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CR2.1

Geophysical measurements offer important baseline datasets as well as validation for modelling and remote sensing products for cryospheric sciences. Applications include the dynamics of ice-sheets, alpine glaciers and sea ice, changes in snow cover properties of seasonal and permanent snow, snow/ice-atmosphere-ocean interactions, permafrost degradation, geomorphic processes and changes in subsurface materials.

In this session we welcome contributions related to a wide spectrum of geophysical- and in-situ methods, including advances in diverse techniques such as radioglaciology, active and passive seismology, acoustic sounding, GPS/GNSS reflectometry or time delay techniques, cosmic ray neutron sensing, drone applications, geoelectrics and NMR. Contributions may concern field applications as well as new approaches in geophysical/in-situ survey techniques or theoretical advances in the field of data analysis, processing or inversion. Case studies from all parts of the cryosphere such as snow, alpine glaciers, ice sheets, glacial and periglacial environments and sea ice are highly welcome. The focus of the session is to compare experiences in the application, processing, analysis and interpretation of different geophysical and in-situ techniques in these highly complex environments.

This session is offered as a PICO: an engaging presentation format that has been successfully tested for this session during the last three years at EGU. All selected contributions will present their research orally, and then further present their research using interactive screens. This results in rich scientific feedback and is an effective tool for communicating science with high visibility.

This is a joined session - we merged with the former session SM5.5 'Active and passive seismic methods for imaging and monitoring the cryosphere'.


++++++++++++++++++++++++++++ Invited Speaker ++++++++++++++++++++++++++++++++++++

Dustin Schroeder: Observing Evolving Subglacial Conditions with Muti-Temporal Radar Sounding

Public information:
Session topics:
A) Glaciers, Englacial and Subglacial: Schroeder (invited), Rix & Mulvaney et al., Yushkova et al., Jansen et al., Delf et al., Church et al., Pettinelli et al., Kufner et al., Mordret et al., Brisbourne et al., Jones et al., Stevens et al.
B) Sea Ice & Ocean Floors: Jakovlev et al., Schlindwein et al.
C) Ice Rheology: Hellmann et al., Booth et al., Ershadi et al., Martin et al.
D) Snow & Firn: Case et al., Pearce et al., Priestley, Capelli et al., Henkel et al.
E) Permafrost: Maierhofer et al., Limbrock et al., Boaga et al., Lyu et al., Valois et al., Majdanski et al.

Besides our EGU2020: Sharing Geoscience Online text-based chat on Mon, 04 May, 08:30–10:15, we are planning an additional video conference (outside the EGU programme) at the same day starting at 18:00/06:00p.m. In this video conference, our invited speaker Dustin Schroeder will give his talk on ‘Observing Evolving Subglacial Conditions with Mutitemporal Radar Sounding’. We will then open a broader discussion on all different topics and methods of our session.
Time: Mon, 04 May, start: 18:00/06:00p.m. Vienna time (CEST) (= 12:00 New York time)
Place/Link: https://rutgers.webex.com/rutgers/k2/j.php?MTID=tc085b8a9bc24b1c04784c81584672fc4
Session password: YvBGu8jV773 (Global call-in numbers: https://rutgers.webex.com/rutgers/globalcallin.php?MTID=t7ddb8d0ab92b0bd317c7e36862494393 Access code: 192 664 533)

@all authors of this session: It would be great if you can help us a bit in our session planning. Therefore, we would like to ask you to complete the following Doodle survey asap: https://doodle.com/poll/sese8bcs57dcfye5
In this survey we would like to know, if you will be able to
a) upload a display until Thu, 30 April
b) participate during our official EGU2020: Sharing Geoscience Online text-based chat on Mon, 04 May, 08:30–10:15 am
c) participate during our additional video conference on Mon, 04 May, 18:00/6p.m. (please pay attention, time was updated!)
Thank you very much for your help.

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Co-organized by HS1.1/SM4
Convener: Franziska KochECSECS | Co-conveners: Nanna Bjørnholt Karlsson, Kristina Keating, Mariusz Majdanski, Emma C. SmithECSECS, Schlindwein Vera, Andreas Köhler
Displays
| Attendance Mon, 04 May, 08:30–10:15 (CEST)

Files for download

Session materials Download all presentations (204MB)

Chat time: Monday, 4 May 2020, 08:30–10:15

Chairperson: E. Smith / M. Majdanski / N. B. Karlsson / F. Koch
D2628 |
EGU2020-179
Dustin Schroeder

Airborne radar sounding is the primary geophysical method for directly observing conditions beneath ice sheet and glaciers at the catchment to continent scale. From single flow-lines to regional surveys to ice-sheet wide gridded topographic datasets, radar sounding profiles provide information-rich constraints on the englacial and subglacial environment. This can include roughness, lithology, hydrology, thermal state, melt, fabric, and structure for both grounded and floating ice. However, the snap-shot view provided by one-time soundings fails to capture subsurface processes across the time-scales over which they evolve and control ice flow.  Doing so requires advancing multi-temporal radar sounding instruments, platforms, and data analysis. For example, point-measurements by ground-based or stationary sounder can be used produce local time-series observations of englacial and subglacial conditions. However, low-cost, low-power active and/or passive radar-sounder networks can dramatically extend the reach and scope of such measurements. Further, repeat surveys by sled-drawn or airborne sounders can capture seasonal and interannual subsurface variations. However, digitization of archival radar film are extending the temporal baseline for such comparison by decades, making multi-decadal studies of subsurface changes possible. Finally, the development of autonomous rover, drone, and satellite sounding platforms and systems promise to enable pervasive, stable, and frequent monitoring of subglacial conditions. Here, we discuss the advances, challenges, and the path forward to observing subsurface conditions across the full range spatial and temporal scales at which they occur.

How to cite: Schroeder, D.: Observing Evolving Subglacial Conditions with Mutitemporal Radar Sounding , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-179, https://doi.org/10.5194/egusphere-egu2020-179, 2019

D2629 |
EGU2020-19599
| Highlight
Julius Rix, Robert Mulvaney, Carlos Martin, Catherine Ritz, and Massimo Frezzotti

Ice domes in the interior of East Antarctica are ideal candidates in the quest for the longest continuous record of climate in polar ice. They are in areas with low surface precipitation, low horizontal advection and large ice thickness. However, the age of the ice near the bottom of the column is very sensitive to subglacial thermal conditions as they can promote basal melting and the loss of the bottommost and oldest ice. Here we report the main findings from a geophysical survey and a shallow, 460m depth, rapid access drilled borehole. We use a low frequency radar, DELORES, to survey the area and detect subglacial melting; a phase sensitive radar, ApRES, to obtain englacial vertical strain-rates and crystal orientation fabrics in selected sites; and, at the drilling site, borehole temperature and water isotope data in the top 460m. Our main findings are: 1) The subglacial topography is characterized by topographic highs criss-crossed by deep valley troughs with typically 0.5-1km difference in height. There is evidence of subglacial melting in the troughs. However the ice stratigaphy, that we survey in detail with DELORES system with 500m grid, drapes over the rough topography and the topographic highs are presently melt-free. 2) The optical birefringence, observed in ApRES polarimetry, shows two aligned crystal orientation fabrics that are typical for glacial periods. This indicate uniform ice-flow conditions during, at least, the last two glacial-interglacial periods and is consistent with the polarimetry from EPICA Dome C. 3) Using the borehole temperature, englacial strain-rates and temperature records from EPICA Dome C we estimate that the geothermal heatflux in the area is 55 +/- 1 mW/m2. Also we find that, due to the delay between basal and surface temperatures, the basal temperature at Little Dome C is currently the coldest and was 0.5 C warmer 80 kyrs ago. We estimate that any topographic high where the ice thickness is below 2810 +/- 10 m was melt-free during the warmest conditions. This information, together with other evidence, lead to choosing the site for the future Beyond Epica – Oldest Ice Core project.

How to cite: Rix, J., Mulvaney, R., Martin, C., Ritz, C., and Frezzotti, M.: Glaciological setting and subglacial conditions at Little Dome C, the future site for Beyond Epica – Oldest Ice Core, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19599, https://doi.org/10.5194/egusphere-egu2020-19599, 2020

D2630 |
EGU2020-21606
Olga Yushkova, Taisiya Dymova, and Viktor Popovnin

Radio echo-sounding is a powerful technique for investigating the subsurface of the glaciers. However, physics underlying the formation of the reflected signal is sometimes oversimplified  in the geophysical glacier studies, leading to wrong results. Various remote sensing techniques use different wavelengths (e.g., 13.575 GHz for CryoSat and 20-25/200-600 MHz for ground-penetrating radar), but it is still not clear which particular wavelengths are the best to detect different characteristics of the ice. Possibly, the results gained using different wavelengths may not coincide but rather complement each other due to frequency dependence of the dielectric permittivity and conductivity of snow, ice and especially water.

Here we attempt to construct an electrophysical model of a cold glacier. This mathematical model considers the variability of the depth profile of the complex dielectric permittivity depending on the frequency of the probing radio signal and the surface temperature. A series of calculations of the reflection coefficients of radio waves from the modelled glacier show that at low temperatures for frequencies above 1 MHz the real part of the dielectric constant of the glacier does not change with frequency and surface temperature, but depends on the glacier structure, while the depth profile of the loss tangent is constant throughout the glacier.  As wavelength decreases, the absorption of radio-waves by the glacier decreases and the frequency dependence of the reflection coefficient becomes a periodic function, its period and amplitude depend on the glacier thickness, the dielectric constant of the bedrock and ice on the surface.

The range of radio-waves from 0.1 to 1 MHz is not optimal for sounding cold glaciers: the absorption of radio-waves by ice is large for studying thick layers of the glacier, and the wavelength does not allow studying thin layers. Hence, reflection from the glacier surface prevails upon reflection of the signal. The small absorption of short radio waves by ice leads to the fact that the frequency dependence of the reflection coefficient of short radio-waves is practically the sum of the partial reflections of radio-waves from the surface and internal snow/firn and firn/ice boundaries. Period and amplitude of oscillations of the function  depend on the depth of the internal boundaries and the gradient of dielectric characteristics of ice, snow, firn and bedrock.

Changes in surface temperature, leading to a change in the loss tangent of the upper glacier layers, are manifested in the phase magnitude of the reflection coefficient of radio-waves:it grows with the temperature. Theoretically, the high-frequency signal reflected from the glacier contains information about the structure of the cold glacier and the depth distribution of the dielectric constant, but to restore the electrophysical parameters of the glaciers, it is necessary to use a broadband signal with smooth spectrum and high digitization speed.

The reported study was funded by RFBR, project number 18-05-60080 (“Dangerous nival-glacial and cryogenic processes and their impact on infrastructure in the Arctic”).

How to cite: Yushkova, O., Dymova, T., and Popovnin, V.: Radio-wave reflectivity from cold glaciers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21606, https://doi.org/10.5194/egusphere-egu2020-21606, 2020

D2631 |
EGU2020-20424
Daniela Jansen, Steven Franke, Tobias Binder, Paul Bons, Dorthe Dahl-Jensen, Olaf Eisen, Veit Helm, Heinrich Miller, Niklas Neckel, John Paden, Daniel Steinhage, and Ilka Weikusat

The North East Greenland Ice Stream (NEGIS) is delineated by well-defined shear margins, which are evident in the gradient of surface velocity field as well as in the surface topography, where they form troughs up to ten meters deep. In the upper part of the ice stream the margins appear not to be linked to bedrock topography. To understand this efficient system of mass transport towards the ocean it is essential to investigate the nature of the shear margins, as here very localized deformation decouples the inner ice stream from the slower flowing surrounding ice sheet. This process is influenced by several factors and feedback mechanisms, including the crystal fabric orientation, strain heating and localization of meltwater. In summary, the shear margins are area-wise a small part of the ice stream itself, but the processes leading to the localization of deformation are of similar importance for ice discharge as the processes enabling fast flow of the main trunk over the bed.

We present results from an airborne radar survey with the AWI Ultra-Wide Band Radar system, covering an area 150 km upstream and 100 km downstream of the deep drilling site on the ice stream (EGRIP). Over the survey area the ice stream accelerates from 12 m/a to 75 m/a. We focus on the signatures of the shear margins in the radar data. In the regions of localized shear, the internal reflections in the radargrams show disturbances in the form of steep undulations, or chevron folds, which are intensified with ongoing shear. As the ice stream has been covered with 36 flow-perpendicular radar sections we are able to show the evolution of these characteristic signatures over the survey area, and thus, as an analog, over time. 3D-representations of the folded stratigraphic layers reveal how new folds are formed when the ice stream widens and how older structures are preserved in the outer part of the main trunk, where they are no longer subject to shear. Furthermore, we link the change of the shape of the internal reflections in the shear zones to a strain rate field calculated from high resolution flow velocities derived by TerraSAR-X data.

How to cite: Jansen, D., Franke, S., Binder, T., Bons, P., Dahl-Jensen, D., Eisen, O., Helm, V., Miller, H., Neckel, N., Paden, J., Steinhage, D., and Weikusat, I.: 3D Structure of NEGIS shear margins from radar stratigraphy , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20424, https://doi.org/10.5194/egusphere-egu2020-20424, 2020

D2632 |
EGU2020-20484
Richard Delf, Robert G Bingham, Andrew Curtis, Satyan Singh, Benjamin Schwarz, and Antonios Giannopoulos

Ground Penetrating Radar (GPR) is widely used on polythermal and temperate glaciers to sound bed topography and investigate the hydrothermal conditions through detection of englacial radar scattering. Water held within micro- and macro-scale pores and ice grain boundaries in ice at the pressure melting point influences the velocity of radar propagation on the scale of the wavelength, and can result in the occurrence of pronounced diffraction patterns in the data. Methods to investigate the water content distribution quantitatively within temperate ice often require the use of multi-offset common mid-point or common source-point survey techniques, which are logistically challenging and expensive. As a result, bed topography estimation is often undertaken using a constant velocity, and, because lateral variations in the the velocity field are unaccounted for, errors in topography are likely.

Here, we present an automated workflow to estimate an englacial radar velocity field from zero offset data and apply the algorithm to GPR data collected on Von Postbreen, a polythermal glacier in Svalbard, using a 25 MHz zero-offset GPR system. We first extract the diffracted wavefield using local coherent stacking to remove scatter and enhance diffractions. We then use the focusing metric of negative entropy to deduce a local migration velocity field from constant-velocity migration panels and produce a glacier-wide model of local (interval) radar velocity. We show that this velocity field is successful in differentiating between areas of cold and temperate ice and can detect lateral variations in radar velocity close to the glacier bed. The effects of this velocity field in both migration and depth-conversion of the bed reflection are shown to result in consistently lower ice depths across the glacier, indicating that diffraction focusing and velocity estimation are crucial in retrieving correct bed topography in the presence of temperate ice.

How to cite: Delf, R., Bingham, R. G., Curtis, A., Singh, S., Schwarz, B., and Giannopoulos, A.: Automated estimation of englacial radar velocity from zero offset data; implications for glacier bed topography retrieval , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20484, https://doi.org/10.5194/egusphere-egu2020-20484, 2020

D2633 |
EGU2020-19365
Gregory Church, Andreas Bauder, Melchior Grab, Cédric Schmelzbach, and Hansruedi Maurer

Surface meltwater is routed through the glacier’s interior by englacial drainage systems into the subglacial drainage system. The subglacial drainage system plays an important control on the glacier sliding velocity. Therefore, studying the evolution of englacial drainage systems throughout the melt season is key to understanding how these englacial drainage systems develop, and how they subsequently feed the subglacial drainage system.

We have conducted 10 repeated ground-penetrating radar using a Sensor & Software pulseEKKO Pro GPR system with 25 MHz antenna between 2012 and 2019 over an englacial conduit network, 90 m below the glacier’s surface, on the Rhonegletscher, Switzerland. These repeated measurements allowed insights into both annual and seasonal changes. We were also able to have direct observations into the englacial conduit network from six boreholes that were drilled in August 2018 using a GeoVISIONTM Dual-Scan borehole camera.

The annual results provided evidence that the englacial drainage network developed between 2012 and 2017. The seasonal evolution of the englacial conduit was studied by inverting the GPR data using an impedance inversion. The impedance inversion delivered reflection coefficients, which provides information on the englacial material properties associated with the englacial conduits. The inversion results provide evidence that during the winter season the englacial network is inactive. During June the englacial network becomes active by transporting surface melt water, and it becomes fully active later in the melt season (August). The reflectivity in summer (June-October) is -0.6, indicating the presence of water within the network. In winter (November-May) the reflectivity is around 0 indicating that the system is neither air or water filled and therefore the system physically closes.

The data processing workflow provided a top and bottom reflection coefficient of the conduit. The travel time between the reflection coefficients can be converted to a thickness when using EM wave velocity of water (from 2018 borehole observations). During the summer months the englacial network is around a quarter wavelength thick (0.3 m), which is approximately the limit of the vertical resolution.

How to cite: Church, G., Bauder, A., Grab, M., Schmelzbach, C., and Maurer, H.: Repeated ground-penetrating radar measurements to detect seasonal and annual variations of an englacial conduit network, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19365, https://doi.org/10.5194/egusphere-egu2020-19365, 2020

D2634 |
EGU2020-22579
Elena Pettinelli, Massimo Pecci, Frank S. Marzano, Marianna Biscarini, Paolo Boccabella, Federica Bruschi, Tiziano Caira, David Cappelletti, Domenico Cimini, Pinuccio D’Aquila, Thomas Di Fiore, Giulio Esposito, Sebastian E. Lauro, Elisabetta Mattei, Angelo Monaco, Gianluca Palermo, Mattia Pecci, Edoardo Raparelli, Marco Scozzafava, and Paolo Tuccella

The Calderone glacier is at present the most southern glacier in Europe (42° 28' 15’’ N). The little apparatus (about 20.000 m2 in surface area) has been giving an interesting response both to short- and long-term climatic variations which resulted in a considerable reduction in surface area and volume. The glacial apparatus is split into two ice bodies (glacierets) since 2000. The two glacierets are located in a deep northward valley below the top of the Corno Grande (2912 m asl) in the centre of the Gran Sasso d’Italia mountain range (Central Italy). Such glacial apparatus has been subjected to a strong reduction, with a loss of total surface area of about 50% and thickness of about 65%with respect to the hypothetical size (about 105.00 m2 and 55 m at the Little Ice Age).

Since early 90s the Calderone glacier has been subjected to several multidisciplinary field campaigns to monitor and evaluate its role as an environmental indicator in the framework of global warming. Starting from historical series related to more than a century of records, the variability of the different glacier properties has been estimated by using classical geomorphologic methods as well as in situ and remote sensing techniques. In particular, the last field campaigns, in 2015, 2016 and 2019, have been carried out using Ground Penetrating Radar equipped with different antenna frequencies, drone-based survey, snow pit measurements and chemical-physical sampling. The measurement campaigns have been complemented by a regional climate analysis, spanning the last fifty years, and snowpack modelling initialized with microphysical snow data (e.g., snow density, crystal shape and size, hardness).  The snowpack chemical analyses include the main and trace elements, soluble inorganic and organic ions, EC/OC and PAH, with different spatial resolution depending on the analytes. We present here the methodological approach used and some preliminary results.

How to cite: Pettinelli, E., Pecci, M., Marzano, F. S., Biscarini, M., Boccabella, P., Bruschi, F., Caira, T., Cappelletti, D., Cimini, D., D’Aquila, P., Di Fiore, T., Esposito, G., Lauro, S. E., Mattei, E., Monaco, A., Palermo, G., Pecci, M., Raparelli, E., Scozzafava, M., and Tuccella, P.: Monitoring the last Apennine glacier: recent in situ campaigns and modelling of Calderone glacial apparatus, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22579, https://doi.org/10.5194/egusphere-egu2020-22579, 2020

D2635 |
EGU2020-1341
Sofia-Katerina Kufner, Alex Brisbourne, Andy Smith, Sridhar Anandakrishnan, Tavi Murray, Rebecca Schlegel, Keith Nicholls, Dominic Hodgson, Michael Kendall, and Ian Lee

Microseismicity, induced by the sliding of a glacier over its bed and through bed deformation, can be used to characterize frictional properties of the ice-bed interface. Together with ice column deformation, these characteristics form the key parameters controlling ice stream flow. Here, we use naturally occuring seismicity to monitor temporal and spatial changes in bed properties at Rutford Ice Stream (RIS), West Antarctica, in order to characterize ongoing basal deformation and sliding. RIS is a significant contributor to the outflow of ice from West Antarctica, with speeds of ~1.1 m/day. Past geological and geophysical surveys, including drilling into the bed itself, have revealed pronounced bed topography and a sharp change in bed character along flow direction from presumably soft deformable to stiffer sediments. These complementary data as well as Rutford’s flow characteristics allow us to interpret the seismic data in their geological context.

Our data consist of three months of seismic recordings from a 35-station seismic network located ~40 km upstream the grounding line of RIS, being collected in the framework of the BEAMISH project during the 2018/19 field season. An event catalogue derived using the QuakeMigrate and Nonlinloc software packages reveals an active seismic environment (~40,000 events in three months) with locally clustered microseismicity. Microseismicity occurs near the ice-bed interface and is concentrated in the transition region between presumed-soft and presumed-hard sediments. Within the more compacted sediments further seismicity occurs, predominantly along topographic lows, which form elongated, flow parallel sub-glacial valleys. Within the regions of activity, seismicity tends to cluster in focused spots of particular high activity. Repeated basal seismicity at spatially restricted locations has been observed before and was interpreted as being caused by ‘sticky spots’ within a more ductile deforming matrix. Our results, showing a close alignment of these sticky spots along structural and topographic boundaries, may indicate that such features form major obstacles for basal glacial sliding. In addition to these spatial variations, the average event frequency varies over time. We estimate an ~15 day periodicity to the activity with as many as 1200 events/day during the active times and as few as ~100 events per day during the more-quiescent times. This roughly corresponds to the period of the spring-neap tidal cycle which has been shown to modulate the horizontal flow velocity of RIS. Time dependent variations in the frequency of microseismicity might suggest the glacial bed affected by these modulations.

How to cite: Kufner, S.-K., Brisbourne, A., Smith, A., Anandakrishnan, S., Murray, T., Schlegel, R., Nicholls, K., Hodgson, D., Kendall, M., and Lee, I.: Bed-character dependent microseismicity clustering at Rutford Ice Stream, West Antarctica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1341, https://doi.org/10.5194/egusphere-egu2020-1341, 2019

D2636 |
EGU2020-11594
Aurélien Mordret, Gauthier Guerin, Diane Rivet, Brad Lipovsky, and Brent Minchew

Part of the movement that occurs on all glaciers in Antarctica is a continuous and stable movement that unloads the ice into the sea. The Whillans Ice Plain (WIP) is a portion of the Whillans ice stream that measures 8000 km² for an ice thickness of 800 meters. This glacier has a unique characteristic of moving thanks to tidally modulated stick-slip events twice a day. The slip speed varies laterally across the glacier.  We measured surface wave velocity variations computed from ambient seismic noise cross-correlation. The cross-correlations make it possible to monitor temporally and spatially the seismic velocities at the bed of the glacier, associated with changes in poro-elastic parameters and frictional properties of the glacial till. We averaged our observations for the 78 stick-slip events of our dataset and managed to achieve a 5 min temporal resolution along the 45 min long slip events. The results show a decrease in velocity of about 9% of the S-wave velocity in the subglacial sediment layer about 30 minutes after the initiation of the slip. This velocity drop mainly affects the central part of the glacier. A 10% increase in porosity could induce this velocity decrease due to dilatancy. Dilatant strengthening results from this porosity increase, which in turn keeps the glacier in a slow-sliding regime. The high rate of seismic cycles on such a large scale makes the Whillans ice stream a unique laboratory to study transient aseismic slips in glacial context but also in active tectonic faults one. 

How to cite: Mordret, A., Guerin, G., Rivet, D., Lipovsky, B., and Minchew, B.: Imaging the poro-elastic properties of glacier beds using ambient seismic noise monitoring : application to Whillans ice stream, Antarctica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11594, https://doi.org/10.5194/egusphere-egu2020-11594, 2020

D2637 |
EGU2020-128
Alex Brisbourne, Andrew Smith, Tavi Murray, Rebecca Schlegel, Keith Nichols, Dominic Hodgson, Sridhar Anandakrishnan, and Sofia Kufner

Ice stream flow is predominantly controlled by sliding over the bed, deformation within the bed and deformation within the ice column. The significance of processes at the bed, now and in the future, remains uncertain due to a lack of knowledge of conditions at the ice stream bed. In the Austral summer of 2018/19, as part of the BEAMISH Project, three holes were drilled to the bed of Rutford Ice Stream to install instruments in the ice column and at the bed, and also sample the bed. Prior to drilling, three seismic profiles were acquired across the bed access sites. These data therefore provide a rare opportunity to compare in situ measurements of ice stream bed conditions with seismic reflection data. The seismic line acquisition was also repeated one year later to investigate any changes at the bed following the drilling and connection to the bed. We will use a combination of imaging, acoustic impedance calculation and wide-angle reflection amplitude variation to characterise the bed conditions using the seismic data.   

How to cite: Brisbourne, A., Smith, A., Murray, T., Schlegel, R., Nichols, K., Hodgson, D., Anandakrishnan, S., and Kufner, S.: Characterising the bed of Rutford Ice Stream, West Antarctica, using reflection seismic profiles, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-128, https://doi.org/10.5194/egusphere-egu2020-128, 2019

D2638 |
EGU2020-8274
Glenn Jones, Bernd Kulessa, Ana Ferreira, Martin Schimmel, Andrea Berbellini, and Andrea Morelli

Basal slip is an important mechanism by which glaciers and ice-sheets flow, and is a major source of uncertainty in simulations of ice-mass loss and sea level rise from the Greenland Ice Sheet (GrIS). Sub-ice geology is a dominant control on ice flow velocity with fast flow often coinciding with the presence of deformable subglacial till eroded from underlying sedimentary rocks. The subglacial geology of Greenland has received relatively little attention thus far and its control on ice flow is poorly understood. Seismic studies of the crust beneath the GrIS have been limited due to a lack of seismic stations and the reliance on earthquake event data. However, in the past decade, there has been a rapid increase in the number of both permanent and temporary seismic stations deployed in Greenland as well developments in ambient noise methods, allowing for improved spatial resolution of crustal geology.

 

Ellipticity measurements, the ratio of the horizontal to vertical component of a Rayleigh wave, have been shown to be particularly sensitive to the geological structure directly beneath the station. Ambient noise H/V measurements have been used for decades in geotechnical and civil engineering for site characterisation, making them a well-suited technique to determine the subglacial geology of the GrIS. Using all available broadband stations deployed on Greenland from 2012 to 2018 we extract Rayleigh wave ellipticity measurement from ambient noise data using the degree-of-polarization (DOP) method where meaningful signals are defined as a waveform with an arbitrary polarization which remains stable for a given time window. We invert these ellipticity measurements in the period range of 4 – 9 s to generate Vs profiles of the first 5 km beneath each station. Our inversions indicate that: (1) off-ice stations along the margins of the GrIS produce a good agreement with the litho1.0 model to within error and (2) an additional subglacial layer 1.0 - 2.0km thick with a Vs < 3.0km is necessary to match the data recorded at several of the on-ice stations. We attribute these observations to the widespread presence of sedimentary rocks beneath the GrIS, potentially capable of sustaining extensive subglacial till layers that can support enhanced basal slip.

How to cite: Jones, G., Kulessa, B., Ferreira, A., Schimmel, M., Berbellini, A., and Morelli, A.: Constraining subglacial geology using ambient noise Rayleigh wave ellipticity , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8274, https://doi.org/10.5194/egusphere-egu2020-8274, 2020

D2639 |
EGU2020-10952
Nathan Stevens, Collin Roland, Dougal Hansen, Emily Schwans, and Lucas Zoet

Parameterization of glacier sliding-laws remains a large source of uncertainty in modeling glacier and ice-sheet flow, requiring validation with experimental and observational data. In the case of ice flowing over a till-free, step-shaped bed, theory predicts bed resistance is independent of glacier sliding speed – a “rate neutral” sliding-law (e.g., Iken, 1981). However, experimental simulation of this system resulted in a notable anti-correlation between sliding speeds and bed resistance – a “rate weakening” sliding-law – that may give rise to basal seismicity (Zoet & Iverson, 2016). To investigate this discrepancy, we conducted a seismic field campaign on Saskatchewan Glacier, which is thought to have a stepped bed like those observed in adjacent glacier forefields. The campaign included a dense, 32 seismometer deployment during the middle of the 2019 melt season, complemented by continuous meteorologic, hydrologic, and GPS observations.

Visual and automated characterization of collected seismic data indicate abundant seismicity near the glacier’s bed. Basal seismicity clusters down-flow from an active moulin and a crevassed region likely connected to the bed. Rates of basal seismicity show a strong diurnal signal, consistently occurring 0.5-4 hours after peak surface melting and subglacial discharge, and continuous GPS data indicate temporary ice-flow acceleration during at least two diurnal seismic cycles. Spikes in seismic rate are also observed during most rain events with shorter response-times than diurnal cycles. The diurnal basal seismic cycle was interrupted by two periods of relative quiescence. The first lasted six days, initiating as mean air temperatures and peak daily subglacial discharge rose, and concluding after mean air temperatures and peak discharge declined. The second lasted one day following an abrupt drop in air temperature and was concurrent with reduced subglacial discharge.

We postulate that rapid surface water delivery to the bed strongly influences basal water pressure near delivery points, triggering bursts of seismicity on parts of Saskatchewan Glacier’s bed. Elevated rates of basal seismicity follow peak hydrologic flux through the subglacial drainage system, indicating that stick-slip motion likely occurs as water pressures fall from a transient. Some seismicity is accompanied by temporary acceleration of the glacier, consistent with results from Zoet & Iverson (2016). The six day period of relative quiescence may reflect reorganization of the subglacial hydrologic system into a more efficient drainage network in seismogenic regions, thus damping water pressure transients. Conversely, the one day quiescent period was likely the result of limited surface-water supply. We propose that temporary transitions from stable to stick-slip sliding occurred when basal water pressure exceed a critical threshold on parts of the bed, as modulated by surface-water supply and subglacial drainage efficiency.

Iken, A. (1981). The Effect of the Subglacial Water Pressure on the Sliding of a Glacier in an Idealized Numerical Model. Journal of Glaciology, 27(97).

Zoet, L. K., & Iverson, N. R. (2016). Rate-weakening drag during glacier sliding. Journal of Geophysical Research: Earth Surface, 121, 1328–1350. https://doi.org/10.1002/2016JF003909

How to cite: Stevens, N., Roland, C., Hansen, D., Schwans, E., and Zoet, L.: Basal Seismicity Forced by Surface-Water Supply on a Stepped-Bed Glacier: Saskatchewan Glacier, Alberta, Canada, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10952, https://doi.org/10.5194/egusphere-egu2020-10952, 2020

D2640 |
EGU2020-6385
Andrey Jakovlev, Sergey Kovalev, Egor Shimanchuk, Evgeniy Shimanchuk, and Aleksey Nubom

Despite the strong attention to the investigations in the Arctic its advance quite slowly. The harsh climatic conditions and big expenses slow down realization of the fieldwork in high latitudes. Therefore, scientists from over the world looks for new technologies, which could optimize and reduce the costs of the fieldworks that aimed at investigation of the geological structure beneath the Arctic Ocean. From March to May 2019 scientific expedition on the Expedition Vessel “Akademic Tryoshnikov” operated by the Arctic and Antarctic Research Institute that belongs to Rosgidromet were conducted in the framework of the program “TransArctica 2019” first stage. In the framework of the seismological experiments 6 temporary seismic stations at 4 different locations were installed on a drifted ice floe in the North Barents Sea. The first aim of the experiment was to elaborate technology of installation of the seismic stations on the drifting ice floes. The second aim was to check if obtained seismological records could be used for registration of the local and remote earthquakes, which are meant to investigate the lithosphere structure in the Arctic regions, and for investigation of the processes within the ice floe.

The stations were installed in the April 2019 on the ice floe near the EV “Akademik Tryoshnikov” that were “frizzed” in the ice floe and drifted together with them. After analysis of the recoded data the following types of the seismic signal generated by processes in the ice were observed:

  • - background signal from bending-gravitational waves with periods from 1 to 30 sec. Swell waves with periods from 17 to 30 sec were observed permanently during the whole period of network operation;
  • - continuous mechanical vibrations (self-oscillations) with a period of up to 2-3 sec;
  • - stick-slip relaxation self-oscillations with a period from 0.1 s to several minutes;
  • - mechanical movements of ice due to compression or stretching of ice caused by chaotic different scales fluctuations in the drift velocity of ice floes;
  • - process of ice fracturing due to compression or stretching of ice.

Results of monitoring of the ice cover has shown that in the most cases there are no direct correlations of processes within the ice floes and local hydrometeorological condition. During the process of ice cover fracturing an increased value of the ice horizontal movement were observed. Analysis of the seismic signal from ice events has shown that stick-slip events preceded origin of the ice fractures.

As a result of the initial analysis of the seismograms several signals from remote and regional earthquakes were detected. For example, an earthquake that according to the ISC bulletin occur at 08:18:23UTC on April 11, 2019 near the Japan (40.35°N, 143.35°E, 35 km depth, MS = 6.0) were detected. A local earthquake that occur approximately at 05:58UTC on April 10, 2019 at a distance of ~500 km. Due to close location of stations to each other the localization of the earthquake is impossible.

This work is supported by the RSCF project #18-17-00095.

How to cite: Jakovlev, A., Kovalev, S., Shimanchuk, E., Shimanchuk, E., and Nubom, A.: Temporary seismic network on drifting ice in the North Barents Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6385, https://doi.org/10.5194/egusphere-egu2020-6385, 2020

D2641 |
EGU2020-6916
| Highlight
Schlindwein Vera, Kirk Henning, Hiller Marc, Scholz John-Robert, and Schmidt-Aursch Mechita

Active and passive seismic monitoring of the cryosphere is mostly done with land seismometers on the surface of ice masses. Seismic monitoring beneath sea ice at the bottom of ice covered oceans has hardly been attempted, because ocean bottom seismometers (OBS) are difficult to recover in perennial sea ice. As a result, for example the tectonic activity of the Arctic mid-ocean ridge system is poorly known. Recently, the ambient seismic noise in long-term seismic records proved a useful tool to monitor the state of the sea ice cover. Since sea ice effectively dampens the formation of wave action, the power in the microseismic noise band, that is mostly generated by ocean wave action, shows seasonal variations which can be explored to study ocean wave climate in relation to the sea ice cover.

From September 2018 - September 2019, we deployed for the first time a network of 4 broadband ocean bottom seismometers at distances of about 10 km at a water depth of roughly 4 km near Gakkel Deep on eastern Gakkel Ridge, Arctic Ocean, from board RV Polarstern. We modified the Lobster-type OBS to include a Posidonia transponder that allowed to accurately track the OBS during descent and ascent and when surfacing underneath an ice floe. We then carefully broke the ice floes until the OBSs appeared in open water and could be recovered.

The network was designed to record local earthquakes along Gakkel Ridge, but it also yields valuable year-round data on the microseismic noise signal at the bottom of the Arctic Ocean in a marginal ice zone.

A first inspection of the data shows a clearly reduced power in the microseismic noise band compared to the Norwegian-Greenland Sea and strongly time dependent noise levels, that may potentially be related to temporary wave action when sea ice retreats during summer. However, the modified OBS structure with a large head buoy fixed to the OBS structure may also be prone to vibrations caused by ocean bottom currents. We will present an initial analysis of the seasonal evolution of the ambient seismic noise that will help to discriminate noise sources and evaluate the potential of such records to monitor the state of the sea ice cover.

How to cite: Vera, S., Henning, K., Marc, H., John-Robert, S., and Mechita, S.-A.: First Ocean Bottom Seismometer network underneath the ice-covered Arctic Ocean: Operational challenges and chances for monitoring the state of the sea ice cover, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6916, https://doi.org/10.5194/egusphere-egu2020-6916, 2020

D2642 |
EGU2020-7087
Sebastian Hellmann, Johanna Kerch, Melchior Grab, Henning Löwe, Ilka Weikusat, Andreas Bauder, and Hansruedi Maurer

Understanding the internal structure of glacier ice is of high interest for studying ice flow mechanics and glacier dynamics. The micro-scale deformation mechanisms cause a reorientation and alignment of the ice grains resulting in a polycrystalline structure with a strong anisotropy. By studying the crystal orientation fabric (COF), details about past and ongoing ice deformation processes can be derived. Usually, obtaining COF requires a work-intensive ice core analysis, which is typically carried out only at a few ice core samples. When similar information can be obtained from geophysical, for example, seismic experiments, a more detailed and more continuous image about the ice deformation would be available.
For checking the suitability of seismic data for such purposes, we have analysed the COF of several ice core samples extracted from Rhone Glacier, a temperate glacier located in the Central Swiss Alps. The COF analysis yield a polycrystalline elasticity tensor for a given volume of ice, from which we predicted seismic velocities for acoustic waves originating from any azimuth and inclination. The seismic data predicted were then verified with ultrasonic experiments conducted along the ice core in the vicinity of the analysed COF. Additional X-ray tomographic measurements yield further constraints about the microstructure, especially about the air bubble content in the ice affecting the data of the ultrasonic experiments. Predicted and measured velocities generally show a good match. This is a very encouraging result, because it suggests that in-situ measurements of seismic velocities can be employed for studying ice deformation. A possible option is to perform seismic cross-hole measurements within an array of boreholes drilled into the glacier ice.

How to cite: Hellmann, S., Kerch, J., Grab, M., Löwe, H., Weikusat, I., Bauder, A., and Maurer, H.: Comparison of seismic velocities derived from crystal orientation fabrics and ultrasonic measurements on an ice core, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7087, https://doi.org/10.5194/egusphere-egu2020-7087, 2020

D2643 |
EGU2020-7709
Adam Booth, Poul Christoffersen, Charlotte Schoonman, Andy Clarke, Bryn Hubbard, Robert Law, Sam Doyle, Tom Chudley, and Athena Chalari

Material anisotropy within a glacier both influences and is influenced by its internal flow regime. Anisotropy can be measured from surface seismic recordings, using either active sources or natural seismic emissions. In the past decade, Distributed Acoustic Sensing (DAS) has emerged as a new, and potentially transformative, seismic acquisition technology, involving determining seismic responses from the deformation of optical fibres. Although DAS has shown great potential within engineering and resources sectors, it has not yet been widely deployed in studies of glaciers and ice masses.

Here, we present results from a glaciological deployment of a DAS system. In July 2019, a Solifos BRUsens fibre optic cable was installed in a 1050 m borehole drilled on Store Glacier in West Greenland. Vertical seismic profiles (VSPs) were recorded using a Silixa iDAS interrogation unit, with seismic energy generated with a 7 kg sledgehammer striking a polyethene (UHMWPE) impact plate. A three-day sequence of zero-offset VSPs (with the source located ~1 m from the borehole top) were recorded to monitor the freezing of the cable, combined with offset-VSPs in along- and cross-flow directions, and radially at 300 m offset.

P-wave energy (frequency ~200 Hz) is detectable through the whole ice thickness, sampled at 1 m depth increments. The zero-offset reflectivity of the glacier bed is low, but reflections are detected from the apparent base of a subglacial sediment layer. S-wave energy is also detectable in the offset VSP records. The zero-offset VSPs show a mean vertical P-wave velocity of 3800 ± 140 m/s for the upper 800 m of the glacier, rising to 4080 ± 140 m/s between 900-950 m. In the deepest 50 m, velocity reduces to 3890 ± 80 m/s. This variation in vertical velocity is consistent with the development of an anisotropic ice fabric in the lowermost 10% of the glacier. The full dataset also contains natural seismic emissions, highlighting the potential of DAS as both an active and passive seismic monitoring tool.

DAS offers transformative potential for understanding the seismic properties of glaciers and ice sheets. The simplicity of the typical VSP geometry makes the interpretation of seismic travel-times less vulnerable to approximations, and thus the derivation of seismic properties more robust, than in conventional surface seismic surveys. As an addition, DAS facilitates VSP recording with unprecedented vertical and temporal resolution. Furthermore, the sensitivity of the optical-fibre to both P- and S-wave particle motion means that a comprehensive suite of acoustic and elastic properties can be inferred.

How to cite: Booth, A., Christoffersen, P., Schoonman, C., Clarke, A., Hubbard, B., Law, R., Doyle, S., Chudley, T., and Chalari, A.: Detecting anisotropy using Distributed Acoustic Sensing and fibre-optic seismology in a fast-flowing glacier in Greenland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7709, https://doi.org/10.5194/egusphere-egu2020-7709, 2020

D2644 |
EGU2020-15923
Mohammadreza Ershadi, Reinhard Drews, Carlos Martín, and Olaf Eisen

Understanding ice flow of ice sheets is not only important to predict their future evolution, but is also required for finding future ice-core sites with an intact stratigraphy and a constrained age-depth relationship of the corresponding climate record. Anisotropic ice flow, induced through the formation of aligned Crystal Orientation Fabric (COF), is in this context important as it may cause ice overturning and folding at larger depths. Here, we use a synthetic radar forward model to explore the feasibility of detecting the crystal orientation fabric orientation and strength using coherent, polarimetric ice-penetrating radar data (ApRES). We compare our results, with ApRES data collected in Antarctica. Some of the sites are located near deep drill ice-core sites (e.g., Dome C), and we validate our approach with ice-core data.

In multilayer models, we distinguish between birefringence (caused by ray propagation through anisotropic COF with unknown strength and orientation) and anisotropic scattering (caused by an unknown depth variability of anisotropic COF). We show analytically that the scattering ratio is determined by the angular dependence of co-polarization extinction nodes. Building on previous work, we infer COF orientation using the depolarization component, and COF strength from the gradient of polarimetric coherence, respectively.

We apply this approach to polarimetric ApRES datasets. We show COF orientation can often reliably be inferred as long as it does not change significantly with depth. Rotation of principal axis with depth, on the other hand, causes a complicated radar response that is not straightforwardly interpreted. At dome positions, where the ice anisotropy develops more gently compared to flank-flow settings, the degree of anisotropy can be estimated with the phase gradient method. This becomes increasingly more difficult for flank-flow settings where phase unwrapping is required. We delineate a number of anisotropic scattering zones which likely correspond to COF patterns changing abruptly. In some cases, boundaries between anisotropic scattering zones coincide with climate transitions within the ice.

We provide our model code in the form of a user-friendly GUI, enabling to quickly explore a wide range of possible COF patterns and their corresponding imprint in the radar data. This is useful both for scientific and educational purposes. Our analysis underlines the potential of coherent, polarimetric radars to infer the COF orientation of ice sheets also away from ice core sites. This will provide important data for the inclusion of ice anisotropy in ice-flow models in the future.

How to cite: Ershadi, M., Drews, R., Martín, C., and Eisen, O.: Application of polarimetric radar to infer ice fabric anisotropy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15923, https://doi.org/10.5194/egusphere-egu2020-15923, 2020

D2645 |
EGU2020-19541
Carlos Martin, Howard Conway, Michelle Koutnik, Catherine Ritz, Thomas Bauska, Reinhard Drews, and M. Reza Ershadi

The climatic conditions over ice sheets at the time of snow deposition and compaction imprint distinctive crystallographic properties to the resulting ice. As the snow gets buried, its macroscopic structure evolves due to vertical compression but retains traces of the climatic imprint that generate distinctive mechanical, thermal and optical properties. Because climate alternates between glaciar periods, that are colder and dustier, and interglacial periods, the ice sheets are composed from layers with alternating mechanical properties. Here we compare ice core dust content and crystal orientation fabrics, from the ice core records, with englacial vertical strain-rates, measured with a phase-sensitive radar (ApRES), at South Pole and EPICA Dome C ice cores. Similarly to previous observations, we show that ice deposited during glacial periods develops stronger crystal orientation fabrics. In addition, we show that ice deposited during glacial periods is harder to vertically compress and horizontally extend, up to about 3 times, but softer to shear. These variations in mechanical properties are typically ignored in ice-flow modelling but they could be critical to interpret ice core records. Also, we show that the changes in crystal orientation fabrics due to transitions from interglacial to glacial conditions can be detected by phase-sensitive radar. This information can be used to constrain age-depth in future ice-core locations.

How to cite: Martin, C., Conway, H., Koutnik, M., Ritz, C., Bauska, T., Drews, R., and Ershadi, M. R.: Climatic imprint in the mechanical properties of ice sheets and its effect on ice flow: Observations from South Pole and EPICA Dome C ice cores, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19541, https://doi.org/10.5194/egusphere-egu2020-19541, 2020

D2646 |
EGU2020-5759
Elizabeth Case and Jonathan Kingslake

Firn densification operates at the boundary between the atmosphere and ice sheets, impacting estimates of ice thickness change, paleoclimate reconstructions, and near-surface hydrology. Direct measurements of firn densification are scarce and firn densification models, which rely mostly on point measurements of density, disagree on the impact of environmental factors like surface temperature and accumulation rate. Phase-sensitive radar systems (pRES) can observe the movement of isochronal layers in firn and ice by tracking the relative two-way travel times (T) of radio waves. In this work, we demonstrate three methods for extracting measurements of densification velocities from pRES data. Method one uses independently measured firn densities to derive compaction velocities. Method two derives vertical velocities in the firn from an inversion that assumes a steady state and exponential density profile. Method three models changes in T using a semi-physical densification model and compares these changes to the pRES observations. We apply each method to radar data from three ice rises in the Weddell Sea Sector of West Antarctica. Results demonstrate how pRES can be used to explore the accumulation dependence of steady-state densification rates. Average accumulation rate is estimated from pRES measurements in areas that are approximately in steady state. Accumulation gradients can be seen across divides (Korff Ice Rise) and densification-rate differences are observed between relatively high (Fletcher Promontory) and low (Skytrain Ice Rise) accumulation environments. With minimal logistic requirements, new pRES systems like autonomous pRES could be inexpensively deployed to monitor firn densification. Furthermore, existing data may contain densification information even in cases when its deployment primarily targeted other processes.

How to cite: Case, E. and Kingslake, J.: Three methods for observing firn densification velocities with phase-sensitive radar, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5759, https://doi.org/10.5194/egusphere-egu2020-5759, 2020

D2647 |
EGU2020-5307
Emma Pearce, Adam Booth, Paul Sava, and Ian Jones

The transformation of snow into ice is a fundamental process in glaciology. The yearly accumulation of fresh snowfall increases the overburden pressure, changing the snow’s properties such that it transitions into firn and pure glacier ice thereafter. Additionally, periods of melt and variations in subsurface and surface conditions can lead to the presence of ice layers and firn aquifers within the firn column. Therefore, firn characteristics provide a tool for evaluating past and present climate conditions relating to the amount of snow accumulation, melt, temperature conditions and the subsequent preservation of the snow.

Due to the importance of relationships between firn and other glaciological processes (e.g., settling, sublimation, recrystallization and other deformation processes) it has not been possible to develop a theoretically-based model which accurately predicts firn properties with depth. Therefore, methods of measuring firn are either intrusive or rely on (potentially unreliable) empirical conversions. Full Waveform Inversion (FWI) may offer a new standard for glaciological seismic modelling, mitigating issues within current seismic modelling techniques and paving the way for the recovery of elastic properties, including density. Constraining firn properties also leads to improved corrections for deeper seismic responses, e.g. glacier bed reflectivity.

 

Using seismic datasets obtained from Norway’s Hardangerjøkulen Ice Cap (60.47°N, 7.49°W) along with varying synthetic firn column scenarios (introducing the presence of ice lenses and firn aquifers), we show how FWI can mitigate the dependence on intrusive techniques and empirical relationships. Furthermore, we compare the robustness of the FWI approaches versus traditional glaciological approaches to velocity model building (Herglotz-Wiechert inversion).

 

How to cite: Pearce, E., Booth, A., Sava, P., and Jones, I.: Seismic Full Waveform Inversion (FWI) to characterise the structure of firn, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5307, https://doi.org/10.5194/egusphere-egu2020-5307, 2020

D2648 |
EGU2020-4851
Alex Priestley

Modelling and monitoring seasonal snow is critical for water resource management, flood forecasting and avalanche risk prediction. Snowmelt processes are of particular importance. The behaviour of liquid water in snow has a big influence on melting processes, but is difficult to measure and monitor non-invasively. Recent work has shown the promise of using electrical self potential measurements as a snow hydrology sensor. Self potential magnitudes can be used to infer both liquid water content of snow and bulk meltwater runoff. In autumn 2018, a prototype self potential monitoring array was installed at Col de Porte in the French Alps, alongside full hydrological and meteorological measurements made routinely at the site. Self potential measurements were taken throughout the following winter, with manual snow pit data obtained in spring 2019. A physically-based snow hydrology model was run for the winter, and an electrical model was coupled to the snow model to create a synthetic set of self potential observations. These synthetic observations were compared to the observed self potential magnitudes to evaluate the effectiveness of the snow model, and to investigate the potential for using the self potential array as part of a coupled geophysical monitoring and modelling system.

How to cite: Priestley, A.: In situ geophysical monitoring of liquid water movement in an Alpine snowpack from self potential signals, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4851, https://doi.org/10.5194/egusphere-egu2020-4851, 2020

D2649 |
EGU2020-17824
Achille Capelli, Franziska Koch, Patrik Henkel, Markus Lamm, Christoph Marty, and Jürg Schweizer

The water stored in the snowpack is a crucial contribution to the hydrological cycle in mountain areas. Estimating the spatial distribution and temporal evolution of snow water equivalent (SWE) in mountain regions is, therefore, a key question in snow hydrology research. For this reason, direct measurements of SWE are still essential, but they are often scarce, not easy to install and maintain, mostly non-continuous or rather expensive. A promising alternative to conventional SWE in-situ measurement methods is a newly developed method based on signals of the freely available Global Navigation Satellite System (GNSS), which can be received with standard low-cost sensors. In general, this measurement technique is based on signal differences between one GNSS antenna buried below the snowpack and one reference antenna above the snow cover. The signal differences reflect the GNSS carrier phase time delay and the GNSS signals strength attenuation within the snowpack, which can be translated into SWE and the snow liquid water content (LWC). So far, this method showed excellent results over several years at the high-alpine test and validation site Weissfluhjoch (Eastern Swiss Alps, 2540 m asl.). Currently, our aim is to assess whether this method is suitable for deriving SWE continuously with reasonable accuracy also at other locations with different characteristics. Therefore, we set up further GNSS sensors at different elevations, where the snow characteristics can vary considerably. At lower elevations the snow cover is normally shallower and is more frequently subject to melt-freeze cycles leading to faster snow aging and different snow densities. Moreover, rapid transition from dry- to wet-snow conditions as well as steep valley sites can be seen as a challenge. In total, we were operating for two season four GNSS stations along a steep elevation gradient (820 m, 1185 m, 1510 m, and 2540 m asl.) within only a few kilometres in the Eastern Swiss Alps. For validation purposes, we monitored SWE and snow height manually and with additional automatic sensors at all locations. We analysed the GNSS SWE derivation accuracy in general and in detail for different meteorological conditions as snowfall, snow settlement, rain on snow and dry or wet snow periods. Eventually, we compared the GNSS results with results from numerical snow cover models.

How to cite: Capelli, A., Koch, F., Henkel, P., Lamm, M., Marty, C., and Schweizer, J.: Assessment of operational monitoring of snow water equivalent measurements with low-cost GNSS sensors, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17824, https://doi.org/10.5194/egusphere-egu2020-17824, 2020

D2650 |
EGU2020-20935
Patrick Henkel, Markus Lamm, and Franziska Koch

The snow water equivalent (SWE) is a key parameter in hydrology. In the past years, the signals of Global Navigation Satellite System (GNSS) receivers were discovered to be very attractive for SWE monitoring. The set-up of GNSS-based SWE monitoring typically consists of two GNSS receivers, whereas one is placed on the ground to sense the signal attenuation and time delay being caused by the snow pack. A second receiver is placed above the snow and serves as reference receiver. The measurements of both receivers are differenced to eliminate the common effect of errors in the satellite orbits and clocks, satellite phase and code biases and atmospheric errors, while the information on the snow is kept.

In this talk, we discuss the replacement of the reference receiver by a virtual reference station (VRS). The VRS is a virtual GNSS reference station, whose corrections are obtained by interpolation of the corrections from multiple surrounding reference stations to achieve a higher accuracy at the user location. The concept of VRS was first developed by Trimble and is widely used in today's real-time kinematic (RTK) positioning receivers. The concept of VRS is also attractive for snow monitoring, since the GNSS reference receiver could be avoided resulting in a lower power consumption and less costs. Moreover, this could be a big advantage for applications in slopes, which are, e.g., potentially avalanche prone. Within the hardware setup of our GNSS SWE sensors, an internet communication link for the reception of the corrections from the VRS corrections at the SWE monitoring site is already available.

However, there are also two challenges: First, the SWE monitoring stations in Alpine areas are typically at a significantly different altitude than the geodetic reference receivers. The differential tropospheric zenith delay is not negligible for altitudinal differences of more than 100 m. Therefore, the differential tropospheric delay needs to be considered either in the determination of VRS corrections or alternatively in the SWE determination. For altitudinal differences of less than 1000 m, the differential tropospheric zenith delay could be approximated by a model with sufficient accuracy. The residual modelling error is projected to the SWE estimate. Second, the use of a VRS instead of a conventional GNSS reference station requires a stronger data link, since the GNSS raw data (pseudoranges, carrier phases and carrier-to-noise power ratio measurements from all tracked satellites) need to be transmitted besides the final SWE results. However, an LTE link is totally sufficient.

Besides the methodology, we will also focus on specific hardware implementations.

How to cite: Henkel, P., Lamm, M., and Koch, F.: Determination of Snow Water Equivalent with only one Global Navigation Satellite System receiver and a Virtual Reference Station, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20935, https://doi.org/10.5194/egusphere-egu2020-20935, 2020

D2651 |
EGU2020-10131
Theresa Maierhofer, Timea Katona, Christin Hilbich, Christian Hauck, and Adrian Flores-Orozco

Permafrost regions are highly sensitive to climate changes, which has significant implications for the hydrological regimes and the mechanical state of the subsurface leading to natural hazards such as rock slope failures. Therefore, a better understanding of the future evolution and dynamics of mountain permafrost is highly relevant and monitoring of the thermal state of permafrost has become an essential task in the European Alps. Geophysical methods have emerged as well-suited to support borehole data and investigate the spatial distribution and temporal changes of temperature and the degradation of permafrost. In particular, electrical resistivity tomography (ERT) has developed into a routine imaging tool for the quantification of ice-rich permafrost, commonly associated with a significant increase in the electrical resistivity. However, in many cases, the interpretation of the subsurface electrical resistivity is ambiguous and additional information would improve the quantification of the ice content within the subsurface. Theoretical and laboratory studies have suggested that ice exhibits a characteristic induced electrical polarization response. Our results from an extensive field programme including many morphologically different mountain permafrost sites now indicate that this IP response may indeed be detected in the field suggesting the potential of the Induced Polarization (IP) method to overcome such ambiguities. We present here Spectral IP (SIP) mapping results conducted over a broad range of frequencies (0.1-225 Hz) at four representative permafrost sites of the Swiss-, Italian- and Austrian Alps. The mapping results have been used to install long-term permafrost monitoring arrays for a better understanding of subsurface variations associated to climate change. All SIP study sites are located at elevations around 2600 - 3000 m and include comprehensive geophysical and temperature data for validation. We focus on the spatial characterization of each site to address different research questions: to (i) reproduce and improve the mapping of the spatial permafrost extent inferred from previous investigations in the Lapires talus slope,Western Swiss Alps, to (ii) improve the geophysical characterization of the Sonnblick monitoring site located in the Austrian Central Alps, to (iii) determine the transition between permafrost and non-permafrost at the Schilthorn site, Bernese Alps, Switzerland, and to (iv) find the best-suited location for a SIP monitoring profile and conduct year-round measurements at the Cime Bianche site, Western Italian Alps. Our various field applications demonstrate the potential of the IP method for characterizing and monitoring permafrost systems in high-mountain environments.

How to cite: Maierhofer, T., Katona, T., Hilbich, C., Hauck, C., and Flores-Orozco, A.: Prospecting alpine permafrost with Spectral Induced Polarization in different geomorphological landforms, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10131, https://doi.org/10.5194/egusphere-egu2020-10131, 2020

D2652 |
EGU2020-20081
Jonas K. Limbrock, Maximilian Weigand, and Andreas Kemna

Geoelectrical methods are increasingly used for non-invasive characterization and monitoring of permafrost sites, since the electrical properties of the subsoil are sensitive to the phase change of liquid to frozen water. In this context, electrical subsurface parameters act as proxies for temperature and ice content.  However, it is still challenging to distinguish between air and ice in the pore space of the rock based on the resistivity method alone due to their similarly low electrical conductivity. This ambiguity in the subsurface conduction properties can be reduced by considering the spectral electrical polarization signature of ice using the Spectral Induced Polarization (SIP) method, in which the complex, frequency-dependent impedance is measured. These measurements are hypothesized to allowing for the quantification of ice content (and thus differentiation of ice and air), and for the improved thermal characterization of alpine permafrost sites.

In the present study, vertical SIP sounding measurements have been made at different alpine permafrost sites in a frequency range from 100 mHz to 45 kHz. From borehole temperature measurements, we know the thermal state of these sites during our SIP soundings, i.e., an active layer thickness of about 4 m at the Schilthorn field site. In order to understand and to calibrate ice and temperature relationships, the electrical impedance was likewise measured on water-saturated soil and rock samples from these field sites in a frequency range from 10 mHz to 45 kHz during controlled freeze-thaw cycles (+20°C to -40°C) in the laboratory.

For field and laboratory measurements, the resistance (impedance magnitude) shows a similar temperature dependence, with increasing resistance for decreasing temperatures. For each sample, the impedance phase spectra exhibit the well-known temperature-dependent relaxation behavior of ice at higher frequencies (1 kHz - 45 kHz), with an increasing polarization magnitude for lower temperatures or larger depths of investigation, respectively. At lower frequencies (1 Hz - 1 kHz), a polarization with a low frequency dependence is observed in the unfrozen state of the samples. We interpret this response as membrane polarization, considering that it decreases in magnitude with decreasing temperature (i.e., with ongoing freezing).

Using the independently measured borehole temperature data, a systematic comparison of the SIP laboratory and field measurements indicates the possibility of a thermal characterization of an alpine permafrost site using SIP.

How to cite: Limbrock, J. K., Weigand, M., and Kemna, A.: Improved thermal characterization of alpine permafrost sites by broadband SIP measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20081, https://doi.org/10.5194/egusphere-egu2020-20081, 2020

D2653 |
EGU2020-5104
Jacopo Boaga, Marcia Phillips, Jeannette Noetzli, Anna haberkorn, Robert Kenner, and Alexander Bast

The characterization of the active layer (AL) in mountain permafrost is an important part of monitoring climate change effects in periglagical environments and may help to determine potential slope instability. Permafrost affects 25% of the Northern Hemisphere and 17% of the entire Earth. It has been studied for decades both in the polar regions and – starting a few decades later – in high mountain environments. Typical point information from permafrost boreholes can be extended to wider areas by geophysical prospecting and provide information that cannot be detected by thermal observations alone.

During Summer 2019 we performed several geophysical surveys at permafrost borehole sites in the Swiss Alps. We focused on electrical resistivity tomography (ERT) and Frequency Domain Electro-magnetic techniques (FDEM) to compare the methods and test the applicability of FDEM for active layer characterization, i.e., its thickness and lateral continuity. ERT provides an electrical image of the subsoil and can discern active layer thickness, changes in ground ice and geological features of the subsoil. From a logistic point of view a contactless method such as FDEM would be preferable : i) it can provide electrical properties of the subsoil with no need of physical electrical contact with the soil; ii) it can cover a wider area of exploration compared to ERT, iii) it is faster and data collection is simpler than with ERT due to lighter instruments and less preparation time needed.

Based on the FDEM surveys at the Swiss permafrost sites we were able to detect the frozen/unfrozen boundary and to achieve results that were in agreement with those obtained from classical ERT and borehole temperature data. The results were promising for future active layer monitoring with the contactless FDEM method.

How to cite: Boaga, J., Phillips, M., Noetzli, J., haberkorn, A., Kenner, R., and Bast, A.: The use of Frequency Domain Electro-magnetometry for the characterization of permafrost active layers: case studies in the Swiss Alps, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5104, https://doi.org/10.5194/egusphere-egu2020-5104, 2020

D2654 |
EGU2020-10334
Chuangxin Lyu, Thomas Ingeman-Nielsen, Seyed Ali Ghoreishian Amiri, Gudmund Reidar Eiksund, and Gustav Grimstad

Abstract. The climate change has aroused great concern on the stability and durability of the infrastructure installed on permafrost, especially for frozen saline clay with a large amount of unfrozen water content at subzero temperature. The joint electrical resistivity and acoustic velocity measurements are conducted for frozen saline sand and onsøy clay with 50% clay content and 20~40 g/L salinity in order to determine the unfrozen water content. A systematic program of tests involves the saline sand with different salinity, natural onsøy clay with the variable of temperature and freezing-thawing cycles and reconstituted onsøy clay with distinctive density and salinity. The data analysis of measurement results in combination with previous joint measurements for frozen soil resolves the effect of temperature, salinity, soil type and freezing-thawing cycles on the acoustic and electrical properties. An increase of temperature, fine content and salinity results in a decrease of both acoustic velocity and electrical resistivity. Electrical resistivity is sensitive to salinity, while acoustic velocity changes substantially near thawing temperature. We also find that both natural and reconstituted clay with similar water content and salinity show quite different acoustic velocity and electrical resistivity, which indicates that ice crystal structures are distinctive between natural and reconstituted samples. Besides, P-wave velocity is much more sensitive to the fabric change or induced cracks than electrical resistance during freezing-thawing cycles.  In the end, acoustic models like the weighted equation (Lee et al., 1996), Zimmerman and King’s model (King et al., 1988) and BGTL (Lee, 2002) are applied to the UWS estimates based on P-wave velocity and electrical models like Archine’s law are adopted based on electrical resistance. Both estimated UWS from different methods is not always consistent. The difference can be up to 20%.

Keywords: Frozen Saline Clay, Acoustic Velocity, Electrical Resistance, Unfrozen Water Saturation

References:

King, M. S., Zimmerman, R. W., & Corwin, R. F. (1988, May). Seismic and Electrical-Properties of Unconsolidated Permafrost. Geophysical Prospecting, 36(4), 349-364. https://doi.org/10.1111/j.1365-2478.1988.tb02168.x

Lee, J. S. (2002). Biot–Gassmann theory for velocities of gas hydrate-bearing sediments.

Lee, M. W., Hutchinson, D. R., Collett, T. S., & Dillon, W. P. (1996). Seismic velocities for hydrate-bearing sediments using weighted equation. Journal of Geophysical Research: Solid Earth, 101(B9), 20347-20358. https://doi.org/10.1029/96jb01886

 

How to cite: Lyu, C., Ingeman-Nielsen, T., Ali Ghoreishian Amiri, S., Reidar Eiksund, G., and Grimstad, G.: Joint Acoustic and Electrical Measurements for Unfrozen Water Saturation of Frozen Saline Soil, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10334, https://doi.org/10.5194/egusphere-egu2020-10334, 2020

D2655 |
EGU2020-11808
remi valois, Nicole Schafer, Giulia De Pasquale, Gonzalo Navarro, and Shelley MacDonell

Rock glaciers play an important hydrological role in the semiarid Andes (SA; 27º-35ºS). They cover about three times the area of uncovered glaciers and they are an important contribution to streamflow when water is needed most, especially during dry years and in the late summer months. Their characteristics such as their extension in depth and their ice content is poorly known. Here, we present a case study of one active rock glacier and periglacial inactive geoform in Estero Derecho (~30˚S), in the upper Elqui River catchment, Chile. Three geophysical methods (ground-penetrating radar and electrical resistivity and seismic refraction tomography) were combined to detect the presence of ice and understand the internal structure of the landform. The results suggest that the combination of electrical resistivity and seismic velocity provide relevant information on ice presence and their geometry. Radargrams shows diffraction linked to boulders presence but some information regarding electromagnetic velocity could be extracted. These results strongly suggest that such landforms contain ice, are therefore important to include in future inventories and should be considered when evaluating the hydrological importance of a particular region.

 

How to cite: valois, R., Schafer, N., De Pasquale, G., Navarro, G., and MacDonell, S.: Geophysical characterization of inactive/active rock glaciers in the semi-arid Andes using seismic, geoelectrics and GPR, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11808, https://doi.org/10.5194/egusphere-egu2020-11808, 2020

D2656 |
EGU2020-8136
Mariusz Majdanski, Artur Marciniak, Bartosz Owoc, Wojciech Dobiński, Tomasz Wawrzyniak, Marzena Osuch, Adam Nawrot, and Michał Glazer

The Arctic regions are the place of the fastest observed climate change. One of the indicators of such evolution are changes occurring in the glaciers and the subsurface in the permafrost. The active layer of the permafrost as the shallowest one is well measured by multiple geophysical techniques and in-situ measurements.

Two high arctic expeditions have been organized to use seismic methods to recognize the shape of the permafrost in two seasons: with the unfrozen ground (October 2017) and frozen ground (April 2018). Two seismic profiles have been designed to visualize the shape of permafrost between the sea coast and the slope of the mountain, and at the front of a retreating glacier. For measurements, a stand-alone seismic stations has been used with accelerated weight drop with in-house modifications and timing system. Seismic profiles were acquired in a time-lapse manner and were supported with GPR and ERT measurements, and continuous temperature monitoring in shallow boreholes.

Joint interpretation of seismic and auxiliary data using Multichannel analysis of surface waves, First arrival travel-time tomography and Reflection imaging show clear seasonal changes affecting the active layer where P-wave velocities are changing from 3500 to 5200 m/s. This confirms the laboratory measurements showing doubling the seismic velocity of water-filled high-porosity rocks when frozen. The same laboratory study shows significant (>10%) increase of velocity in frozen low porosity rocks, that should be easily visible in seismic.

In the reflection seismic processing, the most critical part was a detailed front mute to eliminate refracted arrivals spoiling wide-angle near-surface reflections. Those long offset refractions were however used to estimate near-surface velocities further used in reflection processing. In the reflection seismic image, a horizontal reflection was traced at the depth of 120 m at the sea coast deepening to the depth of 300 m near the mountain.

Additionally, an optimal set of seismic parameters has been established, clearly showing a significantly higher signal to noise ratio in case of frozen ground conditions even with the snow cover. Moreover, logistics in the frozen conditions are much easier and a lack of surface waves recorded in the snow buried geophones makes the seismic processing simpler.

Acknowledgements               

This research was funded by the National Science Centre, Poland (NCN) Grant UMO-2015/21/B/ST10/02509.

How to cite: Majdanski, M., Marciniak, A., Owoc, B., Dobiński, W., Wawrzyniak, T., Osuch, M., Nawrot, A., and Glazer, M.: Geophysical imaging of permafrost in the SW Svalbard – the result of two high arctic expeditions to Spitsbergen , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8136, https://doi.org/10.5194/egusphere-egu2020-8136, 2020