Introduction

It has been known for many decades that temperature, as measured in mines and boreholes, increases with depth. This suggests that the earth's interior is warmer and, therefore, that heat must be flowing upward in the earth. This outflow of heat is almost imperceptible on the earth's surface except at a few localities where heat is sometimes dramatically transferred from the earth's interior to the surface through volcanoes and hot springs. The main source of heat energy within the earth is probably the radioactive decay of long-lived isotopes. A small portion may be a residual effect from the processes that formed the earth.

Worldwide studies of heat flow have provided information on the broad characteristics of thermal conditions beneath the major geological features, including continental rifts, oceanic ridges, subduction margins, and intraplate zones of anomalous thermal activity. Geothermal studies on a regional scale have become all the more important since the energy crisis of the early 1970s when it became imperative to consider geothermal energy as one of several alternative energy sources to displace the use of oil. The broader aspects of geothermal studies including regional heat flow and temperature distribution with depth, exploration for geothermal resources, and heat extraction from thermal reservoirs are discussed in Rybach and Muffler (1981) and Jessop (1990).

On a local scale, thermal measurements (in particular temperature anomalies) have been used to infer the positions of structures such as shallow salt domes, anticlines, faults, fissures, etc.