Roberto Alexandre Vitória de Moraes

Constrained inversion of gravity fields for complex 3-D structures

As part of a research program to develop gravity interpretation tools that can be merged with seismic techniques, a full 3-D complex structural inversion scheme for (possibly multibody) polyhedral models has been developed. The forward modeling algorithm was adopted from previous work. Because the inverse problem is generally very ill posed, several methods of regularizing the inversion were investigated and a combination of the most useful was adopted. The combination includes (i) a structured matrix formulation for the system equations, (ii) an analytical expression for the Jacobian calculation, (iii) first‐derivative damping, (iv) a choice of damping parameter based on a variation of the trust region method, (v) a weighted scheme for parameter correction, and (vi) complete freezing of degrees of freedom found not to influence the gravity field significantly. This combination yields a robust inversion which was successfully demonstrated on data over the Galveston Island salt dome, offshore Texas. Variat...

Integração e análise de dados aerogeofísicos por meio da aplicação de técnicas de processamento digital de imagens e classificação não supervisionada: o exemplo do Greenstone Belt Rio das Velhas, quadrilátero ferrífero, MG

Poor outcrop and deep weathering characterize the Rio das Velhas Greenstone Belt in the southeastern from Brazil. This paper summarizes the use of high-density airborne survey for mineral exploration studies based on interpretation enhancements of magnetic, radiometric and frequency domain electromagnetic data using image-processed methods and an unsupervised classification. The generated products provide new insights and an excellent tool for mapping and trace individual lithological units, improving the information content of the single geophysical channels. The geophysical images were processed using different combinations. The best product was the analytical signal amplitude and inclination integrated by IHS transformation. Information extracted from this image maps the geology and lineament patterns at both regional and local scales. The K-means technique using ten classes was also applied to the geophysical data. These results enhance the lithologies mapped by the field geologists at the 1:100.000. Also shows important host rocks and different gold mineralized geological domains. Such domains host the known gold mineralization, illustrating the utility of these techniques to improve the geological knowledge in the study area.

Determinação das espessuras do Sistema Aquífero Urucuia a partir de estudo geofísico

Resumo O Sistema Aquifero Urucuia (SAU) e um manancial subterrâneo de extensao regional, composto por subtipos de aquiferos inter-relacionados. O aquifero e constituido de quartzo-arenitos e arenitos feldspaticos eolicos bem selecionados, com presenca de niveis silicificados e, em menor proporcao, niveis conglomeraticos relacionados ao Grupo Urucuia, Neocretaceo da Bacia Sanfranciscana, que representa a cobertura fanerozoica do Craton Sao Francisco. O sistema aquifero se estende desde o sul do Piaui ate o noroeste de Minas Gerais, com maior expressao no oeste da Bahia. Apresenta um eixo divisor longitudinal que separa o fluxo subterrâneo para o oeste (bacia hidrografica do Tocantins) e para leste (bacia hidrografica do Sao Francisco). O sistema foi dividido em quatro subsistemas, assim caracterizados: aquifero livre regional; aquifero suspenso local; aquifero confinado ou semiconfinado e aquifero livre profundo. As espessuras totais do sistema aquifero, estimadas a partir da aplicacao de estudos geofisicos, apresentam valores que variam de 100 a 600 m nos 26 pontos do levantamento eletromagnetico no dominio do tempo. As espessuras saturadas variam entre 80 e 460 m. Palavras-chave: Aquifero Urucuia; metodo eletromagnetico; dominio do tempo; hidrogeologia.

Imageamento da subsuperfície baseado na condutividade: obtenção de CDIs para o Sistema Geotem

The objective of subsurface imaging is to transform responses measured by ATDEM methods in an image of the electrical conductivity versus depth (CDI) and with this, to get information on target geometry and conductivity (geoelectrical model). This work presents a methodology to get CDIs over the measurement profile (2-D) used subsequently to visualize the geoelectrical model in 3-D, which would help to integrate and to understand the local geology. To accomplish this, a layered geoelectrical model is used (1-D) together with the depth attained by the maximum electrical induced current concept. The conductivity and the layer characterization are based on a simplified modeling algorithm (early and late asymptote approximations). Resultant software (Sandberg, 1986) was adapted to calculate the depth and to processes GEOTEM data.

Interpretação de dados gamaespectrométricos do Projeto Rio do Sangue, Mato Grosso

This paper presents the results of an extensive processing, interpretation and geological integration of airborne gamma-ray spectrometric data. It comprised data from 30.360 linear kilometers of profiles, spaced 2000 m apart, covering an area of 54.000 km 2 in northwest Mato Grosso State (Rio do Sangue Project, DNPM/DGM/CPRM). Individual and composite images of K, Th, U and TC data channels (RGB and CMY), allowed the interpretation of radiometric units. The integration of geophysical and geological information leads to a better picture of the lithological units present in the project area. Introducao