GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 3.1 571 Results. The inversion models constrained with the PSO approach provided the necessary resolution for imaging the complex subsurface structure of the Larderello system. The results are consistent in terms of RMS, which are even lower than the corresponding ones obtained by geologically constrained inversions. The inversion models image low resistivity anomalies in the central sector of the profiles, in the proximity of a clear subvertical structure along the Cornia River. The upper units are very conductive (3-30 Ωm) due to lithology. The intermediate structural levels (up to 2500 m b.g.l.) coincident mainly with the Tectonic Wedge Complex and most of the Phyllitic Complex are characterized by resistivity values in the range of 40-200 Ωm (mostly 100 Ωm). These results are in perfect agreement with the deep ERT. On the other hand, in the Lago Boracifero area the Micaschist, Gneiss and Intrusive rocks are characterized by high resistivity values in the range of 2500-5000 Ωm as expected. Large low resistivity sub- vertical anomalies, with values of about 150 Ωm, locally interrupt the resistive metamorphic units at depth where an intense vapour dominated hydrothermal circulation is recognized (and exploited). At mid-crustal level, below 6 km depth, a further reduction of resistivity is recognized in all the MT Profiles in the Lago Boracifero area. These anomalies can be ascribed to the partially melted granitic intrusion that act as heat source of the field. Considering this assumption, the heterogeneous distribution of resistivity at this depth can be in part explained as different percentage of melting in the rock volume. The comparison between the resistivity models and the seismological tomography (Batini et al. , 1995) shows a positive correlation between low resistivity anomalies and the low P-wave velocity zone interpreted as a melted intrusion (Fig. 2). Conclusion. The results of the magnetotelluric study of the Larderello field and their interpretation allowed us to contribute on the improvement of the conceptual model of the geothermal system. With regard to the deep structure, we detected a still partial melted igneous intrusion beneath the Lago Boracifero sector acting as heat source, based on the interpretation of the low resistivity anomalies. We highlighted also the fundamental role of a large tectonic structure, i.e. the Cornia Fault, antithetic respect to the northern Apennine and located along the homonymous river. In our opinion, this fault played an important role in the evolution of the Lago Boracifero sector of the field, favouring both the hydrothermal circulation and the emplacement of magma bodies, being possibly deep-rooted in the crust. Fig. 2 - MT Profile 1 plotted in 3D and compared with the seismic low velocity anomaly (Batini et al. , 1995) here filtered for the values below 5 km/s (modified from Santilano, 2017).