In volcanic settings, geothermal reservoirs are characterized by high temperature and permeability, usually leading to high resistivities. However, extensive clay alteration of the enclosing rocks results in low resistivities.
The contrast of resistivity provides an excellent tool for identifying geothermal targets.
Magnetotellurics (MT) is the standard method for mapping the alteration cap. We have carried out geophysical surveys and interpreted the data of geothermal prospects in Italy using passive MT, AMT, passive seismic IPSS and tomography PST technologies. Typically, these have been programs which have integrated gravity data with available CO2 geochemical and geological data.
The magnetotellurics surveys reach great depths in confined spaces.
To magnetotellurics MT, the conditions which cause difficult seismic energy propagation are largely irrelevant and so the method readily images the resistivity of the sub-surface.
The data acquisition system has a receiver unit (Unit MT), two electromagnetic sensors, and four electrodes the placement of 4 electrodes arranged in a cross the disposition of the two orthogonal magnetic sensors at a distance of about 25 m from the instrument receiver.