Shown below are all temperature logs obtained by Virginia Tech in Maryland for the DOE Geothermal Program. It is clear that a wide range of temperatures at any given depth means that some low-temperature geothermal applications might be quite site-specific if the application is critically dependent on reaching a minimum temperature at an economical drilling depth. For example, at a depth of 300 m, the temperature in Maryland can vary between 16 and 29 C, a difference of 13 C. At a depth of only 750 m, the temperature difference can be as great as 20 C.
The relatively large range in subsurface temperatures is attributed to the combination of the low thermal conductivity of the unconsolidated Coastal plain sediments above the crystalline basement, and the occurrence within the crystalline basement of heat-producing post- or syn-metamorphic granitoids. An optimum location for a geothermal resource in the eastern United States is in sediments of the Atlantic Coastal Plain that overlie heat-producing granite basement rock. These will be the locations with the highest temperatures at the shallowest depths. This is the radiogenic model of Costain and others (1980). The post- and syn-metamorphic granites that occur in the southeastern United States have not been metamorphosed, and the heat-producing radioactive isotopes have therefore not been driven off. Most (80%) of the heat produced comes from the radioactive decay of isotopes of U and Th. An optimum location for the development of a low-temperature hydrothermal geothermal resource is where the very permeable Coastal Plain sediments are relatively thick; the geothermal gradients are higher in sediments of low thermal conductivity. For the Crisfield location, the relatively high temperatures are attributed to the presence of a large synmetamorphic heat-producing granitoid beneath Chesapeake Bay. Although this interpretation has not been confirmed by the drill at Crisfield, it is consistent with the negative gravity anomaly that is associated with the post- and syn-metamorphic Late Alleghanian unmetamorphosed granite bodies in many other locations in the eastern United States, where the source of the gravity anomaly has been confirmed by drilling (Costain and others, 1986). The granite bodies are part of the signature of the collision between Africa and North America that formed the Appalachian Mountain System. Although granite was not encountered in the volcanic rocks that make up the shallow crystalline basement at Crisfield, it is most certainly nearby to the west beneath Chesapeake Bay, and probably within the deeper crystalline basement at Crisfield.
References and Further Reading
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Last updated: 11/10/97