LINKING FULL-WAVEFORM SONIC LOGS TO THE HYDROLOGY OF THE KARSTIC BISCAYNE AQUIFER, SE FLORIDA

Full-waveform sonic logging currently is being used with increased frequency to determine acoustic-wave velocities and mechanical properties in boreholes penetrating various rock types. However, results are usually poor in karstic carbonate strata characterized by high values of vuggy porosity and slow compressional wave velocities. Acquisition of sonic-waveform borehole properties and correlation with other borehole geologic data provides qualitative estimates of porosity and permeability. Full-waveform sonic logs were collected in wells fully penetrating the karst Pleistocene limestone that comprise the Biscayne aquifer in southeast Florida. Shear-wave velocity in these limestone strata are similar to ground-water fluid velocity, and difficult to distinguish using an inverse geologic model. Logging activities therefore concentrated on the collection of compressional- and Stoneley-wave velocities. Compressional-wave velocities provide data for a synthetic sonic-porosity log, and Stoneley-wave arrivals produce a Stoneley-wave amplitude log, useful as an indicator of relative permeability.

The resolution of the calculated porosity and relative permeability appears to be adequate to delineate carbonate-diffuse and conduit flow zones. Preliminary log analyses suggest the Biscayne aquifer within the study area can be divided into three Stoneley-amplitude units, with one relatively low-permeability unit lying between two high-permeability units. These three units can be further partitioned into thin zones characterized as carbonate-diffuse flow and conduit flow units. Comparison to digital borehole images show that thick, highly-vuggy zones produce the greatest error in calculated porosity and permeability mostly attributed to increase in vuggy-interval borehole diameter. One-KHz Stoneley-wave and caliper logs were used to identify the depths of the highly-vuggy zones, because Stoneley waves are sensitive to the change in borehole shape. Full-waveform sonic logging surveys appear to be a potential viable logging tool to verify flow zones in karstic aquifers, and thus improve decision making related to aquifer storage and recovery projects and production wells.