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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