Электротомография чаши дренированного термокарстового озера на о. Курунгнах в дельте р. Лены

Evolution of permafrost under thermokarst lakes is an actual question in the light of such problems of cryolythic zone research as greenhouse gas emission, permafrost degradation and cryovolcanism. Recently drained thermokarst lake provide an opportunity to study under-lake permafrost state with ground geophysical methods. This lake located on Kurungnakh island (composed of Yedoma ice complex deposits) in the Lena delta was studied with electrical resistivity tomography. Local low-resistivity anomaly in the central part of the lake was found during previous geophysical research. Main goal of this work is detection of residual thermal effect from a frozen under-lake talik in an electric field. Satellite images of different years show that the drainage has taken place about 30 years ago. The area of the lake was covered in 2016 by high-resolution aerial imagery (5 cm/pixel), then digital elevation model was built using photogrammetry. These data were used for geomorphological description of the alas (depression in permafrost after lake drainage). The alas depth reaches 8 m, its size is about 300 × 500 m. It was formed probably on the last stage of Holocene thermokarst activity and it is relatively shallow in comparison to other typical alases on the island. A number of baijarachs (as a result of polygonal ice wedges thawing) were observed on the alas bottom. A line of the steepest slope marks a coast line of the lake, which allows to estimate a volume of water, which was contained in it earlier. Electrical resistivity tomography was implemented on 8 parallel profiles of 235 m. Measurements were conducted with dipole-dipole array. Basing on 3-dimensional inversion results a 3-dimensional resistivity model of under-lake deposits up to 40 m deep was made. Relatively low resistivity area (16–25 kOhm·m in comparison to 50–100 kOhm·m) was registered at the depth of 15–35 m. It is probably linked to a temperature anomaly (–3...–5 °С in comparison with –8.5 °С average value of the region). Therefore the under lake talik was fully frozen but the rest of temperature anomaly is still observable. 3-dimensional finite-element modeling of talik propagation (500 years) and refreezing (30 years) was done in axisymmetric setting taking phase transition into account. Temperature anomaly up to 0 °С in its center at the depth of 35 m was obtained as a result of the modeling. It qualitatively confirms the interpretation of electrical resistivity tomography data on the residual temperature anomaly below the basin of the drained lake.