The Effects of Different Weather Conditions on the Mobility of Salt Masses Using Interferometry Method of ASAR Time Series Images (Case Study: Larestan Shah-gheyb Salt Dome)

Document Type : Full length article


Assistant Professor of Geography and Urban Planning, Shahid Bahonar University of Kerman, Kerman, Iran


Exploring the behavior of exposed salt structures and its relationship to the environmental factors can be effective in identification of these structures and their environmental effects. Salt layers beneath the sediments, due to their lower viscosity than the surrounding rocks, can flow upward and form different salt structures such as salt pillows, walls and salt domes. Kinematics study of outcropped salt structures has great potential to provide valuable information for engineering and scientific purposes, such as the storage of natural gas. The behavior of salt masses is usually difficult to investigate their instability and extreme variability on the surface. A study of the world's salt domes shows that the evolution of many salt structures is influenced by tectonic factors and regimes. However, the life span of salt masses on the earth's surface is short, but because salts are exposed, they are affected by the processes of the earth surface and undergo various changes. By examining these changes over this short period we can find key and important evidence of evolutionary trends. As a result, it is important for the overall exploration of crustal salt to study the few available sub-aerial salt bodies. Most of our information on salt kinematics and mobility is related to the studies on the Zagros and Alborz salt domes in southern and northern Iran. The Shahgheyb-e-Larestan salt dome is one of the largest exposed saline structures that have been used in the study of salt motility. We have addressed the possible relationship with climate change in this research. Hence, the accrued deformation on this salt dome was identified and measured by time series interferometry technique of ASAR images related to the years of 2003 to 2008.
Materials and methods
InSAR is a remote sensing technique using radar imagery to measure the satellite line of sight (LOS) displacements rate in millimeter to centimeter accuracy. Multiple SAR images are used to generate a set of interferograms and to form a time series after a joint inversion. InSAR time series analysis help us reduce the impact of several noise sources (decorrelation, orbital and DEM errors, atmospheric delays, phase unwrapping errors) on dis-placement rates estimation during the time period spanned by the full dataset  with an accuracy for surface displacement velocity at the mm/yr scale. The short baseline method is used to minimize the spatio-temporal baseline by a combination of interferograms. We have used 22 ESA ASAR C-band radar images acquired by the Envisat satellite between June 2003 and October 2008 from Track 291 (along descending orbits). A small-baseline approach was used to process interferograms. This method was used to calculate average displacement rates in the period with the New Small Baseline Algorithm Subset chain. Eleven individual interferograms were generated using a modified version of the SARScape and the STRM 30-global DEM. The interferograms were corrected from DEM errors.  
Results and discussion  
In order to investigate the relationship between salt kinematics and different weather conditions, the individual maps of 10 different periods between 2003 and 2008 were correlated with the temperature and precipitation data corresponding to the mentioned periods. The radar interferometry method was performed on the pair of images from 2003 and 2008 from the study area to produce the interferogram images.  Several fringes are formed in the interferogram image. Since the used satellite (Envisat) works in the C band, and each obtained fringe is λ/2 equal to 2.8 centimeters, the displacement rate to the satellite's view is obtained by counting the number of fringes. The mechanism of displacement also varies depending on the color cycle (fringes pattern), so that if the color cycle is yellow-blue-red, displacement moving away from the radar. If the color cycle is yellow-red-blue, the displacement has occurred into the radar. The surface displacement rate in the Shah-Gheyb salt dome is 2.3 to 2.8 cm per year, between 2003 and 2008 into the satellite view. Two regions of the salt dome have been uplifted and one region in the northwestern part of the dome has been subsided. Based on the cross-sectional displacement maps, there is an apparent relationship between the hot and cold months of the year and the surface displacement rates of the salts. Corresponding to the two warm months of the year; the rate of uplift is much higher than that of subsidence. The results obtained from the time series analysis and displacement rates indicate that this salt dome is active. It determines the salt motion in the satellite's view of 2.8 cm per year. Based on correlation coefficients, there is a direct relationship between displacement and average temperature. As the temperature increases, the salt uplift rises and the subsidence decreases. But there was no relationship between rainfall and surface changes of the salt. 
In this study, the displacement rate of Shah-Gheyb salt dome was calculated using radar interferometry method in the folded Zagros area. The average uplift rate of salt mass was 2.8 centimeters per year and the average rate of subsidence was 2.2 centimeters per year. This rate of change has been varied in different times according to the temperature of the region. The correlation of different weather conditions with sectional displacement maps revealedthat there are a relative correlation between the salt kinematics and climatic conditions. Therefore, in this salt dome, salt kinematics can be controlled by climate conditions. In addition, the time series analysis has indicated that the difference between the uplift and subsidence has steadily increased in 2003 to 2008. Therefore, secondary salt changes after the outbreak of salt domes on earth were often influenced by climatic conditions and physicochemical properties of salt as well as tectonic conditions. 


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