Magnetotelluric observations over the Chaves geothermal field (NE Portugal)—Preliminary results

Abstract On the Portuguese mainland the most important low enthalpy field occurs in the graben of the Chaves region, where magnetotelluric measurements have been carried out to determine the crustal electrical conductivity distribution. From the impedance tensor decomposition two distinct regional directions (N-S and N65° ± 10°E) have been found. These directions are consistent with the geology and related to the main faults. The apparent resistivity and phase curves derived from the determinant of the impedance tensor 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 range of 100–300 Ω m, from 1-D modelling and 60 Ω m from 2-D model). The 2-D interpretation provides a first estimate of the regional geothermal gradient (28°C km −1 ) in the Hercynian granite. The 1-D models from the soundings located within the graben suggest that local tectonics could play a significant role in the CO 2 extraction and in migration of the deep CO 2 , as indicated from isotopic data.