In order to locate optimum sites systematically, a methodology employing geologic and geophysical methods of investigation was developed and applied. Heat-producing granitoids similar to those in the crystalline basement beneath the Atlantic Coastal Plain are exposed to the northwest in the Piedmont Province. These late Paleozoic granitoids in the central and southern Appalachians are the youngest known to date. A linear relation between surface heat flow, q, and surface heat generation, A, was confirmed for many heat flow sites in granites exposed in the Piedmont of Virginia, North Carolina, South Carolina, and Georgia. Following detailed study of selected heat-producing granitoids exposed in the Piedmont, emphasis shifted to granitoids concealed beneath the thermally insulating sediments of the Atlantic Coastal Plain.
Thermal conductivity varies with sediment type.
This tends to increase thermal conductivity in the deeper part of the section relative to the upper part and thus lowers the geothermal gradient by as much as 15 per cent. Thermal conductivity also increases with compaction. Granites beneath such sediments must be located indirectly by means of geophysical data including gravity, heat flow, and reflection seismology . The unmetamorphosed granitoids at locations PT1, PT2, and PT3 are also heat-producing. We use Hole C25 near Portsmouth, VA, as an example of how to access this geothermal data base.