Investigating the relationship between land subsidence and underground water level changes using radar interferometry (case study: Mashhad city)

The phenomenon of land subsidence can cause irreparable financial and human losses and damage many surface and subsurface structures in urban areas and their suburbs. According to the definition of the Geological Institute of the United States of America, the phenomenon of land subsidence includes the collapse or downward settlement of the earth's surface, which can have a small displacement vector, it is said to occur gradually and instantaneously on a large scale. The most important cause of the regional subsidence of the earth's surface in the sedimentary basins of arid and semi-arid regions is the condensation and compaction of sediments due to excessive extraction of groundwater sources. If watery clay layers are placed between sand layers, this phenomenon will be observed more widely and more acutely. Land subsidence usually occurs with a time delay after the long-term extraction of underground water resources. The amount of subsidence depends on the thickness and compressibility of layers, length of loading time, degree, and type of applied stress. With the decrease of the underground water level, the increase of the effective stress caused by the decrease of the pore water pressure causes subsidence. Usually, it takes time to reduce the water pressure and increase the effective tension; Therefore, following the reduction of the piezometric level, subsidence will occur with a time delay. Subsidence caused by the drop in fluid level mainly takes place in unconsolidated or semi-consolidated sediments that are located in the vicinity of sand layers. In such conditions, inelastic compaction occurs due to the increase of effective stress in the soil, the arrangement of the soil grains is disturbed and the thickness of the vertical layer decreases. In the areas where the sediments are not very thick and the underground water extraction continues indiscriminately, lines and cracks are observed on the surface of the land, especially agricultural land, and it is known as the Shaq phenomenon. The phenomenon of subsidence due to the withdrawal of underground water can have different intensities and destructive effects according to the geological situation and geotechnical characteristics of the region. In the sedimentary basins of arid and semi-arid regions, including the Mashhad Plain, the most important cause of ground subsidence is the density of underground water tables due to excessive water extraction. This situation is especially critical in the area where excessive pumping of aquifers containing sand layers is located between impermeable clay layers and causes subsidence. This phenomenon is also happening in Iran, and it is visible, especially in the eastern and central regions of the country, which suffer from drought and the water supply through underground water extraction has greatly increased. Mashhad Plain is one of the areas with a very high subsidence rate, which needs to be given more attention. Land subsidence is a dangerous global problem, and geometric methods are not suitable for extensive and serious monitoring of land deformation.

Radar interferometry with artificial valve is a remote sensing technique. In which two or more radar images are used to produce digital elevation models or to map the displacement of the earth's surface. In the above article, using radar data and using radar interferometry technique and small baseline time series analysis, the time series of land subsidence phenomenon in Mashhad is monitored and measured.In the present study, using radar interferometry and SBAS time series method, in the period from 2014 to 2021 in Mashhad city, by selecting 38 Sentinel-1 images with a suitable time interval, the average speed of land subsidence and uplift in the studied area was estimated.Then, to determine the main factors of this event, the studied area was investigated in terms of changes in the underground water level and tectonics.

The results of the analysis of the time series of the images showed that in Mashhad, the maximum ground displacement is between -77 and +8 mm, and the areas with subsidence are located in the northern parts of Mashhad, which is recorded between 30 and 30 mm. 77 mm subsidence per year. To determine the cause of this event, tectonic assessment and the status of underground water exploitation in this period and its relationship with land subsidence and earthquake data in the area were investigated.

The results of the research showed that the maximum subsidence rate is 77 mm in the area of Mashhad in the period of 7 years from 2014 to 2021 and is related to the north of Mashhad. It should be noted that while advancing towards the city center from the north, you should look for irreparable damage shortly. To check the accuracy of interferometry and interpret the results and investigate the possible cause of the subsidence of Mashhad city, the trend of changes in the water level in piezometric wells, this factor in the occurrence of subsidence in the region was investigated. The information on piezometric wells was obtained in the city of Mashhad, the results of which strongly showed the water level in the past years, which is based on the cause of subsidence in this area. Only in the center is the temporary rise of the underground water level due to the feeding of the urban Mashhad aquifer.Then, to investigate the tectonic participation in the subsidence event, the history of earthquakes and the fault network of the region were evaluated, which confirmed the absence of the investigated earthquakes (maximum 3.8 on the Richter scale) in the region and its surroundings, and there is no fault network within the area of the subsidence assets. did not have. There is no activity. The probability of supposition and the possibility of technology playing a role in the subsidence event is insignificant. Small cracks are mainly created near mental wells or agricultural water exploitation wells and are scattered much more.