▪ Abstract  Geothermal energy technology is reviewed in terms of its current impact and future potential as an energy source. In general, the geothermal energy resource base is large and well distributed globally. Geothermal systems have a number of positive social characteristics (they are simple, safe, and adaptable systems with modular 1–50 MW [thermal (t) or electric (e)] plants capable of providing continuous baseload, load following, or peaking capacity) and benign environmental attributes (negligible emissions of CO2, SOx, NOx, and particulates, and modest land and water use). Because these features are compatible with sustainable growth of global energy supplies in both developed and developing countries, geothermal energy is an attractive option to replace fossil and fissile fuels. In 1997, about 7,000 MWe of base-load generating capacity and over 15,000 MWt of heating capacity from high-grade geothermal resources are in commercial use worldwide. A key question is whether these levels can grow to a point where geothermal energy is more universally available and thus have a significant impact on global energy supplies in the twenty-first century. Such an achievement will require the economic development of low-grade resources. The current status of commercial and emerging technologies for electric power production and direct heat use is reviewed for the major geothermal resources including hydrothermal, geopressured, hot dry rock, and magma. Typically, high-temperature resources (>150°C) provide base-load generating capacity while lower-temperature resources provide energy for geothermally assisted heat pumps and for direct use in domestic, agricultural, and aquacultural heating applications. Critical development issues relating to resource quality and distribution, drilling costs, and reservoir productivity are discussed in the context of their economic impact on production costs. Advanced drilling and improved heat mining methods are suggested as approaches to increase the worldwide use of geothermal energy by reducing field development costs. With these improvements, lower-grade resources can compete in growing global energy markets that are currently controlled by abundant and low-cost fossil fuels.

Twenty-four magnetotelluric (MT) soundings have been collected in an east-west profile across the center of Long Valley caldera. The average station spacing is approximately I km and appears adequate to sample the important features of the upper crustal and deeper resistivity structures. Additional control on the shallowest resistivity is provided by a continuous profile of time domain electromagnetic soundings coincident with the western portion of the MT line. Our MT data set reveals numerous resistivity structures which illuminate the evolution and present state of the Long Valley system. Many of these have been quantified through two-dimensional (2-D) finite element modeling emphasizing the transverse magnetic (TM) mode. Important structural components include low-resistivity layers 0.5-1.5 km deep under the eastern half of the caldera, beneath the axial graben of the resurgent dome, and under the west caldera moat. Most of this layering appears to lie in post-caldera Early Rhyolite tuffs, and the uppermost unwelded Bishop Tuff. These rhyolite units have been observed to be porous and highly altered and to commonly contain Pleistocene intercalated lacustrine clays. The remainder and majority of the Bishop Tuff appears highlyresistive. A low resistivity layer also occurs below the axial graben near the base of the Bishop Tuff (1.5 km). Hydrothermal fluids or alteration in precaldera volcanic strata or, less likely, carbonaceous metasediments may be the cause of this. Resistive, probably crystalline basement at high levels is apparent beneath the center of resurgence. Low resistivities are modeled at a depth around 5 km below the entire west moat and central graben and may represent a zone of hydrothermal fluids released from magma crystallization, with potential magmatic contributions at greater depths. The correspondence between this low resistivity and teleseismic delay and low density zones found in other studies is quite striking. A subtle anomaly in the transverse electric (TE) mode impedance is weakly suggestive of a midcrustal conductive axis centered beneath the central graben and resurgent dome. However, it cannot be simulated by two-dimensional transverse electric calculations and requires a full three-dimensional evaluation to ensure that the anomaly does not represent resistivity complexity in just the upper few kilometers. A fundamental, caldera-wide 3-D effect is documented by comparison of observed and computed TE impedance and vertical magnetic field data. The abrupt termination of conductive caldera sediments less than 10 km north and south of our profile greatly depresses the observed TE apparent resistivity and vertical magnetic field relative to the model calculations for periods greater than 0.3 s for the central and eastern caldera. Analysis of the TE mode data also suggests that a similar finite-strike effect lies in the response at periods greater than 3 s due to the mid-crustal west moat conductor. The TM mode measurements are judged to also contain some large-scale departure from the ...

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...

at the site involving many missions. There was a reasonable effort to minimize subsurface contamination prob-In a remote site in eastern Idaho, now known as the Idaho National lems. However, the understanding of subsurface flow Engineering and Environmental Laboratory (INEEL) Site, the U.S. Government established a facility to test-fire naval gunnery during and contaminant transport phenomena, and of proper World War II. The mission after the war switched to development disposal techniques, was not sufficient to prevent suband demonstration of nuclear technology. For more than 50 yr, the surface contamination. Even today, our understanding site has been devoted to nuclear energy research. Because of the and our ability to accurately predict subsurface contamiremote location of the site west of Idaho Falls, ID, wastes containing nant flow and transport, especially in the vadose zone, radioactive and hazardous materials were disposed to the subsurface. are severely limited. It was felt that any disposed materials would not travel downward through the vadose zone, which is 100 to 300 m thick, to the underlying Snake River Aquifer. However, some materials have traveled through HISTORY the thick vadose zone and contaminated the aquifer. Other wastes Located in a remote arid area in eastern Idaho on were injected directly into the aquifer. To provide a general backthe northeast-trending Snake River Plain, the U.S. Govground for papers in this special issue of Vadose Zone Journal on ernment established a site to test-fire gunnery that was research at the INEEL, we give a brief historical perspective of work being refurbished at the Naval Ordnance Plant in Pocaconducted at the INEEL Site west of Idaho Falls and associated subsurface contamination issues. We furthermore give an overview tello, ID. The U.S. Navy withdrew approximately 715 of the research papers presented in this special issue.