A synthetic study of acoustic full waveform inversion to improve seismic modelling of firn

Pearce, Emma; Booth, Adam; Rost, Sebastian; Sava, Paul; Konuk, Tugrul; Brisbourne, Alex; Hubbard, Bryn; Jones, Ian F.

doi:10.1017/aog.2023.10

Cited by 3 publications

(1 citation statement)

References 38 publications

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“…10a), FWI is not sensitive to the velocity anomaly and the NRMS error is similar in both cases. Comparison of similar tests between HWI and FWI by Pearce and others (2023) highlights how the HWI is also not sensitive to velocity anomalies at the very near surface. For thicker ice slabs, particularly >2λ (40 m) (Fig.…”

Section: Thickness Variationsmentioning

confidence: 89%

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

et al. 2023

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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.

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Section: Thickness Variationsmentioning

confidence: 89%

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

et al. 2023

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Measuring seismic attenuation in polar firn: method and application to Korff Ice Rise, West Antarctica

Agnew,

Clark,

Booth

et al. 2023

J. Glaciol.

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We present seismic measurements of the firn column at Korff Ice Rise, West Antarctica, including measurements of compressional-wave velocity and attenuation. We describe a modified spectral-ratio method of measuring the seismic quality factor (Q) based on analysis of diving waves, which, combined with a stochastic method of error propagation, enables us to characterise the attenuative structure of firn in greater detail than has previously been possible. Q increases from 56 ± 23 in the uppermost 12 m to 570 ± 450 between 55 and 77 m depth. We corroborate our method with consistent measurements obtained via primary reflection, multiple, source ghost, and critically refracted waves. Using the primary reflection and its ghost, we find Q = 53 ± 20 in the uppermost 20 m of firn. From the critical refraction, we find Q = 640 ± 400 at 90 m depth. Our method aids the understanding of the seismic structure of firn and benefits characterisation of deeper glaciological targets, providing an alternative means of correcting seismic reflection amplitudes in cases where conventional methods of Q correction may be impossible.

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Firn seismic anisotropy in the Northeast Greenland Ice Stream from ambient-noise surface waves

Pearce,

Zigone,

Hofstede

et al. 2024

The Cryosphere

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Abstract. We analyse ambient-noise seismic data from 23 three-component seismic nodes to study firn velocity structure and seismic anisotropy near the EastGRIP camp along the Northeast Greenland Ice Stream (NEGIS). Using nine-component correlation tensors, we derive dispersion curves of Rayleigh and Love wave group velocities from 3 to 40 Hz. These velocity distributions exhibit anisotropy along and across the flow. To assess these variations, we invert dispersion curves for shear wave velocities (Vsh and Vsv) in the top 150 m of the NEGIS using a Markov chain Monte Carlo approach. The reconstructed 1-D shear velocity model reveals radial anisotropy in the firn, with Vsh 12 %–15 % greater than Vsv, peaking at the critical density (550 kg m−3). We combine density data from firn cores drilled in 2016 and 2018 to create a new density parameterisation for the NEGIS, serving as a reference for our results. We link seismic anisotropy in the NEGIS to effective and intrinsic causes. Seasonal densification, wind crusts, and melt layers induce effective anisotropy, leading to faster Vsh waves. Changes in firn recrystallisation cause intrinsic anisotropy, altering the Vsv / Vsh ratio. We observe a shallower firn–ice transition across the flow (≈ 50 m) compared with along the flow (≈ 60 m), suggesting increased firn compaction due to the predominant wind direction and increased deformation towards the shear margin. We demonstrate that short-duration (9 d minimum), passive, seismic deployments and noise-based analysis can determine seismic anisotropy in firn, and we reveal 2-D firn structure and variability.

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A synthetic study of acoustic full waveform inversion to improve seismic modelling of firn

Cited by 3 publications

References 38 publications

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

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

Measuring seismic attenuation in polar firn: method and application to Korff Ice Rise, West Antarctica

Firn seismic anisotropy in the Northeast Greenland Ice Stream from ambient-noise surface waves

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