Land subsidence has been a normal problem in many areas of the world, such as Italy (Del Soldato et al. 2018), Mexico (Osmanoğlu et al. 2011; Yan et al. 2012), Iran (Goorabi et al. 2018; Khorrami et al. 2020), Thailand (Phien-Wej et al. 2006), American (Loesch et al. 2018; Miller et al. 2019), China (Wang et al. 2020; Yu et al. 2020), and so on. And it has a significant impact on social and economic development, due to its damages to urban infrastructure including buildings, railways, highways, subways, and underground facilities (Dong et al. 2014). Thus, land subsidence investigations are essential to figure out the spatial of the affected areas, and the mechanism of land subsidence.
Traditionally, land subsidence is usually monitored by the leveling, total station surveys, and GPS field surveys. However, all these are ground-based measurements at sparse locations, and they cannot achieve a sufficiently high spatial density over large areas. Over the last two decades, Synthetic Aperture Radar Interferometric (InSAR) has proven a remarkable potential tool for observing the Earth's surface (Galloway et al. 1998; Hoffmann et al. 2001; Hooper et al. 2008; Becker et al. 2009). Meanwhile, to overcome the effects of temporal and spatial decorrelation and atmospheric perturbations, advanced InSAR techniques have been developed recently. And the PS-InSAR method has been widely applied in monitoring the subsidence in the urban area (Sun et al. 2017; Zhang et al. 2011; Gido et al. 2020). By measuring phase changes between different images acquired at different times but with the same geometry of observation (so-called interferograms), the deformations in land surface can be measured (Ferretti et al. 2001, 2007; Cian et al. 2019). And the surface displacement with centimetric accuracy, tens meters of spatial resolution, and monthly temporal resolution can be resolved (Xiao et al. 2011).
It is worth noting that the time series of subsidence obtained by the InSAR method are always comparing with the GNSS, leveling, and so on (Hung et al. 2018; Poland et al. 2006; Chen et al. 2017). Although this combination can well verify the monitoring accuracy of subsidence among each other, it is just representing the surface deformation, and the mechanism of land subsidence cannot be fully disclosed. To better understand the mechanism of subsidence, it is critical to measure the compaction of the soils at different depths. Meanwhile, the Distributed Fiber Optic Sensing (DFOS) technology has been successfully applied to many kinds of structural health monitoring, such as tunnels, bridges, concrete piles, rock slope, etc (Li et al. 2004; Zhu et al. 2014; Sang et al. 2019). It has many unique functions, such as non-destructive capability in distributed monitoring and long-distance monitoring, anti-electromagnetic interference, waterproof, corrosion resistance, durability, etc (Wu et al. 2021). To this end, the deep strata deformation characterization can also be investigated detailedly by using the DFOS techniques in the subsidence monitoring (Liu et al. 2020). Meanwhile, combining with the PS-InSAR technique, the mechanism of land subsidence in a certain area can be fully exposed. And it is of great significance to urban planning and management.
In this study, the spatial distribution of the ground surface deformation in Suzhou is investigated by the Persistent Scatterer Interferometry (PSI) technique. The PSI technique can be used to map the location of hazard zones and their ongoing subsidence rate. Meanwhile, the DFOS technique can be used to get the strain distribution of the fiber optical cable in the borehole, which is helpful to pinpoint the main compression layers, determine the compression trends, and allow estimation of land subsidence potential. Combining the two techniques, it can be used to define the risk zones for future ground subsidence and to establish a comprehensive ground survey plan with information on rapidly subsiding areas, detailed time-series deformation and the strata deformation characterization at each borehole. Meanwhile, given the ground subsidence caused by the exploitation of groundwater, it will also provide a practical basis for the rational exploitation and management of groundwater.
The paper is organized as follows: Sect. 2 (Study area) displays the overview of the region; Sect. 3 (Materials and Methods) describes the data used and the methodology employed for the analysis; Sect. 4 (Results and Discussion) presents the results obtained by the two methods and discusses the findings; Sect. 5 (Conclusions) concludes.