Ratio of true vertical component of regional Earth's Heat Flux $\begin{array}{l}j_z({\bf r})\end{array}$ to the subsurface Thermal Conductivity $\begin{array}{l}\lambda_e({\bf r})\end{array}$ at a certain subsurface point $\begin{array}{l}{\bf r}\end{array}$ :

(1)	$\begin{array}{l}\displaystyle G_T({\bf r}) = \frac{j_z({\bf r})}{\lambda_e({\bf r})}\end{array}$

Fig. 1. Schematic view of the Earth's Geothermal Temperature Gradient.

The most common value of Unbiased Geothermal Temperature Gradient (GT) away from tectonic plate boundaries is: $\begin{array}{l}G_T = 0.025 ÷ 0.030 \, \rm °C/m\end{array}$ .

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