Investigação do arcabouço geológico de uma porção da bacia do Rio Piratini utilizando os métodos da eletrorresistividade e de radar de penetração de solo (GPR) – Caçapava do Sul, RS

Abstract

The present work has the objective of investigating the geological framework and the determination of the geometries of the riverbeds in two distinct areas of the Piratini River Basin, southeast portion of Rio Grande do Sul. Based on the contrast of the physical properties of the lithotypes found in the studied areas and integration of shallow geophysical data (eletroresistivity and GPR) was intended to identify and distinguish the lithotypes and when possible define their geometries. The acquisition of the data of eletroresistivity was carried out through the technique of electric walking (CE), with the arrangement dipole-dipole, with spacing of 3 m. For the acquisition of GPR data, a 200 MHz antenna was used with the common offset arrangement. The data processing of eletroresistivity followed the steps of data refinement, interpolation and inversion using the programs, Prosys II and Res2dinv. GPR data processing followed the steps of conversion, processing using the ReflexW, Prism2 programs respectively. Two sections of real electrical resistivity were generated. In area 1 a resistivity variation of 20-4000 (Ωm) was observed, being interpreted as sound rock, altered rock, saprolite with variation of saturation, saturated deposits and fluvial bar. In area 2 a variation of 5-1000 (Ωm), indicated as altered rocks, saprolite with variation of saturation, saturated deposits, presence of blocks and boulders and / or inactive sandy deposits (river bar) and fluvial bar were observed. Two radargrams of area 1, designated B-B' of 32 m and C-C' of 53 m, and three radargrams of area 2, denominated B2-B2' of 115 m, C2-C2' of 48 m D2'-D2 of 78 m. Five facies were interpreted according to the reflector terminations and known patterns, with two boundary surfaces (S1 and S2) and three radarfacies (RA, RB and RC). With the electrical results it was possible to differentiate the subsurface structures in both areas, and the GPR results allowed to determine the depositional architecture of the studied bars. From these results it was possible to integrate the methods and complement the results of both study.