| Summary: | The firn layer covers 98% of Antarctica's ice sheets, protecting underlying glacial ice from the external environment. Accurate measurement of firn properties is essential for assessing cryosphere mass balance and climate change impacts. Characterizing firn structure through core sampling is expensive and logistically challenging. Seismic surveys, which translate seismic velocities into firn densities, offer an efficient alternative. This study employs Distributed Acoustic Sensing technology to transform an existing fiber-optic cable near the South Pole into a multichannel, low-maintenance, continuously interrogated seismic array. The data resolve 16 seismic wave propagation modes at frequencies up to 100 Hz that constrain P and S wave velocities as functions of depth. Using co-located geophones for ambient noise interferometry, we resolve very weak radial anisotropy. Leveraging nearby SPICEcore firn density data, we find prior empirical density-velocity relationships underestimate firn air content by over 15%. We present a new empirical relationship for the South Pole region. © 2024. The Author(s). This is an open access article under the terms of theCreative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. We would like to thank Sheryl Seagraves and Hans Suedhoff of United States Antarctic Program (USAP) for their assistance during the field work. We thank the EarthScope Instrument Center for providing the Propelled Energy Generator for active surveys. We thank Jonathan Kingslake, Benjamin Hills, Zhengbo Li, and Donald Blankenship for insightful discussions. We thank Dr. Emma Pearce and an anonymous reviewer for their constructive suggestions. ZZ was supported by NSF Award 2022920, NSF CAREER Award 1848166, and the Moore Foundation. The representative active source shots data that are used to generate multimode dispersion and the cross ...
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