Geothermal Gradient

 

Geothermal Gradient Formula
\( g_g \;=\;   \dfrac{ T_{bh} - T_s }{ D_{bh} } \cdot 100 \)     (Geothermal Gradient) \( T_{bh} \;=\;   \dfrac{ g_g \cdot D_{bh} }{ 100 }  + T_s   \) \( T_s \;=\;   T_{bh} -  \dfrac{ g_g \cdot D_{bh} }{ 100 } \) \( D_{bh} \;=\; \dfrac{ 100 \cdot (T_{bh} - T_s ) }{ g_g }\)
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	\(ft\)	\(m\)

Geothermal gradient is the rate at which the Earth's temperature increases with depth below its surface.  On average, the temperature increases by about 25°C for every kilometer of depth.

Key Points about Geothermal Gradient

Heat Sources

• Radioactive Decay  -  The primary source of geothermal heat is the decay of radioactive isotopes in the Earth's mantle and crust.
• Residual Heat  -  Heat remaining from the Earth's formation.
• Mantle Convection  -  Heat transferred from the deeper mantle.

Variation with Depth

• Near the surface, the geothermal gradient is relatively high because heat from deeper within the Earth has to pass through the insulating crust.
• At greater depths, especially in the mantle, the gradient tends to be lower because the material is more conductive and convective heat transfer becomes more significant.

Geographical Variation

• Stable Cratons  -  Regions with old, stable crust typically have lower geothermal gradients.
• Tectonically Active Areas  -  Regions with volcanic activity or active tectonics often have higher gradients due to the proximity to magma and increased crustal thinning.

Applications

• Geothermal Energy  -  Understanding where heat from the Earth is used to generate electricity or provide direct heating.
• Oil and Gas Exploration  -  The gradient affects the thermal maturity of organic materials, influencing hydrocarbon formation and the location of oil and gas reserves.

Measurement  -  The geothermal gradient is measured by drilling boreholes and recording temperature at various depths.  These measurements are then used to estimate the temperature increase with depth.