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

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

The TIME project benefits any academic research programme aiming to understand the dynamic controls on the Thwaites Glacier Drainage Basin and, therefore, the wider stability of the West Antarctic Ice Sheet. To date, the characterisation of the Thwaites margins has been largely ignored in numerical ice sheet models; this is despite the fact that the global scientific community considers the collapse of WAIS to be among the most significant risks for coastal environments and cities given the potential consequence for future sea-level rise. Our research therefore addresses a fundamental socio-economic question, and could ultimately influence government policy via contributions to future forecasts of sea-level rise in the coming decades. To facilitate the broadest reach into the wider community, we will continue our collective record of dissemination into the highest-profile open-access scientific literature.
Our programme of public engagement will raise awareness of the role of glaciers in the climate change debate, ensuring that environmental considerations remain on government agendas. This programme involves a continuing commitment to public dissemination, including a 'Polar Science Day' of outreach at each of our five annual science meetings (held successively in Santa Cruz, Cambridge, El Paso, Leeds and Oklahoma). It is the experience of USA partners in the TIME project that more than 1000 community members participate in such educational activities, and we expect to be able to replicate this in each regional event. We will also produce an exhibition based on discovery science in the TIME project, using the Polar Museum in Cambridge as a venue for public outreach. The museum is visited by 40,000 or more members of the public and >100 schools groups each year. Public engagement will also be facilitated by maintaining a project website, featuring "explained" science, field photos, tweets and blogs.
The education and outreach theme will revolve around the central question: Is the WAIS in a state of collapse? This highly relevant question offers rich possibilities for education and outreach for K-12 audiences in the USA and school groups from both primary and secondary education in the UK.