TWO-DIMENSIONAL AND THREE-DIMENSIONAL GROUND PENETRATING RADAR STUDY OF BEDFORM ARCHITECTURE OF AN ANCIENT CARBONATE SHOAL

Dimension-stone quarries in south-central Indiana offer a unique opportunity to study the bedform architecture of an ancient carbonate shoal in the Salem Limestone (Mississippian, Valmeyeran). Previous research on sawed walls of the quarries relied on hand-tracing of bedding surfaces with chalk, digitally photographing the chalked walls, merging the digital photographs into panoramic images of walls, and digitally tracing the bedding surfaces on the panoramic images. This method works well for mapping small exposures but is less useful for study of the overall shoal architecture. The chalking method requires constant tracing and photography as quarry walls are removed. If sections of the wall are damaged, stained, or obstructed, bedding surfaces cannot be seen or traced accurately.

Ground Penetrating Radar (GPR), a geophysical tool used to image the subsurface with radio waves, introduces a new method for collecting bedform data more efficiently and in greater detail. The relatively high frequency signal used in this GPR study (250 mHz) limits the maximum imaging depth, but greatly enhances resolution of small-scale features, such as individual foresets.

The objectives of this study were to demonstrate the ability of GPR to resolve bedding surfaces in a purely carbonate system, explore the characteristics of surfaces causing reflections of the GPR signal, and demonstrate how both 2-D and 3-D GPR data can be integrated with other information to produce a more detailed model of the overall shoal bedform architecture.

Two areas of the Maple Hill Quarry in Bloomington, Indiana, were selected for study. The first exposure consists of four “stair-step” benches each roughly 4 m in height. Two-dimensional GPR surveys were conducted along the tops of these benches. The structures observed in these benches are large shallow trough cross-beds that are over 15 m wide. The second area of the quarry was selected to generate a 3-D model. To construct the 3-D volume, a 5.5-meter by 25-meter grid of one-way GPR lines was run with 0.125-meter spacing between lines. The data show this area has a different bedding character with thinner beds and higher angle foresets.