Characterising ice slabs in firn using seismic full waveform inversion, a sensitivity study

• Published in: Journal of Glaciology
• Main Authors: Emma Pearce, Adam D. Booth, Sebastian Rost, Paul Sava, Tuğrul Konuk, Alex Brisbourne, Bryn Hubbard, Ian Jones
• Format: Article in Journal/Newspaper
• Language: English
• Published: Cambridge University Press 2023
• Subjects: Glacier geophysics; ice thickness measurements; polar firn; seismics; seismology; Environmental sciences; GE1-350; Meteorology. Climatology; QC851-999; Ice Shelf; Journal of Glaciology
• Online Access: https://doi.org/10.1017/jog.2023.30; https://doaj.org/article/3d33748a7d6d42c7a0a45387f71677bb

The density structure of firn has implications for hydrological and climate modelling, and ice-shelf stability. The structure of firn can be evaluated from depth models of seismic velocity, widely obtained with Herglotz–Wiechert inversion (HWI), an approach that considers the slowness of refracted seismic arrivals. However, HWI is strictly appropriate only for steady-state firn profiles and the inversion accuracy can be compromised where firn contains ice layers. In these cases, full waveform inversion (FWI) may yield more success than HWI. FWI extends HWI capabilities by considering the full seismic waveform and incorporates reflected arrivals. Using synthetic firn density profiles, assuming both steady- and non-steady-state accumulation, we show that FWI outperforms HWI for detecting ice slab boundaries (5–80 m thick, 5–80 m deep) and velocity anomalies within firn. FWI can detect slabs thicker than one wavelength (here, 20 m, assuming a maximum frequency of 60 Hz) but requires the starting velocity model to be accurate to ±2.5%. We recommend for field practice that the shallowest layers of velocity models are constrained with ground-truth data. Nonetheless, FWI shows advantages over established methods, and should be considered when the characterisation of firn ice slabs is the goal of the seismic survey.