Assessing subsidence risk of Lordegan county using radar interferometry technique

Introduction

The risk of subsidence can be an important factor in increasing the vulnerability of the center of human activities located in areas with natural hazardous infrastructure such as earthquakes. In recent years, this phenomenon has caused a lot of damage to the plains and cities of Iran, especially to residential buildings, plains and agricultural lands located in Chaharmahal and Bakhtiari provinces. Therefore, monitoring the amount of subsidence and dealing with its influential causes in order to control and manage its vulnerability is of particular importance. So far, various methods have been used such as using GPS, precision instrument alignment and radar interferometric method to measure land subsidence. In the meantime, the radar interferometric method is an up-to-date and effective technique for measuring changes in the earth's surface worldwide. In this article, satellite data and radar interferometric technique have been used to measure the subsidence risk of Lordegan plain and urban settlements. For this purpose, the amount of subsidence in a period of 6 years (from 2017 to 2023) has been evaluated using Sentinel A1 data using SNAP software. The results of field observations indicate that the occurrence of subsidence and proportionally the drop of groundwater level is due to excessive extraction of underground water. The data obtained from the piezometric wells of Lordegan city and Barm spring indicate a sharp drop in the underground water level since 2008. The amount of groundwater dropping has been observed in some areas up to 15 meters. The results obtained from the radar data showed that during a 6-year period, Lordegan city has experienced an amount of subsidence of about 14 to 18 cm. These subsidence amounts are about 3 cm per year, which is at the warning level and beyond the normal state. So, accurate management of the withdrawal of underground water seems essential by considering the significant rate of subsidence and its risk.



Methodology

Radar images obtained from virtual aperture radars (SAR) with the ability to measure the length of the vector from the sensor to the ground surface, are widely used in measurements related to the preparation of digital elevation maps. The technique used in this method is known as radar interferometric technique (InSAR). In radar interferometry, the phase obtained from two images taken from a certain area is interfered to produce an interferometer. In fact, the interferogram is the different product of two radar images. In the interferometry method, the mixed radar images that have the return phase values from the complex to the sensor are combined with each other and an image called the interferometer is produced. Hence, interference is obtained from the phase difference of two images taken at two different times, which are geometrically precisely matched. By using the phase difference information available in the interferometer, which indicates the distance difference between the sensor and the object, it is possible to prepare the deformation variable of the earth's surface or earth's topography. In this research, C-band radar images of the European Space Agency's Sentinel-1A satellite were used in the period of 02/05/2017 to 01/29/2023 at the level of a SLC and in IW mode with high resolution with VV polarization. In addition, in the data processing stage, sub-band IW1 and segment 2 to 3 images were used.



Results and Discussion

The radar interferometry method measures changes in the earth's surface along the line of sight of the satellite. Due to the fact that the major changes in the land surface in subsidence are in the form of height changes, the displacements extracted by the radar interferometric method can be converted into height changes by ignoring the horizontal changes in the ground surface and depicting them in the vertical direction. Each interferogram individually only contains surface changes in one time interval. By using a number of interferograms along with time series analysis, surface changes can be examined over time. Analysis of time series using radar interferometric method is a suitable method for estimating the rate of land surface changes in high spatial resolution. By having two images with different time intervals from satellite images, the rate of subsidence and vulnerability of an area can be calculated at any point and date with the radar interferometric method. It is worth mentioning that the radar interferometry technique is one of the most up-to-date and accurate tools for calculating the rate of land surface changes in the world and is being used all over the world.



Conclusion

In this article, using the radar interferometric technique, the state of the subsidence phenomenon in Lordegan city and the damages caused around the Barm spring in Lordegan city have been discussed. In summary, the most important results obtained are:

• The results obtained from the rainfall data and different hydrographs of the plain showed that, the amount of rainfall and the level of underground water have significantly decreased since 1986, for example, the level of underground water has decreased from a depth of about 15 meters from the ground surface to a depth of about 33 meters.

• The results of the radar interferometry technique to depict the range of subsidence, showed that the total maximum amount of subsidence between the years 2017 and 2023 was about 18 centimeters, which is about 3 centimeters of annual subsidence.

• based on the evidence and investigations carried out on the Barm spring in Lordegan city, it was determined that the volume of water spring has significantly decreased. It should be noted that the construction of heavy structures and other constructions near the spring has also affected the water flow and the reduction of the flow of this spring.

• In recent years, numerous damages have appeared in the city and around the Barm spring, for example, the appearance of numerous and annual cracks on the streets and the path of the old aqueducts, cracks and damage to the stone walls around the spring, damage to buildings.