Using IP-resistivity soundings it is possible to define the structural geometry and the lithological variability within the Urucuia aquifer in the Rio das Fêmeas basin, Bahia. A total of 80 Schlumberger VES, measuring simultaneously the electrical resistivity and the time domain chargeability, were performed within this hydrological basin to a maximum current electrode spacing (AB/2) of 1000 m. Contour maps of the depth to the aquifer substratum, the depth to the phreatic surface, the mean aquifer resistivity (rho), and the upper aquifer polarization (mS) were constructed after the quantitative inversion of the soundings. The aquifer consists of stacked sandstone bodies forming layers of different resistivity overlaying a much more conductive regional shaly substratum. High values of rho and mS, for the aquifer model suggest it is dominantly composed of shaly sandstones but saturated with very fresh groundwater. The reservoir thickness increases from east to west, ranging from about 100 m in the middle of São Desidério County to more than 450 m at the edge of the Serra Geral scarps. The general groundwater flow is from west to east, except in a small portion near the scarps were it is reversed forming a groundwater divide inside this hydrological basin. The jointly inversion of the IP and resistivity data was useful to reduce geoelectrical ambiguities and to better define the water table depths. The achieved results are of great value for planning a sustainable water exploration program for the region, which is being stressed by an accelerated agricultural development.
electrical resistivity; induced polarization; aquifer evaluation
