Subsurface mass monitoring at Theistareykir geothermal field, Iceland, using hybrid gravimetry

Understanding underground mass redistribution in geothermal fields is fundamental to assess the harnessing in geothermal reservoir. Here we apply the hybrid gravimetry method at Theistareykir geothermal field (Northern Iceland). We collected absolute and discrete microgravity measurements yearly within the geothermal field at fixed locations jointly with continuously recorded gravity time series with two superconducting gravimeters (SGs). Data acquisition started in 2017 at the onset of the anthropogenic perturbation. We present and interpret the discrete and continuous gravity datasets from 2017 until 2024 in an attempt to characterize fluid redistribution within the subsurface. The hybrid gravimetry dataset reveal a gravity decrease nearby the extraction area with rates of about −20 μGal per year for the 2017–2022 period, that reduces to few μGal per year in the last two years (2023–2024). An increase in gravity is observed towards the injection area (around 5 to 10 μGal per year). Time-lapse gravity maps reveal a localized gravity decrease (with a maximum of −60 μGal), not collocated with the zone of largest extraction and furthermore an additional trendline of gravity increase (+15 μGal) is observed towards the North. The first suggests a possible lower permeability subsurface zone within the geothermal field, the latter, is in accordance with the direction of the fault system that crosses Theistareykir field, suggesting potential underground fluid pathways. From 2023, although extraction and injection rates did not change, we evidence a change in the gravity trends outside the western part of the geothermal field possibly associated with the 2023 magmatic intrusion. • Hybrid gravimetry method enables tracking of underground mass movements. • Gravity observations remark differences between extraction and injection areas. • Gravity trends suggests preferred structural pathways for the injected fluid. • Attempted mass balance suggests natural recharge within the system. • Gravity trends variations from 2023, possibly associated to magmatic intrusion.