Natural subsurface temperature profile along the True Vertical Direction:
T(z), where
z is True Vertical Depth Sub-Sea (TVDss).
It is essentially a true vertical component of local Geothermal Temperature Field
T(x, \, y, \, z).
Many subsurface studies are focused on rocks below a specific surface area where lateral variation of Geothermal Temperature Field maybe insignificant comparing to true vertical variation.
This leads to simplified version of Geothermal Temperature Field in the form of