Study of subsidence of Abarkooh plain of Yazd using the Synthetic Aperture Radar Interferometry method

The population growth, the development of cities, industry, agriculture, and improper use of resources especially non-renewable resources have led human beings to face the danger of running out of resources. In some cases, in addition to the above, irreversible environmental and geological hazards have occurred due to the overdrawn of resources. Over extraction of groundwater resources is one of the problems that, in addition to human exposure to the risk of water scarcity, also induces risks due to this over-harvesting. The phenomenon of land-subsidence is called subsidence for natural and human reasons. One of the reasons for this phenomenon is human activities, including the overextraction of groundwater resources and the water table drop. Subsidence itself results in some problems such as deep cracks in the ground, well pipes growth and collapse of buildings. One of the common subsidence damages in many plains of Iran is the creation of sinkholes. Sinkholes are deep pits that are mainly involved in the development of karst dissolution and subsequently many risks to urban and environmental areas. The areas affected by the subsidence phenomenon have been identified using the Synthetic Aperture Radar Interferometry (InSAR) method. The interferometry technique uses electromagnetic wave interference to extract information. Radar interferometry uses the phase difference of radar images with a high spatial resolution seeking to produce a digital elevation model of the region and estimate the amount of deformation and displacement of the earthchr('39')s crust. In the radar interferometry technique, complex radar images containing the phase values ​​and amplitude of the wave returning from the complication to the sensor are combined and an image called an interferogram is produced. An interferogram is an image that results from the subtraction of two images taken at two different times that are aligned on each other. An interferogram contains information of the phase difference between the two images, which indicates the difference in distance between the terrain features and the sensor in the two captured images. By having the phase difference value, various parameters such as the amount of ground displacement can be obtained. Persistent Scatterer Interferometry (PSI) time series analysis method uses stable scattering pixels (pixels that have scattering patterns with a constant scattering pattern over time) to extract information and overcome the limitations of radar interferometry. In the present research, an attempt has been made to monitor the subsidence of Abarkooh using the Synthetic Aperture Radar Interferometry method.Abarkooh region is located in Yazd province which is an agricultural region in recent decades and the extraction of groundwater resources has been increased in it. Abarkooh city has two urban points, Abarkooh and Mehrdasht. Excess extraction of groundwater caused numerous subsidence and sinkholes, subsequently. Hydrological studies have determined the rate of groundwater level drop equal to 0.6 m/yr. In this paper, by using 55 Sentinel-1A satellite images in the period of 2017 to 2019, 54 interferograms have been used to process time series analysis based on persistent scatterer after deducting the participatory phases from the interferogram phase. Finally, the average subsidence rate map for the Abarkooh region is estimated. According to the analysis, the maximum subsidence rate of the Abarkooh region is 3 to 4 cm /yr and the maximum subsidence rate of the Mehrdasht region is 6 to 7 cm/yr. Accordingly, by considering the location of the subsidence area in the agricultural lands of the studied region and the decrease of the water table level in that region, one may conclude that the subsidence has happened due to the over-extraction of groundwater resources. In order to investigate the impact of subsidence on the environment, the soil salinity map has been calculated. In general, the salinity of the region is increasing as a factor impacting the degradation of surface soils and desertification. Well pipes growth, sanding of water wells, and reduction of well’s discharge, all are some examples of the consequences of subsidence in the destruction of wells. Other consequences of subsidence in this area include turning the plain into a desert, damaging aquifers, changing the topography of the land, and environmental pollution. Due to the un-uniform distribution of the subsidence areas in ​​the region, the damages to roads, railways, urban and industrial buildings, and energy transmission lines have been predicted. One of the most important subsidence consequences in this area is the creation of deep holes and longitudinal cracks along with the subsidence area. To evaluate the results of the interferometry technique, GNSS station data have been used. Since the data related to this area are available from 2011 until now, it is possible to obtain an accurate evaluation of the InSAR results. The results show that the displacement rate is 1 cm based on the permanent station data (GNSS) and the interferometry measurement results show a displacement rate of 0.7 cm. These values indicate the accuracy of the results of radar interferometry.