عنوان مقاله [English]
Nowadays, differential interferometric synthetic aperture radar (DInSAR) is one the most practical techniques measuring earth surface movement. Using this technology makes continuously monitoring of small earth surface movements with high precision and in a wide range Possible. By studying the ground surface displacement, we can find effective parameters influencing the Earth's displacement process, which will be effective in geophysical modeling and determination of the necessary strategies to deal with this type of phenomenon. Landslide is considered as one of the geophysical phenomenal of sloping instabilities that causes displacement of landslide along the tilt direction. These displacements cause dramatic financial losses and heavy casualties.
Materials and Methods
In this study, the detection of active landslides and their displacement in the Taleghan watershed have been resolved by differential interferometry processing using satellite radar images of Envisat- ASAR satellite in high-pass mode. The required radar images were provided by submitting a research proposal on the University of Delft website while having their consent. Analysis of Taleghan watershed basin radar images for its specifications was done using the EOLI-SA software. 29 images of SAR sensor from Envisat- ASAR satellite were used at present study. Images used are obtained by descending imaging geometry technique (located on the North-South circle of latitude), which are produced by the C band wavelength from the microwave spectrum of electromagnetic waves (λ = 5.6 cm). These images are corrected by Scan Line corrector (SLC) and were collected since 2003 till 2009. Differential interferometry processes were performed to detect landslides locations and calculate their displacements by ENVI 4.2 software. The entire process was carried out through field surveys and a digital elevation model. The final stage of differential interferometry process is transformation of absolute phase to the displacement map and geocoding which is based on the elliptical surface of the WGS84 metric image system.Since there was no direct measurement of dissociated surfaces dislocation in the studied area, the aim of this study was accuracy assessment of archived results by field surveys and overlapping them with the Google Earth software.
Resutls and discussion
Results of processing differential radar interferometry of pair images were collected since July 18, 2003 till March 31, 2009. In this study, a total number of 17 slide bulks were selected in the Taleghan watershed based on field observations. Then, the level of instability of these bulks was analyzed using SAR sensor images for the mentioned period.
Most identified landslides were in the vicinity of residential villages and riverside networks, and their spatial distribution shows that most of the identified areas are matched with previous landslides.In other words, some of the old landslides in the region are still active. Among these slide bulks, Zidasht sliding zone with 7.2 km length is the smallest one on the eastern slope of the basin and the Minavand slippery zone with 4206 km length is located on the western slope of the basin.One of the important points of the chosen sliding bulks is their proximity to residential areas located in the basin. The largest displacement occurred at the surface of Composite slide bulks, as per maximum amount of depression happened in Minavand slide bulk and was 2.5 cm.The spatial distribution of the identified landslides resulted from the processing of coupled radar images and their examination indicates that the largest displacement has happened since 2004 till 2008.Comparison of radar images shows that although these slide bulks were in the northeastern and upstream of hillsides and rivers in the past, they are now located in the western direction, and some landslides which were active in previous periods, haven’t shown any change in their surface during this period and the amount of displacement observed was lower than before.
The results showed that some of the slopes of the Taleghan watershed are still unstable and the compatibleness of landslide site location with some old landslide implies the activeness of old landslides of the region.The findings of this research have been evaluated through field observations and determination of the slide bulks by evaluating radar images. Also results indicated that all sliding surfaces were detected with high precision. Therefore, it can be declared that the differential interferometric synthetic aperture radar method (DInSAR) is an efficient technique for identifying the landslides.Field observations showed that most of identified landslides conform to the old ones. Also displacements at the surface of these landslides determined to be still active. At present, the slide bulks of Minavand and Mehran are considered as the most active and dangerous slide bulks of all kind.