Experimental evidence of resistivity frequency-dispersion in magnetotellurics in the Newberry (Oregon), Snake River Plain (Idaho) and Campi Flegrei (Italy) volcano-geothermal areas

We show that magnetotelluric measurements carried out in volcano-geothermal areas can be significantly affected by the resistivity frequency-dispersion phenomenon. The polarization effects are recognized by comparing magnetotelluric and DC geoelectrical deep soundings performed in the same station. Indeed, the conventional separate analysis of the two data sets sometimes gives unconformable results, especially as it regards the resistivity and thickness of the conductive layers, ascribable to the geothermal fluid-filled rocks. These apparent discrepancies can be overcome by admitting the occurrence of intense induced polarization effects inside the rocks having extensive alterations due to hydrothermal paragenesis. The basic theory of the induced polarization effects over magnetotelluric measurements is at first briefly recalled. Then, three impressive field examples taken from previous experiments performed in the Newberry Volcano, Snake River Plain and Campi Flegrei geothermal areas are presented and discussed.