Investigação por eletroresistividade de ambientes cársticos no município de São João de Pirabas - PA

Abstract

This Thesis discusses the investigation of karst regions using vertical eletrical soundings and dipole-dipole resistivity surveys. The studied area is localized in the northeast of state of Pará, in the district of São João de Pirabas. In this place, during the drilling of water well, the exploration crew from Fundação Nacional de Saúde, faced some problems due to the presence of cavities in the limestone of the Pirabas Formation. In order to analyse this problem, we carried out vertical eletrical soudings using the Schulumberger array. The maximum lenght of AB/2 used was 600 meters. The apparent resistivity sounding curves were initially interpreted with a 1D model computer program. The eletrical soundings located near the well in which karst cavities detected, were interpreted with a 2D model computer program. The algorithm used in the 2D program was the finite element method. The results of the interpretation of eletrical soundings of many models show that the anomalies of resistivities due the presence of karst cavities are very small. Therefore, the method of Schulumberger eletrical soundings does not have enough resolution to identify precisaly the presence of karst cavities at the subsurface. On the other hand, the numerical results for the dipole-dipole surveys show that the technique is by far more efficient than Schulumberger soundings for detection of karst cavities. However, the dipole-dipole method is much more dificult and expensive to be carried out in the field than the Schlumberger sounding method. Also, its interpretation is much more elaborated than the Schlumberger soundings.