Abstract:ABSTRACT. Detecting groundwater in fractured aquifers is always a difficult task. This paper presents the detailed study of a fractured aquifer in Lindóia region, São Paulo, Brazil. The study area was selected after a regional study analyzing the favorability for occurrence of groundwater which classified the area as being of excellent groundwater favorability. The study consisted on fractures mapping based on aerial photographs (scale 1:25,000) and geoelectrical survey (vertical electrical sounding and resist… Show more
“…The methodology of this work involved the integration of remote sensing, structural and electrical resistivity data (Madrucci et al, 2005;Fernandes, 2008;Francese et al 2009). Cartographic data were georreferenced in ArcGIS 10.4 (ESRI, 2015) using the horizontal Datum WGS-84 and the UTM map projection (Zone 24S).…”
The Precambrian metamorphic rocks in the southern region of the Espírito Santo State(southeastern Brazil) is intercepted by ductile and brittle/ductile shear zones over which towns like Guaçuí, Marechal Floriano and Domingos Martins were established. In the years of 2014 to 2016 the Espírito Santo State faced the worst drought in the past 80 years. In situations like that, the only source of fresh water available is underground. Therefore, the purpose of this study was to integrate multisource data for groundwater prospecting and management. The study area comprises the central sectors of the Guaçuí and Batatal shear zones. Methodology involved the interpretation of stereo-pairs for the identification of lineaments, electrical resistivity surveys and geological fieldwork. The results were tentatively validated with available groundwater well production data. Six resistivity profiles with a total length of 2,400 m were surveyed and 15 outcrops were described across these structures. Results have shown that the structural framework of the shear zones is predominantly made of NW to NNW and NE to NNE-trending lineaments. The former is related to brittle tectonics and the latter coincides with the ductile Precambrian fabric of the crystalline basement rocks. A near-surface porous aquifer regarded to the regolith and recent unconsolidated sedimentary deposits presenting resistivity values < 1,372 Ohm.m and depths < 30 m have good potential for groundwater storage. However, results arenot conclusive about the hydrogeological significance of tectonic structures for groundwaterprospection in the study area.
“…The methodology of this work involved the integration of remote sensing, structural and electrical resistivity data (Madrucci et al, 2005;Fernandes, 2008;Francese et al 2009). Cartographic data were georreferenced in ArcGIS 10.4 (ESRI, 2015) using the horizontal Datum WGS-84 and the UTM map projection (Zone 24S).…”
The Precambrian metamorphic rocks in the southern region of the Espírito Santo State(southeastern Brazil) is intercepted by ductile and brittle/ductile shear zones over which towns like Guaçuí, Marechal Floriano and Domingos Martins were established. In the years of 2014 to 2016 the Espírito Santo State faced the worst drought in the past 80 years. In situations like that, the only source of fresh water available is underground. Therefore, the purpose of this study was to integrate multisource data for groundwater prospecting and management. The study area comprises the central sectors of the Guaçuí and Batatal shear zones. Methodology involved the interpretation of stereo-pairs for the identification of lineaments, electrical resistivity surveys and geological fieldwork. The results were tentatively validated with available groundwater well production data. Six resistivity profiles with a total length of 2,400 m were surveyed and 15 outcrops were described across these structures. Results have shown that the structural framework of the shear zones is predominantly made of NW to NNW and NE to NNE-trending lineaments. The former is related to brittle tectonics and the latter coincides with the ductile Precambrian fabric of the crystalline basement rocks. A near-surface porous aquifer regarded to the regolith and recent unconsolidated sedimentary deposits presenting resistivity values < 1,372 Ohm.m and depths < 30 m have good potential for groundwater storage. However, results arenot conclusive about the hydrogeological significance of tectonic structures for groundwaterprospection in the study area.
“…Electrical profiling is indicated to detect lateral variations of subsurface resistivity at one or multiple depths approximately constant, through the shift of the set of electrodes with fixed spacing, which can identify faults and fractures, mapping geological contacts, structures and bodies which have a lateral heterogeneity of electrical resistivity (Madrucci et al, 2005;Keary et al, 2009). In this study the multilevel electric profiling was adopted, which registers the lateral variation of resistivity in multiple depth levels.…”
RESUMO O presente trabalho analisa o efeito da adoção de diferentes configurações do método da eletrorresistividade para investigar a ocorrência de potenciais aquíferos fissurais. Em uma área onde reconhecidamente existe uma zona de fraturas subverticais no embasamento cristalino foram aplicadas as técnicas de múltiplas sondagens elétricas verticais (SEVs), utilizando o arranjo Schlumberger, e o caminhamento elétrico utilizando o arranjo gradiente multinível. Três diferentes configurações foram testadas em uma mesma linha cuja direção é aproximadamente perpendicular aos planos das fraturas. Os resultados obtidos mostram que o caminhamento elétrico multinível pelo arranjo gradiente apresentou maior clareza na detecção da zona de fraturas em relação às configurações utilizando sondagens elétricas verticais. No entanto, tais estruturas subverticais ainda poderiam ser detectadas por sondagem elétrica, desde que adotado um afastamento adequado entre os centros das SEVs. As seções geoelétricas obtidas pelas configurações testadas indicam a presença de uma estrutura acumuladora de água subterrânea do tipo riacho-fenda. Adicionalmente, o caminhamento elétrico pelo arranjo gradiente apresenta vantagens operacionais em relação às demais configurações, sendo mais prático e rápido nos trabalhos de aquisição de dados no campo.Palavras-Chaves: Hidrogeofísica, eletrorresistividade, embasamento cristalino. ABSTRACT This paper analyzes the effect of adoption of different configurations of electric resistivity method to investigate the occurrence of potential fractured aquifers. In an area where there is a sub vertical fracture zone in the crystalline basement the techniques of multiple vertical electrical soundings (VES), using the Schlumberger array, and electrical profiling using the multilevel gradient array, were applied. Three different configurations were tested in a same line whose direction is approximately perpendicular to the fracture plan. The results show that the multilevel gradient array presented better clarity in detecting fracture zone than the settings using vertical electrical soundings. However, such sub vertical structures could still be detected by VES, since it is adopted a proper separation between the VES centers. Obtained geoelectric sections indicate the presence of a groundwater accumulating structure composed by a crack-induced stream. Additionally, the electric gradient profiling presents operational advantages in relation to other arrays, being more practical and fast for data acquisition in the field.Keywords: Hydrogeophysics, electric resistivity, crystalline basement.
“…Por fim, a técnica geofísica de caminhamento elétrico tem sido utilizada para complementar a investigação estrutural de campo (MADRUCCI et al, 2005;RAYNER et al, 2007;WAHN-FRIED, 2010;RATNAKUMARI et al, 2012). O caminhamento elétrico possibilita a localização de descontinuidades do substrato rochoso em escala de sítio, permitindo relacioná-las com a topografia do terreno e com estruturas observadas em afloramentos.…”
No contexto brasileiro, aquíferos cristalinos fraturados têm importância significativa para o abastecimento público e privado nas regiões mais povoadas e de maior relevância econômica, o que justifica a necessidade de se avançar cada vez mais no seu conhecimento. Em geral, entretanto, várias das técnicas e ferramentas adequadas para a investigação desse meio geológico ainda são pouco exploradas na totalidade, tanto para as tarefas de locação de novos poços, como para a previsão do comportamento de contaminantes em situações de investigação de áreas contaminadas. Neste estudo, se pretende chamar a atenção para a aplicação conjunta de técnicas convencionais para o desenvolvimento de um modelo conceitual geológico numa área contaminada em aquífero fraturado, cujos resultados embasam as etapas de investigações subsequentes, mais avançadas. A metodologia integra três técnicas tradicionais e não invasivas, com escalas de investigação se desenvolvendo da regional para a local (top-down): (1) análise de lineamentos; (2) amostragem estrutural em afloramentos; e (3) imageamento geofísico pelo método de caminhamento elétrico. No caso estudado, as três técnicas geraram resultados progressivamente consistentes para a melhor caracterização do aquífero, possibilitando a identificação dos trends estruturais e dos principais grupos de fraturas da região. Assim, os lineamentos identificados guardaram ótima correlação com os trends estruturais identificados em campo; similarmente, zonas de baixa resistividade identificadas nos caminhamentos elétricos coincidiram com a localização dos lineamentos e das zonas de fraturas. A correlação entre os resultados das três etapas foi fundamental para a melhor compreensão da dinâmica de fluxo do aquífero, estabelecendo, em escala local, o modelo conceitual geológico-estrutural e as estruturas que apresentam maior potencial para fluxo de água e, potencialmente, para o transporte de contaminantes.
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