Permafrost and Ground Ice Occurrences at Oketo, Hokkaido.

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Geo-electric-resistivity survey was conducted at the site of permafrost existence on the slope near Lake Kanoko, Oketo, Hokkaido. At that site, ground ice among the sediments was exposed by the previous excavation. The resistivity survey aimed to extend the distribution area by detecting the discontinuity of ground resistivity values. As the results, the high resistivity area in the sediments coincided with the per-existed ground ice body. The long term meaurements in the bore hole which was made at that time of excavation of ground ice indicate the permafrost existence in the layers between 1meter and 5 meter deep. By recent trend of warm climate, this permafrost layers tend to thin.To compare permafrost existence at low altitude area such as Oketo with typical alpine permafrost at Mt. Daisetsu, intensive field surveys were conducted during research periods. The geo-electric-resistivity surveys were also made at the site where typical alpine permafrost existed. Correlation between permafrost thickness and geo-electric-resistivity profiles was made based on the results. The haved topography related to alpine permafrost occurrence named as palsa was observed at that site. The internal structure of palsa was obtained by means of bore hole test and vertical profiling of geo-resistivity survey.Bore hole sampling was made at the site of palsa and obtained samples were subjected to pollen analysis and carbon dating analysis. The results indicated that the sedimentation initiated to last glacial ages and warm interglacial period interval was observed among the layers.