At the present stage of development, use' of geother mal energy saves about 77 million barrels of oil per year worldwide that would otherwise be required for elec trical power generation and direct heat applications. More than a dozen countries are involved in devel opment of geothermal resources. Currently, only the moderate-and high-temperature hydrothermal convec tive type of geothermal system can be economically used for generating electric power. Lower-temperature resources of several types are being tapped for space heating and industrial processing. Geophysics plays im portant roles both in exploration for geothermal sys tems and in delineating. evaluating, and monitoring production from them. The thermal methods, which detect anomalous temperatures directly, and the elec trical methods are probably the most useful and widely used in terms of siting drilling targets, but gravity, mag Geophysical Exploratlon-Geotherma I United States, and to a lesser extent in other countries. Expertise arising from first-hand experience in Iceland, Italy, New Zealand, Mexico, Japan. and the C .S. (primar ily) is being used by the less-developed countries to assist their geothermal efforts. The United Nations (U .N.) sponsors both scientific work and education in underdeveloped countries. Exploration projects using U,N. funding have been carried out in EI Sal vador, Chile, Nicaragua, Turkey, Ethiopia, and Kenya, and the U.N. sponsors geothermal training programs at the united Nations University locations in Iceland, Italy, and New Zealand. La Organizacion Latinoamericana de Energia (OLADE), headquartered in Quito, Ecuador, provides support for geothermal development in Central and South America. In short, an infrastructure for geothermal development is being built throughout the world, and although it is small compared to the corresponding petroleum or minerals infrastructures. It is making important contributions. We review here the application of geophysical methods to geothermal exploration and development and assess the cur rent state-of-the-art. Previous reviews of geophysical appli cations were given by Palrnason (J 1.)76), :v1cNitt \ J976), Meida v and Tonani (1976), Ward (1983bl, and Rapolla and Keller (1984), among others. There are more published ac counts of geophysical work In the geothermal environment than can be discussed or referenced here. We cite a few typical references for each application discussed. We apologize for the emphasis on US. literature, with which we are most familiar, and for omissions in recognizing contributions of many au thors. ~ATLRE OF GEOTHERMAL RESOLRCES Geothermal resources have three common components: (1) a heat source, (2) a reservoir with porosity and permeability. and (3) a fluid to transfer the heat to the surface. In some exploitation schemes, the permeability must be created artifi cially. One useful classification of geothermal resource types is shown in Table 1. Hydrothermal resources, as the term implies, are characterized by natural thermal waters, and are divided into r...
