QUANTIFYING MACROPORE GEOMETRY AND DISTRIBUTION IN THE BISCAYNE AQUIFER USING GIS AND IMAGE ANALYSIS

Image analysis and GIS techniques were used to quantify the geometry and spatial distribution of macropores in a limestone sequence at early stages of karst development. The study was conducted on core and digital borehole images obtained from the Biscayne aquifer in Miami-Dade County, Florida. The Biscayne aquifer is the main water source for South Florida, and owes its high transmissivities and porosities to an interconnected network of dissolution features. Poor core recovery is common due to limestone dissolution, especially across intervals characterized by high flow rates. Thus, hydrologic properties derived from core provide an incomplete picture of aquifer dynamics, and may not characterize the most significant flow pathways in the aquifer. High-resolution televiewers, on the other hand, capture a continuous image of dissolution features intersected by the wellbore. We used remote sensing techniques to analyze the borehole images. The 55 foot section was remapped according to a pore versus matrix classification. Two populations of dissolution features were observed: irregular-shaped macropores and horizontal channels that are continuous across the wellbore. The horizontal channels are at various stages of development, ranging from incipient (small aperture with limestone bridges) to mature (complete separation with apertures up to 50 cm). Geometric attributes of the macropores were obtained in the GIS environment, including macropore area, perimeter, centroid location and dimensions of the best-fit ellipse. The spatial distribution of macropore density and area is not random. Rather, stratigraphic intervals of enhanced macropore development alternate with intervals of lesser development. We used the ArcView Spatial Analyst Extension to generate profile histograms of 2-D porosity, defined as the percentage of surface area occupied by solution cavities within a moving rectangular window. Results identify horizontal solution channels as regions exceeding ~80% porosity. Stratigraphic horizons with 50-80% porosity are regions of enhanced macropore development. Upon further dissolution, these regions may develop into continuous horizontal channels. Thus, image analysis combined with GIS provides a unique perspective on characterizing major flow pathways in the Biscayne aquifer.