AN AUDIOMAGNETOTELLURIC SURVEY OVER THE CHAVES GEOTHERMAL-FIELD (NE PORTUGAL)

In an attempt to define the resistivity model of the Chaves geothermal field in NE Portugal, a detailed survey with scalar audiomagnetotellu ric measurements was performed. The soundings were made in the frequen cy range from 2300 to 4.1 Hz. Electrical resistivity models were deriv ed from the application of 1-D inversion, 2-D trial and error modeling and 2-D inversion procedures. The resistivities inside the geothermal field are low, reaching not more than 30 Omega m and increasing up to 60-150 Omega m beneath the shallow geothermal reservoir. The conducta nce in the low resistivity zone (20-35 S) suggests that the geothermal field is behaving as a local conductivity anomaly, with predominant 2 -D features. The low resistivity structure is associated with sediment ary and metamorphic rocks saturated with highly mineralized water and with fractured zones allowing upward movements of hydrothermal fluids. The range of porosity (13-27%) of the formations associated with the geothermal field was estimated using the modified Archie law. Copyrigh t (C) 1996 CNR.