Geophysical investigations for the characterization of fractured rock aquifers in Itu, SE Brazil

Abstract

This paper presents the results of integrated geophysical investigations to characterize aquifers in fractured rocks in the Granite complex of Itu, SE Brasil, to help locate sites for tubular wells for groundwater exploration. Ground penetrating radar (GPR) profiles, dipole–dipole electric survey, gamma spectrometry, and radon gas emanometry were applied on a same line for a comparative study of these methodologies. The results allowed us to characterize structural discontinuities up to 30 m in depth, such as, dipped, or vertical fractures. The dipped fractures appear as strong GPR reflectors, probably due to the presence of water. Besides, two anomalous regions were identified, one at 50 m and other at 75 m, both showing high attenuation of the GPR signals, and they correspond to the vertical fracture zones. 2D modeling of the dipolar electric survey shows that the two anomalous areas correspond to high electric conductivity zones, and that in the position of 75 m, a vertical fracture zone is clearly identified. In the gamma rays profiles, the values of K and Th concentrations observed in the position of 50 m indicate the presence of vertical fractures filled with clay minerals. On the other hand, in the position of 70 m of the Rn gas emanometry profile, the peak of Rn emanation is five times higher than the regional level, indicating the presence of a vertical fracture zone, clearly filled with fresh water in circulation. The geophysical characterization of the vertical fracture zones with fresh water subsidized the drilling of a well for groundwater exploration in fractured aquifers, which was an important contribution for the local community because the main sources of surface water are contaminated.