On the contrary, Sharm El Maya, an inner zone parallel to the ... Keywords: SAR interferometry, interferometric stacking, ground deformation, Sharm El-Shiekh, ...
Detection of ground deformation in the broader area of Sharm ElShiekh (Sinai Peninsula, Egypt) by SAR interferometry Tarek A. Seleem *a, Michael Foumelisb, Issaak Parcharidisc Suez Canal University, Faculty of Science, Department of Geology, Ismailia, Egypt b Dept. of Geology, University of Athens, Penepistimioupoli-Ilisia, Greece c Department of Geography, Harokopio University of Athens, El. Venizelou 70 Kallithea, 17671 Athens, Greece a
ABSTRACT Sharm El-Shiekh area is located in the most southern part of Sinai Peninsula boarded by the Gulf of Suez to the west and by the Gulf of Aqaba to the east. The present study concerns the application of Multibaseline/Stacking Differential SAR Interferometry (DInSAR) in order to monitor ground deformation rates in the southern part of Sharm El-Shiekh area. The specific technique was applied in order to reduce the influence of atmospheric effects on ground deformation estimates. For this purpose a total number of 24 ENVISAT ASAR scenes covering the period between 2002 and 2008 were processed and analysed. Interferometric results show both patterns of uplift and downlift in the study area. Specifically an area along the coastline with a N-S direction, corresponding to the build up zone of Sharm El-Shiekh, shows average annual subsidence rates between -5 and -7 mm/yr along the line of sight (LOS). On the contrary, Sharm El Maya, an inner zone parallel to the above subsided area, shows slant range uplift of around 5 mm/yr. The obtained results of SAR inteferometry probably indicate the presence of an active fault that affects the coastal zones of Sharm El-Shiekh area. Keywords: SAR interferometry, interferometric stacking, ground deformation, Sharm El-Shiekh, Sinai Peninsula.
1. INTRODUCTION ”Traditional” ground motion monitoring methods are based on field surveys. These methods include optical leveling, Global Position Systems, extensometers etc. During the last two decades the SAR Interferometric technique based on radar satellite data have become a useful tool for ground deformation detection and monitoring [1,2,3,4,5,6,7,8,9]. Radar is an active sensor that alternatively sends out radio waves in form of pulses and records the echoes scattered back by objects (targets) hit by the waves along their travelling path. Each echo (backscattered signal) is a modified version of the transmitted pulse through the reflectivity function of the target(s) seen on the ground. Since the transmitted signal is a complex quantity, also the signal received by the radar is complex. It is composed by the magnitude, which is related to the power scattered back toward the sensor by the target, and the phase, which is expressed by the two-way path distance between the sensor and the target. Both amplitude of the radar signal and phase carry valuable information for the interferometric applications. The magnitude and the phase of the complex interferograms are generally referred to as the degree of coherence (or simply coherence) and the interferometric (InSAR) phase respectively. The coherence measures the degree of correlation between two SAR images. The interferometric phase has values between 0 and 2π (or between -π and π depending on the representation used) and therefore appears as a series of fringes. Two consecutive fringes represent a phase difference of 2π. Areas of low coherence are characterized by noisy interferometric phase. Coherence is a measure of the phase noise or fringe visibility. Since 1992, conventional 2-pass Differential InSAR (DinSAR) has been used by the scientific community to further study and understand specific ground deformation hazards. Normally this interferometric method is used to map ground deformation caused by a natural event like earthquake or landslide etc. The basic idea of differential interferometric processing is to separate the topography and displacement related phase terms allowing, in particular, the retrieval of a differential displacement. This goal is achieved by subtracting the topography related phase using a simulated DEM.
Conventional Differential Interferometric techniques show limitations related to land-cover dependent temporal signal decorrelation and atmospheric propagation effects. Furthermore, the applicability of this interferometric technique depends also on the magnitude of the deformation. Recent developments (end of 90’s) in differential interferometry have demonstrated some potential to overcome some of the above limitations of the conventional technique and also for more accurate and temporally dependent results. The aim of this study concerns the application of Multi-baseline Differential SAR Interferometry and specifically Interferometry Stacking in order to monitoring ground deformation in the southern part of Sharm El-Shiekh area at south Sinai Peninsula (fig. 1).
Figure 1. Location map of the study area (geographic background from Google Earth environment)
2. GEOLOGICAL SETTING 2.1. Lithology Sharm El-Skiekh area occupies the most southern part of the basement rocks of Sinai massif. The exposed rock units ha ve been studied and described by many authors [10,11,12,13,14,15,16,17,18]. The above studies revealed that mainly metamorphic and igneous rocks in addition to the Quaternary cover compose the area (fig. 2). The exposed rock units arranged from oldest to youngest are described as follows: Metamorphic rocks: separated small outcrops represented mainly by metasediements and metavolcanics intruded by younger granites by sharp contacts. Metasediments: are hard, massive, and fine grained with light grey color interbeded, with either metasiltstone or metagreywake in some localities.
Metavolcanics: represented as small relics in the alkaline granitic plutons. These metavolcanics are occasionally interbedded with volcanigenic metasediments . The metavolcanics are represented by meta-andesite, meta-basaltic andesites, and meta-dolerites. Dokhan Volcanics: The term Dokhan volcanics referes to a varicoloured sequence of volcanic flows and pyroclastics of predominately andesitic to rhyolitic composition. At the most southern part of the study area, they form a triangular area around the lower course of Wadi Khashabi and south of it. Older granites: Granitoids are the most dominant rock unit exposed in Sharm El-Shiekh area. The exposures of the older granite are limited in comparison to younger granite covering small areas. The older granites are medium to coarsegrained texture, with grayish color, highly weathered and jointed with dominant NW-SE, NE-SW, and N-S structural trends. They have sharp contacts with both younger and metamorphic rocks. The contact between the older granites and volcanic rocks is structurally controlled. Younger Granites: Younger granites are the most predominant rock unit in the study area. The younger granites are classified into; monzogranites, syenogranites, alkali feldspar granite, and alkali feldspar microgranite. The alkaline granites are more resistant to erosion than other types of younger granites where they form many of the highest and steepest peaks of the study area. The pluton is strongly affected by faulting. The high density of faults is a consequence of position near the southern tip of Sinai Peninsula, where the NNE-SSW fault trend of the Aqaba Rift Valley intersect the NW-SE trending Suez Rift System [11]. 2.2. Tectonics and Seismic activity According to number of studies Sinai Penisula behaves like a rigid block or sub-plate with respect to the African plate [19,20]. The results of the above studies demonstrate that the southwesternward motion of Sinai Penisula with respect to the Arabian plate along the Dead Sea fault system. However Swartz and Arden [21] have postulated that Sinai block moved toward the southeast along the NW-SE faults. Sharm El- Shiekh area is considered one of the most seismically active areas in Egypt which is controlled mainly by the intersection of two major seismic active zones which contribute to the tectonics of the Gulf of Suez rift and Gulf of Aqaba transform tectonics. The Gulf of Suez rift is an asymmetric graben distinguished in the northern, central, and southern basins [22,23]. Sharm El-Shiekh area belongs to the southern part of the Gulf of Suez rift, which in this area, is bordered by N-S to NNW-SSE which mark the transition between the shallow-water Suez basin and the deep (more than 1 km) northern Red Sea basin. The distribution of historical earthquakes shows that the present tectonic activity is concentrated along the southern edge of the Gulf of Suez, near the junction between the Suez rift and the northern Red Sea [24]. On the other hand, the Gulf of Aqaba is a left-lateral transform fault linking the Zagros- Taurus area of plate convergence with the Red Sea opening. Studies [25,26] argue for a 105 km left-lateral movement between Arabia and Sinai. In the region of the Gulf of Aqaba, the main faults trend is N-S to NNE-SSW. They are located on the Sinai and Arabian deformed coastal areas as well as within the Gulf [27,28,]. The 1800m deep Gulf of Aqaba is considered to be a succession of NNE-SSW pull-apart basins [29]. At present, the seismic activity is confined to the Dead Sea area and the Gulf of Aqaba fault. The distribution of historical earthquakes shows that the present tectonic activity is concentrated along the southern edge of the Gulf of Suez, near the junction between the Suez rift and the northern Red Sea. There is no evidence for significant seismcity in the northern and central part of the Gulf of Suez [24]. The largest seismic event occurred in Sharm El-Sheikh area is attributed to the earthquake of Shadwan Island, about 37
Figure 2. Landsat 7 Enhanced Themetic Mapper (ETM) image showing the different rock units in the broader area of Sharm ElShiekh
km south west of Sharm El-Shiekh area (31 March, 1969) with magnitude of 5.9, [30]. As a result of its shock, uplifting of coral reefs by few meters above the present sea level, in addition to fracturing of Quaternary cover, rock slumping, and rock falling were observed in Shadwan Island [30,31]. At Ras Mohamed, in the southern tip of the study area, there are deep and wide fractures that are controlled mainly by NW-SE to WNW-ESE direction that are more or less concordant with the Gulf of Suez tectonics in the form of subsided connected open pits. On the other hand, there are small cracks, which follow the direction of the larger ones. Both types of cracks are interpreted to be as a result of tensional forces due to uplifting accompanied the seismic activity [32].
3. METHOD AND DATA USED Interferometric Stacking aims to combine the information from several differential interferograms in order to extract common information [33,34,35]. The basic procedure involves the computation of linear combinations (generally sums or weighted averages) of differential interferograms. Interferogram stacking is useful in overcoming the following two shortcomings of conventional DInSAR. (i) Low coherency over long temporal separations. If reasonable coherency levels can only be obtained over short time periods (for example, in the case of rural settings in temperate climates) then several short time- period temporallycontiguous interferograms can be summed (subject to data availability) to produce a pseudo-interferogram over a longer period. This enables low magnitude displacements to be monitored over longer periods, where no single coherent interferogram exists. (ii) Atmospheric influence. When multiple differential interferograms exist that brackets an instantaneous event (such as an earthquake or other sudden ground displacement) they can be averaged to increase the (displacement) signal to (atmospheric) noise ratio. This is possible because the displacement signal is constant in each interferogram, whereas the atmospheric signal varies randomly. The average displacement velocity along the stellite’s look direction vdisp is computed as
vdisp=
λ × φcum 4π × tcum
where φcum is the cumulative unwrapped phase and tcum is the total time interval, and the displacement velocity estimation error as
Δvdisp=
λ× n ×E 4π × tcum
here E is the assumed phase error of a single interferograms (e.g. π/2) and λ is the wavelength. Each interferogram has its own phase offset that is determined by averaging interferogram values about the pre-selected reference region. This offset is subtracted from each differential interferogram used in the estimation of the phase rate change. Typically, the error of the phase rate will increase with increasing distance from the reference region as the contribution of the phase errors due to the atmosphere and baseline error increases. In this case a total number of 24 ENVISAT ASAR scenes (table 1) covering the period between 2002 and 2008 were processed and analyzed using the GAMMA software. During the processing the minimum number of interferograms with valid phase values that are required to estimate the phase can be specified. For the removal of the topographic component from the interferometric images, SRTM Digital Elevation Model (1 pixel = 90m) was used.
4. INTERFEROMETRIC PROCESSING The main process steps comprise the generation of co-register Single Look Complex (SLC) images, transformation of DEM to SAR geometry, generation of differential interferograms, filtering, unwrapping, baseline refinement, selection of interferometric pairs to use, reference location selection, averaging by stacking, phase rate to velocity conversion and finally geocoding. The co-registration of complex valued data to identical geometry consists of two main steps. In the first step the registration function is determined. In the second step the registration function is applied in the resampling of the data to the common geometry. The SRTM DEM was resampled from 90x90 m to 40x40 m. The phase unwrapping algorithm that was applied was the Minimum Cost Flow (MCF). This algorithm was used to minimize the total cost associated with phase discontinuities in the scene associated with noise, and layover. The reference region (9x9 pixels) was located 4.5
km westward of Naama city and 7.5 km northwest of Sharm El-Shiekh city. Through the stacking, the estimation of the linear rate of differential phase using the unwrapped differential interferograms was carried out and the stacked phase was converted to displacement. Finally, the deformation map was geocoded from range-Doppler SAR coordinates to UTM map projection. The performed analysis covers a total time span of 8 years. Over urban areas as well as over rocks and arid areas the coherence is high enough allowing interpretation of the interferometric phase even for time intervals of more than 1 year. In this case, it was noted that the coherence of the interferograms independently of the acquisition time intervals was high due to the arid environment of the area. These interferograms were filtered using an adaptive filter to reduce their noise. All the possible combinations with baseline less than 200 m and long time intervals, were computed (in total 68 pairs with an average perpendicular baseline Bp = -51m), unwrapped and stacked in order to generate a single deformation map (fig. 3). A first visual interpretation was carried out in order to identify the main deformation signatures. The resulting interferometric deformation map shows clear deformation patterns of both regional and local character. The deformation is located mainly over a broader area, starting from Sharm El-Shiekh to Naama city at the north. The deformation pattern in the area include displacement both towards (uplift) and away (subsidence) from the satellite. Specifically, over the area of Sharm El-Shiekh, the observed deformation rate is about -4.5 mm/yr while over the Naama city maximum deformation reaches -7 mm/yr (fig 4), contrary to the Sharm El Maya where the observed deformation is towards the satellite with the maximum rates of 4 mm/yr are located one kilometer west of Sharm El-Shiekh. Moreover, local deformation patterns are observed over the area between Naama city and the airport at the north with a maximum of -3.3 mm/yr. Table. 1. List of acquired ENVISAT imagery (Track: 350, Frame: 3051)
Count 1 2 3 4 5 6 7 8 9 10 11 12
Scene 14/03/2003 14/11/2003 19/12/2003 27/02/2004 11/06/2004 24/09/2004 29/10/2004 03/12/2004 11/02/2005 18/03/2005 27/05/2005 05/08/2005
Orbits 05414 08921 09422 10424 11927 13430 13931 14432 15434 15935 16937 17939
Count 13 14 15 16 17 18 19 20 21 22 23 24
Scene 09/09/2005 18/11/2005 23/12/2005 03/03/2006 16/06/2006 29/09/2006 08/12/2006 16/02/2007 23/03/2007 19/10/2007 23/11/2007 11/04/2008
Orbits 18440 19442 19943 20945 22448 23951 24953 25955 26456 29462 29963 31967
5. RESULTS In the present investigation a large number of Envisat ASAR images were available allowing optimizing the data selection with respect to acquisition dates and interferometric baselines. As demonstrated for the Sharm El-Shiekh area the interferogram stacking technique allows reducing errors caused mainly by atmospheric distortions making the measurement of slow deformation velocities feasible. Interferometric processing results show both patterns of uplift and downlift in the study area. Specifically an area along the coastline with N-S to NNE-SSW direction, corresponding to the build up zone of Sharm El-Shiekh and Naama Bay, shows annual average subsidence rates between 5 and 7 mm/yr along the line of sight (LOS). On the contrary, Sharm El Maya, an inner zone, parallel to the above subsided area; shows slant range uplift of 5 mm/yr. The obtained results possibly indicate the presence of an active fault that affects the coastal zones of Sharm El-Shiekh area as the overall
difference in relative motion rates reaches almost 10 mm/yr. a much larger value than the commonly estimated for non tectonic proess.
Naama
Sharm El-Shiekh
Red Sea
Figure 3. Interferometric deformation map of the broader area of Sharm El-Shiekh
Finally, apart from the regional deformation field, localized subsidence phenomena in the form of small “bowls” are observed in the broader area of Naama Bay, which could be attribute to others sources of deformation not related to tectonism such as water over-pumping. Generally, coastal movements are characterized by complex areal patterns due to superimposition of several natural and anthropogenic driving mechanisms that act in different time and spatial scales. Analytic examination of the geological, geomorphological, hydrological and anthropogenic parameters in the area would allow the recognition of their
contribution to the observed deformation field. In addition, the validation of the obtained results through other geodetic measurements such as leveling and D-GPS is of great importance.
Figure 4. Deformation map over Naama area
ACKNOWLEDGEMENT The authors would like to acknowledge the European Space Agency for the high level of collaboration and ENVISAT data provision in the frame of CAT-1 5102 project.
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