Karst, near surface bedrock cavity, fracture, or cave (solution) features, are present near the Crystal Rock Cave system in Erie County, north-central Ohio. Previously, a capacitively-coupled electrical resistivity study of the area was completed to evaluate the methodology for detecting karst features in the subsurface. Resistivity traverses were completed over known subsurface features using different transmitter-receiver spacing, varying the penetration depth of the resistivity survey. The study found that the larger electrode spacing distances and thin, low-clay soils at the Crystal Rock Cave site provided data that clearly identify known sinkhole and cavity features. However, the durability and maneuverability of the towed resistivity array posed problems and resulted in somewhat unreliable datasets in several instances.
To evaluate the findings of the previous resistivity study and assess the ability of ground-penetrating radar (GPR) to identify the same features, a GPR survey was also completed along the same resistivity transects over the Crystal Rock Cave system. The use of GPR is a well-established method of not only detecting subsurface cavities but also mapping the subsurface. The cart-mounted, 400-MHz antenna, GSSI GPR system identified the karst features by detecting the adequately contrasting dielectric properties of the soils, bedrock, and associated air-filled cavities within the shallow subsurface. The shallow (5 m) transects resulted in higher resolution datasets and profiles that clearly show more detail. Deeper (10 m) transects required significant processing and manipulation in order to enhance deeper reflections and to retain detailed, shallow reflections.
During GPR data interpretation and comparison to resistivity pseudosections, similar solution features were discernable and comparable within the subsurface profiles generated by the two methods at numerous locations along transects. Comparison of resistivity pseudosections and GPR profile s during interpretation aided in identifying solution features. The resistivity data along a traverse are affected by features off line, but the resolution and the three-dimensional nature of the recorded GPR signal often allows multiple features (i.e., soil-filled cavity surrounded by bedrock) to be observed within the same space along a transect. In terms of data collection speed and ease, the GPR proved to be better suited than resistivity. However, with the valuable subsurface data both methods provide, they are best utilized in conjunction as companion methodologies.