A MT survey in the Amiata volcanic area: A combined methodology for defining shallow and deep structures.

This paper describes a magnetotelluric (MT) survey carried out on the western edge of the Amiata geothermal region (Tuscany, Italy). In order to enhance the resolution of subsurface conductivity along the survey line and to define the deep features related to geothermal system heat recharge a continuous profiling method was used. The standard wide-band MT data were collected at 28 sites, and 130 high frequency telluric-magnetotelluric (TMT) soundings were carried out at sites adjacent to the main wide-band sites. To tackle the problem of high electrical noise in the area the wide-band data were recorded simultaneously at two sites of the profile (local remote) and at a third far remote and electrically quiet site. Two-dimensional (2D) inversion of both TM and TM-TE data was performed, taking into account topographic and coast effects. The inversion results were able to define both shallow and deep resistivity anomalies whose structural significance is in good agreement with information derived from other geophysical data. Of particular geothermal interest is the evidence of a large conductive anomaly at a depth ranging between 2 and 4 km within the deep metamorphic basement. This anomaly could indicate the presence of a hitherto unidentified deep reservoir that is in communication with the reservoir exploited at present in the Amiata area.