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Glaciological Applications of Seismic Full Waveform Inversion: Insight from a Novel Approach to Evaluating the Seismic Properties of Ice

Dr Adam Booth (SEE), Dr Graham Stuart (SEE), Prof Bryn Hubbard (Aberystwyth University), Dr Alex Brisbourne (British Antarctic Survey), Prof Bernd Kulessa (Swansea University)

Project partner(s): Dr Ian Jones (ION GX Technology) - CASE

Contact email: a.d.booth@leeds.ac.uk

This is a fully funded project and is not part of the DTP competition, applications should be made directly to the hosting department, see http://www.see.leeds.ac.uk/admissions-and-study/research-degrees/how-to-apply/ for more information.

Seismic surveys give insight into the physical properties of the englacial and subglacial properties of ice masses.  Certain quantities, e.g. ice density (Figure 1), can be related to climate models and therefore provide proxies for the climatic evolution of a glacier (Kuipers Munneke et al., 2014; Hubbard et al., 2016).  Such information is critical for predicting the stability of ice masses in a warming climate; these ice masses include the large ice shelves which fringe the Antarctic continent, widely believed to underpin the long-term contribution of Antarctic ice streams to global sea-level rise (DeContro and Pollard, 2016). 

A major limitation of deriving density from seismic velocities is that velocity:density conversions are almost entirely empirical and therefore of questionable accuracy (Booth et al., 2013). The widely-applied Kohnen (1974) conversion is based on lab analysis of a South Pole ice core hence ice is removed from its original temperature/pressure context which, in any case, is likely different for other ice masses. While the general trend of density is likely characterised, greater confidence in the accuracy of inverted parameters would be beneficial.  Full wavefrom inversion (FWI) of a seismic dataset (Virieux and Operto, 2009) represents a promising approach to property estimation that deserves investigation for glaciological applications.

Figure 1.  Schematic representation of ice density derivation from seismic data.  a) Data acquisition in Antarctica.  b) Seismic shot record, inverted for (c) seismic velocity and (d) density via an empirical model.

FWI methods circumvent the need for empirical methods by deriving the underlying physical properties (seismic velocity, density, etc.) from the seismic wavefield itself.  FWI has seen significant development in the hydrocarbons field (Brittan et al., 2013; Bai and Yingst, 2014; Jones, 2015), but its applicability in the glaciological setting is not widely proven.  The development of FWI in glaciology would represent a step-change in seismic capabilities, which could be widely adopted throughout the community.

Research Approach

This studentship aims to explore the transfer of FWI experience between the industrial and glaciological settings, in a three-phase research programme:

     Phase 1) using existing glaciological archives of seismic data, review whether current seismic practice can produce data that are FWI-compatible;

     Phase 2) develop an optimised acquisition strategy to enable FWI;

     Phase 3) undertake field acquisitions to validate FWI performance.

Data available for Phase 1 include seismic and density measurements made on Antarctica’s Larsen C Ice Shelf, during field campaigns of the NERC-funded MIDAS project (Kulessa et al., 2016; Hubbard et al., 2016; Ashmore et al., 2016); additionally, the British Antarctic Survey (BAS) will contribute data from their campaigns on the Antarctic Pine Island Glacier.

During Phase 2, BAS will liaise with the student for guidance on FWI-compliant acquisitions in forthcoming field campaigns, which include a further deployment on Thwaites Glacier and on Rutford Ice Stream.  Here, seismic velocities have important implications for the thermal regime and internal crystal fabric.

In Phase 3, the student will undertake FWI-compliant seismic acquisitions to establish the evolving density of the Hardangerjøkulen ice cap (Giesen and Oerlemans, 2010).  There may also be the opportunity, via the Collaborative Antarctic Science Scheme (CASS) for acquisition in the environs of BAS’s Antarctic Rothera Station.  These new acquisitions will be accompanied by borehole measurements of sonic velocity and density to calibrate the inverted parameters, and compare their accuracy to standard interpretative approaches.

It is also expected that experiences of FWI in glaciology can benefit industrial approaches.  For example, glaciers are structurally simple compared to the geology of a typical hydrocarbon province; furthermore, where borehole control is available, it extends from the ground surface to the target depth therefore offering depth-continuous constraint.

Potential for High Impact Outcomes

FWI is largely untested in glaciological surveying hence any developments will be pioneering steps towards its wider adoption.  The ability to circumvent empirical models is a means of reducing the uncertainty in geophysical analysis, a key requirement in the next generation of predictive climate models.  FWI represented a step-change in seismic capabilities in the industrial setting, and there is every reason to expect similarly significant benefits in glaciology.  A link with established groups, including the British Antarctic Survey, ensures that the research will reach practitioners and lead to high-impact publications that focus on pertinent glaciological themes. 

CASE partner

This studentship has been agreed as a CASE partnership with ION GX Technology.  ION is a leading innovator in full waveform inversion methods, providing the hydrocarbon industry with seismic imaging algorithms to improve the definition of exploration targets.  The CASE supervisor is Dr Ian Jones, an international expert in geophysical imaging and a Senior Geophysical Advisor to ION-GX Technology.  ION GX Technology contribute £1000/year to the research budget of this project, and offer a six-month placement at their Chertsey offices to be distributed over the duration of the studentship.

Training

The student will work primarily under the supervision of Dr Adam Booth and Professor Graham Stuart, in the Institute of Applied Geoscience (IAG) in the University of Leeds School of Earth and Environment.  External supervisors include Professor Bryn Hubbard (Aberystwyth University), an expert in the implementation of borehole methods in glaciology, and geophysicists Dr Alex Brisbourne (British Antarctic Survey) and Professor Bernd Kulessa (Swansea University).  Together with industrial support from Dr Ian Jones, the project offers specific training in:

  • acquisition and quantitative analysis of seismic data in the glaciological setting,

  • design of full waveform inversion approaches,

  • operation of hot-water drilling and borehole surveying equipment,

  • an appreciation of current research themes in glaciology.

Co-supervision will involve regular meetings between all partners.  The successful candidate will have access to a range of MSc-level courses (e.g., in seismic reflection processing) through Leeds’ MSc Exploration Geophysics programme. The student can undertake an MSc-level foundation course in Arctic Glaciology during a one-month study programme at the University Centre on Svalbard, in which participants are trained in theoretical and practical glaciological techniques.  Should a deployment in Antarctica become possible, the student will be trained in Antarctic field procedure by the British Antarctic Survey.

Student Profile

Candidates should have a strong background in geophysics, preferably being familiar with seismic imaging techniques and computational seismic modelling.  Willingness to undertake glaciological field deployments is essential; while a familiarity with glaciological research issues is desirable, these can be honed during the studentship.

References

Ashmore DW et al., 2016. Firn heterogeneity of Larsen C Ice Shelf from borehole optical-televiewing.  International Symposium on the Interactions between Ice Sheets and Glaciers within the Ocean, International Glaciological Society, La Jolla, 10-15th July 2016.

Bai J and Yingst D, 2014; Simulataneous inversion of velocity and density in time-domain full waveform inversion. 84th Annual International Meeting, SEG, Expanded Abstracts, 922-927.

Booth AD et al., 2013; A comparison of seismic and radar methods to establish the thickness and density of glacier snow cover. Annals of Glaciology, 54, 73-82.

Brittan J et al., 2013; Full waveform inversion – The state fo the art.  First Break, 31, 75-81.

DeConto RM and Pollard D, 2016; Contribution of Antarctica to past and future sea-level rise.  Nature, 531, 591-597

Giesen RH and Oerlemans J, 2010; Response of the ice cap Hardangerjøkulen in southern Northay to the 20th and 21st century climates. Cryosphere, 4, 191-213.

Hubbard et al., 2016; Massive subsurface ice formed by refreezing of ice-shelf melt ponds; Nature Communications, 7, 11897.

Jones IF, 2015; Estimating subsurface parameter fields for seismic migration: velocity model building, in: Encyclopedia of Exploration Geophysics. SEG, pp. U1-1-U1-24. Editors: Vladimir Grechka and Kees Wapenaar.

Kohnen H, 1974; The temperature dependence of seismic waves in ice.  Journal of Glaciology, 13, 144-147.

Kuipers Munneke P et al., 2014; Firn air depletion as a precursor of Antarctic ice-shelf collapse.  Journal of Glaciology, 60(220), 205-214.

Kulessa B et al., 2016; Firn air-content of Larsen C Ice Shelf, Antarctic Peninsula, from seismic velocities, borehole surveys and firn modelling. EGU2016-4743, European Geosciences Union, Vienna, Austria.

Virieux and Operto, 2009; An overview of full waveform inversion in exploration geophysics. Geophysics, 74(6), WCC1-WCC26.