Characterizing South Pole Firn Structure With Fiber Optic Sensing.

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. Plain Language Summary: Firn, the layer of compacted snow merging into glacial ice covering Antarctica, acts as an insulating blanket that mitigates environmental perturbations to the polar ice sheet. Understanding the density and seismic characteristics of the firn layer helps scientists better infer its properties and variation, including factors relevant to glacial stability and sea level change. Firn density is the major uncertainty source for measuring ice sheet mass changes via satellite and airborne sensing. Traditional methods of assessing firn density involve drilling or snow pit analyses and are expensive and time‐consuming. We utilize the rapidly developing technology of Distributed Acoustic Sensing to transform a data communication cable near the South Pole into a dense array of seismic sensors, allowing us to noninvasively estimate firn properties by studying seismic waves propagating in the firn to assess its physical properties. Our findings suggest that previous parameterizations overestimate firn density by over 5% and underestimate its air content by over 15% and highlight the value of seismology for advancing glaciological and polar region's climate research. Key Points: Distributed Acoustic Sensing repurposes an 8 km fiber‐optic cable at the South Pole into a dense seismic arrayGathered data resolve 16 dispersion modes at frequencies up to 100 Hz that constrain P‐ and S‐wave velocities in the firn layerPrevious density‐velocity empirical relations overestimate the dry firn density at South Pole [ABSTRACT FROM AUTHOR]