Application of the magnetotelluric method using a remote-remote reference system for characterizing deep geothermal system.

In 1990, the Larderello geothermal field was tomographically imaged using teleseismic data. This study led to the discovery of a P-wave low velocity anomaly, located at a depth of about 7 km. The hypothesized presence of a deep magmatic body is corroborated in this paper by a magnetotelluric (MT) investigation to determine the electrical resistivity of the subsurface. This conclusion is based upon the expectation that a magmatic body will give rise to an increase in electrical conductivity. An MT survey was carried out and simultaneous recordings of the electromagnetic fields were made at sites along two E-W profiles and also at the distant site of Capraia Island. The use of a remote island as a reference location allowed the removal of interference caused by a nearby electric train railway. 4 technique was developed that simultaneously solved for the frequency domain train MT impedance and naturally occurring impedance. This effectively separated the train effect from the natural UT signals. All the data were analyzed in this fashion and were used as input to two-dimensional inversion modelling. The results of inversions were consistent with the results of seismic travel time tomography and other geophysical data.