|Title||Permafrost extent on the Alaskan Beaufort shelf from surface-towed controlled-source electromagnetic surveys|
|Publication Type||Journal Article|
|Year of Publication||2018|
|Authors||Sherman D., Constable S.C|
|Journal||Journal of Geophysical Research-Solid Earth|
|Type of Article||Article|
|Keywords||controlled-source electromagnetics; Geochemistry & Geophysics; hydrate; hydrate deposits; ice-bearing permafrost; inversion; sea; subsea permafrost; thickness; zones|
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 200km 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.
We have demonstrated that our surface‐towed CSEM system can produce maps of the extent of offshore permafrost, which are consistent with well logs. Our surveys were conducted for less than the cost of drilling one borehole, making the surface‐towed CSEM system a cost‐effective method for mapping IBPF in regions without well logs.
Further work could include returning to this location when the Beaufort Sea is ice‐free further offshore, which would allow towing across the edge of IBPF to capture the transition to unfrozen sediments. This will give us a baseline extent to monitor future degradation. Additional data collection very nearshore will be useful inputs to models of coastal erosion, where the state of nearshore subsea permafrost will help determine how quickly the coastline erodes.