Application of the SBAS-DInSAR technique for deformation monitoring in Tunis City and Mornag plain

Geomatics, Natural Hazards and Risk
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/tgnh20
Application of the SBAS-DInSAR technique for
deformation monitoring in Tunis City and Mornag
plain
Anis Chaabani & Benoit Deffontaines
To cite this article: Anis Chaabani & Benoit Deffontaines (2020) Application of the SBAS-DInSAR
technique for deformation monitoring in Tunis City and Mornag plain, Geomatics, Natural Hazards
and Risk, 11:1, 1346-1377, DOI: 10.1080/19475705.2020.1788654
To link to this article: https://doi.org/10.1080/19475705.2020.1788654
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GEOMATICS, NATURAL HAZARDS AND RISK
2020, VOL. 11, NO. 01, 1346–1377
https://doi.org/10.1080/19475705.2020.1788654
Application of the SBAS-DInSAR technique for
deformation monitoring in Tunis City and Mornag plain
Anis Chaabania and Benoit Deffontainesb,c
a
Department of Hydrology and Water Resources Management, Faculty of Meteorology, Environment
& Arid Land Agriculture, King Abdulaziz University, Jeddah, Kingdom of Saudi Arabia; bUniversite
Paris-Est (UPEM), F-77454, Marne-la-Vallee, France; cLab. Int. Assoc. D3E N 536, CNRS-MOST,
France-Taiwan
ABSTRACT
ARTICLE HISTORY
The ground deformation phenomena are the result of the application of natural stress (tectonic) or anthropogenic stress (overexploitation of groundwater, embankment) on the soil. The land
deformation may cause several impacts on the environment and
human life. A good understanding of the deformation behaviour
of the soil is essential to mitigate risks on the infrastructure and
environment. In this study, a radar interferometry is carried out to
analyse the mobility of an urban and suburban area of Greater
Tunis region. The interferometric synthetic aperture radar (SAR)
technique has been proven to be an effective methodology for
detecting and monitoring soil displacement with millimetre accuracy and also, improving our understanding of the current deformations in the study area. Indeed, the application of differential
SAR interferometry made it possible to determine the origin of
the surface deformations related to natural or anthropic phenomena. In this paper, the interferometric method of small baseline
subset (SBAS) developed by Berardino et al. 2002has been
chosen, in order to monitor the spatial and temporal pattern of
the deformation phenomena in Greater Tunis region. The analysis
of Envisat ASAR (2003–2007, descending satellite orbit) and
Sentinel 1B (2016–2018, ascending satellite orbit) SAR data
allowed us to create deformation velocity maps associated with
line of sight displacement time series. The results obtained by
this method showed the existence of subsidence phenomenon in
the urban areas (Tunis City) as well as in the rural areas (Mornag
plain). In the first site (Tunis City), the subsidence rate reaches up
to 13.98 mm/year and 19 mm/year during 2003–2007 and
2016–2018, respectively. The ground deformation that is detected
around the region of Tunis Lake and Sebkhet Essijoumi, is probably due to the nature of the highly compressible and thickness
alluvial deposits and whose substratum depth sometimes reaches
more than 60 m. In the second studied site, ground subsidence
was identified in the whole plain of Mornag. Indeed, the comparison between piezometric data and time series deformations
shows that the trend of soil subsidence is coherent with the
dynamic change in groundwater levels. Furthermore, the analysis
Received 12 December 2019
Accepted 21 June 2020
CONTACT Anis Chaabani
KEYWORDS
Ground deformation;
DinSAR; SBAS; remote
sensing; groundwater;
geotechnics; Tunis
City; Mornag
achaabani@kau.edu.sa
ß 2020 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/
licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is
properly cited.
GEOMATICS, NATURAL HAZARDS AND RISK
1347
of velocity deformation, geological data and hydrogeological
information allows us to associate all the detected settlement patterns to groundwater overexploitation and compressible alluvium
all over the plain. This overexploitation of groundwater in Mornag
plain is mainly caused by the expansion of agricultural and industrial activities and the decrease in annual rainfall in recent years
(from 2003 to 2018).
1. Introduction
Ground subsidence is generally the sudden sinking or gradual settling of the earth’s
surface (Galloway et al. 1999). This phenomenon may be due to natural effects such
as tectonic and compaction of sediments and/or human activities (anthropogenic
processes), such as excessive exploitation of groundwater, natural gas, oil, etc. Land
subsidence may lead to dangerous consequences that gravely impact human life. It
represents a major risk for natural environment and country’s national economy.
Indeed, in urban areas, continuous ground lowering for a long time may cause serious problems to linear infrastructures, buildings and underground facilities. Several
studies have focused on serious damages caused by subsidence in urban or rural
areas, in order to reduce the adverse impact of land subsidence on the environment
and infrastructures (Hu et al. 2004; Cascini et al. 2006; Castellazzi et al. 2016). In this
work, we performed an investigation of the surface deformation phenomena and its
mechanism occurring in Greater Tunis area (Grand Tunis). This study area represents the main socio-economic centre of Tunisia. Under strong demographic pressure, many new constructions and highways were built on the edge of marshy zones
and in unstable wet areas. Rapidly, land subsidence appeared in this area newly
invested by project partners. On the other hand, in the peri-urban area of Mornag
plain, drinking water resources have come under heavy pressure, especially due to
agricultural and industrial activities. As a result, the aquifers of this region are more
and more solicited by pumping. This excessive groundwater extraction has led to a
deficit in groundwater recharge, which has sometimes reached an alarming level.
Indeed, the overexploitation of Mornag aquifer has triggered a reduction of water
storage, which has been resulted in significant regional subsidence and the symptoms
are expressed by superficial soil deformation. Many methods are used to monitor
land subsidence such as global positioning system (GPS), extensometer and traditional levelling (Diao et al. 2018). Generally, these conventional techniques allow us
to perform precise and reliable measurements but they are very time consuming and
have a high cost for installation and maintenance (Ferretti et al. 2007). In order to
address the limitations of classical methods, the differential interferometric synthetic
aperture radar (DInSAR) has proved their capabilities to measure subsidence deformation with a relative low cost and a precise ground deformation monitoring. The
monitoring of land deformation using DInSAR technique is measured by calculating
the phase difference between two synthetic aperture radar (SAR) images captured at
different times (Rosen et al. 2000). In addition, in the last 20 years, this valuable
approach has demonstrated its ability to detect anthropogenic or/and natural
1348
A. CHAABANI AND B. DEFFONTAINES
Figure 1. The geographic location of the region of Tunis. Source: Author
subsidence phenomena (Deffontaines et al. 2017, 2018), with high spatial resolution
and high density of surface measurements, over a wide area and several durations.
Nevertheless, the measuring of ground movements using DInSAR technology may be
affected by many factors such as atmospheric problems and temporal and spatial
decorrelation. To overcome these imperfections, the small baseline subset (SBAS)
technique (Berardino et al. 2002) was used to improve the accuracy of the subsidence
monitoring in Greater Tunis area.
For this purpose, in this study, we tried to detect and monitor ground deformation
affecting the study area through the generation of deformation rate maps and displacement time series. In fact, during this research, we aimed to explore risk areas in
the city of Tunis and its surroundings, particularly the areas located near the Lake of
Tunis and Sebkhet Essijoumi whose soil is being compacted on the one hand, and in
Mornag plain, whose phreatic system has been overexploited over the past
two decades.
2. Study area
Two case studies in Greater Tunis region that monitor ground movement
were presented.
2.1. Tunis City
The city of Tunis covers an area of almost 220 km2 (Figure 1). It is characterized by
high population density which is greater than 4000 persons/km2. The city has been
developed on a series of hills with approximate direction N-S forming a bead corresponding to the places known as Montfleury, Kasbah, Ras Tabia, Notre Dame de
GEOMATICS, NATURAL HAZARDS AND RISK
1349
Figure 2. Assembly of the geological maps (1:50.000) of Ariana, Marsa, Tunis and La Goulette
(Source: modified from ONM, Ferchichi et al. 2017).
Tunis, Saida Manoubia and La Rabta. This zone is limited to the East and West by
two large depressions corresponding respectively to:
Tunis Lake spreads out along the depression of Ariana, part of which is occupied
by Sebkhet of Ariana;
Sebkhet Essijoumi is spread out over the plain of Mannouba-Oued Ellil.
2.1.1. Geotechnics, geology and seismic context
The geotechnical work describes the filling of the depressions of Tunis Lake and
Sebkhet Essijoumi. It is generally a filling of Pliocene and Quaternary clay and sand,
containing a metric level of limestone (El May et al. 2010). The urbanization spreads
across the side of the medina quarter that is located on the shores of Tunis Lake.
This is an old marshy area that experienced in recent years, many disorders on some
buildings. Similarly, on the eastern side of the medina, shanty towns developed and
invaded a large part of the Essijoumi depression.
As regards tectonics, Sebkhet Essijoumi and Tunis Lake correspond to the graben
bordered by faults. The tectonic control would be responsible for the thickening of
the Mio-Plio-Quaternary series belonging to these two depressions. The assembly of
1:50.000 scale geological maps of Ariana, La Marsa, La Goulette and Tunis (Figure
2) shows that the outcrops of the Cretaceous, Jurassic and Eocene carbonate formations constitute the mountainous areas and the Quaternary alluvial deposits constitute
1350
A. CHAABANI AND B. DEFFONTAINES
Figure 3. Study area (Mornag plain). Source: Author.
the most extensive outcrops of the city of Tunis. These cover all the plains and a significant part of the reliefs located in the study area.
Tunisia’s seismicity is moderated compared with other Mediterranean regions such
as Algeria and Morocco (Kacem 2004; Mansour et al. 2013). On 1 December 1970, a
largest earthquake occurred in Tunis City with a magnitude of Richter Scale 5.1 and
caused severe damage to some buildings without causing civilian casualties (Ambraseys
1962; Mellouli 1984; Vogt 1993). However, before the twentieth century, the region of
Tunis was affected by two major earthquakes that were considered destructive. The first
event occurred in December 856, with a magnitude of 6.2 and an intensity of IX, causing
the death of 45,000 victims (Mansour et al. 2013). The second earthquake hit the region
in 1758 with an intensity of IX destroying a large number of houses and leadi (Ferchichi,
Ben Hamouda and Ben Mammou 2017)ng to several deaths (Ambraseys 1962; Mansour
et al. 2013).
2.2. The Mornag plain
The plain of Mornag is located 20 km to the south of Tunis City, which represents
the future extension of the city (Figure 3). It is a coastal zone of the Mediterranean
basin, which is bounded in the South-East by tectonic alignment formed by the
Zaghouan thrust which corresponds to the alignment of the Boukornine, Ressas and
Zaghouan massifs, and in the N-W by the hills of Megrine, Rades, Bir El Kassa and J.
Tella. The plain is mainly drained by the streams of Meliane and El Hma and it
extends from the hills of Khledia upstream to the Gulf of Tunis downstream. This
area is an alluvial plain filled with quaternary deposits whose thickness is variable.
The area of the plain is about 250 km2 and its altitude varies from 0 to 310 m above
sea level. This zone has a very important socio-economic role because it is located in
GEOMATICS, NATURAL HAZARDS AND RISK
1351
Figure 4. Geological map (1:50.000) showing the main outcrops of the Oligo-Quaternary in
Mornag plain (Source: La Goulette and Tunis).
the vicinity of the capital Tunis. The Mornag region has an arid to semi-arid climate
with an annual average rainfall of 450 mm.
2.2.1. Geology and hydrogeology context
Mornag plain appears as a large basin corresponding to the graben controlled by tectonic events according to N-S and NW-SE to N160 directions, which affected the filling of the Mio-Plio-Quaternary (Farhat 2011). The centre of the plain is covered by
recent Quaternary sediments (Figure 4). Despite the density of the surveys carried
out in this area, we deduced that:
The Quaternary is dominated by clays, silty clays and sands;
The cumulative power of the plastic components exceeds 50%;
The thickest series are located in the centre of the basin and more precisely in the
valley of the Wadi Hma whose thickness reaches 600 m at Sidi Saad (Figure 5);
At the confluence (O. Meliane-O. Hma), the thickness of Quaternary sediments is
220 m (Figure 5);
Quaternary is reduced to 13 m upstream (well 7315/2) and 32 m downstream
(well 9676/2).
According to the studies established by Ennabli (1977, 1980), the plain of Mornag
is constituted by an aquifer system. This system is formed by two partially superimposed aquifer levels:
1352
A. CHAABANI AND B. DEFFONTAINES
Figure 5. Variation of thicknesses of the Quaternary sediments (compressible layers) in Mornag
plain (Farhat 2011). Source: Author
The Oligo-Miocene syncline of Khelidia is composed of three confined sandstone
aquifers: the lower Oligocene, the Numidian and the Vindobonian. They are fed
mainly by infiltration of water in the detrital outcrops: Rades, Rarouf, Tella and
Khelidia. These aquifers constitute the deepest reservoir under all the plain.
A significant portion of the groundwater flows downstream into the alluvial deposits of Mornag plain (Jarraya-Horriche 2004);
GEOMATICS, NATURAL HAZARDS AND RISK
1353
The alluvial Mornag plain is an unconfined aquifer, whose groundwater is stored
in recent quaternary series includes a clay-detritic complex and sand with coarser
materials. The detrital sediments of Mornag plain are essentially formed by impermeable to semi-permeable soils, leaving the possibility of communication between
the different layers of the Oligo-Mio-Pliocene and Quaternary.
This plain is fed by direct infiltration of the rain during periods of flooding from
the runoff coming from Miliane and El Hma rivers.
3. Materials and methods
Remote sensing techniques make it possible to carry out deformation analyses over
large areas by exploiting a large number of radar data. In this context, the DInSAR
has established itself as an effective methodology for detecting the small displacements of the ground. This technique exploits the phase difference between two SAR
images acquired under similar geometric conditions but at different dates, to measure
and quantify the movement of soil. Indeed, many DInSAR studies demonstrated the
interest of such an approach in particular to study the oil subsidence (Fielding et al.
1998), the subsidence of urban works, the variations of the piezometric levels of
groundwater (Deffontaines et al. 2004; Fruneau et al. 2005). This technique has also
been used for the study of the surface displacements induced by the ex-mining of
Montceau-les-Mines (Carnec et al. 2001).
Recent work on the interferometric SAR technique aims to study not only single
event deformation phenomena, but also the temporal evolution of identified displacements through the generation of time series of the deformations (Deffontaines et al.
2016; Qiu et al. 2019). These approaches benefit from the availability of extensive
SAR data archives, including those acquired over the past 20 years by SAR sensors
such as Envisat ASAR data provided by the European Space Agency (ESA) (Zhang
et al. 2014). Among these advanced interferometric approaches, we focused on the
SBAS method developed by Berardino et al. (2002). The algorithm of this technique
allows us to obtain extensive, accurate and dense measurements of ground surface
deformation maps (Casu 2009). It also makes it possible to use a large number of
images while avoiding some problems of noise decorrelation (Calamita et al. 2019;
Vicari et al. 2019). Therefore, this method is a tool of high accuracy in the analysis of
surface deformation along the line of sight (LOS), and it can allow a better understanding of the mechanisms generating these displacements.
Indeed, the SBAS approach makes it possible to identify and follow the temporal
evolution of the earth deformation with a high degree of spatial and temporal coverage. In particular, this technique is based on the utilization of a large number of SLC
(single look complex) radar images based on the principle of SAR aperture synthesis
and implements an easy combination of differential interferograms. This process
makes it possible to obtain from multiple unwrapped interferograms, displacement
time series and velocity maps.
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A. CHAABANI AND B. DEFFONTAINES
Table 1. Characteristics of Envisat ASAR and Sentinel 1B SAR data used for the analysed periods
(2003–2007) and (2016–2018).
Satellite
Envisat ASAR
Sentinel 1B
Band
Orbit
Master image
Number of scenes
Acquisition period
k (mm)
Polarization
Angle of incidence
Mode
C
Descending
24 May 2005
17
2003–2007
5.6
VV
23
ASAR IM
C
Ascending
24 December 2017
14
2016–2018
5.6
VV
39
IW
3.1. SAR data
To study soil deformations in the Greater Tunis area, we used two types of SAR
images. First, we exploited the Envisat ASAR radar images that were provided by the
ESA in 2002. The sensor of this radar is characterized by an advanced synthetic aperture to detect surface area variations (ASAR). In this section, we used 17 images in
descending acquires in band in C-band and covering the period from 24 September
2003 to 31 January 2007 (Table 1). The orbital cycle of Envisat ASAR data is 35 days.
The data archives acquired by the Envisat satellite help scientific research on Earth
and enable long-term monitoring of environmental and climatic changes.
As part of the Copernicus programme, the ESA launched on 25 April 2016 the satellite Sentinel 1B (Table 1). The sensor of this satellite proposes acquisitions by SAR
over large areas (Swath ¼ 250 km) with high temporal and spatial resolution. This
new generation of satellites provides open data products to enable scientists to study
changes on the surface of the Earth. In this study, we used 14 Sentinel 1B (C-band)
images in ascending orbit, acquired with the TOPSAR technique and covering the
period from 6 October 2016 to 8 October 2018. They are characterized by a spatial
resolution of 20 m 5 m and with a temporal resolution of 6 days. The interferometric acquisition mode for this type of image is (IW). The general characteristics of this
radar data are presented in Table 1.
3.2. SBAS processing
The processing of SAR data was performed using the SARscape software (version
5.2), which runs on the ENVI platform, as an additional module.
The SARscape software was developed and produced by Sarmap AG, Switzerland,
which was founded in 1988.The interferometric analysis of the Envisat ASAR and
Sentinel 1B sensors were carried out through the SBAS module which is already
integrated in the SARscape software package (Loesch and Vasit Sagan 2018). All
necessary steps to measure displacement rates affecting the study area are shown in
the order below (Figure 6).
3.2.1. Creation of connection graph
At the beginning of this step, a comparison between the spatial and temporal baselines of the radar images was performed. Subsequently, the programme constructs
GEOMATICS, NATURAL HAZARDS AND RISK
1355
Figure 6. SBAS workflow. Source: Author.
connections between radar images whose baselines are small enough to be comparable (Diao et al. 2018). During this step, pairs of images were created and used later
to create interferograms ( SBAS Tutorial 2013; Hu et al. 2014). The result is a network of connections, represented by a connection graph (Figure 7). For this study, a
1000-day time base was used for Envisat ASAR data (2003–2007) and 250 days for
Sentinel 1B data (2016–2018). Figure 7 shows the distance between the different
images as a function of the acquisition date (time plot). These graphs make it possible
to perform a quick visual evaluation of the connections established between the
images and the temporal distribution of the data used.
3.2.2. Interferometry process
During this step, each pair of compatible radar images, identified in the previous
step, is used to create an interferogram. In addition to the radar images and the connection graph, this step requires the use of a good digital terrain model (Hu 2014).
In this study, the digital terrain model SRTM 1-arcsecond provided by NASA with a
resolution of 30 metres was used. In the first step of ‘co-registration’, the SAR images
used in radar interferometry are in Slant Range format, which means that their geometry corresponds to the moment of their capture by the satellite instrument (De
Agostini 2014; Liosis et al. 2018). Indeed, the geometry of the radar image is distorted
because it does not represent its actual size. To solve this problem, the images will
undergo a geometric correction. In fact, this co-registration process resamples each
image with the master image, chosen during the connection graph step, to give them
the same geometry ( SBAS Tutorial 2013). During the second step, a stack of interferograms was created, followed by the process of flattening the interferograms
according to a DTM or control points with a known elevation. Then a Goldstein
adaptive filter was applied into the flattened interferograms to reduce the phase noise
(Goldstein and Werner 1998). The Delaunay MCF method was selected to unwrap
1356
A. CHAABANI AND B. DEFFONTAINES
Figure 7. Connection graph showing perpendicular and temporal baselines of SLC pairs relative to
the master image: (A) Envisat ASAR (master: 24 May 2005); (B) Sentinel 1B (master: 24 December
2017). Source: Author.
interferograms (Costantini 1998; Costantini and Rosen 1999; Pepe and Lanari 2006).
This method is suitable for green and wet areas due to its capacity to connect isolated
groups of high coherence pixel (Hu et al. 2014; Castellazzi et al. 2016). In this context, to get rid signals from low coherence areas, a value of 0.35 is recommended
when using the Delaunay method for unwrapping step ( SBAS Tutorial 2013).
Indeed, all the areas having a coherence lower than the value of the threshold chosen
during this treatment, will be eliminated. The low values can produce particularly
noisy areas in the analysis, which decreases the reliability of the results.
3.2.3. Refining and re-flattening
In this step, ground control points (GCP) with reliable altitude values are required at
the input for the flattening of the interferograms. These GCP must be placed in flat
and supposedly in stable zones, while distributing them homogeneously over the
entire interferogram ( SBAS Tutorial 2013; Gama et al. 2017). In the next operation,
the software corrects for any inaccuracies affected by the orbital data and reflattens
the interferograms to make the phase data more reliable (Zhang et al. 2018). This
step is carried out on the basis of the GCP already created during the previous step
and the DTM digital terrain model (Loesch and Vasit Sagan 2018). Due to the large
number of GCP available for this study, we have chosen the orbital configuration to
refine the interferograms ( GCP Manual 2015).
3.2.4. Inversion first step
During this step, the programme uses the interferogram stack created during the
‘interferometry process’ step to calculate the residual topography and displacement
time series first estimation as a function of the interferograms (Hallden 2017). For
the estimation of the variation of the displacement rate, we choose the linear model (
SBAS Tutorial 2013; Loesch and Vasit Sagan 2018). In the same context, the
GEOMATICS, NATURAL HAZARDS AND RISK
1357
Figure 8. Spatial distribution of surface deformations (LOS) in Tunis City (mm/year) obtained from
Envisat SBAS analysis (2003–2007). Source: Author.
programme also performs a second unwrapping process to improve SAR data for the
next step.
3.2.5. Inversion second step
During this step, atmospheric filtering is performed on the interferometric phase to
provide a more accurate estimation of the final velocity displacement and ameliorate
displacement time series (Gaber et al. 2017). Atmospheric signals have been removed
through temporal and spatial filtering operations (Ferretti et al. 2000 Charef et al.,
2012). Indeed, the algorithm uses two atmospheric filters, high-pass and low-pass, to
correct atmospheric effects and to smooth the temporal signature of the displacement
by respecting certain physical properties of the atmosphere (Kim et al. 2014; Loesch
and Vasit Sagan 2018). The low-pass atmospheric filter eliminates atmospheric effects
that are spatially distributed while the high-pass atmospheric filter suppresses atmospheric effects that are temporally distributed. For this study, the default parameters
proposed by programme is used.
3.2.6. Geocoding
Geocoding occurs when the different results of the previous steps are transformed from
the format (slant range) into geocoded images whose distances will be converted in terms
of distance to the ground. These images will have the same pixel size and correspond to
the same coordinate system. In this study, the result is a geocoded image according to
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A. CHAABANI AND B. DEFFONTAINES
Figure 9. Spatial distribution of surface deformations (LOS) in Tunis City (mm/year) obtained from
Sentinel 1B SBAS analysis (2016–2018). Source: Author.
the UTM (Universal Transverse Mercator) cylindrical map projection Zone 32 N, showing the velocity displacement in the study area, measured in mm/year.
4. Results and discussion
4.1. Tunis City
The processing of data acquired from multiple satellite sensors (SAR) in descending
and ascending orbits has made it possible to distinguish differential movements
affecting several areas in the Tunis City. By visualizing the displacement maps
(Figures 8 and 9), we notice that the results obtained by the interferometric method
SBAS of the Envisat data (2003–2007) and those of the Sentinel 1B method
(2016–2018) generally show a perfect coherence. Indeed, the zones identified by subsidence phenomenon in the first period will continue to move during the second
period but with a different velocity. Several areas affected by surface deformations in
the Greater Tunis area during both periods, revealed the permanence of the subsidence zones. Surface deformations are identified mainly in the northern and northeastern part of Sebkhet Essijoumi and in the western part of Tunis Lake (Figures 8 and
9). The distribution of measuring points within the study area is not homogeneous.
The density of these points is excellent in the city of Tunis and gradually decreases
towards the southern and northern parts of the study area which corresponds to the
presence of very dense vegetation (high vegetation density) such as rural or agricultural areas and also following the loss of coherence between interferograms in these
GEOMATICS, NATURAL HAZARDS AND RISK
1359
Figure 10. Geotechnical map of Tunis City (Source: Guilloux and Nakkouri 1976), modified.
areas. This analysis of Figures 8 and 9 demonstrates a subsidence affecting the south
and north-east of Sebkhet Essijoumi, as well as the western part of Tunis Lake. For
Envisat ASAR data (descending orbit), a negative value of LOS velocity (13.98 mm/
year) is observed, and 19 mm/year (Blue) detected for Sentinel 1B (ascending orbit).
In the eastern part of Sebkhet Essijoumi, 9.91 mm/year is detected for Envisat
ASAR data, and the second analysis of Sentinel 1B indicates a maximum deformation
rate of 14 mm/year. For the other areas around the two previous sites, the map of
LOS velocity shows a dominance of the yellow-green colour which means that the
variation of velocity is weak in these sectors, therefore, they represent the most stable
zones. To determine the origin of these differential movements, it is necessary to
examine the various parameters such as geological and geotechnical factors and
human activities related to recent urban development.
4.1.1. Geotechnical characterization in Tunis City
In order to define the displacement problems that occurred on both sites, we compared the results obtained from the SBAS interferometry method with geotechnical
and geological studies performed in the city of Tunis. The results of these various
research projects, which are mainly related to the geotechnical characterization of
the subsoil.
1360
A. CHAABANI AND B. DEFFONTAINES
Figure 11. Spatial distribution of thicknesses of the muddy soil in the region of Tunis (Source:
Kaaniche 1989), modified.
The first geotechnical map in Tunis City was prepared by Guilloux and Nakkouri
(1976) (Figure 10). This study made possible to evaluate and characterize the plain of
study area, by showing that it is constituted by quaternary alluvial deposits. Indeed,
these sediments are formed of mud, clay and sand. According to this work, these
alluviums are considered as compressive soil (Guilloux and Nakkouri 1976). The geotechnical map shows the subdivision of Tunis City into three zones based on the
properties of the lithological formations, the geological profiles and the geotechnical
characteristics. According to the interferometric results, the areas affected by subsidence in Tunis plain (south and north-east of Sebkhet Essijoumi and the western part
of Tunis Lake) coincide with the red zone whose the substratum is characterized by a
mediocre geotechnical properties (Figure 10).
In the same context, the research conducted by Kaaniche (1989) revealed that the
sediments of the plain of Tunis City (near Tunis Lake and Sebkhet Essijoumi) have
rather poor geotechnical characteristics. The soil and the subsoil of this sector are
constituted by embankments, muddy ground and sandy clay complex. This study
shows that the deposits of this zone are characterized by the presence of sandy bodies
in a deep mud complex whose geometry is in the form of lenses, and it is characterized by tuffy limestones, embankments and complex of sandy clay. The sandy bodies
GEOMATICS, NATURAL HAZARDS AND RISK
1361
Figure 12. Lithological column (reconstituted from core holes) located in instable area of Tunis
City (Source: Klai and Bouassida 2016, modified).
are distributed randomly in the area and they disappear laterally. This disappearance
is responsible for the appearance of the differential settlements in the study area
(Kaaniche 1989). The heterogeneity of the lithology described would have favoured
the differential settlement which causes serious geotechnical disturbances in the
region by affecting the constructions by serious cracks. A geotechnical map was
developed by Kaaniche in the same study area showing the thickness of the muddy
soil in unstable areas (Figure 11). According to the in-situ tests carried out in the
laboratory, the mud complex in the plain of Tunis corresponds to a poor geotechnical
quality characterized by a weak resistance. The depth of this formation gradually
increases towards Tunis Lake from which it can reach 40 metres. This muddy field is
characterized by significant settlements.
The research work carried out by Klai and Bouassida (2016) also addresses the
problems and risks of differential settlements caused by the poor geotechnical quality
of the muddy ground in Tunis region. Figure 12 shows the lithology of the soil in
Tunis City (Sports Complex Park ‘A’ of Mounir Kebaili). This geotechnical section
(reconstituted from core holes) indicates a thin layer of the embankment that overcomes the soft mud of Tunis City whose depth is of the order of 65 m. This type of
saturated soil represents a very low permeability, high compressibility and low resistance characteristics. The weak mechanical characteristics of this muddy soil on the
first 20 metres (depth) caused differential settlements for several constructions in
Tunis. Several degradations of construction works built on the mud of Tunis are due
to the bad quality of subsoil. Indeed, this type of compressible soil causes stability
problems for the foundations of the structures and can also damage the constructions.
For example, the damage caused to the construction of Hotel Africa, which is located
in Avenue of Habib Bourguiba, led to the bankruptcy of the two civil engineering
companies (These companies were responsible for the construction of this hotel). In
fact, the bad choice of the execution technique of the pile foundation is one of the
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A. CHAABANI AND B. DEFFONTAINES
Figure 13. Example of structural damage identified in downtown Tunis (Klai and Bouassida 2016).
problems associated with pile driving into the muddy soil under the effect of their
own weight. Some constructions based on the mud of Tunis City, undergone a geotechnical problems related to excessive settlement.
In fact, Figure 13 shows the settlement of a newly built building (left). The sinking
of this construction caused cracks in the old building (right) (Klai and Bouassida
2016). Another example of disorder observed in this study area. Figure 14 shows a
building located in Tunis City, which began to sink in a compressible ground a few
years after its construction.
Figure 15 demonstrates an old building located in Avenue Hedi Nouira, showing
the inclination of this building a few degrees due to the instability of the ground. The
results achieved from research carried out in Tunis City and from field observations
have revealed the existence of differential movements that have been validated by the
SBAS interferometric method. The joint exploitation of the Envisat (2003–2007) and
Sentinel 1B (2016/2018) datasets, geological and geotechnical information, allowed
the validation of the spatial distribution of differential settlements in the study area.
The settlement phenomena observed coincide with large areas that have relatively
thick alluvium in the study area. These compressive deposits have a potential risk to
subside progressively due to a compaction and under the effect of various loads
related to urbanization, such as artificial embankments and infrastructure. In fact,
these differential movements are the origin of the instability of the soil in the region
of Tunis and thus cause significant damages to buildings.
4.2. Mornag plain
The application of the SBAS interferometric technique in this area has made it possible to detect and measure displacements in the alluvial plain of Mornag.
Displacement maps show the spatial distribution of surface deformations observed
from the two satellites in descending and ascending orbit. The displacements
GEOMATICS, NATURAL HAZARDS AND RISK
1363
Figure 14. Photo example of a building (currently abandoned) was built in the late 80s, located in
front of the old port of Tunis, showing the inclination of this building a few degrees to the East,
(Saad Zaghloul Street). Source: Author.
Figure 15. Photo example of an old building, located in Avenue Hedi Nouira, showing the inclination of this building a few degrees to the East. Source: Author.
highlighted by Envisat ASAR and Sentinel 1B show a permanence subsidence during
both periods. The displacements are measured along the line of the satellite sight.
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A. CHAABANI AND B. DEFFONTAINES
Figure 16. Spatial distribution of surface deformations (LOS) in Mornag plain (mm/year) obtained
from Envisat SBAS analysis (2003–2007). Source: Author.
Figure 17. Spatial distribution of surface deformations (LOS) in Mornag plain (mm/year) obtained
from Sentinel 1B SBAS analysis (2016–2018). Source: Author.
GEOMATICS, NATURAL HAZARDS AND RISK
1365
Figure 18. Irrigated agricultural land in Mornag region. Source: Author.
The negative values (cyan to blue) represent movements away from the satellite
(ground subsidence), while the positive values (yellow to red) represent the movements towards the satellite (terrestrial uplift) (Figures 16 and 17). Indeed, the green
pixels that have velocities between 0.3 and þ0.3 mm/year, are considered stable. For
Envisat ASAR data, in descending orbit the displacement rate detected a long the
LOS direction is 13.98 mm/year (Blue) and 19.4 mm/year (Blue) is observed for
Sentinel 1B data in ascending orbit. Despite the low density of measuring points
made from the interferometric analysis of the Envisat ASAR (2003–2007), we note
that the area affected by negative displacements (subsidence) show a lot of similarity
with the results obtained from the Sentinel 1B data (2016–2018).
This low density is mainly due to the reduced numbers of radar data and the
irregular temporal distribution of the images during the period between 2003 and
2007. Finally, the ground deformation maps produced allowed us to identify a soil
subsidence affecting the extended rural areas of about 70 km2 and to measure ground
variations in the study area within the observed time interval.
4.2.1. Overexploitation of the water table of Mornag plain
The mitigation of the risks related to overexploitation and the protection of water
resources pose complex problems all over the world. Tunisia is one of the most
affected Mediterranean countries by water shortage. Mornag plain is also facing serious problems related to the overexploitation of sub-surface water which is linked to
the increasingly intensive use of water in sectors such as agriculture, industry and the
scarcity of rains resulting from climate change. Indeed, although the climate in this
region remain dry with low precipitation, the plain of Mornag is characterized by
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A. CHAABANI AND B. DEFFONTAINES
Figure 19. Piezometric level of three wells in Mornag aquifer during the period (1965–2000)
(Source: Charef et al. 2012).
intensive and extensive rural activities, whose main source of water is the water table
which is overexploited from underground wells. Over the last three decades, the
increase of irrigated land (12,000 hectares) (Fougeirol et al. 2012) related to the development of some crops (Figure 18), the growth of the deep wells and the decrease in
rainfall over several and consecutive droughts years, have resulted in an increase of
the groundwater exploitation. This intensive use of water caused a decrease in the
piezometric level (Figure 19), sometimes exceeding 1 m/year in shallow water and
deep aquifers located in the centre of Mornag plain (IGIP 2011) and as a result an
increase in salinity (Lassoued et al. 1995).
Overexploitation and depletion of groundwater can make the region extremely vulnerable
to natural hazards. In sum, Mornag region has overexploited these groundwater reserves to
meet the needs of agriculture, industry and the region’s supply of drinking water.
In general, the phenomenon of differential settlement is well known in coastal and
agricultural areas where the combined effect of overexploitation of groundwater and
the natural consolidation of sediments result in þ/ continuous soil subsidence at
different scales. The effectiveness of SBAS approaches in the monitoring and mapping
of surface deformations has been proven by several researchers, showing that subsidence may be related to overexploitation of aquifers throughout the world. Liu et al.
(2017) showed the importance of SBAS technique in the mapping and monitoring
surface deformations. Indeed, they used the ALOS data to demonstrate that the subsidence in the city of Cangzhou, is induced by the excessive pumping of groundwater.
Tomas et al. (2005) used ERS-1 and 2 radar data to measure the extent of groundwater subsidence in the city of Murcia in Spain. Fruneau et al. (2005) studied the
effect of water pumping in the city of Paris (Saint-Lazare station) by comparing piezometric measurements with the results of interferometric analyses.
In this study, due to the unavailability of GPS data and levelling throughout
Mornag plain, groundwater piezometric measurements (Figure 20) were used to analyse changes in groundwater level fluctuation over time. The results of this operation
make it possible to study the correlation between the variations of the piezometric
GEOMATICS, NATURAL HAZARDS AND RISK
1367
Figure 20. The distribution of some groundwater monitoring wells for both periods (2003–2007)
and (2016–2018). Source: Author.
Figure 21. Evolution of piezometric level at selected wells in Mornag plain (2004–2008).
Source: Author.
level of the water table and the ground surface deformations derived from the SBAS
method. Groundwater data were provided by the Directorate General of Water
Resources of Tunisia (DGRE) who is responsible for monitoring piezometric wells
widely distributed in the plain of Mornag.
These data were used to establish the piezometric level variation curves. These piezometric measurements carried out monthly during the period 2004–2018 (Figures 21
and 22), correspond to the period of time series of deformations derived from the
SBAS method. The evolution of the piezometric level of these wells, shows small seasonal fluctuations but the general tendency of these curves indicates a decrease of several metres during the last 20 years. The comparison of time series deformations and
the piezometric measurements recorded at the level of the Pz3 well over the period
(2003–2007) (Figure 23), indicates that the water table level has dropped by about
4 m and the time series for the sample located in the same place reveals the existence
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A. CHAABANI AND B. DEFFONTAINES
Figure 22. Evolution of piezometric level at selected wells in Mornag plain (2009–2018).
Source: Author.
of land subsidence of about 2 cm which occurs in compressible alluvial sediments of
the plain. In the same context, the well Pz6 indicates that the groundwater level
decreased by about 3.9 m in the period (2016–2018) (Figure 24), and that the time
series demonstrates a displacement rate of about 2.3 cm. The temporal evolution of
soil deformations, measured by the SBAS method, therefore shows a good correlation
with the changes in piezometric level in Mornag alluvial plain. Indeed, from the
results achieved from these correlations, we note that the trend of soil subsidence is
consistent with the dynamic change in groundwater level. In fact, the level of groundwater was continually decreasing, sometimes showing fluctuations (seasonal change).
Soil deformations in Mornag area show a decrease in the topographic surface, but
there are some small variations indicating slight uplifts in relation to the increase of
the piezometric level.
In addition, the expansion of agricultural and industrial areas in Mornag plain, the
decrease in rainfall in recent years and the overexploitation of the aquifer have
resulted in a decrease in the level of groundwater. Indeed, the decrease of the piezometric level in a continuous way observed during all this period, caused the differential displacements in Mornag alluvial plain.
As a result, the fluctuation of the groundwater level poses geotechnical problems
resulting from the modification of the physical and mechanical properties of the soil
up to the subsidence of the soil. The alluvial deposits in Mornag plain consists mainly
of clays intermixed with sand and silts and coarser materials. They are widely present
in the study area. These clay units are distinguished by high compressibility, thus representing an important role in the process of subsidence caused by groundwater
extraction and the phenomenon of consolidation.
According to Terzaghi (1923), the phenomenon of consolidation is the result of
the reduction of the water content of a saturated soil and provides that air does not
replace water.The static charges exerted on the ground in the long term, induce the
GEOMATICS, NATURAL HAZARDS AND RISK
1369
Figure 23. The relationship between deformation time series (LOS) for Envisat SAR data and piezometric levels during (2003–2007) at selected well points (Pz1, Pz2, Pz3, Pz5). Source: Author.
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Figure 24. The relationship between deformation time series (LOS) for Sentinel 1B SAR data and
piezometric levels during (2016–2018) at selected well points (Pz4, Pz5, Pz6, Pz9). Source: Author.
GEOMATICS, NATURAL HAZARDS AND RISK
1371
Figure 25. Cross-section profile of Mornag plain showing the geometry of the compressible layers
and the structure of this area (‘Source: Gestion des ressources en eau en Tunisie: bilan et perspectives d’avenir’), modified.
elimination of the interstitial water and thus it causes a settlement. This consolidation
process mainly affects saturated clays because of their low permeabilities.
In fact, settlements caused by external loads are considered a primary consolidation
process (Terzaghi 1923; Polcari et al. 2014). The viscous behaviour of very fine-grained
soils is considered to be one of the main factors responsible of the settlements resulting
from the consolidation process. Generally, the time required to complete the primary
consolidation process depends mainly on the thickness and compressibility of the loaded
soils as well as the hydraulic conductivity (Terzaghi and Peck 1967; Cascini et al. 2007;
Polcari et al. 2014). In our case, the lithology consists mainly of fine-grained sediments
(clay-sands-silts). The thickness of the compressible layer varies depending on its location
in the alluvial plain (Figure 5). In particular, ground settlement is more likely to occur in
the centre of the plain where the thickness of the compressible deposits is deep (Figures
5 and 25). The settlement phenomenon observed in Figure 26 coincides with the alluvial
deposits filled in the centre of Mornag plain. In fact, it can be seen that the increase of
the rate of deformation is related to the variability of the thickness of plio-quaternary
alluvial deposits.
Finally, in order to validate the movements detected in the two sectors of Tunis
City and Mornag plain, we combined the experimental data recorded by the permanent station of the Office of Topography and Cadastre (OTC) located in Charguia
(Tunis), and deformation time series (LOS) calculated using SBAS method during the
period (2003–2018). The Comparison of the curves (Figure 26), shows a similarity
and harmony between land surface deformation (along the LOS) obtained from the
Envisat ASAR and Sentinel 1B SAR data and the surface measurements of the GPS
data. Indeed, this comparison demonstrates the presence of a slight drop in the
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Figure 26. Topographic variations recorded by the permanent station associated to the SBAS time
series of surface deformations (LOS) during the period (2003–2018). Source: Author.
topographic surface over time up to 2 cm registered in the permanent station. There
are also some seasonal changes during this period. Finally, this comparison allows us
to validate the results obtained from the SBAS method in the study area. Knowing
that the mobility of the permanent station can be explained by a depression of the
building where this station is installed. Indeed, it is built on an old marshy area also
corresponding to a compressible soil.
5. Conclusion
The aim of this work was to monitor the deformation phenomena occurring in the
Greater Tunis area, using the Envisat (2003–2007) and Sentinel 1B (2016–2018) SAR
data. In this study, the SBAS technique was exploited to detect surface displacements.
This multi-temporal approach has emerged as an effective methodology for monitoring soil displacements with multi-millimetre accuracy.
Through the analysis carried out by SBAS technique, we have been able to detect
the existence of a differential settlement phenomenon in the urban areas (Tunis City)
as well as in the rural areas (Mornag plain). The Analysis of SAR images in ascending
and descending orbit during both periods in the Greater Tunis area revealed the permanence of the subsidence areas. The SBAS approach has also achieved a high density of measurement points, indicating the ability of this technique to effectively study
surface displacements, even in semi-rural areas such as Mornag plain. The DinSAR
technique that we have applied for subsidence monitoring in urban and semi-rural
areas represents the first step of this work.
The second step of interpretation of these results requires an analytical study these
results requires an analytical study, followed by quantitative comparisons between the
GEOMATICS, NATURAL HAZARDS AND RISK
1373
results on the zones affected by deformation, to see if these displacements are caused
by natural (tectonic) or anthropogenic factors.
In this context, through a comparative analysis between SAR data and geological
and hydrogeological information, we have been able to explore some of the links
between soil subsidence and its main control factors. In particular, a differential
settlement was detected in the first site around the region of Tunis Lake, which is
probably due to the nature of the highly compressible and thickness alluvial sediments and whose substratum depth sometimes reaches more than 60 m. These compressive deposits have a potential risk to subside progressively due to compaction and
under the effect of various loads related to urbanization, such as artificial embankments and infrastructure. For the period of (2003–2007), SBAS technique retrieved
values demonstrate maximum subsidence velocities of 13.98 mm/year, and
19 mm/year for the period of (2016–2018). These differential movements are the
origin of the instability of the soil in the Tunis region and thus cause numerous failures to buildings. In the second site, ground subsidence was detected in the whole
plain of Mornag. Indeed, the analysis of piezometric data and time series deformations revealed that the trend of soil subsidence is consistent with the dynamic change
in groundwater level. The results of the analysis show a decrease in the piezometric
level, sometimes exceeding 3 m/year. This overexploitation of groundwater is mainly
caused by the expansion of agricultural and industrial activities and the decrease in
rainfall in recent years (from 2003 to 2018). The good correlation between results
derived from SBAS and piezeometric level confirms that depletion of groundwater
associated with the consolidation of alluvial sediments are the main driving mechanism of these deformations over the plain. Finally, we recommend this technique for
monitoring subsidence in both urban and peri-urban areas as long as the vegetation
allows it. We also recommend using the DinSAR method to establish risk maps in
urban areas, which saves valuable time and money. Indeed, the establishment of
movement maps through this method makes it possible to overcome the difficulties
of multiplying the number of boreholes and the expensive geotechnical tests. In
Tunisia, the extension programme for urban areas on the margins of wetlands poses
stability problems. The use of interferometry is a very valuable decision support tool.
As part of the geotechnical zonation of Greater Tunis, it would also be interesting to
apply interferometry to highlight the archaeological sites of Carthage.
Acknowledgements
This Project was funded by the Deanship of Scientific Research (DSR), King Abdulaziz
University, Jeddah, under Grant No. (DF-769-155-1441). The authors, therefore, gratefully
acknowledge DSR technical and financial support.
Disclosure statement
No potential conflict of interest was reported by the authors.
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