Our "radiogenic model" is illustrated below. The relatively horizontal sediments shown in the illustration are those of the Atlantic Coastal Plain. The thickness of these sediments is shown in the next figure and has been determined by other drill holes that did encounter basement rocks. "Basement rocks" are defined as either the relatively non-permeable crystalline rocks or Triassic basin sediments similar to those exposed to the west in the Piedmont. We wish to estimate the temperature at the base of the Atlantic Coastal Plain sediments. Most of the exploratory holes drilled during our geothermal program did not reach basement; however, knowing the sediment thickness and making a reasonable assumption about the thermal conductivity of the sediments it is possible to estimate temperatures at greater depths.
As an example, we use our hole C-24 that was drilled to a depth of 315 meters and therefore did not reach basement, and the nearby (37 05' 45'' N, 76 23' 07'' W) drill hole recently completed (November, 2000) by the U.S. Geological Survey near Newport News, VA, that did reach basement at a depth of 626 meters (2,053 feet) (Greg Gohn, U.S. Geological Survey, personal communication, October, 2000). The temperature and gradient profiles for Virginia Tech Hole C-24 are shown in the following figure. At depths shallower than about 250 meters the geothermal gradients are considerably higher that at deeper depths in the hole, and exceed 50oC/Km at some depths. This is because the sediments are less compacted and more porous (water has a very low thermal conductivity). These high geothermal gradients cannot be used to predict temperatures at greater depths. At depths below about 250 meters (in this location in Virginia) the gradient decreases to about 25 oC/Km, and on the basis of data from other locations in Virginia this value is reliable enough to make a prediction of the temperature at a depth of 626 meters. The average value of 25oC/Km for the deeper interval in the hole is shown on the plot below.
The result is shown in the figure below for the assumed deeper geothermal gradient of 25oC/Km.
Where to find the name of a geothermal contractor in your area.
Questions and Answers about Residential Systems (courtesy of WaterFurnace)
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