GEOPHYSICAL MEASUREMENTS OF ELECTRICAL ANISOTROPY AND SECONDARY POROSITY IN THE BISCAYNE AQUIFER USING AZIMUTHAL RESISTIVITY METHODS

Aquifer porosity characterization is often derived from measurements of borehole-gravity, acoustic, neutron, and density logs as well as analysis of core samples. Electrical anisotropy measurements play an important role in estimating the direction of porosity and fluid flow in porous media and can be determined noninvasively. Knowledge of anisotropy in the Biscayne Aquifer rocks may indicate a preferred direction of secondary porosity which has been formed through the dissolution of limestone. In this study azimuthal resistivity measurements at 13 sites in Miami-Dade County, FL were used to investigate anisotropy of the electrical resistivity structure and to estimate the anisotropic secondary porosity of the Biscayne Aquifer. The coefficient of anisotropy ranged from 1.01 - 1.36 with the highest values beneath the Atlantic Costal Ridge near the east coast while lower values are generally in the low elevation areas on the margin of the Everglades to the west. The general trend of minimum resistivity was in the W-E towards NW-SE directions with minor variations. The reported direction of the minimum resistivity from this study is similar to the predevelopment groundwater flow direction as indicated in published modeling studies. The observed electrical anisotropy corresponds to an anisotropic component of the secondary porosity ranging from 1 to 13% which matches other published studies estimating anisotropic secondary porosity in fractured terrains and other karst environments. This study shows that geoelectrical methods can reasonably predict hydrogeological parameters in the surficial aquifer system and provide a key preliminarily assessment tool.