Electrical Resistivity Structures of Southern Tuscany Geothermal Areas, Italy

Abstract

The magnetotelluric data acquired over the last ten years in southern Tuscany indicate that the metamorphic basement hosting the main geothermal reservoirs is characterised by a regional resistivity of a few thousand ohm-m, showing a sharp transition to lower values in the geothermal areas. This is evident in the Mt. Amiata geothermal area, a water-dominated system, and in Larderello, which is a vapour-dominated system. Since the alteration caused by thermal circulation is not extensive, a decrease in resistivity of one order of magnitude would be due almost entirely to fluid circulation, in both the vapour and liquid phase. The heat source of the geothermal systems, created by some shallow and still molten granite intrusions, has been identified beneath the Larderello area, but is much less clear below Mt. Amiata. A very narrow area with no surface manifestations, which does not belong to the known geothermal areas, shows deep features that are very similar to those of Larderello.