Numerical simulation and zoning of land subsidence caused by Solution Mining in the Northwest of Golestan Province

Introduction
Land subsidence is one of the geological hazards that leads to the collapse or lowering of the earth's surface. The deformations on the earth's surface are mostly in the vertical direction and displacement may also be observed in the horizontal direction. Subsidence can be affected by human activities such as tunneling, mineral production, groundwater extraction and fault activity, which cause abundant morphological outcrops on the earth's surface (Sharifikia, 2012). In some cases, land subsidence is caused by the extraction of underground reserves and valuable mineral materials. One of the methods used to produce useful minerals is mineral solutions. If the mineral is solid and soluble in water, the mineral is dissolved in water by injecting water through wells containing the mineral layer, and then pumping and extracting the mineral solution through the extraction wells is carried out. In the extraction cycle, water injection operations are carried out on the one hand and the production of water-soluble materials is carried out in a controlled manner. The United States, Kazakhstan, China, and Uzbekistan use this method to extract potash ore. The Belle Plaine Potash Mine, located in Canada, is the world's first and largest solution mine (Mark et al, 2010). In the case of deep mineral brines that contain valuable elements such as iodine and bromine, the mineral solution extraction method can be used. In this case, due to the nature of the mineral, which is in the form of a mineral solution, the minerals are exploited by drilling deep wells. In order to reduce the effects of subsidence in such conditions, the wastewater from the extraction should be injected into deep layers after processing and separating the mineral. Currently, the mineral solution extraction method is used in our country and in the northwest of Golestan Province. In the northwest of Golestan Province and near the city of Aq Qala, the exploitation of iodized brines at depths of more than 1000 meters is underway, and the exploitation of iodine minerals by extracting deep brines by the private sector has begun since 2008. The purpose of this research is to investigate the numerical values ​​and zoning of land subsidence due to the extraction of iodized brines in the study area. Although subsidence has a relatively high frequency and sequence, it is often difficult to detect and measure accurately due to the very slow and shallow motion of the Earth. Also, the amount of subsidence is usually very small and occurs in an area of ​​one to several kilometers. Such a large area with very little subsidence cannot be analyzed by methods such as geophysical surveys, seismic waves, electromagnetic waves, soil resistivity, and other methods. One promising technology is high-frequency radar. Remote sensing with high-frequency radar can provide the penetration depth and resolution required for accurate detection and identification of such facilities (Klar et al, 2009).
Materials and Methods
In order to investigate the issue of subsidence in the north of the city of Aq Qala in the northwest of Golestan province, remote sensing tools have been used in the first stage. The study area is located 27 kilometers from the city of Aq Qala and in the northwest of Golestan province. In this area, withdrawal from surface and deep aquifers has caused subsidence on the ground surface. Further, through investigations conducted on the wells in the study area, it was determined that the wells drilled by the Ministry of Energy did not extend to the bedrock and in two cases did not even reach the water surface, but the exploration wells for iodine mines and wells drilled by the oil company did extend to the bedrock. By looking at the geological columns prepared in the exploration wells, in other words, the well logging conducted in the area, it is clear that from the south to the north of the studied area, the depth of the bedrock decreases and the water table rises. Also, from the south to the north in the direction of drilling the exploration wells, the thickness of the layers decreases significantly.
Geologically, this area is located on the border between the three tectonic zones of the southern Caspian basin, the southwestern part of the Kopeh Dagh folded belt, and the northern ridge of the Eastern Alborz. The conditions of the study area are influenced by the tectonic history and stratigraphy of these three tectonic zones, and due to the alluvial covers and insignificant outcrops of formations in the study area, information from the exploration wells of the Oil Company and the iodine exploration wells was used.
In this study, in addition to monitoring the subsidence phenomenon using radar interferometry, a numerical simulation method with Plaxis 3D software was used to better understand the subsidence problem and the mechanism of related deformations. Plaxis 3D is a 3D finite element program that was specifically designed to investigate and calculate the settlement of offshore foundations, but in version 1.6, with the addition of consolidation settlement, it gained the ability to investigate settlements resulting from water withdrawal and groundwater level reduction. This program receives simple inputs from the user, combines simple graphics, and automatically creates complex finite element models with advanced output and high flexibility.
Results and Discussion
For modeling the settlement behavior, the Mohr-Coulomb behavioral model has been used according to the geological conditions of the rock layers and the results of rock mechanics tests. The Mohr-Coulomb elasto-plastic model requires five input parameters, namely Young's modulus, Poisson's ratio for soil elasticity, internal shear angle, cohesion for soil ductility, and as the expansion angle. The Mohr-Coulomb model represents an approximate first-order equation of soil or rock behavior. It is recommended to use this model for initial analysis of soil behavior and compare it with other models. For each layer, an average stiffness estimate is constant, and given this constant stiffness, an initial estimate of deformation is obtained with relatively fast calculations. In addition to the model parameters mentioned above, initial soil conditions, such as preconsolidation, play an important role in many soil deformation problems. This method is the simplest method for calculating the consolidated settlement of soil with the most basic available data, which has acceptable accuracy.
In the present study, the extent of land subsidence due to the extraction of deep brines for the production of iodine in the northeast of Aq Qala city - northern Golestan province was investigated by combining radar interferometry and numerical modeling methods. Numerical simulation was performed using Plaxis 3D software and the possible subsidence values ​​were predicted if deep brine extraction continued in the study area. Then, subsidence zoning maps were prepared for the study area using radar images and the actual subsidence values ​​were calculated.
Comparing the results of the simulation with radar zoning maps was able to clearly show the conceptual relationship between subsidence and brine exploitation in a quantitative manner in the study area. Using geological data of the region and Mohr-Coulomb numerical simulation, the range of subsidence changes in the study area is between 0 and 9 centimeters. The above numbers show acceptable agreement with the settlement values ​​obtained from radar images.
Using subsurface geological information, a geometric model of the structure of the formations in the region was formed, and the mechanical and behavioral characteristics of the formations were defined based on the Moore-Coulomb model, and other model requirements were also considered based on the daily pumping rates from the well and the groundwater level. In this model, the settlement of the formations due to brine withdrawal in a one-year period was calculated to be 9 cm, and in order to compare the model results with the actual settlement values, the radar interferometry technique was 
used and subsidence zoning maps were prepared. The settlement value in the exploitation area was calculated to be a maximum of 0.135 m, equivalent to 13.5 cm, which is slightly different from the settlement value obtained in the numerical model. Also, in the case of the settlements observed in the study area, the changes in the settlement value are a function of the withdrawal and recharge of shallow aquifers in water years, and for this reason, in the subsidence zoning maps, we see different values ​​in terms of space and time for the settlements calculated in different water years.
Conclusion
Periodic subsidence monitoring in the study area shows that geological and lithological factors of the formations play a decisive role in the rate of subsidence and surface deformations, and accurate assessment of subsidence is highly sensitive to the geomechanical parameters of the formations.
The calculated values ​​for subsidence based on numerical modeling and remote sensing maps are relatively close to each other, indicating that the mechanical parameters and behavioral model of the formations are close to reality.
Given the continued exploitation in this area and the expansion of exploitation in other areas of the plain and the irrenewability of the deep aquifer, the trend of piezometric level decline in these areas is ongoing and will intensify with increased exploitation.