Defining The Bedrock/Overburden Boundary In Covered Karst Terranes Using Dipole-Dipole Electrical Resistivity Tomography

Sinkhole collapse is one of the main limitations on the development of karst areas, especially<br>where bedrock is covered by unconsolidated material. Studies of sinkhole formation have shown<br>thatsinkholes are likely to develop in cutter zones as a result of subterranean erosion by flowing<br>groundwater. Because of the irregular distribution of pinnacles and cutters on the bedrock<br>surface, uncertainties arise when “hit-or-miss” borehole drilling is used to locate potential<br>collapse sites. A high-resolution geophysical technique capable of depicting the details of the<br>bedrock surface is essential for guiding the drilling program.<br>Dipole-dipole electrical resistivity tomography (ERT) was used to map the bedrock surface at a<br>site in southern Indiana where limestone is covered by about 9 m (meters) of clayey soils.<br>Forty-nine transects were conducted over an area of approximately 42,037 m2. The electrode<br>spacing was 3 m. The length of the transects varied from 81 to 249 m. The tomographs were<br>interpreted with the aid of soil borings. The repeatability of ERT was evaluated by comparing the<br>rock surface elevations interpreted from pairs of transects where they crossed each other. The<br>average difference was 2.4 m, with a maximum of 10 m.<br>The discrepancy between interpreted bedrock-surface elevations for a transect intersection may<br>be caused by variations in the subsurface geology normal to the transect. Averaging the elevation<br>data interpreted from different transects improved the ERT results. A bedrock surface map was<br>generated using only the averaged elevation data at the transect junctions. The accuracy of the<br>map was further evaluated using data from four exploratory boreholes. The average difference<br>between interpreted and actual bedrock surface elevations was less than 0.4 m. The map shows<br>two large troughs in the limestone surface: one coinciding with an existing sinkhole basin, while<br>the other is in alignment with a small topographic valley. Because sinkholes were observed at<br>the same elevation interval in similar valleys in the vicinity, the delineated trough may have<br>implications for future land use at the site.