On the Limitations of Using Polarimetric Radar Sounding to Infer the Crystal Orientation Fabric of Ice Masses

• Published in: Geophysical research letters 2022-01, Vol.49 (1), p.n/a
• Main Authors: Rathmann, Nicholas M., Lilien, David A., Grinsted, Aslak, Gerber, Tamara A., Young, Tun Jan, Dahl‐Jensen, Dorthe
• Format: Article
• Language: English
• Subjects: anisotropic ice; Crystal structure; Crystals; Direction; Fabrics; Glaciation; Glacier flow; Glacier ice; Glaciers; Grain orientation; Ice; Ice crystals; Ice sheets; Incident waves; Local flow; Mathematical analysis; Modelling; Polarimetric radar; Polarimetry; Polls & surveys; Radar; radio wave modeling; Radio waves; Spherical harmonics; Surveys; Tensors; Transfer matrices; Viscosity; Wave propagation
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2021GL096244

We introduce a transfer matrix model for radio‐wave propagation through layered anisotropic ice that permits an arbitrary dielectric permittivity tensor in each layer. The model is used to investigate how crystal orientation fabrics without a vertical principal direction affect polarimetric radar returns over glaciers and ice sheets. By expanding the c‐axis orientation distribution in terms of a spherical harmonic series, we find that radar returns from synthetic fabric profiles are relatively insensitive to the harmonic mode responsible for a nonvertical principal direction; however, only for normally incident waves. Consequently, the strength of this mode might be relatively difficult to infer in glaciers and ice sheets, which in turn has implications for the ability to determine the full second‐order structure tensor, needed to infer the local flow regime, flow history, or to represent the directional viscosity structure of glacier ice for ice‐flow modeling. Plain Language Summary The orientation of ice crystals in glacier ice locally co‐evolve with and can enhance the flow of ice. Methods that allow inferring the crystal structure inside glaciers and ice sheets are, therefore, essential for improving the accuracy and realism of ice‐flow models, and have broad implications for understanding past and present flow regimes. In this work, we introduce a new radio‐wave model and use it to investigate the extent to which radar surveys over glaciers and ice sheets can reveal the orientation information necessary to improve such ice‐flow models and construct a proxy for past flow. We show that conventional polarimetric radar surveys, which look straight down, might be poorly suited for the task; a specific but important component of the grain orientation structure cannot be seen with such surveys. We find, however, that radio waves transmitted at an angle to the surface might overcome this crucial limitation and allow the full crystal structure to be inferred. Key Points Radar returns over glacier ice are, for normal incidence, insensitive to whether or not a vertical principal fabric direction exists The harmonic mode responsible for a nonvertical principal fabric direction might be determined from oblique radar sounding Assuming a vertical principal fabric direction when there is none can substantially affect inferred directional viscosities of glacier ice