Permafrost Extent on the Alaskan Beaufort Shelf From Surface‐Towed Controlled‐Source Electromagnetic Surveys

We have developed a surface‐towed electric dipole‐dipole system capable of operating in shallow water and deployable from small vessels. Our system uses electromagnetic energy from a modulated manmade source to interrogate the underlying resistivity structure of the seafloor. We used this system in the summers of 2014 and 2015 to map subsea ice‐bearing permafrost on the Beaufort shelf along 200 km of coastline, from Tigvariak Island to Harrison Bay. Permafrost is resistive and was found to be anisotropic, likely due to interbedded layers of frozen and unfrozen sediment. Maps of depth to permafrost and its thickness were produced from electrical resistivity inversions and results compared to borehole logs in the area. We observed elevated resistivity values offshore the Sagavanirktok River outflow, supporting the idea that fresh groundwater flow has a preserving effect on submerged permafrost. This system provides a cost effective method that could be used to further quantify permafrost extent, provide a baseline for measurements of future degradation, and provide observational constraints to aid in permafrost modeling studies. Key Points Maps of depth to top of subsea permafrost, its thickness, and its peak resistivity are produced and compared to offshore borehole data Ice‐rich permafrost appears to be anisotropic, likely due to interbedded layers of frozen and unfrozen sediment Permafrost extends the gas hydrate stability zone to between 500 and 1,200 m below the seafloor