A NEW APPROACH IN GEOSTATISTICAL MODELING TO CAPTURE STRATIFICATION OF MACROPOROSITY IN THE BISCAYNE AQUIFER USING BOREHOLE IMAGERY FOR IMPROVED GROUNDWATER FLOW PREDICTION

Accurate characterization of porosity and pore space geometry are important in developing models that predict the response of the Biscayne aquifer to Everglades restoration projects. Optical borehole images (OBI) and variogram-based geostatistical methods have been applied to develop 3D models of the rock and its pore space for use in computation of groundwater flows and estimation of hydraulic conductivity of the Biscayne aquifer. The variogram-based approach successfully captured the gross macroporosity of the rock and its spatial distribution. However, it failed to reproduce the vertical cyclic changes in 0.4 x 0.4 m square by 17 m tall simulations of the carbonate rock mass surrounding a borehole. Variogram analysis of the data suggested a nearly isotropic macroporosity network at OBI variogram sample separation distances (geostatistical “lags”) less than the nominal borehole diameter of 0.2 m. Biases in the structure of the data set led to a situation in which the horizontal correlation was strongest at lags greater than the nominal borehole diameter. This is due to the continuity of macroporous bedding-plane vugs, which are visible in the OBI data and detected by the caliper log, across the borehole.

Variogram analysis of caliper-corrected OBI data provides a two-point statistic that is limited in its ability to capture the geometric shapes of pore spaces and their spatial distributions. Multiple-point statistics, an emerging geostatistical approach, uses observation-based “training image” datasets that provide statistical information needed to characterize the pore space more fully. Multiple-point statistics techniques simulate matches to multiple observations simultaneously and thereby reproduce more realistic patterns. These methods pose computational challenges for the utilization of the OBI data. Multiple-point statistical simulations will use digital OBI and caliper data from Biscayne aquifer boreholes at the L-31N Seepage Management Pilot Project and could lead to more realistic simulation models for the macropore network and subsequently for groundwater flow present at this critical Everglades restoration project. Success should help stakeholders to better predict changes in groundwater flow at seepage management sites and elsewhere in the Greater Everglades hydrologic system.