Subsurface mass modelling at Theistareykir geothermal field, Iceland, using hybrid gravimetry.

Time lapse gravity measurements can give information on underground mass redistribution. Observations are especially valuable over the course of subsurface use, for instance during geothermal exploration of an area. To monitor the mass transfer of underground geothermal fluids associated with the harnessing of a hydrothermal system and to assess its long-term sustainability, we have performed long-term observations at Theistareykir (Icelandic North volcanic zone).   In this study, we model the mass and fluid displacement through the use of the hybrid gravimetry technique. Hybrid gravimetry is a method which consists of the combination of several complementary gravity observations. At Theistareykir, the following experiments are collecting data since 2017: micro-gravity time lapse relative measurements are repeated yearly on a pre-designed network of points; relative gravity measurements are recorded continuously at several multi-parameter stations deployed within and outside the geothermal area. Each station is equipped with a superconducting or a spring gravimeter as well as a GNSS receiver, a broadband seismometer and hydrological and weather sensors. absolute gravity measurements are collected yearly, to constrain the instrumental drift of the relative gravimeters.   Here, we present the complete time series recorded by two superconducting gravity meters at Theistareykir since 2017. Gravity changes associated with potential vertical displacements of the continuous gravity meters are obtained from GNSS data, and removed from the raw data. Similar reductions are performed for other contributions from the meteorological data (pressure, snow height). The reduced time series have been used to obtain an accurate local Earth tide model. Such model is subtracted from the continuous gravity records in order to obtain the gravity residual, sensitive to the geothermal activities (injection, extraction). From the analysis of the gravity time series we notice gravity decrease at the production site. This trend is also visible from the time lapse gravity changes maps, obtained by the integration of micro-gravity data with ground displacement data. Patterns from the spatial maps of gravity changes show gravity increase southwards of the injection area, suggesting drainage of the injected water along the Tjamaras fault. The modelling results are compared with mass changes estimated from the injection and production rates, provided by Landsvirkjun, the operating energy company, thereby constraining the interpretation.   Ongoing work encompasses forward modelling approaches to quantify mass transfers (extraction, injection, recharge, atmospheric losses) within the geothermal system.