عنوان مقاله [English]
The continuous monitoring of land surface deformation and recognition of susceptible area for slope movements, especially in residential area and traffic infrastructures such as roads and railways, is considered as the most effective factors for reducing losses (human and financial) of environmental hazards such as subsidence and landslide. Several techniques such as global position system (GPS), geodesy, tachometry, Leaser scanning and LIDAR had been used to monitor land surface deformation However, their use is limited in wide area due to high cost, time-consuming, and limited surface coverage. Of course, synthetic aperture radar Differential Interferometry (DInSAR), which can be used in all climate conditions during night and day, covers extended earth surface and possesses high spatial and temporal resolution, is regarded as one of the most precise (precision in millimeter) and Cost-effectiveness remote sensing techniques to recognize and monitor land surface deformations, slow movements and slope instability worldwide. Regarding the mechanism, the phase difference of various SAR images with standard format (SLC), taken at different times from a specific area, are calculated and land displacement along the line of sight (LOD) are precisely estimated. In the present study, in order to overcome the limitations of conventional synthetic aperture radar interferometry (InSAR), SUBSOFT software and differential synthetic aperture radar interferometry (DInSAR) based on coherent pixels’ technique (CPT), which is develoed by the Remote Sensing Laboratory (RSLab) group from the Universitat Politècnica de Catalunya (UPC), were utilized to recognize unstable slopes around Lorestan railway. In this regard, 50 descending sentinel-1 images, corresponding to the period from 2015 to 2018 (obtained from European Space Agency) were analyzed. The rate of slope movements (millimeter per year) in satellite trajectory and maximum displacement (28.8 cm/year) were obtained in Tang-e- Haft to Tang-e-Panj stations, by indicating the activation of area with respect to slope movement. The Lorestan Railway is part of the north-south railway of Iran that is 215 km long. it extends from the Momenabad Station in Markazi province to the Tang-e-Haft Station in Khuzestan province and goes across high Zagros Mountains located in the geographical coordinates of 48°, 15' to 49°, 05 ' E, and the latitudes of 32°, 25' to 33°, 30' N. This region has specific characteristics in terms of slope instability and related geomorphological phenomena due to local and regional characteristic of the active Zagros orogenic belt.
In the present study, conventional methods (satellite images, 3D images derived from google earth software, terrain survey, topographic maps and landslide database of area) and advanced techniques (high precision synthetic aperture radar images ASAR) were used to recognize unstable slopes around Lorestan railway. In fact, conventional methods such as land surveing was utilized to recognize large landslide with rapid movement and advanced techniques such as differential radar interferometry (DInSAR) was used to measure slow movement and identify susceptible parts for landslide. Differential radar interferometry (DInSAR) is considered as one of the most important and usable methods, which measures land surface deformation by using phase difference (∆ɸ) of every pixel from a (SLC) image pair, taken individually from a specific area at different time. Fifty descending radar images from sentinel-1 of European Space Agency were used to recognize slope instability in Lorestan railway during temporal range of 2015 to 2018 (recording time of 14 days, track number of 108 and frame number of 482).
Rate linear map and temporal series of slope material displacement in Lorestan railway were prepared by using differential radar interferometry. In order to facilitate the measurement of slope movements, radar images in the area with 2 km radius were selected for calculation, due to the location of the most landside hazard points in Bisheh to Tange e Haft stations and extracting many points from processing. and the highest displacement rate of 28.8 cm per year was obtained of the Tang-e-7 to Tang-5 stations, and finally Several points susceptible for landslide hazard, obtained from final map, were selected through DInSAR method and controlled by field survey to validate differential radar interferometry. Based on the results, this method is considered as a high precision technique to recognize area possessing hazard of slope movement in wide and mountains area such as Lorestan
Lorestan railway is considered susceptible for various landslides due to passing through folded and high Zagros mountainous and alternating geological formations with different resistance. Accessing to some slopes, overlooking railway is regarded impossible due to the presence of impassable segments in area under study. Thus, in the present study, new differential radar interferometry was utilized by using low-pass images of sentinel-1 to recognize area having landslide hazard and measure the slow movement of slope material in Lorestan railway. In addition, in order to verify radar data results, 35 vulnerable points overlooking Lorestan railway were recognized through field survey. Based on the results, radar data and differential interferometry are regarded to detect landslide and calculate their displacement value due to wide coverage, abundant data and high precision. The analysis of temporal series diagrams showed that a maximum of slop material movements occurred in the autumn and spring due to higher rainfall in these seasons compared to others