Complex relationships between surface deformation, surface topography, and top-of-limestone surface in the covered karst Sandhill Reservation, West-central Florida.

Abstract:

The Sandhill Scout Reservation located in Hernando County, Florida, is a local nature preserve that remains relatively untouched by surrounding urbanization. The reservation consists of many individual karst lakes, ponds, and water-filled sinkholes, most of which are connected to the Floridan Aquifer System. Several karst drainage systems and visible limestone outcrops are also found throughout the reserve. In early 2015 observed sinkhole activity at the local pool prompted various geophysical and remote sensing studies near the west end of the reservation. Two metal corner reflectors were installed for continuous monitoring of surface deformation using time-series datasets acquired from Interferometric Synthetic Aperture Radar (InSAR) collected between the years 2015-2018. InSAR time-series datasets reveal millimeter-scale surface subsidence over the three years within meters of the damaged pool. The time-series of the metal reflectors indicate that subsidence is slower over a localized topographic low than a higher point. Other remote sensing techniques applied to the area include Structure from Motion (SfM) drone imaging and LiDAR to provide high-resolution surface models from late 2017 to early 2018. Overall remote sensing techniques have shown that grassy environments are challenging for monitoring subsidence. Ground-penetrating radar (GPR) and electrical resistivity tomography (ERT) surveys show XX-m of topographic variability of the limestone surface beneath surficial sands, over distances of meters to tens of meters. We complete a combined analysis of all methods to address the questions (1) how is surface subsidence irregular in space and time? (2) Is surficial subsidence disconnected from deeper voids by (i) low-density layers separating the surface from deeper sands or (ii) non-vertical raveling in sands? The combination of methods will further the understanding of the interconnectivity and complexities of sinkhole raveling zones to potential subsurface voids.