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 ( \{ x, \, y \}) variation of Geothermal Temperature Field maybe insignificant comparing to true vertical ( \{ z\}) variation.
This allows using Geothermal Temperature Profile
T(z) instead of Geothermal Temperature Field
T(x, \, y, \, z).