A Magnetotelluric Survey in a Reported Geothermal Area in Southern Portugal

Abstract

A geothermal anomaly with heat flow density (HFD) values of about 200 near its center has been reported to exist in southern Portugal. The area where the anomaly is claimed to occur is geologically complex and is crossed by two major tectonic features: the Messejana fault and the Ferreira-Ficalho overthrust. A magnetotelluric (MT) survey was carried out in the region of the highest reported HFD values with the intent of delineating the anomaly in electrical terms and understanding its origin. Thirty-four MT sites were occupied over an area of about 2,500 and, after processing the MT data, electrical resistivity maps at several depths were constructed. Interpretation of the processed data indicates that the HFD values reported for the area of the MT survey do not represent the thermal state of the upper crust in the region. However, low electrical resistivity values are found in zones that coincide at the surface with the Messejana fault and the Ferreira- Ficalho overthrust. This may indicate that fluid flow occurs within these two tectonic features and may generate local high HFD values near the surface.