Observation and modelling of the Saint-Etienne-de-Tinée
landslide using SAR interferometry
||Institut de Physique du Globe
Département Etudes Spatiales
4 place Jussieu 75252 Paris Cedex 05, France
||Institut de Physique du Globe
Département Etudes Spatiales
4 place Jussieu 75252 Paris Cedex 05, France firstname.lastname@example.org
||BRGM, Direction de la Recherche |
3 av. C. Guillemin, BP 6009
45060 Orléans Cedex 2, France
- SAR interferometry has been shown to lead to accurate large-scale
surface displacements mapping. The study of the "La Clapière"
landslide, located in Southern France on the left bank of the
Tinée river, was carried out in order to demonstrate the
capability of interferometry to monitor displacements of small
spatial extension. In a first study, six different interferograms
have been derived from ERS-1 SAR images acquired during the Commissioning
Phase. The coherence of the associated images was shown to remain
significant over most of the surface of the landslide during the
two weeks of the survey. The interferograms, generated on a massively
parallel computer, clearly evidenced deformation fringes associated
with the landslide. They were remarkably similar, and indicated
steady-state displacements over at least 12 days. The displacement
field derived from the interferograms was modeled and shown to
be characterized by a non-uniform displacement gradient from the
top to the bottom. It also revealed a significantly faster motion
of the western part of the landslide. The amplitude of the motion
was shown to be in good agreement with ground measurements. Furthermore,
the interferograms allowed us to evidence a small-scale instability
which could not be observed with discrete ground measurements.
Finally, we present preliminary results obtained on the same site
with images acquired during the second Tandem mission. It provided
the opportunity to extend the study of the landslide, which displacements
are too high to be observed with images acquired on the standard
orbital cycles of 35 days.
- Keywords: SAR differential interferometry - Deformation
field - Saint-Etienne-de-Tinée landslide - Massively parallel
processing - Tandem.
Landslides can be a major threat to populations in mountainous
areas. Even when they occur away from inhabited areas, they can
be a significant hazard and have a serious economic impact by
blocking roads and rivers. The "La Clapière"
landslide is a good example. It is located near Nice, in Southern
France, on the left bank of the Tinée river. This landslide,
which extends over a few km2 between 1100 m and
1700 m, is bounded at the top by a high lobate scarp (fig.1).
It is also characterized by an active scree slope on the NW part.
A competent layer, known as the barre d'Iglière, produces
a sub-horizontal mechanical discontinuity at mid-level in the
Figure 1: The "La Clapière"
It threatens to obstruct the valley, and then may lead to an overflow
of the upstream village of Saint-Etienne-de-Tinée. This
hazard has been mitigated with significant road and tunnel construction.
It has also been monitored by laser-ranging since 1982. Such a
permanent monitoring requires the deployment and servicing of
several tens of laser reflectors as well as daily measurement
operations. We propose in this paper to apply the technique of
SAR interferometry (Zebker et Goldstein, 1986 ; Gabriel et
al., 1988) to this landslide, in order to demonstrate its
capability for studying small scale deformations. In particular,
we compare the characteristics of SAR monitoring derived from
this analysis to those of ground measurements
In a first study, we constructed six interferograms with images
acquired by ERS-1 with a 3-days repeat cycle, on descending orbits,
on August 20, 23, 26, 29 and September 4, 1991.The interferograms
are generated on a massively parallel computer (Connection Machine
5). The effect of topography in each interferogram is removed
using a 5 m x 5 m Digital Elevation Model
(DEM) of the site provided by Institut Géographique National
of France (Massonnet et al., 1993 and 1995). The resulting
differential interferograms are then projected from SAR geometry
to DEM geometry. They correspond to a contour map of the component
of the surface displacement field in the direction of the line
of sight of the satellite. Significant phase variations associated
with the landslide can be detected on these interferograms.
Figure 2: Geocoded differential interferograms.
(a) 23-26 pair (3 days). Bperp= 43 m. (b) 26-04
pair (6 days). Bperp= -298 m. (c) 20-29 pair
(9 days). Bperp= -4 m. (d) 26-04 pair (9 days). Bperp=
248 m. (e) 23-04 pair. Bperp= 291 m. (f)
20-04 pair. Bperp= -301 m
Figure 2 shows the 6 interferograms. The 3-days interferogram
(fig.2a) provides the clearest picture of the landslide, due to
its small baseline and a very short time interval. Its boundaries
are well described, especially the northwestern part and the 2
lobes at the top. The others present fringes with a lower SNR,
because of larger baselines and larger time intervals. All computed
interferograms are shown to be similar. The number of fringes
increases linearly with the elapsed time between the various image
acquisitions, while their overall geometry remains the same. This
suggests that the observed landslide motion is stationary over
the period surveyed. On all six interferograms, NW-SE trending
fringes attest of a downhill movement characterized by a gradient
of displacement from the top to the bottom of the landslide, the
motion decreasing towards the bottom. A full phase rotation is
equivalent to a displacement gradient of 3.9 cm along the
landslide average steepest slope. The fringe intervals are not
constant over the landslide, suggesting both downhill and lateral
variations of the displacement gradient. This gradient changes
from top-to-bottom, especially in the SE part: the gradient is
very low between the intermediate scarp and the "barre d'Iglière"
and seems to increase below this layer. This is consistent with
the hypothesis that this layer behaves as a competent layer that
blocks the movement and maintains some coherence in the upper
part of the massif. From the active scree slope towards the SE,
i.e. towards the right side, one observes a progressive increase
of the fringe separation, indicating a decrease of the displacement
gradient. This variation occurs near the N20° faults that
cut the landslide.
Since interferograms measure only one component of the displacement
(its projection on the slant-range), recovering the 3 components
of the displacement field requires some a priori hypotheses on
the mechanical behaviour of the landslide. Synthetic interferograms
have been computed for two different sliding models which represent
the most important types of slope failure: rotational and translational
slips (Bromhead, 1986 ; Giani, 1992). In rotational slip, the
sliding surface has a spoon shape, and can be approximated by
a circle in vertical cross-section. With translational slip, the
failure surface tends to be planar and roughly parallel to the
The surface displacements associated with this model are similar
to the simple tilt of a rigid block. As already shown by Peltzer
et al. (1994), we observe that the fringe separation, is not
sensitive to the curvature of the sliding surface and only depends
on the rotation of the block. With such a model, the interferogram
analysis appears therefore not to be efficient to estimate the
depth of the sliding surface, a parameter which controls the behaviour
of the landslide and the related hazard. In any case, this type
of model does not seem to describe the St-Etienne-de-Tinée
landslide since it shows fringes with a regular spacing interval
and cannot account for the observed non-uniform gradients displayed
by the interferograms.
A preliminary model based on the 20-29 interferogram was proposed
in a previous study (Fruneau et Achache, 1995). In this model,
elastic deformation along 3 major discontinuities (faults trending
N20°) was superimposed on a uniform gradient of displacement
from top-to-bottom. A rigid block of a few hundred meters was
also included in the eastern part, between the "barre d'Iglière"
and the intermediate scarp. However, when the amplitude of the
displacements is rescaled assuming a constant velocity field,
this model cannot account for the fringes observed on the new
3, 6 and 12 days interferograms derived in the present paper.
Using these additional interferograms, we derived a new model
in which elastic deformation along the major structural discontinuities
is modelled by progressive lateral decrease of the top-to-bottom
displacement gradient (Fruneau et al., 1995b; Fruneau,
1995). Figure 3a displays synthetic fringes produced by such a
displacement field with a gradient ranging from 1.5 cm/100 m
in the west of the slide to 0.5 cm/100 m in the east
above the barre d'Iglière and 1 cm/100 m below
the barre. This variation of the gradient of displacement from
the top to the bottom was introduced to further improve the fit
between observed and synthetic fringes in the eastern part of
the landslide. It may be associated with a swelling of the topography
above the "barre d'Iglière" and is consistent
with the mechanical behaviour of this layer which holds back the
upper part of the landslide. This interferogram (figure 3a) can
be compared with the 23-26 interferogram (figure 2a). The displacement
field of figure 3a can, then, be rescaled by factors 2, 3 and
4 and the resulting fringes (figure 3b, c and d) can be readily
compared with the 6, 9 and 12 days interferograms of figure 2b,
c and e, showing a satisfactory agreement. Furthermore, this modelling
confirms the stationarity of the displacements.
Figure 3: (a)-Synthetic interferogram.
A top-to-bottom gradient of displacement is gradually decreased
across the landslide from 1.5 cm/100 m in the NW to
0.5 cm/100 m in the SE. This interferogram should be
compared with the 3 days interferogram of figure 2a. (b)-The
displacement field is increased be a factor 2 with respect to
figure 3a. To be compared with the 6 days interferogram of figure
2b. (c)-The displacement field is increased by a factor
3 with respect to figure 3a. To be compared to the 9 days interferograms
of figure 2c and d. (d)-The displacement field is increased
by a factor 4 with respect to figure 3a. To be compared to the
12 days interferogram of figure 2e.
Figure 4 displays the difference between modelled and observed
fringes of the 23-26 pair. We observe a nearly uniform phase value
over the area of the landslide, indicating a good agreement between
the two interferograms over most of the sliding zone. At the eastern
top of the slide, figure 4 displays significant phase variations
over a small area. This evidences a small unit in the landslide
which movement is rapid, and which has not been taken into account
by the uniform translational model.
Figure 4: Difference between the real and the
Comparison between SAR and ground measurements
The interferometric analysis provides accurate estimates of the
displacement gradients in close agreement with existing ground
measurements. Figure 5 shows displacement vectors monitored on
ground superimposed on displacement vectors derived from our model
(which gives a smoother representation that the noisy real interferograms).
Figure 5: (a)-Displacement vector measured
on ground by laser telemetry (bleu arrows) and computed from the
model (black arrows) for the 26-04 period. (b)-Same as
figure 5a for the 23-26 period.
Some discrepancies are observed on figure 5b near the bottom of
the slide. This can be explained by the fact that interferometry
provides only the gradients of displacement. Then only relative
displacements can be evaluated because of the discontinuity of
the movements between the landslide and the steady massif and
ground reference points are necessary to determine absolute displacements.
A constant displacement corresponding to the bottom displacement
should be added to our model. In the present case, ground measurements
by laser telemetry reveal a systematic offset with the average
displacement recorded by SAR interferometry. This offset varies
from 2 to 7 mm / day over the duration of the survey
and provides an estimate of the absolute displacement at the bottom
of the landslide.
This shows the complementarity of ground and SAR measurements.
The tandem mission allowed us to circumvent the incompatibility
between the amplitude of the movements and the repeat period of
the standard ERS orbit (35 days). It offered the opportunity to
carry-on our study of the Saint-Etienne-de-Tinée landslide.
Two interferograms calculated with images from the second Tandem
mission give interessant preliminary results. On the 13-14 august
1995 interferogram (fig.6a), we observe a nearly uniform phase
change over the area of the landslide, with respect to the bulbe
of the landslide. The phase change is also observed on the second
couple (10-11 march 1996)(fig.6b), but is less uniform, and clearly
evidences the small block on the upper right part of the landslide,
which was already detected on the previous interferograms of the
previous study. It confirms the high value of displacement of
this block, and then emphasizes its instability.
Figure 6: Geocoded interferograms calculated
with ERS-1 / ERS-2 tandem images. (a)13-14 august
1995 interferogram. Bperp= 50m. (b)-10-11 march 1996 interferogram.
Bperp= 17 m.
SAR interferometry versus ground monitoring
A higher density of "measurements" can be achieved with
SAR interferometry: SAR monitoring provides a continuous displacement
field in comparison with discrete ground measurements. It allows,
in particular, to delineate the limits between the different units
of the landslide. It also allows to detect local instabilities
(in this study we detect a small block at the upper east part)
which may not be disclosed by ground measurements if it has not
been anticipated, so that laser targets can be installed on this
particular block (Achache et al., 1995). Furthermore, ground measurements
suffer from the problem of representativeness of the global motion
by some targets deployed on the site. Ground measurements are
very sensitive to local heterogeneities.
The major limitation of SAR interferometry is the loss of coherence
between the 2 images due either to changes in the orbital geometry
of the two acquisitions, to ground surface changes, or to a too
high gradient of deformation. The two orbital tracks have to be
within a few hundred meters to preserve the coherence . This limits
the number of interferograms which can be produced from satellite
images (among the 10 interferograms which could be generated with
the 5 ERS-1 images of 1991, only 6 have good coherence). Ground-surface
changes also affect directly the contribution of individual ground
targets to the phase. Coherence loss then occurs often in the
presence of vegetation or surface water. We note that the active
scree slope where there is little vegetation remains the most
coherent part. Displacements with high values of gradient, such
as those associated with the landslide, lead also to incoherence
since phase variation across a pixel exceeds one cycle. Orbit
cycles of a few days as well as Tandem configuration allows the
user to overcome this limitation of ERS data.
Of course, interferometry is limited by its "mono-component
vector" evaluation, and by the ambiguous nature of the signal,
which is known within one half of the wavelength only. Furthermore,
it provides only the gradient of displacements, and hence relative
displacements can only be evaluated by remote sensing. Reference
points are necessary to determine absolute displacements.
This investigation demonstrates the capability of SAR interferometry
to monitor surface displacements at the scale required for landslide
monitoring. The constraints of this kind of study are totally
different from the ones associated with earthquakes, due to small
spatial extension and often the high topography encountered. SAR
interferometry demonstrated its capability for studying the deformation
over small areas. We were able to construct several interferograms
on which the landslide is clearly evidenced. These interferograms
show an organized fringe system for an elapsed time as large as
12 days, allowing us to construct a steady-state model of surface
displacements valid for the whole period of observation. A simple
model of translational slide satisfactorily accounts for the observed
interferogram, and suggests the existence of a significant plastic
deformation in the vicinity of the N20° structural discontinuities
cutting the slide. The influence of the major heterogeneities
of the landslide ("barre d'Iglière", intermediate
scarp) on its mechanical behaviour can also be constrained by
the interferometric analysis. It also provides accurate estimates
of the displacement gradients in agreement with ground measurements
and even allows us to detect small blocks with enhanced displacement
which may represent a potential hazard.
- Achache, J., Fruneau, B., and Delacourt, C., 1995:
- Applicability of SAR interferometry for operational monitoring
of landslides, Proceedings of the Second ERS Applications
Workshop, London, 6-8 December 1995, p.165-168.
- Fruneau, B., 1995:
- Interférométrie différentielle d'images
SAR. Application au glissement de terrain de la Clapière,
Thèse d'université, Paris 7, 200p.
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- Détection du glissement de terrain de Saint-Etienne-de-Tinée
par interférométrie SAR et modélisation,
C.R. Acad. Sci. Paris, t.320, Série IIa,
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- Observation and modelling of the Saint-Etienne-de-Tinée
landslide using SAR interferometry, accepted in Tectonophysics.
- Gabriel, A.K., Goldstein, R.M., et Zebker, H.A., 1988:
- Mapping small elevation changes over large areas: differential
radar interferometry, J. Geophys. Res., 94, p.
- Massonnet, D., Rossi, M., Carmona, C., Adragna, F., Peltzer,
G., Feigl, K., et Rabaute, T., 1993:
- The displacement field of the Landers earthquake mapped by
radar interferometry, Nature, 364, p. 138-142.
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Keywords: ESA European
Space Agency - Agence spatiale europeenne,
observation de la terre, earth observation,
satellite remote sensing,
teledetection, geophysique, altimetrie, radar,
chimique atmospherique, geophysics, altimetry, radar,