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

Transcript

1. Iron ore interpretation
   using gravity-gradient inversions
   in the Carajás, Brazil
   Dionisio Uendro Carlos
   Leonardo Uieda*
   Yaoguo Li
   Valéria Cristina Ferreira Barbosa
   Marco Antonio Braga
   Glauco Angeli
   Guilherme Gravina Peres
   

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2. Carajás survey area
   

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6. N1
   plateau
   

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7. N1
   plateau
   Target: hematite
   hard (3.6 g/cm3)
   soft (3.4 g/cm3)
   

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8. N1
   Survey
   System: FTG
   Line spacing: 150 m
   Height: 100 m
   Total survey: ~550 km
   

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9. The data
   

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10. Gzz
    N1 geology
    

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11. Gzz
    N1 geology
    

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12. 3D inversion
    

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13. 2 methods
    Li, Y. (2001), 3-D inversion of gravity gradiometer data, SEG
    Expanded Abstracts, 20, 1470–1473, doi:10.1190/1.1816383
    Uieda, L., and V. C. F. Barbosa (2012), Robust 3D gravity
    gradient inversion by planting anomalous densities, Geophysics,
    77(4), G55–G66, doi:10.1190/geo2011-0388.1
    (1) Planting anomalous densities
    (2) Smooth inversion
    

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14. Planting anomalous densities
    

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15. Observed data
    Mesh
    

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16. Zero
    density contrast
    

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17. Seeds
    (user specified)
    Predicted data
    

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18. Neighbors
    

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19. The best
    New predicted data
    

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20. The best
    New predicted data
    φ=
    √∑
    i=1
    N
    (g
    i
    −d
    i
    )2
    min of Γ=φ+μθ
    and
    compactness
    

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21. The best
    New predicted data
    

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30. Fit
    

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31. Smooth inversion
    

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32. Mesh
    

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33. min φ( p)=φ
    d
    +μ φ
    p
    

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34. min φ( p)=φd
    +μφp
    Densities
    

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35. min φ( p)=φd
    +μφp
    Densities Data misfit
    

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36. min φ( p)=φd
    +μφp
    Densities Data misfit
    Smoothness + depth weights
    

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37. min φ( p)=φd
    +μφp
    subject a⩽ p⩽b
    Densities Data misfit
    Smoothness + depth weights
    

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38. True model
    Recovered
    

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39. Different methods
    • Different approaches
    • Different constraints
    • Common data
    • Common target
    

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40. Inversion parameters
    • Gzz component = 9,053 obs
    • Cell size = 75 m
    – Planting = 581,440 cells
    – Smooth = 1,520,960 cells (larger mesh)
    

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41. Seeds
    45 seeds
    

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42. Results
    

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45. Smooth
    Planting
    

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46. Smooth
    Planting
    

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47. Cross-section
    

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48. Smooth
    

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49. Smooth
    Hematite
    

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50. Smooth
    Hematite
    Jaspilite
    

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51. Planting anomalous densities
    

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52. Conclusions
    • Joint interpretation
    • Preliminary results
    • Compatible solutions
    • Agree with boreholes
    • Concentrated above 300 m
    • Bellow 200 m could be jaspilite
    – Same density contrast
    

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53. Acknowledgements
    Colorado School of Mines, USA
    Observatório Nacional, Brazil
    Vale S.A.
    

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