Estimation of subsidence using radar interferometry technique and groundwater parameters and land use (Case study: shahryar plain)

Subsidence is the vertical movement or gradual subsidence or sudden subsidence of the earth's surface due to various reasons such as dissolution, melting of ice and compaction of deposits, movements of the earth's crust and the release of lava from the solid crust or activities. Humans such as mining, groundwater abstraction or oil occur (Asadzadeh et al., 2016). The phenomenon of subsidence, which may occur as a gradual or sudden subsidence at the earth's surface, is generally due to major factors such as dissolution of subsurface formations, sediment density and subsidence of groundwater, as well as tectonic factors (Chen et al., 2016). Subsidence can be caused by natural geological phenomena such as earthquakes, dissolution of limestone, sedimentary rocks, ice melting and density of deposits, slow crustal movements and lava outflow from the earth's solid crust, or human activity such as mining. Extraction of groundwater fluids such as groundwater, oil or gas (Gallowey and Burbey., 2011). Radar interference method has been successfully used to measure the displacement caused by earthquakes, glacier displacements, and the phenomenon of subsidence and erosion. Land subsidence as a morphological phenomenon, a type of land deformation that is associated with vertical deformation or downward movement of the earth's surface(Sharifikia., 2011).


Methodology
The data used in this study include a combination of satellite data from Sentinel 1 radar sensors as well as ancillary data such as piezometric well data for groundwater level estimation and optical satellite images including Landsat satellite images for 2020 was used to extract land use. In the image processing phase, the time series of 2015 and 2020 of Sentinel 1 satellite in C-bar were used. Using image processing with SARSCAPE 5.2 plugin in ENVI 5.3 software platform and using radar interferometry method, the areas subject to subsidence and the amount of subsidence in each area were determined. In the next step, using survey operations, the effects of subsidence in the area were identified. In order to explain the causes of subsidence in the study area, data related to land use as well as information related to observation wells were compared and statistically analyzed with the results of subsidence in the area.

Results and Discussion
After ensuring that the two images are correct, the resulting phase of the two SLC images is differentiated relative to each other, resulting in an interferometer. Due to the different resolution of this sensor along the range and azimuth and a look in the direction of the range based on the defined mathematical relationship to correct the spectral displacement and Doppler effect was considered. Investigation of subsidence status in the last 6 years, from 2015 to 2020, the amount of subsidence has increased. The red areas have the highest subsidence of 11 cm. The black areas have no subsidence and have a height of 3 cm. When subsidence occurs in the area, it not only decreases, but also has a duality or an ascent. Based on the information obtained from the subsidence map of Shahryar plain, it was found that in the studied period, we see a significant amount of subsidence. In the subsidence map, the highest amount of subsidence was in the eastern part of the city and in the northern part of the city and the lowest amount was in the northwest and northeast of Shahryar plain.

Conclusion
The study of subsidence status in the last 6 years from 2015 to 2020 in the study area shows that the maximum amount of subsidence is 11 cm. In order to reconcile the results of radar interference with other supplementary data, land use as well as piezometric wells in Shahriar plain were used. Co-institutionalization of subsidence plans with land use layers also confirms the maximum occurrence of the subsidence area that the highest rate of subsidence in garden and land uses with a value of 11.54 cm, rangeland with a value of 11.34 cm and residential area uses with a value of They are in the next categories to 10.75 cm. The amount of uplift in all three uses indicates the amount of three cm of subsidence (Table 3). 11 cm subsidence for residential use can provide insecure living conditions for residents and increase their risk of landslides.
Due to the groundwater level of the plain, despite seasonal fluctuations, it has a downward trend. As can be seen, in 2015 the water level was 75.85 meters, which has decreased to 68.52 meters in 6 years in 2020. Which shows that during six years we witnessed a 7 meter drop in groundwater.