The geothermal gradient is the rate at which the Earth's temperature increases with depth. It describes how heat from the planet's interior—generated by radioactive decay and residual heat from its formation—causes temperatures to rise as you move deeper underground. On average, the gradient is about 25–30°C per kilometer (roughly 14–17°F per mile) in the Earth's crust, though this can vary depending on location, tectonic activity, and geological conditions.
Geothermal Gradient Formula |
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\( g_g \;=\; \dfrac{ T_{bh} - T_s }{ D_{bh} } \cdot 100 \) (Geothermal Gradient) \( T_{bh} \;=\; \dfrac{ g_g \cdot D_{bh} }{ 100 } + T_s \) \( T_s \;=\; T_{bh} - \dfrac{ g_g \cdot D_{bh} }{ 100 } \) \( D_{bh} \;=\; \dfrac{ 100 \cdot (T_{bh} - T_s ) }{ g_g }\) |
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| Symbol | English | Metric |
| \( g_g \) = Geothermal Gradient | \(F \;/\; 100\;ft\) | \(C \;/\; 100\;m\) |
| \( T_{bh} \) (Greek symbol sigma) = Maximum Recorded Temperature | \(F\) | \(C\) |
| \( T_s \) = Temperature Near Surface | \(F\) | \(C\) |
| \( D_{bh} \) = Total Depth of Logged Borehole | \(ft\) | \(m\) |
