شناسايي و پايش ناپايداري دامنه اي به روش پردازش اينترفرومتري تفاضلي مطالعه موردي: حوضه آبريز گرمي چاي ميانه

Detecting and monitoring of slope instability based on Differential Synthetic Aperture Radar Interferometry (D-InSAR) Case study: Garmi chai bain-Meianeh

امروزه، اينترفرومتري تفاضلي راداري(DInSAR) به‌عنوان يكي از روش‌هاي كارآمد در اندازه-گيري جابجايي سطح زمين محسوب مي شود. به‌طوري‌كه با استفاده از اين فناوري، امكان پايش حركات كوچك سطح زمين به‌صورت پيوسته، با دقت بالا و در گستره وسيعي امكان‌پذير است. در اين پژوهش، از تكنيك اينترفرومتري تفاضلي بر اساس سري زماني 6 تصوير راداري از سنجنده PALSAR ماهواره ALOS از 2007 تا 2010جهت شناسايي و پايش مناطق ناپايدار دامنه اي حوضه آبريز گرمي چاي استفاده گرديد. نتايج حاصله از اين تحقيق بر اساس مشاهدات ميداني مورد ارزيابي قرار گرفت. بر اساس نتايج اين تحقيق، 20 نقطه ناپايدار در بازه 92 روزه زوج تصوير راداري سال2007 شناسايي شد كه بزرگ‌ترين آن‌ها از نوع لغزش چرخشي بوده و 11- سانتي متر در اين مدت جابجايي داشته است. بر همين اساس، تعداد 25 نقطه لغزشي نيز در سال 2009 شناسايي گرديد كه مهم‌ترين آن، لغزش روستاي سوين با ميزان جابجايي cm5/7 تا 3/10- بوده است. نتايج به‌دست‌آمده در سال 2010 از برخي جهات قابل‌مقايسه با دو دوره قبلي بوده به‌طوري‌كه تعداد مناطق ناپايدار در اين دوره كمتر از دو دوره قبلي بوده و توزيع مكاني مناطق ناپايدار نيز عمدتاً به نيمه شرق حوضه محدودشده است. ميزان جابجايي محاسبه‌شده در اين دوره(cm2 تا 1/5-) نيز كمتر از دو دوره قبل بوده است. مهم‌ترين نقطه لغزشي شناسايي‌شده در دوره اخير، توده لغزشي مجاور سد گرمي بوده كه ادامه فعاليت ساختماني سد را تحت تاثير قرارداده است.

Introduction: Mass movements are a morpho-dynamic phenomenon which is occurred due to different factors in mountainous area. Landslides in various forms are the importance one. Landslides are defined as the movement of a mass of rock, debris or soil along download slope, due to gravitational pull. This state related to region where there is shear stress is greater than shear strength. Landslide hazard and risk management begins with comprehensive landslide detection/mapping, which serves as a basis to understand their spatial and temporal occurrences. Landslide inventories can make use of variety of approaches, ranging from manual image interpretation using digital stereo images coupled with field investigations to automatic identification based on optical remotely sensed images. Visual interpretation of aerial photographs, combined with field investigations remained the major source for landslide inventory map until recently. Although aerial photographs accurately depict details of landslide, they are often not available in a timely manner for the majority of landslide prone areas in the world. Satellite imagery has become an alternative data source since it allows a more economic assessment of large landslide affected areas, as well as a synoptic appreciation of the context whit in which landslides occur. Recent advances in satellite technology have led to the detection of land surface in more accuracy. Microwave remote sensing, such as Synthetic Aperture Radar(SAR) images is very useful for detection and monitoring of landslides, and particularly with Interferometry(InSAR), subtle movement of landslides can be picked up from interfrograms generated from SAR image sequences. Another advantage of SAR data over optical sensor data is its all-weather monitoring ability. The main objective of this research was detection and monitoring of landslides of Garmi Chai basin by InSAR technique. Methodology: This research was performed based on Differential Interferometry (DInSAR) processing technique by SARScape 4.4 software. SAR images used for this research are pair of 2007, 2009 and 2010 PALSAR ALOS satellite images which are provided by European Space Agency (ESA). Obtained results were validated by field observations InSAR processing technique involved: first, all sixth RADAR images converted into SLC format. Then, one of them selected as master image and another images as slave. In the next step, slave Images were Co-registered precisely based on mater image in sub pixel accuracy. So, precise co-registration of coherence interferometric SAR image pairs was used to generation of SAR interferogram. Thus, after registration, the complex interferograms are formed by multiplying each complex pixel of the first image by the complex conjugate of the same pixel in the second image. Differential interferogram (DInSAR) was generated based on phase flattening stage due to applying Digital Elevation Model (DEM). Glodstein filter was applied to reduce phase noses improve phase to noise ratio (SNR). So, Phase unwrapping technique was performed by DMCF algorithm based on enough GCP. Phase unwrapping is a necessary step for the generation of InSAR derived DEM and land surface displacement calculation. Final step of InSAR processing is the Phase to displacement conversion and geocoding. In this step, the absolute calibrated and unwrapped phase values are converted to displacement and directly geocoded into a map projection (WGS84). As final results, land surface displacement was calculated both in slope displacement and vertical displacement direction. Results and Discussion: Obtained results revealed: 20 landslides has detected on 2007(92day time interval). The largest one is a rotational slide which is located near to Benaravan village with 0.43 km2 area and -11cm displacement rate. In this regards, 25 landslides detected in 2009 which Sevin slide is the most important one. Itʹs Maximum and minimum displacements rate is calculated +7.5 to-10.3cm in this period. While, obtained results in 2010 were comparable in some aspects: number of detected landslides and those displacement rates (-5.1 to 2 cm) are less than two previously mentioned periods. Majority detected landslides spatially distributed on eastern parts of Garmi Chai basin. A slide near to Garmy dam is the most important one which has significantly effects on next dam building operations. Conclusion: In this research, InSAR technique was applied for detection and monitoring of slopes instability of Garmi Chai basin due to PALSAR- ALOS images time series from 2007 to 2010. The results showed, some parts of Garmi Chai basin are still unstable. There is a good spatially coincidence between of InSAR derived landslides with former landslide zone. Majority number of observed landslides located near to streams network. This is important from sediment load point of view. Accuracy of obtained results was checked by field observation and GPS. Also, results showed landslides spatially detected in high accuracy. But, as all images were in the ascending mode, therefore, western slopes may be affected by foreshortening distortion. Accordingly, it is suggested to apply both ascending and descending mode to overcome foreshortening affect.