Iron ore interpretation using gravity-gradient inversions in the Carajás, Brazil

Dionisio U. Carlos, Leonardo Uieda, Yaoguo Li, Valéria C. F. Barbosa, Marco A. Braga, Glauco Angeli, Guilherme Peres

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This presentation is about the work Dionisio did for his PhD. He used my planting inversion method on data from his research area in central Brazil. He couldn't make it to the meeting so I ended up giving the talk on his behalf.

Abstract

We have interpreted the airborne gravity gradiometry data from Carajás Mineral Province (CMP), Brazil, by using two different 3D inversion methods. Both inversion methods parameterized the Earth's subsurface into prismatic cells and estimate the 3D density-contrast distribution that retrieves an image of geologic sources subject to an acceptable data misfit. The first inversion method imposes smoothness on the solution by solving a linear system that minimizes an depth weighted L2 model objective function of density-contrast distribution. The second imposes compactness on the solution by using an iterative growth algorithm solved by a systematic search algorithm that accretes mass around user-specified prisms called “seeds”. Using these two inversion methods, the interpretation of full tensor gravity gradiometry data from an iron ore deposit in the area named N1 at CMP shows the consistent geometry and the depth of iron orebody. To date, the maximum depth of the iron orebody is assumed to be 200 m based on the maximum depth attained by the deepest well drilled in this study area. However, both inversion results exhibit a source whose maximum bottom depth is greater than 200 m. These results give rise two interpretations: i) the iron orebody may present its depth to the bottom greater than the maximum depth of 200 m attained by the deepest borehole; or ii) the iron orebody may be 200 m deep and the rocks below may be jaspilite whose density is close to that of soft hematite.

Citation

Carlos, D. U., L. Uieda, Y. Li, V. C. F. Barbosa, M. A. Braga, G. Angeli, and G. Peres (2012), Iron ore interpretation using gravity-gradient inversions in the Carajás, Brazil, SEG Technical Program Expanded Abstracts, pp. 2008–2012, doi:10.1190/segam2012-0525.1

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