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Seismicity at glaciers, ice sheets, and ice shelves provides observational constraint on a number of glaciological processes. Detecting and locating this seismicity, specifically icequakes, is a necessary first step in studying processes such as basal slip, crevassing, imaging ice fabric, and iceberg calving, for example. Most glacier deployments to date use conventional seismic networks, comprised of seismometers distributed over the entire area of interest. However, smaller-aperture seismic arrays can also be used, which are typically sensitive to seismicity distal from the array footprint and require a smaller number of instruments. Here, we investigate the potential of arrays and array-processing methods to detect and locate subsurface microseismicity at glaciers, benchmarking performance against conventional seismic-network-based methods for an example at an Antarctic ice stream. We also provide an array-processing recipe for body-wave cryoseismology applications. Results from an array and a network deployed at Rutford Ice Stream, Antarctica, show that arrays and networks both have strengths and weaknesses. Arrays can detect icequakes from further distances, whereas networks outperform arrays in more comprehensive studies of a particular process due to greater hypocentral constraint within the network extent. We also gain new insights into seismic behaviour at the Rutford Ice Stream. The array detects basal icequakes in what was previously interpreted to be an aseismic region of the bed, as well as new icequake observations downstream and at the ice stream shear margins, where it would be challenging to deploy instruments. Finally, we make some practical recommendations for future array deployments at glaciers. [ABSTRACT FROM AUTHOR]

Seismicity at glaciers, ice sheets and ice shelves provides observational constraint of a number of glaciological processes. Detecting and locating this seismicity, specifically icequakes, is a necessary first step in studying processes such as basal slip, crevassing, and imaging ice fabric, for example. Most glacier deployments to date use conventional seismic networks, comprised of seismometers distributed over the entire area of interest. However, smaller aperture seismic arrays can also be used, which are typically sensitive to seismicity distal from the array footprint and require a smaller number of instruments. Here, we investigate the potential of arrays and array-processing methods to detect and locate seismicity in the cryosphere, benchmarking performance against conventional seismic network-based methods. We also provide an array-processing recipe for cryosphere applications. Results from an array and network deployed at Rutford Ice Stream, Antarctica, show that arrays and networks both have strengths and weaknesses. Arrays can detect icequakes from further distances whereas networks outperform arrays for more comprehensive studies of a process within the network extent, due to greater hypocentral constraint and a smaller magnitude of completeness. We also gain new insights into seismic behaviour at Rutford Ice Stream. The array detects basal icequakes in what was previously interpreted to be an aseismic region of the bed, as well as new icequake observations at the ice stream shear-margins, where it would be challenging to deploy instruments. Finally, we make some practical recommendations for future array deployments at glaciers. [ABSTRACT FROM AUTHOR]

Firn densification profiles are an important parameter for ice‐sheet mass balance and palaeoclimate studies. One conventional method of investigating firn profiles is using seismic refraction surveys, but these are difficult to upscale to large‐area measurements. Distributed acoustic sensing (DAS) presents an opportunity for large‐scale seismic measurements of firn with dense spatial sampling and easy deployment, especially when seismic noise is used. We study the feasibility of seismic noise interferometry (SI) on DAS data for characterizing the firn layer at the Rutford Ice Stream, West Antarctica. Dominant seismic energy appears to come from anthropogenic noise and shear‐margin crevasses. The DAS cross‐correlation interferometry yields noisy Rayleigh wave signals. To overcome this, we present two strategies for cross‐correlations: (a) hybrid instruments—correlating a geophone with DAS, and (b) stacking of selected cross‐correlation panels picked in the tau‐p domain. These approaches are validated with results derived from an active survey. Using the retrieved Rayleigh wave dispersion curve, we inverted for a high‐resolution 1D S‐wave velocity profile down to a depth of 100 m. The profile shows a "kink" (velocity gradient inflection) at ∼12 m depth, resulting from a change of compaction mechanism. A triangular DAS array is used to investigate directional variation in velocity, which shows no evident variations thus suggesting a lack of azimuthal anisotropy in the firn. Our results demonstrate the potential of using DAS and SI to image the near‐surface and present a new approach to derive S‐velocity profiles from surface wave inversion in firn studies. Plain Language Summary: The density distribution (density change with depth) over tens of meters at the top of a glacier is an important feature of ice‐sheet mass balance and palaeoclimate research. It can be estimated using the empirical relationship between density and seismic P‐wave velocity. The P‐wave velocity can be measured using a seismic refraction survey with geophones and active sources. However, refraction seismic surveys are expensive for measurements over large areas. Distributed Acoustic Sensing (DAS) using fiber optic cables to detect seismic waves is an emerging dense spatial sampling seismic acquisition technology. It can be used in conjunction with seismic noise cross‐correlation to make large‐scale measurements easier and cheaper than with conventional geophones. We investigate the feasibility of this approach on Rutford Ice Stream, West Antarctica, and propose two approaches to improve DAS seismic‐noise cross‐correlation results. Surface waves are retrieved by seismic noise cross‐correlation and are used to estimate the S‐wave velocity structure. Our S‐velocity profile resembles an independently measured P‐velocity in‐shape and presents a velocity gradient inflection—related to changes in the snow compaction mechanism. We show that DAS and seismic noise interferometry can be used for future firn measurements, but also more generally in studies of the near‐surface. Key Points: Distributed acoustic sensing (DAS) is used for the first time to derive the S‐wave velocity structure and anisotropy of the firn layer in AntarcticaDAS seismic interferograms are greatly improved through selective stacking and cross‐correlation with a geophoneOur method is suitable for large‐scale measurements and is feasible in the presence of ice lenses where refraction methods are inadequate [ABSTRACT FROM AUTHOR]

The accuracy of sea-level rise predictions is highly dependent on reliably understanding the subglacial environment beneath ice streams. Bedforms result from the interaction between ice and its substrate, and therefore have the potential to shed light on poorly understood basal conditions influencing ice dynamics. However, theoretical models of bedform formation are often based on observations from deglaciated areas or from sparsely-sampled geophysical surveys over glaciated regions. Here, we use high-resolution three- and two-dimensional radar and seismic data to reveal details of the initiation and evolution of a subglacial bedform beneath Rutford Ice Stream, West Antarctica. Radar surveys at 20 m and 50 m line spacing allow detailed imaging of bed topography, including a moat up to 55 m deep, surrounding the upstream end of a 50-m high and >18-km long bedform. Many models rely on either a topographical or a locally resistant seed point to initiate bedform formation. The bedform described here is mostly composed of soft sediment (porosity >0.3) and lacks a boulder or outcrop, suggesting the bedform initiated without a topographic seed point. Sediment at the upstream end of bedforms appears stiffer over a distance of 2.3 km. We suggest sediment inhomogeneities in the initially flat bed cause the deposition of sediment, which, assuming resistant enough, acts as a seed point for bedform extension and moat erosion. The moat's geometry and its truncation of other bedforms suggest that it was eroded after the deposition of surrounding bedforms. These observations from a modern ice stream deliver information of subglacial processes involved in the initiation as well as in situ high-resolution topography and properties of bedforms and moats. Using these observations numerical models can be tested and developed accordingly. • Evidence for a bedform initiating without pre-existing topographic perturbations. • A moat around an elongated bedform revealed from high-resolution radar data. • Constraints of bedform formation mechanism from a modern ice stream. [ABSTRACT FROM AUTHOR]

Observations of surface lowering on Larsen C Ice Shelf (LCIS), Antarctica, have prompted concern about its stability. In this study, an ocean model is used to investigate the extent to which changes in ocean forcing may have influenced ice loss and the distribution of stabilizing marine ice beneath LCIS. The model uses a new bathymetry, containing a southern seabed trough discovered using seismic observations. The modeled extent of marine ice, thought to stabilize LCIS, is in good agreement with observations. Experiments applying idealized ocean warming yield an increase in melting over the southern trough. This is inconsistent with lowering observed in northern LCIS, suggesting oceanic forcing is not responsible for that signal. The marine ice extent and thickness reduces significantly under ocean warming, implying a high sensitivity of LCIS stability to changes in ocean forcing. This result could have wide implications for other cold‐water ice shelves around Antarctica. Plain Language Summary: Satellite observations have revealed a lowering in recent decades of the surface of Larsen C Ice Shelf (LCIS), Antarctica, which has led to concern about its stability. By modeling ocean conditions under LCIS, we investigate the extent to which ocean melting may have caused the ice to thin, leading to the observed lowering, or altered the pattern of marine ice beneath LCIS. Marine ice forms when seawater freezes to the base of the ice shelf, and is thought to stabilize LCIS. The model uses a new seabed data set that contains a wide, deep seabed trough in the south, found by a seismic survey. In modeled ocean warming experiments, an increase in melting is concentrated in this southern region. However, greater lowering has been observed in the north, suggesting that changes in ocean conditions are not responsible for the lowering. The calculated pattern of marine ice at the base of LCIS looks similar to the observed pattern. With a warmer ocean, marine ice is significantly reduced in crucial regions of the ice shelf. This shows that the stability of LCIS is sensitive to changes in ocean conditions and other ice shelves around Antarctica are likely sensitive to these changes too. Key Points: An ocean model with new Larsen C bathymetry shows greatest melting and melt sensitivity where seismic data indicate a deep southern troughThe calculated marine ice distribution under Larsen C Ice Shelf, based on model melt/freeze rates, is in good agreement with observationsA reduction in marine ice with ocean warming implies a threat to Larsen C stability, with wide implications for cold‐water ice shelves [ABSTRACT FROM AUTHOR]

Basal properties beneath ice streams and glaciers are known to be a control for ice flow dynamics, hence knowledge of them is crucial for predicting sea level due to changes in glacial dynamics. Basal properties, processes and topography also drive the formation of subglacial landforms. Bed properties beneath Rutford Ice Stream (West Antarctica) have previously been described using seismic acoustic impedance measurements at a sparse spatial coverage. Here, we derive bed properties in a 15 × 17 km grid of surface radar data with coverage and sampling much higher than previous seismic studies. Bed reflection amplitudes in surface radar data were calibrated using sediment porosities (ranging from 0.4–0.5) derived from seismic acoustic impedance. We find the bed properties are spatially variable, consisting of low porosity material in some areas and soft sediment in other areas. Comparison of seismic and surface radar data imply the low porosity material to be a consolidated sediment or sedimentary rock. Mega‐scale glacial lineations (MSGLs) are ubiquitous on the bed and consist of soft, high porosity, probably deforming sediment, consistent with previous interpretations of MSGLs. We find some MSGLs have high reflectivity on their crest, interpreted as water bodies overlying high porosity sediment, whereas the trough around and the upstream end of some landforms consist of low porosity material. Integrating these different observations, we place constraints on possible explanations for the occurrence of water on the crest of landforms. Plain Language Summary: Physical properties of rocks and sediments as well as the presence of water beneath ice streams and glaciers control the flow of the ice. Understanding glacial flow is important to better constrain predictions of sea‐level changes. Within this study we present physical properties of material beneath Rutford Ice Stream in West Antarctica over a 15 × 17 km area. Our data show areas of high porosity interpreted as wet sediment and areas of low porosity beneath the ∼2 km thick ice stream. Ice movement above areas of soft sediment is assumed to be maintained by sediment deformation, whereas in areas overlain by low porosity material, sliding of the ice over this material is assumed to maintain flow. The topography beneath Rutford Ice Stream shows many elongated landforms, some over 14 km long. We show that these landforms partly consist of soft sediment. Some of these seem to have a water layer on their crest, but it is not well understood why. However, there might be variations in the sediment composition of the landforms enabling the creation of this water layer. The occurrence of materials with different properties and water show us how heterogenous the bed beneath ice streams can be, which is important for modeling ice stream motion. Key Points: Glacier bed properties (water content, porosity) are constrained using surface radar data calibrated with seismic acoustic impedanceResults show sequences of low porosity material (consolidated sediment or rock), soft sediment and free‐water beneath Rutford Ice StreamMega‐scale glacial lineations are pervasive beneath ice streams, some with a free water layer >10 km long and 100 m wide along their crest [ABSTRACT FROM AUTHOR]

Antarctic ice sheet history is imprinted in the structure and fabric of the ice column. At ice rises, the signature of ice flow history is preserved due to the low strain rates inherent at these independent ice flow centres. We present results from a distributed acoustic sensing (DAS) experiment at Skytrain Ice Rise in the Weddell Sea sector of West Antarctica, aimed at delineating the englacial fabric to improve our understanding of ice sheet history in the region. This pilot experiment demonstrates the feasibility of an innovative technique to delineate ice rise structure. Both direct and reflected P- and S-wave energy, as well as surface wave energy, are observed using a range of source offsets, i.e. a walkaway vertical seismic profile, recorded using fibre optic cable. Significant noise, which results from the cable hanging untethered in the borehole, is modelled and suppressed at the processing stage. At greater depth where the cable is suspended in drilling fluid, seismic interval velocities and attenuation are measured. Vertical P-wave velocities are high (VINT=3984±218 m s -1) and consistent with a strong vertical cluster fabric. Seismic attenuation is high (QINT=75±12) and inconsistent with previous observations in ice sheets over this temperature range. The signal level is too low, and the noise level too high, to undertake analysis of englacial fabric variability. However, modelling of P- and S-wave travel times and amplitudes with a range of fabric geometries, combined with these measurements, demonstrates the capacity of the DAS method to discriminate englacial fabric distribution. From this pilot study we make a number of recommendations for future experiments aimed at quantifying englacial fabric to improve our understanding of recent ice sheet history. [ABSTRACT FROM AUTHOR]

Microseismicity, induced by the sliding of a glacier over its bed, can be used to characterize frictional properties of the ice‐bed interface, which are a key parameter controlling ice stream flow. We use naturally occurring seismicity to monitor spatiotemporally varying bed properties at Rutford Ice Stream, West Antarctica. We locate 230,000 micro‐earthquakes with local magnitudes from −2.0 to −0.3 using 90 days of recordings from a 35‐station seismic network located ∼40 km upstream of the grounding line. Events exclusively occur near the ice‐bed interface and indicate predominantly flow‐parallel stick‐slip. They mostly lie within a region of interpreted stiff till and along the likely stiffer part of mega‐scale glacial lineations. Within these regions, micro‐earthquakes occur in spatially (<100 m radius) and temporally (mostly 1–5 days activity) restricted event‐clusters (up to 4,000 events), which exhibit an increase, followed by a decrease, in event magnitude with time. This may indicate event triggering once activity is initiated. Although ocean tides modulate the surface ice flow velocity, we observe little periodic variation in overall event frequency over time and conclude that water content, bed topography and stiffness are the major factors controlling microseismicity. Based on variable rupture mechanisms and spatiotemporal characteristics, we suggest the event‐clusters relate to three end‐member types of bed deformation: (1) continuous creation and seismogenic destruction of small‐scale bed‐roughness, (2) ploughed clasts, and (3) flow‐oblique deformation during landform formation or along bedrock outcrops. This indicates that multiple processes, simultaneously active during glacial sliding, can accommodate stick‐slip behavior and that the bed continuously reorganizes. Key Points: We locate 230,000 micro‐earthquakes, with rupture mechanisms, at the base of a fast‐flowing West Antarctic ice stream within a 3‐month periodEvent distribution is little affected by tidal modulations and indicates basal sliding most affected by bed topography, stiffness, and fluidsEvents occur clustered, likely due to different types of bed deformation: mobile asperities, ploughed clasts and flow‐oblique bed features [ABSTRACT FROM AUTHOR]

Subglacial water plays an important role in ice sheet dynamics and stability. Subglacial lakes are often located at the onset of ice streams and have been hypothesised to enhance ice flow downstream by lubricating the ice–bed interface. The most recent subglacial-lake inventory of Antarctica mapped nearly 400 lakes, of which ∼ 14 % are found in West Antarctica. Despite the potential importance of subglacial water for ice dynamics, there is a lack of detailed subglacial-water characterisation in West Antarctica. Using radio-echo sounding data, we analyse the ice–bed interface to detect subglacial lakes. We report 33 previously uncharted subglacial lakes and present a systematic analysis of their physical properties. This represents a ∼ 40 % increase in subglacial lakes in West Antarctica. Additionally, a new digital elevation model of basal topography of the Ellsworth Subglacial Highlands was built and used to create a hydropotential model to simulate the subglacial hydrological network. This allows us to characterise basal hydrology, determine subglacial water catchments and assess their connectivity. We show that the simulated subglacial hydrological catchments of the Rutford Ice Stream, Pine Island Glacier and Thwaites Glacier do not correspond to their ice surface catchments. [ABSTRACT FROM AUTHOR]

In recent decades, rapid ice-shelf disintegration along the Antarctic Peninsula has had a global impact through enhancing outlet glacier flow, and hence sea level rise, and the freshening of Antarctic Bottom Water. Ice shelf thinning due to basal melting results from the circulation of relatively warm water in the underlying ocean cavity. However, the effect of sub-shelf circulation on future ice-shelf stability cannot be predicted accurately with computer simulations if the geometry of the ice-shelf cavity is unknown. To address this deficit for Larsen C Ice Shelf, west Antarctica, we integrate new water-column thickness measurements with existing observations. We present these new data here along with an updated bathymetry grid of the ocean cavity. Key findings include relatively deep seabed to the south-east of the Kenyon Peninsula, along the grounding line and around the key ice shelf pinning point of Bawden Ice Rise. In addition, we can confirm that the cavity’s southern trough stretches from Mobiloil Inlet to the open ocean. These areas of deep seabed will influence ocean circulation and tidal mixing, and will therefore affect the basal-melt distribution. These results will help constrain models of ice-shelf cavity circulation with the aim of improving our understanding of sub-shelf processes and their potential influence on ice shelf stability. The data set comprises all point measurements of seabed depth and a gridded data product, derived using additional measurements of both offshore seabed depth and the thickness of grounded ice. We present all new depth measurements here as well as a compilation of previously published measurements used in the gridding process. The gridded data product is included in the supplementary material. [ABSTRACT FROM AUTHOR]

The retreating Pine Island Glacier (PIG), West Antarctica, presently contributes ~5-10% of global sea-level rise. PIG's retreat rate has increased in recent decades with associated thinning migrating upstream into tributaries feeding the main glacier trunk. To project future change requires modelling that includes robust parameterisation of basal traction, the resistance to ice flow at the bed. However, most ice-sheet models estimate basal traction from satellite-derived surface velocity, without a priori knowledge of the key processes from which it is derived, namely friction at the ice-bed interface and form drag, and the resistance to ice flow that arises as ice deforms to negotiate bed topography. Here, we present highresolution maps, acquired using ice-penetrating radar, of the bed topography across parts of PIG. Contrary to lower-resolution data currently used for ice-sheet models, these data show a contrasting topography across the ice-bed interface. We show that these diverse subglacial landscapes have an impact on ice flow, and present a challenge for modelling ice-sheet evolution and projecting global sea-level rise from ice-sheet loss. [ABSTRACT FROM AUTHOR]

Ice is one of the most anisotropic natural materials but it is common knowledge in our community that anisotropy in ice is: a) a second order factor that can be ignored, b) easily assimilated into isotropic models by tuning certain parameters, or c) so complicated that it is better to ignore. Interestingly, any measurement of anisotropy in polar ice shows that ice develops crystalline preferred-orientation fabrics that produce strong mechanical, optical and dielectric anisotropic properties. Here, we look in detail at polarimetric radar measurements in different regions of East and West Antarctica. We use a ground-based phase-sensitive frequency-modulated continuous-wave radar (ApRES) and a matrix-based model to study the radio-waves depolarization and anisotropic scattering (Fujita et al, 2006). We find that anisotropy is widespread and an excellent archive of past ice flow conditions. We use it here to extract details of the recent deglaciation of the Ross and Ronny ice shelves over the last thousands of years. Also, because the length of the record is related to the advection time of the ice flow, we use our method to show that a candidate ice core location near Dome C, for the Beyond Epica Oldest Ice Core project, has been under steady ice-flow conditions over the last million years or so. [ABSTRACT FROM AUTHOR]

Acoustic impedance is a measurable quantity that can be recovered using active seismic surveys. Its value depends on the speed of sound in the sediment and the sediment density. For granular sediments there are theories that relate the speed of sound to the effective pressure, the difference between ice overburden and subglacial water pressure. This provides a link between the seismic observable and a key control on basal sliding. For sediments that follow a Coulomb friction law, the higher the effective pressure the higher the shear stress supported by the sediments. In this presentation we investigate the links between acoustic impedance, effective pressure and basal drag that are predicted by theories of the speed of sound in granular materials. We use results for basal drag from an ice flow model inversion in Antarctica to predict how acoustic impedance would vary under the hypothesis that sliding is controlled by a Coulomb sliding law. [ABSTRACT FROM AUTHOR]