Study On The Deformation Characteristics Of Permafrost And The Identification Of Potential Thaw Slumps Based On InSAR Technology

China ranks third in the world in the permafrost area,and the area of permafrost accounts for about 22% of the total area of China.China's permafrost is mainly distributed in the Qinghai-Tibet Plateau,the western mountainous area and the Northeast area.The Qinghai-Tibet Plateau is the world's largest area and highest altitude permafrost area in the middle and low latitudes.Permafrost has brought great challenges to resource development and engineering construction.At the same time,the degradation of permafrost caused by global warming has also damaged the local ecological environment,causing a series of thermal thawing disasters such as slumping.Traditional data collection in permafrost area and thawing slump surveys are carried out by deploying sites in the field and manual surveys,which is costly and restrictive.In addition,the harsh environment of permafrost makes it difficult to carry out data collection on a large scale,but on key areas.In addition,the sampling data by traditional methods only reflects the situation of a single point or a small area,and cannot reflect the spatial distribution characteristics of permafrost from a macro perspective.In recent years,remote sensing has gradually become one of the mainstream methods for permafrost surveys.Remote sensing images and UAV images have achieved great results in the recognition of thaw thump.However,visible spectral remote sensing and UAV can only identify large-scale thaw slumps that have obvious morphological characteristics,and cannot do anything about potential thaw slumps that are in the early stages of development.Interferometric Synthetic Aperture Radar(InSAR)technology has made great progress in the field of surface deformation monitoring.InSAR technology has the advantages of large-scale,all-weather,high-precision,etc..The melting of active layer in summer and freezing in winter will cause slight deformation of the surface due to the density difference in water and ice,which provides conditions for InSAR technology to monitor the small surface deformations in permafrost from a macro perspective.The deformation characteristics of thaw slump also provide conditions for the identification of potential thaw slumps based on InSAR deformation results.This paper uses InSAR technology to carry out the monitoring of surface deformation of permafrost in the upper reaches of the Heihe River in Qilian Mountains from March 2017 to June 2020.Under the condition that the atmospheric correction effect and result accuracy of InSAR are very ideal,we analyzed the distribution law of surface deformation of permafrost under different environments from a macro perspective.On the basis of summarizing the development mechanism and deformation characteristics of thaw slumps,the potential thaw slumps are identified based on InSAR surface deformation.The main results and conclusions are as follows:(1)The atmospheric correction effect and result accuracy of InSAR technology meet the needs of large-scale surveys of the surface deformation in permafrost.The vertical stratification effect of the atmosphere caused by the huge elevation difference in the Qilian Mountains leads to a linear correlation between the interferometric phase and the elevation.Before the atmospheric correction by the InSAR spatio-temporal filter,the regression coefficient of the interferometric phase and the elevation is about-0.007rad/m.After the atmospheric correction by the InSAR spatio-temporal filter,the corrected interferometric phase is not related to the elevation,and the regression coefficient of atmospheric phase separated by the spatio-temporal filter and elevation is about-0.006rad/m.The coherence coefficient of most PS points extracted by InSAR technology is greater than 0.7,which means that the deformation accuracy of PS points is better than 4mm according to the deformation model.At the same time,the regression coefficient of the deformation rate of the homonymous PS point obtained by ascending and descending InSAR is about 0.91,and the correlation coefficient is about 1 when the significance is less than 0.01,indicating that the results obtained by ascending and descending InSAR under different observation geometries have high consistency.All above proved the reliability of the surface deformation data obtained by InSAR technology indirectly.(2)The permafrost of the Heihe River in Qilian Mountains is relatively stable overall,and the deformation rate of most areas is between-2?2mm/a.The areas with larger deformations are mainly distributed along the mountains,and the absolute value of the deformation rate is between 8?20mm/a.The process of freezing and thawing in active layer and the hydraulic erosion are the two major driving factors for the surface deformation in the study area.The surface deformation caused by the freezing and thawing process is cyclical,and the deformation amplitude of the typical area is about10 mm.The deformation caused by thawing in summer and the deformation caused by freezing in winter are basically balanced in one cycle.However,the degradation of permafrost has led to a smaller-scale subsidence trend on the surface,and the cumulative subsidence in 3 years is approximately 2mm.The erosion of glaciers or meltwater leads to severe surface deformation.The surface deformation curve is stepped.The surface deformation mainly occurs in summer but is basically stagnant in winter,which means an obvious seasonal characteristics.The subsidence in summer can reach 10 mm each year.(3)Through the three-step rules of InSAR deformation,the terrain and vegetation conditions,and final affirmance by remote sensing image visual interpretation,34 potential thaw slumps in the upper reaches of the Heihe River in Qilian Mountains were identified.The large-scale thaw slumps proved the reliability of the results.The deformation curve of thaw slumps can reflect the development of thermal thaw slumps and the ablation of underground ice.Thaw slumps with periodic suface deformation curve is certainly influenced by the freezing and thawing process of underground ice,which means that these thaw slumps may develop further,and the scale may continue to expand.Thaw slumps with linear suface deformation curve is sliding down only under the action of gravity,the underground ice degraded seriously,which means that these thaw slumps' development has stopped,and their scale may no longer expand.The potential thaw slumps have the characteristics of large local deformation rate and stable surroundings,and the surface shows periodic sinking,which are very likely to develop into thaw slumps.