Caracterização do subsolo pelos métodos geofísicos de eletrorresistividade e magnetometria terrestre em áreas com histórico de movimento de massa no município de Angra dos Reis, RJ

• Autor(a) principal: Azevedo, Jennyfer Carolliny Souza de
• Data de Publicação: 2021
• Tipo de documento: Dissertação
• Idioma: por
• Título da fonte: Biblioteca Digital de Teses e Dissertações da UERJ
• Texto Completo: http://www.bdtd.uerj.br/handle/1/19870
• topic: Caracterização do Solo; Eletroressistividade; Magnetometria Terrestre; Soil Characterization; Electrical Resistivity; Terrestrial Magnetometry; CIENCIAS EXATAS E DA TERRA::GEOCIENCIAS::GEOLOGIA

The municipality of Angra dos Reis presents a high susceptibility to landslides, with the record of 30 events classified as natural disasters between the years 1991 and 2016. Until 2010, 93 people died as a result of those episodes, which also caused significant economic damages (CEPED, 2013; Brasil, 2013). Based on that and on the presence of active events, 3 areas of study were selected: Bonfim, Caetés and Morro da Carioca. The research involved investigation by means of geophysical methods of electrical resistivity and terrestrial magnetometry, in different seasonal periods, aiming for the characterization of subsoil. The electrical resistivity method identified water concentrations, depth and thickness of soil types, allowing to portray the behavior of subsoil under different water conditions. The terrestrial magnetometry, in turn, reflected the mineralogical composition of the rocks, as well as, geological structures features at the study area. The integration of results with information from available surveys contributed to the definition of geophysical signatures and the correlation of results in other sections. The apparent resistivity values reflect local anthropic actions. The compaction of the soil through the opening/asphalt of the streets contributed to the increase of the electrical resistivity, reducing the intragrain space (and, consequently, the porosity) and reducing the percolation of water. At Morro da Carioca, sandy clay soil exhibits extreme values in several seasonal conditions. In addition, that soil, when in flat and grounded area (at lower altitude), is less compact than the highest portion. Where there is not soil compaction, rates of that analysis allowed to distinguish, according to the preservation of original porosity of the materials, clayey/sandy-clay soil of residual ground. There is orthogneiss of Rio Negro Complex, with the presence of magnetite up to 3% in the rock, at the Morro da Carioca and Bonfim. The Magnetic Susceptibility (MS) in that rock has an average value of 0.30 x 10-6 SI, and the higher rates of the Analytical Signal Amplitude (ASA) match to the presence of shallower blocks or more superficial rocks. The ASA spikes indicate the presence of diabase dykes with MS average of 16 x 10-6 SI, as recorded by Guedes et al. (2004). There is an intrusive granitoid in the orthogneiss of the Rio Negro Complex at Caetés. The highest ASA figures along the section of Procurador Haroldo Fernandes Duarte highway (BR 101) indicate the presence of shallower and partially weathered blocks (orthogneiss). At the section of Caetés Avenue, there is the predominance of low AVA values (<0.5 nT/m) corresponding to the felsic and weathered granitoid mapped at the region, with MS average of 0.05 x 10-6 SI. The employment of terrestrial geophysical methods, integrated with available data, allowed the description of subsoil at neighborhoods with a recent history of landslides, and it will contribute to actions by the Civil Defense of Angra dos Reis.