- Email: [email protected]
SAR interferometry and field data of Randazzo landslide (Eastern Sicily, Italy)
Phys. Chem. Earth (B), Vol. 25, No. 9, pp. 771-780, 2000 64 2000 Elsevier Science Ltd All rights reserved 1464- 1909/00/$ - see front matter
Pergamon
PII: Sl464-1909(00)00100-3
SAR Interferometry
and Field Data of Randazzo Landslide (Eastern Sicily, Italy)
V. Rizo’ and M. Tesauro2 ‘CNR-IRPI, Via Cavour, Roges di Rende, Cosenza, Italy. E-mail: [email protected] 2CNR-IRECE, Via Diocleziano, Napoli, Italy. E-mail: manlio@irecel .na.cnr.it
Received 8 July 1999; accepted 22 February 2000
Abstract. With the aim of improving
our understanding of the development and potential instability of the Randazzo landslide which occurred in the months of March and April, 1996, inclinometric, piezometric and topographical data were collected and compared with the results of differential interferometric elaboration (DIFSAR), up till the end of 1998 using the data of ERSl and ERS2 satellites, that detected displacements of lcm a day. The elaborations enable us to individuate the morphology of the earth mass in movement, and to consider displacements detected in a short time-interval at the end of paroxysmal They show that after the event stage (3-4 April). displacements stopped, or were of such low velocity so as to be not detectable by this technology, apart from in limited areas. These results seem to be in accord with those obtained from inclinomehic measurements, which show displacements all within the range of instrumental error. Recently, a DIFSAR elaboration over a period of seventy days (25 August- 4 November 1998) has revealed a clear image of the entire landslide body. Coming at the end of a particularly dry period and being associated with a discrete lowering of the water table the variations could be the combined result of a small contraction of the flowed masses (not detected by inclinometric measurements) on account of drought and/or a strong variation in the dielectric constant. In either cases the detected variations are caused by the strong difference in physical properties of the flowed masses from the surrounding areas. 0 2000 Elsevier Science Ltd. All rights reserved.
mobilised a prevalently clayey flysch of 24 million cubic meters, 17 million of which flowed along two close channels having a gradient of 2530%, reaching a velocity of 120-2OOm/day. The earth masses were displaced as far as 750-900 meters, and pushing at their base progressively moved the entire foot slope, having a gradient of 16-18% and a total width of 1.3km. The foot, moving at a velocity of 4OmUay along the first and more active flow branch, caused a valley barrage and a lake. At the end the landslide showed two branches, with a total length of 2km, and a height of 450m (Fig.1). The paroxysmal stage developed from 23 March to the beginning of April 1996, and has been described in previous articles (Rizzo, 1996; Basile et al., 1996; Rizzo et al., 1998). Although existence of pre and post-paroxysmal events and deformations was referred to, their degree, beginning and end were not welI defined. In particular it was shown that instability took place before 23 March in the middle part of the slope, where a pile road had recently been constructed (Rizzo, 1996). In order to find more about previous movements, to monitor landslide development and potential instability of its incipient upper part (Rizzo, 1996), inclinometric, piezometric and topographic data were collected and compared to the results of differential interferometric elaboration (DIFSAR). SAR radar is a microwave sensor which enables the production of high definition images (l-1Om) of parts of the Earth’s surface from space. The European satellites that bear such sensors are ERSl and ERM, and utilise the C wave band and a wave length of 5.67cm. They travel one day apart at a height of about 800km returning to the same scene once every 35 days. A type of SAR elaboration is interferometric (IFSAR) which produces three-dimensional maps of the Earth’s surface; this technique is based on the evaluation of the phase difference of the two SAR images (interferogram) of the same area elaborating the data from two different angles, in other words utilising a couple of satellites on different, albeit similar, orbits. A necessary condition for generating an interferogram is that the electromagnetic characteristics of the scene must stay constant between the two acquisitions. A measurement of
1 Introduction
landslide took place on the left flank of Alcantara Valley, at the border of Etna volcano (Sicily, Italy). It
Randazzo
Correspondence to: Rizzo Vincenzo CNR-IRPI Via Cavour - Roges di Rende 87030 COSENZMTALY 771
772
V. Rizzo and M. Tesauro: SAR Interferometry and Field Data
Fig. 1. Instrumentationof Randazzo landslide. The map shows the landslide at the end of paroxysmal stage (April 1996). Data supplied by ANAS and Genio Civile of Catania. 11-H= inclinometric casing of Genie Civile; p&C. pzANAS = piezometers of Genie Civile and ANAS; 1-9, Statinn=
the change is the coherence index between the two different images (that must be~O.3). A further use of SAR is This enables differential interferometry (DIFSAR). researchers to evaluate any variations over time of the altimetric profne of the area under observation in that these variations cause a difference in the optical path and in so doing interfere with the combining of the images. Clearly, in a single interferogram the contribution of topography to the different point of view is joined to the altimetric variation. The aim of the DISFAR methodology is to separate such contributions (Collaro et al., 1997). The difference of phase due to the contribution of ahimetric variation alone (in the slant direction) is translated into an image of displacement (per pixel) with different shades of grey, which express a fraction of the wave length of the entity to be calculated; with greater displacements in wave length it becomes impossible to evaluate this entity. An initial method enables the acquisition of an interferogram from which a “synthetic” one obtained from an external reference map can be subtracted (DEM). Another technique consists in evaluating the topographical contribution starting from another interferogram which does not contain the displacement. To this end three satellites (Al, A2 A3) can be used in order to obtain topographic variations, two
ANAS
henrh
marlrs
(Al, A2) close-ups (master-slave) and a distance one which can be used to analyse topography and displacement (complementary master-slave). In general in temperate climates and in relation to landslides, the gap of 35 days is too long and this leads to considerable loss of coherence. For this reason, in the methodology used to study this landslide, as will be shown in the following section, 4 satellites from the two successive tandems ERSl/ERS2 were employed.
2
Experimental data acquisition
2.1 Topographic data survey
From June 96 to December 96 a network of 9 bench marks were periodically measured in the upper part of the landslide, by traditional topographic instruments with a theoretical accuracy of +/-lcm, in the upper part of landslide under the supervision of ANAS (Italian Road Administration). The data showed absence of movement, having differences in elevation ranging between -1.4 and 0.8 cm. Financial difficulties have made it impossible to cover the planned
V. Rizzo and M. Tesauro:
borehole 2.5
SAR Interferometry
173
and Field Data
borehole
I1
I2
7
24 1.5
1
1
0.5 0 0
10
5
15
borehole
20
25
xl
35
40
I3
depth (m)
borehole I2
borehole 11
10
0
20
30
40
0
20
10
J)
4)
borehole 15
borehole 13 IZ 7 -g ‘i y 0,8 / I
1
2-
13 1 0.5
0
Displacement profile 6s w 66
Fig. 2. Casual errors
Y
n. 1 n-2 0.3
on date bb (L u H
6 act 1997..... 3oJuJNet998...” 12 Dee 1998....
10
20
30
40
so
60
Basefde on 4 Apr 1997 cL u u, “ .‘
(above) and results (below) of inclinometric soundings in the casings 11,12,13,15, during the last two years, showing absence of significant displacements.
V. Rizzo and M. Tesauro: SAR Interferometry and Field Data
774
borehole I1
borebole I2
borehole I3
borebole IS 4 1
14
3,s _ 31 2.5 i----z--t-"
:I
21 1.5 1 1;
3
\I ;I,
0.5 i
,a
0' 0
5
0
depth
1015
20
25
30
3540
45
50
55
60
51015202530354045505560
(ml
Displacement 66 u 66
profile u 66 u
n-1 n-2 n.3 0.4
on u
date LL u
19 Ott 1996 . . .. . Baseme on 9 Sept 1996 25 Nov 19!%....” on3ODec19!?6 28 Jan 1997.....” ‘& u 14 Dee 1997.... LI
Fig. 3. Inclinotnetxic results between end of 1996 and beginning of 1997, referred to work-steps having different basefiles
@elaboration of data supplied by Genio Civile).
V. Rizzo and M. Tesauro:
SAR Interferometry
and Field Data
borehole 11
borehole I2
borehole I3
borehole I5 --
20
-
15 IO 5.
o-5 -0.5
e
5
10
15
2b5
30
Z-5
40
46
0
10
15
20
25
30
35
40
-
45
50
-10 -15 . -20 ( -2s
-2
5
55
m I
s -____
depth On) Profile n. 5: casing axis from the vertical on date 2S Nov 1996 (Basefile on 9 Sept 1996) CI a_bz u 0 64 66 66 7Oct1997( L( c1 - ) I
I
Fig. 4. Elaboration
of different
work-steps datafiles using the vertical as common reference baseline. The comparison of
various profiles proves that the displacements obtained in Fig 3 was not significant being affected by large casual errors.
GPS network (Baldassarre et al., 1997) or carry out other topographic surveys over longer periods of time. 2.2. Inclinometric sounding In the summer of 1996 five inclinometric casings were placed and monitored in three steps, having different basefiles, instruments and supervision. In the first two steps, with base-files on 9 Sept and 30 Dee 1996 the soundings and evaluation were made by a private company, under the supervision of the local Public Works Authority (Genio Civile of Catania); the third step with base-file on 09 April was made by CNR-IRPI of Cosenza. The probe utilized by Public Works Authority mounted a Wheastone bridge type sensor (SEGEA Company); while CNR-IRPI utilized more precise equipment mounting a servo-accelerometer (SINCO model n.50325). All the acquired data were processed using our program comparing the data-files in different ways: 1) either referring all the data files to the initial data in order to have the sequence of relative tube displacement; 2) or comparing at every date the tube position in respect to me vertical instead to the initial reading and subtracting the results in order to have deformation, in order to compare the different work-steps. Bore-hole soundings carried out on several
occasions on the same date, gave the amount of casual error, distributing in a triangular area with a maximum value located at the top of the casing, ranging between 0.7cm and 2.5cm (Fig 2). On the basis of evaluation of the data it was established that the casing deformation obtained by simple comparisons the initial readings (the respective step-work basefiles), as considered by the Public Works Authority (Fig. 3), was not significant being affected by relevant casual errors; such conclusion was point out looking at the difference from the vertical of the casing axis (Fig.4). The subsequent CNRIRPI data elaboration, from April 1997 to December 1998, showed very small casing displacement, being near to the field of previously detected errors (Fig 2). 2.3. Ground water level measurement and rainfall data At the same time, from 1996 to 1998, water levels in open standpipes and bore-holes supplied by ANAS, were measured. The collected data was then compared with rainfall amount at the Montelaguardia Station (Skm from the site; 690m above sea level). The monthly rainfall showed a remarkable reduction in 1998 accompanied by a general lowering of ground-water levels in the monitored holes (Fig 5).
V. Rizzo and M. Tesauro:
116
SAR Interferometry
and Field Data
Monthly mln data In the monitored time Interval
D
monthly rainfall number of rainy days In the month
t
!
Landslide
1
16
-
14
-
12
-
10
: 8
7-l
t
month
WATER
LEVEL
Very
dry period
-
pz8/GC
5 pz9lANAS
__
moderate
of groundwater
5 x 6
month pr3fGC
I
pz4Kic
I
m pzZANAS
e pz3iANAS
I
pz4/ANAS
0 pz5lANAS
D pzl OIANAS
P ptl
I
lowering
)_ % = 6
6 ii3 z
@ % 5
__----I) pzl EC Q pzZ/GC
and
pzS/GC
m pzG/GC I
pz6fANAS
level
g N
--.
I
s a 5
__-
0 pr7IGC q pz7lANAS
1 IANAS
Fig. 5. Lowering of groundwater levels (below) and monthly rainfall (above) showing dry period, after landslide.
Table 1. Characteristics of utilized satellite data.
Date February 1996
Orbitn
ERSI-ERS2
ERS I-ERS2
27-28
24165-4492
FG3IlE
BII
BI
141
-70 m
-138 m
April 1996
3-4
24666-4993
141
-55 m
-126 m
May 1996
7-8
25167-5494
141
-61 m
-129 m
August-Nov 1998
26-4
17518-18520
141
-10 m
-33 m
(Collaro et al., 1997). However, coherence maps can help 3
DIFSAR methodology and results
The DIFSAR technique has already been demonstrated to be an important tool for earth displacement analysis (lv%Wsonnetet al., 1993). In the case of landslides, lack of coherence is the most relevant limitation in its application
assess the quality of the results; to this aim a shorter gap
between satellite times would be helpful. In the case under study we used ERS-l/ERS-2 acquisitions (Tab. 1) and as a consequence the sampling rate was very long (35 days).All the 35day interferograms were affected by lack of. coherence due to temporal de-correlation,
V. Rizzo and M. ‘fesauro:
Fig. 6. Two DIFSAR elaborations of the slant direction (prey tone denotes the topography of April and the data distribution. Note the differences at movement after 4 April,
SAR Interferometry
and Field Data
landslide displacements between 3 and 4 April. The white and black pixels are absence of displacements). In 6A=3-4 April DIFSAR, using the topography of of May for the difference; the picture shows geomorphologic interpretation of landslide foot in the two elaborations as consequence of topography change
777
respectively lowered and lifted area in May. In 6B=3-4 April DIFSAR using displaced masses on the base of pixel from May to June, due to landslide
178
V. Rizzo and M. Tesauro:
SAR Interferometry
although the baselines were small enough for DIFSAR applications. The only coherent interferograms were those in tandem (ERS-l/ERS-2), being only one day apart. In this case the problem of temporal correlation is negligible and coherence is lost only from the high velocity of mass movements. The topographic contribution was removed by using a second tandem acquisition and scaling the unwrapped phase in order to take into account the different baseline values. The methodology used in this case employed four readings of the same scene: two for the estimation of the typography and two for displacement (Collar0 et al., 1997). The use of four readings was motivated by the difficulty in having three images availableand generating two interferograms with coherence indices above 0.3. On the other hand with four images, coherent in pairs, it is easier since ERSl and ERS2 are separated by only 24 hours.
and Field Data
Note that Table 1 also includes a 70 day repeat ERS-2 acquisition which shows a high coherence level due to the dry weather in that period. Only three elaborations were successful in detecting landslide movements. Although the landslide rapidly became dormant, the data furnished a relevant contribution to the understanding some aspects of its evolution, such as: a) it was seen that the deformations pointed out in previous works (Rizzo, 1996; Rizzo 1998), developed in January and February 1996 had only local or temporary relevance and/or low velocity, not being detected by SAR elaboration (DIFSAR on 27-28 Febrary 1996); b) . a similar DIFSAR on 3-4 April 1996, using the topography of May) shows the same tendency to lowering in the upper part, but a remarkable change of movement polarity at the foot of the landslide branches (detectable by a colour change from predominantly white to black
Fig. 7. DIFSAR of 7-8 May (above) and showing local movements (evidenced by circles; R is a georeferenced). DIFSAR over a dry period of 70 days (25 Aug-4 Nov 1998) showing a clear image of landslide body (left).
V. Rizzo and M. Tesauro:
cJ.ozc
I--
f
I
SAR Interferometry
and Field Data
I
0.015
0
010
5.005
7’-
s.000
0.005
0
150
Fig. 8. Earth displacement profiles in the satellite slant direction and relative to the A and B elaboration showing similar lowering in the upper part of landslide (going far from satellite) of about Icm
200
of Figure 6 (3-4 April 1996).
780
V. Rizzo and M. Tesauro: SAR Interferometry and Field Data
(Fig 6A), probably due to the topographic variation between April and May; c) it was seen that near the end of paroxysmal stage (DIFSAR of 3-4 April 1996; Fig 6A), the movements were active over all the landslide body, including the masses along the scarp of upper incipient slide; d) at the same time the upper part of the landslide showed a general tendency to lowering (white pixels), while along the flows and at landslide foot prevailed the uplifting and a distribution interpretable as the result of different kinematism of sliding bodies (the morphological interpretation is given in the same figure; Fig. 6B); e) the slope foot, at the base of the last flow branch (flow 2 in Fig 1) and until 3-4 April 1996, was still not involved in movement; f) the instability in the upper part of landslide appears to continue over the highest scarp, over a triangular area, bordered by faults (Rizzo, 1996), probably due to tectonic dependence (Figs 6A and 6B). g) it was seen that after April 1996 and until second half of 98 (DIFSAR of 7-8 May and after) the landslide had already stopped, except at a few local points, involving 5 sites for lo-20 pixels at the most (Fig 7); h) the amount of lowering in the upper part of the landslide (of the masses placed along the landslide crown) was measured showing homogeneous displacements of about I2cm a day (3-4 April; Fig 8); i) the DIFSAR of 26Aug-4Nov 1998, over a period of 70 days, shows a clear image of the two flow bodies due to physical variation (Fig. 7). The variation may be explained as a consequence of very small movements -that were not detected by inclinometric soundings (
4 Conclusion The data furnished further interesting information on landslide displacements, as well on potential of DIFSAR methodology in this field. At present the loss of coherence due to the time existing between two successive SAR covers of the scene (the availability of ERSl/ERS2 tandem pair is occasional) represents the most serious drawback to the large scale use of this technology. In the examined event the DIFSAR elaboration offered good coherence and significant results, in particular using
tandem data (over two days), and helped us outline the landslide kinematics in the sampled times. On the base of acquired data it was seen that postparoxysmal movements of this landslide were limited to very restricted areas. Pre-paroxysmal movements, described in previous works were not detected, probably being limited in extension and having very slow velocity; but at the same time the low scene coherence, as consequence of wet weather, drastically reduced the number of elaborations . Regarding inclinometric sounding, the data show the importance of accuracy and quality of equipment in the value and significance of results. The data elaborations indicate that all the inclinometric casings, after September 1996 and until the end of 1998 were not subjected to significant deformations. At the same time after a very dry season at the end of 1998 a DIFSAR elaboration shows in the phase of movements a clear images of the landslide, which theoretically should be a consequence of small movements over all the landslide. The latter may be explained by a mass contraction of the unsaturated part over all the landslide body or ascribed to an homogeneous variation of its dielectric constant. on the effectiveness of the detected landslide movements. In any case the DIFSAR methodology has vast potential and a very high pixel accuracy. We would like to thank the geologist F. Fragale for help in editing. This work is financed by GNDCI: publication II. 2086.
Acknowledgements.
References Baldassarre, G., Caprioli. M., and Rizzo, V., Utilization of GPS methodology for a control network and georeference of a vast and complex landslide, Eastern Sicily, Italy, Proc ISthICC Cartographic Conference, Stockholm, June, 1997, vo1.3,1533-1540,1997. Basile, G., Ferrara, V., and Pappalardo, G., Mmanismi e fasi evolutive della frana di Randaxzo nell’aha valle de1 Fiutm Alcantara, Sicilia NE, Proc “La pcevenxione delle catastrofi idrogeologiche: il contribute della ricerca scientifica”, AIba, Italy, November, Edited by Fabio Luino, GNDCI, Vol.l,125-134, 1996. Collaro, A., Franceschetti, G., Sansosti, E. and Tesauro M., Interferometria diffefenxiale e sue applicaxioui. Rot. 97th Anttual AEI Meeting, Session 4, Baveno, Italy, May, t-6.1997. Rizzo, V.. Eventi idrologici Ixedisponenti e caratteristiche cinematiche di uu fenomeno fianoso complesso, Proc “La pfevenxione delle catastrofl idrogeologiche: il coutributo della ticerca scientifica”, Alba, Italy, November, Edited by Fabio Luino, GNDCI, Vol.l,31-44,1996. Rixzo. V., and Terrauova, 0.. Triggering factors aud development of a complex landslide, Randaxzo, northeastern Sicily, Italy”, Pm. 8th International IAEG Congress, Balketna, Rotterdam 1661-1668,199s.
Pergamon
PII: Sl464-1909(00)00100-3
SAR Interferometry
and Field Data of Randazzo Landslide (Eastern Sicily, Italy)
V. Rizo’ and M. Tesauro2 ‘CNR-IRPI, Via Cavour, Roges di Rende, Cosenza, Italy. E-mail: [email protected] 2CNR-IRECE, Via Diocleziano, Napoli, Italy. E-mail: manlio@irecel .na.cnr.it
Received 8 July 1999; accepted 22 February 2000
Abstract. With the aim of improving
our understanding of the development and potential instability of the Randazzo landslide which occurred in the months of March and April, 1996, inclinometric, piezometric and topographical data were collected and compared with the results of differential interferometric elaboration (DIFSAR), up till the end of 1998 using the data of ERSl and ERS2 satellites, that detected displacements of lcm a day. The elaborations enable us to individuate the morphology of the earth mass in movement, and to consider displacements detected in a short time-interval at the end of paroxysmal They show that after the event stage (3-4 April). displacements stopped, or were of such low velocity so as to be not detectable by this technology, apart from in limited areas. These results seem to be in accord with those obtained from inclinomehic measurements, which show displacements all within the range of instrumental error. Recently, a DIFSAR elaboration over a period of seventy days (25 August- 4 November 1998) has revealed a clear image of the entire landslide body. Coming at the end of a particularly dry period and being associated with a discrete lowering of the water table the variations could be the combined result of a small contraction of the flowed masses (not detected by inclinometric measurements) on account of drought and/or a strong variation in the dielectric constant. In either cases the detected variations are caused by the strong difference in physical properties of the flowed masses from the surrounding areas. 0 2000 Elsevier Science Ltd. All rights reserved.
mobilised a prevalently clayey flysch of 24 million cubic meters, 17 million of which flowed along two close channels having a gradient of 2530%, reaching a velocity of 120-2OOm/day. The earth masses were displaced as far as 750-900 meters, and pushing at their base progressively moved the entire foot slope, having a gradient of 16-18% and a total width of 1.3km. The foot, moving at a velocity of 4OmUay along the first and more active flow branch, caused a valley barrage and a lake. At the end the landslide showed two branches, with a total length of 2km, and a height of 450m (Fig.1). The paroxysmal stage developed from 23 March to the beginning of April 1996, and has been described in previous articles (Rizzo, 1996; Basile et al., 1996; Rizzo et al., 1998). Although existence of pre and post-paroxysmal events and deformations was referred to, their degree, beginning and end were not welI defined. In particular it was shown that instability took place before 23 March in the middle part of the slope, where a pile road had recently been constructed (Rizzo, 1996). In order to find more about previous movements, to monitor landslide development and potential instability of its incipient upper part (Rizzo, 1996), inclinometric, piezometric and topographic data were collected and compared to the results of differential interferometric elaboration (DIFSAR). SAR radar is a microwave sensor which enables the production of high definition images (l-1Om) of parts of the Earth’s surface from space. The European satellites that bear such sensors are ERSl and ERM, and utilise the C wave band and a wave length of 5.67cm. They travel one day apart at a height of about 800km returning to the same scene once every 35 days. A type of SAR elaboration is interferometric (IFSAR) which produces three-dimensional maps of the Earth’s surface; this technique is based on the evaluation of the phase difference of the two SAR images (interferogram) of the same area elaborating the data from two different angles, in other words utilising a couple of satellites on different, albeit similar, orbits. A necessary condition for generating an interferogram is that the electromagnetic characteristics of the scene must stay constant between the two acquisitions. A measurement of
1 Introduction
landslide took place on the left flank of Alcantara Valley, at the border of Etna volcano (Sicily, Italy). It
Randazzo
Correspondence to: Rizzo Vincenzo CNR-IRPI Via Cavour - Roges di Rende 87030 COSENZMTALY 771
772
V. Rizzo and M. Tesauro: SAR Interferometry and Field Data
Fig. 1. Instrumentationof Randazzo landslide. The map shows the landslide at the end of paroxysmal stage (April 1996). Data supplied by ANAS and Genio Civile of Catania. 11-H= inclinometric casing of Genie Civile; p&C. pzANAS = piezometers of Genie Civile and ANAS; 1-9, Statinn=
the change is the coherence index between the two different images (that must be~O.3). A further use of SAR is This enables differential interferometry (DIFSAR). researchers to evaluate any variations over time of the altimetric profne of the area under observation in that these variations cause a difference in the optical path and in so doing interfere with the combining of the images. Clearly, in a single interferogram the contribution of topography to the different point of view is joined to the altimetric variation. The aim of the DISFAR methodology is to separate such contributions (Collaro et al., 1997). The difference of phase due to the contribution of ahimetric variation alone (in the slant direction) is translated into an image of displacement (per pixel) with different shades of grey, which express a fraction of the wave length of the entity to be calculated; with greater displacements in wave length it becomes impossible to evaluate this entity. An initial method enables the acquisition of an interferogram from which a “synthetic” one obtained from an external reference map can be subtracted (DEM). Another technique consists in evaluating the topographical contribution starting from another interferogram which does not contain the displacement. To this end three satellites (Al, A2 A3) can be used in order to obtain topographic variations, two
ANAS
henrh
marlrs
(Al, A2) close-ups (master-slave) and a distance one which can be used to analyse topography and displacement (complementary master-slave). In general in temperate climates and in relation to landslides, the gap of 35 days is too long and this leads to considerable loss of coherence. For this reason, in the methodology used to study this landslide, as will be shown in the following section, 4 satellites from the two successive tandems ERSl/ERS2 were employed.
2
Experimental data acquisition
2.1 Topographic data survey
From June 96 to December 96 a network of 9 bench marks were periodically measured in the upper part of the landslide, by traditional topographic instruments with a theoretical accuracy of +/-lcm, in the upper part of landslide under the supervision of ANAS (Italian Road Administration). The data showed absence of movement, having differences in elevation ranging between -1.4 and 0.8 cm. Financial difficulties have made it impossible to cover the planned
V. Rizzo and M. Tesauro:
borehole 2.5
SAR Interferometry
173
and Field Data
borehole
I1
I2
7
24 1.5
1
1
0.5 0 0
10
5
15
borehole
20
25
xl
35
40
I3
depth (m)
borehole I2
borehole 11
10
0
20
30
40
0
20
10
J)
4)
borehole 15
borehole 13 IZ 7 -g ‘i y 0,8 / I
1
2-
13 1 0.5
0
Displacement profile 6s w 66
Fig. 2. Casual errors
Y
n. 1 n-2 0.3
on date bb (L u H
6 act 1997..... 3oJuJNet998...” 12 Dee 1998....
10
20
30
40
so
60
Basefde on 4 Apr 1997 cL u u, “ .‘
(above) and results (below) of inclinometric soundings in the casings 11,12,13,15, during the last two years, showing absence of significant displacements.
V. Rizzo and M. Tesauro: SAR Interferometry and Field Data
774
borehole I1
borebole I2
borehole I3
borebole IS 4 1
14
3,s _ 31 2.5 i----z--t-"
:I
21 1.5 1 1;
3
\I ;I,
0.5 i
,a
0' 0
5
0
depth
1015
20
25
30
3540
45
50
55
60
51015202530354045505560
(ml
Displacement 66 u 66
profile u 66 u
n-1 n-2 n.3 0.4
on u
date LL u
19 Ott 1996 . . .. . Baseme on 9 Sept 1996 25 Nov 19!%....” on3ODec19!?6 28 Jan 1997.....” ‘& u 14 Dee 1997.... LI
Fig. 3. Inclinotnetxic results between end of 1996 and beginning of 1997, referred to work-steps having different basefiles
@elaboration of data supplied by Genio Civile).
V. Rizzo and M. Tesauro:
SAR Interferometry
and Field Data
borehole 11
borehole I2
borehole I3
borehole I5 --
20
-
15 IO 5.
o-5 -0.5
e
5
10
15
2b5
30
Z-5
40
46
0
10
15
20
25
30
35
40
-
45
50
-10 -15 . -20 ( -2s
-2
5
55
m I
s -____
depth On) Profile n. 5: casing axis from the vertical on date 2S Nov 1996 (Basefile on 9 Sept 1996) CI a_bz u 0 64 66 66 7Oct1997( L( c1 - ) I
I
Fig. 4. Elaboration
of different
work-steps datafiles using the vertical as common reference baseline. The comparison of
various profiles proves that the displacements obtained in Fig 3 was not significant being affected by large casual errors.
GPS network (Baldassarre et al., 1997) or carry out other topographic surveys over longer periods of time. 2.2. Inclinometric sounding In the summer of 1996 five inclinometric casings were placed and monitored in three steps, having different basefiles, instruments and supervision. In the first two steps, with base-files on 9 Sept and 30 Dee 1996 the soundings and evaluation were made by a private company, under the supervision of the local Public Works Authority (Genio Civile of Catania); the third step with base-file on 09 April was made by CNR-IRPI of Cosenza. The probe utilized by Public Works Authority mounted a Wheastone bridge type sensor (SEGEA Company); while CNR-IRPI utilized more precise equipment mounting a servo-accelerometer (SINCO model n.50325). All the acquired data were processed using our program comparing the data-files in different ways: 1) either referring all the data files to the initial data in order to have the sequence of relative tube displacement; 2) or comparing at every date the tube position in respect to me vertical instead to the initial reading and subtracting the results in order to have deformation, in order to compare the different work-steps. Bore-hole soundings carried out on several
occasions on the same date, gave the amount of casual error, distributing in a triangular area with a maximum value located at the top of the casing, ranging between 0.7cm and 2.5cm (Fig 2). On the basis of evaluation of the data it was established that the casing deformation obtained by simple comparisons the initial readings (the respective step-work basefiles), as considered by the Public Works Authority (Fig. 3), was not significant being affected by relevant casual errors; such conclusion was point out looking at the difference from the vertical of the casing axis (Fig.4). The subsequent CNRIRPI data elaboration, from April 1997 to December 1998, showed very small casing displacement, being near to the field of previously detected errors (Fig 2). 2.3. Ground water level measurement and rainfall data At the same time, from 1996 to 1998, water levels in open standpipes and bore-holes supplied by ANAS, were measured. The collected data was then compared with rainfall amount at the Montelaguardia Station (Skm from the site; 690m above sea level). The monthly rainfall showed a remarkable reduction in 1998 accompanied by a general lowering of ground-water levels in the monitored holes (Fig 5).
V. Rizzo and M. Tesauro:
116
SAR Interferometry
and Field Data
Monthly mln data In the monitored time Interval
D
monthly rainfall number of rainy days In the month
t
!
Landslide
1
16
-
14
-
12
-
10
: 8
7-l
t
month
WATER
LEVEL
Very
dry period
-
pz8/GC
5 pz9lANAS
__
moderate
of groundwater
5 x 6
month pr3fGC
I
pz4Kic
I
m pzZANAS
e pz3iANAS
I
pz4/ANAS
0 pz5lANAS
D pzl OIANAS
P ptl
I
lowering
)_ % = 6
6 ii3 z
@ % 5
__----I) pzl EC Q pzZ/GC
and
pzS/GC
m pzG/GC I
pz6fANAS
level
g N
--.
I
s a 5
__-
0 pr7IGC q pz7lANAS
1 IANAS
Fig. 5. Lowering of groundwater levels (below) and monthly rainfall (above) showing dry period, after landslide.
Table 1. Characteristics of utilized satellite data.
Date February 1996
Orbitn
ERSI-ERS2
ERS I-ERS2
27-28
24165-4492
FG3IlE
BII
BI
141
-70 m
-138 m
April 1996
3-4
24666-4993
141
-55 m
-126 m
May 1996
7-8
25167-5494
141
-61 m
-129 m
August-Nov 1998
26-4
17518-18520
141
-10 m
-33 m
(Collaro et al., 1997). However, coherence maps can help 3
DIFSAR methodology and results
The DIFSAR technique has already been demonstrated to be an important tool for earth displacement analysis (lv%Wsonnetet al., 1993). In the case of landslides, lack of coherence is the most relevant limitation in its application
assess the quality of the results; to this aim a shorter gap
between satellite times would be helpful. In the case under study we used ERS-l/ERS-2 acquisitions (Tab. 1) and as a consequence the sampling rate was very long (35 days).All the 35day interferograms were affected by lack of. coherence due to temporal de-correlation,
V. Rizzo and M. ‘fesauro:
Fig. 6. Two DIFSAR elaborations of the slant direction (prey tone denotes the topography of April and the data distribution. Note the differences at movement after 4 April,
SAR Interferometry
and Field Data
landslide displacements between 3 and 4 April. The white and black pixels are absence of displacements). In 6A=3-4 April DIFSAR, using the topography of of May for the difference; the picture shows geomorphologic interpretation of landslide foot in the two elaborations as consequence of topography change
777
respectively lowered and lifted area in May. In 6B=3-4 April DIFSAR using displaced masses on the base of pixel from May to June, due to landslide
178
V. Rizzo and M. Tesauro:
SAR Interferometry
although the baselines were small enough for DIFSAR applications. The only coherent interferograms were those in tandem (ERS-l/ERS-2), being only one day apart. In this case the problem of temporal correlation is negligible and coherence is lost only from the high velocity of mass movements. The topographic contribution was removed by using a second tandem acquisition and scaling the unwrapped phase in order to take into account the different baseline values. The methodology used in this case employed four readings of the same scene: two for the estimation of the typography and two for displacement (Collar0 et al., 1997). The use of four readings was motivated by the difficulty in having three images availableand generating two interferograms with coherence indices above 0.3. On the other hand with four images, coherent in pairs, it is easier since ERSl and ERS2 are separated by only 24 hours.
and Field Data
Note that Table 1 also includes a 70 day repeat ERS-2 acquisition which shows a high coherence level due to the dry weather in that period. Only three elaborations were successful in detecting landslide movements. Although the landslide rapidly became dormant, the data furnished a relevant contribution to the understanding some aspects of its evolution, such as: a) it was seen that the deformations pointed out in previous works (Rizzo, 1996; Rizzo 1998), developed in January and February 1996 had only local or temporary relevance and/or low velocity, not being detected by SAR elaboration (DIFSAR on 27-28 Febrary 1996); b) . a similar DIFSAR on 3-4 April 1996, using the topography of May) shows the same tendency to lowering in the upper part, but a remarkable change of movement polarity at the foot of the landslide branches (detectable by a colour change from predominantly white to black
Fig. 7. DIFSAR of 7-8 May (above) and showing local movements (evidenced by circles; R is a georeferenced). DIFSAR over a dry period of 70 days (25 Aug-4 Nov 1998) showing a clear image of landslide body (left).
V. Rizzo and M. Tesauro:
cJ.ozc
I--
f
I
SAR Interferometry
and Field Data
I
0.015
0
010
5.005
7’-
s.000
0.005
0
150
Fig. 8. Earth displacement profiles in the satellite slant direction and relative to the A and B elaboration showing similar lowering in the upper part of landslide (going far from satellite) of about Icm
200
of Figure 6 (3-4 April 1996).
780
V. Rizzo and M. Tesauro: SAR Interferometry and Field Data
(Fig 6A), probably due to the topographic variation between April and May; c) it was seen that near the end of paroxysmal stage (DIFSAR of 3-4 April 1996; Fig 6A), the movements were active over all the landslide body, including the masses along the scarp of upper incipient slide; d) at the same time the upper part of the landslide showed a general tendency to lowering (white pixels), while along the flows and at landslide foot prevailed the uplifting and a distribution interpretable as the result of different kinematism of sliding bodies (the morphological interpretation is given in the same figure; Fig. 6B); e) the slope foot, at the base of the last flow branch (flow 2 in Fig 1) and until 3-4 April 1996, was still not involved in movement; f) the instability in the upper part of landslide appears to continue over the highest scarp, over a triangular area, bordered by faults (Rizzo, 1996), probably due to tectonic dependence (Figs 6A and 6B). g) it was seen that after April 1996 and until second half of 98 (DIFSAR of 7-8 May and after) the landslide had already stopped, except at a few local points, involving 5 sites for lo-20 pixels at the most (Fig 7); h) the amount of lowering in the upper part of the landslide (of the masses placed along the landslide crown) was measured showing homogeneous displacements of about I2cm a day (3-4 April; Fig 8); i) the DIFSAR of 26Aug-4Nov 1998, over a period of 70 days, shows a clear image of the two flow bodies due to physical variation (Fig. 7). The variation may be explained as a consequence of very small movements -that were not detected by inclinometric soundings (
4 Conclusion The data furnished further interesting information on landslide displacements, as well on potential of DIFSAR methodology in this field. At present the loss of coherence due to the time existing between two successive SAR covers of the scene (the availability of ERSl/ERS2 tandem pair is occasional) represents the most serious drawback to the large scale use of this technology. In the examined event the DIFSAR elaboration offered good coherence and significant results, in particular using
tandem data (over two days), and helped us outline the landslide kinematics in the sampled times. On the base of acquired data it was seen that postparoxysmal movements of this landslide were limited to very restricted areas. Pre-paroxysmal movements, described in previous works were not detected, probably being limited in extension and having very slow velocity; but at the same time the low scene coherence, as consequence of wet weather, drastically reduced the number of elaborations . Regarding inclinometric sounding, the data show the importance of accuracy and quality of equipment in the value and significance of results. The data elaborations indicate that all the inclinometric casings, after September 1996 and until the end of 1998 were not subjected to significant deformations. At the same time after a very dry season at the end of 1998 a DIFSAR elaboration shows in the phase of movements a clear images of the landslide, which theoretically should be a consequence of small movements over all the landslide. The latter may be explained by a mass contraction of the unsaturated part over all the landslide body or ascribed to an homogeneous variation of its dielectric constant. on the effectiveness of the detected landslide movements. In any case the DIFSAR methodology has vast potential and a very high pixel accuracy. We would like to thank the geologist F. Fragale for help in editing. This work is financed by GNDCI: publication II. 2086.
Acknowledgements.
References Baldassarre, G., Caprioli. M., and Rizzo, V., Utilization of GPS methodology for a control network and georeference of a vast and complex landslide, Eastern Sicily, Italy, Proc ISthICC Cartographic Conference, Stockholm, June, 1997, vo1.3,1533-1540,1997. Basile, G., Ferrara, V., and Pappalardo, G., Mmanismi e fasi evolutive della frana di Randaxzo nell’aha valle de1 Fiutm Alcantara, Sicilia NE, Proc “La pcevenxione delle catastrofi idrogeologiche: il contribute della ricerca scientifica”, AIba, Italy, November, Edited by Fabio Luino, GNDCI, Vol.l,125-134, 1996. Collaro, A., Franceschetti, G., Sansosti, E. and Tesauro M., Interferometria diffefenxiale e sue applicaxioui. Rot. 97th Anttual AEI Meeting, Session 4, Baveno, Italy, May, t-6.1997. Rizzo, V.. Eventi idrologici Ixedisponenti e caratteristiche cinematiche di uu fenomeno fianoso complesso, Proc “La pfevenxione delle catastrofl idrogeologiche: il coutributo della ticerca scientifica”, Alba, Italy, November, Edited by Fabio Luino, GNDCI, Vol.l,31-44,1996. Rixzo. V., and Terrauova, 0.. Triggering factors aud development of a complex landslide, Randaxzo, northeastern Sicily, Italy”, Pm. 8th International IAEG Congress, Balketna, Rotterdam 1661-1668,199s.