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Title: A gravity model for the Coso geothermal area, California

Abstract

Two- and three-dimensional gravity modeling was done using gridded Bouguer gravity data covering a 45 {times} 45 km region over the Coso geothermal area in an effort to identify features related to the heat source and to seek possible evidence for an underlying magma chamber. Isostatic and terrain corrected Bouguer gravity data for about 1300 gravity stations were obtained from the US Geological Survey. After the data were checked, the gravity values were gridded at 1 km centers for the area of interest centered on the Coso volcanic field. Most of the gravity variations can be explained by two lithologic units: (1) low density wedges of Quarternary alluvium with interbedded thin basalts (2.4 g/cm{sup 3}) filling the Rose Valley and Coso Basin/Indian Wells Valley, and (2) low density cover of Tertiary volcanic rocks and intercalated Coso Formation (2.49 g/cm{sup 3}). A 3-D iterative approach was used to find the thicknesses of both units. The gravity anomaly remaining after effects from Units 1 and 2 are removed is a broad north-south-trending low whose major peak lies 5 km north of Sugarloaf Mountain, the largest of the less than 0.3 m.y. old rhyolite domes in the Coso Range. Most of this residualmore » anomaly can be accounted for by a deep, low-density (2.47 g/cm{sup 3}) prismatic body extending from 8 to about 30 km below the surface. While some of this anomaly might be associated with fractured Sierran granitic rocks, its close correlation to a low-velocity zone with comparable geometry suggests that the residual anomaly is probably caused a large zone of partial melt underlying the rhyolite domes of the Coso Range. 12 refs., 9 figs.« less

Authors:
;
Publication Date:
Research Org.:
Lawrence Berkeley Lab., CA (USA)
Sponsoring Org.:
USDOE; USDOE, Washington, DC (USA)
OSTI Identifier:
5684468
Report Number(s):
LBL-28878; CONF-900823-13
ON: DE91011874
DOE Contract Number:
AC03-76SF00098
Resource Type:
Conference
Resource Relation:
Conference: Annual meeting of the Geothermal Resources Council and international symposium on geothermal energy, Kailua Kona, HI (USA), 20-24 Aug 1990
Country of Publication:
United States
Language:
English
Subject:
15 GEOTHERMAL ENERGY; GEOTHERMAL FIELDS; GEOLOGIC MODELS; COSO HOT SPRINGS; GRAVITY SURVEYS; HEAT SOURCES; MAPS; RHYOLITES; THREE-DIMENSIONAL CALCULATIONS; TWO-DIMENSIONAL CALCULATIONS; CALIFORNIA; FEDERAL REGION IX; GEOPHYSICAL SURVEYS; IGNEOUS ROCKS; NORTH AMERICA; ROCKS; SURVEYS; USA; VOLCANIC ROCKS; Geothermal Legacy; 150301* - Geothermal Exploration & Exploration Technology- Geophysical Techniques & Surveys

Citation Formats

Feighner, M.A., and Goldstein, N.E. A gravity model for the Coso geothermal area, California. United States: N. p., 1990. Web.
Feighner, M.A., & Goldstein, N.E. A gravity model for the Coso geothermal area, California. United States.
Feighner, M.A., and Goldstein, N.E. Wed . "A gravity model for the Coso geothermal area, California". United States. doi:. https://www.osti.gov/servlets/purl/5684468.
@article{osti_5684468,
title = {A gravity model for the Coso geothermal area, California},
author = {Feighner, M.A. and Goldstein, N.E.},
abstractNote = {Two- and three-dimensional gravity modeling was done using gridded Bouguer gravity data covering a 45 {times} 45 km region over the Coso geothermal area in an effort to identify features related to the heat source and to seek possible evidence for an underlying magma chamber. Isostatic and terrain corrected Bouguer gravity data for about 1300 gravity stations were obtained from the US Geological Survey. After the data were checked, the gravity values were gridded at 1 km centers for the area of interest centered on the Coso volcanic field. Most of the gravity variations can be explained by two lithologic units: (1) low density wedges of Quarternary alluvium with interbedded thin basalts (2.4 g/cm{sup 3}) filling the Rose Valley and Coso Basin/Indian Wells Valley, and (2) low density cover of Tertiary volcanic rocks and intercalated Coso Formation (2.49 g/cm{sup 3}). A 3-D iterative approach was used to find the thicknesses of both units. The gravity anomaly remaining after effects from Units 1 and 2 are removed is a broad north-south-trending low whose major peak lies 5 km north of Sugarloaf Mountain, the largest of the less than 0.3 m.y. old rhyolite domes in the Coso Range. Most of this residual anomaly can be accounted for by a deep, low-density (2.47 g/cm{sup 3}) prismatic body extending from 8 to about 30 km below the surface. While some of this anomaly might be associated with fractured Sierran granitic rocks, its close correlation to a low-velocity zone with comparable geometry suggests that the residual anomaly is probably caused a large zone of partial melt underlying the rhyolite domes of the Coso Range. 12 refs., 9 figs.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Wed Aug 01 00:00:00 EDT 1990},
month = {Wed Aug 01 00:00:00 EDT 1990}
}

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  • Obvious surface manifestations of an anomalous concentration of geothermal energy at the Coso Geothermal Area, California, include fumarolic activity, active hot springs, and associated hydrothermally altered rocks. Abundant Pleistocene volcanic rocks, including a cluster of thirty-seven rhyolite domes, occupy a north-trending structural and topographic ridge near the center of an oval-shaped zone of late Cenozoic ring faulting. In an investigation of the thermal regime of the geothermal area, temperatures have been obtained to depths up to 133 m in 22 boreholes with measurements being made at least four times in each borehole. Geothermal gradients ranged from 24/sup 0/C/km to 450/supmore » 0/C/km. The high gradients arise from convecting hot water and magma which are cooled at shallow depth by conduction of heat to the surface. Thermal conductivity measurements ranging from 3.2 to 10.5 mcal/cm-sec-/sup 0/C were made on both cores and drill cuttings. The resultant heat flow values of 2.5 to 18 HFU are typical of geothermal areas. The actual process by which heat is transferred is rather complex; however, the heat flow determinations can be divided into two groups. The first group, less than 6.5 HFU, are indicative of regions with primarily conductive regimes although deep-seated mass transfer is implied. The second group, greater than 6.5 HFU, are characteristic of regions with considerable convective heat transfer in the shallow subsurface. The high heat flow values are essentially restricted to the central rhyolite dome field and the associated surface thermal manifestations. Heat transferred by convection of water would be rapidly exhausted if it were not intermittently supplied with energy from depth; therefore, the heat flow data substantiate the concept that the ring structure and associated volcanic rocks are products of a large magma chamber that has periodically erupted lava during the past one million years.« less
  • Remote sensing studies have been made in and adjacent to the Coso geothermal field using TM FCC satellite imagery, 1:100,000 scale, US Geological Survey orthophotos, 1:24,OOO scale, and proprietary black-and-white photography by California Energy Company, Inc., at various scales including black-and-white positive film transparencies at a scale of 1:6,000. These studies have been made in an attempt to understand the complex geology seen on the surface and to try to improve the method of locating geothermal wells. The tectonic history indicated by remote sensing, gravity, magnetic, refraction and reflection seismic studies indicates structure caused by a period of thrusting andmore » folding followed by local and regional slumping and collapse, which is continuing today. During Sevier/Laramide orogeny, the Sierra Nevada Mountains were thrust eastward over Rose Valley/lndian Wells Valley. Relatively thin granitic/metamorphic plates were folded to form the Coso Range and thrust eastward over Coso Wash. In turn, the Argus Range to the east was thrust eastward over Panamint Valley. As soon as topographic relief, developed by the thrusting, was high enough, the entire area started to collapse, in some cases using the original thrust planes for slumping. The granitic/metamorphic rocks forming the surface of the Coso Range anticline slumped eastward into Coso Wash and westward toward Rose Valley. The Sierra front slumped eastward into Rose Valley/Indian Wells Valley, and the entire range may have had westward on the original thrust faults. The thin basalts on the east side of Coso Wash slumped westward into Coso Wash. Several basaltic eruptions of 3-4 million years ago used the zone between slump blocks as eruption sites. Regional slumping for the entire area toward the southeast is indicated on satellite imagery.« less
  • In this study, we studied high-resolution relative locations and full moment tensors of microearthquakes (MEQs) occurring before, during and following Enhanced Geothermal Systems (EGS) experiments in two wells at the Coso geothermal area, California. The objective was to map new fractures, determine the mode and sense of failure, and characterize the stress cycle associated with injection. New software developed for this work combines waveform cross-correlation measurement of arrival times with relative relocation methods, and assesses confidence regions for moment tensors derived using linear-programming methods. For moment tensor determination we also developed a convenient Graphical User Interface (GUI), to streamline themore » work. We used data from the U.S. Navy’s permanent network of three-component digital borehole seismometers and from 14 portable three-component digital instruments. The latter supplemented the permanent network during injection experiments in well 34A-9 in 2004 and well 34-9RD2 in 2005. In the experiment in well 34A-9, the co-injection earthquakes were more numerous, smaller, more explosive and had more horizontal motion, compared with the pre-injection earthquakes. In the experiment in well 34-9RD2 the relocated hypocenters reveal a well-defined planar structure, 700 m long and 600 m high in the depth range 0.8 to 1.4 km below sea level, striking N 20° E and dipping at 75° to the WNW. The moment tensors show that it corresponds to a mode I (opening) crack. Finally, for both wells, the perturbed stress state near the bottom of the well persisted for at least two months following the injection.« less
  • The purpose of this study was to test the hypothesis that a steam-filled fracture geothermal reservoir exists at Coso Hot Springs KGRA, as proposed by Combs and Jarzabek (1977). Gravity data collected by the USGS (Isherwood and Plouff, 1978) was plotted and compared with the geology of the area, which is well known. An east-west trending Bouguer gravity profile was constructed through the center of the heat flow anomaly described by Combs (1976). The best fit model for the observed gravity at the anomaly was obtained by combining the gravitational effect of the known geology and that of a hypotheticalmore » geothermal reservoir. The reservoir proposed in this study was modeled at a cylindrical disk of radius 12,500 ft., height 5000 ft., depth 5000 ft., and steam-filled porosity of 5%. Such a reservoir at the temperature suggested by hydrochemical geothermometers would be capable of sustaining a commercial-sized electric generation project.« less
  • Recently completed environmetal studies performed within the region surrounding the Coso KGRA in California included detailed assessment of public attitudes and concerns toward geothermal development. The results of these studies of local issues were incorporated into plans for mitigating public concerns and increasing public acceptance of the proposed leasing and development of 72,640 acres within and adjoining the KGRA. Salient issues, attitudes, and concerns expressed over development at Coso are summarized, and a series of conclusions is offered regarding their general applicability to public acceptance and support of geothermal development elsewhere.