We utilize a magnetotelluric profile in the Travale geothermal field, in Tuscany, 
Italy, to obtain a resistivity model using 2-D inversion of the series and parallel 
(S-P) invariant impedances. The idea is to experiment the performance of the S-
P responses as compared with 2-D inversion of conventional TE and TM 
impedances. Resembling TE and TM responses, S-P are complementary, as 
series impedance is more sensible to galvanic effects while parallel is to 
inductive effects. Moreover, as both responses are rotation invariants, they 
overcome the trouble of selecting a rotation angle or using a tensor 
decomposition technique. For S-P inversion we used a Gauss-Newton algorithm 
designed to minimize the data misfit at the same time as the model is kept as 
smooth as possible. The trade-off between data misfit and model roughness is 
balanced by a regularization factor. We change this factor in a search for the 
model with the best tradeoff between data misfit and model roughness. TE and 
TM impedances were inverted using the algorithm of Rodi and Mackie, after 
rotation of data toward the main strike direction of the area. The resulting TE-
TM and S-P model shows many similarities in defining resistivity anomalies at 
both shallow (500 m) and deep (1-5 km) depth. The 2D models show resistivity 
anomalies that correlate with zones of high permeability and fluid content 
representing exploited geothermal reservoirs. The shallow resistivity anomalies 
show a good correlation with the shallow geothermal reservoir, located in 
carbonate units. The anomalies are particularly visible in correspondence of the 
faults, suggesting an increase of permeability, a pathway for fluids and possible 
related alteration minerals. The most conspicuous feature of the 2D models is 
the presence of low resistivity anomalies inside the resistive basement at a 
depth of 1-4 km b.g.l. This deep resistivity anomaly corresponds to the deep 
fractured and highly productive geothermal reservoir located in the 
metamorphic rocks. This deep reservoir is made of sparse fractures and hosts 
superheated steam.

Campus SIAF, SP del Monte Volterrano
Localita' Il Cipresso
Volterra, Italy