Geoelectrical baseline model of the subsurface of the Hontomín site (Spain) for CO2 geological storage in a deep saline aquifer: a 3D magnetotelluric characterisation

The magnetotelluric (MT) method was used to characterise the underground research laboratory (URL) for CO₂ storage in a deep saline aquifer at Hontomín (Spain). A total grid of 109 closely-spaced broadband MT sites was acquired in the study area covering an areal extent of 3×5km². Different three-dimensional (3D) inversion codes were employed to invert the MT data in the period range of 0.001-10s (frequency range 1000-0.1Hz), with all of them giving similar results. The final preferred 3D model validates a previously published two-dimensional (2D) MT study and is supported by a variety of multidisciplinary data (e.g., well log, 3D seismic and hydrogeochemistry data). The 3D model constitutes the baseline electrical resistivity model of the site that will be used for the future time-lapse electromagnetic (EM) monitoring experiments of the URL. The 3D resistivity distribution shows the dome-like structure of the saline aquifer and images fracture regions, thus identifying the most likely leakage pathways and consequently, the monitoring requirements of the Hontomín site.