oa Preliminary Results from an Airborne Electromagnetic Survey of Permafrost in the Area of fort Yukon, Ak

The U.S. Geological Survey undertook a helicopter frequency domain electromagnetic (HFEM) survey in July 2010 of the area of fort Yukon, Alaska for the purpose of mapping the distribution of permafrost. Concerns over the impacts of climate change have recently energized research on permafrost and the potential impacts that thawing permafrost may have on groundwater flow, infrastructure, forests, and contaminant transport. there is typically little knowledge known about the three-dimensional distribution of permafrost, including thickness and where openings or taliks occur. This study maps the distribution of permafrost in selected areas near fortYukon in order to develop hydrogeologic Information and Interpretation for development of a groundwater model in the YukonBasin. An additional benefit is the collection of an anchor dataset of 3D permafrost distribution which can be compared to future similar data collections to determine a volumetric change over time which in turn can be correlated to climate change.the HFEM system used six frequencies from 140,000 hertz to 400 hertz, providing data for imaging electrical properties at varying depths. Approximately 1800 line kilometers of data were acquired in typical block style near Ft.Yukon and in long reconnaissance lines along the Yukon River flood plain and the Porcupine River tributary. in preliminary processing, the airborne data were converted to depth sections of electrical resistivity using a numerical transform. Electrical properties of earth materials are impacted by temperature and the presence of ice causing them to become more resisitive as the water freezes. A simplified conceptual model of areas of discontinuous permafrost is that high resistive areas have permafrost and low resistive areas are permafrost free. Results from the survey show continuous areas of high electrical resistivity indicating the presence of permafrost intermixed with areas of low electrical resistivity under the Yukon River and some lakes indicating areas that are not frozen. the area of the loess hills on the margins of the Yukon River has very high electrical resistivity indicating that they contain a relatively large amount of frozen water. A drill hole in the Ft.Yukon area drilled in the late 1990s found the thickness of permafrost was about 100 m. the HEM survey shows discontinuous high resistivities that extend to this depth. Drill hole electrical logging showed that the basement rocks in this area are conductive shale. Some shallow areas of low resistivity correlate with gravel pits and may offer insight into temporal changes in the character of the permafrost. the flood plain of the Porcupine River appears more resistive that the Yukon River flood plain which can indicate more permafrost and resistive bedrock. Further Interpretation and ground truth is required to verify the interpreted distribution of permafrost. However, this is a first look into the distribution of the permafrost in the areas around fortYukon and is a demonstration of the application of HFEM to permafrost mapping as part of an integrated climate change study. Such surveys conducted in other parts of the Arctic region will enable comparisons and contrasts of permafrost distribution on a world wide scale.