3D resistivity model of the los Humeros geothermal field in Mexico, based on magnetotelluric data

• A magnetotelluric survey explores the 3D electrical resistivity distribution in the Los Humeros super-hot geothermal system (SHGS). • The resistivity model clearly reveals a highly conductive anomaly associated with the shape and depth of the cap-rock above the geothermal reservoir. • The change from high conductivity to more resistive rocks at the caprock’s base corresponds to the top of the andesitic sequence hosting the reservoir. • Our model provides a subsurface picture of the major faults and structures mapped on the surface and cut by the wells. • The resistivity model shows that at the northern end of the Maxtaloya corridor, formed by the Antigua and the Humeros faults, the top of the andesitic rocks is shallow, and houses the hottest wells. A joint geothermal project of a European and Mexican consortium (the GEMex project) was carried out between 2016 and 2020 to develop geothermal energy in the easternmost region of the Trans-Mexican Volcanic Belt. GEMex project was funded by the European Union’s Horizon 2020 and SENER-CONACYT Mexican Energy Sustainability programs. One of the key outcomes is the 3D resistivity model of the Los Humeros geothermal field in Mexico, a potentially superheated geothermal reservoir, where fluid temperature reaches close to 400 °C in some areas, based on magnetotelluric (MT) data. The model reveals a shallow resistive zone linked to post-caldera volcanic rocks; underlying this horizon is an updoming conductive anomaly caused by smectite-rich hydrothermally altered rocks that act as cap-rock. Beneath this seal-cap, a deep dome-shaped resistive anomaly is observed, likely corresponding to andesitic reservoir rocks (50–100 Ohm-m) and deeper basement rocks, possibly limestone and granodiorite. This structure aligns with temperature measurements from boreholes, where the 250–300 °C isotherms follow the shape of the top of the resistive dome. The obtained 3D model successfully identifies deep faults that facilitate hot fluid circulation and define the structural limits of the Los Potreros caldera. Faults within the caldera collapse significantly disrupt the cap-rock and the top of the andesites that host the reservoir. This information, together with results derived from other geophysical, geological, and geochemical methods, will help to infer the location and depth at which high-temperature fluids might be found. Resistivity distribution in the Los Humeros geothermal area. The white circles represent MT soundings. UTM coordinates in meters, and depth in m a.sl.