GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 3.1 569 THE DEEP STRUCTURES OF THE LARDERELLO GEOTHERMAL FIELD (ITALY): RESULTS FROM A NEW ELECTROMAGNETIC STUDY A. Santilano 1 , A. Godio 2 , E. Rizzo 3 , A. Manzella 1 1 IGG-CNR, Pisa, Italy 2 Department of Environment, Land and Infrastructure Engineering (DIATI), Politecnico di Torino, Italy 3 IMAA-CNR, Potenza, Italy Introduction. The intrinsic complexity of geothermal systems and the need of an accurate integration of the geophysical parameters with the geological and hydrogeological properties still represent a challenge of the exploration geophysics. It is in such a scenario, we focused our research on the Larderello geothermal field (Tuscany, Italy), in order to relate the results of electromagnetic (EM) surveys with an integrated modelling of the system. Larderello is the oldest field under exploitation in the world. The production of geothermal electricity, in its modern meaning here was born. A century of industrial and scientific researches were not enough to understand all the geological, chemical and physical features of this complex system, and to solve the critical issues that are currently debated in the scientific community. The main target of this research is the improvement of the knowledge on the deep structures of the Larderello field, formulating an accurate conceptual model including its deep roots with a focus on the heat source of the system and deep crustal fluids, the tectonics and its relation with the hydrothermal circulation. We acquired new magnetotelluric (MT) and Time Domain EM (TDEM) data close to Lago Boracifero. The data integrate the MT datasets previously acquired in the frame of exploration and scientific projects. We also propose an integrated approach to improve the reliability of the 2D MT inversion models, by using external information from the geological model as well as an innovative probabilistic analysis of MT data. The Lago Boracifero sector and the scientific challenge. We considered the Larderello system as a “Convective intrusive geothermal play” as proposed by Santilano et al. (2015). The exploited hydrothermal reservoirs are hosted in sedimentary, mostly carbonates, units and underlying crystalline rocks. Superheated steam is present to depth over 3.5 km with temperatures exceeding 350°C. Strong seismic reflectors in the metamorphic complexes, i.e. the H and K horizons, have been explained with rock fracturing and presence of fluids, even if their nature is not known. The heat source of the system is related to shallow igneous intrusions. The Larderello intrusive bodies, cored in several deep wells, can be classified as two-mica granites with ages ranging from 3.8 Ma to 1.3 Ma (Dini et al. , 2005). In the Lago Boracifero sector (Fig. 1), a regional culmination of the seismic K-horizon occurs at depth of about 3 km b.g.l. Very high temperature and high pressure systems (> 400°C and pressure > 290 bar), possibly at supercritical condition are inferred at or close the depth of the K-horizon, and are shallow enough to be explored with current technologies. A pioneering deep drilling project (DESCRAMBLE, EU H2020) at supercritical conditions is now ongoing. Many scientific issues are still debated on the physics of the geothermal processes in this field. We particularly refer: i) to the anomalous low electrical resistivity values that were estimated locally in a vapour dominated crystalline reservoir (theoretically resistive), ii) to the uncertainties on the deep structures of the field, such as the occurrence of a molten magmatic intrusion and its geometry and iii) the role of faults in the hydrothermal circulation. Nowadays, the tectonic fluids pathways are not clearly figured out by the operator. The magnetotelluric and time domain EM dataset. The geophysical study counts the analysis of old and new MT datasets for an amount of 40 soundings in the Lago Boracifero. area. This sector was not investigated by Magnetotellurics, despite its scientific relevance. In 2016, we carried out a new MT survey in the SW sector of the Larderello field near Lago Boracifero. Due to the near-field effect of the electrified railways in southern Tuscany, we installed a permanent remote station in the Capraia Island, located 80 km far from the area of interest. The broad-band time series were acquired with high-resolution, multi-channel 32-bit receivers in the range of 0.0001 Hz to 1 kHz, (Zonge system). The two perpendicular horizontal