Characterization of seasonal variation of the active layer above permafrost with reverse time migration of GPR profiles near the Qinghai-Tibet Highway

The permafrost on the Qinghai-Tibet Plateau (QTP) is critically sensitive to temperature changes. The balance of heat transfer between the land and the atmosphere in permafrost regions were disturbed by the construction of man-made infrastructures, such as the Qinghai-Tibet Highway (QTH), the towers of power lines and telecommunications. Obviously, the characteristics of permafrost have been altered near the QTH. Reversely, thawing and degradation of the permafrost have a serious impact to the stability of QTH. Ground-penetrating radar (GPR) surveys can detect the subsurface structure of permafrost layer near the QTH that differentiate the situations in summer and winter and enable us to analyze the feature of permafrost table and active layer thickness (ALT) in different seasons. Consequently, it can provide the characterization of the relationship between the permafrost thawing and stability of QTH. Numerous studies have used the GPR to detect the permafrost and collected GPR data in summer in previous studies. However, due to the high loss of GPR signal in summer time caused by the blocking nature of the melted active layer limited the detection of formation features in greater depth. In this paper, we collected GPR data near the QTH in Beiluhe region in both summer and winter seasons and used the finite-difference reverse time migration (RTM) algorithm to process the GPR data. From the results of RTM profiles, we find that: 1) the RTM profiles in winter can provide high resolution interpretation image and clearly register the active layer/permafrost boundary and lithological interfaces within the active layer; 2) the top boundary of the permafrost and ALT have significant change near the QTH, which may be caused by the degradation of the surface vegetation and higher temperature on sides of QTH. With the accumulation of more GPR surveys, we can potentially analyze more details and wider coverage for the characteristics of permafrost top boundary and ALT in different seasons and provide more objective input to reduce the adverse impact from engineering constructions.