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| CATEGORIES ABOUT GEOTHERMAL GRADIENT | |
| geological processes | |
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| structure of the earth | |
| geothermal energy | |
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The geothermal gradient is the heating up of the Earth's top soil and rock layer the deeper you go down, for instance by descending into a Cave or Drilling a well. The geothermal gradient is measured by taking a series of bottom open-hole temperature measurements during the drilling of a borehole. To achieve accuracy the drilling fluid needs time to reach the ambient temperature. This is not always achievable for practical reasons. In stable tectonic areas in the tropics a temperature-depth plot will converge to the annual average surface temperature. However in areas where deep Permafrost developed during the Pleistocene a low temperature anomaly can be observed that persists down to several hundred metres. {Link without Title} The Suwalki cold anomaly in Poland has led to the recognition that similar thermal disturbances related to Pleistocene-Holocene climatic changes are recorded in boreholes throughout Poland. In areas of Holocene uplift and erosion (Fig. 1) the initial gradient will be higher than the average until it reaches an inflection point where it reaches the stabilized heat-flow regime. If the gradient of the stabilized regime is projected above the inflection point to its intersect with present-day annual average temperature, the height of this intersect above present-day surface level gives a measure of the extent of Holocene uplift and erosion. In areas of Holocene subsidence and deposition (Fig. 2) the initial gradient will be lower than the average until it reaches an inflection point where it joins the stabilized heat-flow regime. In deep boreholes the temperature of the rock below the inflection point generally increases with depth at rates of the order of 20º K/km or more. Fourier's law of heat flow applied to the Earth gives. q = Mg where q is the heat flux at a point on the Earth's surface, M the thermal conductivity of the rocks there, and g the measured geothermal gradient. A representative value for the thermal conductivity of granitic rocks is M = 3.0 W /mºK. Hence, using the global average geothermal gradient of 0.02º K/m we get that q = 0.06 W/m2. This estimate, corroborated by thousands of observations of heat flow in boreholes all over the world, gives a global average of 63 mW/m2. Thus, if the geothermal heat flow rising through an acre of granite terrain could be efficiently captured, it would light four 60 watt light bulbs. If that rate of heating were constant it would soon reach the point where all known rocks would melt. We know, however, that the earth's Mantle is solid because it transmits S-waves . The temperature gradient dramatically decreases when the mechanism of thermal transport changes from conduction, as within the rigid tectonic plates, to convection, as in the portion of Earth's mantle that convects. Despite its solidity, most of the Earth's mantle behaves over long time-scales as a fluid, and heat is transported by advection, or material transport. Thus, the geothermal gradient within the bulk of Earth's mantle is of the order of 0.1° C per kilometer, and is determined by the adiabatic gradient associated with mantle material. This heating up can be both beneficial or detrimental in terms of Engineering : Geothermal Energy can be used as a means for generating Electricity , by using the heat of the surrounding layers of rock underground to heat water and then routing the steam from this process through a Turbine connected to a generator. On the other hand, Drill Bits have to be cooled not only because of the Friction created by the process of Drilling itself but also because of the heat of the surrounding rock at great depth. Also, very deep Mines like some Gold mines in South Africa need the air inside to be cooled and circulated to allow miners to work at such great depth. |
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