Subsidence Monitoring along Tehran Railway: An Analysis of InSAR and Ground Observations

Land subsidence is recognized as one of the most perilous natural occurrences, often resulting from human negligence in water extraction, underground mining, and various other factors. This phenomenon poses a significant threat, specifically in sensitive areas such as railway systems, where irreparable damage can transpire. Notably, subsidence-induced cracks have emerged along several railway routes, including Tehran-Mashhad, Tehran-Varamin, and Isfahan to Shiraz, jeopardizing the integrity of these lines. Consequently, comprehensive monitoring of subsidence and deformation in both temporal and spatial dimensions becomes imperative for effective event management. To accurately assess the deformation patterns of such phenomena, a thorough analysis of the instantaneous time series within the study region is essential. In recent times, Synthetic Aperture Radar Interferometry (InSAR) has emerged as a widely adopted technique for precisely measuring crustal deformation.

This study focuses on examining the rate of land subsidence along the railway lines in the Tehran region, utilizing InSAR and Sentinel-1 satellite imagery spanning the period from 2017 to 2020. The analysis involved processing a total of 46 images and generating 158 interferograms through the application of time series analysis and employing the Small Baseline Subset (SBAS) technique. GMTSAR software was used to create a time series and a displacement map from the interferograms.  To ensure the credibility and comprehension of the research findings, diverse datasets were utilized, including the Iranian Permanent GPS Network for Geodynamics (IPGN) data, the data sourced from the Shamim network of the Land Registry Organization, the measurements from piezometric wells, and the soil characteristics derived from drilling boreholes.

The analysis of the interferometry time series reveals the occurrence of subsidence in specific areas within the case study. The most significant subsidence was observed along the Karaj-Kordan and Maleki-Aprin railway lines, with a deformation rate of approximately 139 mm/year along the line of sight (LOS). Notably, the validation process considering the errors associated with each method yielded relatively satisfactory results. Furthermore, an investigation was conducted to explore the relationship between subsidence, groundwater withdrawal, and soil type. This investigation utilized data from 12 piezometric wells located in the Tehran and Karaj plains, as well as information gathered from drilling boreholes in the study region. The overall findings indicate that the primary cause of subsidence in the region is attributed to a decline in groundwater withdrawal.

Upon analyzing the relationship between annual water loss, subsidence, and the soil characteristics within the region, it was determined that the primary cause of subsidence is the withdrawal of groundwater in areas characterized by thick deposits of fine-grained sediments. The proposed approach in this study highlights the effectiveness of utilizing the InSAR technique for initial evaluations of subsidence along linear infrastructures like railway lines. However, it is advised to employ more precise methods in subsidence-affected regions. Given the relatively limited resolution of Sentinel-1 imagery, it is recommended to utilize images with smaller pixel sizes when assessing linear structures such as roads or railways. Additionally, accurate leveling techniques can be employed to enhance the precision and identification of subsidence areas.