Integrated geophysical methods in a hydrogeological study in the Northern Coastal Plain of Rio de Janeiro State - Brazil

Abstract

A geophysical pilot study has been undertaken in the Northern Coastal Plain of Rio de Janeiro State (Brazil), near Rio das Ostras Town, 200 km NE from Rio de Janeiro City. This region is characterized by both scarcity of drinking water and the existence of saline intrusions, which is caused mainly by the presence of a complex Quaternary geology and excessive aquifer exploitation. The research was performed by employing magnetic and electromagnetic methods (time - TEM and frequency - FEM domains), in order to evaluate the effectiveness of these methods under these particular geological conditions, and thus to allow the formulation of a more comprehensive hydrogeological research in the region in future. FEM profiles and one-dimensional (1D) interpretation of TEM data were capable of depicting different geological formations, to locate the main aquifer and to reveal the fresh-saline water contact. On the other hand, the magnetic method showed the presence of a NE-SW direction geological fault, which is located just in the geological contact between sand and clay, coincidentally with the fresh-saline water contact. Our results evince a promising future for using electromagnetic geophysical methods in this region in small scale surveys, as well as it confirmed that a precursory regional survey, chiefly using the magnetic method, is essential in the indication of most suitable places to locate productive fresh-water wells.