Investigation of land subsidence, using time series of radar images and its relationship with groundwater level changes (Case study: Karaj metropolis)

The Earth's solid crust has not been fixed throughout the history of geology but is constantly changing under the influence of internal and external factors. These elevation changes in the form of subsidence and elevation significantly increase the trend of environmental hazards that affect of these hazards causes severe damage to the natural environment and human effects (Zare Kamali et al., 2017).According to the US Geological Survey, the phenomenon of subsidence involves the collapse or subsidence of the earth's surface, which can have a small displacement vector, which is said to occur gradually and instantly on a large scale (Geological Survey, 2011). The most important cause of subsidence of surface areas in sedimentary basins of arid and semi-arid regions is the compaction and compaction of sediments due to improper extraction of groundwater aquifers (Zhou et al., 2015). If wet clay layers are located between the sand layers, this phenomenon will be observed more widely and acutely (Liu et al., 2006). Land subsidence usually occurs with a time delay after prolonged abstraction of groundwater resources (Scott, 1979). The amount of subsidence depends on the thickness and compressibility of the layers, the duration of loading, the degree and type of stress (Signa et al., 2014).Research dataThe purpose of this study is to monitor land surface changes and identify areas with subsidence and uplift and identify the rate of these changes in the area and finally to investigate the causes and factors affecting the occurrence or exacerbation. For this purpose, first the Sentinel-1 radar image series with suitable time and place difference (maximum 150 meters) including 25 images (Table 1) in the period 2014 to 2021 with Ascending direction and also 12.5 meters Dem related to ALOS PALSAR sensor from the space agency site. Europe (Vertex) was prepared.In addition to radar data, information about the trend of changes in the level of piezometric wells in the region and also the average monthly precipitation of the region to compare and investigate the causes of subsidence in the study area were provided by the Water Resources Management Company and the Meteorological Organization, respectively.

In the present study, using the SBAS time series method in the period 2014 to 2021 in the metropolitan area of Karaj by selecting 25 Sentinel-1 images with appropriate time and place distance, the average subsidence velocity, and land subsidence in the study area was estimated.To determine the main factors of this event, the study area was examined in more detail in terms of mining, tectonic activity, land use in terms of construction of massive structures, and finally changes in groundwater level. Then to investigate the causes and factors. The decreasing trend of groundwater level and precipitation in the region during the last 6 years was also examined.

The results of time series analysis of interferometric images show that in Karaj metropolis the number of land surface changes was between -145 to +15 mm and the areas with subsidence in the northwestern part of the study area are Mehrshahr which recorded subsidence between 100 and 145 mm. To determine the main factors of this event, the Mehrshahr region was examined more precisely in terms of mining, tectonic activity, land use in terms of construction of buildings and massive structures, and finally changes in groundwater level, which showed that only water level changes. Underground in the region has a trend corresponding to subsidence in the Mehrshahr area. To investigate the causes and factors of the declining trend of groundwater level, rainfall in the region during the last 6 years was examined. On groundwater recharge and eventually subsidence.

The importance and increasing monitoring of land status, especially in order to identify the occurrence of subsidence that has occurred in recent years and following the drought and over-exploitation of groundwater resources have caused more attention by researchers in this regard. In the present study, due to the importance of the subject and the study area, the rate of subsidence and its relationship with groundwater level changes were evaluated. The results showed that in the metropolis of Karaj in the period of 6 years 2014 to 2021, the highest rate of land movement was between -145.27 to +15 mm.Therefore, according to the results of the present study and the direct relationship between subsidence in Karaj metropolis with groundwater decline, it seems necessary to monitor the use of groundwater in the region, especially in agriculture, because the current trend in the use of groundwater Recent droughts will lead to irreparable risks of subsidence in the region.