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SUBSIDENCE DETECTION USING INSAR AND GEODETIC MEASUREMENTS IN
NORTH-WEST OF IRAN
Morteza Sedighi (1)
, Siavash Arabi(1)
, Hamid Reza Nankali(1)
, Masomeh Amighpey(1)
, Farokh Tavakoli(1)
, Ali
Soltanpour(1)
,Mehdi Motagh(2)
(1)National Cartographic Center (NCC), Meraj Ave. – Azadi Sq. - Tehran -IRAN, Email:Sedighi@ncc.org.ir (2)Tehran University, Karegar-e-Shomali St.(Kouye Daneshgah) -Tehran -IRAN
ABSTRACT
Geodetic measurements i.e., repeated leveling
measurements of first order leveling network of Iran and
continuous GPS measurements of Iranian Permanent
GPS Network of Iran (IPGN), showed that there is
subsidence in the north-west of Iran. In this paper we try
to find the area and rate of subsidence in Ghara-Geshlagh in north-west of Iran using InSAR and
geodetic techniques. The result of InSAR technique
show a better understanding on this phenomenon in
Salmas and Ghara-Gheshlagh area and have a good
consistency with accurate geodetic measurements.
1. INTERODUCTION
The subsidence of the Earth surface is a phenomenon
that occurs in some places in the world which overuse
underground sources of water. Iran has semi-arid and
arid climate and the rate of rainfall is lower than the
mean rate in the world. Now we are encountered by over-exploitation of groundwater in agricultural areas
and also for extending the cities and industrial areas.
From the past, geodetic measurements such as leveling
survey have been used for subsidence investigation to
map the height variations. It can measure the height
variations with very high precision, but only provides us
the data at limited numbers of stations along the routes.
When GPS came up with a fast and accurate technique
for measuring the earth surface displacement, we
attained a faster and cheaper measurement with respect
to leveling. The main advantage of GPS is the capability to measure real time 3D component displacements but,
similar to leveling, these measurements are also limited
to some scattered points (permanent GPS stations or
campaign GPS networks).
For studying a wide area deformation such as
subsidence phenomena, we have to design a dense
geodetic (leveling and/or GPS) network to estimate the
area and the rate of displacement. Although the geodetic
techniques have good accuracy for detecting the Earth
surface deformation but these techniques are not
increasingly competent for the large-scale deformation monitoring such as subsidence. Compared with leveling
and GPS measurements, which provide information at
the specific observation points, the Interferometric
Synthetic Aperture (InSAR) technique can provide
accurate subsidence information over widespread areas
with high spatial resolution. In the other words, the
InSAR provides an effective tool to detect and measure
the magnitude and spatial variation of the earth surface
displacement such as subsidence especially when it is combined with geodetic techniques. Indeed, InSAR only
measures displacement along the satellite Line Of Sight
(LOS) direction, and it is most sensitive to vertical
motion. Therefore it is an appropriate technique for
measuring subsidence.
In the InSAR technique two complex SAR images
(recorded by antennas mounted on satellite or airplane)
are combined to generate surface deformation maps
and/or DEM (Digital Elevation Model). The
interferograms are generated by multiplication of the
first image (Master) to the complex conjugate of the
second image (Slave). The result of this multiplication contains the phase difference of two images. This fringe
pattern reflects the ground displacement that has
occurred between the two acquisition times (Master and
Slave). The observed phase (∅int ) is the sum of several
contributions (Eq. 1):
∅int = ∅topography + ∅displacement + ∅atmospher +
∅orbit + ∅noise (1)
These components are topographic phase, surface
displacement signal (phase), atmospheric delay, orbital
phase, and noise respectively. The orbital error (affected
phase by orbital parameters) can be removed using
precise orbital parameters e.g., DEOS orbital
parameters. The topographic term is removed from each
interferogram using DEM e.g., Shuttle Radar
Topography Mapping (SRTM) DEM. The displacement
signal is the component that we attempt to extract from interferograms. Each fringe cycle in an interferogram
corresponds to half the radar wavelength (that is 28 mm
for ENVISAT satellite data) ground displacement along
the LOS direction.
2. STUDY AREA
National Cartographic Center (NCC) of Iran has
established and re-measured the first order levelling
_____________________________________________________ Proc. ‘Fringe 2009 Workshop’, Frascati, Italy, 30 November – 4 December 2009 (ESA SP-677, March 2010)
network of Iran during the past 20 years. Also NCC has
established 110 permanent GPS stations (since 2004) to
monitor ground surface movement for geodynamic
purposes (Figs. 1a, 1b). Repeated levelling (on some
levelling lines) and (some) permanent GPS
measurements showed that there are subsidences in
many parts of Iran. Salmas and Ghara-Gheshlag area as
a large agricultural zone is one of the subsiding areas located in the north-west of Iran (north-west of Urumieh
Lake) (Fig. 2).
3. DATA
The levelling line (AGAH) from Khoy to Urumieh
measured in two epochs in 1989 and 2005. These
measurements revealed subsidence in some levelling
benchmarks near Salmas. The subsidence rate
determined by levelling measurements during 17 years
is about 6 cm/year (Fig. 3). Also the GPS permanent
station in Ghara-Gheshlagh (GGSH) showed subsidence
with the same rate with nonlinear behaviour (Fig. 4).
Figure 3. Elevation changes in Levelling line (AGAH)
1989-2005
Figure 1a. First Order Levelling Network of Iran and
Detected Subsidence Areas Figure 1b. Iranian Permanent GPS Network
(IPGN)
m
Figure 4. GPS time series of GGSH permanent station –
h component
In order to study the temporal behaviour of the
deformation in high spatial resolution, the InSAR observations acquired by ENVISAT satellite are used to
investigate land subsidence in Salmas and Ghara-
Gheshlagh area caused by groundwater
overexploitation. Our dataset consists of 11 images
acquired by ENVISAT satellite chosen from descending
track 92 and ascending track 228 passes from August
2003 to November 2007. The spatial and temporal
coverage of these images are presented in Fig. 5&
Tab.1.
Figure 5. Location of ENVISAT images
Table 1. ENVISAT data used in this project No. Orbit Number Acquisition Date
1. 8162 2003-09-22
2. 9164 2003-12-01
3. 10667 2004-03-15
4. 11669 2004-05-24
5. 15312 2005-02-02
6. 16815 2005-05-18
7. 17817 2005-07-27
8. 20186 2006-01-09
9. 26198 2007-03-05
10. 28202 2007-07-23
11. 29705 2007-11-05
The selected interferograms have the perpendicular
baseline less than 400m and the time span from 70 days
to 8 months (The normal orbital cycle for each satellite
is 35 days). In this paper analysis of 6 interferograms
are presented.
4. PROCESSING
The InSAR images were acquired from ESA, are
processed using InSAR processing software "DORIS"
with Delft orbits (DEOS). The topographical effect in
the interferograms was removed from each
interferogram using a 30m SRTM-DEM. The
"Goldstien filter" used for interferogram filtering.
"SNAPHU" software was used for unwrapping the interferograms, i.e., converting from phase to
displacement along the satellite line of sight. By
considering that the displacement is a pure vertical
movement, the displacements were projected into the
vertical direction.
Fig. 6 shows six interferograms of the study area. As it
was expected, the fringes cover whole parts of plain
between Salmas and Ghara-Gheshlagh.
Figure 6. Four interferogram in Salmas area with
temporal baseline between 70-240 day
5. TIME SERIES ANALYSIS OF SUBSIDENCE
AREA BASED ON LEAST SQUARES
METHOD
For obtaining a time series of subsidence in the study
area, Least squares method is used for adjusting the
observations. The result of this adjustment can give us a
better view of subsiding (from the availability of SAR
images). Each interferogram provides the displacement
of the earth surface for a period of time between master
and slave images. The time of first image is considered
as the origin of subsidence (zero subsidence) and then
the absolute subsidence for other images are computed
with respect to the origin using Least square technique.
The result of this adjustment is represented in Fig. 7.
Also, the DEM of the area is represented in Fig. 8.
6. RESULTS
As it was expected, subsidence was observed in Salmas,
Ghara-Gheshlagh plain. Although the GPS and levelling
measurements show about 6cm/year subsiding rate for
some points in the area which is consistent with results
from InSAR for the same points, new results from
INSAR have provided an extended spatial pattern of the
subsidence with highlighting the points with maximum
rate of subsidence.
7. ACKNOWLEDGEMENT
We would like to thank the European Space Agency
(ESA) that provides SAR images (ENVISAT data)
according to the proposal 6244 (C1P.6244).
Figure 7. Adjusted subsiding maps
Figure 8. DEM map of study area
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