Spatial analysis of the amount of landslides using radar interferometric technique in order to reduce hazards (Study area: Sarbaz area in Isfahan province)

Introduction
Landslides are one of the most complex and at the same time harmful phenomena, as a result of which the slope material is moved from the sloping surfaces and relatively deep wounds are left on the surface of the slopes. A phenomenon near cultural features, such as residential areas, roads, etc., causes financial damage and many casualties. The use of geographic information systems in the first place, followed by the use of satellite image analysis technology, is of great help to this part of management. For this purpose, the D-InSAR radar interferometry technique, with its frequent, continuous wide ground coverage and high temporal and spatial resolution, as one of the most accurate and least expensive remote sensing techniques, to display the displacements that occur in the ground surface is provided. The present study investigated the existence of landslides in the study area of Sarbaz village by means of Sentinel 1 sensor radar images and using radar interferometric technique in the period of 2017 and 2018. After various processes that were done in ENVI software and SarScape plugin, the amount of displacement and landslide of the area was obtained, and according to the results obtained, it can be said that Sarbaz area is an area with a high level of activity in terms of landslides. Also, 3 points have been introduced as points with high sensitivity, which were found to be in the medium to high risk level. The obtained results showed that Sarbaz region had a displacement of -0.069 in the period of 06/05/2017 to 12/14/2017, in fact it had a displacement of nearly 7 cm, in the period of 12/14/2017 to 12 In 06/2018, the amount of displacement was -0.035, which means that nearly 4 centimeters of displacement occurred in the region, and between 12/06/2018 and 21/12/2018, the displacement in the region was equal to -0.064, which It shows that there has been 6 cm displacement in the studied area during this time period. At the end, a map of sensitive points was prepared and presented.
Deformation of the soil surface is a dynamic process on the Earth's surface, this process occurs naturally or due to human intervention. One of the forms of this deformation is landslide [7]. The definition of landslide is the general and deep movements of the entire soil layer on the surface of the mother earth, which cause heavy damages every year, sometimes it is not possible to compensate for these damages, and we need to spend a lot of time and money. The speed of their operation and their extent often creates spectacular and sometimes catastrophic phenomena, and they may affect tens or hundreds of thousands of cubic meters of rock and soil in one place. This phenomenon works mostly in isolated sediments [10]. The effects of domain instability are mass movements on a small to large scale. Range movements may be minor and unique to the fall of a single piece of rock, or they may be very large and catastrophic. The damage caused by this type of movement is more in developed countries, but 95% of the deaths caused by it happen in developing countries. Landslides are among the most devastating natural disasters in steep areas, the damage to residential areas and economic infrastructure, as well as human casualties caused by landslides are increasing worldwide [9]. Iran, with its mainly mountainous topography, tectonic activity and high seismicity, diverse geological and climatic conditions, has the main natural conditions for creating a wide range of landslides [4]. By using morphotectonic indicators, it is possible to help planners by providing maps in predicting the places that will become unstable due to future earthquakes. By using morphometric analysis and using geomorphic indices and rank parameters of drainage networks, the active tectonics of a region can be investigated [5]. Among the presented methods, we can mention tachometry, leveling, short-range photogrammetry, observations from geodetic networks [GPS], radar interferometry, and lidar. Although the first use of radar data in investigating the instability of the earth dates back to the mid-1990s, in recent decades, due to the availability of radar data with a long time span compared to the past, and the development and expansion of new methods such as techniques Interferometry has drawn the attention of researchers to the use of these methods [4]. Among the remote sensing techniques, differential radar interferometry [DINSAR], which has the ability to work in all bad weather conditions and the length of day and night, is considered as one of the effective and efficient techniques in monitoring slow changes in the earth's surface [11]. Due to the fact that natural phenomena and hazards will cause damage in different regions, therefore, it is important and necessary to investigate these issues in different time frames and regions in the country. This research was conducted for the first time in the studied region. and the purpose of doing it is to investigate the amount of landslides in Sarbaz village in Isfahan province using radar satellite images, remote sensing techniques and radar interferometry. Therefore, this research is innovative in terms of being new and up-to-date.
Materials and methods
In the current research, in order to monitor and determine the amount of displacement caused by the movement of the sliding mass of Sarbaz village, the radar interferometric method and Sentinle-1, radar satellite images with C-band wavelength in the ascending orbit mode have been used. Radar interferometry or interferometry is the method of using the phase difference of signals returned from the earth's surface in two SAR images taken with a time delay or with parallax from a region to extract height or information related to changes in the earth's surface. In order to implement the DInSAR method, one should first pay attention to two principles between each pair of radar images (main image and secondary image), the first is the temporal baseline and the second is the spatial baseline. After choosing suitable interferometer pairs based on temporal and spatial baselines, in the next step of processing these images, and in order to remove the topography effect from the interferometer, the method of preparing the interferometer along with the digital height model was used. The output of this stage of the work is in the form of a differential interferogram in the form of a flattened interferogram in which the stationary phase and the topographic phase are removed from the interferogram. Since the flattened interferometer contains noise that lowers the visual quality of the fringes, the adaptive filter is used for this purpose.
Discussion and Results
Identifying areas prone to landslides and zoning the potential of landslides is very necessary and important in order to avoid these areas and implement prevention and control methods. In order to implement the DINSAR method in this research, the capabilities and quality of the radar images used to prepare the interferometer were checked by calculating their temporal and spatial baseline values.
The amount of displacement in these maps is shown in the form of a numerical range between negative values and positive values in meters(Figure 2). The level of the range and slopes and positive values indicate the accumulation of sediments at the foot of the range and slopes. According to the maps of phase transformation to displacement of the landslide and the analysis of images in different time intervals, the metamorphosis of the landslide surface can also be explained well.
 
 Table 1. The results of numerical processing of landslides and accumulation of sediments




Image


Image date


The direction of the satellite


Time interval of two images
(Day)


Baseline
(Meter)


displacement (centimeter)


General displacement
(centimeter)




1


05/06/2017


Ascending


192


123,689


Accumulation of sediments 1.6


8.6




14/12/2017


Ascending


7   slide




2


14/12/2017


Ascending


180


75,574


Accumulation of sediments 5


8.5




12/06/2018


Ascending


3.5   slide




3


12/06/2018


Ascending


192


98,088


Accumulation of sediments
2.2


8.6




21/12/2018


Ascending


6.4    slide




Fig. 1. General map of landslides and accumulation of sediments
According to the results obtained and determining the amount of landslides in the region, we have reached a positive correlation with the results of similar works, according to the results of similar works in the field of landslides that Sadeghi et al. (2022), Casagli et al. (2016), Hammand et al.(2019), Deming et al.(2020), it is concluded that by using remote sensing and radar interferometric technique, the amount of landslides in different areas can be determined with high accuracy and low cost. achieved and provided the results to relevant authorities to provide appropriate solutions.
Conclusion
In the new decade, the use of new technologies in the field of urban management as well as natural disaster management has had a growing trend. The use of geographic information systems in the first place, followed by the use of satellite image analysis technology, is of great help to this part of management. The present study calculated amount of landslides in Sarbaz village in Semiram city of Isfahan province by using Sentinel 1 radar satellite data and with radar interferometric technique. It is caused by landslides or, on a smaller scale, human activities. Negative numbers and sections indicate falls on slopes and other areas.  According to the obtained results, it can be said that Sarzab region is an area with a high level of landslide activity. Also, 3 points have been introduced as high sensitivity points, which were found to be in the medium to high risk level. The obtained results showed that Sarbaz region had a displacement of -0.069 in the period of 06/05/2017 to 12/14/2017, in fact it had a displacement of nearly 7 cm, in the period of 12/14/2017 to 12 In 06/2018, the amount of displacement was -0.035, which means that nearly 4 centimeters of displacement occurred in the region, and between 12/06/2018 and 21/12/2018, the displacement in the region was equal to -0.064, which It shows that there has been 6 cm displacement in the studied area during this time period.