Ice shelf structure derived from dispersion curve analysis of ambient seismic noise, Ross Ice Shelf, Antarctica

Abstract

S U M M A R YAn L-configured, three-component short period seismic array was deployed on the Ross Ice Shelf, Antarctica during November 2014. Polarization analysis of ambient noise data from these stations shows linearly polarized waves for frequency bands between 0.2 and 2 Hz. A spectral peak at about 1.6 Hz is interpreted as the resonance frequency of the water column and is used to estimate the water layer thickness below the ice shelf. The frequency band from 4 to 18 Hz is dominated by Rayleigh and Love waves propagating from the north that, based on daily temporal variations, we conclude were generated by field camp activity. Frequency-slowness plots were calculated using beamforming. Resulting Love and Rayleigh wave dispersion curves were inverted for the shear wave velocity profile within the firn and ice to ∼150 m depth. The derived density profile allows estimation of the pore close-off depth and the firn-air content thickness. Separate inversions of Rayleigh and Love wave dispersion curves give different shear wave velocity profiles within the firn. We attribute this difference to an effective anisotropy due to fine layering. The layered structure of firn, ice, water and the seafloor results in a characteristic dispersion curve below 7 Hz. Forward modelling the observed Rayleigh wave dispersion curves using representative firn, ice, water and sediment structures indicates that Rayleigh waves are observed when wavelengths are long enough to span the distance from the ice shelf surface to the seafloor. The forward modelling shows that analysis of seismic data from an ice shelf provides the possibility of resolving ice shelf thickness, water column thickness and the physical properties of the ice shelf and underlying seafloor using passive-source seismic data.

Cryoseismology

“…The coherence condition is often met for small networks (less than 1 km aperture) on glacial ice at frequencies below 10–18 Hz. If surface phases dominate the wavefield, determining phase velocities at different frequencies yields a dispersion relationship, which can be used to estimate ice thicknesses and determine structure as it was done locally for the Greenland Ice Sheet [ Walter et al , ] and for the Ross Ice Shelf [ Diez et al , ]. Equivalently, MacAyeal et al [] used waveform coherence to identify seismic, hydroacoustic, and flexural gravity waves recorded on the top of a tabular iceberg in Antarctic.…”

“…, v -kinematic viscosity of water (1.787 × 10 −6 m 2 s −1 at 0 ∘ C), 0 -density of water (1000 kg m −3 ), Q iw is a quality factor, and f is a characteristic resonant frequency. tomography [Zhan et al, 2014;Diez et al, 2016]; and (iii) phase velocity measurements using naturally occurring glacier seismicity [Walter et al, 2015a];…”

Tsunami and infragravity waves impacting Antarctic ice shelves

“…Relatively high beam power in the 0.02-0.025 Hz band at low slowness in Figure 4b (this energy is below swell frequencies) indicates that some higher-frequency IG wave energy is converted into higher phase velocity extensional Lamb waves that propagate horizontally within the ice shelf. This energy has a slowness of about 0.35 s/km, corresponding to a phase velocity of about 2800 m/s, which is about 1.8 times the shear wave speed in ice [Diez et al, 2016]. Differences between Figures 4a and 4b at slowness <1 s/km reflect differences in incident gravity wave spectral energy distributions, with lower extensional Lamb-wave beam Journal of Geophysical Research: Oceans 10.1002/2017JC012913 power in Figure 4a consistent with lower IG band energy impacting the RIS during the tsunami than during the strong IG event, which has spectral levels near 0.02 Hz that are about 5 dB higher ( Figure 3).…”

“…Each seismic station was equipped with self‐leveling three‐component Nanometrics T120 PHQ seismometers and Quanterra Q330 digitizers. Preliminary results from 2 weeks of data collected in November 2014 from three stations (DR09, DR10, and DR13) suggested that IG waves generate water/ice coupled flexural‐gravity waves that propagate southward from the shelf front [ Bromirski et al ., ; Diez et al ., ]. However, three closely situated stations within 10 km of each other were insufficient to adequately describe the gravity wave‐induced signal propagation characteristics or their spatial variability.…”

Antarctic Ice Properties Revealed From Teleseismic P Wave Coda Autocorrelation

“…This technique was also used to constrain an internal layer (Wittlinger & Farra, 2012, 2015 or a sedimentary layer sandwiched between ice and crustal bedrock (Anandakrishnan & Winberry, 2004;Chaput et al, 2014). The second class of methods use the spectral ratio between vertical and horizontal wave forms to reveal resonance peaks of P or S waves reverberated within a thin layer of snow (e.g., Lévêque et al, 2010) or ice (e.g., Diez et al, 2016) or a water layer beneath an ice shelf (Zhan et al, 2014). Most recently, Yan et al (2018) presented a successful application of the horizontal-to-vertical (H/V) spectral ratio method to estimate the ice thickness beneath a large number of seismic stations distributed over Antarctica.…”