The total installed capacity of geothermal generating plants in the world today is approximately 620 MW, distributed among the following five countries: Italy, 340; New Zealand, 190; USA, 50; USSR, 30; and Japan, 10. Although exceedingly small in comparison with the world's total generating capacity from conventional sources, the rapid growth of this new industry is reflected by the fact that more than half of this present geothermal capacity has been installed during the last 10 years. The success of these installations is stimulating worldwide interest in geothermal energy, and exploration projects are now underway in Mexico, El Salvador, Chile, Turkey, Kenya, China (Taiwan), and the Philippines.

The principal incentives for development of geothermal power are: (a) the lack of more conventional sources in the market area, and (b) the competitive economic position of geothermal power even in those areas where other sources are available. Geothermal sources generate low-cost power even at capacities under 100 MW, making them particularly advantageous in market areas where power demands are still low. Low steam pressures make it necessary to use small generating units, i.e., on the order of 25 MW, but total capacities of several hundred megawatts can be expected from a single steam field.

All the thermal areas now under investigation share a common regional geologic setting: the areas are located in orogenic zones, where late Tertiary or Quaternary volcanism has taken place. The thermal areas, however, are not necessarily in close proximity to volcanic centers. Tectonically the regions are characterized by vertical movements, both uplift and subsidence, which have taken place on normal faults. Fault blocks, tilted consistently in one direction, appear to be more common than horst and graben structures.

Variations in local structure, stratigraphy, and hydrology result in considerable differences in the geologic characteristics of individual steam fields. Fault zones, permeable strata, or a combination of both, can form thermal fluid reservoirs. Although it is now generally agreed that the heat sources are shallow intrusive bodies and the thermal fluid is at least 95% meteoric in origin, there are still many fundamental problems as yet only partly answered. How is the heat transfer actually accomplished? Do phase changes occur in the thermal fluid under natural conditions or only when the system is under exploitation? What are the roles of convection currents and caprocks in forming an economically exploitable deposit? What factors determine the life expectancy of the field?

Geothermal exploration methods have not advanced far beyond the stage of merely drilling on hot springs, except in Italy where geothermal gradient surveys have been applied successfully. Recent results from deep resistivity surveys, however, indicate that this method holds considerable promise. Much progress has been made in understanding the chemistry of thermal systems and in the near future this knowledge should form the basis of effective exploration methods.

Although the successful development of geothermal resources offers a great challenge to exploration geologists and engineers, it offers no less a challenge to power legislators, planners, and administrators. Because natural steam must be utilized when and where it is produced, successful development requires the closest cooperation between the exploration groups and the power marketing and distribution sector. Rapid development of geothermal power cannot be expected until these two groups, and the legislators who control their activities, arrive at a mutual understanding of each other's problems.

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