ORIGINAL ARTICLE Identification of buried sinkholes using refraction tomography at Ft. Campbell Army Airfield, Kentucky I. Camilo Higuera-Dı ´az Æ Philip J. Carpenter Æ Michael D. Thompson Received: 16 October 2006 / Accepted: 13 February 2007 / Published online: 7 March 2007 Ó Springer-Verlag 2007 Abstract Karst aquifers are highly susceptible to con- tamination, with numerous points of entry for contaminants through recharge features such as sinkholes, swallow holes and solutionally enlarged fractures. These recharge features may be filled or obscured at the surface, requiring the use of geophysical or remote sensing techniques for their identification. This study uses seismic refraction data col- lected at the Ft. Campbell Army Airfield (CAAF), Ken- tucky, USA, to test the hypothesis that refraction tomography is a useful tool for imaging bedrock depres- sions beneath thick overburden (greater than 20 m of unconsolidated sediment). Southeast of the main taxiway of CAAF seismic velocity tomograms imaged a bedrock low, possibly a closed depression, at a depth of 25 m that had been earlier identified through delay-time analysis of the same refraction data. Tomography suggests the bedrock low is about 250-m wide by 10-m deep at its widest point. High rates of contaminant vapor extraction over the wes- tern extension of this feature suggest a high concentration of contaminants above, and within, this filled bedrock low, the base of which may contain solutionally enlarged frac- tures (i.e. karst conduits) that could funnel these contami- nants to the upper or lower bedrock aquifers. This study thus demonstrates the viability of seismic refraction tomography as a tool for identification of filled sinkholes and bedrock depressions in karst areas. Keywords Geophysics Á Ground water Á Carbonate hydrology Introduction Karst systems pose unique challenges to contaminant characterization and remediation. Fracture and void sys- tems within the rock provide conduits for migration of contamination, as do bedding plane partings and weath- ered zones. In areas where the karst is mantled by unconsolidated sediment and/or residuum, the topography of the karstic bedrock surface can be an important target for identifying points of contaminant infiltration and entrapment. Topographic lows in the karst surface pro- vide a greater column of overburden and residuum in which to trap contaminants. However, these lows may also be associated with hydraulically transmissive zones where rocks have been preferentially weathered because they are fractured or contain solution openings. Con- taminants that do manage to migrate through the thicker overburden might easily percolate to bedrock aquifers via these zones. This paper explores the utility of seismic refraction tomography in characterizing buried karst topography in an urban and/or industrial setting where electrical or electromagnetic methods would likely fail. Specifically, this study tests the hypothesis that seismic refraction tomography may be used to identify and image depres- sions in the bedrock surface, even when they are buried by 20 m or more of unconsolidated sediment. Tomogra- phy also allows lateral velocity variations and vertical gradients to be imaged, as opposed to most conventional refraction interpretation methods, which require constant velocity layers. I. C. Higuera-Dı ´az (&) Á P. J. Carpenter Department of Geology and Environmental Geosciences, Northern Illinois University, DeKalb IL 60115, USA e-mail: ihiguera@niu.edu; higuera@geol.niu.edu M. D. Thompson Energy Systems Division, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL, USA 123 Environ Geol (2007) 53:805–812 DOI 10.1007/s00254-007-0693-y