Defesa de dissertação de mestrado
Student: Marianna Tagnin
Program: Geophysics
Título: "Induced Polarization method for the characterization of weathering mantles in crystalline terrains: Case study in Extrema – MG"
Advisor: Prof. Dr. Carlos Alberto Mendonça
Judging Committee
- Prof. Dr. Carlos Alberto Mendonça – IAG (advisor);
- Dr. Otávio Coaracy Brasil Gandolfo - IPT-SP;
- Dr. Cassiano Antonio Bortolozo – CEMADEN.
Abstract
Structures that cover the crystalline bedrock, known as weathering mantle or regolith, can store water in the interstitial space, acting as granular aquifers and, when over fractured aquifers, can supply them. Geophysical methods are effective in characterizing such structures, in particular the Resistivity (ER) and Induced Polarization (IP) methods, which are sensitive to the pore space properties and the pore fluid chemistry. This research aims to evaluate the applicability of the IP method, in addition to the ER method, in characterizing the weathering mantle overlying the crystalline bedrock in the headwater sub-basin of Ribeirão das Posses. The study area is part of the Cantareira System, an important water reservoir for the metropolitan region of São Paulo – SP, which has been one of the objects of study by the Center for Water and Food Security in Critical Zones (Centro para Segurança Hídrica e Alimentar em Zonas Críticas, CSHAZC) from the FAPESP program for the creation of Science Centers for Development (Centros de Ciência para o Desenvolvimento, CCD-SP). This research investigated areas in the alluvial plain, along the hillslopes and in a spring close to the sub-basin’s watershed divide. IP parameters reduced ambiguity in the interpretation of electrical resistivity (ER) data, allowing for the identification of features in the weathering mantle that were not highlighted by resistivity. Since IP is sensitive to physicochemical changes at the grain-fluid interface, geoelectric responses were associated with possible weathering degrees of the rock. Fresh rocks are more resistive and less polarizable. On the other hand, rocks with a greater degree of alteration show a decrease in resistivity and an increase in polarizability. The normalized chargeability IP parameter enabled distinguishing lithological features associated with a higher concentration of dispersed clay in the medium, more pronounced in the alluvial plain. Although variation of this parameter was smaller along the hillslopes, it was observed that areas associated with more weathered rocks exhibited slightly higher normalized chargeability than areas of hard rocks, suggesting greater clay content in the saprolite due to weathering. The results suggest a possible relationship between the area’s topography and the formation and preservation of local regolith against erosive processes. Hillslope areas presented shallow unconsolidated surface coverage, with the upper portions possibly removed by erosion, while flattened areas exhibit more developed weathering mantles. More efficient water percolation in regions with a lower topographic gradient promoted weathering at greater depths, thickening the regolith and enabling water accumulation, thus forming a potential granular aquifer capable of recharging the underlying fractured aquifer. This research underscores the relevance of acquiring IP data alongside ER data for the characterization of weathering mantles in crystalline terrains.
Palavras-chave: Induced Polarization, Resistivity, Weathering mantle, Regolith, Hillslope characterization