State-of-the-art geophysical exploration for geothermal resources

At the present stage of development, use of geothermal energy saves about 77 million barrels of oil per year worldwide that would otherwise be required for electrical power generation and direct heat applications. More than a dozen countries are involved in development of geothermal resources. Currently, only the moderate- and high-temperature hydrothermal convective 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 important roles both in exploration for geothermal systems and in delineating, evaluating, and monitoring production from them. The thermal methods, which detect anomalous temperatures directly, and the electrical methods are probably the most useful and widely used in terms of siting drilling targets, but gravity, magnetics, seismic methods, and geophysical well logging all have important application.Advances in geophysical methods are needed to improve cost effectiveness and to enhance solutions of geologic problems. There is no wholly satisfactory electrical system from the standpoint of resolution of subsurface resistivity configuration at the required scale, depth of penetration, portability of equipment, and survey cost. The resolution of microseismic and microearthquake techniques needs improvement, and the reflection seismic technique needs substantial improvement to be cost effective in many hard-rock environments. Well-logging tools need to be developed and calibrated for use in corrosive wells at temperatures exceeding 200 degrees C. Well-log interpretation techniques need to be developed for the hard-rock environment. Borehole geophysical techniques and geotomography are just beginning to be applied and show promise with future development.