We adapt from volcano seismology an automated method of locating icequakes with poorly defined onsets and indistinguishable seismic phases, which can be tuned to either body or surface waves. The method involves (1) the calculation of the root-mean-squared amplitudes of the filtered envelope signals, (2) a coarse-grid search to locate the hypocentres of the seismic events using their amplitudes and (3) refinement of hypocentre locations using an iteratively damped least-squares approach. First, we calibrate the adapted method by application to real data, recorded using a network of six passive seismometers, in response to surface explosions in known locations on the western margin of the Greenland ice sheet. Second, we present a seismic modelling experiment simulating rapid supraglacial lake drainage driven hydrofracture through 1 km thick ice. The test reveals horizontal and vertical location uncertainties of ∼121 m and 275 m, respectively. Since seismic emissions from glaciers and ice sheets often have complex waveforms akin to those considered here, our adapted method is likely to have widespread applicability to glaciological problems.

High-resolution seismic reflection (HRSR) data acquired over the Pesotum Bedrock Valley in central Illinois, USA, helped construct the seismic stratigraphy of a valley fill and the overlying sediments. Integrating these data with drilling and borehole geophysics allowed us to develop a seismo-stratigraphic classification for sediments on undulating and folded bedrock. Seven seismo-stratigraphic units that overlie the bedrock surface were characterized. Seismic units A and B include glacial sediments of multiple Pleistocene glaciations above the Pesotum Bedrock Valley, which completely mask the feature. Seismic units C–F, the valley fill, primarily include tills and glacial lake sediment deposited during the earliest Pleistocene glaciations and preglacial alluvium and colluvium that is draped over in situ weathered bedrock. The preservation of conformable-lying glacial and preglacial deposits and paucity of sand and gravel in the buried valley strongly indicate that little or no incision by glacial meltwaters has occurred. These observations contrast markedly with interpretations from buried valleys elsewhere in North America and northern Europe where valley fills contain significant deposits of sand and gravel in tunnel valleys. The HRSR data assisted the characterization and analysis of heterogeneous sedimentary sequences over a buried valley where existing subsurface information was limited. The extent of Pleistocene-age glacial lakes is inferred from the lateral continuity of silt and clay units.

The electrical properties of water ice impact the study of diverse frozen environments, in particular the radar sounding of ice masses. The high-frequency response of meteoric polar ice depends partly on the bulk concentration of ammonium (NH4+), but the nature of this response has been unclear. Here we use broadband dielectric spectroscopy to investigate the electrical response of laboratory-frozen solutions. By analyzing the relaxation frequency of these samples and its temperature dependence, we show that the mobility of Bjerrum D-defects formed in the ice lattice by ammonium is 1.4 ±0.8 x 10–9m2 V–1 s–1 at -20°C, or about an order of magnitude smaller than that of Bjerrum L-defects formed by chloride. However, co-substitution of both ions increases the ice-lattice solubility of chloride by a factor of ∼7, causing an enhanced conductivity response due to greater concentrations of Bjerrum L-defects. Thus, despite its low mobility, ammonium can also affect the high-frequency electrical response of polar ice, but its covariance with chloride must be considered.

Pine Island Glacier (PIG), West Antarctica, has been experiencing acceleration in its flow speed and mass loss for nearly two decades, driven in part by an increase in the delivery of relatively warm Circumpolar Deep Water (CDW). However, at present, the configuration of the sub-ice-shelf cavity and bed conditions beneath the PIG ice shelf that dictate such oceanic influences remain poorly understood. Here, we use aerogravity data and ocean bottom depths measured by an autonomous underwater vehicle (AUV) to model the bathymetry and sediment layer thickness beneath the PIG ice shelf. Results reveal that the deep basins, previously found by AUV on both landward and seaward sides of a submarine ridge, extend substantially to the north and south. The water column thickness of the basins reaches 400-550 m on the landward side of the ridge and 500-600 m on the seaward side. The sediment layer covers the whole expanse of the seabed beneath the ice shelf, and the thickness is in the range ∼200-1000 m. The thinnest sediments (<200 m) are found on the seaward slope of the submarine ridge, suggesting that erosion by advancing ice may have been concentrated in the lee of the topographic high.

The porosity of a sea-ice pressure ridge keel is an important but poorly known variable, relevant for determining the mass budget and evolution of the Arctic sea-ice cover. Determination of keel porosity from drillholes is time-intensive and only yields limited information because of their limited lateral extent. Since the porosity within a keel equals its liquid water content, surface-nuclear magnetic resonance (surface-NMR) methods can be used to estimate porosity within such features. Surface-NMR tomography measurements were made in April 2011 using seven surface coil positions across a first-year pressure ridge on landfast sea ice near Barrow, Alaska, USA. The inversion results indicate water contents of 30 ± 7% and 40 ±10% in the ridge’s shallow and deep parts, respectively. These values are much higher than those obtained from drillholes, which are ∼10% and ∼27%, respectively. In contrast to drilling, surface-NMR tomography yields average porosity values for the entire subsurface volume. However, the inversion process is sensitive to the electrical conductivity distribution; uncertain conductivity estimates limit the reliability of the inverted water contents. Nevertheless, the results suggest that ridge porosities obtained from invasive measurements such as drilling may lead to substantially overestimated sea-ice volume.

Radio-echo sounding surveys over the Greenland ice sheet show clear, extensive internal layering, and comparisons with age–depth scales from deep ice cores allow for dating of the layering along the ice divide. We present one of the first attempts to extend the dated layers beyond the ice core drill sites by locating the depth of the Bølling–Allerød transition in >400 flight-lines using an automated fitting method. Results show that the transition is located in the upper one-third of the ice column in the central part of North Greenland, while the transition lowers towards the margin. This pattern mirrors the present surface accumulation, and also indicates that a substantial amount of pre-Holocene ice must be present in central North Greenland.

A method to estimate the flexural stiffness and effective elastic modulus of floating ice is described and analysed. The method is based on the analysis of water pressure records at two or three locations below the bottom of floating ice when flexural-gravity waves propagate through the ice. The relative errors in the calculations of the ice flexural stiffness and the water depth are analysed. The method is tested using data from field measurements in Tempelfjorden, Svalbard, where flexural-gravity waves were excited by an icefall at the front of the outflow glacier Tunabreen in February 2011.

This study uses boreholes, ground temperature monitoring and ground-penetrating radar (GPR) in order to understand the internal structure and composition of a rock glacier in the upper Choapa valley, northern Chile. The rock glacier is a small valley-side feature, 200 m long and ranging between 3710 and 3780 ma.s.l. Two boreholes were drilled down to depths of 20 and 25 m, respectively, using the diamond drillhole technique. An ice-rock mixture was encountered in the boreholes, with heterogeneous ice content averaging 15-30%. Data from common-midpoint (CMP) and constant-offset (CO) GPR surveys acquired, respectively, near the boreholes and across the whole rock glacier were processed to highlight the internal stratigraphy and variations in the radar-wave velocity. The GPR profiles depict a rock glacier constituted of stacked and generally concordant layers, with a thickness ranging from 10 m in its upper part to ∼30m towards its terminus. The CMP analysis highlights radar-wave velocities of 0.13-0.16 m ns–1 in the first 20 m of the structure. Larger vertical and lateral velocity variations are highlighted from CO data, reflecting the heterogeneous composition of the rock glacier and the likely presence of unfrozen water in the structure. Given the average air temperature registered at the site (+0.5°C), the near-melting-point temperature registered in the boreholes over more than a year and the presence of locally high water content inferred from GPR data, it is thought that the permafrost in the rock glacier is currently degrading.

We show that geophysical methods offer an effective means of quantifying snow thickness and density. Opportunistic (efficient but non-optimized) seismic refraction and ground-penetrating radar (GPR) surveys were performed on Storglaciären, Sweden, co-located with a snow pit that shows the snowpack to be 1.73 m thick, with density increasing from ∼120 to ∼500 kg m–3 (with a +50 kg m–3 anomaly between 0.73 and 0.83 m depth). Depths estimated for two detectable GPR reflectors, 0.76 ±0.02 and 1.71 ± 0.03 m, correlate extremely well with ground-truth observations. Refraction seismic predicts an interface at 1.90 ± 0.31 m depth, with a refraction velocity (3730 ± 190 ms–1) indicative of underlying glacier ice. For density estimates, several standard velocity-density relationships are trialled. In the best case, GPR delivers an excellent density estimate for the upper snow layer (observed = 321 ± 74 kg m–3, estimated = 319 ± 10 kgm–3) but overestimates the density of the lower layer by 20%. Refraction seismic delivers a bulk density of 404 ±22 kgm–3 compared with a ground-truth average of 356 ± 22 kg m–3. We suggest that geophysical surveys are an effective complement to mass-balance measurements (particularly for controlling estimates of snow thickness between pits) but should always be validated against ground-truth observations.

We report on the application of optical televiewing (OPTV) to the uppermost 630 m of the North Greenland Eemian Ice Drilling (NEEM) deep ice borehole, Greenland. The resulting log reveals numerous natural and drilling-related properties, including the integrity of the borehole casing and its joints, the presence of drill-tooth scoring on the ice wall of the borehole and the presence of regularly repeated layering, interpreted to be annual, to a depth approaching 200 m. A second OPTV log was acquired from a nearby shallow borehole. With the exception of the uppermost ∼10 m, this log shows a gradual decrease in luminosity with depth, interpreted as a decrease in light scattering with firnification. This shallow log also clearly images annual layers, allowing the construction of an age-depth scale. Comparing this with an independent core-based scale reveals that the OPTV record yields an age of 1724 at the deepest common point of both scales (80 m), 13 years older than the core-based record at 1737. However, all of this deviation accrues in the uppermost ∼30 m of the OPTV record where highly reflective snow saturates the luminosity of the borehole image, an artefact that can be reduced by further adaptation of the OPTV system.

On RADARSAT imagery, the southern margin of the onset zone of Bindschadler Ice Stream, West Antarctica, manifests a multi-banded feature, with brightness varying across the bands and oscillating along each band. Ground-based radar profiles across the margin reveal folds in the firn stratigraphy associated with this pattern and provide evidence for correlation between the depth of shallow isochrones and the RADARSAT backscatter intensity on each profile, allowing us to interpret the banded feature for firn-layer geometry in three dimensions. We use a kinematic model of isochrone depth evolution to show how layer folding and the band expression may result from deformation and advection in the near-surface flow field at ice-stream margins, even with steady flow. The model predicts the formation of longitudinally patterned bands when the ice-stream acceleration fluctuates along flow. Concerted study of the planform and stratigraphy of other RADARSAT-detected features on the ice sheets may help us understand their origin.

The Woore Moraine, situated within the Cheshire Plain, is one of the largest glacial landforms in Britain. However, its key characteristics, internal structure and glaciological significance are poorly understood due to limited exposure of its key geological features. This paper focuses on the results of a suite of complementary geophysical techniques (ground-penetrating radar, electrical resistivity tomography and seismic refraction) that were used to image the interior structure of the moraine and help determine its origin. Geophysical imaging reveals a two-layer moraine structure comprising a thin and heterogeneous carapace of intercalated sediments overlying a core of subglacial clay-rich diamicton and proglacial sands deformed by a series of fore- and back-thrusts. These features indicate that the moraine is glaciotectonic in origin, involving initial compression of proglacial sediments against a bedrock obstruction and subsequent modification due to overriding.

Ice streams provide major drainage pathways for the Antarctic ice sheet. The stress distribution and style of flow in such ice streams produce elastic and rheological anisotropy, which informs ice-flow modelling as to how ice masses respond to external changes such as global warming. Here we analyse elastic anisotropy in Rutford Ice Stream, West Antarctica, using observations of shear-wave splitting from three-component icequake seismograms to characterize ice deformation via crystal-preferred orientation. Over 110 high-quality measurements are made on 41 events recorded at five stations deployed temporarily near the ice-stream grounding line. To the best of our knowledge, this is the first well-documented observation of shear-wave splitting from Antarctic icequakes. The magnitude of the splitting ranges from 2 to 80 ms and suggests a maximum of 6% shear-wave splitting. The fast shear-wave polarization direction is roughly perpendicular to ice-flow direction. We consider three mechanisms for ice anisotropy: a cluster model (vertical transversely isotropic (VTI) model); a girdle model (horizontal transversely isotropic (HTI) model); and crack-induced anisotropy (HTI model). Based on the data, we can rule out a VTI mechanism as the sole cause of anisotropy – an HTI component is needed, which may be due to ice crystal a-axis alignment in the direction of flow or the alignment of cracks or ice films in the plane perpendicular to the flow direction. The results suggest a combination of mechanisms may be at play, which represent vertical variations in the symmetry of ice crystal anisotropy in an ice stream, as predicted by ice fabric models.

We have investigated the speed of compressional waves in a polythermal glacier by, first, predicting them from a simple three-phase (ice, air, water) model derived from a published ground-penetrating radar study, and then comparing them with field data from four orthogonally orientated walkaway vertical seismic profiles (VSPs) acquired in an 80 m deep borehole drilled in the ablation area of Storglaciären, northern Sweden. The model predicts that the P-wave speed increases gradually with depth from 3700ms–1 at the surface to 3760ms–1 at 80m depth, and this change is almost wholly caused by a reduction in air content from 3% at the surface to <0.5% at depth. Changes in P-wave speed due to water content variations are small (<10 ms–1); the model’s seismic cold–temperate transition surface (CTS) is characterized by a 0.3% decrease downwards in P-wave speed (about ten times smaller than the radar CTS). This lack of sensitivity, and the small contrast at the CTS, makes seismically derived water content estimation very challenging. Nevertheless, for down-going direct-wave first arrivals for zero- and near-offset VSP shots, we find that the model-predicted travel times and field observations agree to within 0.2 ms, i.e. less than the observational uncertainties.

The environments underneath ice sheets are of high scientific interest. Wireless sensors offer the prospect of sustained, distributed remote sensing in the subglacial environment. Typically, wireless sensor networks use radio-frequency (RF) electromagnetic communications, but these are highly attenuated in wet environments. In such environments, acoustic communications may be more power-efficient. Here we review the literature on acoustic and RF attenuation through ice and other relevant media, and present the results of new experiments on acoustic attenuation in glacial ice. Link budgets for communications from a range of subglacial environments show that acoustic communications are a viable strategy for transmission through water and ice where RF is too highly attenuated to be detected. Acoustic communication at 30 kHz is predicted to be possible through 1 km of glacial ice, using a 1 W transmitter. Such a strategy may be appropriate for shallow ice-stream environments around the Antarctic and Greenland ice sheet margins.

Amplitude-versus-angle (AVA) analysis of a seismic reflection line, imaged 13 km from Russell Glacier terminus, near the western margin of the Greenland ice sheet (GrIS), suggests the presence of sediment at the bed. The analysis was complicated by the lack of identifiable multiples in the data due to a highly irregular and crevassed ice surface, rendering deeper seismic returns noisy. A modified technique for AVA processing of glacial seismic data using forward modelling with primary reflection amplitudes and simulated multiple amplitudes is presented here. Our analysis demonstrates that AVA analysis can be applied to areas with noisy seismic returns and indicates that sediment underlies the seismic study site. Our data are inconsistent with the common assumption that the GrIS is underlain only by hard bedrock, but consistent with the presence of subglacial sediment with porosity between 30% and 40%. As analysis and modelling of ice-sheet dynamics requires a sound knowledge of the underlying basal materials, subglacial sediment should be taken into account when considering ice dynamics in this region of the GrIS.

Active seismic imaging of glaciers and ice sheets is important for constraining inputs to climate models, such as englacial ice fabric and the nature of the basal interface. However, acquiring high-quality seismic data is time-consuming and resource-intensive. Using traditional single-element geophones requires ideal weather conditions (e.g. light winds) and excellent source coupling. In addition, deploying and retrieving these geophones is slow and cumbersome. We have developed a four-element ‘georod’ that enhances signal levels by 20–30dB in a variety of conditions, including blowing snow and poorly coupled source detonations. The long, slender design of these georods makes them easy to deploy and retrieve, allowing researchers to acquire greater line-kilometers of seismic data during field campaigns that are commonly time-constrained.

A passive seismology experiment was conducted across the main overdeepening of Storglaciären in the Tarfala valley, northern Sweden, to investigate the spatial and temporal distribution of basal microseismic waveforms in relation to known dynamics of this small polythermal sub-arctic glacier. The high ablation rate made it difficult to keep geophones buried and well coupled to the glacier during the experiment and reduced the number of days of good-quality data collection. The characterization of typical and atypical waveforms showed that the dominant waveforms were from near-surface events such as crevassing. Waveforms resembling basal microseismic signals were very rare, and seldom observed on more than two seismic stations simultaneously. The analysis of waveforms, amplitudes and particle motions suggested a near-field origin for most events. Even though basal sliding is known to occur in the overdeepening, no convincing examples of basal waveforms were detected, suggesting basal microseismic signals are rare or difficult to detect beneath polythermal glaciers like Storglaciären. We discuss the reasons for failing to locate basal signals, consider the origin of common waveforms and make recommendations for setting up passive seismology experiments on glaciers with high ablation rates.

To investigate recent glacier changes in the Himalayan region, we carried out GPS and ground-penetrating radar (GPR) measurements at Yala Glacier, a benchmark glacier in Nepal. Glacier surface elevation and ice thickness were surveyed along a 1.5 km profile from the glacier top to the terminus. Ice flow velocity was measured at five locations by surveying stakes for either 1 year or 4 day periods. Obtained surface elevation and ice velocity were compared with those measured in 1982 and 1996. The mean ice thickness along the radar profile was 36 m in 2009 and the ice has been thinning at rates of-0.69 ±0.25 and -0.75 ± 0.24 m a-1 during the periods 1982-96 and 1996-2009, respectively. The thinning rate increases down-glacier, reaching a magnitude up to -1.8 m a-1 near the terminus from 1996 to 2009. The ice velocity has reduced by >70% from 1982 to 2009 in the lower half of the glacier. By assuming a constant driving stress over the glacier, the total ice volume in 2009 was estimated as 0.061 km3. Our results indicate that Yala Glacier has lost ∼40% of its ice volume over the last 27 years and that the rate of the mass loss has accelerated over the last decade.