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The density structure of firn has implications for hydrological and climate modelling, and ice-shelf stability. The structure of firn can be evaluated from depth models of seismic velocity, widely obtained with Herglotz–Wiechert inversion (HWI), an approach that considers the slowness of refracted seismic arrivals. However, HWI is strictly appropriate only for steady-state firn profiles and the inversion accuracy can be compromised where firn contains ice layers. In these cases, full waveform inversion (FWI) may yield more success than HWI. FWI extends HWI capabilities by considering the full seismic waveform and incorporates reflected arrivals. Using synthetic firn density profiles, assuming both steady- and non-steady-state accumulation, we show that FWI outperforms HWI for detecting ice slab boundaries (5–80 m thick, 5–80 m deep) and velocity anomalies within firn. FWI can detect slabs thicker than one wavelength (here, 20 m, assuming a maximum frequency of 60 Hz) but requires the starting velocity model to be accurate to ±2.5%. We recommend for field practice that the shallowest layers of velocity models are constrained with ground-truth data. Nonetheless, FWI shows advantages over established methods, and should be considered when the characterisation of firn ice slabs is the goal of the seismic survey. [ABSTRACT FROM AUTHOR]

The debris that covers the ablation areas of high-elevation debris-covered glaciers contributes to the distinctive features and processes occurring both on and within such glaciers. Despite recent advances, knowledge of the subsurface environments of high-elevation debris-covered glaciers is still extremely limited. In particular, targeted field-based data are needed to parameterise and refine the projections of these glaciers in numerical models. Here, we outline the current understanding of the internal properties of high-elevation debris-covered glaciers based on direct field-based methods and suggest future research directions for field-based studies. [ABSTRACT FROM AUTHOR]

Distributed Acoustic Sensing (DAS) is increasingly recognised as a valuable tool for glaciological seismic applications, although analysing the large data volumes generated in acquisitions poses computational challenges. We show the potential of active-source DAS to image and characterise subglacial sediment beneath a fast-flowing Greenlandic outlet glacier, estimating the thickness of sediment layers to be 20–30 m. However, the lack of subglacial velocity constraint limits the accuracy of this estimate. Constraint could be provided by analysing cryoseismic events in a counterpart 3-day record of passive seismicity through, for example, seismic tomography, but locating them within the 9 TB data volume is computationally inefficient. We describe experiments with data compression using the frequency-wavenumber (f-k) transform ahead of training a convolutional neural network, that provides a ~300-fold improvement in efficiency. In combining active and passive-source and our machine learning framework, the potential of large DAS datasets could be unlocked for a range of future applications. [ABSTRACT FROM AUTHOR]

The density structure of firn has implications for hydrological and climate modelling and for ice shelf stability. The firn structure can be evaluated from depth models of seismic velocity, widely obtained with Herglotz-Wiechert inversion (HWI), an approach that considers the slowness of refracted seismic arrivals. However, HWI is appropriate only for steady-state firn profiles and the inversion accuracy can be compromised where firn contains ice layers. In these cases, Full Waveform Inversion (FWI) can be more successful than HWI. FWI extends HWI capabilities by considering the full seismic waveform and incorporates reflected arrivals, thus offering a more accurate estimate of a velocity profile. We show the FWI characterisation of the velocity model has an error of only 1.7% for regions (vs. 4.2% with HWI) with an ice slab (20 m thick, 40 m deep) in an otherwise steady-state firn profile. [ABSTRACT FROM AUTHOR]

Characterising the structures within glaciers can give unique insight into ice motion processes. On debris-covered glaciers, traditional structural glaciological mapping is challenging because the lower glacier is hidden by the supraglacial debris layer. Here, we use high-resolution optical televiewer (OPTV) image logs from four boreholes drilled into Khumbu Glacier, Nepal, to overcome this limitation and investigate englacial structural features within a Himalayan debris-covered glacier. The OPTV logs show structural features that are up to an order of magnitude thinner than those observed at the glacier surface and reveal five structural units: (I) primary stratification of ice; (II) debris-rich planes that conform with the primary stratification; (III) water-healed crevasse traces; (IV) healed crevasse traces; and (V) steeply dipping planes of basally derived fine sediment near the glacier terminus. The OPTV logs also reveal that the primary stratification both decreases in dip with depth (by up to 56° over 20 m) and rotates with depth (by up to 100° over 20 m) towards parallelism with the proximal lateral moraine. This transformation and the presence of relict layers of basally derived sediment raised into an englacial position – possibly involving thrusting – near the glacier's now stagnant terminus reveal a previously more dynamic glacier regime. [ABSTRACT FROM AUTHOR]

The debris that covers the ablation areas of high-elevation debris-covered glaciers contributes to the distinctive features and processes occurring both on and within such glaciers. Despite recent advances, knowledge of the subsurface environments of high-elevation debris-covered glaciers is still extremely limited. In particular, targeted field-based data are needed to parameterise and refine the projections of these glaciers in numerical models. Here, we outline the current understanding of the internal properties of high-elevation debris-covered glaciers based on direct field-based methods and suggest future research directions for field-based studies. [ABSTRACT FROM AUTHOR]

Distributed Acoustic Sensing (DAS) is increasingly recognised as a valuable tool for glaciological seismic applications, although analysing the large data volumes generated in acquisitions poses computational challenges. We show the potential of active-source DAS to image and characterise subglacial sediment beneath a fast-flowing Greenlandic outlet glacier, estimating the thickness of sediment layers to be 20–30 m. However, the lack of subglacial velocity constraint limits the accuracy of this estimate. Constraint could be provided by analysing cryoseismic events in a counterpart 3-day record of passive seismicity through, for example, seismic tomography, but locating them within the 9 TB data volume is computationally inefficient. We describe experiments with data compression using the frequency-wavenumber (f-k) transform ahead of training a convolutional neural network, that provides a ~300-fold improvement in efficiency. In combining active and passive-source and our machine learning framework, the potential of large DAS datasets could be unlocked for a range of future applications. [ABSTRACT FROM AUTHOR]

The density structure of firn has implications for hydrological and climate modelling and for ice shelf stability. The firn structure can be evaluated from depth models of seismic velocity, widely obtained with Herglotz-Wiechert inversion (HWI), an approach that considers the slowness of refracted seismic arrivals. However, HWI is appropriate only for steady-state firn profiles and the inversion accuracy can be compromised where firn contains ice layers. In these cases, Full Waveform Inversion (FWI) can be more successful than HWI. FWI extends HWI capabilities by considering the full seismic waveform and incorporates reflected arrivals, thus offering a more accurate estimate of a velocity profile. We show the FWI characterisation of the velocity model has an error of only 1.7% for regions (vs. 4.2% with HWI) with an ice slab (20 m thick, 40 m deep) in an otherwise steady-state firn profile. [ABSTRACT FROM AUTHOR]

Subglacial hydrology modulates basal motion but remains poorly constrained, particularly for soft-bedded Greenlandic outlet glaciers. Here, we report detailed measurements of the response of subglacial water pressure to the connection and drainage of adjacent water-filled boreholes drilled through kilometre-thick ice on Sermeq Kujalleq (Store Glacier). These measurements provide evidence for gap opening at the ice-sediment interface, Darcian flow through the sediment layer, and the forcing of water pressure in hydraulically-isolated cavities by stress transfer. We observed a small pressure drop followed by a large pressure rise in response to the connection of an adjacent borehole, consistent with the propagation of a flexural wave within the ice and underlying deformable sediment. We interpret the delayed pressure rise as evidence of no pre-existing conduit and the progressive decrease in hydraulic transmissivity as the closure of a narrow (< 1.5 mm) gap opened at the ice-sediment interface, and a reversion to Darcian flow through the sediment layer with a hydraulic conductivity of ≤ 10−6 m s−1. We suggest that gap opening at the ice-sediment interface deserves further attention as it will occur naturally in response to the rapid pressurisation of water at the bed. [ABSTRACT FROM AUTHOR]

Subglacial hydrological systems require innovative technological solutions to access and observe. Wireless sensor platforms can be used to collect and return data, but their performance in deep and fast-moving ice requires quantification. We report experimental results from Cryoegg: a spherical probe that can be deployed into a borehole or moulin and transit through the subglacial hydrological system. The probe measures temperature, pressure and electrical conductivity in situ and returns all data wirelessly via a radio link. We demonstrate Cryoegg's utility in studying englacial channels and moulins, including in situ salt dilution gauging. Cryoegg uses VHF radio to transmit data to a surface receiving array. We demonstrate transmission through up to 1.3 km of cold ice – a significant improvement on the previous design. The wireless transmission uses Wireless M-Bus on 169 MHz; we present a simple radio link budget model for its performance in cold ice and experimentally confirm its validity. Cryoegg has also been tested successfully in temperate ice. The battery capacity should allow measurements to be made every 2 h for more than a year. Future iterations of the radio system will enable Cryoegg to transmit data through up to 2.5 km of ice. [ABSTRACT FROM AUTHOR]

Direct measurements of spatially distributed vertical strain within ice masses are scientifically valuable but challenging to acquire. We use manual marker tracking and automatic cross correlation between two repeat optical televiewer (OPTV) images of an ~100 m-long borehole at Derwael Ice Rise (DIR), Antarctica, to reconstruct discretised, vertical strain rate and velocity at millimetre resolution. The resulting profiles decay with depth, from −0.07 a−1 at the surface to ~−0.002 a−1 towards the base in strain and from −1.3 m a−1 at the surface to ~−0.5 m a−1 towards the base in velocity. Both profiles also show substantial local variability. Three coffee-can markers installed at different depths into adjacent boreholes record consistent strain rates and velocities, although averaged over longer depth ranges and subject to greater uncertainty. Measured strain-rate profiles generally compare closely with output from a 2-D ice-flow model, while the former additionally reveal substantial high-resolution variability. We conclude that repeat OPTV borehole logging represents an effective means of measuring distributed vertical strain at millimetre scale, revealing high-resolution variability along the uppermost ~100 m of DIR, Antarctica. [ABSTRACT FROM AUTHOR]

While hot-water drilling is a well-established technique used to access the subsurface of ice masses, drilling into high-elevation (≳ 4000 m a.s.l.) debris-covered glaciers faces specific challenges. First, restricted transport capacity limits individual equipment items to a volume and mass that can be slung by small helicopters. Second, low atmospheric oxygen and pressure reduces the effectiveness of combustion, limiting a system's ability to pump and heat water. Third, thick supraglacial debris, which is both highly uneven and unstable, inhibits direct access to the ice surface, hinders the manoeuvring of equipment and limits secure sites for equipment placement. Fourth, englacial debris can slow the drilling rate such that continued drilling becomes impracticable and/or boreholes deviate substantially from vertical. Because of these challenges, field-based englacial and subglacial data required to calibrate numerical models of high-elevation debris-covered glaciers are scarce or absent. Here, we summarise our experiences of hot-water drilling over two field seasons (2017–2018) at the debris-covered Khumbu Glacier, Nepal, where we melted 27 boreholes up to 192 m length, at elevations between 4900 and 5200 m a.s.l. We describe the drilling equipment and operation, evaluate the effectiveness of our approach and suggest equipment and methodological adaptations for future use. [ABSTRACT FROM AUTHOR]

The phase-sensitive radio-echo sounder (pRES) is a powerful new instrument that can measure the depth of internal layers and the glacier bed to millimetre accuracy. We use a stationary 16-antenna pRES array on Store Glacier in West Greenland to measure the three-dimensional orientation of dipping internal reflectors, extending the capabilities of pRES beyond conventional depth sounding. This novel technique portrays the effectiveness of pRES in deriving the orientation of dipping internal layers that may complement profiles obtained through other geophysical surveying methods. Deriving ice vertical strain rates from changes in layer depth as measured by a sequence of pRES observations assumes that the internal reflections come from vertically beneath the antenna. By revealing the orientation of internal reflectors and the potential deviation from nadir of their associated reflections, the use of an antenna array can correct this assumption. While the array configuration was able to resolve the geometry of englacial layers, the same configuration could not be used to accurately image the glacier bed. Here, we use simulations of the performance of different array geometries to identify configurations that can be tailored to study different types of basal geometry for future deployments. [ABSTRACT FROM AUTHOR]

Large numbers of small valley glaciers on Svalbard were thicker and more extensive during the Little Ice Age (LIA), demonstrated by prominent ice-cored moraines up to several kilometres beyond present-day margins. The majority of these glaciers have since experienced a long period of strongly negative mass balance during the 20th century and are now largely frozen to their beds, indicating they are likely to have undergone a thermal transition from a polythermal to a cold-based regime. We present evidence for such a switch by reconstructing the former flow dynamics and thermal regime of Tellbreen, a small cold-based valley glacier in central Spitsbergen, based on its basal sequence and glaciological structures. Within the basal sequence, the underlying matrix-supported diamict is interpreted as saturated subglacial traction till which has frozen at the bed, indicating that the thermal switch has resulted in a cessation of subglacial sediment deformation due to freezing of the former deforming layer. This is overlain by debris-poor dispersed facies ice, interpreted to have formed through strain-induced metamorphism of englacial ice. The sequential development of structures includes arcuate fracture traces, interpreted as shear planes formed in a compressive/transpressive stress regime; and fracture traces, interpreted as healed extensional crevasses. The formation of these sediment/ice facies and structures is indicative of dynamic, warm-based flow, most likely during the LIA when the glacier was significantly thicker. [ABSTRACT FROM AUTHOR]

We report on the application of optical televiewing (OPTV) to the uppermost 630m 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 ~30m 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. [ABSTRACT FROM AUTHOR]

The article discusses the study which evaluates the technique of Digital optical televiewing (OPTV) of the hot-water-drilled boreholes englacial ice and debris structures in the terminus region of midre Lovénbreen, Svalbard, Norway. It notes that the resulting OPTV logs successfully reveal several visually distinctive englacial ice properties and deformation structures It adds that the combination of the OPTV logs with surface mapping result in the identification of eight separate structural elements can be interpolated into grids with sediment layers intercalated in primary stratification.