Fractures and fabrics in glacier ice: Sensitivity of seismic anistropy for antarctic ice masses

Lead Research Organisation: University of Leeds
Department Name: School of Earth and Environment

Abstract

Accurate predictions of the contribution of Antarctic ice to sea-level rise require reliable estimates of how ice dynamics will evolve. This takes place within a complex set of feedbacks between atmosphere, ice and ocean systems. For the ice system, we require knowledge of the internal stress regime. Glacier flow is expected to accelerate under warmer temperature regimes, leading to stress-state changes within the ice mass. Geophysics is a valuable means of estimating the present-day stress regime, such that it can then be supplied to predictive computational models of future ice mass evolution.

Englacial stress-states can be measured from seismic anisotropy. The ice crystal is itself strongly anisotropic hence, a bulk anisotropic fabric is formed when a stress regime causes crystals to align. For such fabrics, seismic energy will propagate more quickly when travelling orthogonal to the layer, than when travelling along it. This defines a regime of vertical transverse isotropy (VTI), in which the observed variation of velocity is only with the obliquity of the incidence angle, rather than with azimuth.

Anisotropy may also arise when there are preferentially-aligned crevasses. Seismic energy propagating through a crevassed zone will travel more slowly than that travelling through intact ice. This defines horizontal transverse isotropy (HTI), where velocity varies with the azimuth relative to the crevasse orientation. Each of these regimes is an indicator of fast glacier flow, hence it is important to develop effective strategies for monitoring the development of these regimes.

Ice anisotropy can be characterised from surface seismic reflection data, however there has been little analysis of the sensitivity of the acquisition geometry to varying anisotropic regimes. This is especially important to consider since field logistics often demand a compromised acquisition strategy. Building reliable models of englacial anisotropic fabrics, and simulating the seismic response to them, will lead to a set of seismic acquisition guidelines for any given glacier target.

Objectives

In this project, you will consider the modelling and detectability of anisotropic fabrics in two specific Antarctic ice masses, both of which are considered critical for regional ice stability.
- HTI fabrics will be explored with relation to the intensity of basal crevassing in Larsen C Ice Shelf, on the Antarctic Peninsula. The stress regime of Larsen C is of interest given the potential link between the iceberg calving in 2017 and the stability of the wider shelf.
- VTI fabrics will be simulated for flow regimes of Thwaites Glacier, a major outlet of the West Antarctic Ice Sheet. Specifically, these investigations will be focused around the shear margin of the glacier, which marks the onset of fast glacier flow. A robust measurement of anisotropy will contribute to the understanding of the controls on fast glacier flow.

Anisotropy models will be developed using a discrete fracture formulation of the wave equation, implemented in WAVE software; these models will be compared against the signatures of anisotropy in real seismic data. An archive of azimuthal seismic datasets already exists for Larsen C Ice Shelf. Data from Thwaites Glacier will be acquired in two field campaigns, which you will join in 2022, to record novel 3-D seismic reflection data.

You will undertake this project under the guidance of a team of Leeds scientists, who are leading experts in glaciology, seismic modelling and anisotropic analysis. Specific objectives of the project include, but are not limited to:
1. Use of seismic modelling methods to simulate the response to anisotropic ice fabrics and crevasses.
2. Assessment of the sensitivity of the seismic response to the intensity of the ice fabric and fractures, and the seismic acquisition geometry.
3. Analysis of new field data from Larsen C Ice Shelf and Thwaites Glacier. In the latter case, this processing

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
NE/S00677X/1 01/08/2018 01/10/2024
2438285 Studentship NE/S00677X/1 01/10/2020 31/03/2024 Andrew Pretorius