Research On Qinghai-Tibet Permafrost Environment And Engineering Using High Resolution SAR Images

The Qinghai-Tibet Plateau(QTP),the highest plateau in the world,has the largest extend of permafrost,which affects its surrounding environment and climate directly through atmosphere and hydrological process.And the global and local climate changes affect the stability of QTP permafrost.With global warming,the QTP is suffering from many serious permafrost degradation problems,such as the decrease of permafrost area,the increase of the active layer thickness(ALT)and average ground temperature.And those degradation problems would cause serious environmental problem and melt disasters such as soil moisture loss,land desertification,reduced vegetation coverage and engineering stability of permafrost,which would threaten the ecological environment and local people's life and property safety.Therefore,it is significant to study the permafrost environment with large area scale and high precision.In this dissertation,we use the high resolution SAR images to study QTP permafrost environment and engineering with the Beiluhe permafrost area as study site.Specifically,soil moisture,deformation of permafrost and engineering and the active layer thickness in QTP have been studied and analysed in details.The objective is to study QTP permafrost environment in a more detail way with high resolution SAR images,which could provide more new detailed understandings of the QTP for people and gives scientific basis for protecting of degradation and ecological environment.The following innovations are obtained in this dissertation:1.A time-series approach using time series radar images with different incidence angles is proposed to retrieve soil moisture at QTP with complex surface environment.First,the relationship of the soil moisture and radar backscattering signal has been analysed and a linear relationship has been found between them.Then,the sencitivity of the radar backscattering signal to soil roughness has been discussed.Generally,it is assume that in dry condition soil roughness occupy the major of the surface radar backscattering signal.In this thesis,the sensitivities of three terms ?low0,?low0/?high0 and exp(?low0/?high0)dB to soil roughness have analysed.The result shows that the term exp(?low0/?high0)dB is a good indicator of the surface roughness conditions,which is used as the component of soil roughness in radar signal.Combing the linear relationship between soil moisture and radar backscattering signal,the soil moisture retrieving model is proposed and applied in Beiluhe area.The soil moistures of the study area from 2014 to 2016 have been obtained using the proposed model.The results indicate that the soil moisture in Beiluhe area shows obvious seasonal variation,with the maximum soil moisture in summer season and minimum value in winter season.The retrieval result from the time-series method is validated against in situ measured soil moisture,with the RMSE and Bias of 4.2%and 4.05%,respectively.The proposed model can be used for large area soil moisture retrieving in QTP area.2.The deformation of permafrost and Qinghai-Tibeti railway engineering have been obtained and analyzed using DInSAR and time-series InSAR method with high resolution SAR images.In the DInSAR step,the structural features and the deformation of the QTR in high resolution SAR image are analysed.And a preliminary understanding of the deformation characteristics of permafrost in the study area has been obtained.In the time-series InSAR processing step,based on the process of the permafrost freezing and thawing,the deformation is assumed to be a sum of linear deformation component and seasonal deformation component.And the seasonal deformation model based on the simplified Stefan equation has been adopted to measure the seasonal displacement.The result indicates that most of the study area show obvious subsidence,with the mamximum subsidence velocity of-15mm/year.And most of the study area had seasonal displacements of up to 65mm.The deformation feature of Qinghai-Tibet railway,highway,the experimental embankment segment and the sand soil control-measurements have been analyzed and discussed.The deformation asymmetry between the raiway has been observed in the high resolution deformation map,which is is related to side slope thermal effect.The difference of the deformation rate betweent the two railyway embankment can be reaches to 10mm/year.In the experimental railway subgrade,displacements with varying measurements of cooling are different,which shows their dissimilar cooling characteristics.3.To decrease the errors from the assumption that the soil porosity is constant and the soil water content is saturated in retrieving ALT from InSAR technique,an ALT retrieving model have been proposed.This model has considered the soil porosity and soil moisture in different soil types.Considering the landform and soil types of the study area,the ground objects are classified into two classes using SAR amplitude map:alpine meadow and alpine desert.Then for those two ground feature types,the underground soil moisture models have been proposed based on the measured data.Combing the retrieved seasonal deformation from InSAR method,the active layer thickness retrieval model is further established.The experimental results indicate that this model could obtain the ALT,which is verified by field measured data.The ALT shows difference between the alpine meadow and alpine desert areas,with the mean ALT of 1.5m in alpine meadow area and 3m in alpine desert area.