Internal structure and trend of glacier change assessed by geophysical investigations

Ground-penetrating radar (GPR) is a useful tool for mapping the thickness, morphology and structure of alpine glaciers. Englacial information obtained during a field survey of the Koxkar Glacier in Tian Shan, China, in June 2008 was retrieved from GPR profile data. At least four specific types of structures can be identified from the GPR reflection image: (1) hyperbolae from point sources, (2) irregular linear distributed features, (3) a dense chaotic return zone above a clearly defined bed reflection and (4) linear reflection within the ice body. The interpretation of a typical GPR image was validated using waveform analysis and a finite-difference time-domain numerical model. Through a series of data analyses, these types of reflection characteristics were strongly related to englacial melting. Considering also the GPR result of the debris thickness distribution at the terminus, it is concluded that strong glacial ablation does not occur at the terminus but at a position higher than the terminus where there is a thin layer of debris and relatively high temperatures. Meanwhile, there are many lakes receiving mostly englacial discharge that greatly erode the ice body through melt-water discharge in some regions, and a thick layer of debris insulating the ice body at the terminus; these features may differentiate the retreat of the Koxkar Glacier from the general retreat of glaciers. In the future, with the ablation of glaciers, the central positions of intense ablation may be disconnected and there may be a dead zone at the terminus.