The presence of deep fracture permeability in crystalline rocks is now recognized as widespread and therefore potentially important for groundwater geothermal heat pump applications. Precision temperature logs are a direct indicator of fluid flow in holes that intersect fracture conduits. Such data are the only known geological or geophysical method of detecting those fractures that are hydraulically conductive. Regional reflection seismic data image the subsurface structural geometry and therefore provide, when combined with a knowledge of the geologic framework and tectonic history of an area, a measure of the general geologic complexity and fracture potential of a region. Available seismic data provide a useful guide to where the rocks are known to be highly reflective and would therefore be good targets for future, more high resolution, seismic studies that are designed to image fracture zones in more detail. A number of examples of thermal and seismic data sets already available from the crystalline rocks of the eastern United States are being prepared for display at this web site.
It is well known that precision temperature logs can detect hydraulically conductive fractures in crystalline (or any other) rocks. If a hole penetrates a hydraulically conductive fracture, then water can enter the hole at a slightly higher or lower temperature depending upon whether the water has been flowing upward or downward, respectively, in the fracture. Precision temperature probes can detect differences in temperature of just a few thousandths of a degree. If the temperature log is differentiated with respect to depth to obtain the geothermal gradient, then the depths at which water is entering or leaving the hole from a fracture zone are more easily identified. The anomalous temperatures result in abrupt changes in a plot of the temperature gradient versus depth. Under these conditions, heat is transported by convection rather than by conduction. Convection is a much more efficient way to transfer heat. The use of precision temperature measurements to detect water flowing into or out of fractures is the only geologic or geophysical method of identifying those fracture zones that are hydraulically conductive at depth.
rule of thumb generally followed by hydrologists from the
early 1900s to the mid 1980s was to not drill greater than about 250 to 300
feet (Ellis, 1906) in the Piedmont. In the Atlanta, GA, area, however, Cressler
and others (1983) found that many wells with large yields penetrated horizontal
openings at depths of 400 to 600 feet below the surface. Cressler and others
(1983, p. 19) interpreted these openings as stress-relief fractures that formed
as a result of erosional unloading. They noted that "The chance of obtaining
large well yields from stress-relief fractures is significantly increased by
drilling to about 620 feet." The next major advance in knowledge of the
Piedmont groundwater system resulted from a statistical analysis by Daniel
(1989) of more than 6,000 well records from the Piedmont and Blue Ridge
Provinces of North Carolina. Daniels found that well yields increase with depth
to a much greater depth than previously thought, and that well yields increase
dramatically as well diameter increases. Text from Heath (1989).
Data from water-bearing fractures from 227 wells in metamorphic and igneous rocks in Coastal Maine indicate that there is no evidence that fracture yield or fracture density decrease with depth in at least the upper 600 feet (Loiselle and Evans, 1995). Intercrystalline porosity is absent in these rocks. Porosity is entirely a result of brittle fracturing. Groundwater flow in these rocks is localized along fractures. Thus, with regard to the implications for groundwater resource exploration and groundwater heat pump applications, the available data do not justify imposing limits on well depths when drilling for water-bearing fractures, supporting the conclusions from Daniel (1989).
In the crystalline basement rocks beneath the Savannah River Site in South Carolina, major zones of fracture permeability have been detected by packer tests and confirmed by temperature logging. Horizontal fractures have been correlated across the site at depths as great as 550 meters (see unpublished figures below from Costain (1976).
|Stations 12 - 1095|
|Stations 1045 - 2178|
The ADCOH Site area is shown in the following figure. Exploratory Holes #1 and #2 are separated by about ? km. These holes were logged for temperature by Virginia Tech. This study was part of the ADCOH Project to drill the first ultradeep hole in the United States for scientific studies (Hatcher and others, 1988), similar to the ambitious program still being pursued by the Russians. For various reasons, the deep hole was never drilled; however, a great deal of geological and geophysical information was obtained as part of the preliminary site selection studies. Some of the data are displayed at this web site. Go here for an interactive version of the following map to access the heat flow data, or here to view seismic data..
Temperature and temperature gradients from Holes #1 and #2 are shown below. Groundwater circulation is apparent in both holes over the depth interval 170-200 meters, as well as at shallower depths. Hole #1 was strongly artesian and finally had to be capped off to stop the flow, which originated abruptly from a depth of about 170 m. In both holes, the geothermal gradient returns to its regional value of about 20 C/Km below a depth of about 200 m, suggesting that groundwater circulation is minimal or absent from a depth of 200 meters to the bottom of the hole. The coincidence of the geothermal gradients below a depth of about 200 m suggests that the same lithologic unit was encountered in Holes 1 and 2 over this deeper depth interval.
All four holes are shown in the following figure. It is apparent that a fracture/weathered zone of regional extent was encountered in each hole at a depth of about 170 m. Indeed, Hole #3 had to be abandoned because of difficult drilling conditions encountered near and at this depth. Further discussions of this area and how the fracture zone relates to lithologic units are in preparation for publication (Costain and Hatcher, 1997, in preparation).
The fracture zone appears to be related to relief of overburden stress.
Hole MCC-1 is one of the deepest (> 2,000 m) holes logged by Virginia Tech in the southeastern U.S. Large anomalies in the temperature gradient log (click on MCC-1 on the map below) indicate several zones of convection between depths of 500 and 1,500 m. For other hotmap items, go to Georgia map.
|Costain, J.K., and Decker, E.R., 1987, Heat flow at the proposed Appalachian Ultradeep Core Hole (ADCOH) site: Tectonic Implications, Geophysical Research Letters, v.14, No. 3, p. 252-255.|
|Hatcher, Jr., R.D., Williams, R.D., Edelman, S.H., Costain, J.K., Coruh, C., Phinney, R.A., Roy-Chowdury, K., Decker, E.R., Zoback, M.D., Moos, D., and Anderson, R.N., 1988, The Appalachian Ultradeep Core Hole (ADCOH) Project, in Deep Drilling in Crystalline Bedrock, v. 2, edited by A. Boden and K.G. Eriksson, Springer-Verlag, p. 117-154.|
|Heath, R.C., 1989, The Piedmont Groundwater System, in Proceedings, Conference on Ground Water in the Piedmont of the Eastern United States, published by Clemson University, Clemson, SC 29634-0357, (803) 656-4073.|
|Lampshire, L.D., Çoruh, C., and Costain, J.K., 1994, Crustal structures and the eastern extent of lower Paleozoic shelf strata within the central Appalachians: A seismic reflection interpretation, Geol. Soc. Amer. Bull., v. 106, 1-18.|
|Loiselle and Evans, 1995, Fracture density distributions and well yields in Coastal Maine, Ground Water, v. 33, No. 2, March-April, p. 190-196.|
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