Hybrid Microgravity Monitoring of the Theistareykir Geothermal Reservoir (North Iceland)

The Theistareykir geothermal field is located in North Iceland on the Mid-Atlantic ridge. A power plant produces 90 MWe using two 45 MWe turbines in operation since autumn 2017 and spring 2018, respectively. We performed hybrid microgravity measurements from 2017 to 2019 to monitor the short-term mass redistribution induced by geothermal production. Time-lapse microgravity surveys conducted each summer with a Scintrex CG5 gravimeter reveal the spatial gravity variations with respect to a reference, where the temporal gravity changes are monitored by absolute gravity measurements done with FG5#206 from Micro-g Solutions. In parallel, continuous gravity changes are recorded by a network of several GWR Instruments iGrav superconducting gravimeters and spring gravimeter, located in the injection and production areas. A height correction is applied to the gravity data using InSAR and GNSS measurements. We notice a regular residual gravity decrease in the production area versus a stable behaviour in the injection area. Time-lapse gravity measurements reveal a minimum residual decrease of − 38 ± 10 µGal (1 µGal = 10 –8 m s −2 ) in 2019 with respect to 2017. Simplistic forward modelling of the produced geothermal fluid using a multiple Mogi sphere model can partly explain the residual gravity decrease. This suggest that a significant part of the injected geothermal fluid flows away, maybe drained by the Tjarnarás fault to the South where an increase of the water table level is observed. However, further modelling work is needed to confirm this.