Howard P. Ross

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.

Geophysical Investigations of the Cove Fort-Sulphurdale Geothermal System, Utah

Abstract The Cove Fort-Sulphurdale KGRA is part of one of the largest thermal anomalies in the western United States. Since 1975 an extensive data base has been developed which includes the results of detailed and regional geologic, gravity, magnetic, seismic, and resistivity investigations. Geologic studies have delineated the major tectonic elements of the thermal system and have led to the recognition of large-scale gravitational glide blocks that act as a leaky cap to portions of the geothermal system. Gravity and magnetic data have delineated major throughgoing structures beneath alluvium and basalt cover, and have indicated the importance of the Cove Fort-Beaver graben in localizing the geothermal reservoir. The presence of these structures and a high level of microearthquake activity suggest other target areas within the larger thermal anomaly. Electrical resistivity surveys and thermal gradient holes both contribute to the delineation of the known reservoir. Four deep exploration wells which test the geothermal system were drilled between 1975 and 1979. One well, CFSU 42-7, recorded temperatures of 178°C. The high cost of drilling, high corrosion rates, low reservoir pressures, and the apparent limited extent of the high-temperature reservoir led to a premature conclusion in 1980 that the field was not economic for large-scale electric power production. More recent drilling in the vicinity of CFSU 42-7 resulted in the discovery of high-temperature (2Q0°C?) geothermal fluids at a depth of approximately 350 m. A well-head generator was installed and power production is expected in 1985. Additional development of the geothermal reservoir is anticipated in the 1985 to 1987 time frame.

Roosevelt Hot Springs Geothermal System, Utah--Case Study

The Roosevelt Hot Springs geothermal system has been undergoing intensive exploration since 1974 and has been used as a natural laboratory for the development and testing of geothermal exploration methods by research organizations. This paper summarizes the geological, geophysical, and geochemical data which have been collected since 1974, and presents a retrospective strategy describing the most effective means of exploration for the Roosevelt Hot Springs hydrothermal resource. The bedrock geology of the area is dominated by metamorphic rocks of Precambrian age and felsic plutonic phases of the Tertiary Mineral Mountains intrusive complex. Rhyolite flows, domes, and pyroclastic rocks reflect igneous activity between 0.8 and 0.5 m.y. ago. The structural setting includes older low-angle normal faulting and east-west faulting produced by deep-seated regional zones of weakness. North to north-northeast-trending faults are the youngest structures in the area, and they control present fumarolic activity. The geothermal reservoir is controlled by intersections of the principal zones of faulting. The geothermal fluids that discharge from the deep wells are dilute sodium chloride brines containing approximately 7,000 ppm total dissolved solids and anomalous concentrations of F, As, Li, B, and Hg. Geothermometers calculated from the predicted cation contents of the deep reservoir brine range from 520 to 531°F (271 to 277°C). Hydrothermal alteration by these fluids has produced assemblages of clays, alunite, muscovite, chlorite, pyrite, calcite, quartz, and hematite. Geochemical analyses of rocks and soils of the Roosevelt Hot Springs thermal area demonstrate that Hg, As, Mn, Cu, Sb, W, Li, Pb, Zn, Ba, and Be have been transported and redeposited by the thermal fluids. The geothermal system is well expressed in electrical resistivity and thermal-gradient data and these methods, coupled with geologic mapping, are adequate to delineate the fluids and alteration associated with the geothermal reservoir. The dipole-dipole array seems best suited to acquire and interpret the resistivity data, although controlled source AMT (CSAMT) may be competitive for near-surface mapping. Representations of the thermal data as temperature gradients, heat flow, and temperature are all useful in exploration of the geothermal system, because the thermal fluids themselves rise close to the surface. Self-potential, gravity, magnetic, seismic, and magnetotelluric survey data all contribute to our understanding of the system, but are not considered essential to its explorati n.

Exploration Strategy for High-Temperature Hydrothermal Systems in Basin and Range Province

A 15-phase strategy of exploration for high-temperature convective hydrothermal resources in the Basin and Range province features a balanced mix of geologic, geochemical, geophysical, hydrologic, and drilling activities. The strategy, based on a study of data submitted under the Department of Energys Industry Coupled Case Study Program, provides justification for inclusion or exclusion of all pertinent exploration methods. With continuing research on methods of exploration for, and modeling of, convective hydrothermal systems, this strategy is expected to change and become more cost-effective with time. The basic strategy may vary with the geology or hydrology. Personal preferences, budgetary constraints, time and land position constraints, and varied experience may cau e industrial geothermal exploration managers to differ with our strategy. For those just entering geothermal exploration, the strategy should be particularly useful; many of its elements may apply in other geologic settings.

Electrical resistivity surveys, ascension Island, South Atlantic Ocean

Abstract Reconnaissance electrical resistivity surveys, completed on Ascension Island as one part of an integrated geothermal exploration program, define a 5 km2 area of low resistivity within the central part of the island. Dipole-dipole lines help to define the vertical resistivity structure and suggest the incursion of seawater beneath much of the area surveyed. A zone of 5–10 Ωm indicated by numerical modeling suggests possible alteration and geothermal brines that may have mixed with seawater, above a geothermal reservoir. Temperature gradients in shallow drillholes and a deep exploration well indicate the presence of a deep geothermal system in this area.

Aeromagnetic survey and interpretation, Ascention Island, South Atlantic Ocean

Abstract A detailed aeromagnetic survey of Ascension Island, which was completed in February and March of 1983 as part of an evaluation of the geothermal potential of the island, is described. The aeromagnetic map represents a basic data set useful for the interpretation of subsurface geology. An in situ magnetic susceptibility survey was also carried out to assist in understanding the magnetic properties of Ascension rocks and to aid in the interpretation of the aeromagnetic data. The aeromagnetic survey was interpreted using a three-dimensional numerical modeling program that computes the net magnetic field of a large number of vertically sided prisms. Multiple source bodies of complex geometry were modeled and modified until a general agreement was achieved between the observed data and the computed results. The interpretation indicates northeast- and east-trending elongate bodies of much higher apparent susceptibility than adjacent rocks. The relationship to mapped geologic features such as volcanic vents, dikes and faults suggests that these magnetic sources are zones of increased dike density and of other mafic intrusives emplaced along structures that fed the many volcanic centers. A large magnetic source on the northeastern portion of the island may be the intrusive equivalent of trachyte lavas present at the surface. A low-magnetization area, mainly north and west of Green Mountain, appears to be the most likely area for the presence of a geothermal system at moderate (1–3 km) depth.

Review of Well Logging in the Basin and Range Known Geothermal Resource Area

A discussion is presented of applications and interpretations of well logs to Basin and Range Province geothermal exploration and development. Problems experienced in use of conventional oilfield tools and techniques are reviewed, and methods to circumvent these problems are illustrated. Particular examples focus on log responses and matrix effects in complex lithologies. 36 refs.

Roosevelt Hot Springs, Utah Geothermal Resource--Integrated Case Study: ABSTRACT

The Roosevelt Hot Springs geothermal resource is located along the western margin of the Mineral Mountains, approximately 19 km northeast of Milford, in southwestern Utah. To date, seven producing wells have been drilled by Phillips Petroleum Co. and Thermal Power Co. Construction will soon begin on the first stage of a 120-megawatt power plant. Detailed geologic mapping and the study of well logs and drill cuttings indicate that the geothermal reservoir is a fracture-controlled, liquid-dominated system. The host rocks of the reservoir are Precambrian metamorphic rocks and various Tertiary intrusives. The reservoir is mainly localized between the range front and an alluvial covered horst block, along which fluids have migrated to the surface forming an elongate north-trending dome of siliceous sinter. The reservoir is an area of high heat flow (over 1,000 mW/sq mi) and low near-surface electrical resistivity (less than 10 ohm-m). Aeromagnetic, gravity, and reflection seismic data help define the geologic structure within and around the alluvium covered reservoir. Trace element geochemistry shows that arsenic, lithium, and mer ury are enriched along fluid pathways of the geothermal system. Mercury concentrations greater than 20 ppb occur only at temperatures less than 225°C and reflect the present thermal configuration of the field. The system was efficiently explored using detailed geologic mapping in combination with thermal gradient studies and dipole-dipole resistivity. End_of_Article - Last_Page 982------------

Reflection Seismic Surveys for Basin and Range Geothermal Areas--An Assessment: ABSTRACT

Several state-of-the-art reflection seismic surveys have been completed in high-temperature geothermal areas of the northern Basin and Range province. The survey data have been made public through the Department of Energy/Division of Geothermal Energy Industry Coupled and Exploration Technology programs. Data were studied for the Stillwater, Dixie Valley, Beowawe, San Emidio, and Soda Lake resource areas. Reflection quality, and hence usefulness of the reflection method, can be highly variable in the complex basin and range environment. Certainly survey design and proper processing are required to enhance the quality of the data. The most severe geologic condition appears to be the presence of surface, or near surface, layered volcanic rocks. These result in strong early reflections, substantial ringing and poor energy penetration to depth, as at Beowawe. In areas of thick alluvial cover, or Tertiary gravels and lake bed sedimentation (San Emidio, Soda Lake, Stillwater), data quality is often sufficient to map basin border faults and major displacements on volcanic or bedrock surfaces beneath 2,000 to 4,000 ft (609 to 1,219 m) of cover. Faulting is indicated primarily by the systematic termination of coherent reflections. Diffraction patterns are sometimes recognized but commonly obscured by the complex faulting and lithologic variations. The identification of a given reflector across major structures and accurate time-to-depth conversion are difficult interpretational problems. Excellent data quality at Stillwater and Dixie Valley should contribute to the development of these resources. End_of_Article - Last_Page 982------------

Strategy of Exploration for High Temperature Hydrothermal Systems in Basin and Range Province: ABSTRACT

A 15-phase strategy of exploration for high temperature convective hydrothermal resources in the Basin and Range province features a balanced mix of geologic, geochemical, geophysical, hydrologic, and drilling activities. The strategy is based on a study of data submitted under the Department of Energys Industry Coupled Case Study Program. Justification for inclusion in or exclusion from the strategy of all pertinent geoscientific methods is given. With continuing research on methods of exploration for and modeling of convective hydrothermal systems, this strategy is expected to change and become more cost-effective with time. Variations on the basic strategy are to be expected where the geology or hydrology requires it. Personal preferences, budgetary constraints, time nd land position constraints, and varied experience may cause industrial geothermal exploration managers to differ with our strategy. For those just entering geothermal exploration, the strategy is expected to be particularly useful. End_of_Article - Last_Page 799------------