Crystallographic preferred orientations (CPOs) are particularly important in controlling the mechanical properties of glacial shear margins. Logistical and safety considerations often make direct sampling of shear margins difficult, and geophysical measurements are commonly used to constrain the CPOs. We present here the first direct comparison of seismic and ultrasonic data with measured CPOs in a polar shear margin. The measured CPO from ice samples from a
Ice streams and glaciers are localised regions of high ice flow velocity inside otherwise mostly stationary ice masses of Antarctica and Greenland
The presence of a CPO results in anisotropic mechanical properties and so influences the viscous behaviour of ice significantly
A better understanding of CPO patterns in glacier shear margins is therefore highly desirable to accurately determine their mechanical properties. Ice core drilling, the primary direct information source for CPO, is however rarely performed on fast-flowing ice because of difficulties in access and on-site safety. Geophysical studies, e.g. seismic
A continuous ice core of
After completion of drilling and core retrieval, the open borehole was used to conduct a vertical-seismic-profile (VSP) experiment to constrain seismic properties of the near-surface glacier ice, with a particular focus on seismic anisotropy. To complete the link between seismic anisotropy of the ice volume around the borehole and CPO measurements from the core, multi-azimuthal ultrasonic velocity measurements
Seismic phase velocities for a single crystal dependent on angle
This is the first study of the horizontal-cluster CPO type observed in shear margins with seismic methods. Analyses of seismic, ultrasonic and measured CPO datasets are combined to assess the potential of active-source seismic surveys for the constraint of shear margin anisotropy.
A VSP dataset was recorded at the Priestley drill site using a three-component borehole seismometer (built by ESS Earth Sciences, Victoria, Australia) with a pneumatic clamping system which was installed at depths
Multi-azimuth VSP survey parameters.
Geode data recording for each shot in the field was initiated by a Geometrics switch trigger taped to the sledgehammer handle. Quality control of traces in the field found that this trigger type produces inconsistent zero times; i.e. repeat shots for a given source–receiver pair exhibit different arrival times. To enable the determination of absolute velocities, the recorded signal from surface geophones, collocated at the shotpoints, was used to define shot times. The Geode was set to record
Polarisation patterns indicate a ringing effect of the pneumatic borehole seismometer, where phase arrivals are followed by a tail of mono-frequent oscillations (see traces in Fig.
P- and S-wave first-arrival signals were recorded with a high signal-to-noise ratio, and phase arrival times can be clearly identified. Picking of seismic phase arrivals is performed manually for each shot to determine travel times: one first-arrival pick is made on the surface geophone trace at the shot location in addition to picks of P- and S-wave arrivals on the borehole seismometer traces. The P- and S-wave travel time
Observed P-wave travel times along the seismic profiles are presented in Fig.
Differences in travel times between the profiles become apparent for shots at offsets
Observed S-wave travel times are presented in Fig.
VSP travel times observed along the four shot profiles.
Seismic velocities are calculated from the presented travel times for different depths
Velocity uncertainties
Ultrasonic experiments were performed inside a freezer at the temperature
Travel time measurements across the ice core were made in azimuthal increments of
The ultrasonic source signal pulse was created by a JSR Ultrasonics DPR300 pulser unit and shows a dominant frequency
At each azimuth
Waveforms recorded on sample
Waveforms recorded using S-wave transducers set for vertical vibration are shown in Fig.
Waveforms from S-wave transducers set for horizontal vibration are shown in Fig.
Arrival times
Ultrasonic P- and S-wave velocities of the three core sections studied are shown in Fig.
The largest P- and S-wave velocities are observed in sample
Ultrasonic
An attempted measurement of
Ultrasonic multi-azimuth velocities for different core sections relative to the macroscopic glacier flow direction (
The observed high degree of seismic anisotropy in VSP seismic and multi-azimuth ultrasonic data is consistent with the observation of strong CPO in the retrieved core samples from the site. EBSD measurements on core samples constrain the CPO to be characterised by a strong clustering of
CPO models are created using the MTEX toolbox for MATLAB
Upper-hemisphere stereographic projection
Seismic properties of individual crystals are characterised by the elasticity tensor
Model parameters
Horizontal-cluster CPO model parameters.
The ability of a CPO model to explain measured seismic anisotropy is assessed by introduction of a misfit between synthetic forward-modelled seismic properties and observations.
We found that CPO-predicted seismic velocities are consistently faster than observed velocities, an effect which can be attributed to the absence of grain boundary effects
We consider the elasticity data from
Misfits
The model misfit of the VSP data is calculated using
Best-fitting CPO model parameters and uncertainties informed by VSP data.
Figure
Multi-azimuth P-wave observed velocity anisotropy (symbols) and model results (dashed lines) along the different profiles. Symbol colour shows sensor depths.
Fast S-wave anisotropy
Multi-azimuth S-wave observed velocity anisotropy (symbols) and model results (dashed lines) along the different profiles. Symbol colour shows sensor depths.
Misfits
Figure
Misfit surfaces of seismic velocities showing fit to model parameters of the horizontal-cluster CPO for sample
Best-fitting CPO model parameters and uncertainties informed by ultrasonic data. The notation
The best-fitting models of the three core samples are identified by the minimum in the misfit sum
Measured (black) and modelled (red) CPO geometries.
Both the VSP data and the ultrasonic data are best matched by CPOs with cluster azimuths between
The inter-sample variation in CPO cluster orientations inside the shallow ice at the site
Figure
Observed, modelled and CPO-predicted anisotropy for sample
The observed, modelled and forward-modelled velocity anisotropy for sample
Observed, modelled and CPO-predicted anisotropy for sample
This azimuthal difference between EBSD-measured CPO and CPO models based on ultrasonic data is best explained by small-scale variation within the samples. There is a potential azimuthal error of up to about
The seismic VSP data record wavelengths that are larger than the scale of individual samples and therefore do not resolve a variation in cluster orientation at this scale. The CPO model informed by VSP velocities provides an averaging of any given variation in CPO properties over the entire sampled depth range. The CPO model found to best explain the VSP data is characterised by a slightly larger cone opening angle compared to results of the individual ultrasonic measurements and therefore exhibits slightly weaker anisotropy.
The VSP seismic and ultrasonic datasets presented in this study have fundamentally different acquisition geometries which ultimately determine the observed velocity variation due to CPO. The constraint of CPO models from seismic anisotropy is consequently highly sensitive to the sampling geometry.
The acquisition geometry of the VSP survey will have a critical control on the ability to distinguish different CPO patterns. For example the Flow and Perp lines show equally good P-wave (Fig.
The ultrasonic data offer a dense sampling of velocities along azimuths in the horizontal direction. The given CPO type with horizontal
The influence of azimuthal sampling on model results is investigated by sub-sampling of the multi-azimuth ultrasonic dataset. Figure
Parameter results using downsampled velocity measurements with azimuthal spacing of
Models informed by
Our downsampling analysis shows that in an ideal survey geometry, which is in this case given by the sampling of a horizontal-cluster CPO by horizontal velocity measurements, a realistic CPO model can be created from sampling in up to
The VSP survey CPO modelling presents ambiguity in cluster orientation if only P-wave velocities are considered, as shown in Fig.
The difficulty in reconstructing a realistic CPO model from P-wave velocities in the VSP dataset is a consequence of poor azimuthal sampling. The VSP survey is characterised by coverage of ray paths from a range of incidence angles from multiple azimuths. For the given horizontal-cluster CPO this geometry is clearly not as sensitive to anisotropy as the dense azimuthal sampling of horizontal velocities, which characterises the ultrasonic measurements. The inclusion of P- and S-wave phase information mitigates this shortcoming of the VSP data by resolving the ambiguity in cluster orientation and identifying a CPO model which is in agreement with measured CPO. The study of all available seismic phases should therefore become standard in seismic CPO constraints in ice, rather than the commonly encountered focus on P-wave velocities.
For the horizontal-cluster CPO, the variation in seismic velocities with incidence angle is highly dependent on the azimuth. Therefore, VSP data might be unable to constrain this CPO if azimuths with strong variation are not sampled. The difficulty in finding a correct CPO model from VSP P-wave velocities could be a consequence of this problem, highlighted in Fig.
We have conducted a vertical-seismic-profile (VSP) experiment and laboratory ultrasonic experiments aimed at measuring the seismic anisotropy of ice from the lateral shear margin of the Priestley Glacier, Antarctica, and at linking these data to seismic anisotropy model predictions based on measured crystallographic preferred orientations (CPOs) in EBSD data.
Horizontal-cluster CPO models are informed by P-wave and S-wave velocity anisotropy data from a
Azimuthal ultrasonic P-wave and S-wave velocity measurements, made in
The ultrasonic data have been downsampled to larger azimuthal increments (
VSP seismic data, ultrasonic data and travel time picks are available at
DJP and CLH led the Priestley Glacier project. DJP and FL designed the seismic and ultrasonic experiments. FL collected the multi-azimuth ultrasonic data. HS collected the axial ultrasonic data. SF wrote MTEX code used to simulate CPOs. All authors except CLH and SF were involved in seismic data collection in the field. FL wrote the manuscript in collaboration with DJP. All authors edited the manuscript.
The contact author has declared that none of the authors has any competing interests.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
We would like to acknowledge the logistics support from Antarctica New Zealand and field support from staff at Scott Base, Mario Zucchelli and Jang Bogo research stations. We would also like to thank Brent Pooley for building the ultrasonic rig. We would like to acknowledge Adam Booth for the editorial work and the reviewer Chao Qi and the anonymous reviewer for their insightful comments.
This research has been supported by the Marsden Fund (grant no. UOO1716), the Korea Polar Research Institute (grant no. PE22430) and the New Zealand Antarctic Research Institute (Early Career Researcher Seed Grant, grant no. NZARI 2020-1-3).
This paper was edited by Adam Booth and reviewed by Chao Qi and one anonymous referee.