Phillip M. Wright

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.

Magnetotelluric transect of Long Valley caldera: Resistivity cross‐section, structural implications, and the limits of a 2-D analysis

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 1 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 upp...

Shallow thermal structure and hydrology of Ascension Island, South Atlantic Ocean

Seven coreholes, 63 to 533 m deep, were drilled on Ascension Island to determine the shallow thermal structure in preparation for siting a deep geothermal exploration well. These holes were used to evaluate specific geothermal exploration targets defined by previously completed geological mapping, aeromagnetic and electrical resistivity surveys. The highest temperature measured was 54.4°C at 203 m below sea-level. Measured temperature gradients reached up to 72°C/km; heat flow, up to 124 m W/m2. The higher heat flow values are associated with the two silicic volcanic centers on the island. The water table in all wells is at approximately sea-level. Groundwater samples from different depths in corehole LDTGH, located in the central part of the island, show a zone of brackish water overlying water with the salinity of seawater. Both waters retain the cation signature of seawater, but show elevated silica concentrations. Mixing calculations and chemical geothermometry indicate temperatures as high as 143°C.

Geothermal development in the U.S.A. and future directions

The geothermal industry presently has an operating generation capacity of about 2,300 megawatts and generates about 17 billion kilowatt-hours per year in the United States. Although the domestic market is stagnant due to restructuring of the electricity industry and to the very low competing price of natural gas, the industry is doing well by developing geothermal fields and power plants in the Philippines and Indonesia. The industry strongly supports the Department of Energy research program to develop new and improved technology and help lower the costs of geothermal power generation.

Overview of geothermal resource development with emphasis on engineering aspects

Geothermal energy is an important alternative energy resource that is commercially viable at some of the high-grade geothermal sites today. Although the worldwide geothermal resource base is very large, only a small fraction of these resources can be used economically due to inadequacies in technology. The geothermal industry is currently evaluating the state of the technology, and embarking on an R&D program, in cooperation with the US Department of Energy, that is aimed at improving technology and allowing more of the available resource base to be developed.