Abstract

A database of borehole geophysical logs and other types of data files were compiled as part of studies of water availability and assessment of brackish- and saline-water resources (Williams et al., 2016). The database contains 4883 logs from 1248 wells in Florida, Georgia, Alabama, South Carolina, and from a limited number of offshore wells of the eastern Gulf of Mexico and the Atlantic Ocean. The logs can be accessed through a download directory organized by state and county for onshore wells and in a single directory for the offshore wells.

The objective of this study was to improve the overall understanding of the available fresh, brackish, and saline water resources for ongoing and potential future development. Specific tasks were to (1) develop a digital georeferenced database of borehole geophysical data to enable analysis and characterization of fresh, brackish, and saline aquifers (see locations in Figure 1), (2) identify and map the regional extent of the fresh to saline aquifer systems and describe the thickness and character of hydrologic units that compose these systems, and (3) delineate salinity variations at key well sites and along section lines to provide a regional depiction of the freshwater-saltwater interfaces. Electrical resistivity and induction logs, coupled with a variety of different porosity logs (sonic, density, and neutron), were the primary types of borehole geophysical logs used to estimate the water quality in brackish and saline formations. The results from the geophysical log calculations were compared to available water-quality data obtained from water wells and from drill-stem water samples collected in test wells.

Gamma ray and resistivity markers were used to map the base of the aquifer system throughout much of the peninsular area with an example of these markers from a well in De Soto County, Florida (Fig. 2). Mapped units include a low-resistivity zone near the base of the aquifer system often denoted by a basal gamma ray marker, a massive dolostone unit within the Oldsmar Formation, and the Glauconite marker unit. Although geophysical log response varies considerably throughout the area, individual high and low-permeability zones within the aquifer system could be mapped generally along distinct lithologic units such as shown for a well in Citrus County, Florida (Fig. 3). In this well the aquifer contains permeable zones near the top of the carbonate rock sequence, in the middle of the sequence (Avon Park Formation) and near the base of the system (Oldsmar Formation).

Using point data compiled from geophysical logs and water samples from wells, digital surfaces and thicknesses of selected hydrogeologic units of the Floridan aquifer system were developed to define an updated hydrogeologic framework (Williams and Dixon, 2015). The dataset contains structural surfaces depicting the top and base of the aquifer system, its major and minor hydrogeologic units and zones, geophysical marker horizons, and the altitude of the 10,000-milligram-per-liter total dissolved solids boundary that defines the approximate fresh and saline parts of the aquifer system. The thicknesses of selected major and minor units or zones were determined by interpolating points of known thickness or from raster surface subtraction of the structural surfaces. Additional data contained include clipping polygons; regional polygon features that represent geologic or hydrogeologic aspects of the aquifers and the minor units or zones; data points used in the interpolation; and polygon and line features that represent faults, boundaries, and other features in the aquifer system.