Abstract:
We use polarimetric radar sounding to investigate variation in ice crystal orientation fabric within the near-surface (top 40-300 m) of Rutford Ice Stream, West Antarctica. To assess the influence of the fabric on ice flow, we use an analytical model to derive anisotropic enhancements of the flow law from the fabric measurements. In the shallowest ice (40-100 m) the azimuthal fabric orientation is consistent with flow-induced development and correlates with the surface strain field. Notably, toward the ice-stream margins, both the horizontal compression angle and fabric orientation tend toward 45 degrees relative to ice flow. This result is consistent with theoretical predictions of flow-induced fabric under simple shear, but to our knowledge has never been observed. The fabric orientation in deeper ice (100-300 m) is significantly misaligned with shallower ice in some locations, and therefore inconsistent with the local surface strain field. This result represents a new challenge for ice flow models which typically infer basal properties from the surface conditions assuming simplified vertical variation of ice flow. Our technique retrieves azimuthal variations in fabric but is insensitive to vertical variation, and we therefore constrain the fabric and rheology within two end-members: a vertical girdle or a horizontal pole. Our hypotheses are that fabric near the center of the ice-stream tends to a vertical girdle that enhances horizontal compression, and near the ice-stream margins tends to a horizontal pole that enhances lateral shear.
ApRES radar data were collected as part of the BEAMISH Project (NERC AFI award numbers NE/G014159/1 and NE/G013187/1). Tom Jordan would like to acknowledge support from EU Horizon 2020 grant 747336-BRISRES-H2020-MSCA-IF-2016.
Keywords:
anisotropy, fabric, ice flow, polarimetry, radar, rheology
Jordan, T., Martin, C., Brisbourne, A., Schroeder, D., & Smith, A. (2020). Radar characterization of ice crystal orientation fabric and anisotropic rheology within Rutford Ice Stream, 2017-2019 (Version 1.0) [Data set]. UK Polar Data Centre, Natural Environment Research Council, UK Research & Innovation. https://doi.org/10.5285/d5b7e5a1-b04d-48d8-a440-c010658ec146
| Access Constraints: | No restrictions apply. |
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| Use Constraints: | This data is supplied under Open Government Licence v3.0 http://www.nationalarchives.gov.uk/doc/open-government-licence/version/3/. |
| Creation Date: | 2020-11-03 |
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| Dataset Progress: | Complete |
| Dataset Language: | English |
| ISO Topic Categories: |
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| Parameters: |
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| Personnel: | |
| Name | UK Polar Data Centre |
| Role(s) | Metadata Author |
| Organisation | British Antarctic Survey |
| Name | Tom M Jordan |
| Role(s) | Technical Contact, Investigator |
| Organisation | Univeristy of Bristol |
| Name | Alex M Brisbourne |
| Role(s) | Investigator |
| Organisation | British Antarctic Survey |
| Name | Carlos Martin |
| Role(s) | Investigator |
| Organisation | British Antarctic Survey |
| Name | Dustin M Schroeder |
| Role(s) | Investigator |
| Organisation | Stanford University |
| Name | Andrew M Smith |
| Role(s) | Investigator |
| Organisation | British Antarctic Survey |
| Parent Dataset: | N/A |
| Reference: | Brennan, P. V., Lok, L. B., Nicholls, K., & Corr, H. (2014). Phase-sensitive FMCW radar system for high-precision Antarctic ice shelf profille monitoring. IET Radar Sonar Navigation, 8 (February 2013), doi: 10.1049/iet-rsn.2013.0053 Brisbourne, A. M., Martin, C., Smith, A. M., Baird, A. F., Kendall, J. M., & Kingslake, J. (2019). Constraining Recent Ice Flow History at Korf Ice Rise, West Antarctica, Using Radar and Seismic Measurements of Ice Fabric. Journal of Geophysical Research: Earth Surface, 124 , 175-194-373. doi: 10.1029/2018JF004776 Jordan, T. M., Schroeder, D. M., Castelletti, D., Li, J., & Dall, J. (2019). A Polarimetric Coherence Method to Determine Ice Crystal Orientation Fabric From Radar Sounding: Application to the NEEM Ice Core Region. IEEE Transactions on Geoscience and Remote Sensing, doi: 10.1109/TGRS.2019.2921980 |
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| Quality: | Polarimetric data quality is established from the coherence magnitude, which provides a metric for sections of the ice column where fabric can be reliably estimated. Fabric estimates could be made in the top 100-200 m of the ice column at 18 of the 20 measurement sites (maximum depth ~ 100 m near the shear-margin, and ~200 m in the ice-stream centre). | |
| Lineage: | Polarimetric data acquisition follows Brisbourne et al. 2019. This paper describes the quad-polarized measurement technique that was applied in the field. ApRES signal processing follows Brennan et al. 2014. This paper describes the processing steps applied to the raw data to generate ice-depth profiles for the phase and amplitude of radar wave. Polarimetric data processing follows Jordan et al. 2019. This paper describes the coherence methodology that is used to determine the quality of the radar data for fabric estimation. |
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| Temporal Coverage: | |
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| Start Date | 2017-01-20 |
| End Date | 2017-01-20 |
| Start Date | 2019-12-05 |
| End Date | 2019-12-05 |
| Spatial Coverage: | |
| Latitude | |
| Southernmost | -78.13825 |
| Northernmost | -78.1049 |
| Longitude | |
| Westernmost | -83.92791 |
| Easternmost | -83.58778 |
| Altitude | |
| Min Altitude | N/A |
| Max Altitude | N/A |
| Depth | |
| Min Depth | N/A |
| Max Depth | N/A |
| Latitude | |
| Southernmost | -78.10663 |
| Northernmost | -77.79755 |
| Longitude | |
| Westernmost | -84.52104 |
| Easternmost | -84.00194 |
| Altitude | |
| Min Altitude | N/A |
| Max Altitude | N/A |
| Depth | |
| Min Depth | N/A |
| Max Depth | N/A |
| Location: | |
| Location | Antarctica |
| Detailed Location | Rutford Ice Stream |
| Sensor(s): |
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| Data Collection: | Radar: ApRES (British Antarctic Survey's Autonomous phase-sensitive Radio-Echo Sounder) - chirps of 200 to 400 MHz in time intervals of 1 s. |
| Distribution: | |
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| Distribution Media | Online Internet (HTTP) |
| Distribution Size | 765 MB |
| Distribution Format | netCDF |
| Fees | N/A |
| Distribution Media | Online Internet (HTTP) |
| Distribution Size | 1.3 kB |
| Distribution Format | ASCII |
| Fees | N/A |
| Data Storage: | Each measurement site has its own folder containing raw quad-polarized ApRES data. The folders are labelled A1-A10 for Transect A and B1-B10 for Transect B - for example Transect_A/A1. Within each folder are 5 binary data files saved in order HH, VH, VV, HV, HH, where H and V corresponds to polarization orientations aligned with magnetic East and North respectively. The data files are labelled as: YYYY-MM-DD_XXXXXX, e.g. 2017-01-20_175547. Site coordinates, and file names (_XXXXXX extensions) are provided in CSV summary tables: Transect A: RIS-TransectA-apres-2016-17.csv, Transect B: RIS-TransectB-apres-2019-20.csv. |