Delineation of macroporous zones in the unsaturated portion of the Miami Limestone using ground penetrating radar, Miami Dade County, Florida

•We use ground penetrating radar to rapidly characterize porosity in unsaturated karst limestone.•We demonstrate the ability of GPR to characterize porosity and the water table.•Study shows relationship of EM wave velocity to partially saturated limestone. The Atlantic Coastal Ridge in Dade County is an ooid shoal formed during the late Pleistocene. The ridge is composed of the eogenetic karst Miami Limestone, characterized by a heterogeneous distribution of porosity that can manifest as large touching-vug macroporous features with sizes in the meters and tens of meters. Direct evidence for the presence of such large dissolution features is only visible when exposed to the surface which may be problematic in urbanized areas. For that reason it is critical to detect any potential precursors for such dissolution features at the subsurface level. The purpose of this study was to investigate the ability of ground penetrating radar to detect areas of macroporosity by quantifying changes in electromagnetic wave velocity and relate them to changes in porosity after application of the complex refractive index model (CRIM). In order to constrain volumetric water content (VWC) in the CRIM, an experiment using Miami Limestone samples was designed to understand: (1) the range of typical VWCs; and (2) the effect of capillary fringe on water table elevation. The results show several areas where increases in EM wave travel time associated with contrasts in porosity exceeding 40% cannot be explained by changes in VWCs typically shown in the Miami Limestone. This study may help understand porosity variability in the unsaturated part of the Miami Limestone and if expanded to larger scales may aid groundwater flow models by better capturing distributions of macroporous areas that may contribute to direct recharge of the Biscayne aquifer.