The study and monitoring of an active landslide using differential interferometry synthetic aperture RADAR (Case study: Noghol landslide, Semirom)

Landslide is defined as the downward movement of rock masses, debris and earth materials. This condition occurs where shear stress of materials exceeds its shear strength and is commonly known as slope failure. In other words, landslides are surface processes occuring in mountainous areas to various scales. Based on the kind and scale, landslides have significant effects on the environment. Identification and the effect of active landslides as one of the aspects of erosion threatening residential areas, farmlands, dams and destruction of roads have great significance. Launching satellites with Synthetic Aperture Radar (SAR) sensors onboard, is purposely built for differential interferometry (DInSAR) applications opening new opportunities for mapping and monitoring slow ground deformations and even high speed landslides. For this purpose, the technique of differential interferometry synthetic aperture radar containing extensive and constant earth coverage with great time and place resolution as one of the most accurate and least expansive remote sensing techniques has been presented to identify and illustrate displacement on the earth surface. This research is aimed to identify active domain of Noghol landslide and to estimate its activities done due to the sliding mass in the area.
Padena area in Semirom is situated in the southern part of Esfahan province. This area due to its special geological, climatic and geomorphological characteristics provides favorable condition for occurrences of various mass movements. The Noghol landslide is also situated in Padena- Semirom. The study area is located at about 300 km south of Esfahan in 51° 39' 10" E and 30° 52' 39" N.
Thus, the surface displacement done by landslide in Hanna city, 70 km south of Semirom by using differential interferometry synthetic aperture radar (DInSAR) technique has been calculated. For this purpose 10 RADAR images of ASAR sensor from ENVISAT satellite have been chosen and then processed with DInSAR method.
Consequently, the preliminary processing consist of paired images totally 9 interferometer images have been identified to be processed in 2004, 2010 and 2011 to be considered as a base to provide landslide displacement maps. Based on the results obtained from the ASAR RADAR images processed in the current research, the maximum displacement of Noghol landslide related to pairs images on 3 August 2011 and 31 December 2011 with a period of 150 days has been estimated 8 cm. Also, the average displacement rate among the interferogram built during the years 2004, 2010 and 2011 due to this landslide has been estimated about 2 cm per month. Within the research, in addition to monitor and figure out the amount of displacement due to sliding mass during the obtaining RADAR images, the results taken from analyzing images during different time periods, indicate the landslide displacement. Also, increasing the domain of the landslide gradually and with simultaneous increasing in time period between pairs interferometry during the image dates have been used so that the least displacement done on the sliding mass was related to pair images of 20 May 2010 and 24 June 2010 with a period of 35 days and about 3 cm. So, we can say that the domain of the slide activity is bound from north and north east of sliding mass toward center and south and ultimately to the south west side of the sliding mass along the Marbor river. With expanding the active domain of landslide, the amount displacement of landslide has been grown up. The above issue can be an indication the activity of Noghol landslide during obtaining the RADAR images within the study in various steps. It can also be mentioned that the results obtaining from the differential interferometry of ASAR RADAR images with respect to activity domain of the landslide with the greatest displacement within the Noghol sliding mass have been in accord with the field observations. So, based on the above conclusions and with respect to the surface of rupture and slip materials resulted from field observations and movement patterns and monitoring of landslide obtained from the analyses of phase to displacement maps of landslide and also with respect to the landslide activities during the various steps so far, it can be stated the Noghol landslide would be a kind of complex rotational slide (slump). According to the above conclusions, the total movement orientation of the landslide would be toward the river from the east to the west.
The research results implies the activity of Noghol landslide during obtaining the RADAR images in the area. So that this landslide occupies an area of about 11 hectares. Due to the occurrence of landslide in rural areas and farmlands especially apple orchards have affected and heavy damages have been incurred and a large amount of productive farming soil destroyed. Since the landslide is adjacent with the hydrological network of the area, it is considered as the main source of sedimentation. The results of the research indicates that the radar images as well as differential interferometry contain significant potential to identify slope unstable areas, monitoring landslides and also estimating the amount of displacement. Meanwhile, RADAR images (ASAR) contain superior capability for providing displacement maps of differential interferometry with more details about the area landslide because of containing shorter wavelength (c band).