Assessment of Land Subsidence Time Series in Kermanshah Plain using InSAR Methods

ABSTRACT
Various natural phenomena have had a significant impact on the quality of human life since long ago. One of these types of natural phenomena is the deformation and displacement of the earth's surface, including subsidence. Subsidence is a morphological phenomenon that occurs under the influence of the downward movement of the earth. The salient features of radar images and the acceptable accuracy of the radar interferometric method have provided a powerful tool for researchers in investigating land subsidence. For this reason, 35 radar images of the Sentinel 1 sensor in the ascending orbit and transit 174 in the period from 2016 (June) to 2021 (January) were used to investigate the land subsidence in the Kermanshah plain. To analyze the time series of these images to prepare the average annual subsidence map in the plain, the radar interferometric technique was used under PSI and SBAS approaches. The results show the maximum land subsidence of 100 mm in the SBAS method and 10 mm in the PSI method in the west and northwest of the plain for 6 years. Finally, the maximum range was investigated in terms of geology and geo-hydrology. The results of the investigations showed that the land use of the maximum land subsidence area includes irrigated and rainfed agricultural lands, with the highest amount of water withdrawal in the agricultural sector, along with an average drop in the water level of 8 meters in 20 years in wells with a large thickness of fine-grained sediments. Is. In general, land subsidence in the area is affected by human and natural factors
Extended Abstract
Introduction
Studying and monitoring the displacement field caused by the changes in the shape of the earth's surface is one of the essential and practical studies in various topics such as geology, geomorphology, and geophysics.In the meantime, land subsidence is one of the destructive geological phenomena that can cause irreparable financial and human losses. In fact, land subsidence is a type of change in the shape of the earth's surface, which is associated with a vertical deformation or downward movement of the earth's surface so that the surface materials settle gradually or precipitately. This phenomenon is a problem and a danger threatening global societies.The phenomenon of subsidence can have surface morphological effects. For this purpose, identifying and reducing the consequences of the subsidence phenomenon requires a monitoring system. In order to evaluate and accurately measure this phenomenon, several methods have been presented, and the radar interferometry technique was introduced as one of the methods of processing radar images in active remote sensing, a useful tool in monitoring the displacements of the earth's surface. So that for several years, the salient features of radar images and the acceptable accuracy of radar interferometric methods have provided researchers with a powerful tool for investigating land subsidence. For this reason, 35 radar images of the Sentinel 1 sensor in the ascending orbit and transit 174 from 2016 (June) to 2021 (January) were used to investigate the land subsidence in the Kermanshah plain.In order to analyze the time series of these images to prepare the average annual subsidence map in the plain, radar interferometric technique was used under PSI and SBAS approaches.
 
Methodology
Artificial aperture radar interferometry is a remote sensing technique. Two or more radar images are used to produce a digital elevation model or prepare a land surface displacement map. In this technique, the phase difference between two waves is measured, which is attributed to the change in the distance between the sensor and the ground target or the displacement of the ground surface. Currently, there are three general methods for limitations and analysis of interferometer time series, which are hybrid, small baseline length, and permanent scatters. In the following article, the time series of the land subsidence phenomenon in Kermanshah Plain is monitored and measured using radar data, radar interferometry technique, and time series analysis of small baseline and persistent scatterers.In the small baseline method, only pairs of images are used whose vertical component is less than the critical value of the baseline. Also, their time baseline should be minimum at the same time. In this way, only interferograms with suitable quality are formed. In the method of persistent scatterer, the selection of permanent scattered pixels with constant scattering behavior in time can eliminate the limitations of the traditional radar interferometry method, and the possibility of measuring the displacement of the earth's surface even provided a few mm. Then, the results of these two methods, which are the average annual subsidence map in the desired time period, are examined in order to explain the connection and cause of the subsidence that occurred on the level of the plain, hydrogeological, and geological data.
 
Results and discussion
In order to investigate the behavior pattern of the earth's surface in the long term, time series analysis methods were used using a small baseline and persistent scatterer. To accomplish this task, among the many images and interferograms, 35 radar images from the Sentinel 1 sensor were selected in the period from 2016 (June) to 2021 (January), and 88 interferograms that had a suitable spatial and temporal baseline were selected using the lowest baseline method. Thirty-three interferometers were selected in the method of the persistent scatterer, and they were covered in the interferometry process. After obtaining the interferogram images, the noises in the interference mapping should be removed so that the remaining noise is only caused by the earth's surface's displacements, resulting in the average map of the displacements of the earth's surface in the desired time period. The evaluation of the map obtained in the small baseline method indicates maximum subsidence of 100 mm per year and 10 mm in the persistent scatterer method in the western and northwestern parts of the plain. Finally, the hydrogeological data (number of wells allowed, type of consumption, amount of harvesting, drop in the level of piezometer wells) and geology (geological log of wells) and land-use of the plain were investigated in order to investigate the cause of subsidence in the plain. These surveys showed the impact of human and natural factors' impact on the subsidence in the plain.
 
Conclusion
In this research, the time series of land subsidence in Kermanshah plain was measured in 2016 (June)-2021 (January) with two approaches, small baseline, and persistent scatterer. The results of the two-timeseries show the maximum land subsidence of 100 mm per year in the SBAB method and 10 mm per year in the PS method in the western and northwestern parts of Kermanshah Plain. In the maximum range, the number of wells has a high density, and most of the wells' water extraction is for the agricultural sector. The land use map of the region also confirms that the maximum land subsidence area has 62% (911 km) area of the plain. The selected wells evaluated in the maximum range of land subsidence also show the amount of water level drop 8 meters per year. From the point of view of geology, these wells have the thickness of sediments, which is about 20 to 37 meters. These cases express the conclusion that the area is affected by human factors (land use, indiscriminate extraction from the surface of the well, number of wells) as an aggravating factor and natural factors (reduction of atmospheric precipitation, continuation of drought, type of sediments on the plain) in next to each other has caused subsidence phenomenon.
 
Funding
There is no funding support.
 
Authors’ Contribution
All of the authors approved thecontent of the manuscript and agreed on all aspects of the work.
 
Conflict of Interest
Authors declared no conflict of interest.
 
Acknowledgments
We are grateful to all the scientific consultants of this paper.