Imaging high-temperature geothermal reservoirs with ambient seismic noise tomography, a case study of the Hengill geothermal field, SW Iceland

Sánchez‐Pastor, Pilar; Obermann, Anne; Reinsch, Thomas; Ágústsdóttir, Þorbjörg; Gunnarsson, Gunnar; Tómasdóttir, Sigrún; Hjörleifsdóttir, Vala; Hersir, Gylfi Páll; Ágústsson, Kristján; Wiemer, Stefan

doi:10.1016/j.geothermics.2021.102207

Cited by 18 publications

(14 citation statements)

References 91 publications

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“…Heath et al, 2022). These observations are in good agreement with previous tomography studies (Wagner, 2019;Sánchez-Pastor et al, 2020). Also, laboratory measurements show that P-wave velocities decrease with increasing temperature in a systematic way that generally fits the predictions of the Gassmann equation, implying that fluid characteristics, with modifications that allow for the presence of bubbles and microfracturing, account for much of the seismic velocity reduction (Jaya et al, 2010).…”

Section: -D Seismic Velocity Models In the Hengill Region In The Cont...supporting

confidence: 90%

“…To make the comparison easier, we scale the size of the S symbols by 1/(Vp/Vs) in Figure 4, with Vp/Vs=1.75 being the average ratio of the 1-D inversion. The southwestern region shows positive delay times, which agrees with results of an ambient noise Rayleigh-wave tomography of the area that indicates reduced shear-wave velocity anomalies South of Central Hengill (Sánchez-Pastor et al, 2020).…”

Section: Station-delay Timessupporting

confidence: 86%

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Seismicity and 3-D body-wave velocity models across the Hengill geothermal area, SW Iceland

Obermann

Ágústsdóttir

et al. 2022

Front. Earth Sci.

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We image shallow crustal structures and analyze seismicity patterns in the Hengill high-enthalpy geothermal area in SW Iceland, exploiting a temporary densification of the seismic network 2018 to 2020. Using a subset of 6,300 high-quality manually picked P- and S-phases, we compute a minimum 1-D model for the region. Our results suggest that the most consistent and accurate hypocenter locations are derived from a joint inversion of P and S arrival times for the Hengill area. We demonstrate that this minimum 1-D model in combination with SeisComP detection and location algorithms can be used to produce fully-automated yet high-quality earthquake catalogs. Our analysis established that both the induced and natural seismicity in the Hengill area occurs in several distinct, spatially constrained clusters. In production and injection areas, the depth of the clusters is at about 2 km, near the bottom of the production and injection wells. These are most likely triggered by the injection and induced by the production, respectively. Outside of these clusters, the seismicity is generally deeper, with the depth of the deepest seismicity indicating the brittle-ductile transition zone. This zone is encountered at about 4 km near the center of the Hengill volcanic area and deepens with increasing distance from its volcanic center, to about 7 km in the southernmost region. A spatial analysis of b-values shows slightly increased values in areas with numerous injection wells and slightly decreased values in production areas. Three-dimensional crustal imaging of VP, VS, VP/VS shows a SE-NW trending fast velocity that extends, at 1–3 km depth between the extinct Grensdalur volcanic center and the presently active Hengill volcanic center. The fastest velocities are found in the NW corner of the Grensdalur volcanic center coinciding with a gravity high and probably reflecting dense solidified magmatic intrusion(s). This trend coincides with traces of geothermal surface manifestations, a shallow lying low resistivity anomaly and an aero-magnetic low. All these anomalies are caused by high temperature at some point in the geological history of the area and are most likely due to migration of the crustal accretion and volcanic activity between the two volcanic centers. Below-average VP/VS ratios at similar depth, coincide with the main production field. We suggest that this anomaly is caused by the extensive fluid extraction, which lowers the pore-pressure in the field and consequently increases the steam dominated zone, leading to lower Vp/Vs ratios. Most of the earthquakes are within the Vp/Vs low and at the boundary of the high and low Vp/Vs anomalies, which might indicate a region of good permeability.

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Section: -D Seismic Velocity Models In the Hengill Region In The Cont...supporting

confidence: 90%

Section: Station-delay Timessupporting

confidence: 86%

Seismicity and 3-D body-wave velocity models across the Hengill geothermal area, SW Iceland

Obermann

Ágústsdóttir

et al. 2022

Front. Earth Sci.

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“…The field is nested between three volcanoes: Hengill to the north, Hrómundartindur to the north-east, and Graensdalur to the east. Mount Hengill was last active around 2000 years ago, while Hrómundartindur was last active circa 10,000 years ago, and Graensdalur has been extinct for 300,000 years (Foulger & Toomey, 1989;Jousset et al, 2011;Sánchez-Pastor et al, 2021). The area forms the junction of three tectonic systems: The Reykjanes Peninsula oblique rifting system (RP), the South Iceland Seismic Zone (SISZ) and the West Volcanic Zone (WVZ) (Tomasdóttir, 2018; Figure 1 lower left panel).…”

Section: Geological Contextmentioning

confidence: 99%

“…Hverahlíð: Hverahlíð is located in the south of the Hengill region and is considered a subsystem of Hellisheiði. It hosts the most powerful wells of the Hengill field and is associated with a shallow (2-3 km depth) low velocity anomaly (Sánchez-Pastor et al, 2021). The area is also characterized by a relative abundance of strike-slip faults linked to the SISZ, making the area likely even more permeable than the rest of the field (Franzson et al, 2010).…”

Section: Hellisheiðimentioning

confidence: 99%

Pseudo-prospective forecasting of induced and natural seismicity in the Hengill geothermal field

Ritz,

Mizrahi,

Clasen Repollés

et al. 2024

Preprint

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The Hengill geothermal field, located in southwest Iceland, is host to the Hellisheiði power plant, with its 40+ production wells and 17 reinjection wells. Located on a tectonically active area, the field experiences both natural and induced seismicity associated to the power plant operations. To better manage the risk posed by this seismicity, the development of robust and informative forecasting models is paramount. In this study, we compare the forecasting performance of a model developed for fluid-induced seismicity (the Seismogenic Index model) and a class of well-established statistical models (Epidemic-Type Aftershock Sequence). The pseudo-prospective experiment is set up with 14 months of initial calibration and daily forecasts for a year. In the timeframe of this experiment, a dense broadband network was in place in Hengill, allowing us to rely on a high quality relocated seismic catalogue. The seismicity in the geothermal field is characterised by four main clusters, associated with the two reinjection areas, one production area an area with surface geothermal manifestations but where no operations are taking place. We show that the models are generally well suited to forecast induced seismicity, despite some limitations, and that a hybrid ETAS  model accounting for fluid forcing has some potential in complex regions with natural and fluid-induced seismicity.

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“…Ambient noise tomography (ANT) is an alternative method that is receiving increasing attention in geothermal exploration due to its increased resolution at shallow depth (e.g. Lehujeur et al, 2017;Martins et al, 2020bMartins et al, ,2020aGranados et al, 2020;Planès et al, 2020;Sánchez-Pastor et al, 2021). ANT is based on the reconstruction of Green's functions between different receiver pairs retrieved from the crosscorrelation of long duration ambient noise records (Wapenaar, 2004;Shapiro et al, 2005;Wapenaar and Fokkema, 2006).…”

Section: Introductionmentioning

confidence: 99%

Ambient seismic noise monitoring and imaging at the Theistareykir geothermal field (Iceland)

Toledo

Obermann

Verdel

et al. 2022

Journal of Volcanology and Geothermal Research

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Imaging high-temperature geothermal reservoirs with ambient seismic noise tomography, a case study of the Hengill geothermal field, SW Iceland

Cited by 18 publications

References 91 publications

Seismicity and 3-D body-wave velocity models across the Hengill geothermal area, SW Iceland

Seismicity and 3-D body-wave velocity models across the Hengill geothermal area, SW Iceland

Pseudo-prospective forecasting of induced and natural seismicity in the Hengill geothermal field

Ambient seismic noise monitoring and imaging at the Theistareykir geothermal field (Iceland)

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