Publication Date
4-1-2018
Abstract
Grand Caverns Natural National Landmark lies in the southeastern Shenandoah Valley and is home to the oldest show cave in the United States. The park and adjacent private lands include a complex of five known caves: Grand Caverns, Madison Cave, Steger’s Fissure, Jefferson Cave, and Fountain Cave, all encompassed within the northern section of Cave Hill. The cave complex lies below a series of sinkholes that run in two approximately north to south parallel lines. One sinkhole, the site of a U.S. Geological Survey study, was selected as a location of interest due to the possible existence of a perched aquifer between it and Grand Caverns below. The southern section of Cave Hill lacks sinkholes and large known caverns, although a few smaller caves exist in the area. The southern section does contain a large karstic swale feature, which we selected as a second location of interest to look for unknown void spaces associated with groundwater. Understanding groundwater distribution can give insight into the relationship between the geomorphologic karst features at the surface and the caves below. We conducted an electrical resistivity survey to investigate the spatial relationship between groundwater and karstic features at both locations. Electrical resistivity is useful in karst because we can generate a model of resistivity distribution in the subsurface that allows us to identify groundwater and void spaces based on the conductive nature of water and the high resistance of air filled voids. A total of ten resistivity lines, consisting of 14, 28, and 56 -electrodes spaced 6.25 m apart, were deployed in and/or around the features at both locations. Both dipole-dipole and Schlumberger arrays were collected during each deployment; these arrays were then merged and inverted using AGI EarthImager 2D-Inversion Software. Inverted resistivity sections were correlated to geologic cross-sections, high resolution airborne-derived LiDAR digital elevation models, known surface features, and cave depth survey data. By using the different data types, we were able to correlate many of the features present in the resistivity sections. The results indicate that bedding geometry and rock type are the dominate factors that define groundwater distribution and type of karstic features found within Cave Hill. In the northern section, the images showed several perched aquifers, two of which were dry, situated above the Grand Caverns and correlated with individual sinkholes. We also imaged the water table approximately 61 meters below the surface. The images correlated to the digital elevation model and cross-sections suggests that the sinkholes formed along a calcareous arenite confining ridge to the east which blocks the northwest flow of surface water off Cave Hill. In the southern section, the resistivity sections suggest that groundwater flows along bedding planes with trend of a antiform fold axis which is situated approximately parallel to and beneath the swale feature. The images also show that long and narrow void spaces exist beneath the areas of groundwater saturation on the flanks and within the swale. These void spaces appear larger in areas where water enters beneath the swale and in areas where the swale diverges course.
Rights Information
DOI
https://doi.org/10.5038/9780991000982.1041
Investigating Subsurface Void Spaces and Groundwater in Cave Hill Karst Using Resistivity
Grand Caverns Natural National Landmark lies in the southeastern Shenandoah Valley and is home to the oldest show cave in the United States. The park and adjacent private lands include a complex of five known caves: Grand Caverns, Madison Cave, Steger’s Fissure, Jefferson Cave, and Fountain Cave, all encompassed within the northern section of Cave Hill. The cave complex lies below a series of sinkholes that run in two approximately north to south parallel lines. One sinkhole, the site of a U.S. Geological Survey study, was selected as a location of interest due to the possible existence of a perched aquifer between it and Grand Caverns below. The southern section of Cave Hill lacks sinkholes and large known caverns, although a few smaller caves exist in the area. The southern section does contain a large karstic swale feature, which we selected as a second location of interest to look for unknown void spaces associated with groundwater. Understanding groundwater distribution can give insight into the relationship between the geomorphologic karst features at the surface and the caves below. We conducted an electrical resistivity survey to investigate the spatial relationship between groundwater and karstic features at both locations. Electrical resistivity is useful in karst because we can generate a model of resistivity distribution in the subsurface that allows us to identify groundwater and void spaces based on the conductive nature of water and the high resistance of air filled voids. A total of ten resistivity lines, consisting of 14, 28, and 56 -electrodes spaced 6.25 m apart, were deployed in and/or around the features at both locations. Both dipole-dipole and Schlumberger arrays were collected during each deployment; these arrays were then merged and inverted using AGI EarthImager 2D-Inversion Software. Inverted resistivity sections were correlated to geologic cross-sections, high resolution airborne-derived LiDAR digital elevation models, known surface features, and cave depth survey data. By using the different data types, we were able to correlate many of the features present in the resistivity sections. The results indicate that bedding geometry and rock type are the dominate factors that define groundwater distribution and type of karstic features found within Cave Hill. In the northern section, the images showed several perched aquifers, two of which were dry, situated above the Grand Caverns and correlated with individual sinkholes. We also imaged the water table approximately 61 meters below the surface. The images correlated to the digital elevation model and cross-sections suggests that the sinkholes formed along a calcareous arenite confining ridge to the east which blocks the northwest flow of surface water off Cave Hill. In the southern section, the resistivity sections suggest that groundwater flows along bedding planes with trend of a antiform fold axis which is situated approximately parallel to and beneath the swale feature. The images also show that long and narrow void spaces exist beneath the areas of groundwater saturation on the flanks and within the swale. These void spaces appear larger in areas where water enters beneath the swale and in areas where the swale diverges course.
