downhole measurement

In geology, any one of a number of experiments performed by instruments lowered down a borehole. Such instruments may be detectors that study vibrations passing through the rock from a generator, or may measure the electrical resistivity of the rock or the natural radiation.

Geophysicists measured heat flow at 10,000 boreholes worldwide 1986-1994 to gather information on climate change. Surface air temperature affects the temperature of underlying rock. Heat travels so slowly that the first 500 m/1,640 ft of the Earth's crust provides a record of the ground temperature for the last thousand years.