Three-dimensional GPR imaging of complex structures in covered karst terrain

3D GPR surveys in the covered karst terrain of west-central Florida, USA, reveal surprising geometries of surficial sediments. Several meters of surficial sands overlie progressively more clay-rich sediments, which in turn overlie weathered limestone. The top of a clay-rich horizon produces an exceptionally clear GPR reflector visible from depths between 0.5 and ~8 meters. The geometry of this horizon as it drapes over underlying weathered limestone suggests that depressions are not conical, but instead more complex troughs that surround domed stratigraphic highs. One possible interpretation is that dissolution in underlying limestone is concentrated in elongated fracture zones rather than in columnar or spherical voids. Groundwater recharge in this area is concentrated into conduits that breach the clay-rich units that overlie the limestone aquifer. This study suggests that the conduits themselves may be quasi-planar features rather than cylindrical in form. These geometric complexities were not apparent from 2D profiles alone. Their reconstruction required 3D surveys with 250 MHz and 500 MHz antennas, with 10-cm line spacings. Careful corrections for antenna positions and 3D migrations were key processing steps.