Introduction
Many cities in the world have land subsidence problems because of environmental issues related to urban development. The arid and semi-arid climatic conditions prevailing in most of the regions of Iran and the need for the increasing industrial, agricultural, and drinking water exploitation of underground water resources, as well as the development of urbanization, have provided a suitable infrastructure for the occurrence of land subsidence in Iran, especially in Tehran. Despite the capability of conventional techniques to measure land subsidence, these methods are time-consuming and somehow difficult. In many cases, accurate measurements are not possible, especially in dense urban areas. Satellite radar interferometry (InSAR) is one of the most accurate remote sensing techniques to obtain information and display ground surface displacements. Due to the use of satellite data and its repeatability, it is possible to monitor land subsidence in the desired location in the shortest time and at an optimal cost. This study aims to assess land subsidence in Tehran, Iran, using the InSAR technique.

Methods
In this analytical survey study, 31 images of Sentinel-1 were processed using the time series of persistent scatter interferometry (PSI) technique in 2022. In the pre-processing stage, after converting the images into a readable format by the SARScape module based on the ENVI platform, images were determined based on the border of the study area. An image with lower spatial and temporal correlation during the time series was selected as a reference image. In the second step, the geometrical registration of the images and the production of the interferometer were performed. In the third step, using the set of interferometers obtained from the previous step as well as the amplitude dispersion index (ADI), persistent scatterer (PS) pixels whose fuzzy behavior was constant over time were selected. In the fourth step, considering that the phase difference of two InSAR images for each PS includes components such as the phase caused by the atmosphere, the phase caused by orbital errors, the phase caused by topography, and the phase caused by noise, the mentioned phases were identified to estimate the phase caused by the displacement of the earth's surface by subtracting these phases from the interferometer phase. In the final step, a map of the average annual rate of subsidence in Tehran metropolis was obtained. After validating the results of InSAR using the observations of the Global Navigation Satellite System, the relationship between land subsidence and changes in the underground water levels was examined, as the most important factor of surface changes in the study area, using the regression analysis. 

Results
The findings showed that the land subsidence pattern had a decreasing trend by moving from the plain to urban areas. The highest land subsidence, with a rate of -43 mm per year, occurred in the southern and southwestern parts of Tehran. Districts 10, 11, 12, 16, 17, 18, 19, and 20, which comprise about 26% of Tehran’s population, were affected by land subsidence. The drop in the water level of observation wells was considerable in the areas where the highest land subsidence occurred. The subsidence level had a decreasing trend from the south to the north of Tehran, where districts 1, 3, and 4 had the lowest subsidence rate in 2022. Another finding was the decrease of the underground water level from the north to the south of Tehran, indicating that the water depth in the southern areas of Tehran has decreased due to human activities such as water pumping. The results of regression analysis showed the high relationship between land subsidence and changes in the underground water levels.

Conclusion
The results of this study showed that the increasing in pumping water from wells in the study area for various uses such as drinking, industrial, and agricultural has led to the escalation of land subsidence. Correct and efficient management of water resources in urban and non-urban areas of Tehran metropolis is mandatory with the knowledge that most observation wells have experienced a drop in water level and some of them have dried up. The use of InSAR technique and Sentinel-1 data is helpful in determining the rate and range of land subsidence.

Ethical Considerations

Compliance with ethical guidelines
The current research has been done with the full knowledge of the authors about the process of conducting the research and the points of research ethics have been fully observed in it.

Funding
This research did not receive any grant from funding agencies in the public, commercial, or non-profit sectors. 

Authors' contributions
All authors equally contributed to preparing this article.

Conflicts of interest
The authors declared no conflict of interest.

Acknowledgements
The authors would like to thank the National Cartographic Center of Iran and the regional water department of Tehran for providing the data needed to conduct this research.




References