This document summarizes a seismic study of the Mt. Toc landslide site in Italy. Reflection seismic, refraction seismic, and surface wave techniques were used to image the complex geology. P-wave and SH-wave reflection and refraction data were acquired using vibroseis and weight drop sources. Surface wave data were also recorded. The data were processed, integrated with geology, and interpreted to develop a seismic velocity model and identify key geologic formations. The study demonstrated that seismic methods can be applied to investigate landslide environments, though the complex geology presents challenges that require an integrated approach combining different seismic techniques and auxiliary information.

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