Evaluation and zoning the subsidence of Namin city using radar interferometric method and Aras multi-criteria technique

As a morphological phenomenon, land subsidence is a type of change in the shape of the earth's surface, which is associated with a vertical deformation or downward movement of the earth's surface, as well as the gradual settlement or removal of surface materials. The phenomenon of subsidence is due to various causes, including natural factors such as earthquake, volcanoes, fault activities, subsidence due to rising sea levels, dissolution in rocks or following human activities, including; Indiscriminate removal of fluids from the ground, such as; Water, oil and gas occur. The environmental consequences of the subsidence phenomenon are irreversible, costly, and destructive, and include creating a gap on the surface of the earth, damaging human structures, etc. Foundations, streets, bridges, roads and power and sewage transmission lines, destruction of irrigation systems and fertile agricultural soils and damage to wells. According to the statistics announced in Iran, the adverse effects caused by subsidence are not low and are rapidly developing and spreading in different plains across the country, and the lack of timely management and control of its factors can cause loss of life. And leave an irreparable loss.



Methodology

In this research, the images of Sentinel 1, which captures images in the C-band range of microwaves, have been used. Then the necessary processing was done through the SARSCAPE 5.2 plugin in ENVI 5.3 software and the technique used in this research to determine the amount of subsidence is the differential interferometric method with the combined opening of two frequent or non-frequent passes. In radar interferometry, the phase difference of two images from a specific time zone and baseline is determined. To zone the risk of subsidence, in the stage of data collection, the influencing components must first be specified and the required data and information must be collected and classified based on them. For this purpose, in this study, first, the effective factors (including slope, lithology, land use, precipitation, distance from the city and village, distance from the river, distance from the fault, and drop in the underground water level), according to the natural and human conditions of the region was identified. In the next step, information layers related to each of the factors were prepared in the geographic information system environment. In the next step, the weighting of the investigated factors was done according to the Critic method, and the final analysis was done using the Aras method.



Results and Discussion

After the interferometry steps, the phase difference has been converted into vertical displacement in the metric system. During the surveys conducted, the amount of subsidence between the years 2016 and 2020, during the last 5 years, shows that according to the above map, the highest amount of subsidence in the central, north-western, and north-eastern parts, amounting to 0.37 mm, it is concentrated. Investigation and analysis of land use and subsidence map and field studies confirm the maximum occurrence of subsidence in agricultural areas, and urban and rural areas, respectively. The presence of fertile plains and the flow of the Qarasu river in Namin city has provided a good opportunity for agricultural and animal husbandry activities, and the areas with agricultural land use have a high potential for subsidence due to the exploitation of underground water areas. After extracting the maps of each of the criteria and applying the relevant weights obtained by the Critic method and by implementing the different stages of the Aras technique, the maps of the subsidence risk zoning in 5 categories from very high to very low risk The risk is achieved. According to the result of using the Aras method, 168.01 square kilometers of the range area is in the very high-risk class and 222.31 square kilometers is in the high-risk class. Most of the high-risk and high-risk class in terms of the possibility of subsidence is located in the central and semi-western parts. Examining the high-risk and high-risk points introduced by the Aras algorithm shows; In terms of the criteria of land use, agricultural use, urban and industrial areas account for the largest percentage of the area of areas with very high and high risk. According to the land use map of the study area, a major part of the city's land has been allocated to agricultural use, which has caused overexploitation of the underground water table in recent years. In addition, a large part of the water needed for drinking and the industrial sector is also supplied from underground water sources, and this, in turn, is involved in the reduction of the water level and the increase in the risk of subsidence in the studied area.





Conclusion

The evaluation of the subsidence rate of the region shows that the highest subsidence rate was in the central, north-west, and north-east areas of the range and the results show that the trend of the groundwater level drop is consistent with the subsidence rate and in the areas where the water level drop is the highest. has existed, the amount of subsidence has also been higher. According to the results of subsidence risk zoning; The criteria of water level drop, and land use, respectively, with a weighting factor of 0.186 and 0.168, were assigned the most weight, and therefore, it can be said that the most important factor involved in increasing the amount and potential of subsidence in the study area of exploitation. It is a waste of underground water. Finally, it can be acknowledged that because land subsidence can cause irreparable financial and human losses, long-term solutions in the form of modifying water resources management methods and short-term solutions in the form of prevention From extraction of underground water tables, creating of underground dams to increase the water level, curbing surface water and implementing artificial feeding plans, confiscating unauthorized drilling equipment, preparing and installing smart meter devices, organizing drilling companies, strict control of water consumption and change The consumption pattern should be taken into consideration by relevant officials as well as residents.