MAGNETOTELLURIC OBSERVATIONS OVER THE CHAVES GEOTHERMAL-FIELD (NE PORTUGAL) - PRELIMINARY-RESULTS

On the Portuguese mainland the most important low enthalpy field occur s in the graben of the Chaves region, where magnetotelluric measuremen ts have been carried out to determine the crustal electrical conductiv ity distribution. From the impedance tenser decomposition two distinct regional directions (N-S and N65 degrees +/- 10 degrees E) have been found. These directions are consistent with the geology and related to the main faults. The apparent resistivity and phase curves derived fr om the determinant of the impedance tenser were used to obtain 1-D and 2-D interpretations. The regional models indicate the existence of a conductive layer at a depth between 7 and 12 km (resistivity in the ra nge of 100-300 Ohm m, from 1-D modelling and 60 Ohm m from 2-D model). The 2-D interpretation provides a first estimate of the regional geot hermal gradient (28 degrees C km(-1)) in the Hercynian granite. The 1- D models from the soundings located within the graben suggest that loc al tectonics could play a significant role in the CO2 extraction and i n migration of the deep CO2, as indicated from isotopic data.