Large Scale Subsidence Monitoring In Coal Mining Area Based On SAR/InSAR

The exploitation of coal resources has promoted the rapid growth of the national economy,which also caused more and more serious ecological and environmental problems.Among all disasters,the surface subsidence resulting from underground coal seam mining is thought the root.Therefore,the real-time monitoring of subsidence phenomenon has an important practical significance for ecological environment restoration and early warning of geological disasters.Synthetic aperture radar interferometry(InSAR),with millimeter monitoring precision,provides an effective way to realize the dynamic monitoring of surface deformation.However,the characteristics of mining subsidence,such as small deformation range,high velocity,large scale,nonlinearity and discontinuity,as well as the inherent imaging conditions of SAR,have severely restricted the application of InSAR related technologies in mining subsidence monitoring.In particular,the failures to extract large-scale and three-dimensional deformation information are regarded as the main bottlenecks.In view of the above limitations,this paper explores how to use SAR images to extract the surface deformation of mining area accurately and comprehensively from different perspectives,based on the research ideas of "theoretical analysis-model construction-simulation experiment-instance verification-analysis and evaluation".The key research contents and achievements in this thesis are as follows:(1)This paper summarizes the current research status and limitations of existing InSAR related technologies in mining subsidence monitoring.Meanwhile,the principles and features of SAR imaging,the theoretical basis of InSAR deformation detection and the main sources of errors are described.The main influencing factors of deformation monitoring with InSAR are analyzed from the aspects of coherence,deformation gradient and 3D deformation decomposition.Under the conditions of full mining,this paper established a theoretical function model between the critical vertical displacements of mine that can be monitored with two single looking complex images and the mining conditions,as well as the parameters of the SAR satellite system.(2)In view of the research results of multi-baseline InSAR for constructing high-precision DEM,a large-scale surface deformation extraction method based on DEM “construction-difference” and without being limited by detectable deformation gradient is formed.At the same time,this paper studies the accuracy and influencing factors of deformation monitoring of the proposed method and verifies the effectiveness of the method through concrete examples.By comparison with the subsidence prediction results,the accuracy of surface deformation detected by the proposed method is not superior to the subsidence prediction value when the geological conditions and mining working face information are perfect.However,for mines with incomplete mining information,the proposed method can obtain a complete subsidence basin.Especially for monitoring the long-term ground surface subsidence of the mining area,the proposed method is not limited by the conventional InSAR deformation detection gradient.In the end,some measures to improve the capability of deformation monitoring are given: choosing suitable SAR images for DEM extraction and improving the accuracy of DEM.(3)In this paper,a phase unwrapping method for high-density differential interference fringes in mining area is proposed.This method achieves the purpose of reducing the number of interference fringes by pre-removing the interfering phase of surface deformation with prior knowledge to realize the phase unwrapping and obtain the deformation of the ground surface.The simulation experiment results without noise conditions show that the maximum difference between the unwrapped phase and the true phase obtained by the proposed method is only 0.05 rad(corresponding to about 0.3 mm of surface deformation),which confirms its theoretical feasibility.At the same time,it is pointed out that the two main errors of proposed method are the resampling error of the predicted surface deformation phase and the phase unwrapping error of the residual deformation phase.On the other hand,the real SAR imaging experiment also proves that the proposed method can not only overcome the incapacity of the conventional phase unwrapping algorithm in rapidly deforming the surface of mining area,but also give full consideration to the mining subsidence mechanism.Compared with the simple predicted results,the surface deformation obtained by the proposed method can better represent the real spatial distribution of surface deformation.(4)In this paper,the sub-band InSAR is applied to monitor the mining subsidence and the deformation extraction process based on sub band interference is formed.Sub-band InSAR method can reduce phase unwrapping difficulty to achieve large-scale surface deformation extraction by increasing the analog wavelength and reducing the number of interference fringes.The simulation experiment under no-noise condition shows that this method can theoretically be applied to the monitoring of ground subsidence after the mining working face is fully exploited.Its deformation monitoring accuracy can reach 94.3% of that conventional InSAR,which can meet the requirements of mining subsidence monitoring.However,due to the limitation of the SAR original bandwidth,the simulated radar wavelength cannot be optimally selected according to the surface deformation,which makes the sub-band interference fringes hardly formed.For the high-coherence regions where the surface deformation exceeds one-fifth of the analog wavelength,the sub-band InSAR processing method gradually shows the superiority in large-scale deformation monitoring.It is considered that it is difficult to realize the dynamic monitoring of the subsidence in the mining area relying solely on simple sub-band InSAR processing for the currently operating SAR satellite system,which is mainly affected by the image bandwidth and the coherence.(5)In view of the fact that the conventional InSAR technology can only obtain the deformation along the radar line-of-sight,this paper explores the method of solving the 3D deformation of the ground surface with the joint subsidence prediction model and deduces the functional relationship between the surface vertical subsidence caused by underground mining,arbitrary horizontal displacement and LOS displacement theoretically.The 3D deformation fields of the three concrete working faces after being mined in Fengfeng Mining Area were obtained by using the proposed method,and the surface subsidence and incline caused by the mining of 152under31 S working face were verified.The results show that the maximum vertical displacement difference is 12 mm and the average difference is 3.2mm.The maximum difference between adjacent observation stations is 0.31mm/m,and the average difference is 0.10mm/m,which fully meets the requirement of mining subsidence monitoring.The horizontal displacements of the ground surface in the east-west and north-south direction are 106 mm and 73 mm,respectively.The spatial distributions of the horizontal displacements are in accordance with the basic law of mining subsidence.These prove the feasibility and effectiveness of the proposed method.(6)This paper explores the application of InSAR time series analysis method on assessment of mining-induced damage to buildings and takes the Qiannantai Village in Jiulong mine as a case study.The results show the evolution of the damaged area of the village.Benefit from the obvious backscattering characteristics of artificial buildings in SAR images,the influence of incoherence is small;on the other hand,the InSAR technology based on surface measurement has the ability of high-precision deformation monitoring,the boundary affected by mining in the construction area can be accurately delineate.Therefore,it is considered that the InSAR time series analysis method has unique advantages and can provide a more objective,accurate and comprehensive assessment result for the damage situation in the construction area,which will provide a new way for the technical appraisal of mining damage.