Document Type : Research Paper

Authors

• Kourosh Shirani ¹
• Mehrdad Pasandi ²

¹ Associate Professor, Department of Soil and Water Management Engineering, Soil Conservation and Watershed Management Research Institute, Agricultural Research, Education and Extension Organization (AREEO), Tehran, Iran

² Associate Professor, Department of Geology, University of Isfahan, Isfahan, Iran

Abstract

Introduction
Land subsidence, a significant geological hazard, poses widespread risks to infrastructure, agriculture, and the environment. This phenomenon may result from factors such as excessive groundwater extraction, mining activities, oil/gas extraction, or natural causes like sediment compaction and tectonic movements. Accurate monitoring of land subsidence using advanced technologies is essential for mitigating its adverse effects. Interferometric Synthetic Aperture Radar (InSAR), particularly the Persistent Scatterer InSAR (PSInSAR) technique, is one of the most advanced methods for monitoring ground deformation. With millimeter-level precision, it enables the detection of subtle land movements and serves as a critical tool for long-term subsidence monitoring over large areas. In this study, Sentinel-1 satellite data (ascending, descending, and combined modes) and the PSInSAR technique were utilized to assess and map land subsidence risk in major watersheds of Isfahan Province, including Isfahan-Borkhar, Najafabad, Northern Mahyar, Southern Mahyar, and Kuhpayeh-Sejzi. By providing detailed insights into the extent and severity of subsidence, this research identifies spatial and temporal patterns, offering crucial information for policymaking and risk management.
 
Materials and methods
The study area encompasses extensive watersheds that include the metropolis of Isfahan, as well as significant agricultural lands and residential areas. The Sentinel-1 radar data spanning from 2014 to 2023 were used, including ascending and descending imagery, to resolve displacement ambiguities caused by the directional nature of movement. Initial data processing involved co-registration of radar images to align pixels accurately and generate interferograms for phase change extraction. Persistent scatterers (PS) were identified using the Amplitude Dispersion Index (ADI) and phase stability analysis. Atmospheric and orbital errors were corrected using statistical models and inversion techniques to eliminate biases. Temporal analysis of ground displacement was conducted to calculate deformation trends, with data georeferenced for spatial interpretation. Validation was carried out by comparing results with ground-based data and independent sources. Final outputs included cumulative subsidence maps, annual subsidence rates, and risk zoning maps highlighting areas prone to land subsidence.
 
Results and discussion
The findings reveal that subsidence in the study area ranged from negligible levels to 55 cm over the nine-year observation period. Annual subsidence rates in parts of the Isfahan-Borkhar and Southern Mahyar watersheds reached 60 mm per year. Combining ascending and descending data improved accuracy and enabled the separation of vertical and east-west horizontal displacement components. The highest cumulative subsidence was observed in urban and agricultural zones of the Isfahan-Borkhar watershed and in clayey sediment areas within the Southern Mahyar watershed. Risk zoning maps indicate that the Isfahan-Borkhar and Southern Mahyar watersheds have the largest areas classified as high-risk. The other watersheds predominantly exhibit moderate to low-risk zones. The maps demonstrate a strong correlation between severe subsidence and land use (urban and agricultural areas) as well as geological features (clayey sediments and alluvial deposits).
 
Conclusions
The application of PSInSAR for monitoring land subsidence in Isfahan Province provides valuable insights into the patterns and trends of this phenomenon. The results highlight severe and ongoing subsidence in the Isfahan-Borkhar and Southern Mahyar watersheds, necessitating urgent planning and management measures to mitigate the associated risks. The link between subsidence and anthropogenic factors, such as excessive groundwater extraction, and geological characteristics underscores the need for integrated planning for sustainable water resource management and land use. The risk zoning maps presented in this study serve as essential tools for policymakers and urban planners to optimize risk management strategies. Future research should focus on continuous monitoring and the development of predictive subsidence models to address this issue effectively.

Keywords

• Ascending and descending data
• PSInSAR interferometry
• Sentinel-1
• Land subsidence