The increase of subsurface temperature with depth (the geothermal gradient) is a fundamental, universally observed geophysical property of the Earth and does not require academic citation.
The claim states that subsurface temperature increases predictably with digging depth due to the geothermal gradient. This is a well-established and universally understood physical phenomenon of the Earth, qualifying as common knowledge. No citations are needed for basic everyday or textbook scientific observations of this nature.