CHARACTERIZATION OF POROSITY IN THE UNSATURATED ZONE OF THE MIAMI LIMESTONE USING GROUND PENETRATING RADAR, MIAMI, FLORIDA

The Miami Limestone in south Florida is characterized by a marked heterogeneous distribution of porosity due to the presence of dissolution features ranging from meters to tens of meters in extent. In coastal part of Miami, a relict shoal of the eogenetic karst Miami Limestone has created a topographic high known as the Atlantic Coastal Ridge which is the site for many urban infrastructure developments. We selected an area that contains a series of dissolution and collapse features, with the largest known being Smather’s Cave. This research investigates the ability of ground penetrating radar (GPR) to rapidly delineate areas of high porosity by quantifying changes in the electromagnetic wave velocity in the unsaturated part of the Miami Limestone using the Complex Refractive Index Model. The approach assumes that lateral variability in travel times from the ground surface to the water table are due to changes in water content (i.e. saturation) within the limestone column as related to changes in porosity. In order to constrain the hydrologic properties in the subsurface, laboratory experiments were conducted to measure the typical ranges of volumetric water content and the effect of the capillary fringe on water table elevation. Our results show several areas where electromagnetic wave variability and thus porosity of nearly 40% cannot be explained simply by changes in saturation within the Miami Limestone. This work shows the potential for rapidly quantifying porosity for the unsaturated portion of the Miami limestone at the field scale. Furthermore the approach can be easily extrapolated to larger scales of measurement to assist current models of ground water flow in the near coastal areas or help predicting how changes in sea level rise could potentially influence such models.