Temperature measurements below the Earth's surface; a history of records

The first documentation of temperature records in the subground derives from the cellar of the Observatory of Paris where the records started in 1671. An important problem in the seventeenth and eighteenth centuries was the solution of the effect of the radiation of the Sun on the temperature of the Earth. Jean Baptiste Joseph Fourier (1768-1830) contributed with the theoretical treatment of heat conduction problems at the beginning of the nineteenth century. At the same time, several special solutions which are applied to the Earth were reported. The contemporarily developed technique of measurement used mercury thermometers which are optimized for borehole records. Thermocouples and electrical resistances were also introduced in the nineteenth century. Modern electronics using semiconductors or quartz oscillators achieved a remarkable progress in resolution and accuracy of records in the Earth's subsurface. An early temperature log was reported in 1871 when a borehole near Berlin, Germany reached a depth of 1,270 meters. The temperature data supported a parabolic law of temperature variation with depth, which caused a general discussion about the temperature in the Earth's interior. The increasing interest in thermal problems yielded not only linear functions but also two-dimensional temperature distributions as well as time-dependent functions. On the basis of subsurface temperature records at Tbilisi, Georgia, a map of isopleths was constructed in 1901 and a map of tautochrones of daily variations at Beograd, Yugoslavia in 1911. After special attention to the effect of inhomogeneities and water movements on the temperature field in the subsurface, a recent topic has been the evaluation of climate variations and global warming by temperature records in the subsurface.