http://www.renewableenergyaccess.com/rea/news/ate/story?id=49161
U.S. Geothermal Resource Map
The geothermal resources map of the United States below shows the estimated subterranean temperatures at a depth of 6 kilometers. To determine the Earth's internal temperature at any depth below the capabilities of normal well drilling, multiple data sets are synthesized. The data used for this figure are: thermal conductivity, thickness of sedimentary rock, geothermal gradient, heat flow, and surface temperature.
Also see geothermal resource maps.