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| USGS Coastal Carolina Project 2002 - Kitty Hawk |
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An important part of well construction is to determine the character and the thickness of the different layers of material penetrated by the well. Information on materials penetrated is recorded in the form of "logs." The logs most commonly prepared for supply wells are (1) drillers logs, and (2) geophysical (electric) logs.
Drillers logs consist of written descriptions of the material penetrated by wells. Geophysical logs provide indirect information on the character of rock layers. The most common type of geophysical log, the type normally referred to as an electric log, consists of a record of the rock units and of the spontaneous electrical potentials generated in the borehole.
The spontaneous potential log (which is usually referred to as the SP log) is a record of the differences in voltage between an electrode at the land surface and an electrode in the borehole. The resistivity log is a record of the resistance to the flow of an alternating electric current offered by the rock layers and their contained fluids and the fluid in the borehole.
Several other types of geophysical logs are available, including gamma-ray logs that record the rate of emission of gamma rays by different rock layers.
Taken from Heath, R.C., 1980, Basic elements of ground-water hydrology with reference to conditions in North Carolina: U.S. Geological Survey Water-Resources Investigations Open-File Report 80-44, 86 p.