Fractures and Fabrics in Glacier Ice: Sensitivity of Seismic Anisotropy for Antarctic Ice Masses

Highlights

  • Antarctic field deployment, acquiring new geophysical data on Thwaites Glacier, as part of the ITGC’s TIME project
  • Development of cutting-edge geophysical methods in the assessment of Antarctic ice-mass stability
  • Collaboration with an international team of glaciological and geophysical experts

NOTE: This PhD project is funded separately to those advertised through the main NERC PANORAMA DTP competition.  You may apply for this project while also applying for any other from the DTP.  For eligibility details and how to apply please see https://phd.leeds.ac.uk/project/467-fractures-and-fabrics-in-glacier-ice:-sensitivity-of-seismic-anisotropy-for-antarctic-ice-masses.

 

Contacts

Lead supervisor: Dr Adam Booth (a.d.booth@leeds.ac.uk)

Co-supervisors from: Dr Sjoerd de Ridder (s.deridder@leeds.ac.uk), Dr Mark Hildyard (m.hildyard@leeds.ac.uk), Dr Andy Nowacki (a.nowacki@leeds.ac.uk)

For application queries, contact env-pgr@leeds.ac.uk

 

Project Summary

Accurate predictions of the contribution of Antarctic ice to global sea-level rise require reliable predictions of how the dynamics of glaciers and ice masses will evolve.  The evolution of these dynamics 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 changes in stress state within the ice mass.  Geophysical methods are 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.

The stress state within a glacier can be measured by considering the variation of seismic velocities with propagation direction, itself termed anisotropy. Several processes can give rise to anisotropy within a glacier. The ice crystal is itself strongly anisotropic hence, a bulk anisotropic fabric is formed when a stress regime causes crystals to align (Diez et al., 2015). For such fabrics, seismic energy will propagate more quickly when travelling orthogonal to the layer, than when travelling along it (e.g., Figure 1a). 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.

Figure 1. Schematic diagrams of anisotropic fabrics within ice. a) VTI regime within an ice sheet, caused by ice crystal alignment. b) HTI regime within a floating ice shelf, caused by aligned basal crevasses. For both models, slower seismic propagation would be observed in the direction indicated by the red source/receiver geometry.

 

Anisotropy may also arise when there are preferentially-aligned crevasses (Figure 1b). Seismic energy propagating through a crevassed zone will travel more slowly than that travelling through intact ice (Bradford et al., 2013). 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 (Diez et al., 2015; Smith et al., 2017), 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 (Figure 2). 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 (Borstad et al., 2017).
  • VTI fabrics will be simulated for the flow regime of Thwaites Glacier, a major outlet of the West Antarctic Ice Sheet (Scambos et al., 2017). 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.

Models of anisotropy will be developed using a discrete fracture formulation of the wave equation, implemented in the WAVE software; these models will ultimately be compared against the signatures of anisotropy in real seismic reflection 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: as the student on this project, you will join the field deployment in 2022 to record highly novel 3-D seismic reflection data. The 3-D coverage in these data also allow for unprecedented imaging quality of a complex glacier system (Figure 3).

Figure 2. Seismic acquisition and data from Larsen C Ice Shelf, on the Antarctic Peninsula. The stress regime of the ice shelf is of interest following the calving of Iceberg A68 in 2017 (upper image, © Adrian Luckman, Swansea University). Example seismic data show reflections from the base of the ice shelf and the underlying sea-bed.
Figure 3. Schematic seismic acquisition design for Thwaites Glacier survey. Your deployment of a 3D nodal seismic system, as part of the TIME project’s deployment in 2022, enables unique insight into the azimuthal variation of ice properties

 

You will undertake this project under the guidance of a team of Leeds scientists, who are leading experts in glaciology, seismic modelling (de Ridder et al., 2018) and anisotropic analysis. Specific objectives of the project include, but are not limited to:

  1. Use (and adaptation, where appropriate) of continuum and discrete fracture modelling methods to simulate the seismic response to anisotropic ice fabrics and crevasses.
  2. Assessment of the sensitivity of the seismic response to both the intensity of the ice fabric and fractures, and to the seismic acquisition geometry.
  3. Analysis of new field data from Larsen C Ice Shelf and Thwaites Glacier, considering the observations in objectives (1) and (2). In the latter case, this processing will be applied both for high-resolution imaging of the glacier bed and for determining basal anisotropy.

 

Apply for this Project?

The closing date for applications is 13th January 2020, with interviews to be held in Leeds in mid-February.  You can submit an application via the University of Leeds “Applying for research degrees” page.

 

Student Profile

You should have a passion for geophysical data analysis and a desire to apply it in glaciological research. You should have a background in seismic data analysis, being familiar with data processing routines, and potentially experienced with field surveying and/or computational seismic modelling.  You should also be prepared to undertake a deep-field Antarctic deployment, potentially with smaller field deployments in support of methods development.

Eligibility

Eligibility follows standard guidelines for NERC-funded PhD research. Eligibility is therefore limited to UK or EU nationals who have ordinarily been resident in the UK for 3 years immediately prior to the commencement of the project. You must be able to commence PhD study before 1st August 2020.

Potential for high impact outcomes

Research into the anisotropic properties of ice is a vibrant research field, not least because new paradigms in seismic acquisition technology are unlocking new interpretative potential from seismic datasets. Both of the study sites in this project are of global research interest, particularly the Thwaites Glacier which is seen as the ‘vulnerable underbelly’ to the stability of the West Antarctic Ice Sheet. We anticipate that research outputs from this project will be publishable in leading high-impact international journals, and will be of great interest to the international research community.

 Training

During this project, you will work in the School of Earth and Environment, University of Leeds, principally under the supervision of Dr Adam Booth, an expert in glacier seismic acquisition and analysis. Depending on the experience and preferred focus of the successful applicant, co-supervision can be provided by any of:

  • Dr Mark Hildyard, a computational seismologist and developer of the WAVE discrete fracture modelling software (Hildyard, 2007),
  • Dr Sjoerd de Ridder, an expert in seismic imaging and data inversion, and
  • Dr Andy Nowacki, a seismologist with specific interests in characterising seismic anisotropy.

Additionally, you will work with the wider glaciological community in the Thwaites Glacier TIME (Thwaites Interdisciplinary Margin Evolution) project, which will include a post-doctoral researcher based at Leeds, and visits to other colleagues at collaborating institutions around the UK (e.g., Dr Poul Christoffersen, Scott Polar Research Institute) and USA (e.g., Prof Slawek Tulaczyk, University of California Santa Cruz). The project provides high-level specific training including:

  • the significant research themes in the disciplines of glaciology and seismic imaging,
  • use of state-of-the-art seismic computational approaches,
  • fieldwork procedures for deep-field Antarctic deployments, and
  • integration of learning from synthetic and real-data observations to determine the reliability of geophysical parameters.

Co-supervision will involve regular meetings between all partners.  The successful candidate will have access to a broad spectrum of training workshops facilitated by Leeds Doctoral Training Programme. They can also access MSc-level modules in Earth and Environment, drawn from our MSc Exploration Geophysics programme.

 

References

Borstad C, McGrath D and Pope A (2017); Fracture propagation and stability of ice shelves governed by ice shelf heterogeneity. Geophysical Research Letters, 44, 4186-4194.

Bradford JH, Nichols J, Harper JT and Meierbachtol T (2013); Compressional and EM wave velocity anisotropy in a temperate glacier due to basal crevasses and implications for water content estimation. Annals of Glaciology, 554(64), 2013.

de Ridder SAL and Maddison JR (2018); Full wavefield inversion of ambient seismic noise. Geophysical Journal International, 215(2), 1215-1230.

Diez A and Eisen O (2015); Seismic wave propagation in anisotropic ice – Part 1: Elasticity tensor and derived quantities from ice-core properties. The Cryosphere, 9, 367-384.

Hildyard M (2007); Manuel Rocha Medal Recipient: Wave interaction with underground openings in fractured rock. Rock Mechanics and Rock Engineering, 40(6), 531-561.

Scambos TA and 22 others (2017); How much, how fast? A science review and outlook for research on the instability of Antarctica’s Thwaites Glacier in the 21st century. Global and Planetary Change, 153, 16-34.

Smith EC, Baird AF, Kendall JM, Martin C, White RS, Brisbourne AM and Smith AM (2017); Ice fabric in an Antarctic ice stream interpreted from seismic anisotropy. Geophysical Research Letters, 44, 3710-3718.