LiDAR Imagery Identifies Karst Features
As part of the Illinois Height Modernization Project, the Illinois State Geological Survey (ISGS) is receiving high-resolution LiDAR elevation data on a county-by-county basis (http://isgs.illinois.edu/nsdihome/webdocs/ilhmp/). LiDAR (an acronym for Light Detection And Ranging) is an imaging technique that incorporates pulsating laser light from a low-flying aircraft to the ground surface. The reflected light pulses are detected by instruments that record their location in three dimensions. The "first return" recorded by a LiDAR sensor is the first object contacted by a laser pulse, such as a tree, building rooftop, or a ground feature. When a laser pulse encounters a soft target, such as forest canopy, a portion of the laser beam continues downward and reflects from the underlying tree branches and trunk providing a "second return." If conditions are ideal, the last return represents the ground or "bare earth" surface. To maximize the probability of acquiring the bare earth return data, LiDAR is collected during the portion of the year when leaves are off the trees(leaf-off season).
The ISGS scientists are using LiDAR elevation data acquired in 2008 to better understand the fabric and geometry of the crevice karst aquifers that underlie Jo Daviess County, Illinois, and constitute its groundwater resources. Figure 1, is a portion of a U.S. Geological Survey (USGS) digital orthophotograph acquired in early April 2005 over the southeast quarter of the Bellevue, Illinois, Quadrangle in western Jo Daviess County. The 1.5-square mile area is characterized by 400 feet of local relief, and the steep slopes are dominated by dense forest cover. The underlying bedrock is dolomite of Silurian age. This type of landscape obscures the surface topography and makes detection of geologic features difficult or impossible using traditional aerial photography or topographic maps.
Figure 2, is a color relief-shaded image of the same geographic area produced from the LiDAR bare earth return data. Note the distinctive, roughly circular features situated below the crests of several of the slopes, which are completely obscured in the aerial photograph. These are cover-collapse sinkholes, which range from approximately 20 to 70 feet in diameter. The sinkholes are positioned en echelon (in a step-like manner) along nearly east-west–trending lineaments in sediment overlying the Silurian dolomite. Road cuts in the area reveal that these aligned sinkholes probably formed along nearly east-west–trending crevices that range from one to more than six feet in width. Collapse of sediment into these large crevices creates the sinkholes that can be directly observed on the LiDAR imagery. Many of the sinkholes are 10 feet or less in depth and therefore are not shown as depression contours on the existing USGS 7.5-minute topographic maps for the area. However, these features are highly visible on the bare earth LiDAR imagery because of the high vertical resolution of the elevation data. Figure 3, merges the USGS digital orthophotograph and the LiDAR imagery showing the location of the sinkholes beneath the forest cover.