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Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
555
DOI: 10.4408/IJEGE.2013-06.B-53
3D GEOPHYSICAL IMAGING OF THE VAJONT LANDSLIDE
AND OF ITS SURROUNDINGS
R
obeRto
FRANCESE
(*)
, M
assiMo
GIORGI
(*)
, G
ualtieRo
BÖHM
(*)
, a
ndRea
BISTACCHI
(**)
,
a
ldino
BONDESAN
(***)
, M
atteo
MASSIRONI
(***)
& R
inaldo
GENEVOIS
(***)
(*)
OGS - Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - Trieste, Italy
(**)
Università degli Studi di Milano Bicocca - Department of Earth and Environmental Sciences - Milan, Italy
(***)
Università degli Studi di Padova - Dipartimento di Geoscienze- Padua, Italy
Contact author: roberto.francese@inogs.it
slide. In the eastern lobe the geophysical images clear-
ly outlined several detachment planes that disrupt the
continuity of the pre-landslide geology. The seismic
and the resistivity tomography obtained inverting the
data resulted comparable indicating a proper choice of
the measuring techniques and of the field parameters.
K
ey
words
: Vajont landslide, 3D geophysical imaging, sei-
smic velocity, electrical resistivity, Southern Alps
INTRODUCTION
The Vajont area has been studied in details before
and after the occurrence of the 1963 catastrophe. It
is well known that in the late evening of October 9th
the northern slope of the Monte Toc collapsed causing
almost 2000 victims mainly in the Piave valley where
the huge wave that overtopped the dam wiped out the
town of Longarone and several other small villages.
The discovery of the palaeolandslide in 1959 (G
iudici
& s
eMenza
, 1960), at the time of the dam construc-
tion, was a major impulse in the study of geology, of
seismicity and of morphology of the Vajont valley
(M
ülleR
, 1959; G
iudici
& s
eMenza
, 1960; s
eMenza
,
1960). Many others data were collected in the fol-
lowing 50 years (R
ossi
& s
eMenza
, 1965, M
aRtinis
,
1978; H
endRon
& P
atton
, 1985; s
eMenza
& G
HiRotti
,
2000) as the Vajont was one of the most catastrophic
landslides occurred in the modern times in the western
world. An updated and comprehensive review is given
by G
enevois
& G
HiRotti
(2005).
ABSTRACT
The 1963 collapse of the northern slope of the
Monte Toc in the Vajont reservoir is probably one
of the most studied landslides worldwide. During
the various studies several numerical models have
been proposed to explain the collapse dynamics but
a comprehensive and reliable insight into the failure
kinematics is still missing. A major step forward in
the understanding of the landslide mechanisms is rep-
resented by the reconstruction of the geometry of the
different geological units within the landslide body
and of the associated physical properties. A large scale
geophysical experiment based on 2D and 3D seismic
and resistivity was undertaken to address the issue.
Prior to the main survey electrical resistivity and lon-
gitudinal and transversal wave velocity of the litho-
logical units involved in the landslide were measured
along the exposure of the rock wall below the village
of Casso. The geophysical images of the two landslide
lobes showed a very good correlation with the refer-
ence section. Particularly a conductive unit located in
the bottom part of the stratigraphic sequence resulted
an excellent geophysical marker. Some important
structural records, recently defined on the sliding sur-
face, were imaged in the geophysical profiles of the
western lobe indicating a limited internal deformation
along the east-west axis during the slide. In the deeper
part of the western lobe the geophysical image ap-
pears rather complicated because of the partial overlap
of the various lithological units occurred during the
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R. FRANCESE, M. GIORGI, G. BÖHM, A. BISTACCHI, A. BONDESAN, M. MASSIRONI & R. GENEVOIS
556
International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
GEOLOGICAL SETTING
The stratigraphy of the Vajont valley is comprised
of the typical Jurassic-Cretaceous carbonate sequences
of the South-Eastern Alps. The different formational
units have been widely described in several papers
(c
aRloni
e
M
azzanti
, 1964; s
eMenza
, 1965; M
aRti
-
nis
, 1978) with minor differences in the geological in-
terpretation. The thickness of the formations changes
along and across the valley primarily due to a depo-
sitional hiatus dated to the Late Jurassic - Early Cre-
taceous. The Jurassic sequence (Fig. 1) is comprised
of the massive Vajont Limestone (350-400 m), of the
layered cherty limestone of the Fonzaso Formation
(10-40 m) and of the nodular limestone of the Am-
monitico Rosso Formation (0-15 m). This latter unit is
not continuous and is reported only by some authors.
The Cretaceous sequence is comprised of the Soc-
cher Limestone (200-250 m) and of the layered marly
limestones and marls of the Scaglia Rossa Formation
(ca. 300 m). The units involved in the landslide are the
upper part of the Fonzaso Formation and the Soccher
Limestone Formation. In these last two Formations
R
ossi
& s
eMenza
(1965) further defined six lithologi-
cal members indicated with letters from a to f in Fig. 1.
In the upper part of the Fonzaso Formation (unit
a’) thin clayey interbeds (from few centimetres to dec-
imetres) are reported in the layered cherty limestone
(H
endRon
& P
atton
, 1985). These interbeds were indi-
cated by H
endRon
& P
atton
(1985) as the stratigraphic
level where the primary sliding occurred caused by
overpressured groundwater. There are just few out-
crops of this unit as during the collapse the unit was
somewhat crunched along the sliding surface. Unit a’
is comprised of an alternation of layered cherty lime-
stone and marly limestones. According to R
ossi
& s
e
-
Menza
(1965) the bottom level of unit a’’ is represented
by the nodular limestones of the Ammonitico Rosso
Formation. Unit b is a thin conglomerate layer and is
very important because it is a stratigraphic marker both
within the landslide and in the surroundings outcrops.
Units c, d and e are comprised of massive limestones
grading to layered marly and cherty limestones. Finally
unit f is comprised of layered marly cherty limestones.
The landslide body is comprised of three major
lobes (s
eMenza
, 1965): the Massalezza lobe and two
separate masses (defined as the “eastern lobe” and the
“western lobe”) failed ad a latter time after the wash-
out of the Massalezza lobe.
Although several extensive studies were carried
out since the 1963 event and many authors proposed
different theories to explain the failure and the collapse
behaviour (K
ilbuRn
& P
etley
, 2003; P
etley
& P
et
-
ley
, 2006) some of the factors controlling triggering
and dynamic of the landslide are still not completely
clear (b
istaccHi
et alii, in this volume). Among the
major issues there is the velocity of the sliding mass
itself that caused such an unexpected large wave.
The accuracy of the geological (R
ossi
& s
eMenza
,
1965) and topographic data (s
eMenza
, 2001) collected
before the failure allow for a detailed modelling of the
geometry and partially also the dynamics of the mass
movement. An important improvement of the collapse
model and an initial validation of the associated theo-
ries could be obtained by a better characterization of the
geometry and of the physical properties of the different
geological units embedded in the landslide body. For the
above reasons a major geophysical exploration program
of the Vajont landslide and of its surroundings, based on
2D/3D seismic and resistivity imaging, was undertaken.
The geophysical experiment focuses on two points: a)
comparing P- and S-wave velocity and resistivity fields
of the various lithological units inside and outside the
landslide zone; b) mapping the hidden geometry of the
various lithological units within the landslide body.
Presently very few geophysical data are avail-
able for the Vajont area. Some P-wave profiles were
collected before the 1963 collapse, on the palaeolan-
dslide, to assess the rock quality and few others dur-
ing the following years to confirm the reliability of the
pre-1963 results.
The new geophysical investigation reported here
was designed and conducted integrating the re-inter-
preted geological (b
istaccHi
et alii, in this volume)
and structural data (M
assiRoni
et alii, in this volume) as
well as aerial radiometry and aerial and terrestrial laser
scanning data. These last two datasets were extremely
useful to constrain the inversion of resistivity and seis-
mic data and model the deformation of the electrical
field caused by the rough topography of the landslide
mass. The reliability of the new geophysical images is
discussed in details and a correlation between lithol-
ogy and physical parameters is proposed both for the
outcropping in-situ bedrock sequence and for the land-
slide mass. This 3D physical model of the landslide
introduces a series of new constrains for an accurate
numerical simulation of the landslide kinematics.
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3D GEOPHYSICAL IMAGING OF THE VAJONT LANDSLIDE AND OF ITS SURROUNDINGS
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
557
ments on the body of the Vajont landslide strongly
depends upon the definition of a reliable and accu-
rate physical reference model of the upper part of the
Fonzaso Formation and of the Soccher Limestone
Formation. This model represents the key to tie the
geophysical response (in this case seismic and elec-
trical properties) to lithology and to understand the
buried geometry of the different units in the landslide
and furthermore to get some insight in the degree of
fracturing of the landslide mass itself. The coherency
of the rocks could be used as a first indicator of the
their geo-physical properties. Hard rocks (HR in Fig.
1) are probably characterized both by high velocity
and high resistivity while soft rocks (SR in Fig. 1)
are low velocity and low resistivity. In this simpli-
fied approach units a’, a’’ and f could be classified
as moderately soft rocks while units b, c, d and e are
mostly referable to moderately hard rocks.
Some boreholes, drilled after the 1963 event
(Fig. 3), could be used as a major constrains in as-
sisting geophysical data analysis and processing.
The majority of these boreholes reached the depth of
Under the structural point of view a review of
available data and new observations has been recently
completed (M
assiRoni
et alii, in this volume). In the
northern slope of the Monte Toc there is the interfer-
ence of the well known E-W trending Erto syncline
(G
iudici
& s
eMenza
, 1960) and of a N-S trending syn-
cline with it’s axis elongated along the pre-landslide
Massalezza valley. The interference is clearly visible
in the exposed sliding surface where a series of small
and medium scale N-S trending structures fold the
Erto syncline. The same folding pattern is also visible
outside the landslide area (b
istaccHi
et alii, in this vol-
ume). These structural features appear to have a strong
influence on kinematics of the landslide (c
astellanza
et alii, in this volume). In particular the association of
the stratigraphy and of the N-trending bedding planes
with the curved shape of the sliding surface appeared
to be the key factors for the 1963 collapse.
THE GEOPHYSICAL DATABASE
The effectiveness of the geophysical experi-
ments and of the associated 2D and 3D measure-
Fig. 1 - Stratigraphy of the Vajont landslide and of its surroundings (left), members indicated with letters from a to f have been
originally defined by R
ossy
& s
emenza
(1965) and were involved in the landslide. Am.R. = Ammonitico Rosso. SLS =
Sliding surface. Unit b (the conglomerate layer) is a stratigraphic marker and it appears as a compact layer located
just above the terrace. The horizontal scale represents an attempt of rock quality assessment based on lithology and
bedding thickness. SR: Soft Rocks; MSR: Moderately Soft Rocks; MHR: Moderately Hard Rocks; HR: Hard Rocks.
Members a’’ to f as they appear in the rock wall below the village of Casso (right)
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558
International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
quite realistic and more and less similar to the values
measured during the second seismic survey.
Seismic velocity and electrical resistivity of a
medium depend mostly upon lithology (density), po-
rosity/fracturing and fluid content (t
elfoRd
et alii,
1990; d
obRin
& s
avit
, 1988). Fluid content in the Va-
jont landslide body is almost zero due to the very high
permeability of the collapsed mass. This is confirmed
by the absolute absence of water in the boreholes.
Repetitive attempts to measure the water level dur-
ing the geophysical campaign were made but a water
table has been never detected. The groundwater table
in the landslide body is probably controlled by the
water level in the residual lake. In the system there
are still two unknowns (lithology and porosity/frac-
turing) and one equation (P-wave velocity or resistiv-
ity). To reduce the uncertainty resistivity and P-wave
velocity should be measured on the same formations
but outside the landslide.
On the rock wall below the village of Casso (Fig.
1) there is a full exposure of units from a’’ to f (almost
the entire sequence involved in the landslide). A refer-
ence geophysical profile was collected here to gain a
specific insight in the properties of the different geo-
logical units. The only missing unit is a’ that’s covered
by the talus deposits.
the in-situ bedrock but, unfortunately, the stratigra-
phy of several wells is not reported in the scientific
literature.
The few geophysical data available for the Va-
jont area refer to P-wave profiles. A seismic survey
was conducted immediately after the discovery of the
paleolandslide (c
aloi
& s
Padea
, 1960) in the north-
ern slope of the Monte Toc. Survey results showed
surprisingly high (5000-6000 m/s) P-wave velocity
values in the uppermost layers indicating potential
data inconsistencies (s
eMenza
, 1965). A second seis-
mic survey was carried out immediately later than
the discovery of the perimetrical crack in October
1960 (s
eMenza
, 2001). More realistic P-wave ve-
locity values (2500-3000 m/s) were measured dur-
ing this second survey. After the 1963 collapse the
Court of Belluno required a new seismic survey with
the major goal of collecting data inside and outside
the landslide area. P-wave velocity was measured at
few locations inside (from borehole to borehole) and
outside the landslide area (M
oRelli
& c
aRabelli
,
1965). The two datasets are not directly compara-
ble because the travelpath in the boreholes crosses
different geological units. P-wave velocity values
of the Cretaceous sequence outside the landslide
range from 2100 m/s to 3000 m/s. These numbers are
Fig. 2 - Geophysical imaging on the rock wall below the village of Casso. The measuring lines have been deployed on the
exposures of Fig. 1. Lithological units a’’ to f in the Soccher Limestone Formation (left). Resistivity response (middle)
and P-wave seismic response (right)
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3D GEOPHYSICAL IMAGING OF THE VAJONT LANDSLIDE AND OF ITS SURROUNDINGS
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
559
channels were not connected to the amplifier. The bot-
tom 8 channels of the resistivity line were deployed in
the talus deposits (Fig. 2).
Seismic data were collected on the Massalezza
lobe only (lobe A in Fig. 3) using a DMT Summit mod-
ular system with more than 200 double channel A/D
conversion units. Each receiving station was equipped
with a three component 10-Hz geophone to detect the
incoming signal. The three component sensors were
laid out along four lines (L100, L200, L300, L400).
The average station spacing was 10.0 m for a total of
276*3 live channels (Fig. 3). Elastic waves were gen-
erated and propagated into the ground using a truck-
mounted IVI Minivib vibrating source (both in P-wave
and S-wave modes). The sweep was set to 15 s with a
frequency ranging from 10 to 350 Hz. The triggering
signal was radio transmitted to the command console.
The source stations were located along the boundary
roads with an average interval of 20 m (Fig. 3). The
resulting dataset was comprised of 162 shots covering
an area of approximately 500,000 square metres.
Recorded seismic data were generally of good
quality; first breaks, in P-wave mode, were sharp and
GEOPHYSICAL DATA ACQUISITION
AND PROCESSING
Geophysical data acquisition was somewhat chal-
lenging because of the complex morphology of the
landslide and the associated complications in cou-
pling geophones and electrodes especially when the
soil was comprised of hard rocks.
The reference profile (Fig. 1 and Fig. 2) was com-
prised of a 24-channel seismic line and of a 48-elec-
trode resistivity line. In the seismic signal was de-
tected using 10-Hz three component sensors spaced of
10 m while the electrodes, in the resistivity line, were
spaced of 5 m and data were recorded using the Wen-
ner and the Dipole-Dipole configurations. Coupling of
the seismic sensors and of the electrodes was achieved
drilling a series of holes in the rock wall. Geophones
were firmly tightened to climbing screws while the
electrodes were hammered into holes filled with con-
ductive medical gel. Data resulted good quality and
the tomographic inversion of both the two datasets
was carried out with a minor misfit (less than 5%).
The seismic line resulted a little bit shorter (210 m) as
compared to the electrical line because the bottom 4
Fig. 3 - Key features of the geophysical campaign on the Vajont landslide (rem. Pole = remote pole; stratigraphy a
/
na =
borehole with “available / not available” stratigraphy)
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International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
easy to pick even at offsets larger than 500 m while in
S-wave mode the signal was slightly lower amplitude.
In P-wave mode a total of about 55000 travel-times
were picked and pre-processed prior to invert the data
using a combined 3D refraction/reflection tomogra-
phy. Inversion was carried out with the CAT3D propri-
etary software. A staggered grid approach (v
esnaveR
& b
oHM
, 2000) was utilised to improve the horizontal
and vertical resolution in the inverted dataset.
Resistivity data were collected on the two landslide
lobes (A and B in Fig. 3) using a 48-electrodes Syscal
R1 system. A 2D line, with 20 m electrode separation,
was collected approximately in the middle of the lobe
and recorded in Wenner configuration. Additional 3D
data were collected in Pole-Dipole configuration with
four remote poles located in the vicinity of the bound-
ary of the single lobe. Each resistivity volume was
comprised of approximately 6000 data-points. Several
others 2D lines, with 5 m electrode spacing, covered
significant zones within the landslide. In order to mini-
mize the effects of important changes in the water con-
tent resistivity data were collected in separate sessions
during early spring and middle autumn after three days
of heavy rain. The wet soil conditions significantly im-
proved the coupling of the electrodes.
Resistivity data also resulted of good quality and
just few points needed to be removed from the data-
set prior to run the 2D/3D inversion. Bad data-points
removal was mostly focused on the Q values larger
than zero and on the high geometric factors (K). Few
data points were further removed because of instru-
mental noise. The inversion was carried out using the
package ERTLAB+ that is based on a sophisticated
reweight of the inversion parameters after each itera-
tion (M
oRelli
& l
a
b
Recque
, 1996).
RESULTS AND DISCUSSION
THE REFERENCE PROFILE
Unit a’’ in the lower part of the rock wall appears
to be low-velocity (2200-2700 m/s) and low-resistivi-
ty (0.5-1.1 Kohm*m). The raising of the near surface
resistivity in the elevation interval 780-800 m is due to
the coarse pebbly deposits of the talus debris. Unit b,
due to its reduced thickness (less than 4 m), is outside
the resolution capability of both the two techniques.
Units c, d and e in the middle part of the rock wall
appear to have a similar response. In these units the
velocity (3400-3800 m/s) and the resistivity (2.5-4.5
Kohm*m) are higher as compared to unit a’’ indicat-
ing the massive character of the limestones. At least
three very resistive domains are visible in the resistiv-
ity image. The consistency of the rocks in these zones
is probably very high and much higher with respect to
the average of the middle portion of the rock wall. At
the top of the rock wall, where unit f outcrops, there
is a sudden lowering of both the resistivity (0.5-1.0
Kohm*m) and the velocity (2300-2800 m/s).
THE LANDSLIDE BODY
In the inverted 2D and 3D dataset the vertical
and horizontal resolution could be approximately 10
m in the vicinity of the geophysical lines. The depth
of investigation of both the seismic and the resistiv-
ity data, although sufficient to detect the in-situ bed-
rock, was not adequate to resolve it.
Resistivity in the landslide body ranges from
0.15-0.20 Kohm*m to values larger than 3.50-4.00
Kohm*m.
The resistivity image along the 2D profile ERT1
(Fig. 4) appears to be quite complicated. The tri-di-
mensional geometry of the geological structures as-
sociated with the nature of the electrical field that has
a similar sensitivity below and on both the two sides
of the profile generated a complex response. Resistiv-
ity values (ranging from 0.25 to about 3.0 Kohm) ap-
pear to be slightly lower as compared to the reference
profile. This is probably related to the high degree of
fracturing of several units within the landslide. Unfor-
tunately along profile ERT1 there are no borehole data
available to constrain the interpretation. Stratigraphy
of borehole S5E was not found in the literature
In order to achieve a reliable interpretation the
deep conductive unit a’’ (Fig. 4) was utilized to de-
fine the geological layout along the profile. The most
prominent structure is a narrow syncline fold located
in the middle portion of the profile with its axis prob-
ably elongated along the old Massalezza valley. The
structure is consistent with the one of the in-situ bed-
rock visible in the scarp area. The bottom layers are
probably belonging to the a’’ unit while in the top lay-
ers there are the typical lithologies of the c unit; the
high resistive uppermost layer is a thick fluvio-glacial
deposit exposed at different spots in the Massalezza
valley. In the left portion of the profile are visible some
resistive bodies that could be associated with a partly
undifferentiated c-d-e lithological sequence. The near
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3D GEOPHYSICAL IMAGING OF THE VAJONT LANDSLIDE AND OF ITS SURROUNDINGS
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
561
angle detachment planes (see for example at the x-
coordinate interval 200-300 m) that could explain the
larger thickness of the c, d and e units in depth.
The resistivity image along profile ERT5 (Fig. 5),
that intersects lobe B, shows a framework more and
less comparable with the post-failure geological map
of R
ossi
& s
eMenza
(1965). The outcropping litho-
logical layers are represented by an undifferentiated
unit a covering almost the entire Col Tramontin (lo-
cated approximately in the centre of lobe B of Fig.
3) and by some smaller spots belonging to the unit
b. The general structure appears to be folded with
some major internal discontinuities. In this case also
the stratigraphy of boreholes S6C and S11D was not
available to constrain interpretation. The geometry
of the bottom layers of lithological unit a’’ was uti-
lized as a geophysical marker to define the geological
layout. In the reference section (Fig. 1) unit a’ is not
exposed and hence there are no indications about its
possible geophysical response. Regarding the thick-
ness of lithological units a’ and a’’ former indications
from c
aRloni
& M
azzanti
(1964) suggest a total
surface conductive unit, visible in the interval 50-200
m, according to R
ossi
& s
eMenza
(1965) still belongs
to units c, d and e. In this case either the mass is very
fractured or unit d in the landslide is more conductive
than in the reference section. In the eastern section of
the profile, starting from the x-coordinate 550 m, there
is a clear overlapping of a near surface conductive
body on the eastern flank of the deep a’’ unit.
In profile ERT1 the resistivity contacts are folded
with an apparent N-S trend (Fig. 4). According to
M
assiRoni
et alii (in this volume) these structures,
clearly visible in the sliding surface, were surprisingly
preserved in the landslide body confirming the hy-
pothesis of a sliding mode with marginal disturbance
of structural features. Furthermore the c-d-e sequence
exhibits a larger thickness as compared to the refer-
ence section. This is probably due to a duplication
of the units caused by some detachment phenomena
occurred, during the failure, along existing or newly
developed discontinuities. Some of these high angle
discontinuities have already been reported by R
ossi
& s
eMenza
(1965) but there are also some others low
Fig. 4 - 2D Resistivity image along tomography ERT1 (top). Profile is oriented from W to E (see Fig. 3). Preliminary inter-
pretation (bottom). Letters from a to f represent the lithological units as defined by R
ossi
& s
emenza
(1965); Electri-
cal resistivity in unit a’’, ranges from 0.50 Kohm*m to 1.10 Kohm*m while in the c-d-e sequence it ranges from 1.7
Kohm*m to values larger then 2.0 Kohm*m. The lowering of the resistivity in the c-d-e sequence (as compared to the
reference section) is probably due to severe fracturing. qd: quaternary deposits. SLS: sliding surface. The groundwa-
ter table (GWT) probably corresponds to the water level in the residual lake (635 m a.s.l.) The investigated landslide
mass could be then considered dry. See text for description
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R. FRANCESE, M. GIORGI, G. BÖHM, A. BISTACCHI, A. BONDESAN, M. MASSIRONI & R. GENEVOIS
562
International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
Fig. 5 - 2D resistivity image along tomography ERT5 (top). Profile is oriented from SW to NE (see Fig. 3). Preliminary inter-
pretation (bottom). Letters from a to f represent the lithological units as defined by R
ossi
& s
emenza
(1965); Electrical
resistivity in unit a’’, ranges from 0.50 Kohm*m to 1.10 Kohm*m. qd: quaternary deposits. Reference resistivity values
for unit a’ are not available. SLS: sliding surface. The groundwater table (GWT) probably corresponds to the water level
in the residual lake (635 m a.s.l.) The investigated landslide mass could be then considered dry. See text for description
Fig. 6 - Comparison between 3D seismic (tomography (top) and 2D electrical tomography (bottom) response. The profiles are
oriented from W to E. The geophysical data are presented as 10m by 10m cell scalars without interpolation. The two
sections are computed along the trace of profile ERT1. See text for description
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3D GEOPHYSICAL IMAGING OF THE VAJONT LANDSLIDE AND OF ITS SURROUNDINGS
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
563
deep resistive layer below a conductive layer requires
very large AB spacing to be sampled by an electrical
field that is forced into the conductor. The seismic data
are reliable because there is a geophone line exactly
along the Massalezza ditch (Fig. 3) and the sources
are located on the nearby road. According to the pre-
vious consideration a high velocity layer (or high
resistivity) below unit a’’ is quite difficult to explain
without assuming the presence of a detachment plane
that duplicate the sequence. In the right side of the ve-
locity profile there is an east-verging structure that is
also visible in the resistivity section. The near surface
high-resistivity values that have no correspondence in
the seismic image are due to the presence of debris
with a large percentage of voids. In the left side of
the velocity profile there are some observable differ-
ences between P-wave velocities and resistivity val-
ues. The near surface is low velocity in the uppermost
30 m and this low velocity layer is laterally continu-
ous while in the electrical section two high-resistivity
zones are visible. The difference is probably due to the
averaging effect of the algorithm (diving wave) used
to invert the seismic volume. The stratigraphy of the
key boreholes is not presently available to constrain
the geophysical interpretation. A possible strategy to
minimize this limitation was to compare the different
geophysical responses and also to compare the geo-
physical data with the surface geology.
CONCLUSIONS
The 2D and 3D geophysical parameterization of
large landslide accumulation masses represents a great
challenge because the displacement of the geological/
lithological units generally occurring during the failure
and often corresponding to a high complexity in the
spatial distribution of the physical properties within the
landslide body. The Vajont landslide makes no differ-
ence and because of its size is also more complicated.
The geophysical investigation was undertaken with a
large effort and it is still in progress but the authors are
aware that in various areas of the landslide the reso-
lution should be increased. This last issue is vital to
support a reliable subsurface reconstruction and hence
proper modelling of the landslide dynamics. Although
geology has been studied in details before and after the
failure the geometry of the buried units it is still par-
tially unknown. Borehole stratigraphy is only partially
reliable because there are no evident markers and also
thickness of about 80-90 for the undifferentiated unit
a. The maximum thickness of unit a’’ is around 50
m. According to several authors (R
ossi
& s
eMenza
,
1965; M
aRtinis
, 1978; H
endRon
& P
atton
, 1985)
unit a’ is similar to unit a’’ with the sole difference
of the presence of some thin interbedded clays in the
basal layers. The resistivity signature of units a’ and
a’’ should be then comparable. Regarding these con-
siderations the increase of the resistivity values in the
deeper portion of profile ERT5 appears rather diffi-
cult to explain. There are basically two ways to inter-
pret this behaviour. In the first case the upper part of
unit a’ is more resistive than expected (Fig. 5) while
in the second and more realistic case in the deeper
portion of profile ERT5 unit a’’ overthrusts the resis-
tive terrains belonging to the c-d units. As mentioned
above several high and low angle discontinuities are
clearly visible in the profile. These planes disrupt the
former continuity of the geological layers generating
an ensemble of pop-up like structures. Unfortunately
on this lobe the sliding surface is not constrained by
post-landslide data and also during the failure oc-
curred a partial overlap of the eastern lobe on the
western one. The deeper portion of the geophysical
image could be then really complex.
The P-wave velocity values in the landslide body
range from 700-1000 m/s to values larger than 3500-
4000 m/s in those spots where the rocks are still very
compact. Seismic data resulted comparable to electri-
cal data with a reasonable degree of confidence.
The correspondence between resistivity and P-
wave velocity has been analysed in details along
profile ERT1 (Fig. 6) on lobe A of the landslide. The
P-wave profile was extracted from the 3D subsurface
velocity model while the resistivity data belongs to a
standard 2D acquisition. A 3D resistivity volume of
lobe has been also generated with the aid of four re-
mote poles (Fig. 3) and although the data quality is
encouraging it has not been yet analysed in details.
The syncline structure below the Massalezza ditch
is visible also in the seismic data but it is not com-
pletely resolved as in the resistivity image probably
because the geophone line is located 60 m northern of
the electrode line. Right in the middle of the P-wave
section there is a high-velocity bottom layer that is
not visible in the resistivity section. The high-velocity
layer is located at a depth where the signal to noise ra-
tio of the resistivity data is very low. It is known that a
background image
R. FRANCESE, M. GIORGI, G. BÖHM, A. BISTACCHI, A. BONDESAN, M. MASSIRONI & R. GENEVOIS
564
International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
the 1963 failure and before at the time of the former
landslide was acting in the north-south direction. This is
clearly visible in resistivity profile ERT5, collected on
lobe B, where the entire geophysical layout appears to
be affected by folds with east-west axes.
The quaternary coverage, mostly glacial and talus
deposits with a thickness of few to several metres,
which could be a major obstacle to reconstruct subsur-
face geology, did not create any problem for the geo-
physical investigation. These deposits are generally in
dry conditions or just wet after a rainfall. The presence
of an abundant silty matrix allowed for a better cou-
pling of the sensors resulting in a minor distortion of
the field to be measured.
Further work that will be undertaken in a very
near future includes (1) geophysical parameterization
of unit a’, (2) inversion of the electrical data into a
3D resistivity volume of the entire landslide, (3) cor-
relation of the seismic and of the resistivity responses
to improve interpretation and finally (4) processing of
S-wave data in order to estimate the elastic parameters
of the landslide body.
ACKNOWLEDGMENTS
We acknowledge the Friuli Venezia-Giulia Re-
gion for providing the funding for the project (project
35935/2010). A special thank to Alessia Rosolen for
her personal support and to Ketty Segatti. A final
thank to Giovanni Rigatto for his assistance during
field operations.
correlating well information in such a laterally variable
environment is very critical. The microfossil logs in
the boreholes confirm the existence of several detach-
ment planes that cause sequence duplications.
The different units involved in the landslide, due
to the lithological changes were expected to have a
distinct geophysical signature. The seismic and re-
sistivity profiles collected along the rock wall below
Casso confirmed this hypothesis and the landslide se-
quence from the bottom to the top is a sort of sandwich
of conductive (low velocity) - resistive (high velocity)
- conductive (low velocity) layers. No reference data
were collected for the 35-40 m thick bottom unit a’ as
it was not exposed along the reference section.
Resistivity and seismic imaging within the land-
slide body confirmed the general observations from the
reference section. The conductive unit a’’, because of
its position in the sequence that is just below the strati-
graphic marker b, was chosen as a geophysical marker
for the initial interpretation. The pre-landslide stratigra-
phy appears well preserved in the very near surface lay-
ers, but in depth the geophysical response often exhibits
complex images. In the subsurface of lobe A both the
resistivity and the seismic images along profile ERT1
traced a syncline structure that was also recognized in
the exposed record of the sliding surface. In addition in
the same profile the geophysical layout shows a series
of other small-scale folds compatible with a stress field
acting along the east-west direction. These structures
are probably pre-landslide as the stress occurred during
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