GNGTS 2016 - Atti del 35° Convegno Nazionale

GNGTS 2016 S essione 3.1 499 Deep electrical resistivity model of the Larderello geothermal field (Italy): preliminary results of the FP7 IMAGE experiment L. Capozzoli 1 , G. De Martino 1 , V. Giampaolo 1 , A. Godio 2,3 , A. Manzella 3 , F. Perciante 1 , E. Rizzo 1 , A. Santilano 2,3 1 CNR-IMAA, Potenza, Italy 2 Politecnico di Torino, Italy 3 CNR-IGG, Pisa, Italy 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 MonterotondoMarittimo town (Fig. 1). In order to improve the quality of theMT 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, multi- channel 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 kmwith 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 km 2 . Therefore, the described system permitted to record several drops of potential