ABSTRACT Bakaninbreen, a 17-km long glacier in southern Svalbard, underwent a prolonged surge during 1985--95. The surge front, ˜50 m high, propagated progressively downstream, and by 1995 had halted ˜1.7 km from the glacier margin.... more
ABSTRACT Bakaninbreen, a 17-km long glacier in southern Svalbard, underwent a prolonged surge during 1985--95. The surge front, ˜50 m high, propagated progressively downstream, and by 1995 had halted ˜1.7 km from the glacier margin. Ground-penetrating radar (GPR) surveys located just downstream of the front suggested that ice and bed were at temperatures below the pressure melting point; surveys located just upstream of the surge front showed warm ice at the bed (at the pressure melting point), and zones of scattering at the sides of the glacier interpreted as warm ice in shear zones. These results have been used to suggest that the surge involved a thermally regulated soft bed mechanism, with the rate of propagation of the front controlled by friction at the margins and the rate of warming of the bed. However, the interpretation at Bakaninbreen was based on GPR and seismic surveys located within 1 km of the surge front. GPR surveys further upstream were conducted to identify spatial variation in the structure and thickness of the scattering zones; to image any marginal shear zones; to attempt to differentiate between water and sediment inclusions as causes for the scattering; and to assess the water content of any warm ice. Twenty-seven 100 MHz crossglacier common offset (CO) radar lines were collected, typically 1 km in length, as well as shorter multi-frequency and multi-polarisation CO and common midpoint (CMP) surveys. The distribution of scattering does not simply reflect shear margins at the sides of the glacier, nor is it confined to the areas of thickest ice. Two unexpected attributes of the regions are: (i) their distribution changes markedly over relatively short distances up and downstream ( ˜200 m); (ii) some are isolated both from the glacier bed and the surface. The CMP surveys show the water content of the ice would be ˜3-4% dependent on the inclusion model used. Warm ice could result from (1) strain heating during fast flow; (2) water ``injection'' from basal or surface crevasses; or (3) changes in the melting point with depth. Sediment can be added to the ice from the glacier bed or from surface sources. We assess which of these best explains the distribution of scattering within the glacier.
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Interactions between glaciers and the ocean are key for understanding the dynamics of the cryosphere in the climate system. Here we investigate the role of hydrostatic forces in glacier calving. We develop a mathematical model to account... more
Interactions between glaciers and the ocean are key for understanding the dynamics of the cryosphere in the climate system. Here we investigate the role of hydrostatic forces in glacier calving. We develop a mathematical model to account for the elastic deformation of glaciers in response to three effects: (i) marine and lake‐terminating glaciers tend to enter water with a nonzero slope, resulting in upward flexure around the grounding line; (ii) horizontal pressure imbalances at the terminus are known to cause hydrostatic in‐plane stresses and downward acting torque; (iii) submerged ice protrusions at the glacier front may induce additional buoyancy forces that can cause calving. Our model provides theoretical estimates of the importance of each effect and suggests geometric and material conditions under which a given glacier will calve from hydrostatic flexure. We find good agreement with observations. This work sheds light on the intricate processes involved in glacier calving an...
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Quantifying glacier contribution to sea-level rise is difficult due largely to a scarcity of long-term mass balance observations. Of more than 160,000 glaciers worldwide only about 40 have mass balance records longer than 20 years. We... more
Quantifying glacier contribution to sea-level rise is difficult due largely to a scarcity of long-term mass balance observations. Of more than 160,000 glaciers worldwide only about 40 have mass balance records longer than 20 years. We have developed a new method that will significantly increase the distribution and spatial/temporal resolution of mass balance records by combining historical aerial photographs and
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Research Interests: Geology and Reflectivity
ABSTRACT Calving glaciers have been identified as having a crucial role in the mass balance of the Greenland Ice Sheet, with acceleration and retreat of these glaciers resulting in major mass loss from the ice sheet interior, leading to a... more
ABSTRACT Calving glaciers have been identified as having a crucial role in the mass balance of the Greenland Ice Sheet, with acceleration and retreat of these glaciers resulting in major mass loss from the ice sheet interior, leading to a corresponding sea level rise. The ability to reproduce observed glacier behaviour in calving models is very desirable, but this is hindered by the difficulty of obtaining appropriate field measurements, combined with the complex interaction of the possible controls on iceberg calving. Our project brings together experts in glaciology, Global Navigation Satellite Systems (GNSS) technology and processing, and wireless networking, to design, install and operate a wireless network of GNSS sensors at the margin of a heavily crevassed Greenland outlet glacier. The network will provide velocity and elevation data of unprecedented resolution in time and space for the key marginal area of the glacier, where recent changes in glacier dynamics appear to have initiated. These will be analysed in conjunction with contemporaneous auxiliary data, such as surface and airborne lidar measurements of surface topography, crevasse spacing and calving rates, to yield new insights into processes active at the margins of tidewater glaciers. Our major field campaign will be in summer 2013, with a network of approximately 20 GNSS sensors being deployed, and a suite of ancillary data being collected in tandem. In preparation, we deployed a small test network of three GNSS sensors along the Helheim Glacier flowline in summer 2012, and here we present results from these sensors as a demonstration of the detail we expect to obtain in our main field season. The deployment of our GNSS sensors in summer 2012 coincided with a large calving event. We have no direct observations of this event; however, 250-500 m of ice was lost from the northern half of the calving front during the period 22-24th July, inferred from MODIS imagery. This retreat coincided with a significant glacial earthquake and increased noise in the fjord on the evening of 25th July giving a probable timing for the calving event. Our data shows a similar step change in glacier velocity to those demonstrated by previous authors, and also shows evidence of tidal signals in both the horizontal and vertical velocity components within 800 m of the calving front, suggesting that part of the glacier was at or very near flotation during this period.
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A series of surface ground-penetrating radar profiles at 100 MHz have been collected over an englacial channel system 'Crystal Cave' on the tidewater glacier, Hansbreen, Svalbard. The aim of the surveys was assess radar as a... more
A series of surface ground-penetrating radar profiles at 100 MHz have been collected over an englacial channel system 'Crystal Cave' on the tidewater glacier, Hansbreen, Svalbard. The aim of the surveys was assess radar as a method for determining the size and shape of the channel system and its fill, and to map inaccessible parts of the channel system. It is possible to descend into the main channels of the system from moulins along its course and a detailed map of the accessible regions has been made. This mapping allows ground-truthing of the radar interpretation. The channel system consists of multiple channels at different elevations. The channels generally have low gradient sections linked by near vertical shafts. In common with other channels in Svalbard glaciers, the channels probably initiated as a supraglacial features progressively downcutting into the ice and filling with compressed snow from above. We show that ground-penetrating radar can be successfully used...
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Differential radar interferometry has proven to be an excellent method to measure displacements associated with geophysical phenomena such as glacier flow, subsidence, tectonic plate motion, and earthquake displacement. Since the... more
Differential radar interferometry has proven to be an excellent method to measure displacements associated with geophysical phenomena such as glacier flow, subsidence, tectonic plate motion, and earthquake displacement. Since the technique utilizes the interferometric phase, it is limited in cases where large displacements in the slant range direction result in complete decorrelation. Similarly, if the surface deformation causes rotation of
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Research Interests: History, Botany, Earth Sciences, Geology, Glaciology, and 15 moreLaw, Climatology, Multidisciplinary, Ecology and Environment, Natural History Museum, Supervised Classification, Satellite Imagery, PLoS one, Glacier, Population Model, Glacier mass balance, Robust Regression, Ground Truth, Remote Sensing Image, and Breeding season
The rapid drainage of supraglacial lakes around the ablation zone of the Greenland Ice Sheet forms an important link between water at the surface and the ice sheet base, allowing surface meltwater to reach the bed and hence increase... more
The rapid drainage of supraglacial lakes around the ablation zone of the Greenland Ice Sheet forms an important link between water at the surface and the ice sheet base, allowing surface meltwater to reach the bed and hence increase glacial velocity. The conduits formed by lake drainages may remain open during the remainder of the melt season providing a pathway for further meltwater to reach the base. We investigated the drainage behavior of lakes from all regions of the Greenland Ice Sheet for the period 2005–2009. We mapped the evolution of 2600 lakes from 3704 MODIS images detecting a mean of 263 drainage events per year, of which 61% occurred in the south‐west region. Only 1% of lake drainages occurred in the rapidly thinning south‐east region. Our results show marked differences between the hydrology of the different regions of the ice sheet, with few lake drainages occurring in the regions where the highest dynamic mass loss is occurring. In the south‐west and north‐east, lak...
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A new implementation of a calving model, using the finite-element code Elmer, is presented and used to investigate the effects of surface water within crevasses on calving rate. For this work, we use a two-dimensional flowline model of... more
A new implementation of a calving model, using the finite-element code Elmer, is presented and used to investigate the effects of surface water within crevasses on calving rate. For this work, we use a two-dimensional flowline model of Columbia Glacier, Alaska. Using the glacier’s 1993 geometry as a starting point, we apply a crevasse-depth calving criterion, which predicts calving at the location where surface crevasses cross the waterline. Crevasse depth is calculated using the Nye formulation. We find that calving rate in such a regime is highly dependent on the depth of water in surface crevasses, with a change of just a few metres in water depth causing the glacier to change from advancing at a rate of 3.5 kma–1 to retreating at a rate of 1.9 km a–1. These results highlight the potential for atmospheric warming and surface meltwater to trigger glacier retreat, but also the difficulty of modelling calving rates, as crevasse water depth is difficult to determine either by measure...
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Synchronous acceleration and thinning of south-east (SE) Greenland glaciers during the early 2000s was the main contributor in causing the annual mass loss from the ice sheet to double. During the period 2003-2005, SE... more
Synchronous acceleration and thinning of south-east (SE) Greenland glaciers during the early 2000s was the main contributor in causing the annual mass loss from the ice sheet to double. During the period 2003-2005, SE Greenland's outlet glaciers were dramatically thinning, accelerating, and retreating. Then, in 2006, two of the largest outlet glaciers in the sector, Helheim and Kangerdlugssuaq, were reported
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The number of gigaton‐sized iceberg‐calving events occurring annually at Greenland glaciers is increasing, part of a larger trend of accelerating mass loss from the Greenland Ice Sheet. Though visual observation of large calving events is... more
The number of gigaton‐sized iceberg‐calving events occurring annually at Greenland glaciers is increasing, part of a larger trend of accelerating mass loss from the Greenland Ice Sheet. Though visual observation of large calving events is rare, ∼60 glacial earthquakes generated by these calving events are currently recorded each year by regional and global seismic stations. An empirical relationship between iceberg size and MCSF, a summary measure of glacial‐earthquake size, was recently demonstrated by Olsen and Nettles (2019), https://doi.org/10.1029/2019JF005054. However, MCSF is known to be sensitive to choices made in modeling the seismic source. We incorporate constraints on the seismic source from laboratory studies of calving and test multiple source time functions using synthetic and observed glacial‐earthquake waveforms. We find that a simple, fixed time function with a shape informed by laboratory results greatly improves estimates of earthquake size. The average ratio of...
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What happens beneath a glacier affects both the way it flows and the landforms left behind when it retreats. Unfortunately, although the subglacial environment is one of the most critical to understanding ice flow and the processes of... more
What happens beneath a glacier affects both the way it flows and the landforms left behind when it retreats. Unfortunately, although the subglacial environment is one of the most critical to understanding ice flow and the processes of bedform formation, it is also the most difficult to study. As part of the RABID project on Rutford Ice Stream, West Antarctica
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Interest is growing in the use of ground penetrating radar (GPR) methods for quantifying subsurface properties (e.g. porosity, water content) derived from EM wave interval velocity, vINT. This velocity is usually calculated from Dix's... more
Interest is growing in the use of ground penetrating radar (GPR) methods for quantifying subsurface properties (e.g. porosity, water content) derived from EM wave interval velocity, vINT. This velocity is usually calculated from Dix's Equation with velocity and time picks obtained from semblance analysis of reflection moveout times in common midpoint (CMP) data. However, this process leads to imprecision and
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Areas of clay-rich tills in the UK are attractive sites for landfills. The relatively impermeable clays act as a barrier between landfill contents and surrounding permeable materials, but tills often contain sand and gravel deposits,... more
Areas of clay-rich tills in the UK are attractive sites for landfills. The relatively impermeable clays act as a barrier between landfill contents and surrounding permeable materials, but tills often contain sand and gravel deposits, which may be water-bearing and/or hydraulically connected to aquifers or surface water bodies. The sands and gravels may be missed by borehole and trial-pit led
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ABSTRACT Models of ground penetrating radar (GPR) propagation velocity are often used to quantify physical subsurface properties (e.g., layer thickness, porosity, water content). A common approach is to conduct semblance analysis on... more
ABSTRACT Models of ground penetrating radar (GPR) propagation velocity are often used to quantify physical subsurface properties (e.g., layer thickness, porosity, water content). A common approach is to conduct semblance analysis on common midpoint (CMP) gathers, in which successive pairs of stacking velocity and travel-time are input to Dix's Equation, to obtain a velocity estimate, termed interval stacking velocity. However, the precision in interval stacking velocity is seldom reported since it is cumbersome to obtain an analytic expression for precision particularly since Dix's Equation has four independently resolved degrees of freedom. We present Monte Carlo simulations as a means of expressing interval stacking velocities, and its derivative quantities, as a probability density functions (PDFs). These are built using Gaussian distributions of pseudo-random samples from within the 50% contour around successive coherence responses; precision is summarised using the median and inter-quartile range of each PDF. To verify this method's accuracy, we simulate CMP data in which travel-times are computed for reflections from two horizontal horizons at depths of 5 and 10 m, with isotropic interval velocity of 0.1 m/ns, for source-receiver offsets from 0 to 30 m. The Monte Carlo simulation produces 107 manifestations of interval stacking velocity and layer thickness, and yields estimates of 0.1±0.003 m/ns and 4.99±0.16 m, respectively. The simulation is repeated for real CMP data, acquired with 50 MHz antennas, for establishing the precision in interval stacking velocity and speculative estimates (in the absence of extensive borehole logs) of layer thickness and fractional porosity (via the complex refractive index method, CRIM). The site comprises Quaternary sediment, in which the water table is observed at ~2 m depth, overlying Cambrian basement at 10-15 m depth. The resolution of stacking velocity in semblance analysis decreases with depth, as reflected in PDFs of interval stacking velocity and layer thickness. However, whilst interval stacking velocity for the shallowest (dry) medium is more precise than that of deeper (saturated) media (±0.9% and ±4.6%, respectively), it has the least precise porosity estimate (±8.0% vs. ±3.0%). This implies that estimates of porosity may be superior where water, rather than air, occupies pore spaces. Monte Carlo simulations are recommended as a convenient means of expressing the precision in GPR velocity, or any physical properties derived from it, particularly in the absence of extensive borehole control.
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ABSTRACT As part of a multi-disciplinary, multi-national project investigating the ice-dynamic implications of rapidly draining supraglacial lakes on the West Greenland Ice Sheet, we have conducted a series of seismic reflection... more
ABSTRACT As part of a multi-disciplinary, multi-national project investigating the ice-dynamic implications of rapidly draining supraglacial lakes on the West Greenland Ice Sheet, we have conducted a series of seismic reflection experiments immediately following the rapid drainage of Lake F in the land-terminating Russell Glacier catchment to [1] isolate the principal mode of basal motion, and [2] identify and characterise the modification of that mode as forced by ingress of surface-derived meltwaters. Lake F had a surface area of ~3.84 km2 and drained entirely in less than two hours at a maximum rate of ~ 3300 m3 s-1, marked by local ice extension and uplift of up to 1 m. Two seismic profiles (A and B) were acquired and optimised for amplitude versus angle (AVA) characterisation of the substrate. All seismic data were recorded with a Geometrics GEODE system, using 48 vertically-orientated 100-Hz geophones installed at 10 m intervals. 250 g pentalite charges were fired in shallow auger holes at 80 m intervals along each line, providing six-fold coverage. Profile A targets the subglacial hydrological basin into which the Lake-F waters drained, and reveals a uniform, flat glacier bed beneath ~1.3 km of ice, characterised by the presence of a very stiff till with an acoustic impedance of 4.17 ± 0.11 x 106 kg m-2 s1 and a Poisson's ratio of 0.06 ± 0.05. In profile B, to the southeast of Lake F in an isolated subglacial hydrological basin, ice thickness is 1.0-1.1 km and a discrete sedimentary basin is evident; within this feature, we interpret a stratified subglacial till deposit, having lodged till (acoustic impedance = 4.26 ± 0.59×106 kgm-2 s-1) underlying a water-saturated dilatant till layer (thickness
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ABSTRACT Retreat since the 1940s surge of Kongsvegen, Svalbard has exposed sediment structures on the glacier surface and along a 1 km long, 5--20 m high, grounded cliff section near the terminus. Glacier ice provides an almost ideal... more
ABSTRACT Retreat since the 1940s surge of Kongsvegen, Svalbard has exposed sediment structures on the glacier surface and along a 1 km long, 5--20 m high, grounded cliff section near the terminus. Glacier ice provides an almost ideal target for ground-penetrating radar (GPR) because low attenuation means that penetration up to 100s metres can be achieved. Reflections result from changes in the dielectric properties of the ice, typically due to changes in sediment or water content of the ice. Coincident GPR and real-time kinematic GPS data were collected on Kongsvegen by towing the instruments behind a moving snowscooter. Grids (~ 100 m x 50 m) of closely spaced common offset GPR lines (~ 0.25 m in line and 1 m between line spacing) were imaged over prominent englacial sediment features. The data were tied through a time stamp at each radar trace. The data have been 2-D interpolated at each time interval onto a regular grid to allow visualization. Prior to interpolation each line was despiked, and filtered to remove low frequency noise and horizontal banding resulting from ringing in the antennae. After interpolation the data were topographically corrected and migrated before visualization. The grids imaged two sub-horizontal reflectors. The upper reflector is strong and continuous, dips gently upglacier, and is interpreted to be the base of the glacier or the top of a basal ice layer. The lower reflector is discontinuous and is interpreted either as a thermal boundary (between frozen and unfrozen sediments) or as a décollement layer within the sedimentary bed material. Above the basal reflector a series of strong upglacier dipping faults were imaged, which do not cross the basal reflector. Weaker features occur between the basal reflector and the lower reflector. We interpret these structures in terms of the surge history of the Kongsvegen glacier complex.
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Over the past 70 years, many different components of the cryosphere have been imaged with a variety of radar systems using increasingly sophisticated processing techniques. These systems use various pulse lengths, signal frequencies and,... more
Over the past 70 years, many different components of the cryosphere have been imaged with a variety of radar systems using increasingly sophisticated processing techniques. These systems use various pulse lengths, signal frequencies and, in some cases, modulated signals. The increasing diversity of radar systems has created the potential for confusion due to the use of non-consistent terminology. Here we provide an overview of state-of-the-science radar technologies and suggest a simplified and unified terminology for use by the cryosphere community. We recommend a terminology that is target independent but specifies the characteristics of the signal. Following this recommendation, commercial impulse systems that penetrate the subsurface should be referred to as ground-penetrating radar (GPR), and pulse radars as radio-echo sounding (RES). Continuous-wave (CW) radar systems should be referred to as ground-penetrating CW radars. We further suggest any additional characterisation of t...
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Basal properties under ice streams and glaciers are known to control the flow dynamics of ice. Hence knowledge of them is crucial for making predictions of sea-level due to changes in glacial dynam...
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Repeatable and strong seismoelectric signals were recorded on Glacier de Tsanfleuron, Switzerland, using a vertical sounding geometry. Electromagnetic waves are inferred to be generated by electrokinetic conversion of seismic energy... more
Repeatable and strong seismoelectric signals were recorded on Glacier de Tsanfleuron, Switzerland, using a vertical sounding geometry. Electromagnetic waves are inferred to be generated by electrokinetic conversion of seismic energy within the snow pack and near the dry‐wet ice and ice‐bed interfaces. A simple gradient‐based scheme allows such electrokinetic interface responses (EIRs) to be isolated from noise. EIRs depend sensitively on the azimuthal orientation of the receiving array of electrical dipoles. Seismoelectric techniques promise to allow mapping even of thin water‐bearing strata within or beneath glaciers or frozen ground, estimation of hydraulic or fluid properties of such strata, as well as monitoring of ice fracturing or basal properties and processes at improved spatial resolution.