USE OF GROUND-PENETRATING RADAR, DIGITAL OPTICAL BOREHOLE IMAGES AND CORES FOR CHARACTERIZATION OF THE KARSTIC BISCAYNE AQUIFER, SOUTHEASTERN FLORIDA

Ground-penetrating radar is a useful tool in the characterization of shallow carbonate aquifers. This technique was applied to karstic carbonate rocks (Miami Limestone and Fort Thompson Formation) of the upper Biscayne aquifer in southeastern Florida. Resultant ground-penetrating radar profiles showed the following geologic features: (1) large-scale porosity, (2) paleo-outliers, (3) paleotopographic relief on subaerial exposure surfaces, (4) low-angle accretion bedding and (5) vertical stacking of upward-shallowing cycles. These features were then ground-truthed by comparing them to features recognized in digital optical borehole images, cores, and outcrops.

Using a pixel-counting technique, vuggy porosity was measured for 470 meters of digital optical borehole images. These images were recorded in the upper Biscayne aquifer from 41 coreholes in an approximately 230-square km area contiguous to the eastern boundary of the Everglades. Analysis of the borehole imaging logs suggests that geologic depositional cycles, rock fabric, and quantity and type of vuggy porosity are all interrelated and that karst-related conduit flow is the principal mechanism of ground-water movement in the upper Biscayne aquifer.

Findings indicate that conduit-flow paths within the Fort Thompson Formation are produced by well-connected, solution-enlarged pore space. Characteristics of the solution-enlarged pore space vary as a result of depositional textures, diagenesis in a meteoric-water system, and vertical position within stacked lithofacies that combine to form upward-shallowing cycles. Thin, vertical solution pipes can be associated with fresh-water or tidal-flat deposits or both that commonly cap cycles and contain a low permeability matrix. These pipes could provide a network of passageways for vertical ground-water flow across the low-permeability cycle caps. Middle portions of cycles are relatively non-vuggy. Well-connected pelecypod molds or irregular vugs or both are mainly in the lower portion of cycles. Horizontal conduit flow appears to be largely within the vuggy porosity at the base of each cycle.