Exploration for geothermal resources includes evaluation of the volcanic history of areas, regional hydrology, geochemistry of hot springs, and certain selected geophysical methods that determine temperature, heat flow, and structure of prospective areas.

Geothermal energy is used mostly for electric-power generation with current worldwide installed capacity of about 1,000 Mw. The only geothermal area in the world completely developed by private enterprise is at the Geysers in northern California, where it has proved to be a viable, mechanically reliable, and environmentally acceptable resource, competing economically with alternative forms of power generation in Pacific Gas and Electric Company's system.

Modern drilling for natural steam was started in the Geysers area in 1955. Pacific Gas and Electric's first power plant, with a 12.5-Mw electric-generating capacity utilizing this steam, went into operation in 1960. By 1970, four units of 82 Mw were on stream. With the annual completion of 110-Mw generating facilities beginning in 1971, the Geysers field is expanding rapidly and it produces today 400 Mw, with the field estimated to be in excess of 1,000 Mw in size.

The National Petroleum Council estimates that by 1985 about 15,000 Mw of geothermal power can be developed in the western United States. With improved exploration, drilling and utilization technology, and modification of certain institutional barriers it has been estimated that geothermal power may be of the order of 75,000 Mw by the year 2000.

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