The magnetotelluric (MT) method is used to estimate the electrical characteristics 
of earth structures using naturally occurring electromagnetic fields. It is 
particularly suited to the task of exploring areas of high heat flow since these 
areas are commonly associated with dynamic activities such as magma emplacement, 
crustal fracturing, and the circulation of hot, electri¬cally conductive fluids. 
Southern Tuscany, which is characterized by a high heat flow and the presence of 
two of the most important geothermal areas of the world, Larderello and Mt. Amiata, 
is an excellent test site for resistivity characterization. The region is 
characterized by sequences of sedimentary, metamorphic and igneous rocks, and the 
main geothermal  reservoirs are located within the metamorphic basement, at depths 
of more than 2 km, at pressures of up to 70 bar and temperatures between 300 and 
350 °C. 
The MT surveys in southern Tuscany were undertaken primarily for geothermal and 
deep crustal exploration. For a long time MT surveys have encountered significant 
difficul¬ties that mainly stem from three factors: 1) the high level of 
electromagnetic signals from industrial and cultural sources, which interfere with 
the natural fields used in the method; 2) the presence of very conductive shallow 
formations, and hence the necessity to acquire data for long periods; and 3) the 
naturally-occurring structural complexity commonly found in these areas, which 
requires two- or three-dimensional MT modelling for interpretation purposes, thus 
significantly increasing the amount of effort required by modelling. 
The picture emerging from these MT surveys is that of a resistivity structure that 
is only partly related to the heat flow regime of the area. A very low resistivity 
has been found below the steam-dominated geothermal system of Larderello and below 
areas that have no clear connection to any geothermal system, whereas this 
reduction of resistivity is less conspicuous below the water-dominated geothermal 
system of Mt. Amiata. 
The reasons of the resistivity variation in this region are an interesting subject: 
resistivity varies with lithology, both as matrix and alteration minerals, and with 
fluid distribution and state. The latter are in turn created and controlled by 
temperature, pressure and tectonic processes. In Tuscany rock matrix should not 
provide strong variation since resistivity changes from metamorphic to granitic 
rocks are little. Moreover, the most anomalous area is Larderello where the 
exploited geothermal fluid is superheated steam, which by itself should not 
contribute to a resistivity reduction. Partial melting reduces resistivity, and 
this effect is very probable in the medium-lower crust where teleseismic tomography 
defined low velocity bodies. However, melts are not present at the depths of 
geothermal reservoir where many resistivity anomalies are located.
What remains are two possible explanations for the resistivity reduction from 103 
to 100 ohm-m observed in Larderello: the effect of alteration minerals and the 
presence of brines at liquid phase whose interconnection is sufficient to produce 
electrolytic conduction. The former is a very known effect in volcanic rocks and 
appears to be by far the main source of resistivity reduction even in water 
dominated systems. These aspects are now under study.

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