Passive Seismology: Lightweight and Rapid Detection of Arctic Subsea and Sub‐Aquatic Permafrost.

Low sea levels during the last Ice Age exposed millions of square kilometers of Arctic shelves which have been subsequently submerged, creating subsea permafrost. In onshore settings, permafrost can also exist beneath water bodies such as coastal lagoons, rivers, and thermokarst lakes. We explored passive seismology as a method for mapping unfrozen sediment thickness above subsea and sub‐aquatic permafrost. We present passive seismic data collected with the Mobile Ocean Bottom Seismic Instrument (MOBSI) from the Beaufort Sea near Tuktoyaktuk in Canada, Ivashkina Lagoon on the Bykovsky Peninsula, as well as a lake and river in the Lena Delta, Siberia, Russia. We use borehole data and frost probe measurements to identify permafrost‐related H/V measurement peaks and calibrate shear wave velocities for frequency‐to‐depth conversion. We employ the shortest path and maximum signal amplitude to connect peaks and generate geological profiles. The MOBSI detected the ice‐bonded permafrost table beneath the Beaufort Sea, as well as beneath a Siberian lake and lagoon. At Tuktoyaktuk, an ocean bottom seismometer revealed a 5% scatter about the peak frequency for three‐minute time windows and over 8 hr of recording time. With peak frequencies ranging from 4.9 ± 0.2 Hz to 27.6 ± 1.4 Hz, the depth to subsea permafrost ranged from 1.4 ± 0.1 m bsl at the shoreline to 14.0 ± 0.4 m bsl 240 m offshore. Given an accurate shear wave velocity, our findings highlight that MOBSI deployment times as short as 3 min are adequate for detecting Arctic subsea and sub‐aquatic permafrost. Plain Language Summary: During the Ice Age, low sea levels exposed large areas of the Arctic, creating perennially frozen ground known as permafrost. When sea levels rose, this permafrost became submerged. Permafrost can also exist under bodies of water like coastal lagoons, rivers, and lakes. However, we have limited direct measurements of its characteristics and how it is changing over time. To address this, we used a technique called passive seismology to map the thickness of unfrozen sediment above subsea and sub‐aquatic permafrost. We collected passive seismic data using the Mobile Ocean Bottom Seismic Instrument (MOBSI) in the Beaufort Sea near Tuktoyaktuk in Canada, Ivashkina Lagoon on the Bykovsky Peninsula, and a lake and river in the Lena Delta, Siberia, Russia. This method involves recording ambient seismic noise to identify differences in the speed of shear waves in the subsurface. These boundaries with contrasting velocities are associated with geological interfaces like the top of permafrost. We used data from boreholes and frost probe measurements to calibrate and verify our passive seismic results. Key Points: We present passive seismic data from Arctic lake, river, lagoon, and subsea permafrost settingsWe applied the maximum H/V amplitude and shortest path to connect H/V peaks and interpret subsurface shear wave velocity contrastsPassive seismic H/V ratio measurements of just a few minutes of recording time can be used to investigate the depth of submerged permafrost [ABSTRACT FROM AUTHOR]