Three-Dimensional Magnetotelluric Characterization of the Travale Geothermal Field (Italy)

The geoelectrical features of the Travale geothermal field (Italy), one of the most productive geothermal fields in the world, have been investigated by means of three-dimensional (3D) magnetotelluric (MT) data inversion. This study presents the first resistivity model of the Travale geothermal field derived from derivative-based 3D MT inversion. We analyzed MT data that have been acquired in Travale over the past decades in order to determine its geoelectrical dimensionality, directionality, and phase tensor properties. We selected data from 51 MT sites for 3D inversion. We carried out a number of 3D MT inversion tests by changing the type of data to be inverted, the inclusion of static-shift correction at some sites where new time-domain electromagnetic soundings (TDEM) were acquired, the grid rotation, as well as the starting model in order to assess the connection between the inversion model and the geology. The final 3D model herein presents deep elongated resistive bodies between the depths of 1.5 and 8 km. They are transverse to the Apennine structures and suggest a correlation with the strike-slip tectonics. Comparison with a seismic velocity model and well log data suggests a highly-fractured volume of rocks with vapor-dominated circulation. The outcome of this study provides new insights into the complex geothermal system of Travale

On the optimization of electromagnetic geophysical data: Application of the PSO algorithm

Particle Swarm optimization (PSO) algorithm resolves constrained multi-parameter problems and is suitable for simultaneous optimization of linear and nonlinear problems, with the assumption that forward modeling is based on good understanding of ill-posed problem for geophysical inversion. We apply PSO for solving the geophysical inverse problem to infer an Earth model, i.e. the electrical resistivity at depth, consistent with the observed geophysical data. The method doesn't require an initial model and can be easily constrained, according to external information for each single sounding. The optimization process to estimate the model parameters from the electromagnetic soundings focuses on the discussion of the objective function to be minimized. We discuss the possibility to introduce in the objective function vertical and lateral constraints, with an Occam-like regularization. A sensitivity analysis allowed us to check the performance of the algorithm. The reliability of the approach is tested on synthetic, real Audio-Magnetotelluric (AMT) and Long Period MT data. The method appears able to solve complex problems and allows us to estimate the a posteriori distribution of the model parameters

Particle Swarm Optimisation of Electromagnetic Soundings

We discuss through synthetic and real data some the application of PSO in electromagnetic soundings. The suggested approach can be easily adapted to resistivity soundings (RS), time domain soundings (TDEM) , magneto-telluric (MT) and audio-magneto-telluric survey (AMT). We propose an overview on the PSO for solving 1D problems with a priori information and/or lateral constraints. The application of PSO on AMT data is suggested by the high speed of convergence to a problem's solution respect other evolutionary methods. Application on the synthetic dataset allow us to analyze the relevance of the setting parameters, and to select the optimal solutions when a priori information or additional constraints are introduced. We demonstrate how PSO could be an effective approach in AMT data processing (1D). The results can be selected as starting model for a subsequent gradient-based inversion

STOCHASTIC INVERSE MODELING OF MAGNETOTELLURIC DATA FROM THE LARDERELLO-TRAVALE GEOTHERMAL AREA (ITALY)

This work presents the two-dimensional stochastic inverse modelling of a magnetotelluric profile from the Larderello geothermal area (Italy). For the first time, the algorithm Particle Swarm Optimization was applied to this kind of field data to investigate a complex electrical structure without the initial assumption given by an external starting model driving the inversion. The outcome was in good agreement with results of previous research with the advantage of a lower data misfit as well as the contribution that the modelling was not initially constrained by a priori information (e.g., from well-log or other geophysical methods) that, in geothermal areas, can be unavailable or even misleading

Electromagnetic and DC methods for geothermal exploration in Italy, state-of-the-art, case studies and future developments

eothermal energy is a renewable and eco-compatible resource suitable for base-load power and thermal production, which means a daily continuous energy production. In the past few years this source has been of interest for governments, companies and research institutes worldwide that are working for the increase of geothermal exploitation with the aim of reducing greenhouse gas emissions and fossil fuels consumption. Italy was the first country (in 1913) where geothermal energy was exploited for industrial power production and is now the sixth-largest geothermal electricity producer in the world (Bertani, 2015). The geothermal potential of Italy, both for power production and direct uses, is really huge due to particular geological conditions; elsewhere it is mostly underexploited for non-technical barriers. In Italy, many industrial and scientific exploration projects have been carried out in the last few years for assessing shallow and deep geothermal resources. ElectroMagnetic (EM) methods play a fundamental role in the geothermal exploration due to particular sensitivity of the subsurface electrical resistivity (hereby resistivity) to hydrothermal circulation, thermal regime and rocks alteration. Many papers have been published on the study of geothermal areas by EM methods worldwide (Meju, 2002; Spichak and Manzella, 2009; Muñoz, 2014 and references therein). In this paper, we propose an updated state-of-the-art of the main electromagnetic and direct current methods for geothermal exploration in Italy, describing innovative case studies and including a discussion about the direction of new researches. The Magnetotellurics (MT) represents the most common and effective method for investigating deep geothermal reservoirs. A case study in southern Tuscany is herein described. We will also focus the attention on the resistivity measurements for shallow geothermal exploration by means of Airborne EM (AEM), Transient or Time Domain EM (TEM or TDEM) and Electrical Resistivity Tomography (ERT). Among the various scientific projects for geothermal exploration that the Italian National Research Council (CNR) carried out, the VIGOR project (evaluation of the geothermal potential of Regions of Convergence) for Southern Italy provided the occasion of detailed geoelectromagnetic studies for assessing shallow and deep geothermal resources (Manzella et al. 2013a, VIGOR website). Some cases study of the VIGOR project are briefly described as: i) the innovative application of Airborne EM data acquired over large areas in Sicily and applied to the assessment of shallow geothermal potential and ii) a Deep Electrical Resistivity Tomography (DERT) acquired on a thermal area in Calabria region