«Temperature Tools The temperature log can be used to give a useful profile of the thermal conductivity of rocks adjacent to the borehole and a measurement of the local geothermal gradient when the borehole is allowed to stabilize for a period of time (at least several days) after drilling ceases, assuming there is no convection in the borehole. (judithcurry.com)
But according to the article on geothermal gradient, «the geothermal gradient within the bulk of Earth's mantle is of the order of 0.5 kelvin per kilometer,». (judithcurry.com)
«Departures from the expected increase in temperature with depth (the geothermal gradient) can be interpreted in terms of changes in temperature at the surface in the past, which have slowly diffused downward, warming or cooling layers meters below the surface.» (judithcurry.com)