Ground deformation due to steam cap processes at Reykjanes, SW-Iceland: Effects of geothermal exploitation inferred from interferometric analysis of Sentinel-1 images 2015-2017

The Reykjanes geothermal system is a high-temperature sea water system situated in SW-Iceland. Interferometric analysis of the Sentinel-1 satellite synthetic aperture radar data has been used to determine a time-series of ground deformation induced by geothermal utilization between April 2015 and October 2017. Surface displacements have been estimated at coherent pixels, indicating a steady and linear subsidence within a subcircular bowl centred on the well field at a maximum near-vertical rate of about 25 mm yr -1, together with horizontal contraction. The average line-of-sight displacement rates from ascending and descending tracks are inverted to determine the characteristics of the deformation source at depth, modelling the geothermal reservoir as a body of simple geometry within an elastic half-space. The results indicate a deformation source at about 1-km depth contracting at a rate of (0.7-0.9) × 10 5 m 3 yr -1 during the 2015-2017 period. Using pressure and temperature monitoring data at 900-m depth as well as an analysis of the reservoir structure and rock properties, we infer that the recent estimated volume change can be attributed to processes in the steam cap situated in the topmost part of the geothermal reservoir, in the 800-1200 m depth range. Processes involve a combination of compaction under pressure decrease and/or thermal contraction due to cooling of the rocks within or near the steam cap. The steam cap expanded in response to a sudden pressure drop resulting from the increase in extraction of geothermal fluids for a new power plant in 2006.