scholarly journals Observing the subglacial hydrology network and its dynamics with a dense seismic array

2021 ◽  
Vol 118 (28) ◽  
pp. e2023757118
Author(s):  
Ugo Nanni ◽  
Florent Gimbert ◽  
Philippe Roux ◽  
Albanne Lecointre
Keyword(s):  
Water Flow ◽  
Climate Warming ◽  
Ice Sheets ◽  
Drainage System ◽  
Seismic Source ◽  
Seismic Array ◽  
Glacier Flow ◽  
Dense Seismic Array ◽  
Seismic Source Location ◽  
Subglacial Drainage

Subglacial water flow strongly modulates glacier basal motion, which itself strongly influences the contributions of glaciers and ice sheets to sea level rise. However, our understanding of when and where subglacial water flow enhances or impedes glacier flow is limited due to the paucity of direct observations of subglacial drainage characteristics. Here, we demonstrate that dense seismic array observations combined with an innovative systematic seismic source location technique allows the retrieval of a two-dimensional map of a subglacial drainage system, as well as its day-to-day temporal evolution. We observe with unprecedented detail when and where subglacial water flows through a cavity-like system that enhances glacier flow versus when and where water mainly flows through a channel-like system that impedes glacier flow. Most importantly, we are able to identify regions of high hydraulic connectivity within and across the cavity and channel systems, which have been identified as having a major impact on the long-term glacier response to climate warming. Applying a similar seismic monitoring strategy in other glacier settings, including for ice sheets, may help to diagnose the susceptibility of their dynamics to increased meltwater input due to climate warming.

Investigating Spatio-temporal Changes in Subglacial Hydrology from Dense Array Seismology.

2020 ◽  
Author(s):  
Ugo Nanni ◽  
Florent Gimbert ◽  
Philippe Roux ◽  
Albanne Lecointre
Keyword(s):  
Water Flow ◽  
Seismic Noise ◽  
Water Pressure ◽  
Noise Analysis ◽  
Drainage System ◽  
Dense Array ◽  
Noise Sources ◽  
Spatio Temporal ◽  
Surrounding Areas ◽  
Subglacial Drainage

<p>Subglacial hydrology strongly modulates glacier basal sliding, and thus likely exerts a major control on ice loss and sea-level rise. However, the limited direct and spatialized observations of the subglacial drainage system make difficult to assess the physical processes involved in its development. Recent work shows that detectable seismic noise is generated by subglacial water flow, such that seismic noise analysis may be used to retrieve the physical properties of subglacial channelized water flow. Yet, investigating the spatial organisation of the drainage system (e.g. channels numbers and positions) together with its evolving properties (e.g. pressure conditions) through seismic observations remains to be done. The objective of this study is to bring new insights on the subglacial hydrology spatio-temporal dynamics using dense array seismic observations.</p><p>We use 1-month long ground motion records at a hundred of sensors deployed on the Argentière Glacier (French Alps) during the onset of the melt season, when the subglacial drainage system is expected to strongly evolve in response to the rapidly increasing water input. We conduct a multi-method approach based on the analysis of both amplitude and phase maps of seismic signals. We observe characteristic spatial patterns, consistent across those independent approaches, which we attribute to the underlying subglacial drainage system.</p><p>The phase-driven approach shows seismic noise sources that focuses in the along-flow direction as the water input increases. We identify this evolution as the development of the main subglacial channel whose position is coherent with the one expected from hydraulic potential calculations. During periods of rapid changes in water input (5 days over 31) and concomitant glacier acceleration the amplitude-driven approach shows spatial pattern highly consistent with the seismic noise sources location. At this time, we suggest that the spatial variations in the amplitude are representative of the water pressure conditions in subglacial channels and surrounding areas. Our spatialized observations therefore reveal the spatio-temporal evolution of the subglacial drainage system together with its changing pressure conditions. We observe, for instance, that channels develop at the very onset of the melt-season and rapidly capture the water from surrounding areas. Such unique observations may allow to better constrain the physics of subglacial water flow and therefore strengthen our knowledge on the dynamics of subglacial environments.</p>

scholarly journals Channelized subglacial drainage over a deformable bed

1994 ◽  
Vol 40 (134) ◽  
pp. 3-15 ◽  
Author(s):  
Joseph S. Walder ◽  
Andrew Fowler
Keyword(s):  
Water Pressure ◽  
Ice Sheets ◽  
Drainage System ◽  
West Antarctica ◽  
Plausible Assumption ◽  
Basal Ice ◽  
Ice Stream B ◽  
Steep Slopes ◽  
Subglacial Drainage

AbstractWe develop theoretically a description of a possible subglacial drainage mechanism for glaciers and ice sheets moving over saturated, deformable till. The model is based on the plausible assumption that flow of water in a thin film at the ice-till interface is unstable to the formation of a channelized drainage system, and is restricted to the case in which meltwater cannot escape through the till to an underlying aquifer. In describing the physics of such channelized drainage, we have generalized and extended Röthlisberger’s model of channels cut into basal ice to include “canals” cut into the till, paying particular attention to the role of sediment properties and the mechanics of sediment transport. We show that sediment-floored Röthlisberger (R) channels can exist for high effective pressures, and wide, shallow, ice-roofed canals cut into the till for low effective pressures. Canals should form a distributed, non-arborescent system, unlike R channels. For steep slopes typical of alpine glaciers, both drainage systems can exist, but with the water pressure lower in the R channels than in the canals; the canal drainage should therefore be unstable in the presence of channels. For small slopes typical of ice sheets, only canals can exist and we therefore predict that, if channelized meltwater flow occurs under ice sheets moving over deformable till, it takes the form of shallow, distributed canals at low effective pressure, similar to that measured at Ice Stream B in West Antarctica. Geologic evidence derived from land forms and deposits left by the Pleistocene ice sheets in North America and Europe is also consistent with predictions of the model.

scholarly journals Glacier Speed-Up Events and Subglacial Hydrology on the Lower Franz Josef Glacier, New Zealand

2021 ◽  
Author(s):  
◽  
Laura M. Kehrl
Keyword(s):  
New Zealand ◽  
Water Pressure ◽  
Drainage System ◽  
Temporal Variations ◽  
Ice Flow ◽  
Speed Up ◽  
Glacier Velocity ◽  
Glacier Flow ◽  
Flow Velocities ◽  
Subglacial Drainage

<p>The contribution of glacier mass loss to future sea level rise is still poorly constrained (Lemke and others, 2007). One of the remaining unknowns is how water inputs influence glacier velocity. Short-term variations in glacier velocity occur when a water input exceeds the capacity of the subglacial drainage system, and the subglacial water pressure increases. Several studies (Van de Wal and others, 2008; Sundal and others, 2011) have suggested that high ice-flow velocities during these events are later offset by lower ice-flow velocities due to a more efficient subglacial drainage system. This study combines in-situ velocity measurements with a full Stokes glacier flowline model to understand the spatial and temporal variations in glacier flow on the lower Franz Josef Glacier, New Zealand. The Franz Josef Glacier experiences significant water inputs throughout the year (Anderson and others, 2006), and as a result, the subglacial drainage system is likely well-developed. In March 2011, measured ice-flow velocities increased by up to 75% above background values in response to rain events and by up to 32% in response to diurnal melt cycles. These speed-up events occurred at all survey locations across the lower glacier. Through flowline modelling, it is shown that the enhanced glacier flow can be explained by a spatially-uniform subglacial water pressure that increased during periods of heavy rain and glacier melt. From these results, it is suggested that temporary spikes in water inputs can cause glacier speed-up events, even when the subglacial hydrology system is well-developed (cf. Schoof, 2010). Future studies should focus on determining the contribution of glacier speed-up events to overall glacier motion.</p>

scholarly journals Channelized subglacial drainage over a deformable bed

1994 ◽  
Vol 40 (134) ◽  
pp. 3-15 ◽  
Author(s):  
Joseph S. Walder ◽  
Andrew Fowler
Keyword(s):  
Water Pressure ◽  
Ice Sheets ◽  
Drainage System ◽  
West Antarctica ◽  
Plausible Assumption ◽  
Basal Ice ◽  
Ice Stream B ◽  
Steep Slopes ◽  
Subglacial Drainage

AbstractWe develop theoretically a description of a possible subglacial drainage mechanism for glaciers and ice sheets moving over saturated, deformable till. The model is based on the plausible assumption that flow of water in a thin film at the ice-till interface is unstable to the formation of a channelized drainage system, and is restricted to the case in which meltwater cannot escape through the till to an underlying aquifer. In describing the physics of such channelized drainage, we have generalized and extended Röthlisberger’s model of channels cut into basal ice to include “canals” cut into the till, paying particular attention to the role of sediment properties and the mechanics of sediment transport. We show that sediment-floored Röthlisberger (R) channels can exist for high effective pressures, and wide, shallow, ice-roofed canals cut into the till for low effective pressures. Canals should form a distributed, non-arborescent system, unlike R channels. For steep slopes typical of alpine glaciers, both drainage systems can exist, but with the water pressure lower in the R channels than in the canals; the canal drainage should therefore be unstable in the presence of channels. For small slopes typical of ice sheets, only canals can exist and we therefore predict that, if channelized meltwater flow occurs under ice sheets moving over deformable till, it takes the form of shallow, distributed canals at low effective pressure, similar to that measured at Ice Stream B in West Antarctica. Geologic evidence derived from land forms and deposits left by the Pleistocene ice sheets in North America and Europe is also consistent with predictions of the model.

A Multi-Physics Experiment with a Temporary Dense Seismic Array on the Argentière Glacier, French Alps: The RESOLVE Project

2021 ◽  
Author(s):  
Florent Gimbert ◽  
Ugo Nanni ◽  
Philippe Roux ◽  
Agnès Helmstetter ◽  
Stéphane Garambois ◽  
...  
Keyword(s):  
Water Flow ◽  
Template Matching ◽  
Water Discharge ◽  
Seismic Array ◽  
Global Navigation Satellite Systems ◽  
Seismic Velocities ◽  
Stick Slip ◽  
French Alps ◽  
Wide Range ◽  
Dense Seismic Array

Abstract Recent work in the field of cryo-seismology demonstrates that high-frequency (&gt;1  Hz) seismic waves provide key constraints on a wide range of glacier processes, such as basal friction, surface crevassing, or subglacial water flow. Establishing quantitative links between the seismic signal and the processes of interest, however, requires detailed characterization of the wavefield, which, at high frequencies, necessitates the deployment of large and dense seismic arrays. Although dense seismic array monitoring has recently become increasingly common in geophysics, its application to glaciated environments remains limited. Here, we present a dense seismic array experiment made of 98 three-component seismic stations continuously recording during 35 days in early spring 2018 on the Argentière Glacier, French Alps. The seismic dataset is supplemented with a wide range of complementary observations obtained from ground-penetrating radar, drone imagery, Global Navigation Satellite Systems positioning, and in situ measurements of basal glacier sliding velocities and subglacial water discharge. We present first results through conducting spectral analysis, template matching, matched-field processing, and eikonal-wave tomography. We report enhanced spatial resolution on basal stick slip and englacial fracturing sources as well as novel constraints on the heterogeneous nature of the noise field generated by subglacial water flow and on the link between crevasse properties and englacial seismic velocities. We outline in which ways further work using this dataset could help tackle key remaining questions in the field.

scholarly journals Changes in geometry and subglacial drainage derived from digital elevation models: Unteraargletscher, Switzerland, 1927–97

2005 ◽  
Vol 40 ◽  
pp. 20-24 ◽  
Author(s):  
Urs H. Fischer ◽  
André Braun ◽  
Andreas Bauder ◽  
Gwenn E. Flowers
Keyword(s):  
Water Flow ◽  
Digital Elevation Models ◽  
Drainage System ◽  
System Structure ◽  
Water Drainage ◽  
Drainage Pattern ◽  
Glacier Surface ◽  
Digital Elevation ◽  
Bed Changes ◽  
Subglacial Drainage

AbstractDigital elevation models of the bed and surface of Unteraargletscher, Switzerland, are used to reconstruct the theoretical pattern of basal water drainage for the years 1927, 1947, 1961 and 1997, during which period the glacier was thinning and receding. The theoretical drainage pattern for 1997 compares well, in a broad sense, with the locations of active moulins and the hydraulic connection status of boreholes drilled to the glacier bed. Changes in the basal water-flow pattern over the period 1927–97 that are revealed by the theoretical reconstructions of the subglacial drainage system structure are likely to have resulted from changes in glacier geometry. Concurrent with the retreat and thinning of the glacier, the height of medial moraines increased, probably due to the insulating effect of the debris cover reducing the melt of the underlying ice. This increase of moraine heights has led to the formation of hydraulic barriers at the glacier bed such that water flow has become channelized beneath the ice along drainage axes that parallel the course of the medial moraines on the glacier surface.

scholarly journals Glacier Speed-Up Events and Subglacial Hydrology on the Lower Franz Josef Glacier, New Zealand

2021 ◽  
Author(s):  
◽  
Laura M. Kehrl
Keyword(s):  
New Zealand ◽  
Water Pressure ◽  
Drainage System ◽  
Temporal Variations ◽  
Ice Flow ◽  
Speed Up ◽  
Glacier Velocity ◽  
Glacier Flow ◽  
Flow Velocities ◽  
Subglacial Drainage

<p>The contribution of glacier mass loss to future sea level rise is still poorly constrained (Lemke and others, 2007). One of the remaining unknowns is how water inputs influence glacier velocity. Short-term variations in glacier velocity occur when a water input exceeds the capacity of the subglacial drainage system, and the subglacial water pressure increases. Several studies (Van de Wal and others, 2008; Sundal and others, 2011) have suggested that high ice-flow velocities during these events are later offset by lower ice-flow velocities due to a more efficient subglacial drainage system. This study combines in-situ velocity measurements with a full Stokes glacier flowline model to understand the spatial and temporal variations in glacier flow on the lower Franz Josef Glacier, New Zealand. The Franz Josef Glacier experiences significant water inputs throughout the year (Anderson and others, 2006), and as a result, the subglacial drainage system is likely well-developed. In March 2011, measured ice-flow velocities increased by up to 75% above background values in response to rain events and by up to 32% in response to diurnal melt cycles. These speed-up events occurred at all survey locations across the lower glacier. Through flowline modelling, it is shown that the enhanced glacier flow can be explained by a spatially-uniform subglacial water pressure that increased during periods of heavy rain and glacier melt. From these results, it is suggested that temporary spikes in water inputs can cause glacier speed-up events, even when the subglacial hydrology system is well-developed (cf. Schoof, 2010). Future studies should focus on determining the contribution of glacier speed-up events to overall glacier motion.</p>

P-wave backazimuth anomalies observed by a small-aperture seismic array at Pinyon Flat, southern California: Implications for structure and source location

1996 ◽  
Vol 86 (2) ◽  
pp. 470-476 ◽  
Author(s):  
Cheng-Horng Lin ◽  
S. W. Roecker
Keyword(s):  
Southern California ◽  
Seismic Array ◽  
P Wave ◽  
Small Aperture ◽  
Data Set ◽  
Single Interface ◽  
Seismic Networks ◽  
Dense Seismic Array ◽  
Beamforming Technique

Abstract Seismograms of earthquakes and explosions recorded at local, regional, and teleseismic distances by a small-aperture, dense seismic array located on Pinyon Flat, in southern California, reveal large (±15°) backazimuth anomalies. We investigate the causes and implications of these anomalies by first comparing the effectiveness of estimating backazimuth with an array using three different techniques: the broadband frequency-wavenumber (BBFK) technique, the polarization technique, and the beamforming technique. While each technique provided nearly the same direction as a most likely estimate, the beamforming estimate was associated with the smallest uncertainties. Backazimuth anomalies were then calculated for the entire data set by comparing the results from beamforming with backazimuths derived from earthquake locations reported by the Anza and Caltech seismic networks and the Preliminary Determination of Epicenters (PDE) Bulletin. These backazimuth anomalies have a simple sinelike dependence on azimuth, with the largest anomalies observed from the southeast and northwest directions. Such a trend may be explained as the effect of one or more interfaces dipping to the northeast beneath the array. A best-fit model of a single interface has a dip and strike of 20° and 315°, respectively, and a velocity contrast of 0.82 km/sec. Application of corrections computed from this simple model to ray directions significantly improves locations at all distances and directions, suggesting that this is an upper crustal feature. We confirm that knowledge of local structure can be very important for earthquake location by an array but also show that corrections computed from simple models may not only be adequate but superior to those determined by raytracing through smoothed laterally varying models.

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