65ª Reunião Anual da SBPC

A. Ciências Exatas e da Terra - 6. Geociências - 2. Geofísica

USE OF ELECTRICAL RESISTIVITY IN THE ESTIMATION OF UNDERGROUND AQUIFERS IN THE REGION OF CABO FRIO, RJ.

Vinicius Vianna Riguête - Faculdade de Geologia da Universidade do Estado do Rio de Janeiro – UERJ Dr. Alan Freitas Machado - Instituto de Física – UERJ Dr. Claudio Elias – UERJ - Instituto de Física – UERJ Dr. Arnaldo Santiago – UERJ - Instituto de Física – UERJ Alex Muniz Rodrigues - Instituto de Física – UERJ

INTRODUÇÃO:

The present work aims to identify coastal underground aquifers in the district of Tamoios, Cabo Frio region in Rio de Janeiro state, by using the method of electrical resistivity, as well as show the seasonality effect in the studied area. The region is characterized by fine sediments such as sand and silt of quaternary age, and extensive coastal sand ridges reworked by marine transgression and regression. These sediments are associated with fluvial and coastal deposits. The purpose of electrical resistivity’s method is to measure the subsurface resistivity by making measurements on the ground surface. Resistivity quantifies how strongly a material opposes the flow of electric current, and is the inverse of conductivity.

OBJETIVO DO TRABALHO:

The project has as one of its main objectives to alert local people of the existence of these aquifers in order to prevent their contamination, as well as help the region’s government in the correct use of this water for the eventual supply of urban communities. Spreading the uses of geophysical methods is also an important point.

MÉTODOS:

The purpose of electrical resistivity’s method is to measure the subsurface resistivity by making measurements on the ground surface. The method consists in injecting current in the subsurface by two current electrodes (often called A/B or C1/C2) and measuring the voltage difference by two potencial electrodes (often called M/N or P1/P2). This will give you the value of an apparent resistivity (the resistivity will change when changed the location and separation of the electrodes), that can be calculated by diving the voltage (V) by the current injected(I), and multiplying by a factor that changes for each array. The array used in the field work was the dipole-dipole with constant 5 meter distance between each electrode. Horizontal profiling was used with roll-along method, moving the hole array with constant electrode spacing horizontally along surface. During the work, it was used the Syscal Kid Switch 24 equipment to obtain the measurements. The work was done in 4 days and measurements were obtained from a line of approximately 970 meters in the district of Tamoios, region of Cabo Frio.

RESULTADOS E DISCUSSÃO:

After inversion of the data and creation of a pseudosection, data were interpreted. The pseudosection clearly shows a drop in the value of resistivity from 6 meters depth. This top layer of soil with higher resistivity values are sediments such as sand and silt, characteristic of the region. From about 6 meters, the drop in the value of resistivity, and thus a more conductive material, shows a great chance of a shallow aquifer. Another point observed in the study was the effect of seasonality on the resistivity values. When compared with a previous work done in the same area but at different times of the year, the drop in the resistivity values occurred in different depth limits.

CONCLUSÕES:

Since many rocks, freshwater and saltwater have ranges of resistivity values already determined, the resistivity method, unlike others, can give a very accurate result. The objective was achieved, considering that it was possible the location of underground aquifers.

Palavras-chave: Horizontal Profiling, Dipole-Dipole, Conductivity.