A caliper log measures the diameter of a cased well or of an open borehole and is used to determine the integrity and length of the well casing, to determine well diameter and screen locations, and to locate fractures in rock wells. It is also used to calibrate downhole flowmeter logs when estimating volumetric flow rates from permeable zones in limestone wells, and to identify washout zones that may influence the response of other logs.
A temperature log measures the temperature of water in a well or of drilling fluid in an open borehole. Ground-water temperature increases with depth and so the log can be used to determine the geothermal gradient in an area. Temperature logs are also used to make corrections to resistivity logs and have been used to identify water-bearing fractures in rock wells of the Piedmont (Gellici, J.A., 1989, Borehole geophysics in the Piedmont of South Carolina: in Ground Water in the Piedmont—Proceedings of a conference on ground water in the Piedmont of the Eastern United States, Clemson University, pp. 510-525).
A fluid resistivity log measures the resistivity of water in a well or of drilling fluid in an open borehole. Resistivity is inversely correlated with the concentration of dissolved solids in water. The log is used to determine the depth of the freshwater/saltwater interface in aquifers that have been intruded by saltwater and to estimate the salinity and dissolved solids concentration of water.
A gamma-ray log measures the gamma radiation emitted from rocks and sediments penetrated by a well. It is used to infer different types of lithology, such as clay, marl, sand, and limestone. Sand and limestone typically form the aquifers of the Coastal Plain, and clay and marl form the confining units. Consequently, the log is very useful in delineating the depth and thickness of aquifers and confining units of the Coastal Plain. In the Piedmont region, the log is used to differentiate rock types and to determine the thickness of the saprolite and transition zones. The log has been used to identify high-radiation zones in rock wells (see Gellici, J.A., 1989) and is commonly used in the Low Country to identify the top of the Upper Floridan aquifer (Hayes, L.R., 1979, The ground-water resources of Beaufort, Colleton, Hampton, and Jasper Counties, South Carolina: South Carolina Water Resources Commission Report No. 9, 91 p).
A single-point resistance log measures the electrical resistance of formation water and of sediments and rocks penetrated by the borehole. It is used to delineate the depth and thickness of aquifers and confining units of the Coastal Plain and to determine the location of fractures in rock wells of Piedmont region. Owing to its high vertical resolution, the log is useful at identifying thin beds of clay, sand, and limestone.
A spontaneous potential log records small electrical voltages that occur in the borehole and in the sediments penetrated by the borehole caused by water chemistry and mineralogy. The log is used primarily to indicate relative permeability and is especially useful in coastal counties where stronger voltages occur from a more mineralized ground water.
The three resistivity logs—16-inch short normal, 64-inch long normal, and 72-inch lateral—measure the electrical resistivity of the formation water and of the sediments and rocks penetrated by the borehole. The three logs differ in their vertical resolution and in the distance to which the electrical current penetrates the formation, both a function of the spacing of the electrodes. Generally, the 16-inch short normal can differentiate thin beds but penetrates only into the invaded zone of the formation; consequently, it is influenced by the borehole fluid and doesn’t provide a measure of the true resistivity of the formation. The 64-inch normal, on the other hand, cannot differentiate thin beds but penetrates deeper into the formation and can therefore get a measure of the true resistivity of the formation.
All three logs are used to delineate the aquifers and confining units of the Coastal Plain. The 64-inch normal has been used to estimate the total dissolved solids concentration of water in sand aquifers of the Coastal Plain (Newcome, Roy, Jr., 1990, Using electric logs to predict ground-water quality in the sand aquifers of the South Carolina Coastal Plain: South Carolina Water Resources Commission, Open-File Report No. 35, 10 p).