Self-potential electric (SP) measurements made in wells and at the surface along traverses crossing aquifers in fractured crystalline rocks in the state of Bahia, Brazil, revealed the presence of strong anomalies related to groundwater movements. By considering a simple geometric-hydraulic model for a phreatic aquifer, in an essentially 2-D structure, and using basic equations for ionic concentration and fluid (or heat) coupled flows, it was found that electrokinetic effects developed along fracture walls can produce anomalies of amplitude comparable to those observed in the field. Estimates obtained for other coupled processes, such as the electrochemical and the thermo-electric effects, suggest that the electrokinetic effect is the most important component for setting up SP currents in fractured zones of crystalline rocks. Based on this result, a joint interpretation of pseudoresistivity sections and SP profiles was performed and the results show that the SP method can be used as a complementary geophysical technique to the resistivity method, allowing to distinguish areas of relative high permeability within fractured rocks.
Groundwater; Self-Potential; Fractured rocks
