Report of the modelling workshop in Glasgow » back

1) Summary

The workshop „Microdynamic Modelling of Ice and related materials“ took place in Glasgow at the School of Geographical and Earth Sciences from the 8th to the 11th of May 2011. The main aim of the workshop was an interdisciplinary exchange between the Ice-Community and the Microstructural Modelling Community with an emphasis on the „Elle“ modelling platform and related numerical models. Organizers of the workshop were Daniel Koehn from Glasgow, Paul Bons from Tübingen, Ilka Weikusat from the AWI in Bremerhaven, Hans de Bresser from Utrecht University and Albert Griera from Spain. The workshop consisted of a one-day field trip to the Scottish Highlands with an emphasis on glacial landscape and volcanic complexes. The excursion was followed by two days of talks from a large variety of researchers. On the final day the group came together to discuss future research projects, publications and open questions in Ice related research in order to define new research directions. During the 17 talks on days 2 and 3 we had invited talks from field researchers, experimentalists and numerical modellers from a range of fields. These included geology, glaciology, material sciences, engineering and physics. It became clear that a multidisciplinary approach is needed to proceed further in the modelling of Ice. The workshop led to fruitful discussions and new finding of partners in addition to the publication of a special issue in the Journal of Structural Geology. We were explicitly invited to bring this special issue together with three leading review articles on natural ice microstructures, experiments and numerical modelling. The main aim is to have a larger impact on the Earth Science Community that deals with rheology and microstructure of rocks.

2) Scientific content of and discussion at the event

The workshop started on the second day with an introduction and welcome by Daniel Koehn followed by a detailed account on the MicroDICE initiative by Paul Bons. With this workshop we want to target the core aim of the ESF Research Network Programme Micro-DICE, namely discuss how we can reach a better understanding of the dynamic grain-scale processes in ice to understand how these processes influence the properties of ice and how predictive models can be constructed that are based on grain-scale ice dynamics. In order to achieve this goal we wanted to merge researchers from the ice-community with the Microstructural Modelling community in the Earth and Material Sciences.

The introduction of Paul Bons was followed by a review of natural ice microstructures by Sergio Faria from Göttingen. He gave a talk on the „State of the art and open questions in ice microstructure research“ which dealt mainly with ice cores from Antartica. He showed numerous thin-sections of ice and detailed logs through the ice cores. He then tried to combine the microstructures with the rheological large scale ice models. His results clearly indicated how the grain size changes through the core and how cloudy bands with inclusion start to form further down the core. At the base of the ice-sheet the ice layers become folded developing into a large (up to 500m thick) shear zone where the ice flows strongly.

Sergio Faria also gave the second talk on „Deformation structures in natural ice“ instead of Ilka Weikusat who could not attend the meeting. In this impressive talk Sergio Faria showed how deformation structures can be observed in natural ice thin-sections. The ice also came mainly from cores from Antartica. Sergio Faria showed how grain boundary migration, grain growth and subgrain formation can be observed in natural ice thin-sections. From his observations it became clear that the microstructure of natural ice does not necessarily show clear evidence of a purely power-law rheology with migration of dislocations. This means that the microstructure does not necessarily support current models of ice deformation. Such a finding already led to a deep discussion between the modellers and the researchers that observe the microstructures.
This first part of the workshop was focused on natural microstructures. It was followed by two modelling presentations that were directly involved in ice sheet modelling. The first presentation was given by Maurine Montagnat from Grenoble on „Modelling ice and rock deformation“. She gave a review on mainly dislocation dynamics modelling of ice sheet and glacier flow. Maurine Montagnat showed in detail how the models are derived and how dislocation dynamics is then used to model large scale deformation. The presentation was followed by a discussion on the methods.

The second presentation that dealt with ice modelling was concerned with a larger scale and three-dimensional models. Fabien Gillet-Chaulet from Grenoble gave an overview presentation on his models „3D modelling of ice flow“. His talk was following the presentation of Maurine Montagnat because the models are based on the same principles. He showed how large scale three-dimensional ice deformation models can be performed. The two numerical modelling talks were followed by a discussion on the importance of the microstructural observations presented earlier. This led to the important question of whether or not ice flows only by dislocation creep and has a power law rheology or if diffusion creep is also possible especially at small grain sizes that are observed in thin-sections.

The second half of day one was dealing with an overview of numerical microstructural models. The first talk was an overview of the Elle modelling environment by Paul Bons from Tübingen. He showed how the numerical model is used to simulate microstructure development in rocks, materials and also ice. His talk led to interesting discussion on how to implement additional routines from other workshop participants into the Elle environment and also on how to couple the microstructural modelling with large-scale deformation.

The second talk in the afternoon was a direct example of ice microstructure modelling in Elle. Jens Roessiger from Tübingen gave a talk on „Modelling ice deformation microstructures“ where he was mainly concerned with the movement of air bubbles through ice. This is an important research field in ice microstructure modelling and illustrates how powerful the Elle environment is.

The final talks on day one were concerned with more Elle applications. Gema Llorens from Barcelona gave a talk on folding of rocks with Elle and produced structures that are very similar to ice folds that can be observed in the lowest parts of large ice sheets where ice layers become folded and strongly sheared. Daniel Koehn from Glasgow showed how Elle can be used to model fluid-rock interaction as well as brittle-ductile transitions where material flows and fractures at the same time. In discussions after these two talks many possibilities became clear on how to model ice deformation, microstructures and rheology with Elle.

The last talk on day one was a direct application of dislocation modelling with an FFT approach in Elle of ice flow. Albert Griera from Barcelona showed his impressive models that can be used to model real dislocation microstructures in ice in three dimensions. The following discussion focused again on the applicability of these microstructure models for the large ice sheet models.

Day two of the workshop started with an experimental series. Hans de Bresser from Utrecht University gave an introduction on „The role of grain size in the flow of ice, background and hypothesis“. He showed how natural rocks generally tend to change the grain size in such a way that the lie in a field that is neither purely controlled by diffusion creep nor dislocation creep. The rather lie in the area in between the two mechanisms suggesting that both are active. His talk was followed by an investigation of Sabrina Diebold from Utrecht on the same problem but with ice as material. She illustrates that ice also behaves similar to other rock types and seems to change the grain size such that not only dislocation creep is active. This led to numerous discussions on how to model the rheology and flow of such systems. Is dislocation dynamics modelling enough to understand the rheology, especially when we see that diffusion creep is also active?

The second part of day one consisted of three applied talks on ice deformation. Mark Peternell from Mainz showed how ice is deformed in experiments and how the crystallographic orientations of grains can be mapped live during an experiment. He focuses on see-through experiments where he can observe the microstructure directly. His talk was followed by a talk of Aine Ni Bhreasail from Imperial College London on micromechanics of Permafrost thaw. She showed with experiments how ice can crack pores. She was followed by Alexandra Seymour-Pierce from University College London on „Indentation of Ice Surfaces“ working on friction of ice and related flow and fractures. All talks showed the importance of experimental observations and comparison with numerical models.

In the afternoon of day 3 of the workshop the talks were concluded by 4 modelling presentation coming from material sciences and illustrating additional modelling methods. Aitor Luque from CERS in Spain showed „Multi-scale modelling, from molecular to FEM of copper and/or ice creams“. He showed a whole range of models that can be used and combined on different scales depending on the desired questions. His talk was followed by a presentation by Senthil Vel from Maine University. He showed how „Modelling the bulk elastic properties and seismic anisotropy of polycrystalline materials“ can be performed. The most interesting approach was an up-scaling of micro-structural data to the km scale. As was discussed after his talk this is also of great interest to the ice modellers in order to merge the microstructure with the large-scale model.

Frank Wendler from Karlsruhe showed „Phase field models of ice crystals“ and illustrated how the growth and dissolution of crystals can be modelled. Most interesting where his approaches to include complex kinematics and he showed clearly that the kinematics completely changed the growth process.

The final presentation was by Ryszard Staroszczyk from Poland on “Theoretical modelling of migration recrystallization in polar ice by using a micro-mechanical approach” showing how microstructures in ice can be modelled in small and large scale. This was again another approach based on micromechanics and dislocation glide.

3) Assessment of the results and impact of the event on the future direction of the field

In order to have an impact on the Earth Science community we will publish a special issue that came out of the workshop. The following is a list of papers that will be submitted to this issue by September 2011.

Journal of Structural Geology MicroDICE Meeting, Special issue

1.    Review of Natural Ice Microstructures (Sergio Faria, Ilka Weikusat et al.)
2.    Review of Ice Experiments (Chris Wilson, Montagnat, et la.)
3.    Review in Ice Microstructural Modelling (Griera Albert et al.)
4.    A new grain growth law for ice (Diebold, de Bresser, Durham and Stern)
5.    Modelling grain growth in ice in the presence of bubbles (Roessiger et al.)
6.    Interaction of Fracturing and Flow in Ice (Koehn et al.)
7.    Homogenization of bulk thermoelastic properties of rock and ice microstructures (Senthil Vel et al.)
8.    Simultaneous recording of microstructures and c-axis orientations in glacial and polycrystalline ice during 2D in situ deformation experiments (Peternell, Russel-Head and Wilson)
9.    Concentration of dust in cloudy bands by grain boundary migration (Bons, Schmatz, Roessinger, Koehn et al.)

Description of content: An understanding of the dynamic properties of ice in glaciers and ice sheets is of fundamental importance in the Earth Sciences and remains a major challenge. In order to understand the large-scale rheological behaviour of such systems it is important to realize that the microstructure on the grain and sub-grain scale and its dynamic change is a crucial input parameter for numerical models. Microstructural modelling of Earth Materials has become an important tool in the last years where techniques from Physics and Material Sciences are increasingly used in the Earth Science community. This special issue is a based on the outcomes of a workshop of the ESF Research Network Programme Micro-DICE, which was hold to discuss how we can reach a better understanding of the dynamic grain-scale processes in ice to understand how these processes influence the properties of ice and how predictive models can be constructed that are based on grain-scale ice dynamics. In order to give an introduction on microstructures in ice, experiments on microstructure development and numerical simulations we start the issue with three reviews, namely on natural ice microstructures, experiments and numerical simulations. These are followed by a variety of research papers focusing on ice related studies.

The results of the meeting were discussed in detail on the fourth day of the workshop. Numerous new partners emerge during the workshop, which is already reflected by the publications in the special issue.

Major questions that emerged from the workshop:

•    How important is the microstructure for the large-scale model? During the workshop we have seen numerous models on microstructure development and how microstructure in real ice look like. But how important is the microstructure really for the large-scale models and how can small scale models be used for an upscaling to large scale rheology?

•    How important is diffusion creep? Experiments and also some microstructural observations indicate that diffusion creep may be important (plus grain boundary sliding). However, we cannot model a combination of power law creep and dislocation movement with diffusion creep and grain boundary sliding. This should be a major aim in the future.

•    Is it enough to model ice deformation based on dislocation dynamics. We can only answer this question when we have developed full scale models that can deal with such a behaviour or if the experiments can clearly show that. However, it is crucial for large scale ice models to know if the underlying assumptions are correct.

•    How can we model transitions from brittle to ductile behaviour? A new research field that we will proceed is the combination of fracturing and flow in ice in order to model ice calving. This will be a combined approach from Glasgow and Grenoble.

•    How important are cloudy bands and how do they develop? Another interesting research field that we will follow up and that is also included in the special issue is the development of cloudy bands. We are currently working on the Elle environment in order to make these models possible.

4) Final programme of the meeting

Sunday, 8th of May,
Field trip and discussions

Monday, 9th of May
9:00 to 9:15 Introduction (Daniel Koehn)
9:15 to 9:30 The MicroDICE innovative (Paul Bons)
9:30 to 10:10 State of the art and open questions in ice microstructure research (Sergio Faria)
10:10 to 10:50 Deformation structures in natural ice (Ilka Weikusat, Sergio Faria)
10:50 to 11:30 Coffee and Discussion break
11:30 to 12:10 Modelling ice and rock deformation (Maurine Montagnat)
12:10 to 12:50 3D modelling of ice flow (Fabien Gillet-Chaulet)
13:00 to 14:00 Lunch
14:00 to 14:40 The Elle modelling environment (Paul Bons)
14:40 to 15:10 Modelling ice deformation microstructures (Jens Roessiger)
15:10 to 15:50 Coffee and Discussion break
15:50 to 16:20 Modelling physico-chemical interactions and hydrofracturing with Elle (Daniel Koehn)
16:20 to 17:00 FFT-methods to model ice deformation (Albert Griera)

Tuesday, 10th of May
9:00 to 9:40 The role of grain size in the flow of ice (background and hypothesis) (Hans de Bresser)
9:40 to 10:20 The role of grain size in the flow of ice (new experiments) (Sabrina Diebold)
10:20 to 11:00 Coffee and Discussion break
11:00 to 11:40 Ice deformation experiments (Mark Peternell)
11:40 to 12:00 Micromechanics of Permafrost thaw (Áine Ní Bhreasail)
12:00 to 12:20 Indentation of ice surfaces (Alexandra Seymour-Pierce)
12:30 to 13:30 Lunch
13:30 to 14:10 Multi-scale modelling, from molecular to FEM of copper and/or ice creams (Aitor Luque)
14:10 to 14:50 Modelling the bulk elastic properties and seismic anisotropy of polycrystalline materials (Senthil Vel)
14:50 to 15:30 Coffee and Discussion break
15:30 to 16:10 Phase field models of ice crystals (Frank Wendler)
16:10 to 17:00 Theoretical modelling of migration recrystallization in polar ice by using a micro-mechanical approach (Ryszard Staroszczyk)

Wednesday, 11th of May