Airborne Electromagnetic Surveys for Baseline Permafrost Mapping and Potential Long-Term Monitoring

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, ecosystems, and contaminant transport. There is typically little known at watershed or regional scales about the three-dimensional distribution of permafrost, including its thickness and the distribution of taliks (unfrozen zones), and other permafrost features thereby impeding the assessment of consequences of permafrost degradation. Airborne remote sensing methods for mapping permafrost are attractive, particularly in arctic and subarctic studies where ground access is difficult and ecosystems are fragile. As part of its Climate Effect Network (CEN) research and observation effort in the Yukon River Basin, the U.S. Geological Survey (USGS) has initiated an effort to map permafrost using airborne geophysics to complement hydrologic and biogeochemical studies in the study area. Interpretation of airborne geophysical data will be integrated with other remotely sensed data to supply critical hydrogeologic information needed for refining groundwater flow models in the Yukon Flats Basin. Airborne surveys also provide baseline data for estimating 3D permafrost distribution that can be compared to future permafrost surveys to estimate a volumetric change over time. In June 2010, the USGS conducted a helicopter frequency domain electromagnetic (HFEM) survey in the area of Fort Yukon, Alaska to map permafrost distribution. Flight line data processing has been completed that includes data leveling and a simple transformation to resistivity-depth along the flight lines. Preliminary resistivity-depth images from the survey can be qualitatively compared with known permafrost features and used to establish new permafrost features. Electrical properties of earth materials are impacted by temperature and the presence of ice causing them to become substantially more resistive when frozen. The area of the loess hills on the margins of the Yukon River has very high electrical resistivity. Since loess normally has low resistivity, the results suggest that the loess hills contain a relatively large proportion of frozen water, which is consistent with observations. In the area of Fort Yukon, the HFEM survey shows high resistivities extending to depth, likely indicative of thick permafrost; this corresponds well to observations from a borehole drilled in the area in the late 1990s that detected permafrost to a depth of about 100 m. In contrast, the Yukon River and its floodplain are not associated with deep resistive sediments suggesting a lack of deep permafrost, at least within the depth range of the HFEM mapping. Further refinement of depth images from flight line data, integrated interpretation and ground truth is required to understand the signatures from the HFEM survey and implications for permafrost studies. A future use of this data will be a comparison to other remotely sensed information on land cover and ecosystem performance in attempts to establish a link between datasets that can be extrapolated to other arctic and subarctic areas.