10 research outputs found
Investigating the internal structure of glaciers and ice sheets using Ground Penetrating Radar
Ice penetrating radar (IPR) is a key tool in understanding the internal geometry and nature of glaciers and ice sheets, and has widely been used to
derive bed topography, map internal layers and understand the thermal state
of the cryosphere. Modern glacier and ice-sheet models facilitate increased assimilation of observations of englacial structure, including glacier thermal state
and internal-layer geometry, yet the products available from radar surveys are
often under-utilised. This thesis presents the development and assessment of
radar processing strategies to improve quantitative retrievals from commonly
acquired radar data.
The first major focus of this thesis centres on deriving englacial velocities
from zero-offset IPR data. Water held within micro- and macro-scale pores
in ice has a direct influence on radar velocity, and significantly reduces ice
viscosity and hence impacts the long-term evolution of polythermal glaciers.
Knowledge of the radar velocity field is essential to retrieve correct bed topography from depth conversion processing, yet bed topography is often estimated assuming constant velocity, and potential errors from lateral variations
in the velocity field are neglected. Here I calculate the englacial radar velocity
field from common offset IPR data collected on Von Postbreen, a polythermal
glacier in Svalbard. I first extract the diffracted wavefield using local coherent
stacking, then use the focusing metric of negative entropy to deduce a local
migration velocity field from constant-velocity migration panels and produce
a glacier-wide model of local radar velocity. I show that this velocity field is
successful in differentiating between areas of cold and temperate ice and can
detect lateral variations in radar velocity close to the glacier bed. The effects of
this velocity field in both migration and depth-conversion of the bed reflection
are shown to result in consistently lower ice depths across the glacier, indicating that diffraction focusing and velocity estimation are crucial in retrieving
correct bed topography in the presence of temperate ice.
For the thesis’ second major component I undertake an assessment of automated techniques for tracing and interpreting ice-sheet internal stratigraphy. Radar surveys across ice sheets typically measure numerous englacial
layers that can be often be regarded as isochrones. Such layers are valuable for extrapolating age-depth relationships away from ice-core locations,
reconstructing palaeoaccumulation variability, and investigating past ice-sheet
dynamics. However, the use of englacial layers in Antarctica has been hampered by underdeveloped techniques for characterising layer continuity and
geometry over large distances, with techniques developed independently and
little opportunity for inter-comparison of results. In this paper, we present
a methodology to assess the performance of automated layer-tracking and
layer-dip-estimation algorithms through their ability to propagate a correct
age-depth model. We use this to assess isochrone-tracking techniques applied
to two test case datasets, selected from CreSIS MCoRDS data over Antarctica
from a range of environments including low-dip, continuous layers and layers
with terminations. We find that dip-estimation techniques are generally successful in tracking englacial dip but break down in the upper and lower regions
of the ice sheet. The results of testing two previously published layer-tracking
algorithms show that further development is required to attain a good constraint of age-depth relationship away from dated ice cores. I make the recommendation that auto-tracking techniques focus on improved linking of picked
stratigraphy across signal disruptions to enable accurate determination of the
Antarctic-wide age-depth structure.
The final aspect of the thesis focuses on Finite-Difference Time-Domain
(FDTD) modelling of IPR data. I present a sliced-3D approach to FDTD
modelling, whereby a thin 3D domain is used to replicate modelling of full 3D
polarisation while reducing computational cost. Sliced-3D modelling makes use
of perfectly matched layer (PML) boundary conditions, and requires tuning
of PML parameters to minimise non-physical reflections from the model-PML
interface. I investigate the frequency dependence of PML parameters, and
establish a relationship between complex frequency stretching parameters and
effective wavelength. The resultant parameter choice is shown to minimise
propagation errors in the context of a simple radioglaciological model, where
3D domains may be prohibitively large, and for a near-surface cross-borehole
survey configuration, a case where full waveform inversion may typically be
used
The desmin mutation R349P increases contractility and fragility of stem cell-generated muscle micro-tissues
Aims Desminopathies comprise hereditary myopathies and cardiomyopathies caused by mutations in the intermediate filament protein desmin that lead to severe and often lethal degeneration of striated muscle tissue. Animal and single cell studies hinted that this degeneration process is associated with massive ultrastructural defects correlating with increased susceptibility of the muscle to acute mechanical stress. The underlying mechanism of mechanical susceptibility, and how muscle degeneration develops over time, however, has remained elusive. Methods Here, we investigated the effect of a desmin mutation on the formation, differentiation, and contractile function of in vitro-engineered three-dimensional micro-tissues grown from muscle stem cells (satellite cells) isolated from heterozygous R349P desmin knock-in mice. Results Micro-tissues grown from desmin-mutated cells exhibited spontaneous unsynchronised contractions, higher contractile forces in response to electrical stimulation, and faster force recovery compared with tissues grown from wild-type cells. Within 1 week of culture, the majority of R349P desmin-mutated tissues disintegrated, whereas wild-type tissues remained intact over at least three weeks. Moreover, under tetanic stimulation lasting less than 5 s, desmin-mutated tissues partially or completely ruptured, whereas wild-type tissues did not display signs of damage. Conclusions Our results demonstrate that the progressive degeneration of desmin-mutated micro-tissues is closely linked to extracellular matrix fibre breakage associated with increased contractile forces and unevenly distributed tensile stress. This suggests that the age-related degeneration of skeletal and cardiac muscle in patients suffering from desminopathies may be similarly exacerbated by mechanical damage from high-intensity muscle contractions. We conclude that micro-tissues may provide a valuable tool for studying the organization of myocytes and the pathogenic mechanisms of myopathies