Airborne geophysical and geological data applied to the selection of groudwater potential areas in crystalline rocks of Ceara, Brazil / Dados aerogeofisicos e geologicos aplicados a seleção de areas favoraveis para agua subterranea no dominio cristalino do Ceara, Brasil

AUTOR(ES)
DATA DE PUBLICAÇÃO

2008

RESUMO

The location of water well in Northeast Brazil is usually based on the interpretation of structural lineaments in aerial-photographs and electromagnetic and electrical resistivity ground geophysics surveys. However, the success of these techniques is variable, and the small coverage area is another limitation. This Thesis favors a different approach, the use of airborne geophysical surveys and spatial modeling to map groundwater potential and quality in the State of Ceará, Brazil. The data sets comprise a local helicopter survey, a regional airborne survey as well as multispectral images from the Enhanced thematic Mapper sensor (ETM+)/Landsat-7 and field data. The thesis was developed in three stages: i) the processing of airborne magnetic, electromagnetic and radiometric data; ii) a geostatistical modeling to produce groundwater electrical conductivity maps, and; iii) groundwater favorability modeling based on the association of high- and low-yield wells and geophysical, structural and spectral properties. The magnetic anomaly data were filtered using the regional and local survey to enhance through-going structures whose gradients crosscut rocks below 400 m depth. The helicopter electromagnetic data (HEM) were processed so that shallow (up to 15 m depth) conductive structures and lithologic units were enhanced. Inversion modeling applied to electromagnetic data unveiled a conductive soil horizon over a resistive bedrock at depths greater than 15 m which restricts the detection of deeper conductive anomalies. The regional radiometric data together with ETM+ images characterized the soil texture and relative contents of Fe-oxide/clay-mineral/vegetation density ratios. Structural data were interpreted from aerialphotographs and from ETM+/Landsat-7 data. Kriging and stochastic simulation techniques were applied to electrical conductivity data from water-wells to generate a groundwater electrical conductivity map. The HEM data were used as an external variable to characterize the variability of groundwater electrical conductivity associated with crystalline aquifers. Validation tests comparing predicted values with measurements in new wells confirmed the competence of these models in locating fresh groundwater sources in bedrock. The probability ratio and weights of evidence techniques quantitatively described the association of each value of magnetization, electrical conductivity, geophysical gradients, structural lineaments, radiometric data and multiespectral data to different well yields. The groundwater favorability models imply the existence of structural lineaments in an E-W direction, with non-magnetic properties, which are pervasive below 250 m depth but conductive from up to 15 m below the surface. The E-W lineaments are cut by conductive N-S structures. Field data confirm that the geophysical trends are extensional brittle structures. High yield classes are more associated with coarse-grained soils (high potassium, medium thorium and uranium) where alteration minerals are predominantly clay-minerals as opposed to Fe-oxide minerals

ASSUNTO(S)

geofisica - ceara (estado) geophysics geophysics hydrogeology geofisica - processamento de dados hidrogeologia

ACESSO AO ARTIGO

Documentos Relacionados