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GNGTS 2016 Sessione 3.1 Deep electrical resistivity model of the Larderello geothermal field (Italy): preliminary results of the FP7 IMAGE experiment L. Capozzoli1, G. De Martino1, V. Giampaolo1, A. Godio2,3, A. Manzella3, F. Perciante1, E. Rizzo1, A. Santilano2,3 CNR-IMAA, Potenza, Italy Politecnico di Torino, Italy 3 CNR-IGG, Pisa, Italy 1 2 The paper describes the exploration of a deep geothermal systems based on integration of different electrical resistivity surveys (DC and MT). The integrated approach applied in an area of the Larderello site, where there is the oldest field in the world under exploitation for power production. It is located in southern Tuscany (Italy) and the actual installed capacity is about 795 MWe (Bertani, 2015). A vapour-dominated system is exploited to depth over 3500 m, with temperatures exceeding 350°C, from two different reservoirs. The Larderello test site has been investigated by many geological and geophysical data of previous exploration projects but nowadays several critical issues on deep features of the field are still matter of debate, e.g., permeability distribution in the hydrothermal reservoir and the presence of fluids at supercritical condition at depth. Previous MT studies were carried out in southern Tuscany in the frame of exploration and research projects (Fiordelisi et al., 1998; Manzella, 2004; Manzella et al., 2006). Recently, Santilano et al. (2015) proposed a re-analysis of old MT data acquired by Enel in the ’90 by integrating geological and geophysical data in order to set a well-constrained a-priori model and to check inversion results supporting the interpretation. The analysis of the data integration confirmed the effects of strong resistivity inhomogeneity at the depth of the crystalline basement, where a resistive behaviour is expected. Despite of the lithological features of the reservoirs and the vapour state of the geothermal fluids, low resistivity anomalies recognized. This study regards new electromagnetic acquisition in the area. First, a new broadband magnetotelluric (MT) survey was carried out in the SW sector of the Larderello field near Monterotondo Marittimo town (Fig. 1). In order to improve the quality of the MT data potentially affected by near-field noise effect of the electrified railways, we installed a permanent remote station in the Capraia Island. We carried out 22 MT station, using high-resolution, multichannel 32-bit receivers able to record broadband time-series from 0.0001 to 1 kHz, using a Zonge International Inc equipment. The two perpendicular horizontal components of the electric and magnetic fields on surface were measured with a L-shaped configuration of 100 meters electrical dipoles and two Zonge Ant/4 magnetometers. For each site, we recorded at least 17 hours using the sampling rate of 256 Hz. In addition, we acquired over one hour with a sampling rate of 4096 and 1024 Hz. The preliminary results of MT data processing shows that, although some of the soundings resulted too affected by noise, most of them are suitable for imaging the resistivity distribution at depth. Furthermore, we designed a new experimental high resolution Surface-Hole Deep Electrical Resistivity Tomography (SHDERT) in the Venelle 2 well (kindly provided by Enel GP), that is located in the central sector of the MT survey area. The aim of the SHDERT acquisition was to obtain a detailed deep electrical resistivity distribution (until 2 km), in order to constrain and improve the MT inversion. The Venelle 2 well was accessible down to 1.6 km with a temperature up to 250°C and a metallic casing down to 1 km. We designed an in-hole electrical cable both to stand high temperature and to have flexible metallic electrodes to be located along the open-hole portion (1050 m-1600 m). In details, we carried out the SHDERT by using a dipole transmitting station that injected a direct current (5-10 A) into the ground and a multichannel receiver to record the drop of potential. The injected dipoles were located only on the surface and the receiver ones were distributed both in the Venelle 2 well and on the surface to cover an area around 16 km2. Therefore, the described system permitted to record several drops of potential 499 GNGTS 2016 Sessione 3.1 Fig. 1 – Schematic geological map of the Larderello geothermal area, simplified from the geological maps at scale 1:10000 of Tuscany Region. 1) Quaternary deposits; 2) Neoautochthonous terrigenous deposits (Miocene-Pliocene); 3) Ligurian and sub-Ligurian Flysch complex (Jurassic-Eocene); 4) Tuscan Nappe formations (Upper Trias-Miocene); 5) Metamorphic Units (Paleozoic); 6) Remote site on Capraia Island; 7) Study area; 8) MT site acquired in IMAGE; 9) MT site previously acquired; 10) Position of surface current electrodes for the acquisition of the DERT-SHDERT; 11) Venelle 2 area. simultaneously for each current injection. The crucial task was the data processing, considering the large distance between the Tx and Rx systems that strongly reduces the signal to-noise ratio. To overcome this drawback, for each quadripole position the corresponding voltage signal was filtered, stored and processed with advanced statistical packages (Colella et al., 2004; Rizzo et al., 2004, Tamburiello et al., 2008). The integration of MT model and experimental DC resistivity measurements improved the knowledge on the deep structures of the Larderello field. Acknowledgements. This study is part of the EU FP7-funded Integrated Methods for Advanced Geothermal Exploration (IMAGE) Project under grant agreement n° 608553. We thank the colleagues that supported the fieldwork during the MT and DC surveys. We thank Enel Green Power for the precious technical and logistical support on carrying out the borehole experiment. References Bertani, R.: Geothermal Power Generation in the World 2010-2014, Update Report. Proceedings of World Geothermal Congress 2015, Australia. Colella A., Lapenna V., Rizzo E. (2004). High-resolution imaging of the High Agri Valley basin (Southern Italy) with Electrical Resistivity Tomography. Tectonophysics, 386, 29-40. Fiordelisi, A., Mackie, R., Manzella, A. and Zaja, A.: Electrical features of deep structures in Southern Tuscany (Italy), Annals of Geophysics (1998), 41(3), 333-341. Manzella, A.: Resistivity and heterogeneity of Earth crust in an active tectonic region, Southern Tuscany (Italy). Annals of Geophysics (2004), 47(1), 107-118. Manzella, A., Spichak, V., Pushkarev, P., Sileva, D., Oskooi, B., Ruggieri, G. and Sizov, Y.: Deep fluid circulation in the Travale geothermal area and its relation with tectonic structure investigated by a magnetotelluric survey. Proceedings, Thirty-First Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California (2006). Rizzo E., Colella, A., Lapenna, V. and Piscitelli, S. (2004). “High-resolution images of the fault controlled High Agri Valley basin (Southern Italy) with deep and shallow Electrical Resistivity Tomographies”. Physics and Chemistry of the Earth, 29, 321-327. 500 GNGTS 2016 Sessione 3.1 Tamburriello, G., Balasco, M., Rizzo, E., Harabaglia, P., Lapenna, V. and Siniscalchi, A. Deep electrical resistivity tomography and geothermal analysis of Bradano foredeep deposits in Venosa area (Southern Italy): first results. Annals of Geophysics (2008), 51 (1), 203-212 Santilano, A., Godio, A, Manzella, A., and Dini, I.: Electrical Resistivity Structures and their Relation to Geological Features at the Larderello Geothermal Field (Italy). Proceeding of Near Surface Geoscience (2015a), Turin (Italy). 501