Determination of crystal orientation fabric from seismic wideangle data

It is known from ice core analyses that the crystal orientation fabric (COF) of ice sheets is anisotropic and changes over depth. A better understanding of these anisotropies as well as their remote detection is important to optimize flow models for ice. Here we show how seismic wideangle measurements can be used to determine the COF remotely. We demonstrate the principle formalism how observed seismic traveltimes can be related to COF properties by a forward model and then apply the formalism to field data. The eigenvalues that describe the ice fabric of the ice core EDML (Dronning Maud Land, Antarctca) are set into a relationship with the elasticity tensor. From the elasticity tensor the expected seismic velocities and reflection coefficients are calculated. Additionally we calculate the value eta from the Thomsen-parameters epsilon and delta. The value eta gives a measure of the anisotropy of vertical transverse isotropic (VTI)-media and is an important tool for the NMO-correction of anisotropic data. The approximation of reflection horizons as hyperbolas is not valid anymore in anisotropic media. The calculation of the moveout is therefore performed by a 4th order NMO-correction with the rms-velocity and the effective eta value as variables. This approach is applied to data from a wideangle survey shot at Halvfarryggen, Dronning Maud Land, Antarctica. From this data we derived rms-velocities and effective eta values. These values were than recalculated to interval velocities and interval eta values to give a hint on the measure of anisotropy of the different layers. The results give first insight into the anisotropies at Halvfarryggen

"Basal conditions of Kongsvegen at the onset of surge - revealed using seismic vibroseis surveys" in the IASC Workshop on the dynamics and mass budget of Arctic glaciers - Abstracts and program booklet.

Kongsvegen is a well-studied surge-type glacier in the Kongsfjord area of northwest Svalbard. Long-term monitoring has shown that the ice surface velocity has been increasing for the past 4 years; presenting a unique opportunity to study the internal ice structure, basal conditions and thermal regime that play a crucial role in initiating glacier surges. In April 2019, three-component seismic vibroseis surveys were conducted at two sites on the glacier, using a small Electrodynamic Vibrator source (ElViS). Site 1 is in the ablation area and site 2 near the equilibrium line, where the greatest increase in ice surface velocity has been observed. Initial analysis indicates the conditions at the two sites are significantly different. At site 1 the ice is around 220 m thick, sitting on a relatively flat and uniform bed, with no clear change in the bed reflection along the profile. There is a horizontally layered sediment package ∼60 m thick underlaying the bed. The ice column shows no internal layering. By contrast at site 2 (Fig. 1), where the ice is around 390 m thick, there is much more complex internal ice structure. Clear internal ice reflections are visible between 150-250 m depth – where we expect a transition from cold to temperate ice. Further reflections in the 100 m above the bed indicate there could be shearing or sediment entrainment in this area. Below the bed, cross-cutting layers are clearly visible and the bed reflection itself shows changing reflection polarity – suggesting water or very wet sediment is present in some areas. This suggests ice movement by basal sliding and shearing is likely

Sediment features at the grounding zone and beneath Ekström Ice Shelf, East Antarctica, imaged using on-ice vibroseis

The grounding zone, where an ice sheet becomes a floating ice shelf, is known to be a key threshold region for ice flow and stability. A better understanding of ice dynamics and sediment transport across such zones will improve knowledge about contemporary and palaeo ice flow, as well as past ice extent. Here we present a set of seismic reflection profiles crossing the grounding zone and continuing to the shelf edge of Ekström Ice Shelf, East Antarctica. Using an on-ice vibroseis source combined with a snowstreamer we have imaged a range of sub-glacial and sub-shelf sedimentary and geomorphological features; from layered sediment deposits to elongated flow features. The acoustic properties of the features as well as their morphology allow us to draw conclusions as to their material properties and origin. These results will eventually be integrated with numerical models of ice dynamics to quantify past and present interactions between ice and the solid Earth in East Antarctica; leading to a better understanding of future contributions of this region to sea-level rise

On-ice vibroseis and snowstreamer systems for geoscientific research

We present implementations of vibroseis system configurations with a snowstreamer for over-ice long-distance seismic traverses (>100 km). The configurations have been evaluated in Antarctica on ice sheet and ice shelf areas in the period 2010–2014. We discuss results of two different vibroseis sources: Failing Y-1100 on skis with a peak force of 120 kN in the frequency range 10–110 Hz; IVI EnviroVibe with a nominal peak force of 66 kN in the nominal frequency range 10–300 Hz. All measurements used a well-established 60 channel 1.5 km snowstreamer for the recording. Employed forces during sweeps were limited to less than 80% of the peak force. Maximum sweep frequencies, with a typical duration of 10 s, were 100 and 250 Hz for the Failing and EnviroVibe, respectively. Three different concepts for source movement were employed: the Failing vibrator was mounted with wheels on skis and pulled by a Pistenbully snow tractor. The EnviroVibe was operated self-propelled on Mattracks on the Antarctic plateau. This lead to difficulties in soft snow. For later implementations the EnviroVibe with tracks was put on a polyethylene (PE) sled. The sled had a hole in the center to lower the vibrator baseplate directly onto the snow surface. With the latter setup, data production varied between 20 km/day for 6-fold and 40 km/day for single fold for 9 h/day of measurements. The combination of tracks with the PE-sled was especially advantageous on hard and rough surfaces because of the flexibility of each component and the relatively lose mounting. The systems presented here are suitable to obtain data of subglacial and sub-seabed sediment layers and englacial layering in comparable quality as obtained from marine geophysics and land-based explosive surveys. The large offset aperture of the streamer overcomes limitations of radar systems for imaging of steep along-track subglacial topography. With joint international scientific and logistic efforts, large-scale mapping of Antarctica's and Greenland's subglacial geology, ice-shelf cavity geometries and sea-bed strata, as well as englacial structures can be achieved

Detecting the subglacial conditions at Store Glacier, West Greenland, using a combined seismic-radar survey

As part of the research project RESPONDER, we performed two combined radar-seismic surveys to identify the bed conditions and suitable drilling locations at Store Glacier, a marine-terminating glacier in West Greenland. The two sites at 30 (Low Site) and 60 km (High Site) upstream of the snout of the glacier are thought to be part of the same subglacial drainage system but have different conditions both at the surface and at the base. As the ice-bed contact in the seismic data was sometimes difficult to identify we used the radar (Ground Penetrating Radar) data for confirmation. At the Low Site in the ablation zone, the surface is icy and crevassed. The five 2 to 3 km long seismic profiles show a large subglacial trench (width 2 km, depth 350 m) orientated in flow direction. The basal conditions vary with patches water, whether or not present in saturated sediments or exclusively at the base, both at the along-flow and across-flow profiles but they appear mainly at the sloping sides of the trench. The NE side of the trench contains a 100 to 150 m thick stratified sequence of softer, less consolidated sediments. At the High Site at equilibrium line, the surface is snowy with two frozen supra-glacial lakes. The two seismic profiles show less topography but have a similar patchy character. Despite thicker ice the ice-bed contact is much clearer visible in the seismic data which we contribute to a better coupled snowstreamer. The 5 km along-flowprofile has a flat base consisting of sediments. A clear single englacial reflection following the shape of the base can be seen at 85% depth of the ice column, possibly the Holocene-Wisconsin transition. At the 1.7 km across-flow profile there is a 130 m rise of the bed from S to N. Judging by the strength of the basal reflection the sediments at the northern side are softer then at the southern side

On the evolution of an ice shelf melt channel at the base of Filchner Ice Shelf, from observations and viscoelastic modeling

Ice shelves play a key role in the stability of the Antarctic Ice Sheet due to their buttressing effect. A loss of buttressing as a result of increased basal melting or ice shelf disintegration will lead to increased ice discharge. Some ice shelves exhibit channels at the base that are not yet fully understood. In this study, we present in situ melt rates of a channel which is up to 330 m high and located in the southern Filchner Ice Shelf. Maximum observed melt rates are 2 m yr−1. Melt rates inside the channel decrease in the direction of ice flow and turn to freezing ∼55 km downstream of the grounding line. While closer to the grounding line melt rates are higher within the channel than outside, this relationship reverses further downstream. Comparing the modeled evolution of this channel under present-day climate conditions over 250 years with its present geometry reveals a mismatch. Melt rates twice as large as the present-day values are required to fit the observed geometry. In contrast, forcing the model with present-day melt rates results in a closure of the channel, which contradicts observations. The ice shelf experiences strong tidal variability in vertical strain rates at the measured site, and discrete pulses of increased melting occurred throughout the measurement period. The type of melt channel in this study diminishes in height with distance from the grounding line and is hence not a destabilizing factor for ice shelves.</p

Comparison of elastic moduli from seismic diving-wave and ice-core microstructure analysis in Antarctic polar firn

We compared elastic moduli in polar firn derived from diving wave refraction seismic velocity analysis, firn-core density measurements and microstructure modelling based on firn-core data. The seismic data were obtained with a small electrodynamic vibrator source near Kohnen Station, East Antarctica. The analysis of diving waves resulted in velocity–depth profiles for different wave types (P-, SH- and SV-waves). Dynamic elastic moduli of firn were derived by combining P- and S-wave velocities and densities obtained from firn-core measurements. The structural finite-element method (FEM) was used to calculate the components of the elastic tensor from firn microstructure derived from X-ray tomography of firn-core samples at depths of 10, 42, 71 and 99 m, providing static elastic moduli. Shear and bulk moduli range from 0.39 to 2.42 GPa and 0.68 to 2.42 GPa, respectively. The elastic moduli from seismic observations and the structural FEM agree within 8.5% for the deepest achieved values at a depth of 71 m, and are within the uncertainty range. Our observations demonstrate that the elastic moduli of the firn can be consistently obtained from two independent methods which are based on dynamic (seismic) and static (tomography and FEM) observations, respectively, for deeper layers in the firn below ∼10 m depth

Comparison of elastic moduli from seismic diving-wave and ice-core microstructure analysis in Antarctic polar firn

The densification of firn depends on the elastic properties of firn, processes which are still not fully explained by the usual models. Geophysical methods provide spatially distributed data, while the analysis of firn cores is restricted to finite locations, but with a different vertical resolution. In this study, we compared elastic moduli in polar firn derived from refraction seismic velocity analysis and vertical density profiles from the firn-core measurements to elastic properties derived from microstructure modelling based on firn-core data. The seismic data were obtained with a small electrodynamic vibrator source (ElViS) near Kohnen Station, East Antarctica. The analysis of divingwaves resulted in velocity–depth profiles for P-, SH- and SV-wave velocities. Elastic moduli of firn were derived by combining P- and S-wave velocities and densities obtained from firn-core measurements. P-wave velocities derived from diving-wave analysis range from 2060 m s−1at 10 m depth to 3400 m s−1at 70 m depth, S-wave velocities from 1250 m s−1 to 1700 m s−1, respectively. The structural finite-element method (FEM) was used to calculate the components of the elastic tensor from firn microstructure derivedfrom X-ray tomography of firn-core samples at depths of 10, 42, 71 and 99 m. Shear and bulk moduli range from 0.39 GPa to 2.42 GPa and 0.68 GPa to 2.42 GPa, respectively. The elastic moduli from seismic observations and the structural FEM agree within 8.5% for the values derived at a depth of 71 m, and are within the uncertainty range. Our study demonstrates that elastic moduli of firn can be consistently obtained from two independent methods, which are based on dynamic (seismic) and static (tomography and FEM) observations, respectively. The agreement of the results for both methods indicates that elastic properties in firn can be acquired as spatially distributed data with the seismic approach, supported by local density information. Thus, information about elastic properties can be derived over larger lateral distances than would be possible with the static method. This enables the analysis of the firn and conclusions of the densification models might be drawn from observations of spatial and temporal changes in elastic properties