9oct 1 petronio-reflection seismic

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  • 1. Reflection Seismic and Surface Wave analysis on complex heterogeneous media: the case of Mt. Toc landslide in Vajont valley Lorenzo Petronio 1, Jacopo Boaga 1 2 and Giorgio Cassiani 2 OGS – Istituto Nazionale di Oceanografia e Geofisica Sperimentale, Trieste 2 Dipartimento di Geoscienze - Università di Padova International Conference Vajont, 1963-2013 Thoughts and analyses after 50 year since the catastrophic landslide October 8-10, 2013, Padua, Italy
  • 2. Outline Vajont dam and monte Toc landslide • Geology • Targets • Experiment: - Reflection seismic - Surface wave analysis • Data integration and results
  • 3. Vajont dam and monte Toc landslide • Vajont dam, 264.6 m (1957 – 1959) • landslide - 9 October 1963 • around 2000 victims Nord 0 1 Km
  • 4. Monte Toc landslide • 260 millions m3 (2000 x 1000 x 130 m) • slide: 500 m, 110 Km/h
  • 5. Geology N S Da Riva et al., 1990
  • 6. Targets • Can seismic methods be used in this landslide environment? • Feasibility study for further studies/applications In the frame of “Strategic research project GEO-Risks”- “Geological and Hydrogeological processes: monitoring, modelling and impact in North-East Italy” Problems/limitations • • • • Rough topography and difficult logistics Karst area Strong heterogeneities (also lateral) Seismic impedance contrasts?
  • 7. Seismic survey: Walkaway test and experiment preparation • A priori information collection • Scouting • Walkaway test • Data analysis • Experiment design
  • 8. Seismic survey • Reflection seismic (P- and SH- wave) • Refraction seismic (P- and SH- wave) • Surface wave
  • 9. Seismic survey: reflection/refraction seismic • P-wave reflection/refraction seismic L1 256 ch. 10 Hz z (fixed spread), 2 m, 510 m 24 ch. 4.5 Hz z (fixed spread) - 4 m 125 shots (vibroseis upsweep 14 s, 5-250 Hz) – 4 m L2 162 ch. 10 Hz z (fixed spread) – 2 m, 322 m 24 ch. 4.5 Hz z (fixed spread) – 4 m 81 shots (vibroseis upsweep 14 s, 5-250 Hz) – 4 m • SH-wave reflection/refraction seismic L1 113 ch. 10 Hz x (fixed spread) – 4 m, 448 m 113 ch. 10 Hz y (fixed spread) – 4 m 50 shots (vibroseis upsweep 14 s, 5-250 Hz) – 8 m
  • 10. Seismic survey: surface wave • Surface wave L1 256 ch. 10 Hz z (fixed spread) – 2 m, 510 m 24 ch. 4.5 Hz z (fixed spread) - 4 m 48 ch. 4.5 Hz z (fixed spread) – 10 m 2 remote stations 3ch 1 Hz x, y, z 9 shots (weight drop – about 240 Kg; H=6, 10 and 14 m)
  • 11. Acquisition: seismic sources Mini-vibroseis Weight drop
  • 12. Acquisition: seismic receivers 10 Hz geophones (Z and 3C) 10 Hz and 4.5 Hz geophones (Z) 1 Hz geophone (3C)
  • 13. Acquisition: data recording DMT Summit telemetry system + 24 ch. compact unit Sampling rate: Data length: 1 ms Vibroseis, 16 s 2 s correlated data (GF) Weight drop, 10 s GPS synchronization Orion remote stations Sampling rate: Data length: 2 ms Weight drop, 10 s GPS synchronization Geode seismograph (48 ch.) Sampling rate: Data length: 1 ms Weight drop, 10 s Synchronized with DMT
  • 14. Acquisition: in field quality control DMT acquisition + QC Theoretical vs. real sweep Vibroseis sweep QC Correlated data
  • 15. Common shot gathers: L1 data
  • 16. Reflection seismic: data processing Crosscorrelation Data editing Geometry First break picking Static correction Band pass filtering Spherical divergence compensation Deconvolution S/N improvement actions (*) CDP sorting Velocity analysis NMO correction CDP stack Prestack migration (Kirchhoff) Time to depth conversion (refraction velocities) (*) based on the high spatially sampled data
  • 17. Data processing: examples Raw data Processed data
  • 18. Reflection seismic: pre-stack migration Line L1 W X (m) E
  • 19. Reflection seismic: data validation (time) Pre-stack migration Super CSG vibroseis CSG weight drop
  • 20. First break picking: P- velocities Real data picking vs. synthetic data (direct modelling) P wave P model
  • 21. First break picking: SH- velocities Real data picking vs. synthetic data (direct modelling) SH wave SH model
  • 22. Surface wave: weight drop data Remote station data Near offset Far offset Multichannel data
  • 23. P- and S- velocities: from refraction and surface wave analysis
  • 24. Line 1: data interpretation W X (m) E
  • 25. Line 1: data interpretation W X (m) E
  • 26. Line 1: data interpretation W Soccher Fm. Fonzaso Fm. Vajont Fm. R3 X (m) E
  • 27. Line 1: data interpretation W X (m) E
  • 28. Line 1: data interpretation W X (m) E
  • 29. Line 1: data interpretation W X (m) E
  • 30. Line 1: data interpretation
  • 31. Conclusions • Walkaway test for the tuning of the acquisition parameters (i.e., geometry) is necessary to obtain reliable data • The integration of different seismic techniques (supported by geology) is a key factor for data validation and interpretation • Reflection/refraction seismic and surface wave analysis can be used as investigation tools in complex area