Shear-wave anisotropy across the geothermal field of Milos, Aegean volcanic arc

Sachpazi, Maria

doi:10.1111/j.1365-246x.1991.tb01426.x

Cited by 9 publications

(4 citation statements)

References 16 publications

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“…From that research we note that the delay times can be highly variable, depending upon the geology conditions and the tectonic features of a given region. For instance, KANESHIMA (1990) reported delay times of up to 0.21 seconds for various regions in Japan; SACHPAZI and HIRN (1991) observed mean delay times of 0.05 seconds for a geothermal field in Greece; BIANCO et al (1996) In this study, it is found that regardless of the hypocenter location of the earthquakes, the observed delay times are between 0.0 and 0.6 seconds. The overall mean value of these measurements was found to be equal to 0.35 seconds.…”

Section: Delay Times Between Fast and Slow S Wavesmentioning

confidence: 62%

Seismic Anisotropy Observations in the Mexicali Valley, Baja California, México

González

Munguía

2003

Pure appl. geophys.

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A study of seismic anisotropy was performed using data from earthquakes of the Mexicali Valley. The investigated region encompasses the Cerro Prieto Geothermal Field (CPGF), one of the most important fields in the world. The results showed that at most of our stations the average polarization directions of the fast S waves range from N14°W to N17°E. A N-S polarization direction was obtained for the whole area by averaging the polarization directions from all stations used. In terms of the EDA hypothesis, this average trend agrees with the postulated state of stress for southern California, and with fault plane solutions for events of the Mexicali Valley. Notorious deviations from the N-S global trend were found southeast of the CPGF, with polarization trends between N25°E and N67°E, and in the geothermal field, with polarization directions between N7°W and N14°W. The polarization results for these zones indicated stress conditions that are different from the more regional stress pattern.The delay times that were measured between the fast and slow shear waves reached values of up to 0.6 sec, with a mean value of 0.35 sec. Consistent with our polarization results, the larger delay times (0.2-0.6 sec) were found in the CPGF. Smaller or null values were observed at the periphery of the study area. No temporal trends in the delay times were apparent, as shown by data from the two stations that recorded the larger number of events.Overall, we conclude that the splitting effects of this study result from a shallow anisotropy volume. The splitting results are thus interpreted as caused by the preferred orientation of vertical fluid-filled microcracks aligned in a direction that is parallel to the regional stress field. The stronger splitting effects that were observed in the area of the CPGF were found consistent with the geothermal reservoir that is embedded in the sedimentary cover of the zone, at depths of 1 to 4-5 km from the surface. We thus believe that such marked splitting effects have a direct relation with the reservoir of the CPGF.

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Section: Delay Times Between Fast and Slow S Wavesmentioning

confidence: 62%

Seismic Anisotropy Observations in the Mexicali Valley, Baja California, México

González

Munguía

2003

Pure appl. geophys.

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“…Monitoring shear-wave splitting is thus an important means to determine the direction and evaluate the bulk density of subsurface fractures in hydrocarbon or geothermal reservoirs (Cliet et al, 1991;Mueller,1991;Sachpazi and Him, 1991;Sato et al, 1991;Crampin, 1993). In this paper, we report on the determination of fracture patterns and crack density in the Coso geothermal field, California, by shear-wave splitting inspection and analyses of a large number of microearthquake seismograms recorded by a downhole, three-component seismic network.…”

Section: Introductionmentioning

confidence: 98%

Characterization of geothermal reservoir crack patterns using shear‐wave splitting

Lou

Rial

1997

GEOPHYSICS

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Microearthquakes recorded by a downhole, threecomponent seismic network deployed around the Coso, California, geothermal reservoir since 1992 display distinctive shear-wave splitting and clear polarization directions. From the polarizations we estimated three predominant subsurface fracture directions, and from the time delays of the split waves we determined tomographically the 3-D fracture density distribution in the reservoir.

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“…Until now VSPs have generally been used for these studies (Cliet et al, 199I;Lefeuvre et al, 1992). Anisotropy has also been observed and related to crack density and local stress field by using shear-wave splitting of shallow earthquake data in geothermal areas (Kaneshima et al, 1988; Sachpazi et al, 1991). In the present study, we propose to use the induced seismicity observed in the hydrocarbon field of Lacq (France) for estimating the mean characteristics of the anisotropy.…”

Section: Introductionmentioning

confidence: 94%

Anisotropy at the Lacq hydrocarbon field (France) from shear‐wave splitting

Herquel

Wittlinger

1994

Geophysical Research Letters

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Three‐component seismograms from local earthquakes located beneath the reservoir of the Lacq hydrocarbon field have been analysed for shear‐wave splitting. Shear‐waves recorded at the station USI located on top of the hydrocarbon reservoir show a clear splitting in an average polarization direction N30°–40°E which is different from the present‐day local and regional stress direction. This direction seems approximately parallel to the Tertiary period stress field in this area which corresponds to the measured main crack directions in the gas reservoir. At station LDB, located outside of the anticlinal structure, no splitting was detected but due to a poor signal‐to‐noise ratio, only nine events were available and anisotropy could be absent or obscured. For a 250 m thick oil or gas field the velocity anisotropy is estimated at about 6–10%, which is in the same range as values obtained from VSPs in other hydrocarbon fields. Monitoring induced‐seismicity by three component stations could be a useful tool to characterize hydrocarbon fields in term of the rock's average properties in a large volume.

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Shear-wave anisotropy across the geothermal field of Milos, Aegean volcanic arc

Cited by 9 publications

References 16 publications

Seismic Anisotropy Observations in the Mexicali Valley, Baja California, México

Seismic Anisotropy Observations in the Mexicali Valley, Baja California, México

Characterization of geothermal reservoir crack patterns using shear‐wave splitting

Anisotropy at the Lacq hydrocarbon field (France) from shear‐wave splitting

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