Prospecção de água em aquíferos fraturados através do imageamento elétrico – estudo de caso no seridó da Paraíba e do Rio Grande do Norte.

Abstract

In order to mitigate the effects of drought and provide a more reliable water supply, both public and private-sector entities have attempted to utilize water wells across the semiarid regions of northeast Brazil. However, it is difficult to find groundwater because the regional geology is predominantly composed of crystalline rocks with little or no hydraulic conductivity and no accessible productive aquifer layers of sand, gravel, or soil. Instead, groundwater is present and accessible only in fractured zones where cracks, fractures, and discontinuities have propagated throughout the rock formations. Productive water wells are limited to only those wells that intercept water-filled fractures. The States of Paraíba (PB) and Rio Grande do Norte (RN) are largely composed of these nonporous rock layers: approximately 80% of PB and 60% of RN contain a stratigraphy mostly composed of crystalline units. Geophysical methods have demonstrated their decisive and dependable role in the goal of developing productive water wells in areas with crystalline bedrock. Through the use of electrical resistance imaging through multi-electrode arrays, surveys were conducted at 14 potential water well sites. At each location, a survey line was established consisting of over 80 electrodes and data was acquired through an automatic 2-D survey system collecting thousands of data points at varying depths across this survey line. The acquired data points were filtered to eliminate noise, processed through inversion using a software program, and resulted in a 2-dimensional profile of resistance values in a vertical section of the earth extending across the length of the survey line. This 2-D profile was interpreted based on the geologic observations at each location, taking into account the orientation and behavior of the geological structures and formations at each location. Following this collection, processing, and interpretation of the data, a single well was drilled at each site at the location indicated by the resistivity data and a chemical analysis was performed of the water at each well. The relatively thin surface layer of weathered rock in the majority of these locations does not have any bearing of the productivity of the wells, but does affect flowrates when they are located near surface drainage zones. In regards to the groundwater quality, the water samples from most of the wells were outside the established limits of water suitable for human consumption, being classified as saline or salty water. This requires the usage of treatment or desalination in order to attain potable water standards. Finally, in both Paraíba and Rio Grande do Norte, in addition to demonstrating the effectiveness of electrical resistance imaging for groundwater exploration in crystalline aquifers, we analyzed the resistivity profiles of each survey location and determined parameters of resistivity values that indicate the presence of groundwater. These results are of great importance to the body of 10 knowledge of the semiarid region, enable a deeper understanding of the behavior of subsurface fractures, and assist in the geophysical research of groundwater in similar regions across the world.