PRELIMINARY-RESULTS OF A MAGNTEOTELLURIC SURVEY OVER A GEOTHERMAL ANOMALY IN PORTUGAL

During summer 1990 a magneto-telluric (MT) survey was conducted over a geothermal anomaly discovered a few years ago in south central Portugal. This anomaly, known as the Alentejo Geothermal Anomaly (AGA), exhibits heat flow density values (greater than 160 mW m2) at its centre. The area covered by the MT survey was about 2500 km2, and 34 MT sites were occupied. The preliminary results indicate that the central area of the AGA coincides with a region of high electrical resistivity which continues to depths greater than 10 000 m. As no intense hydrothermal manifestations are known in the study area, it is suggested that the heat flow density anomaly is the result of high heat production in some of the geological formations that outcrop in the region. Two major faults cross the area, intersecting at approximately a right angle. Low-resistivity zones appear to be associated with the fault zones, and a possible interpretation is that these conductive zones may be related to fluids contained in fractures surrounding the faults.