ИСТИНА

More than a fourth of the global land area is covered by permafrost, where ground temperatures remain below 0°C for at least two consecutive years. Any infrastructure on this frozen ground is associated with the risk of thaw subsidence due to the melting of ice bodies (so called ground ice). Therefore it is necessary to know exactly the position of the ground ice within the subsurface. Non-destructive geophysical methods are widely used in permafrost environments because frozen ground is very sensitive to mechanical and thermal forces. Ground-penetrating radar (GPR) is a geophysical method, which uses reflected radio waves to probe the ground. Ground-penetrating radar measurements consist of a transmitter and a receiver in a fixed geometry, which are moved over the surface to detect reflections from subsurface features. The investigated medium can be represented as a layered model with constant electrical properties within a layer, and ice wedges – as the local objects within layers, with different properties. If radar waves meet a boundary between two materials with a different permittivity a part of the energy is reflected. In permafrost areas the efficiency of ground-penetrating radar is based on the strong dielectric contrast between liquid water in unfrozen sediments and ice, that's why it is usually used here to estimate the thickness of active layers – top part of soil that thaws during the summer and freezes again during the autumn. However, when investigating specific types of ground ice, ground penetrating radar data typically show complex reflection patterns. To accurately image such complex subsurface structures sophisticated procedures of processing and interpretation are required. A prominent type of ground ice are ice wedges, which exhibit a characteristic wedge-shaped geometrical structure and form in successive thaw- and freeze cycles during several years. Any construction in the permafrost zone is difficult and expensive because the risk of ice wedge thawing. The incorrect determination of the position and size of ice-wedges can lead to disaster. The specific polygonal grid on the ground indicates the frost cracking and ice wedge forming. The areas with clearly visible polygonal pattern ground are usually noted on satellite images, but sometimes there are relict deep buried ice wedges without visible polygonal grid. In taiga zone they can be covered by conflagrations and not clearly marked in relief - there aren’t any indicators of ice wedges on the surface. At the same time on such sites there are ice wedges visible in the river outcrops. Identification of such “hidden” ice wedges especially close to existing or prospective construction areas is an extremely important problem. During the work on my PhD project I examined the potential of imaging size and geometry of ice bodies on GPR data. I focused on numerical and physical modeling to solve a direct problem, then I used various methods of analysis to describe how the ice wedges characteristics (such as depth, size and shape) are displayed on the ground-penetrating radar profiles. To verify my assumptions on the real data I took part in several expeditions, where I studied different permafrost types with GPR – Yamal Peninsula (nearshore permafrost), Finnish Lapland (discontinuous permafrost on peatlands) and Eastern Siberia (thick continental permafrost). The research efforts led to the development of a methodology for estimation with certain limitations the size and shape of ice bodies within permafrost using ground-penetrating radar.