شبیه سازی عددی و پهنه بندی فرونشست زمین ناشی ازاستخراج محلول های معدنی در شمال غرب استان گلستان

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 A, et al. 2009).

Materials and Methods

In the present study, the issue of subsidence in the north of Aq Qala city in the northwest of Golestan province has been investigated using remote sensing tools. The study area is located 27 kilometers from Aq Qala city and in the northwest of Golestan province. In this area, withdrawal from surface and deep aquifers has caused subsidence on the ground surface. An examination of the drilled wells shows 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 of iodine mines and the oil company continued to the bedrock. A general look at the columns prepared in the exploration wells, in other words, the well logging carried out in the area, shows that from the south to the north of the study area, the depth of the bedrock decreases and the water table becomes higher, and also from the south to the north in the direction of drilling the exploration wells, the thickness of the layers decreases significantly.

This area is geologically located on the border between 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, the methods of subsidence analysis include radar interferometry and numerical modeling. Radar interferometry is a method for combining Sentinel 51 images taken from radar sensors mounted on aircraft in order to prepare elevation maps, displacements, and changes in the ground surface, as well as determine the speed of target movement. Two Sentinel 1 images are taken before and after the displacement of the ground surface. Any displacement in the ground surface causes a change in the sensor distance (Dong et al. 2018). Sentinel-1 imagery was developed by the European Space Agency and began imaging on 14 April 2013 and is expected to last 7 years. The satellite is a 693 km orbital satellite and images in the C-band at a wavelength of 5.55 mm and with a temporal resolution of 12 days. The most important product of Sentinel-1 is single-view data with a spatial resolution of 20 x 5 m (Rucci et al. 2012). In this study, in addition to monitoring the subsidence phenomenon using radar interferometry, a numerical simulation method using 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 takes simple inputs from the user, combines simple graphics, and automatically creates complex finite element models with advanced output and high flexibility.

Discussion and Results

For modeling the settlement behavior, the Mohr-Coulomb MC 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 E Young's modulus, ν Poisson's ratio for soil elasticity, ϕ internal shear angle, C cohesion for soil ductility, and ψ 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 soil consolidation settlement with the most basic available data, which has acceptable accuracy (Khosh Akhlagh, 2015).

In the present study, the amount 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 methods, radar interferometry and numerical modeling. Numerical simulation was performed using Plaxis 3D software and the possible subsidence values were predicted if extraction from deep brines 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 the radar zoning maps was able to clearly show the conceptual relationship between subsidence and the exploitation of brines quantitatively in the study area. Using geological data of the region and numerical simulation of Mohr-Coulomb, the range of subsidence changes in the study area is between 0 and 9 centimeters. The above numbers show acceptable agreement with the settlement value obtained by radar images.

After creating the shown geometric model, the mechanical and behavioral characteristics of the formations were defined based on the Mohr-Coulomb model and based on the daily pumping values from the well and the groundwater level, other requirements of the model were also considered and finally the maximum possible amount of subsidence due to extraction in a one-year period is predicted to be 9 centimeters. Based on the radar interferometry technique, the subsidence values in the study area have been calculated and the maximum subsidence value in the exploitation area has been calculated to be 0.135 meters, equivalent to 13.5 centimeters.

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 will continue and intensify with increased exploitation.