Evaluation of kermanshah plain subsidence time series using small-based radar interferometry technique (SBAS)

Extended Abstract



Introduction

Investigation and monitoring of displacement field due to deformations of the earth's surface is one of the most important and applied studies in various topics such as geology, geomorphology, geophysical. Land subsidence is one of the destructive geological phenomena that can cause irreparable financial and life losses. In fact, subsidence is a morphological phenomenon that in the logical state of this phenomenon includes collapse or downward sitting of the earth's surface, which can also have a small horizontal displacement vector (Solari et al., 2018). This phenomenon has been charted as a serious crisis in most plains of the country in recent years. Therefore, identifying and reducing the consequences of subsidence phenomenon requires a monitoring system. Several methods have been proposed to accurately evaluate and measure this phenomenon, which radar interferometry technique was introduced as one of the methods of radar image processing in active remote sensing, a useful tool in monitoring the placements of the earth's surface (Sadeghi et al., 2012).So that over the past several years, the use of radar interferometry technique to evaluate and monitor land subsidence has increased and its time series algorithms such as PSI and SBAS have provided the possibility of measuring ground displacements (Abu pit et al., 2010). Therefore, in this study, we tried to estimate the time series of kermanshah plain subsidence during 2016-2021 using Sentinel 1 satellite imagery and using radar interferometry technique under small baseline length approach.



Methodology

Radar interferometry with artificial valve is a remote sensing technique. In which two or more radar images are used to produce digital elevation models or to map the displacement of the earth's surface. In this technique, the phase difference between two different waves is measured and this phase difference is attributed to the change in distance between the sensor and the ground target or the displacement of the earth's surface. Ideally, each interferogram should have only a phase difference caused by the displacement of the earth. For this reason, other components must be removed or deducted so that the remaining phase can be obtained only from displacement. In the above article, using radar data and using radar interferometry technique and small baseline time series analysis, the time series of land subsidence phenomenon in Kermanshah plain is monitored and measured. In this method, only visual couples are used that the vertical component of the baseline is less than the critical value of the baseline. Also, their time baseline is minimized at the same time. In this way, only interferences are formed that have good quality. Then, the results of this method, which is the average annual subsidence map in the desired period, are investigated to explain the relationship between the cause of subsidence occurring on the plain surface with changes in groundwater level and the thickness of fine sediments.



Results and Discussion

In order to investigate the behavior pattern of the earth surface in the long term, we used time series analysis using small baseline method. To do this, among multiple images and interferograms, 36 radar images were selected from sentinel 1 sensor at the time interval of 2016 (June) to 2021 (January) and selected by the lowest baseline of 88 interferograms that had a suitable spatial and temporal baseline and were dispersed in the interferometry process. After obtaining the interferogram images, the noises in the mapping interference must be removed so that the residual noise is only due to the displacements of the earth's surface, resulting in the average map of the earth's surface displacements in the desired timeframe. Evaluation of the obtained map indicates maximum subsidence of -100 mm/year in the western parts of the plain. Then, by preparing a map of water level drop in the plain, the relationship between these two variables was investigated, which did not show a significant correlation between them, so by examining another effective factor in the plain surface, we realized that in addition to the drop in the water level, the thickness of fine sediments should also be considered because by summing these two factors together, the surface subsidence. The earth intensifies. This is well known in the western parts of the plain

Conclusion

The maximum subsidence calculated between 2016 and 2021 is 10 cm, which covers an area of about 1.67% of the total area. Then, in order to explain the cause of subsidence occurring in the region, we investigated the relationship between groundwater level drop and subsidence in the region. By evaluating 54 piezometer wells in the plain, it has been observed that the maximum subsidence range is not in accordance with the maximum drop in water level. Therefore, we were looking for another effective factor in the plain surface that by studying the thickness of fine sediments (clay and silt) in the maximum subsidence range, we saw a high thickness of fine sediments which has experienced a moderate drop from the water surface about 5 meters per year. Therefore, it is not always possible to see the linear relationship between water level drop and land subsidence.

investigation of steady state index showthebarchan dunes, with annual displacement rate more than 12 meters,have the maximum number and highest rates of displacement, and have the lowest steady state. Thus this group is demanding the more attention and planning requirements for stabilization of quicksand and environmental management of mobile barchan dunes.