Geysers Geothermal Area Research Articles

A data-driven artificial neural network model for the prediction of ground motion from induced seismicity: The case of The Geysers geothermal field

Ground-motion models have gained foremost attention during recent years for being capable of predicting ground-motion intensity levels for future seismic scenarios. They are a key element for estimating seismic hazard and always demand timely refinement in order to improve the reliability of seismic hazard maps. In the present study, we propose a ground motion prediction model for induced earthquakes recorded in The Geysers geothermal area. We use a fully connected data-driven artificial neural network (ANN) model to fit ground motion parameters. Especially, we used data from 212 earthquakes recorded at 29 stations of the Berkeley–Geysers network between September 2009 and November 2010. The magnitude range is 1.3 and 3.3 moment magnitude (Mw), whereas the hypocentral distance range is between 0.5 and 20 km. The ground motions are predicted in terms of peak ground acceleration (PGA), peak ground velocity (PGV), and 5% damped spectral acceleration (SA) at T=0.2, 0.5, and 1 s. The predicted values from our deep learning model are compared with observed data and the predictions made by empirical ground motion prediction equations developed bySharma et al. (2013)for the same data set by using the nonlinear mixed-effect (NLME) regression technique. For validation of the approach, we compared the models on a separate data made of 25 earthquakes in the same region, with magnitudes ranging between 1.0 and 3.1 and hypocentral distances ranging between 1.2 and 15.5 km, with the ANN model providing a 3% improvement compared to the baseline GMM model. The results obtained in the present study show a moderate improvement in ground motion predictions and unravel modeling features that were not taken into account by the empirical model. The comparison is measured in terms of both theR2statistic and the total standard deviation, together with inter-event and intra-event components.

Integrated tomographic methods for seismic imaging and monitoring of volcanic caldera structures and geothermal areas

In this paper we present innovative methodologies for seismic monitoring of volcanic structures in space and time (4D) which can possibly evolve toward an unrest stage. They are based on repeated phase and amplitude measurements done on active and/or passive seismic data including shots, vibrations, earthquakes and ambient noise in order to characterize the structure of the volcano and track its evolution through time. The characterization of the medium properties is performed through the reconstruction of an image of the elastic and anelastic properties of the propagation medium crossed by seismic waves. This study focuses on the application of specific tomographic inversion methods to obtain high quality tomographic images. The resolution of the tomographic models is influenced by the number and spatial distribution of data. The expected resolution thus guides the setup of, for example, active seismic surveys. To recognize and monitor changes in the properties of the propagation medium without performing an active survey we identify a fast proxy based on the time evolution of the Vp/Vs ratio. The advantages and limitations of the methods are discussed through synthetic tests, resolution analysis and case studies in volcanic areas such as the Campi Flegrei (southern Italy) and The Geysers geothermal area (California).

Accurate estimation of seismic source parameters of induced seismicity by a combined approach of generalized inversion and genetic algorithm: Application to The Geysers geothermal area, California

AbstractThe accurate determination of stress drop, seismic efficiency, and how source parameters scale with earthquake size is an important issue for seismic hazard assessment of induced seismicity. We propose an improved nonparametric, data‐driven strategy suitable for monitoring induced seismicity, which combines the generalized inversion technique together with genetic algorithms. In the first step of the analysis the generalized inversion technique allows for an effective correction of waveforms for attenuation and site contributions. Then, the retrieved source spectra are inverted by a nonlinear sensitivity‐driven inversion scheme that allows accurate estimation of source parameters. We therefore investigate the earthquake source characteristics of 633 induced earthquakes (Mw 2–3.8) recorded at The Geysers geothermal field (California) by a dense seismic network (i.e., 32 stations, more than 17.000 velocity records). We find a nonself‐similar behavior, empirical source spectra that require an ωγ source model with γ > 2 to be well fit and small radiation efficiency ηSW. All these findings suggest different dynamic rupture processes for smaller and larger earthquakes and that the proportion of high‐frequency energy radiation and the amount of energy required to overcome the friction or for the creation of new fractures surface changes with earthquake size. Furthermore, we observe also two distinct families of events with peculiar source parameters that in one case suggests the reactivation of deep structures linked to the regional tectonics, while in the other supports the idea of an important role of steeply dipping faults in the fluid pressure diffusion.

Ground‐Motion Prediction Equations for The Geysers Geothermal Area based on Induced Seismicity Records

Abstract The Geysers geothermal field in Northern California, which has been actively exploited since the 1960s, is the world’s largest geothermal field. The continuous injection of fluids and the consequent stress perturbations induce seismicity that is clearly felt in the surrounding communities. In order to evaluate seismic hazard due to induced seismicity and the effects of seismicity rate level on the population and buildings in the area, reliable ground‐motion prediction equations (GMPEs) must be developed. This paper introduces the first GMPEs specific for The Geysers area in terms of peak ground velocity (PGV), peak ground acceleration (PGA), and 5% damped spectral acceleration SA( T ) at T =0.2 s, 0.5 s, and 1.0 s. The adopted non‐linear mixed‐effect regression technique to derive the GMPE includes both fixed and random effects, and it permits to account for both inter‐event and intra‐event dependencies in the data. Site‐specific effects are also estimated from the data and are corrected in the final ground‐motion model. We used data from earthquakes recorded at 29 stations of the Berkeley‐Geysers network during the period September 2007 through November 2010. The magnitude range is 1.3≤ M w ≤3.3, whereas the hypocentral distances range between 0.5 km and 20 km. The comparison of our new GMPE for The Geysers with a standard model derived in a different tectonic context shows that our model is more robust when predictions have to be made for induced earthquakes in this geothermal area.

Geochemistry of volcanic rocks from the Geysers geothermal area, California Coast Ranges

The Geysers geothermal reservoir in the California Coast Ranges is the world's largest economically used geothermal system. Surface exposure of volcanic rocks and extensive drill penetration of a > 300 km 3 subsurface plutonic body (Geysers Plutonic Complex or GPC) that underlies the reservoir allow unique insights into the evolution of a volcanic–plutonic magma system. We present compositional data for major elements, trace elements, and Nd, Sr and Pb isotopes of late Pliocene to early Pleistocene lavas that are spatially related to the geothermal reservoir and cores from coeval subsurface plutonic rocks. Isotopic ratios of dacitic to rhyolitic lavas from Cobb Mountain, Pine Mountain and Tyler Valley, and core samples from the shallow microgranite phase of the GPC range between 0.70343–0.70596 ( 87Sr / 86Sr), 0.512813–0.512626 ( 143Nd / 144Nd), and 19.084–19.189 ( 206Pb / 204Pb). These compositions fall between values for two major isotopic end-members: (1) coeval basalt of Caldwell Pines that has compositional affinities to magmas extracted from a subcontinental mantle-wedge, and (2) regional crustal rocks equivalent to those of the Franciscan and Great Valley sequence. Model calculations suggest that lower crustal intrusion of basalt triggered crustal assimilation in mass proportions of ∼5 : 1 to produce magmas of intermediate composition. Because rhyolites are isotopically indistinguishable to dacites, we interpret them as partial melts derived from precursor intrusions of granodioritic compositions. Our data suggest that two major mantle and crustal reservoirs continually interacted over the > 1 Ma duration of volcanism and pluton emplacement at the Geysers. Individual magma pulses, however, evolved independently, arguing against the presence of a single long-lived magma body.

Reservoir depletion at The Geysers geothermal area, California, shown by four‐dimensional seismic tomography

Intensive geothermal exploitation at The Geysers geothermal area, California, induces myriads of small‐magnitude earthquakes that are monitored by a dense, permanent, local seismometer network. Using this network, tomographic inversions were performed for the three‐dimensional Vp and Vp/Vs structure of the reservoir for April 1991, February 1993, December 1994, October 1996, and August 1998. The extensive low‐Vp/Vs anomaly that occupies the reservoir grew in strength from a maximum of 9% to a maximum of 13.4% during the 7‐year study period. This is attributed to depletion of pore liquid water in the reservoir and replacement with steam. This decreases Vp by increasing compressibility, and increases Vs because of reduction in pore pressure and the drying of argillaceous minerals, e.g., illite, which increase the shear modulus. These effects serendipitously combine to lower Vp/Vs, resulting in a strong overall effect that provides a convenient tool for monitoring reservoir depletion. Variations in the Vp and Vs fields indicate that water depletion is the dominant process in the central part of the exploited reservoir, and pressure reduction and mineral drying in the northwest and southeast parts of the reservoir. The rate at which the Vp/Vs anomaly grew in strength in the period 1991–1998 suggests most of the original anomaly was caused by exploitation. Continuous monitoring of Vp, Vs, and Vp/Vs is an effective geothermal reservoir depletion monitoring tool and can potentially provide information about depletion in parts of the reservoir that have not been drilled.

Fractal clustering of induced seismicity in The Geysers geothermal area, California

We analyse earthquakes recorded at The Geysers geothermal field in California, an area where industrial activity induces seismicity. The seismicity is characterized by the seismic b-value and D, the fractal dimension of earthquake hypocentres measured from sliding windows containing 200 events. We study a group of events strongly clustered around an injection well. Over most of the time period examined we find a positive correlation between b and D. However, during the initiation of injection into a new well we find instead a negative correlation. The differences in correlation are statistically significant at the 1σ level but only marginally so at the 2σ level. These results provide evidence for a transient change in the seismic mechanisms operating, and may be explained by a change from conditions of slow stress loading to rapid loading as a result of the build-up of the rate of water injection into the reservoir.

Source processes of industrially‐induced earthquakes at The Geysers geothermal area, California

Microearthquake activity at The Geysers geothermal area, California, mirrors the steam production rate, suggesting that the earthquakes are industrially induced. A 15-station network of digital, three‐component seismic stations was operated for one month in 1991, and 3,900 earthquakes were recorded. Highly‐accurate moment tensors were derived for 30 of the best recorded earthquakes by tracing rays through tomographically derived 3-D [Formula: see text] and [Formula: see text] structures, and inverting P- and S-wave polarities and amplitude ratios. The orientations of the P- and T-axes are very scattered, suggesting that there is no strong, systematic deviatoric stress field in the reservoir, which could explain why the earthquakes are not large. Most of the events had significant non‐double‐couple (non-DC) components in their source mechanisms with volumetric components up to ∼30% of the total moment. Explosive and implosive sources were observed in approximately equal numbers, and must be caused by cavity creation (or expansion) and collapse. It is likely that there is a causal relationship between these processes and fluid reinjection and steam withdrawal. Compensated linear vector dipole (CLVD) components were up to 100% of the deviatoric component. Combinations of opening cracks and shear faults cannot explain all the observations, and rapid fluid flow may also be involved. The pattern of non-DC failure at The Geysers contrasts with that of the Hengill‐Grensdalur area in Iceland, a largely unexploited water‐dominated field in an extensional stress regime. These differences are poorly understood but may be linked to the contrasting regional stress regimes and the industrial exploitation at The Geysers.

Industrially induced changes in Earth structure at the Geysers Geothermal Area, California

Industrial exploitation is causing clearly‐measurable changes in Earth structure at The Geysers geothermal area, California. Production at The Geysers peaked in the late 1980s at ∼3.5 × 10³ kg s−1 of steam and 1800 MW of electricity. It subsequently decreased by about 10% per year [Barker et al., 1992] because of declining reservoir pressure. The steam reservoir coincides with a strong negative anomaly (∼0.16, ∼9%) in the compressional‐to‐shear seismic wave speed ratio VP/ VS, consistent with the expected effects of low‐pressure vapor‐phase pore fluid [Julian et al., 1996]. Between 1991 and 1994 this anomaly increased in amplitude by up to about 0.07 (∼4%). This is consistent with the expected effects of continued pressure reduction and conversion of pore water to steam as a result of exploitation. These unique results show that VP/VS tomography can easily detect saturation changes caused by exploitation of reservoirs, and is a potentially valuable technique for monitoring environmental change. They also provide geophysical observational evidence that geothermal energy is not a renewable energy source.

Non‐double‐couple earthquake mechanisms at the Geysers Geothermal Area, California

Inverting P‐ and S‐wave polarities and P:SH amplitude ratios using linear programming methods suggests that about 20% of earthquakes at The Geysers geothermal area have significantly non‐double‐couple focal mechanisms, with explosive volumetric components as large as 33% of the seismic moment. This conclusion contrasts with those of earlier studies, which interpreted data in terms of double couples. The non‐double‐couple mechanisms are consistent with combined shear and tensile faulting, possibly caused by industrial water injection. Implosive mechanisms, which might be expected because of rapid steam withdrawal, have not been found. Significant compensated‐linear‐vector‐dipole (CLVD) components in some mechanisms may indicate rapid fluid flow accompanying crack opening.

Three‐dimensional seismic image of a geothermal reservoir: The Geysers, California

Three‐dimensional seismic travel‐time tomography of The Geysers geothermal area, in the coast ranges of northern California, shows a strong (−9%) anomaly in VP/VS, the ratio of the compressional and shear wave speeds, that is not evident in VP alone and corresponds closely to the most intensively exploited part of the geothermal reservoir. This anomaly probably indicates low pore pressure and relatively dry conditions, caused partly by boiling of pore water as steam is extracted. Steam pressure decreases over the last decade have probably caused seismologically measurable changes in wave speeds. Tomographic measurement of VP/VS is a promising technique both for identifying geothermal resources and for monitoring them during exploitation.

Origin of petroleum and mercury in the Culver-Baer cinnabar deposit, Mayacmas District, California

The Culver-Baer mercury deposit (Mayacmas district) is located within the strike-slip Mercuryville fault zone at the margin of the Geysers geothermal area where serpentine + magnetite protolith is altered to silica-carbonate rock. Silica-carbonate rock is silica rich at the center of the deposit and consists of the assemblage quartz + magnesite + pyrite. This assemblage is cut by numerous quartz-chalcedony veins and breccia veins composed of quartz + chalcedony + or - cinnabar + or - petroleum + or - millerite + or - barite. The cinnabar, which is chemically pure, and the petroleum, which consists predominantly of polycyclic aromatic hydrocarbons, occur preferentially along the hanging walls of the breccia veins. Outward from the deposit center there is a zonation to carbonate-rich silica-carbonate rock which consists of magnesite + or - quartz + or - pyrite. At the deposit margin itself, magnesite + or - magnetite is the dominant stable assemblage. All rock samples reveal a consistent temporal evolution from magnesite veinlets to silica veinlets.Geochemical modeling indicates that the mineralogical zonation from serpentinite to silica-rich silica-carbonate rock in the center of the deposit can arise from the interaction of serpentinite with a CO 2 -rich, H 2 O-CO 2 fluid (PCO 2 > or =30 bars at 200 degrees C). Petrographic and analytical data suggest that other than CO 2 , the only components added to the system during alteration and mineralization were H 2 S, Hg, and petroleum. Isotopic and biological marker data support the hypothesis that all of these components may have been derived from local sedimentary rocks in response to heating and transported to the site of deposition in a gas phase. The association of petroleum and cinnabar, particularly along the hanging walls of breccias, is most likely due to a common source and transport in a relatively buoyant fluid rather than due to a direct chemical link. Because there is no evidence for the introduction of any other components, these data suggest that the Culver-Baer deposit may be a fossil condensate zone akin to the model of White et al. (1971).

Categorization of Nocturnal Drainage Flows within the Brush Creek Valley and the Variability of Sigma Theta in Complex Terrain

Abstract The monthly frequencies of nocturnal drainage flows in the Brush Creek Valley were estimated over the period August 1982–January 1985 for the purpose of evaluating the representativeness of the drainage flows observed during a few intensive study periods. These estimates were made on the basis of data from three short meteorological towers situated in the valley. The highest frequencies were observed during the July to October timeframes: 30%–40% during 1983 and 5%–25% during 1984. Of the ten experimental nights when intensive investigations were conducted within the Brush Creek Valley, seven were during strong drainage flow periods and three were during weaker drainage flow periods. The variability of a σθ in complex terrain areas was investigated since this parameter is often used to estimate diffusion of pollutants. Measurements made during strong drainage flow periods within two valleys in The Geysers geothermal area in northern California and within the Brush Creek Valley yielded median hour...

Hydrogeologic reconnaissance of the beowawe geysers geothermal area, Nevada

The Beowawe Geysers in north-central Nevada are the discharge from a hydrothermal-convection system in a region of high heat flow. The site of thermal-fluid upflow (at about 18 kg/s before drilling and well testing) appears to be related to the intersection at depth of two major fault zones. Assuming steady-state conditions, recharge within the drainage basin could account for both thermal and nonthermal ground-water discharge. Circulation of thermal fluid to depths exceeding 5 km is required to attain estimated temperatures of more than 220°C.

Extensional tectonics at The Geysers Geothermal Area, California

Comparisons between the yearly volume of steam withdrawn from The Geysers, California, geothermal reservoir and the number of earthquakes indicate that most of the seismicity is induced. To establish the inducing mechanism, an estimate of the stress field orientation was calculated from inversion of 210 fault plane solutions. Below 1 km depth The Geysers is undergoing uniaxial extension with the least principal stress σ3 oriented near horizontal at approximately 105° azimuth. The consistency between the local and regional stress fields indicates that the regional tectonic stress is much larger than the stresses induced locally through geothermal activities. The stress data are equivocal as to whether earthquakes are induced due to conversion of aseismic to seismic slip or by increased shear stresses arising from reservoir contraction. The observed tendency for downward growth of seismicity may arise from a preferential fracture orientation that is near vertical in conjunction with a stress state where the gradient of σ1 exceeds that of σ3. Evidence of extension at The Geysers and in the surrounding Clear Lake region indicates a direct relationship to the presence of the Quaternary Clear Lake Volcanics.

Induced seismicity in the Geysers geothermal area, California : Eberhart-Phillips D; Oppenheimer, D H J Geophys ResV89, NB2, 10 Feb 1984, P1191–1207

Induced seismicity in the Geysers geothermal area, California : Eberhart-Phillips D; Oppenheimer, D H J Geophys ResV89, NB2, 10 Feb 1984, P1191–1207

Transport and Diffusion in the Transition Layer and Planetary Boundary Layer for Drainage Flows in Anderson and Putah Creeks, California, during ASCOT 1980

Abstract Approximately 100 constant-volume, superpressured balloons (tetroons) were tracked in the Anderson and Putah Creek drainages of the Geysers geothermal area of northern California as part of the ASCOT (Atmospheric Studies in Complex Terrain) program. These tetroons were tracked by radar. Each tetroon was equipped with a transponder which provided a 100 MHz shift in frequency, providing a means of discriminating the tetroon-transponder broadcast signal from radar signals reflected from the hard terrain. Tetroons were used to provide direct measurements of trajectories of individual air parcels, to provide transport speeds of individual air parcels and to provide direct measurements of the turbulence associated with an individual parcel of air. In addition, clusters of tetroons were released, providing direct measurement of the very low frequency turbulence associated with air flow meander within the two valleys. All tetroon flights were in the transition layer between the katabatic drainage flow an...

Induced seismicity in The Geysers Geothermal Area, California

A simultaneous inversion for hypocenters, velocities, and station delays was performed from the P arrival times of 55 microearthquakes and four explosions distributed throughout a 90‐ by 100‐km region centered at The Geysers geothermal area in northern California. The resulting one‐dimensional layered model shows velocity increasing with depth from 4.4 km/s at the surface to 5.9 km/s at 8 km and deeper. We relocated 7215 earthquakes which occurred between May 1975 and February 1982, using the new velocity model. These relocations show a significant spatial clustering near production wells in the steam field that was not apparent in earlier studies. We infer from the spatial and temporal pattern of seismicity that the seismicity at The Geysers is induced by geothermal production activities. With the expansion of power production by approximately 70% in 1979–1980, seismic activity has developed near the new production areas. Seismicity has also expanded northwestward of the present production area, although there are no active steam wells in this area. Statistical cross correlations between the number of earthquakes per month and both steam withdrawal per month and injection volume per month were calculated. For the wells studied, we find no consistent pattern of correlation between injection and seismicity. Likewise, we find little or no correlation between steam withdrawal and seismicity for steam wells in production longer than 7 years. Two mechanisms remain plausible for inducing seismicity at The Geysers. Volumetric contraction due to mass withdrawal may perturb the stress field and cause faulting in reservoir rock already near failure due to the regional stress field. Alternatively, aseismic deformation due to regional tectonic stress may be converted to stick‐slip(seismic) deformation due to an increase of coefficient of friction along fault surfaces, such as could occur from exsolution of dissolved silica into fractures. Neither mechanism would be likely to show strong correlation between production and seismicity rates. However, we expect seismicity to continue in The Geysers and for seismicity to appear in areas where new production occurs.

Cooling tower visible plume rise analyses by time integrated photographs

Cooling tower plume rise measurements are required for evaluation and validation of cooling tower plume rise models. In August 1981, five cooling tower plume defining experiments were conducted at the Geysers Geothermal Area in California during both valley nocturnal drainage flow conditions, and diurnal limited vertical mixing conditions on a mountain ridge. Sequential 300-s time integrated photographs of the plumes were made for several hours during each experiment. Time lapse movies were also made during the daytime experiments. Meteorological conditions were measured during all experimental periods by tethersonde and rawinsonde. The time integrated photographs have been analyzed to provide values of plume rise heights vs time for each of the experiments. Predictions by one version of the Argonne National Laboratory plume rise model are within a factor of 2 of the plume rise calculated from the photographs.

Cooling tower plume rise analyses by airborne lidar

As part of the Atmospheric Studies in Complex Terrain (ASCOT) program, five cooling-tower plume experiments were conducted at The Geysers geothermal area in California during August 1981. The experiments were designed to investigate plume behavior during conditions of valley nocturnal drainage flow and daytime vertical mixing on a mountain ridge. The Airborne Lidar Plume and Haze Analyzer (ALPHA-1) system was used during the experiments to observe plume geometry and aerosol structure of the boundary layer. Subvisible plume rise derived from the backscatter signatures is related to visible plume-rise results recently published. This analysis indicates that the lidar-observed plumes are about 10 times higher than visually observed plumes. The lidar-observed plume rise seemed to correspond with heights of elevated aerosol layers that typically indicate presence of temperature inversions. Pictorial displays are presented which provide information on atmospheric behavior over mountainous terrain.