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Moment tensor inversion in strongly heterogeneous media Pyhäsalmi ore mine, Finland

Moment tensor inversion in strongly heterogeneous media Pyhäsalmi ore mine, Finland. D. Kühn, V. Vavryčuk September 2010 AIM, Bratislava. Pyhäsalmi ore mine, Finland. microseismic monitoring: since January 2003 safety of the underground personnel optimisation of mining process

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Moment tensor inversion in strongly heterogeneous media Pyhäsalmi ore mine, Finland

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  1. Moment tensor inversion in strongly heterogeneous mediaPyhäsalmi ore mine, Finland D. Kühn, V. Vavryčuk September 2010 AIM, Bratislava

  2. Pyhäsalmi ore mine, Finland • microseismic monitoring: • since January 2003 • safety of the underground personnel • optimisation of mining process • network: • 12 1-C geophones • + 6 3-C geophones (ISS) • 3-D geometry • sampling rate: < 3000 Hz • events: • 1500 events /months (including blasting) • -2 < Mw < 1.5

  3. Inconsistent polarities of P-wave first onset

  4. Wavefield modelling • E3D: viscoelastic 3-D FD code • (Larsen and Grieger, 1998)

  5. Wavefield modelling • 2-D section • strong interaction with mining cavities: reflection, scattering, conversion • healing of wavefronts 620 m

  6. Wavefield modelling synthetic seismograms • - complex waveforms • strong coda • complex secondary arrivals • scattering effects stronger on amplitudes than travel times, since size of heterogeneities (cavities, access tunnels) same order or smaller than wavelengths • arrival times computed by Eikonal solver still fit (wavefronts heal quickly after passing a cavitiy) observed seismograms

  7. Influence of source depth (beneath cavity)

  8. Influence of source depth

  9. Influence of source depth

  10. Influence of source depth

  11. Influence of source depth

  12. Source depth → ray path

  13. Ray path → onset polarity

  14. Geophone network (artificial)

  15. Influence of source mechanism

  16. Influence of source mechanism

  17. Influence of source mechanism

  18. Influence of source mechanism

  19. Influence of material properties within cavity

  20. Influence of material properties within cavity P-wave: 6300 m/s, S-wave: 3700 m/s

  21. Influence of material properties within cavity P-wave: 4500 m/s, S-wave: 2600 m/s

  22. Influence of material properties within cavity P-wave: 4000 m/s, S-wave: 2310 m/s

  23. Influence of material properties within cavity P-wave: 3500 m/s, S-wave: 2020 m/s

  24. Influence of material properties within cavity P-wave: 2000 m/s, S-wave: 1150 m/s

  25. Influence of material properties within cavity P-wave: 1000 m/s, S-wave: 580 m/s

  26. Moment tensor inversion

  27. Moment tensor inversion observed amplitudes moment tensor

  28. Moment tensor inversion observed amplitudes moment tensor

  29. Moment tensor inversion observed amplitudes moment tensor source mechanism

  30. Moment tensor inversion:amplitudes versus waveforms

  31. Moment tensor inversion Representation theorem point source Amplitude inversion Waveform inversion frequency domain decomposition of the moment-time functions source-time function moment tensor

  32. Amplitude & waveform inversions • Amplitude inversion • homogeneous model of the medium • Green’s functions calculated using ray theory • inversion of P-wave amplitudes (20-30) • frequencies: 250-500 Hz • Waveform inversion • 3-D heterogeneous model of the medium • Green’s functions calculated using FD code • inversion of full waveforms (15-20) • frequencies: 50-100 Hz

  33. Example: event 12 Location 24-Dec-2005 07:00:51.6230 • relatively far from • the cavities • (strong inhomogeneities) • good ray coverage Event 12

  34. Data ch 22 ch 7 ch 3 ch 14 ch 8 ch 21 ch 30 ch 15 ch 2 Complex waveforms, strong reflections, in some cases difficult to identify the S wave

  35. Fit of amplitudes O– theoretical amplitude O– observed amplitude 21 P-wave amplitudes

  36. Fit of waveforms good fit phase misfit amplitude misfit ch 1 ch 13 ch 4 ch 2 ch 20 ch 10 ch 9 ch 21 ch 14 ch 22 ch 15 ch 7 ch 23 ch 8 amplitude misfit ch 30 ch 11 ch 3

  37. Comparison amplitude inversion waveform inversion strike = 156º dip = 84º rake = 166º DC = 75% CLVD = -14% ISO = 11% strike = 145º dip = 80º rake = 133º DC = 47% CLVD = -22% ISO = 31%

  38. Summary • structure model in mines is usually very complex • earthquake source is complex (single forces, non-DC moment tensors) Accurate determination of source parameters is extremely difficult !

  39. Summary • Amplitude inversion: • its applicability is limited (simple Green’s functions are not adequate) • Full waveform inversion: • complex Green’s functions can be calculated by 3-D FD codes (accurate model needed) • sensitive to time shifts due to mislocation or due to inaccurate medium model • computationally demanding

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