Active Microwave Remote Sensing Monitoring Of Permafrost Surface Deformation And Active Layer Thickness Along The Qinghai-Tibet Engineering Corridor

As the largest elevational permafrost region in the middle and low latitudes,the Qinghai-Tibet Plateau is known as the"Third Pole"and one of the"Asian Water Towers".It is an important ecological security barrier,resource reserve,and alpine biological germplasm resources bank in China,and has been widely concerned by scientists.In order to improve the living standards of people in permafrost regions,a series of engineering facilities have been built on the Qinghai-Tibet Plateau,such as Qinghai-Tibet Railway,Qinghai-Tibet Highway,Qinghai-Tibet Powerlines,and Golmud-Lhasa Products Oil Pipelines,among others.In recent years,climate warming has led to rapid degradation of permafrost on the Qinghai-Tibet Plateau,such as manifested in surface subsidence,thickening of the active layer,and development of retrogressive thaw slumps,among others.In the mean time,the construction of engineering facilities will also disturb the hydrothermal stability of the underlying permafrost and active layer,thereby exacerbating its degradation.However,the degradation of permafrost may threaten the stability and safety of engineering facilities.At present,although active microwave remote sensing has been widely used to monitor the degradation of permafrost on the Qinghai-Tibet Plateau,it has relatively little application in quantitative research on the impact of engineering construction and operation.Therefore,it is urgent to carry out high-temporal-spatial resolution,high-precision,long-term,and automated monitoring of permafrost along the Qinghai-Tibet Engineering Corridors(QTEC),in order to provide key basic data and technical support for the safe operation and maintenance of engineering facilities.In this paper,we use Interferometric Synthetic Aperture Radar(In SAR)and Ground Penetrating Radar(GPR)technologies to carry out research on permafrost monitoring along the QTEC.The main research results include the following aspects:The average interannual deformation rate in Fenghuoshan Mountains region from 2004 to 2010 was-3.5±5.1 mm/year,and the average seasonal deformation amplitude was 9.22±6.48 mm.The average deformation rate for the hinterland of the QTP along the QTEC(HQQ)from 2014 to 2023 is-6.92±5.16 mm/year,and the average seasonal deformation amplitude is 12.15±8.09 mm.About 53%of the HQQ showed significant subsidence(<-5 mm/year),and long-term subsidence would increase the water content atop of the permafrost table due to the melting of subsurface ice.Both deformation rate and amplitude showed strong correlations with altitude,slope,slope angle,surface thawing index and precipitation.In the permafrost section along the Qinghai-Tibet Engineering Corridor,a total of 57 GPR profiling data were collected during 2018～2021,with a cumulative length of 58.5 km,achieving a refined field investigation of the active layer thickness(ALT)along the QTEC.The results show that the ALT along the engineering route ranges from 1.25 to 6.7 m,with an average at ALT of 2.49±0.57 m,and the standard deviation range of ALT estimated by each GPR data is 0.07 to 1.2 m.Combining the GPR data and In SAR data,a high-resolution(30 m)ALT dataset for the HQQ was estimated by the Cat Boost model(R~2=0.86,RMSE=0.22 m).The results show that the ALT in the HQQ ranges from 1.50 to 5.52 m,with an average of 2.4±0.28 m.The combined results indicate that ALT in the HQQ is strongly heterogeneous at both large and fine scales.The large-scale deformation results along the QTEC show that the operation of the Qinghai-Tibet Railway mainly affects the changes of permafrost in the 1300 m buffer zone on the east and west sides,while the Qinghai-Tibet Highway affects the1100 m and 1400 m buffer zones on the east and west sides,respectively.Due to the difference in the solar radiation of the roadbed on the east and west slopes,the settlement on the east slope is greater than that on the west slope.The transmission tower of Qinghai-Tibet Powerlines mainly affects the 300 m buffer zone,which can reduce the temperature of the roadbed and surrounding permafrost to a certain extent.By removing the engineering impact area to establish the corresponding natural deformation benchmark,it was found that the operation of the Qinghai-Tibet Railway increased the settlement relative to the natural deformation by 1.1%.The Qinghai-Tibet Highway has reached a dynamic equilibrium with the natural deformation,and the transmission towers of the Qinghai-Tibet Powerlines can effectively decrease the settlement by 5.1%.The results of the long-term(2004～2023)deformation in the Fenghuoshan area show that the QTEC has significant subsidence in the Beiluhe River-Fenghuoshan Mountains-Tuotuohe River section.The settlement of different engineering in different periods is inconsistent,which is mainly related to the amount of protection measures,construction time sequence,and human activities disturbance.Comparing the natural deformation benchmarks in different periods,it is found that the Qinghai-Tibet Railway disturbed the underlying permafrost during the construction period(2004～2006).As an important medium tool for the construction of the Qinghai-Tibet Railway,frequent material transportation and human activities of the QTH lead to aggravated subsidence.During the initial operation period(2006～2010)of the Qinghai-Tibet Railway,the original hydrothermal balance of the underlying permafrost was disturbed,but during the relatively stable operation period(2014～2023),active cooling measures have played an effective protective role to a certain extent.Due to the later construction of the Qinghai-Tibet Poerlines,rich construction experience and relatively small human disturbances resulted in relatively small overall settlements.Evaluating the effectiveness of thermosyphon,hollow block revetment roadbeds,and highway land bridge on the Qinghai-Tibet Highway and thermosyphon,ventilated pipes,sand control measures,and railway land bridge on the Qinghai-Tibet Railway.The results show that the engineering protection measures basically achieve the initial design effect,with the thermosyphon roadbed effectively reducing the settlement by10±22%compared to ordinary roadbed,and the land bridges reducing the settlement by 12±36%.Overall,83.6%of the Qinghai-Tibet Railway can effectively reduce the settlement caused by the engineering operation,and even significantly lower the temperature of the underlying permafrost.87.6%of the sections of the Qinghai-Tibet Highway have reached a balanced state;50.9%～61.9%of the transmission towers of the Qinghai-Tibet Poerlines can significantly reduce the permafrost temperature of the overlying permafrost at the tower foundation.This study revealed the spatial distribution characteristics of ALT along the QTEC based on active microwave remote sensing data,and successfully estimates the high-resolution(30m)ALT data in the HQQ.Through long-term and large-scale deformation data,the impacts of the facilities along the QTEC at different time periods were revealed,and the effectiveness of different protection measures was quantitatively evaluated,as well as the overall protection performance of engineering construction.These research results provide strong data support for engineering maintenance and protection,and provide important scientific information and technical reference for the comprehensive construction of Qinghai-Tibet Expressway.