RADAR INTERFEROMETRY FOR MONITORING LAND SUBSIDENCE DUE TO
OVER-PUMPING GROUND WATER IN CRETE, GREECE
S. P. Mertikas(1), E.S. Papadaki(1)
The Technical University of Crete, GR- 73 100, Chania, Crete, Greece, Email :[email protected]
(1)
ABSTRACT
The level of water in several wells at the Messara valley,
Crete, Greece has dropped 40 meters over the last 20
years. Anecdotal reports describe cracks in the concrete
foundations of some residential structures around the
valley. These also suggest that ground subsidence may
exceed one centimeter, at least, over a period of
recharging and withdrawal ground water.
Conventional differential SAR interferometry using
ERS-SAR and ALOS-PALSAR images, as well as the
stacking technique have been applied to monitor this
ground deformation. The used images covered the period
from 1992 to 2000 and 2007-2009, respectively.
A total of 29 ERS-1&2 SAR and 7 ALOS images have
been used for forming interferograms. Image pairs with
Doppler difference less than 0.20 pulse repetition
frequency, and perpendicular baseline smaller than 100m
have been used for processing. A Digital Elevation
Model with 20-m pixel size and ±7 m height accuracy
has also been incorporated in interferometric processing.
Atmospheric artifacts have been compensated by using
image stacking.
The valley to be monitored is densely cultivated and
irrigated. Thus, loss of coherence in images has been
observed in the C-Band and could not be overcome.
Consequently, interferometric results with the ERS-SAR
images have been limited. On the other hand, processing
of L-band data has brought up a ground deformation that
amounted to a subsidence of at least 4 cm/yr. The
correlation of the observed ground deformation with
respect to water pumping and other geological
parameters has also been investigated.
1. INTRODUCTION
Land subsidence could be induced by pumping excessive
ground water in a region. Such deformation has already
been observed in wide areas all over the world. The Po
Valley in Italy, the Antelope and the San Joaquin
Valleys in the USA, as well as the Bangkok in Thailand
are some example cases where this type of deformation
is taking place [7]. The phenomenon occurs especially in
non-homogeneous
porous
media
environments.
Lowering of the level of ground water leaves
underground caves which are unable to support the land
overlying them. Hence, land subsidence occurs. In urban
_____________________________________________________
Proc. ‘Fringe 2009 Workshop’, Frascati, Italy,
30 November – 4 December 2009 (ESA SP-677, March 2010)
areas, subsidence could be exaggerated because of the
additional weight imposed by the constructions above.
There, buildings may crack and sometimes collapse,
while roads could twist and brake.
Measuring subsidence is commonly carried out by using
conventional ground-based techniques, such as precise
levelling and geodetic surveys. Nevertheless, new
remote-sensing techniques for monitoring ground
deformation have recently emerged. Such a powerful
tool is the differential interferometry (DInSAR) using
Synthetic Aperture Radar (SAR) satellite images on
repeat orbits. Several studies on monitoring ground
subsidence by DInSAR have already been conducted,
especially in the arid, south-western parts of the United
States, where agricultural irrigation greatly shrinks
aquifers; perhaps irrecoverably ([1], [2], [3] and [6]). A
detailed literature review on the application of DInSAR
techniques for monitoring subsidence could be found in
[9].
The aim of this work is to detect and map ground
deformation at the Messara valley in Greece by using
differential SAR interferometry. The area under
investigation is situated in the south of the Heraklion
prefecture of Crete in Greece. During the past 20 years,
the volume of ground water in storage is decreasing.
Over-exploitation of water aquifers, for irrigation and
water supply of cities, has led to a dramatic drop of 40
meters in the level of groundwater. Anecdotal reports of
cracks in concrete foundations, in residential structures
around the Messara valley, suggest that ground
subsidence induced by such intense pumping, exceeds a
centimeter over a one winter-summer rechargewithdrawal cycle.
Experience however suggests that little information is
available about the exact volume of water withdrawn or
consumed in the area. Only in 1990, the authorities in
Heraklion prefecture of Crete issued some 500 licenses
for drilling and pumping water wells [8]. On the other
hand, the local population increases. These issues raise
great concern over the sustainability of the available
water resources in Crete with a possible desertification in
the horizon. Therefore, monitoring ground subsidence
using precise data can help us in evaluating, and
consequently managing properly the balance between
supply (inflow) and demand (recharge) in water aquifers.
2. GENERAL SETTING OF THE AREA
The Messara valley covers an area of more than 300 km2
in south east Crete (Fig. 1). North of it, there exists the
highest mountain of Crete, the “Idi” mountain range,
with a peak altitude of 2456 m. The Asterousia mountain
chain, south of the Messara basin, rises 600 m in the
west to 1200 m in the east and constitutes the
southernmost mountain range of Europe (Fig. 2). The
climate is dry and sub humid.
The Asterousia Mt
Figure 2. The main crops cultivated at the Messara
basin are olive groves and vineyards. The Asterousia
Montains are shown in the background.
From the geographical and hydrogeological point of
view, Messara is distinguished into the eastern part,
represented by the Praitoria basin, and the western part
which consists of the basins of Asimi, Vagionia, Moires,
Pompia and Tympaki. All these areas have been named
after the neighbouring small towns (Fig.5).
The Messara basin is covered mainly by Quarternary
alluvial clays, silts, sands and gravels with thickness
from a few metres to 100 m or more. The
inhomogeneities of the Plain deposits give rise to great
variations in the hydrogeologic conditions even over
small distances. The northern slopes are mainly siltymarly Neogene formations while the southern slopes are
mainly schists and limestone Mesozoic formations.
3. METHODOLOGY
MONITORING
Figure 1. The Figure above shows the location of the
area under investigation in the island of Crete,
Greece. The lower image is a LANDSAT TM false
colour composite image showing the broader area of
Messara Valley, Crete, Greece (near infrared=red,
visible red=green and visible blue=blue).
Around 80% of the area in Messara is used for
agricultural irrigation (Fig. 2). The valley is the largest
and most productive region of the island of Crete.
FOR
DEFORMATION
Conventional differential SAR interferometry using
ERS-SAR and ALOS-PALSAR images, as well as the
stacking technique have been applied to monitor this
ground deformation. The used images covered the period
from 1992 to 2000 and from 2007 to 2009, respectively.
At the first phase, the conventional DInSAR technique
has been applied by processing a total of 29 SAR images
from the European Remote Sensing satellites 1&2.
Images were acquired in descending mode and were
available in the Single Look Complex (SLC- CEOS)
format. The frame title was Track 422/Frame 2897.
These SAR images cover the time interval 1992-2000.
They have been used to form 406 potential master-slave
image pairs for monitoring land subsidence.
The number of available interferograms has been
reduced using the following sorting criteria:
1. Image pairs having perpendicular baseline smaller
than 50 m have been selected for processing.
2. Pairs exhibiting a Doppler difference greater than 0.2
Pulse Repetition Frequency (PRF), have not been taken
into account.
3. To avoid loss of image coherence, interferograms
spanning more than 3 years have been excluded for
processing.
4. Pair-wise logic has been performed for eliminating
atmospheric and orbital fringes [5].
Interferometric processing has been performed using the
DIAPASON software (originally developed at CNES,
France). The precise determination of satellite orbits
have been based on the Delft orbit archive, available at
A
Digital
http://www.deos.tudelft.nl/ers/precorbs.
Elevation Model (DEM) has been used to eliminate
topographic artifacts. The available DEM had a 20-m
pixel resolution and a vertical accuracy of ±7m. It was
produced using SPOT-4 stereopair images.
The stacking technique, i.e., summing or averaging of
multiple differential interferograms into a single
interferogram, has also been carried out. This technique
has the advantage of overcoming the two main
shortcomings of conventional DInSAR: 1) the loss of
coherence occurring when long temporal baselines are
used, and 2) the atmospheric interference mainly due to
the wet component of the troposphere.
Stacking is justified because reasonable coherence levels
can only be achieved over short time intervals.
Therefore, averaging as many short-time intreferograms
as possible may finally form a single pseudointerferogram corresponding to a longer period.
Moreover, the atmosphere is a temporally and spatially
uncorrelated propagation medium. Consequently, the
more SAR image pairs are applied in stacking, the higher
the possibility for the elimination of the atmospheric
artifacts. The presence of atmospheric artifacts in a
particular date does not necessarily preclude the use of
that image in a stack; as long as the master image is
being used the same number of times as the slave does.
The result of adding deformation interferograms with
common images can be compared with one
interferogram produced using the first and last image.
Atmospheric influence is thus limited to the atmospheric
interference present in these two final images [4]. Also,
when adding interferograms, the unwrapping of the
individual interferometric pairs is not necessary.
A large number of master-slave interferometric pairs and
several stacking combinations have been thus produced.
Finally, it has been concluded that the C-band frequency
has not been efficient in producing reliable results for the
land subsidence of the area because of the dense
vegetation and irrigation applied in Messara Valley.
Consistent loss of coherence has been readily noticed for
most of the interferograms in the C-band. Small
temporal baselines have not helped in mapping the land
subsidence expected. Temporal baselines have been
either characterized by atmospheric perturbations at the
acquisition time of one or both of the images used to
form an interferometric pair, or have been too small, e.g.,
with 1-month temporal separation. On the other hand,
long temporal baselines have led to temporal
decorrelation and total loss of coherence in
intereferograms within the valley.
In the next phase, differential SAR intereferometry has
been applied using the ALOS (Advanced Land
Observing Satellite) PALSAR (Phased Array type Lband Synthetic Aperture Radar) images, acquired by
Japan's latest Earth observation satellite. The idea of
using ALOS imagery has been based on the fact that the
interferometric signal of L-band (λ=23.5 cm) is expected
to be more coherent on vegetated areas than the C-band
(λ = 5.6 cm). In this preliminary analysis, three ALOS
data sets have been used for processing (Tab. 1). For
computational purposes, processing has been carried out
using only the area of interest and not the full frame
images.
The potential disadvantages of L-band versus C-band
are: First, the lower fringe rate, measured in cm instead
of mm, may result in less precise monitoring for the land
subsidence; and second, the ionospheric refraction could
be 16.5 times worse in L-band than in C-band frequency.
The path delays in radar signals, caused by water vapour
in the troposphere, are independent of wavelength. So,
this distortion should affect both bands, equally [4].
The ALOS images (Tab. 1) had been acquired in
ascending mode (~9:00 p.m.). This should overcome
both the ionospheric and the tropospheric contamination
to a quite satisfactory level. In this work, the ionospheric
path delay has been taken into consideration. It is
proportional to the number of electrons per unit area in a
column extending from the earth surface to the satellite.
This columnar electron density count is referred to as the
Total Electron Content (TEC). It is measured in TEC
units, or TECU. Each TECU equals 1016 electrons/m2.
The TEC units as determined in this work are presented
in Table 1. Results have been based on the International
Reference Ionosphere model (http://iri.gsfc.nasa.gov).
Table 1: ALOS PALSAR images used in this
investigation.
Orbit
5190
15926
14584
Date
14/1/2007
19/1/2009
19/10/2008
Polarization mode
Fine Beam Single
Fine Beam Single
Fine Beam Double
TECU
2
1.9
1.6
The zenith ionospheric correction, dion, is inversely
proportional to the square of the radar frequency:
d ion
k ⋅ (TEC )
=
f2
(1)
where k is a constant and equal to k=–0.40250 m
GHz2/TECU. The negative sign of k is justified by the
fact that a decrease in TEC results in an increase of
phase delay [4].
In the case of DInSAR, an interferogram is the
difference between two SAR acquisitions acquired at
different days, but at the same local time. Therefore,
identical TEC values will cancel out the effect of
ionosphere in the interferogram.
At the L-band frequency of ALOS, with approximately
1.27 GHz, the ionospheric delay in the radar signals
amounts to about 25cm for each TEC unit variation in
the ionosphere. In other words, a 1 TECU increase
between the master and slave images yields a 25cm
phase advance, i.e., 25cm should be added to the
interferometric values determined.
4. RESULTS
The valley to be monitored is densely cultivated and
irrigated (wet ground surface). Thus, loss of coherence in
images has been observed in the C-Band and could not
be overcome. Consequently, interferometric results with
the ERS-SAR images have been limited. Decorrelation
has been observed for most of the image pairs, spanning
time intervals greater than 6 months. In addition, the
atmosphere has significantly deteriorated the radar signal
for even smaller time intervals.
Figure 3. The ERS-SAR pseudo-interferogram
corresponding to the period 20/6/1993 and 10/11/1999.
Fig.3 presents the linear combination (in this case the
wrapped sum in complex domain) of six ERS SAR
images. Those cover the following periods 1) 20/6/1993-
23/8/1995; 2) 23/8/1995-22/8/1995; 3) 22/8/19957/8/1996; 4) 7/8/1996-16/10/1996; 5) 16/10/19961/10/1997; and 6) 1/10/1997-10/11/1999. Summation of
these interferograms at consecutive periods has aimed at
eliminating the atmospheric interference and at
providing a land deformation signal covering the entire
time interval 1993-1999. The use of the tandem image
pair taken on 22/8/1995-23/8/1995 and applied in
reversed chronological order has been made only for
eliminating the atmospheric noise. No land subsidence is
expected to be masked using this reversed one-day
interval in the stacking chain.
As has been mentioned earlier, the atmosphere in that
particular linear combination of images should be
limited to the first and last images only, specifically the
images acquired on 20/6/1993 and 10/11/1999. Note that
all individual pairs chosen for processing have been
characterized with high values for the altitude of
ambiguity (ha) and small differences in Doppler values
(Δdop). Nonetheless, consistent loss of coherence has
been evident in the broader area of interest. The signal
has been enhanced in a couple of regions (see the areas
indicated by the white arrows in Fig.3). However, for
these features, an atmospheric perturbation at the
acquisition time of the first image acquired on 20/6/1993
has been the most probable explanation for it.
On the other hand, the longer wavelength of ALOS Lband has overcome the problem of incoherence caused
by wet ground surface because of irrigation and
vegetation at the valley. Even for long time intervals, the
land subsidence has been very well mapped on the final
interferograms with ALOS imagery (Fig. 4). The
noticeable fringes shown in Fig.4 cannot be attributed to
topographic artefacts. Because they appear in both
interfograms, regardless of the different values for the
height of ambiguity (ha = -585m and -97m, respectively).
In ALOS PALSAR data, one full colour cycle
corresponds to λ/2 ≈ 11.8 cm of surface displacement
and along the line of sight (LOS) between the ground
target and the satellite.
MASTER
14/1/2007
MASTER
14/1/2007
SLAVE
19/10/2008
SLAVE
19/1/2009
ha (m)
-585
ha (m)
-97
ΔdopPRF
-0.00131
ΔdopPRF
+ 0.00802
Figure 4. ALOS PALSAR processing has mapped quite
well the land subsidence occurring at the Messara valley
even for long temporal baselines.
Figure 5 .Unwrapped interferogram for the two year
time span, overlain by the drainage system and the
main small towns of the area.
At the last phase, correlation of the DInSAR results
using ALOS data, with respect to water pumping has
been examined. More particularly, in the framework of
INTERISK project, an interregional telematic network
for the management of Risks in the Balkan and South
Meditteranean area, and the BEWARE-CRINNO project
(Crete Innovative region), the Region of Crete has
initialized a telematic system consisting of five
meteorological stations and 30 more stations for
measuring both underground and surface water. These
stations are placed in the main water wagons of the
island.
Fig. 6 shows the groundwater level fluctuations recorded
below the station placed at Praitoria area. The station is
established at an altitude of 44.91m above sea level
surface. The general decline of ground water level is
easily observed.
Unwrapping of the 2-year interferogram (2007-2009) (as
shown in Fig.5) has resulted in a land subsidence of
~8cm/yr along the direction of the line of sight. Taking
into account the 0.1 decrease in TEC units between the
two image acquisitions (see Table 1), a path delay for
ionosphere equals to 2.5cm. Consequently, this value
should be subtracted from the unwrapped values
presented in Fig.5.
The highest rate of subsidence is located close to
Praitoria basin area and is of the order of ~5cm/yr. It is
clear that the broader area of Messara undergoes strong
land deformation.
Figure 6. Groundwater level fluctuations (shown in
meters) recorded at the Praitoria station and for the
period 2005-2008.
It appears (Fig. 6) that the level of water table keeps
dropping during the last 4 years to a considerable
amount. In particular, a worrying decrease of about 18 m
has been marked during the period October 2006–
October 2008. The rate of the decrease is considerably
high during the hydrological cycle May-October, i.e.,
during the summer seasons. As a result of this, the
Region of Crete has decided upon the rejection of all the
applications requesting new water wells in the area.
Similar dramatic data can be found in annual reports
given by the Region of Crete. It suffices to say that: 1) in
the case of Pompia, a drop in the level of water table by
11m has been observed over the period October 2006October 2007 and by 19m over the period October 2005October 2007; 2) at the Moires station, a decrease of
about 4 m has been recorded during the period October
2006-October 2007 and of 13m over the period October
2005-October 2007; 3) a lower decrease of 1.5m has
been marked at the Tympaki station during the period
October 2005-October 2007; 4) at Asimi, a 11m decline
of the groundwater’s level has been observed over the
period October 2004-October 2005; and 5) on October
2005, the sensor’s readings at the Vagionia station
indicated that the groundwater level has reached the
drilling floor. Undoubtedly, the estimated drop to such
low levels raises serious concern over the sustainability
of water resources in the area. To invert the situation
detected, special actions and measures should be taken
immediately.
and Pompia basin areas and the respectively high
groundwater level decrease recorded over the period
2005-2008.
5. CONCLUSIONS
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(1998). Detection of aquifer system compaction
and land subsidence using interferometric synthetic
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7. Tomás, R., Márquez, Y., Lopez-Sanchez, J.M.,
Delgado, J., Blanco, P., Mallorquí, J.J., Martínez,
The continuity of ERS missions for almost 15 years
provides a unique data archive for monitoring slow land
deformations caused by over-pumping water resources.
However, the short wavelength (~ 5 cm) of ERS signals
brings about a temporal decorrelation in the images over
long time intervals. Also, these radar signals cannot
penetrate extreme atmospheric conditions. Preliminary
results of classical DInSAR and stacking ERS SAR
images has indicated that the C-band has not been
capable of mapping the land deformation at the Messara
valley. Atmospheric corrections using in situ
measurements of temperature, pressure and humidity
have not been carried out. This is because the coherence
levels have been extremely low, even for interferograms
spanning from a few months to less than a year.
On the other hand, the ALOS PALSAR images and its
unique capability to penetrate vegetation canopy seems
to have mapped quite well land subsidence. An
improvement in temporal correlation has been far more
than obvious. However, atmosphere is still an issue that
should be further investigated. First, the TEC values
determined here account for a zenith ionospheric delay.
These values should be recomputed in the radar’s line of
sight. Tropospheric interference should also be
examined.
Lastly, geological data should be investigated to justify
the differences between the relatively small land
deformation revealed by DInSAR in the case of Asimi
Plans for future research include processing of more
ALOS PALSAR images, at variant time intervals and
polarization modes.
Two Global Navigation Satellite Systems reference
stations will be established in the broader area of interest
to monitor independently land subsidence with
millimeter accuracy. The first GNSS station will be
placed close to the region exhibiting the highest
subsidence rate, i.e., the Praitoria or the Moires basin,
while the second one will be placed in an area of general
equilibrium, ~20 km northern of the valley.
Acknowledgements. This work has been supported by
the FP7-REGPOT-2008-1, Project No. 229885 (SOFIA),
sponsored by the European Commission. The ERS SAR
and ALOS PALSAR images have been provided by the
European Space Agency- ESA in the framework of
Category-1 Scientific Research Project C1P-4373.
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