By Michael T. Laitta
AUTHOR: Student (Hydrogeology), U.S. Geological Survey, 3039 Amwiler Road, Peachtree Business Center, Atlanta, GA 30360-2824.
REFERENCE: Proceedings of the 2001 Georgia Water Resources Conference, held March 26-27, 2001, at The University of Georgia, Kathryn J. Hatcher, editor, Institute of Ecology, The University of Georgia, Athens, Georgia., p. 764-767.

Introduction

To characterize ground-water flow and movement of saltwater into freshwater zones, the U.S. Geological Survey (USGS), in cooperation with the Georgia Department of Natural Resources, Environmental Protection Division, is developing regional ground-water flow and solute transport models for the Floridan aquifer system and shallower aquifers in coastal Georgia and adjacent parts of South Carolina and Florida. In support of these modeling efforts, structure-contour and thickness maps are being constructed using seismic, geophysical, lithologic, and paleontologic data in a 37,000-square mile (mi²) area that includes a 8,000-mi² offshore region (fig. 1). Constructing these maps is a challenge because (1) the spatial distribution of onshore data is distinctly more dense than that of offshore data; and (2) different types of data are used to identify hydrogeologic units in different areas. Prior to this study, no attempts had been made to integrate and construct a seamless geospatial hydrogeologic forename linking both onshore and offshore interpretations. Objectives of this study include identifying areas with insufficient data coverage; evaluating how well various hydrogeologic interpretations coalesce; and determining the extent to which offshore hydrogeologic data reflect known structural features.

Sources of Hydrogeologic Data

Principle sources of onshore hydrogeologic data include Miller (1986) and Clarke and others (1990). Miller (1986) delineated the hydrogeologic framework of the Floridan aquifer system based on lithologic, paleontologic, geophysical, and hydrologic data for 662 wells in the Coastal Plain Province of Alabama, Florida, Georgia, and South Carolina. The structure-contour and thickness maps of the Floridan aquifer system encompass an area of about 70,000 mi² and constitute the basic data for the Floridan aquifer system for this study. Clarke and others (1990) used natural gamma logs as the principal basis to correlate Miocene and younger geologic units that overlie the Floridan aquifer system in 13 percent of the original study area and represent about 10 percent of the area of interest in this study. Clarke and others (1990) described the hydrogeologic framework and water quality of the surficial aquifer and upper and lower Brunswick aquifers and updated earlier interpretations of the Upper Floridan aquifer (Miller, 1986). Clarke and others (1990) used the "A," "B," "C," and "D" (Wait, 1962, 1965) geophysical markers from 500 wells to approximate the altitude of the top of the upper and lower Brunswick aquifers. Other sources used to supplement this investigation include more recent paleontologic data (L.E. Edwards and R.E. Weems, U.S. Geological Survey, written commun., 2000; H.E. Gill, U.S. Geological Survey (retired), written commun., 2000; and Henry and Foyle, Georgia Southern University, written commun. 2000).

In the offshore region, sources of data include (1) seismic data from Henry and Idris (1992); (2) borehole data from McClelland Engineers, Inc. (1984—report to the U.S. Department of the Navy); and (3) biostrati-graphic data from Huddlestun (1993). Seismic data collected by Henry and Idris (1992) were correlated with biostratigraphic data obtained from eight Tactical Air Command Test Sites (TACTS) offshore drilling sites (Manheim, 1992) and from AMCOR 6002 exploratory borings (Hathaway and others, 1981). Seismic records were interpreted to identify time-stratigraphic (geologic) units bound at the top and bottom by unconformities. Key reflectors, assumed to represent formational contacts, were traced along the grid and correlated with contacts identified in the TACTS, Savannah Light Tower (SLT), and AMCOR 6002 offshore borings (fig. 1). Offshore structure contours then were linked to four onshore boreholes—GAS 90 (Chatham County), GAT 90 (Chatham County), Chatham13 (Chatham County) and Cumberland Island 1 (GGS 3426) (Camden County).

Method of Study

Onshore and offshore data were compiled into a Geographic Information System data base by scanning and digitizing contour maps and manually entering altitudes of the tops of hydrogeologic units based on the distribution of borehole data. Digital data sets then were projected into an Albers Equal Area projection and adjusted to the North American Datum of 1983 (NAD83). Contour data sets then were clipped to the model boundaries and edited to remove areas of overlap. Each data set was gridded to form a digital elevation model (DEM) at a resolution of 1,000 square meters, and was contoured using a consistent contour interval of 500 feet. In order to create a seamless surface, the gridded models were combined to form one triangulated irregular network (TIN) surface, which was then re-gridded and contoured. To estimate the degree of fit between offshore seismic data (Henry and Idris, 1992) and onshore geophysical data (Clarke and others, 1990), stratigraphic horizons from four boreholes common to both data sets were correlated with biostratigraphic horizons (L.E. Edwards and R.E. Weems, written commun., 2000) and in an earlier study conducted by Huddlestun (1993) (fig. 2).

Preliminary Results–Upper Floridan Aquifer

The previously described procedure was attempted for the Upper Floridan aquifer, for which substantial data are available in the form of previous map interpretations and abundant borehole data. Initial results indicate a reasonable match between the three input interpretations (fig. 3). The merged interpretations produced a seamless structure-contour surface that retained known structural features such as the Gulf Trough, Beaufort High, and the offshore Sea Isle Escarpment (Huddlestun, 1993) (figs. 1, 3). Correlation between the four onshore boreholes show general agreement in all but the Cumberland Island 1 (GGS3426) borehole (fig. 2).

Figure 3. Three interpretations (A–C) used to construct a (D) seamless onshore/offshore structure-contour map of the top of the Floridan aquifer system.

Ongoing Work

This same procedure is currently being used to estimate the configuration of the altitude of the upper and lower Brunswick aquifers. The objective of this ongoing work is to produce a three dimensional, seamless onshore–offshore distribution of selected geologic units within the study region.

Literature Cited

Clarke, J.S., Hacke, C.M, and Peck, M.F., 1990, Geology and ground-water resources of the coastal area of Georgia: Georgia Geologic Survey Bulletin 113, 106 p., 12 plates.

Hathaway, J.C., Schlee, J.S., Poag, C.W., Valentine, P.C., Weed, E.G.A., Bothner, M.H., Kohout, F.A., Manheim, F.T., Schoen, R., Miller, R.E., and Schultz, D.M., 1981, The 1976 Atlantic margin coring project of the U.S. Geological Survey: U.S. Geological Survey Open-File Report 81-0239, 217 p.

Henry, V.J., and Idris, F.M., 1992, Offshore minerals assessment studies on the Georgia Continental Shelf—Phase 2: Seismic Stratigraphy of the TACTS Area and Evaluation of Selected Sites for Economic Hard Minerals Potential: Georgia Geologic Survey Project Report 18, 143 p.

Huddlestun, P.F., 1993, A revision of the lithostratigraphic units of the Coastal Plain of Georgia—The Oligocene: Georgia Geologic Survey Bulletin 105, 67 p., 5 plates.

Manheim, F.T. (ed.), 1992, Geology, stratigraphic relationships, and chemical composition of phosphatic drill cores (TACTS Boreholes) from the continental shelf off Georgia: Georgia Geologic Survey Project Report 17, 47 p.

McClelland Engineers, Inc., 1984, Field and laboratory report ocean bottom survey, air combat training range, Naval Air Station, Charleston, TACTS: Houston, Texas, McClelland Engineers, Inc., report for the U.S. Department of the Navy to Brown and Root Development Inc., variously paged.

Miller, J.A., 1986, Hydrogeologic framework of the Floridan aquifer system in Florida and in parts of Georgia, South Carolina, and Alabama: U.S. Geological Survey Professional Paper 1403-B, 12 p.

Wait, R.L., 1962, Interim report on test drilling and water sampling in the Brunswick area, Glynn County, Georgia: Georgia Geologic Survey Information Circular 23, 46 p.

Wait, R.L., 1965, Geology and occurrence of fresh and brackish water in Glynn County, Georgia: U.S. Geological Survey Water-Supply Paper 1613-E, 94 p., 4 plates.