief, which together can be used to estimate the elevation error.
We conducted a sensitivity analysis of the impacts of horizontal and vertical measurement errors on a digitally-interpolated bedrock topography map for a 3-county region east of St. Louis. The goal is to characterize the resolution of the subsurface information. The data set includes water-well records but also the better-constrained geologic, geotechnical, and mineral boring records, and outcrop descriptions. The data were compiled from 7.5-minute quadrangle mapping projects completed over the past 12 years. Quantitative estimates of the horizontal errors were assigned to the updated locations. For each location, minimum and maximum land surface elevations were determined from a DEM using an area defined by this horizontal error. Versions of the bedrock topographic model were then re-interpolated using, alternatively, these minimum and maximum values. A comparison was then made between these different surface models. Because magnitude of the total error was small relative to the size of the regional map features, large-scale features were reasonably resolved in all of them. Locally, model results varied with factors including error magnitude, data distribution and density, and the combined variability between local relief in land surface and observed bedrock elevations from the well data.
