Evaluating the Slope Instability of Ardabil-Sarein-Sarab Roads using Radar Interference

The danger of amplitude movements is considered as one of the hazards that has occurred in many areas in recent years. Identifying the areas exposed to amplitude movements and estimating its rate plays an important role in managing and controlling this phenomenon. Radar interference technique is an efficient method in measuring ground surface displacement which makes it possible to monitor small movements of the earth surface continuously, with high accuracy and in a wide range. This technology has become very common in the study of natural disasters of the earth, including slope displacement, subsidence, earthquakes and volcanic activity. This technique compares the phase taken from two radar datasets at two different times and. Besides, creating an interrogram, it is able to measure changes on the earth surface over time. In the current study, the radar images of 2015 and 2020 have been applied in order to identify and measure landslides. SARSCAPE software has been used to process information. The land-use map of the study area was extracted using Landsat 8 image and object-oriented classification method. The findings reveal that radar images have a good potential to detect the instability of slopes and to calculate their displacement. During the study period, the maximum amount of material movement has been recorded as 21 cm, indicating the area is active in terms of amplitude movements. The overlap of the landslide map with the land use layer also confirms the maximum occurrence of landslides in the use of vegetation and rainfed agriculture.
Extended Abstract
1-Introduction
Continuous monitoring of the earth surface changes and identifying the areas prone to slope movements are recognized as the effective factors to reduce casualties and natural hazards such as landslides and slope movements, especially in human settlements and communication infrastructure such as roads and railways (Elliott and Choudhari, 1999). So far, several techniques such as using the Global Positioning System, geodesy and tachometry, mapping cameras, laser scanning and lidar have been proposed to monitor surface changes (Hooper et al., 2004). However, due to the high cost of implementation, high time consumption and limited coverage of these methods, the use of these methods in a wide range is not economically effective.
2-Materials and Methods
 In this study, Sentinel 1 images were used to capture images in the C-band range of microwaves. Then the necessary processes were performed through SARSCAPE 5.2 plugin in ENVI 5.3 software. A differential interferometric method with a combined opening of two frequent or non-frequent passes has been used in this research to determine the amount of amplitude movements is. One of the most basic steps in radar interferometry processing is to select the right image pair. Several factors such as sensor frequency, spatial baseline, temporal baseline as well as spatial overlap in the direction of sensor movement are effective in selecting image pairs.
3-Results and Discussion
The Positive values of amplitude movements ​​indicate the amount of displacement in the direction of satellite sensors, while negative values ​​signify the amount of displacement in the opposite direction of the sensor. The material displacement of the measured slopes in the study area shows a maximum of 21 cm and a minimum of 3 cm in the study area in 2015 to 2020. The highest slope movement is recorded for Neir road and Sarein, while the lowest is found for the exit road of Ardebil. Radar interference method is a very accurate method that can detect amplitude movements using two images of an area at different time intervals very accurately in centimeters and even in millimeters. The results of field research also confirm the very high accuracy of satellite images. Besides, it is possible to monitor small movements of the earth's surface continuously with high accuracy and on a large scale using this method. Radar images have detected amplitude movements in the study area with great accuracy. Also, compiling the map of slope movements with land use obtained from Landsat images taken in spring and in June shows that the main slope movements occurred in the use of vegetation. The study area has the highest rainfall and snowmelt in the spring which directs water infiltration into the soil, and, consequently, the slope support is lost. As a result, road construction is vulnerable to the shocks and shakes which are mainly caused by the movement of heavy vehicles in the area. Falling causes the stability of the slope to be lost, which in turn leads to sloping movements in the study area. The accuracy of the radar interferometry technique in monitoring and detecting amplitude movements has been confirmed by many researches around the world.
4-Conclusion
The findings reveal that radar images have a good potential to detect the instability of slopes and to calculate their displacement. The maximum amount of material movement is 21 cm in the study area indicating the area is active in terms of amplitude movements. Land use maps were performed using Landsat 8 image using object-oriented classification in the study area. The results of adapting the land-use map and the slope movement map in Table 3 show that the highest rate of slope movement is recoded for rainfed vegetation and agriculture with -0.21 cm and residential area with -0.19 cm, respectively. In some cases, these accidents can cause heavy and irreparable losses due to high population density or the expansion of the radius of the collapsed lands. Also, observing and comparing slope and geological movements demonstrates that most of the slope movements occur in the areas in which rocks are mostly volcanic andesites, gypsum and marl. However, slight landslides mainly occur in areas with alluvial sediments and river deposits. The highest percentage is related to sedimentary rock classes with tuff interlayers. Moreover, the results of digital elevation and landslide model show that altitudes of more than 2000 meters have the highest share of landslides, also at altitudes of 1500 meters, landslides have been significant due to the instability of the slopes against climatic and environmental factors.  In terms of land-use, the floors related to vegetation and rainfed agriculture areas are influenced by rock fall which is due to the infiltration and conduction of rainwater to the lower classes.