InSAR Application in GeohazardsHersh Fattahi Master science student of Remote Sensing K.N.Toosi university of Technology Faculty of Geodesy and Geomatic hersh_fattahi@yahoo.com M.J. Valadan Zoej Assistant professor K.N.Toosi university of Technology Faculty of Geodesy and Geomatic Maryam Dehghani Remote Sensing PHD student K.N.Toosi University of Technology Faculty of Geodesy and Geomatic Geological Survey of Iran(GSI) Abstract: InSAR is a radar technique for combining SAR complex images to form interfrogram and utilizing its phase contribution to earth's topography, surface movement and target velocity. InSAR is able to produce DEM with the precision of a couple of ten meters whereas its movement map results have subcentimeter precision. In this article we have considered InSAR theory and studied its applications in geohazards like earthquakes, ground subsidence, volcano and landslide. Eventually, we have demonstrated this technique ability to distinguish movements due to earthquake and ground subsidence. Introduction: InSAR is a synonym for Interferometry Synthetic Aperture Radar. This radar technique combines complex SAR images, recorded by antennas mounted on airplanes or satellites, to generate DEM or surface deformation maps [2]. The first terrestrial use of InSAR to generate DEM was reported by Graham (1974) [1]. Combining multiple interferograms by Gabriel (1989) demonstrated that differential interferometry can be used to detect and estimate sub centimeter surface perturbations [1]. The first earthquake displacement map was reported by Massonet (1993). He used the available DEM to remove the interferogram's topographic phase [1]. Zebker, in 1994, combined three radar images to separate dynamic and topographic effects [1]. Thus he could evaluate the surface displacement only by radar images. Thereafter, most researchers used InSAR for different purposes especially in geohazard applications which some of them are indicated here. Nowadays InSAR ability to generate topographic and displacement maps due to a variety of applications such as earthquakes, mining, landslide, volcanoes and other has been proven. Although there are other facilities like GPS, total stations, laser altimeters for these purposes, a comparison between InSAR and these approaches given in the following reveals that InSAR is an unavoidable technique. Laser altimeters can generate high resolution DEM and low resolution displacement maps in contrary with InSAR. However, it should be considered that most laser altimeters, record narrow swaths. Therefore, in order to construct a DEM using laser altimeter, more overlapping images are required compared to the INSAR method. Displacement map precision obtained by terrestrial surveying using GPS and total stations is comparable with or better than InSAR results. In general GPS provides better estimation of horizontal displacement and with permanent benchmarks it allows slow deformations to be monitored for years without concerning about surface decorrelation [5]. The most important InSAR advantages over GPS and total stations are wide continuous coverage without fieldwork. Therefore, wide and continuous coverage, high precision, cost effective and feasibility of recording data with no weather restriction are main causes to using InSAR. However, it is important to keep in mind that the InSAR displacement result is in the radar line of sight direction and to decompose this vector to vertical and horizontal components , the terrestrial data or extra interferograms with different imaging geometry are required. SAR Interferometry Principles: Interferogram is 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. The factors which contribute to the obtained phase difference are imaging geometry, topography, surface displacement, atmospheric change and noise. Two principle concepts for the introduction of InSAR are spatial and temporal baseline. In spatial baseline approach, the images are taken from two different positions at the same time. However, the images are acquired from the same position at different times using temporal baseline. While using satellite images, mixed baseline will occur that both baselines exist. Using multiple pair baselines is a solution to separate topographic and dynamic components. Topographic Mapping: Topographic maps can be derived by single pass or two pass interferometry. Single pass system is implemented by mounting two antennas in an along track situation on a fuselage. Interferometric phase is computed by Eq. (1) as follows:  While is the interferometric phase, φ1 and Φ2 are the master and slave phases, λ is the radar wavelength and ρ1 and ρ2 are distances between antennas and target. In case of using common transmitter p is one and for the other cases it gets the value of two.  Fig.1: Imaging geometry Regarding to the imaging geometry in fig.1, look angle and interferometric phase will be related as follows:  Where θ is the radar look angle , α is the angle between baseline and horizon and B is the length of baseline. Considering the imaging geometry target elevation (h) and across track position (y) are calculated by Eq. (3) and (4):  Where H is the platform elevation. It's evident that look angle (Θ ) can be calculated from interferometric phase using Eq. (2). Therefore target elevation will be specified by Eq. (3). Performance Limitation: Phase noise and error sources in measuring baseline orientation ( α) lead to uncertainty in InSAR results. Phase noise is a high frequency error while baseline orientation error causes the total tilt of the image. The other sources of error are the baseline decorrelation and motion compensation. When spatial baseline is larger than critical baseline, coherency will be low and baseline decorrelation will occur. Two pass interferometry is the conventional method while using satellite images. In this system, spatial baseline is not well defined and the temporal decorrelation due to the surface changes during imaging may occur. Displacement Mapping By InSAR: When no displacement occurs between two epochs of observations, interferometric phase is calculated as:  Now consider that surface has displaced by a phenomena like earthquake during two successive observations. In this case interferometric phase is calculated as follows:  Where Δρ is the radar line of sight component of the displacement. By scaling the prime interferogram by the ratio of the parallel components of the baseline and subtracting from the second interferogram, resultant equation is dependent only on the displacement of the surface:  However, the observed phase data are wrapped, which are restricted in a 2 modulus, and they must be unwrapped to their true absolute phase values [15, 17]. Converting the wrapped values to the unwrapped phases is considered as phase unwrapping process that can be implemented by various techniques. Path-following (or integration-based) methods and least squares methods are the mostly-used techniques of phase-unwrapping [17]. Interferometric phase is much more sensitive to surface changes than to topography. Thus while InSAR generates DEM with the precision of a couple of ten meters, its displacement results have sub centimeter precision. Multi pass interferometry is affected by the atmospheric water vapor changes [16]. Spatial and temporal changes due to the 20% of relative humidity produce an error of 10cm in deformation. Moreover, for those pair images with inappropriate baseline the error introduced to the topographic maps is almost 100m. In topographic mapping this error can be reduced by choosing interferometric pairs with relatively long baselines, while in the displacement case the solution is the averaging independent interferograms [16]. InSAR Application in Geohazards: Regarding InSAR abilities in movement detection, most researchers have used this technique to monitor the surface displacement due to natural hazards such as earthquake, flood, volcano and man induced reasons like mining. Landslide Monitoring: Measuring displacement due to high rate landslide by InSAR is difficult as images coherency will be low. But in low slip regions that images correlation has been relatively kept, InSAR can evaluate surface deformation due to land slide. Fruneau et al. estimated a 30mm per day displacement due to the Laclapiere landslide. Ground Subsidence Monitoring: Ground subsidence is subsiding or collapse of the land surface by natural or human-induced activities causes. When ground surface collapses into under ground cavities produced by the solution of soluble material in the ground water, natural subsidence occurs. Most current subsidence is human-induced and is related to underground mining. This phenomenon causes the surface deformation and local ecosystem changes and influences the man-made facilities. Ching-chung chang and et al. at 2003 used DInSAR( Differential InSAR ) technique to determine ground subsidence [7]. They obtained DEM of the area using ERS1 and ERS2 images with one day difference whereas they used JERS1 images to determine displacements. This group used GIS to interpret DInSAR results.  Fig.2: DInSAR result on JERS images Fig.2 shows the DInSAR results on JERS images. In this figure brighter regions show more displacement and darker regions illustrate the lower displacement. Earthquake Displacement Monitoring: Recently InSAR abilities to determine pre, co and post seismic movements have been demonstrated by researchers. Indeed due to wide coverage and being cost effective most earthquakes are studied by InSAR. The results obtained by InSAR are applied in order to modeling faults. Massonet in 1993 used InSAR to determine surface displacement due to Landers earthquake. Zebker in 1994 applied another InSAR method to determine Landers displacement. Actually, he used three SAR images, instead of using DEM, in order to eliminate the topographic contribution in the interferograms. Bam earthquake with the magnitude of 6.5 took place in dec of 2003. James Jackson, Barry Parsons et al. determined displacement caused by this earthquake using InSAR [12]. They generated the DEM of the area applying ERS1/2 images with a one-day delay and estimated pre and co seismic displacements using ASAR images. Fig.3 demonstrates the related interferogram that includes four lobes. Since the displacement in the east is greater than that in the west, the related lobes are larger.  Fig.3 : Interferogram of Bam region The displacements measured along the radar line of sight direction are 30 cm and 16 cm at south east and north east lobes of the interferogram, respectively. However, the displacement related to the western part of the area is about 5cm along the radar line of sight direction. Volcanic Activity Monitoring: The spatial distribution of surface deformation, derived from InSAR , enables us to produce the detailed mechanical models in order to enhance the study of magmatic and tectonic processes. These studies will improve our understanding on how the volcanoes work and our capability to predict future eruptions and the associated hazards. Before most volcanic eruptions surface deformation in response to increasing pressure from magma chambers occurs. Surface deformation analysis with seismicity and other information provide significant inputs for studying magma dynamics supposing that studying most volcanoes along with field work is difficult and costly. Therefore, the most appropriate choice to study these phenomena is InSAR technique. Zhong Lu et al. have studied Alaska volcanoes and obtained satisfying results [13]. Conclusions: InSAR is able to generate topographic and displacement maps. Widely-covered, continuous and high precision displacement maps, derived from InSAR, have encouraged researchers to use this technique for studying geohazard. Iran is a country with active tectonic which annually causes lethal events. Therefore, studying the movement rates of surface ground structures such as faults plays an important role in crisis management. Also there are a lot of natural resources in Iran. Exploitation of these resources leads to ground subsidence that affects man-made equipments and local ecosystems. Cost-effective, high precision, wide and continuous observations, compared to GPS discontinuous observations are some of InSAR characteristics that encourage us to use InSAR to monitor these geohazard. References:
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