3D and 4D seismic imaging of the Nesjavellir geothermal field, Iceland
Authors: Amoroso Ortensia , Napolitano Ferdinando , Convertito Vincenzo , De Matteis Raffaella , Ágústsdóttir Thorbjörg , Hersir Gylfi Páll , Hjörleifsdóttir Vala , Capuano Paolo 
Published: March 2021

&lt;p&gt;The Nesjavellir geothermal field in the Northeastern part of the Hengill central volcano, South West Iceland, has been exploited since 1990. Geothermal energy is currently produced by Reykjav&amp;#237;k Energy (OR) at two power plants around Hengill, at Nesjavellir to the northeast and at Hellishei&amp;#240;i to the southwest. Part of the surplus geothermal water from both plants goes into injection wells, and in analogy with the nearby Hellishei&amp;#240;i power plant the re-injection of geothermal gases into basaltic formations is planned in Nesjavellir. Currently, a test of deep fluid injection is conducted in preparation of the experimental re-injection of carbon dioxide and hydrogen sulphide. The seismicity recorded in the study area is due to volcano-tectonic processes, natural geothermal activity as well as induced seismicity due to production and injection.&lt;/p&gt;&lt;p&gt;The aim of this work is to seismically image the production area of the Nesjavellir geothermal plant. Where the elastic properties of the propagation medium are investigated through the 3D and 4D seismic tomography and the b-value.&lt;/p&gt;&lt;p&gt;The available dataset in Nesjavellir consists of 6906 seismic events extracted from &amp;#205;SOR&amp;#8217;s catalogue, with local magnitude -0.8&amp;#8804;M&lt;sub&gt;L&lt;/sub&gt;&amp;#8804;3.8 recorded between October 2016 and June 2020. The earthquakes were relocated in a 1D velocity model optimized for the area. We used tomographic method in which the P- and S-arrival times are simultaneously inverted for earthquakes location and velocity parameters estimation. Re-located earthquakes are further analysed to image the b-value in the investigated volume. Time variations of the seismic properties of the medium are observed in terms of V&lt;sub&gt;P&lt;/sub&gt;, Vs and V&lt;sub&gt;P&lt;/sub&gt;/Vs ratio obtained from the 4D tomography.&lt;/p&gt;&lt;p&gt;The results indicate that seismicity in Nesjavellir is mainly concentrated in three different clusters: two are located at shallow depths (1-2 km) while the third reaches down to 6 km depth. The three clusters of earthquakes are striking SW-NE and are all dipping to the west. Both the P- and S-velocity obtained models show lateral variation in E-W direction. A high V&lt;sub&gt;P&lt;/sub&gt;/Vs ratio value is observed at shallow depths (due to low Vs values) and high V&lt;sub&gt;P&lt;/sub&gt;/V&lt;sub&gt;S&lt;/sub&gt; ratio is observed between 3.5 and 6 km depth (due to high V&lt;sub&gt;P&lt;/sub&gt; and low Vs values). From the b-value mapping we observe low values (less than 1) at shallow depths and high values where the rate of small magnitude events is considerably higher. For each timestep we observe variations in V&lt;sub&gt;P&lt;/sub&gt; and V&lt;sub&gt;S&lt;/sub&gt; velocities that seem to be correlated with the fluids involved in field operation.&lt;/p&gt;&lt;p&gt;This work has been supported by the S4CE (&quot;Science for Clean Energy&quot;) project, funded by the European Union&amp;#8217;s Horizon 2020 - R&amp;I Framework Programme, under grant agreement No 764810 and by PRIN-2017 MATISSE project, No 20177EPPN2, funded by the Italian Ministry of Education and Research.&lt;/p&gt;