Gas emissions and crustal deformation from the Krýsuvík high temperature geothermal system, Iceland

The Krýsuvík volcanic system is located on the oblique spreading Reykjanes Peninsula, SW Iceland. Since early 2009 the region has been undergoing episodes of localized ground uplift and subsidence. From April–November 2013, we operated near-real time monitoring of gas emissions in Krýsuvík, using a Multi-component Gas Analyzer System (Multi-GAS), collecting data on gas composition from a fumarole (H₂O, CO₂, SO₂, H₂S). The dataset in this study, comprises a near-continuous gas composition time series, the quantification of diffuse CO₂ gas flux, analytical results for direct samples of dry gas, seismic records, and GPS data. Gas emissions from the Krýsuvík geothermal system were examined and compared with crustal deformation and seismicity. The gas emissions from the Krýsuvík system are H₂O-dominated, with CO₂ as the most abundant dry gas species, followed by smaller concentrations of H₂S. The average subsurface equilibrium temperature was calculated as 278 °C. This is consistent with previous observations made through sporadic spot sampling campaigns. In addition, the semi-continuous Multi-GAS dataset reveals higher variations in gas composition than previously reported by spot sampling. The diffuse soil CO₂ flux is found to be variable between the three studied degassing areas in Krýsuvík, ranging from 10.9–70.9 T/day, with the highest flux in Hveradalir where the Multi-GAS station is located. The total flux is estimated as 101 T/day. Comparison between Multi-GAS and geophysical data shows that peaks of H₂O-rich emissions appears to follow crustal movements. Coinciding with the H₂O-rich peaks, SO₂ is detected in minor amounts (~0.6 ppmv), allowing for calculations of H₂O/SO₂, CO₂/SO₂ and H₂S/SO₂ ratios. This is the first time SO₂ has been detected in the Krýsuvík area. The large variations in H₂O/CO₂ and H₂O/H₂S ratios are considered to reflect variable degassing activity in the fumarole. The activity of the fumarole appears less intense during intervals of low or no recorded seismic events. The H2₂O/CO₂ and H₂O/H₂S ratios are lower, presumably due to H₂O condensation affecting the steam jet before reaching the Multi-GAS inlet tube

Injection-induced surface deformation and seismicity at the Hellisheidi geothermal field, Iceland

Induced seismicity is often associated with fluid injection but only rarely linked to surface deformation. At the Hellisheidi geothermal power plant in south-west Iceland we observe up to 2 cm of surface displacements during 2011–2012, indicating expansion of the crust. The displacements occurred at the same time as a strong increase in seismicity was detected and coincide with the initial phase of geothermal wastewater reinjection at Hellisheidi. Reinjection started on September 1, 2011 with a flow rate of around 500 kg/s. Micro-seismicity increased immediately in the area north of the injection sites, with the largest seismic events in the sequence being two M4 earthquakes on October 15, 2011. Semi-continuous GPS sites installed on October 15 and 17, and on November 2, 2011 reveal a transient signal which indicates that most of the deformation occurred in the first months after the start of the injection. The surface deformation is evident in ascending TerraSAR-X data covering June 2011 to May 2012 as well. We use an inverse modeling approach and simulate both the InSAR and GPS data to find the most plausible cause of the deformation signal, investigating how surface deformation, seismicity and fluid injection may be connected to each other. We argue that fluid injection caused an increase in pore pressure which resulted in increased seismicity and fault slip. Both pore pressure increase and fault slip contribute to the surface deformation

Deformation due to geothermal exploitation at Reykjanes, Iceland

Ground deformation in utilized geothermal areas is often attributed either to pressure decrease or temperature decrease in the geothermal reservoir. A new geothermal power plant at Reykjanes began operation in May 2006 and local deformation caused by geothermal utilization was observed shortly thereafter. We use images acquired by the Envisat and TerraSAR-X (TSX) satellites, between 2003 and 2016, as well as available GNSS data, to derive constraints on the cumulative ground displacement at the Reykjanes geothermal area, Iceland, and compare these results to production data acquired from observation wells in this region. We employ interferometric analysis of synthetic aperture satellite radar images (InSAR), using a combined persistent scatterer and small baseline approach, on both ascending and descending Envisat and TSX satellite tracks covering the 2003–2016 period. Time series of range change along line-of-sight (LOS) from the ground to the satellite show the characteristics of on-going ground deflation in the vicinity of the Reykjanes power plant. In the 2005–2008 period, the main area of deformation was 4 km long by 2.5 km wide, aligned along the Reykjanes fissure swarm, but in the period 2009–2016 it is more circular in shape and ~2 km wide. LOS displacement rates have remained relatively steady in time, although slightly faster in the 2005–2008 period than the 2009–2016 period. The average LOS velocities from ascending and descending tracks are decomposed into estimates of near-vertical and near-east displacements. The inferred maximum subsidence since the start of production is ~260 mm. Horizontal displacements show contraction towards the center of deflation of up to ~140 mm. Geodetic modeling is undertaken using sources of simple geometry within an elastic halfspace to determine the optimal sources for the observed contraction throughout 2005–2016. For the earlier period modeled utilizing ENVISAT interferograms (16 June 2005–16 August 2008) the optimal source is a Yang model with a strike of 58° and a source depth of 2.2 km. The calculated volume change associated with the observed contraction is −2.3 × 106m3. For the latter period, utilizing TSX interferograms (24 September 2009–17 August 2016), the preferred source is a Mogi-type model at a depth of 1.2 km and the modeled volume change is −1 × 106m3