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Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
593
DOI: 10.4408/IJEGE.2013-06.B-57
REFLECTION SEISMIC AND SURFACE WAVE ANALYSIS
ON COMPLEX HETEROGENEOUS MEDIA:
THE CASE OF MOUNT TOC LANDSLIDE IN THE VAJONT VALLEY
L
orenzo
PETRONIO
(*)
, J
acopo
BOAGA
(**)
& G
iorGio
CASSIANI
(**)
(*)
OGS -Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - Borgo Grotta Gigante 42/C - 34010 Sgonico, Italy
(**)
University of Padova - Department of Geosciences - 35137 Padova, Italy
ties to be used in future rupture and slide simulations.
K
ey
words
: Reflection seismic; Refraction seismic; FTAN; P-
wave; S- wave; landslide
INTRODUCTION
Italy’s Vajont rockslide is one of the widely known
and most tragic examples of a natural disaster in-
duced by human activity. It is commonly considered
as a reference event for risk evaluation, as well as for
rock mechanics studies, as its study laid the basis of
modern Engineering Geology. The 9 of October 1963
a catastrophic landslide occurred on the northern slope
of Mount Toc (Vajont valley, Northern Italy). The land-
slide was induced by an artificial reservoir designed for
electrical production. A rock mass of approximately
270 million m
3
collapsed into the reservoir generating
a wave that surmonted the dam and swept into the -
‘Piave’ - valley below, causing the death of approxi-
mately 2000 people. At that time, over US$16 million
was paid in civil lawsuits brought in respect of personal
injury and death. The bibliography on Vajont (S
uper
-
chi
et alii, 2010; 2012) shows how many studies were
conducted for re-evaluating the failure mechanisms in
the last decades. Recently, due to the exceptional incre-
ment of new tools for publishing and sharing scientific
papers, new knowledge methods and techniques for
rock mass analyses are at our disposal. Nevertheless no
deep geophysical seismic study of the entire landslide
body has been conducted, due to the very difficult lo-
ABSTRACT
In the framework of the Research Strategic Project
Geo-Risks "Geological and hydrological processes:
monitoring, modeling and impact in the North-East
Italy", a seismic reflection survey and surface waves
analysis were performed on the Mt. Toc landslide in the
Vajont Valley, Italy. The aim of the geophysical meas-
urements was to obtain a 2D geophysical model of the
entire landslide body down to the sliding surface depth
and the characterization of rock seismic velocities. Due
to the critical environment, another aim of the project
was also to explore the feasibility and limitations of
these geophysical tools in large landslide studies.
The seismic reflection survey was conducted
along two lines: L1 and L2, that are 510 and 322 m
long respectively, with a - ‘Vibroseis’ - operating in
the vertical mode (P wave) and vertical geophones.
L1 was also acquired with - ‘Vibroseis’ - in horizontal
mode (SH) and horizontal geophones.
The Frequency Time Analysis of surface wave
was also adopted get another independent estimation
of seismic velocity profile (shear wave) of the land-
slide body. For the surface wave study a 250 Kg drop
source was used, in order to generate low frequency
excitation. The large amount of recording channels
furnished an image of the subsoil for the entire land-
slide body down to the target depth. Results are in
good agreement with the borehole data and geological
interpretations. Shear and compressional wave pro-
files allow a comparison with rock mechanical proper-
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L. PETRONIO, J. BOAGA & G. CASSIANI
594
International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
bedding planes, causing the outcropping of the Fonzaso
Formation (G
enevoiS
& G
hirotti
, 2005). The failure was
indicated to be principally confined within the 0.5-18 cm
thick, clay-rich layers, which were observed to be con-
tinuous over large areas of the failure surface (h
endron
& p
atton
, 1985). Fig. 2 shows a sketch of the 1963 land-
slide as reported by r
oSSi
& S
emenza
(1965).
SEISMIC SURVEYS
The main aim of the project was to explore the fea-
sibility and limitations of seismic methods applied to the
study of a complex large landslide.
In Spring 2011 a combined reflection/refraction (P-
and SH- wave) and surface wave survey was conducted
to evaluate both the buried geometry after the tragic Va-
jont landslide and the physical-mechanical properties of
the materials (p
etronio
et alii, 2011).
gistic problems of operating on the big landslide. This
work represents a preliminary approach on this kind of
geophysical characterization for the Mt. Toc landslide.
The techniques employed are reflection survey in
P- and SH- wave together with controlled source sur-
face wave analysis. In the spring of 2011 we acquired
along the landslide body: N. 2 reflection seismic lines
with a - 'Vibroseis' - source in P wave and vertical geo-
phones; N.1 reflection seismic survey with 'Vibroseis'
source in SH and horizontal geophones; N.1 surface
wave seismic survey with a heavy drop-weight source
and low frequency geophones.
Despite the critical landslide environment, the
large amounts of active channels, the short receiver
interval and appropriate seismic sources allow a sat-
isfying imaging of the landslide structure. Results
are in good agreement with borehole data, geological
interpretations and mechanical properties of rocks, as
estimated by other research groups.
GEOLOGICAL SETTINGS
The study area is located in the South-Eastern-Alps
that, including the Dolomite region, are separated from
the Orogenic wedge of the Alps s.s. by an important fault
system known as the Insubric (or Periadriatic) linea-
ment (Fig.1). The stratigraphic sequence outcropping in
the Vajont and adjoining valleys (‘Zemola’ and ‘Tuora’
at East, ‘Mesazzo’ at the West), covers the upper Trias
(‘Dolomia principale’) to Eocene (‘Flysch’). It is formed
by the following units: ‘Dolomia Principale’ formation,
‘Soverzene’ formation, ‘Igne’ formation, ‘Vajont’ lime-
stone, ‘Fonzaso ‘and ‘Socchèr’ formations, ‘Scaglia
Rossa’, ‘Erto’ Marls and Eocenic flysch. The Jurassic
and Cretaceous rocks (limestones and marls mainly of
the Socchér Formation), which were involved in the
landslide movement, present various degrees of fractur-
ing. These formations slid down along the “chair-like”
Fig. 1 - The studied area (N-E Italy) and the 3D hillshade
lidar overview of the Northern slope of Mount
Toc. In dotted black the landslide detachment,
in
red are the locations of the seismic investiga-
tions (see Fig. 3)
Fig. 2 - Schematic evolution of the 1963 landslide. Pre
and post event, modified from R
ossi
& s
emenza
(1965). Circle red is the investigated portion
Fig. 3 - Position map. L1 and L2 are seismic reflection
lines performed at the top of the landslide body.
R3, R5 and R6 are post-event boreholes used for
data correlation
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REFLECTION SEISMIC AND SURFACE WAVE ANALYSIS ON COMPLEX HETEROGENEOUS MEDIA:
THE CASE OF MOUNT TOC LANDSLIDE IN THE VAJONT VALLEY
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
595
SURFACE WAVE SURVEY
Surface wave analysis on Mt. Toc landslide was
REFLECTION SURVEY
The seismic lines location was forced by the
very difficult logistical site conditions, especially
in terms of source access difficulties, complexity
of the landslide main body environment and study's
needs. The aim was mainly addressed to a feasibil-
ity evaluation of the seismic method in such a pe-
culiar and complex environment. For these reasons
we used a large number of recording channels with
short trace interval in fixed spread configuration.
This approach leads to obtaining an high spatial
density, mandatory for applications on such highly
heterogeneous fields. Seismic line acquisition pa-
rameters are reported in Tab. 1. Data processing
sequence is reported in Tab. 2.
The reflection analysis on single common shot
gather reveals a complex wavefield; beyond first arriv-
als, clearly observable even at large offsets (400 -500
m), significant ground roll is present. Reflected events
are detectable in many common-shot gathers, even if
they present complex patterns and limited lateral con-
tinuity (Fig. 4). We perform cdp stack, pre-stack time
migration (Fig. 5) and depth conversion.
Tab. 1 - Reflection seismic acquisition parameters
Tab. 2 - Processing flow
Fig. 4 - L1 data example: common-shot gathers (P wav
e)
Fig. 5 - L1: Kirchhoff pre-stack time migration
Fig. 6b - Weight drop data recorded by 1 Hz 3C geopho-
ne. 3C seismograms (1=inline H, 2=crossline
H, 3=Z) and spectra b) far offset data (bottom):
Fig. 6a - Weight drop data recorded by 1 Hz 3C geophone.
3C seismograms (1=inline H, 2=crossline H,
3=Z) and spectra: a) near offset (top)
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L. PETRONIO, J. BOAGA & G. CASSIANI
596
International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
wave velocities inversions. As the example of Fig. 7
shows, the velocities inversion at 50-70 m depth is in
very good agreement with an expected level of frac-
tured layers of the Soccher formation. This will play a
relevant role in mechanical interpretation of the land-
slide, since the identification of shear and compres-
sional wave allows the estimation of the Poisson ratio
on site to be compared with lab analysis.
Figure 11 shows the interpretation of the L1 line
guided by two borehole information.
F
erri
et alii (2011) performed uniaxial and tri-
axial compressive tests on rock specimen to evaluate
strength and elastic properties of the Mt. Toc landslide
sequence. To perform on site mechanical estimation
with geophysics is crucial, due to the representative-
ness limits of specimen geotechnical characterization
at the landslide scale.
In Fig. 12 Poisson’s ratio values obtained from
laboratory measurements are compared with dynamic
performed by the use of a weight-drop controlled
source. We adopted a 250 kg weight release by the arm
of a crane at 6, 10 and 14 meters of height at 9 differ-
ent locations along the L1 line. Seismic signals were
recorded along the L1 line by 72 vertical geophones
(4 m of trace intervals) with a natural frequency of
4.5 Hz along a portion of the L1 line and by 3 low
frequency vertical receivers with natural frequency of
1 Hz, located at fixed positions (Fig. 6).
Low frequency receivers were used to perform a
surface wave dispersion analysis (n
azarian
et alii,
1983) with the FTAN technique (Frequency-time
analysis as in d
ziewonSky
et alii, 1969; L
evShin
et
alii, 1972, 1992; k
nopoFF
, 1972; k
nopoFF
& p
anza
,
1977; n
unziata
et alii, 1999). Figure 7 shows the
structure models as results of the Hedgehog non lin-
ear inversion adopted (v
aLyuSv
et alii, 1968; p
anza
,
1981; B
oyadzhiev
et alii, 2008).
REFRACTION SURVEY
P- and SH-velocities were computed from first ar-
rivals recorded during the acquisition of the P- and
SH-wave reflection survey along the L1 line.
Fig. 8 and Fig. 9 show the first arrival pickings
performed on P- and SH-wave, respectively. Direct
modelling was adopted to obtain simplified 1D ve-
locity models used to convert time to depth reflection
data and to support the data interpretation.
DISCUSSION AND CONCLUSIONS
Shear waves velocities of seismic refractions
were compared with the velocities as derived by sur-
face waves analyses. Fig. 10 shows the line drawing
of L1 line with the superposition of seismic velocity
models and boreholes data.
Surface wave analyses such as FTAN, as opposed
to refraction seismic, was able to recognize shear
Fig. 7 - Example of 2 FTAN solutions for eastern and
western receivers on Mt. Toc landslide
Fig. 8 - L1 (P-wave): first break picking
Fig. 9 - L1 (SH-wave): first break picking
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REFLECTION SEISMIC AND SURFACE WAVE ANALYSIS ON COMPLEX HETEROGENEOUS MEDIA:
THE CASE OF MOUNT TOC LANDSLIDE IN THE VAJONT VALLEY
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
597
reasons promising tools for others large landslide
case studies.
values computed by seismic velocities. Static and dy-
namic values present interesting relationship, espe-
cially for the rupture zone characterization.
The very detailed reflection/refraction seismic
together with surface waves methodologies, proved
to be able to characterize the seismic properties of
the Vajont 1963 landslide mass. These results are
particularly relevant if compared to the hard lo-
gistic environments of the large landslide. Despite
the difficult geophone coupling and limited sources
employments, the geophysical surveys show good
agreement with borehole data and allow a 2D imag-
ing of the shear zone. In particular the joint use of
very detailed refrection/refraction seismic and sur-
face waves analysis seems to considerably improve
the subsoil mechanical evaluation and are for these
Fig. 10 - L1 line depth section with velocities profiles. in
black Vp, in red Vs from refraction, in green Vs
from surface waves. On the left the R3 borehole
stratigraphy with in blue the ‘Soccher’ formation,
in orange upper fractured ‘Fonzaso’ formation, in
green lower ‘Fonzaso’ unit
Fig. 11 - Interpretation of seismic line L1. R3 and R6 are
boreholes used for correlation. In blue the Soccher
formation; in orange Fonzaso upper formation, in
green lower Fonzaso formation, in grey the Vajont
Formation. Lines profiles are as above in black
Vp, in red Vs from refraction, in green Vs from sur-
face waves. In dashed thick red the sliding surface
Fig. 12 - Static and dynamic Poisson’s ratio comparison
on Fig. 10
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