Investigação de reservatórios de água subterrânea na região central do estado do Rio Grande do Sul através do método geoelétrico da resistividade

Abstract

The state of Rio Grande do Sul (RS) has its economy based on agriculture and livestock, which are strongly dependent on water offer. However, RS have been suffering prolonged droughts, and a current solution to mitigate this problem is the underground water prospection. The geophysical method of electrical resistivity has been applied in Brazil and worldwide for this purpose, since it allows the identification of different lithologies found in subsurface according to electrical properties of the materials, with no need of excavations.The geophysical method of electrical resistivity was used to study lithological layers in the Guarani Aquifer System recharge zone. Geoelectric surveys were carried out in 21 research lines, with a length between 50 and 125 meters. The method has been optimized to reduce data acquisition time. The data collected in the field (electrical current and voltage measurements) were processed and inverted with Res2Dinv® software, generating 2D electric profiles, which were compared with tubular wells lithological profiles, with data obtained from the refraction seismic technique and with hydrogeological data. The distance between the line and the well was also considered. The standard deviation of the electric current measurements was between 0 and 10.41 mA, and between 0.08 and 10.53 mV for voltage. Most of the electric profiles (12) presented high compatibility with the well data, 5 presented reasonable compatibility and in 4 of them the analysis could not be complete. Some lines of investigation are located at the edge between the granular and fractured aquifers, which may explain non-conformities between lithological and electrical profiles. The depth of investigation was between 13 and 21 m, within the expected values according to theoretical models. Through the electrical profiles, geological structures with potential for groundwater storage were identified by the presence of fractures and the estimation of the degree of saturation of the rock.