Application of the Interferometry Synthetic Aperture Radar (InSAR) in monitoring and evaluating landslides (case study: Ahar-Varzeghan region)

Introduction
Risk management and landslide assessment begin with a comprehensive identification and monitoring of their movements and mapping them, which can be used as a basis for achieving knowledge about their spatial and temporal distribution. The integrated vision of the radar remote sensing images has made them a powerful tool for preparing distribution maps, assessing the risk of landslides, and evaluating the instabilities reoccurring at various time intervals. Therefore, this study aimed to identify, monitor, and measure the extent to which the region is at the risk of slope instabilities in the period of 2009-2011 in the Ahar-Varzeghan region in the Ahar Chay heights using the Interferometry Synthetic Aperture Radar (InSAR) technique.
Data and Method
The studied area is a part of Ahar Chay watershed with an area of 1593 km2, which makes it the largest watershed in the East Azerbaijan province. The said area has an east longitude of 35´ 46° to 10´ 47° and north latitude of 20´ 38° to 45´ 38°. This area is located on northern slopes at an altitude of more than 2730 m, and in the Alpaut and Chaligh areas with an altitude of less than 1260 m.
Various types of data were used in this research, including radar imagines, ENVISAT-ASAR, and data captured by Global Positioning System (GPS) in field research. Besides, topographic maps of 1.25000 associated with the country's land surveying organization and geological maps of 1:250000 related to the country's geological and mineral exploration organization were used to examine the area's geology and morphology. Besides, SAR scape ENVI4.8 and Arc/GIS software environments were used to prepare and process radar images, and provide the displacement maps associated with the landslides.
INSAR is a remote sensing technique developed to study the earth's surface movements in the 1992 Landers, California earthquake. When this technique is adopted, two or more SAR images are used to generate a digital elevation model or prepare a map of the movements of the earth's surface. Also, this technique measures the phase difference between two different waves, and this phase difference is attributed to the change in the distance between the sensor and the earth target or the movements of the earth's surface. SAR images show the movements of the earth's surface in the direction of the satellite's line of sight. However, SAR images can be used in both ascending and descending modes, which enable them to measure the earth's surface movements in three directions: vertically, from the north, and east. The following steps must be taken to measure the movements of the earth's surface and/or develop a digital elevation model using the InSAR technique:
SAR image formation:
-          Interferogram generation
-          Ambiguous phase extraction
-          Phase recovery in the generated interferogram
-          Elimination of the effect of the topographic phase on the interferogram
-          Displacement phase extraction
Results and Discussion
The InSAR technique has been used in this study to identify the range of slope instabilities and the rate of displacement of unstable landslides. Based on the research findings, radar images and differential interferometry processing methods could potentially identify unstable active areas and calculate the rate of displacement. In this study, the results of the ASAR radar image processing were used to calculate the rate of displacement from 2009 to 2011. Figure 9 showed the displacement rate for the period of 2006-2011. It should be noted that positive and negative numbers in this figure indicated the rate of upward and downward movements, respectively. According to this figure, the highest and lowest rates of downward movements were associated with 2009 and 2011, respectively. In other words, the rate of downward movements had decreased by 2011. However, the rate of upward movement in the region was the highest for the years 2009 and 2010. Also, the lowest rate of upward movement recorded for this area was in 2011.
Conclusion
The present study utilized the InSAR technique to identify the range of landslides and to estimate the displacement of unstable landslides. The results of this study showed that radar images and differential interferometry processing methods could potentially identify unstable active areas and calculate the rate of displacement. In this study, the results of the ASAR radar image processing were used to calculate the rate of displacement between 2009 and 2011. Then, the validity of the results of the InSAR technique was measured by adapting the displacement maps to field operations and Google Earth. According to the research findings, the highest rate of fall or downward movement occurred in approximately 8 cm in the southern areas. Moreover, the highest rate of elevation or upward movement occurred in about 5 cm in the east northern areas in 2009. The highest rate of falls in this year was associated with areas with old landslides, among which the villages of Depigh, Nasirabad, and around Sariaraghan were more affected than other areas. These instabilities in the village of Depigh were mostly located around the bed of the Ahar Chay river, in a way that severe landslides in this area caused enormous rocks to accumulate on the surface of the river bed.