Investigating the Amount of Ground Surface Displacement in the Gotvand Olya Dam Using Differential Radar Interference Technique

Satellite imagery is considered as one of the most important tools of land resource management, due to the wide vision that emerges from one area and its regular repeated coverage. Using remote sensing technology, in particular radar interferometry (InSAR), it is possible to study the movements and changes caused by phenomena such as earthquakes, volcanoes, glaciers, landslides, salt diopters, and other irregular phenomena. In this research, differential radar interferometry was used to illustrate the displacement of the Earth's surface within the Gotvand Dam area and to investigate the short-term and long-term changes in the incident. There have been several studies conducted to investigate the effects of water reservoirs of lakes and dams, as well as the processing of satellite images of landslides and subsidence. For example, Haghighat-Mehr et al. (2010), using Radar interferometry technique to determine the subsidence rate and Landsat slides of the Hashtgerd plain, used 4 images of the ENVISAT radar during the 4-month period (July 11, 2008 to October 24, 2008), and estimated that the maximum rate of downslope in the plain was 35 mm per month. In addition, Jennat et al. (2009) used the radar interferometry method to determine the surface deformation in the Golpayegan Plain. In the current research, differential radar interferometry was used to illustrate the displacement of the Earth's surface within the Gotvand Dam area and to investigate the short-term and long-term changes in the incident. The term interferometry is derived from the combination of two words including interference and measurement meaning that two waves interact on the surface of the earth. Radar interferometry technique is the combination of two electromagnetic waves on the surface of the earth and the synthetic valve radar technique (Dehghan, 1391). This technique is based on the difference in the ground-back signal phases in two SAR images taken with a time delay or with parallax from an area to extract altitude or information on land surface changes. In order to do this research, 4 images of the SLC Band C, the ASAR satellite ENVISAT for the period of 2007 to 2011, were taken as a research project from the European Space Center with an area of 100 × 100 km2. Through processing of the images with a time interval of one year and more than one year, 4 independent interferograms were obtained. The SRTM DEM region with a resolution of 90 m was used to remove the topographic effects of interferograms. Interfragm processing was performed using Sarscape software. The DOR-VOR file containing the DORIS satellite image capturing information from the European Space Agency was used to correct image distortion caused by the gravitational force of the moon on images. In the first phase of the analysis chain, the data was designed with the aim of constructing the interferometer. Due to the different resolution of this sensor along the radial mile radius (7/80 m) and azimuth (4/5 m), the power dissipation information in line with the range can be converted to the resolution of the earth according to equation (7). A differential interferometer derived from the removal of the topographic effects in the previous stage contains noises that result from the time difference or the time interval received by two radar images. It can also be due to the difference in the basis of the spatial base as well as the resulting spots that have no signal and may result in lower interference imaging; therefore, it is necessary to remove the effects of noise from interference by implementing adaptive filters. Subsequently, the final results of this phase, with the help of Digital Ground Modeling (DTM), were transmitted to Arc GIS for thematic maps. Finally, the results of the landslide detection at the region level, by using field survey and its adaptation to existing information, was controlled and evaluated in the Google Earth environment.
Result, Discussion and The results of this study indicated that the maximum displacement rate in the region reaches about 3.5 cm per year (from 2007 to 2011). To obtain the final landslide map in the area, after processing the pair of radar images, the existing displacements were identified and in the software environment, these changes were derived from each interferometer, combined, and their common areas were deleted. Also, landslides that occur parallel to the flight of the satellite cannot be removed because the landslides in the direction of vision (LOS) of the PALSAR sensor can be picked up. Based on the results of the radar interferometry, the maximum subsidence level in the study area is estimated to be about 3.5 cm per year. In addition, the maximum subsidence rate in Gachsaran and Anabal salt mine is 3.5 cm.