DInSAR analysis of differential ground subsidence affecting urban

Rivista italiana di Telerilevamento - 2008, 40 (2): 103-113
DInSAR analysis of differential ground subsidence affecting
urban areas along the Mexican Volcanic Belt (MVB)
Paolo Farina1, Jorge Alejandro Avila-Olivera2, Victor Hugo Garduño-Monroy3
and Filippo Catani1
Università degli Studi di Firenze, Dipartimento di Scienze della Terra, via G. La Pira 4 - 50121 Firenze, Italy. E-mail: [email protected]
2
Instituto de Geofísica, UNAM, C.U., Ciudad Universitaria, Del. Coyoacán,
México D.F 04510, Mexico City, Mexico
3
Instituto de Investigaciones Metalúrgicas, UMSNH, C.U., Av. Rey Tariacuri 374-D, Morelia, Mexico
1
Abstract
Most of the urban areas located in the Central and Northern sectors of the Mexican Volcanic
Belt (MVB) during the last two decades registered differential ground subsidence along
preferential directions parallel to the regional fault system. The combination of the consolidation of soft terrains due to groundwater withdrawal with the presence of regional and
local systems of normal faults has been addressed as the controlling factors of the spatial
distribution of the ground movements. In this paper we present the preliminary results of a
DInSAR analysis carried out over the main cities of the MVB using ENVISAT data spanning the period 2003-2006. The analysis revealed the presence of ground displacements
along the same linear structures as identified in the field, reaching at some locations velocities up to 6-7 cm/year.
Keywords: land subsidence, DInSAR, Envisat ASAR
Riassunto
La maggior parte dei centri urbani ubicati nel settore centro-settentrionale della Mexican
Volcanic Belt sono interessati da oltre 20 anni da fenomeni di subsidenza che hanno provocato cedimenti differenziali lungo discontinuità orientate secondo direzioni coincidenti con
quelle delle principali famiglie di discontinuità tettoniche della zona. Tali movimenti sono
imputabili al consolidamento del terreno indotto dall’emungimento della falda combinato
con la presenza di strutture tettoniche pre-esistenti. Il presente lavoro mostra i risultati preliminari di un’analisi interferometrica differenziale di dati Envisat realizzata sui principali
centri urbani della MVB. L’analisi ha rilevato la presenza di deformazioni lungo le principali faglie con velocità massime fino a 6-7 cm/anno.
Parole chiave: subsidenza, DInSAR, Envisat ASAR
Introduction
Since the 1980s, habitants of some cities built-up in lacustrine or fluvio-lacustrine depressions of the Central and Northern sectors of the Mexican Volcanic Belt (MVB), a complex
volcanogenetic zone E-W oriented, some 20-200 km wide and 1,000 km long, started to
notice that their houses, pavements of the streets, pipelines and railways were affected
by differential ground settlements oriented along preferential directions [Aranda-Gómez,
1985; Trujillo-Candelaria, 1985; Martínez-Reyes and Nieto Samaniego, 1990; Pasquaré et.
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DInSAR analysis of land subsidence in the Mexican Volcanic Belt
Figure 1 - Location map of
the Mexican Volcanic Belt
(MVB). Orange dots indicate the location of the cities
investigated in this paper.
Al., 1991; Trejo-Moedano and Baini, 1991; Trujillo-Candelaria, 1991; Silva-Mora, 1995;
Lermo-Samaniego et al., 1996; Garduño-Monroy et al., 1998; Garduño-Monroy et al.,
1999]. The most relevant cases of this phenomenon are represented by the cities of Morelia, Celaya, Querétaro, Salamanca and Aguascalientes. These ground settlements represent
a major threat for the utility and transportation networks crossing the urban areas, for the
safety of private buildings, but also for the historical parts of the cities, some of which,
namely Morelia and Querétaro, have been included in the UNESCO World Heritage List
due to their historical and cultural relevance. The results of the geological, geophysical and
hydro-geological investigations carried out in the last years have related ground settlements
to the over-exploitation of groundwater resources that, generating a decrease of the pore
water pressure in the solid phase of the soil, causes the consolidation of the soft sediments
representing the upper filling of the depressions. The combination of groundwater withdrawal with the presence of regional and local systems of tectonic discontinuities has been
addressed as the controlling factor of the displacement spatial distribution. Indeed, the main
damaged buildings and structures have been detected along preferential directions which
are parallel to the regional fault system. Based on the elements necessary for the generation
of such a phenomenon local geologists refer to it as Process of Subsidence-Creep-Fault
(PSCF) [Garduño-Monroy et al., 2001]. Subsidence because the movement is supposed
to be primarily connected to the consolidation of the lacustrine and fluvio-lacustrine sediments caused by the aquifer withdrawal; creep to denote that this type of deformation is
slow, continuous through time and aseismic; and fault because of the presence of tectonic
structures buried by sediments that control the spatial distribution of ground settlements.
Despite the existence of several detailed studies focused on specific cities located in the
MVB [Garduño-Monroy et al., 2001; Rojas et al., 2002; Pacheco et al., 2006], there is a
lack in the knowledge of the spatial extension and on the temporal evolution of such a phenomenon over the whole Central and Northern sectors of the MVB at regional scale. For
the detection and measurement of these ground movements conventional differential SAR
interferomety (DInSAR) was implemented, focusing preliminarily on the cities of Morelia,
Querétaro and Celaya (Fig. 1). These urban settlements are affected by the Process of Subsidence-Creep-Fault (PSCF) along linear discontinuities oriented in different directions:
Morelia is characterized by a NE-SW faulting system, typical of the MVB, while Querétaro
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Rivista italiana di Telerilevamento - 2008, 40 (2): 103-113
and Celaya are encompassed by a NNW-SSE faulting. After a description of the main features of the adopted InSAR processing, preliminary results showing the spatial distribution
of the detected ground displacements, their magnitude and the correlation with geological
and hydro-geological observations are presented for three cities of the MVB. Finally, some
concluding remarks about the impact of the DInSAR analysis on the phenomena comprehension and a description of the work still in progress are given.
Envisat DInSAR analysis
In order to retrieve ground displacements over the studied area a conventional DInSAR
analysis has been carried out using ASAR scenes acquired by Envisat during the time interval 2003-2006. At the base of the DInSAR technique is the differential interferogram: a
complex image displaying the change in signal phase between a pair of SAR scenes. The
basic model that defines signal phase is:
[1]
where φ = phase; ψ = reflectivity of an object or pixel; r = the distance between the satellite
and the target; α = the atmospheric contribution, and λ = signal wavelength.
It follows that, when looking at change between a pair of scenes, the phase shift Δφ can be
expressed as:
[2]
For both images, if ψ remains the same and if we have identical atmospheric contribution
as well as low noise energy, then the expression for phase shift becomes:
[3]
where Δr is the range variation possibly due to target motion.
By dealing with only two images the identification of atmospheric artefacts and their quantification is an impossible task without using external information. In some cases (e.g. high
deformation rates or peculiar spatial patterns of deformation) the impact of the atmosphere
on the differential interferometric phase can be neglected. Apart from atmospheric distortions, it is worth to be noticed that different effects reduce, up to compromise in some cases,
the quality of the interferometric analyses. First of all, temporal decorrelation of the signal
induced by the variations of the dielectric properties of the targets between the two acquisitions [Zebker and Villasenor, 1992]. Vegetated areas, due to the movement induced by the
wind and to the seasonal growth, cause significant phase decorrelation of SAR couples over
periods longer than few months, while urban areas maintain high levels of coherence over
longer time intervals. In addition, spatial decorrelation of the signal induced by the spectral
shift caused by different acquisition geometries limits good InSAR results to short baseline
interferograms [Rosen et al, 2000].
For the analysis of the SAR scenes over the MVB urban areas, in order to limit the effect
of temporal and spatial decorrelation of the phase the selection of the suitable data pairs
within the ESA archives has been driven by the cross check on perpendicular baseline vs.
time interval. The selected dataset, consisting of 10 scenes for two consecutive frames, span
the time interval from July 2003 up to May 2006. The ASAR raw data were first processed
to full resolution Single-Look Complex (SLC) images and then, after the co-registration
of the SLC images, interferometric processing was carried out to 5 azimuth and 1 range
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DInSAR analysis of land subsidence in the Mexican Volcanic Belt
looks with commonband filtering. Among all possible suitable combinations of interferometric pairs, as reported in Tab. 1, high coherence over the urban areas has been preserved
only on interferograms with a maximum spanning interval of 490 days. The coherent pairs
have been processed through the “two pass interferometry” method in order to obtain the
displacement component of the interferometric phase [Massonnet and Feigl, 1998]. The
phase contribution related to the topography has been eliminated by creating a synthetic
interferogram containing only the topographic term of the phase from a pre-existing DEM
and then subtracting it from the real interferograms. The portion of the topographic component of the phase effectively removed using such a method depends on the ratio between
the height of ambiguity, namely the sensitivity to topographic relief of a pair of SAR images, and the DEM vertical accuracy [Massonnet and Feigl, 1998]. To this aim an accurate
DEM of the area with a 30 m posting and interferometric pairs characterized by small
baselines have been employed. Strong atmospheric artefacts affecting the interferometric
phase have been noted on differential interferograms with acquisitions during the rainfall
season. These couples have been excluded from further processing. For the other suitable
pairs the particular pattern of the measured signal and its presence on multiple independent
observations suggested a negligible impact of the atmosphere on the retrieved deformations, at least for the late summer and winter couples. After an adaptive filtering, on the best
differential interferograms phase unwrapping, the process of restoring the correct multiple
of 2π to each point of the interferogram, has been performed using different approaches
based on the characteristics of the identified pattern of deformation [Werner et al., 2002]. A
region growing algorithm has been applied in case of high phase gradients and discontinuities and a minimum cost flow algorithm for areas with sparse urban fabric and low phase
gradients. In order to avoid the introduction of further errors, areas of low phase correlation
and consequent difficulty in phase unwrapping have been masked out, limiting the analysis
to reliable information. Finally, the unwrapped differential phase has been converted into
displacement in cm along the satellite line-of-sight and geocoded according to the projections and postings of the employed DEM. The final results of the interferometric processing
Table 1 - List of the processed interferometric pairs.
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Orbits
Date
Δ days
Bperp (m)
7137-14151
12072003-13112004
490
18
7137-19662
12072003-03122005
875
342
7137-20664
12072003-11022006
945
213
7638-14151
16082003-13112004
455
57
14151-20664
13112004-11022006
455
195
14652-17157
18122004-11062005
175
98
14652-19662
18122004-03122005
350
-47
14652-20664
18122004-11022006
420
-176
17157-19662
11062005-03122005
175
-145
19662-20664
03122005-11022006
70
-129
20664-22167
11022006-27052006
105
78
19662-22167
03122005-27052006
175
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Figure 2 - Differential interferogram of
Morelia overlaid to a multi-look SAR
amplitude image in SAR coordinates
spanning the time interval 18/12/200403/12/2005.
consist of several multi-temporal displacement maps measured along the line-of-sight of
Envisat descending orbits (23° from the vertical, circa E-W directed).
Morelia
The fluvio-lacustrine depression upon which the city of Morelia has been built-up is characterized by a bedrock made up of different sequences of volcanic materials, starting from
lavas, breccias and pyroclasts of andesitic composition placed in the Middle Miocene (12
My) followed by a sequence of ignimbrite. Overlying the ignimbrite a lacustrine, eventually becoming fluvial, deposit is present with a thickness of about 60 m and ages from the
Miocene (8 My) to the Pleistocene. At the top of this terrigenous sequence outcrop the
volcanic products of the volcanoes Quinceo, Las Tetillas and Cerro del Águila, consisting
of lava flows, breccias and pyroclasts of basaltic or andesitic composition. All the above
described units appear affected by NE-SW and E-W faults. In Morelia, differential ground
subsidence started in 1983 when ground gashes with very small displacement started to
form. Subsequently these gashes evolved to form a network of normal faults that generated
morphological scarps with vertical throws up to 1 m high in some parts of the city. The
most important discontinuities affected by differential settlements are represented by the
Central Camioniera cracking and the La Colina cracking. Since 1983 differential ground
movements affected a significant number of buildings, some of them to such an extend,
that they had to be demolished. Other man-made structures such as streets, drainage and
water pipes were also damaged. The long term differential interferograms processed over
the city of Morelia, spanning a time interval of 350-450 days, clearly revealed the presence
in the urban area of phase signals related to ground displacements along the satellite line
of sight away from the sensor, as shown in Figure 2. Two different displacement patterns
were detected: small but strict variations of the phase along preferential directions a few
kilometres long, mainly oriented along the NE-SW direction and sub-circular bowls with a
diameter of a few hundred meters. The best differential interferograms, in terms of coherence and absence of clear atmospheric artefacts, after phase unwrapping and geolocation
have been converted into ground displacement map along the satellite l.o.s. By importing
these maps within a GIS environment and overlaying them with other thematic layers, such
as a detailed topographic map, the location of the declared wells and the most evident faultings, as mapped by field surveys carried out in the last years, it has been possible to notice
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Figure 3 - a) Line-of-sight displacement map of Morelia overlaid to a topographic map, spanning
the time interval 12/07/2003-13/11/2004; b) line-of-sight displacement map of Morelia overlaid to a
topographic map, spanning the time interval 18/12/2004-03/12/2005. A coherence threshold of 0.5
has been used for phase unwrapping. Figures show also the location of the main faults as mapped in
the field in the last years (1999 and 2005).
a clear correlation between the registered movements and the location of the main tectonic
structures (Fig. 3).
In particular, movements were registered along the La Paloma fault, the Central Camioniera
cracking and La Colina cracking. Displacement rates measured by the DInSAR analysis
between the 2003 and the 2005 range between -1.0 and -3.5 cm/year. Apart from the movements along the described faults the sub-circular pattern of deformation was interpreted as
a subsidence bowl connected to the presence of an important well for the supply of the municipal aqueduct. Field surveys carried out during July 2006 allowed to check the presence
of damage to buildings and man-made structures along the areas detected by the DInSAR
analysis, where already in the past evidences of the movement were identified (Fig. 4).
Currently, the displacement maps provided by the DInSAR processing are analyzed along
with ancillary data, such as piezometric maps and stratigraphic data, in order to understand
the spatial distribution of the movements and to support the interpretation of the causes of the
differential settlements.
Figure 4 - Picture taken during the
summer 2006 showing the morphological evidence of differential
ground settlements along the Central Camioniera cracking in the
city of Morelia.
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On the other hand, the InSAR measurements will be validated through the acquisition of GPS
measurements on a network of benchmarks installed since 2005.
Querétaro
The valley of Querétaro represents from a geologic point of view a composed tectonic
structure delimited by NNW and ENE oriented faults, the most important of which are the
Oriental and the San Bartolomé faults. At a more local scale the city of Querétaro is settled
within a smaller graben bounded two N-S oriented faults, namely the 5 de Febrero fault or
Falla Central to the East and the Tlacote fault to the West of the city. The graben is filled
with sequences of andesitic lavas and basalt masses of Miocene-Pliocene ages and tuffs
covered by granular materials of fluvial or lacustrine origin. The near surface stratigraphy
consists of an upper layer of black clay overlying a sequence of pyroclastic and fluvio-lacustrine silts and sands intercalated with lenses of pyroclastic materials. Such a granular
material represents the multi-layered semi-confined aquifer system present in this sector
of the valley. Groundwater resources in the Querétaro area have been strongly exploited
in the last 20 years, with common depletion rates of the water table around 3 m/year and a
maximum decline of the water level in the central part of the valley, located at the footwall
of the main tectonic discontinuities, up to 160 m [Carreón-Freyre et al., 2005].
The most important fault in relation to the PSCF and its impact on the built-up area is
represented by the 5 de Febrero fracture. The first cracks were noticed in the early 1970s
and by the beginning of the 1980s the cracks evolved in a faulting [Trejo-Moedano and
Baini, 1991; Pacheco et al., 2006]. Currently the superficial evidence of these movements
consists of a fault scarp with a vertical offset of more than 1 m at some locations along its
trace [Rojas et al., 2002]. The DInSAR analysis over Querétaro identified a deformation
field characterized by two separate domains: a stable area corresponding to the Eastern portion of the city and, separated by a clear discontinuity oriented in the NNW-SSE direction
with a length of more than 3 km, a sector on the Western side of the city corresponding to
the industrial district affected by ground movements, as indicated by the retrieved narrow
interferometric fringes (Fig. 5). By unwrapping the interferometric phase along a section
Figure 5 - a) Differential interferogram of Querétaro overlaid to a multi-look SAR amplitude image
in SAR coordinates spanning the time interval 16/08/2003-13/11/2004, the white dotted line represents the location of a cross section, b) differential interferogram of Querétaro overlaid to a multilook SAR amplitude image in SAR coordinates spanning the time interval 18/12/2004-03/12/2005.
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DInSAR analysis of land subsidence in the Mexican Volcanic Belt
Figure 6 - Displacement profile related to the time periods 16/08/200313/11/2004 obtained through phase
unwrapping along the AB section
located across the 5 de Febrero discontinuity.
deformation rates across the discontinuity up to 6.8 cm/year between 2003 and 2004 were
measured (Fig. 6). The geolocation of the differential interferograms and the comparison
with the main tectonic structures, as reported by several authors, confirmed that the discontinuities correspond to the main fault of the Querétaro city, namely the 5 de Febrero fault.
Field checks identified a clear morphological evidence of the movement along such a tectonic structure, even if due to the industrial nature of that portion of the city the number of
damaged buildings was not so significant.
Indeed, most of the damage were detected on the transportation network, such as disrupted
pavement along the 5 de Febrero avenue and significant curvature of the railway tracks to
Celaya.
Celaya
The city of Celaya is located on a large alluvial and fluvio-lacustrine plain resting upon a
basement of Cretaceous rocks of volcanic and sedimentary origin that outcrops in the Sierra
de Guanajuato. From the base to top three sequences can be recognized: a sequence of py-
Figure 7 - a) Differential interferogram of Celaya overlaid to a multi-look SAR amplitude image in
SAR coordinates spanning the time interval 11/02/2006-27/05/2006; b) differential interferogram of
Celaya overlaid to a multi-look SAR amplitude image in SAR coordinates spanning the time interval
16/08/2003-13/11/2004.
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roclastic and terrigenous layers with a thickness up to 50 m; an alternation of andesitic and
basaltic lava flows with thickness ranging from a few meters to over 300 m with associated
pyroclastic products and a granular sequence from the Pliocene-Pleistocene made up of an
alternation of clays, sands, conglomerates and pyroclastic products with thicknesses from
a few meters to 200 m. Within this sequence several levels of clays are embedded forming
the main aquitards. The faulting associated with PSCF in Celaya has a NNW-SSE direction.
Damage has been registered in the downtown area starting from the 1980s. Several buildings located along the discontinuities were completely destroyed and the trace of the main
roads crossing the tectonic structures are affected by vertical offsets of more than 2 meters
at some locations. Even in this case the interferometric analysis revealed ground displacements in the middle of the urban area, distribuited along two major discontinuities circa NS oriented (Fig. 7) with rates of 2-3 cm/year. In addition, two clear subsidence bowls were
identified in the Northern part of the city, probably connected to the industrial activity and
the consequent ground-water exploitation.
Due to the presence of discontinuous surface deformation along several parallel fractures
located in a narrow portion of the urban areas some difficulties have been encountered in
the correct propagation of the phase unwrapping algorithms in the analysis of long-term
interferograms. Such a problem has been preliminarily overcome by using interferograms
spanning a shorter time interval (Fig. 8).
Field surveys carried out over the areas affected by movements, as revealed by the satellite
analysis, allowed to identify clear evidences of vertical movements, with scarps 2 m high at
some locations along the main faults (Fig. 9).
Figure 8 - Line-of-sight displacement map of Celaya overlaid to a topographic
map, spanning the time interval 11/02/2006-27/05/2006. A coherence threshold of
0.5 has been used for phase unwrapping.
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Figure 9 - Picture taken during the
summer 2006 showing the morphological evidence of differential
ground settlements along the main
fault in the city of Celaya.
Conclusions
The DInSAR analysis carried out over the cities of Morelia, Querétaro and Celaya described in the paper revealed the presence of differential ground displacements along linear
discontinuities corresponding to the tectonic structures identified in the field and reported
in the literature. These outcomes confirm how the regional and local tectonic structures
of the MVB play a controlling role in the spatial distribution of the ground settlements.
Displacement rates up to 6-7 cm/year have been registered in the city of Querétaro, while
in Morelia and Celaya the registered velocities reached few centimetres per year. Furthermore, the obtained deformation maps enabled to extend the mapping of the main faultings
even over portions of the urban areas where the discontinuities were not yet identified by
field surveys, due to their still scarce superficial evidences. The presented results, which
represent the preliminary outcomes of a study focused on all the urban settlements located
in the Central and Northern parts of the MVB affected by differential land subsidence, will
be integrated in the future with ancillary data and validated with the available in-situ measurements, in order to improve the understanding of the on-going phenomena.
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