scholarly journals Detecting and characterising an englacial conduit network within a temperate Swiss glacier using active seismic, ground penetrating radar and borehole analysis

2019 ◽  
Vol 60 (79) ◽  
pp. 193-205 ◽  
Author(s):  
Gregory Church ◽  
Andreas Bauder ◽  
Melchior Grab ◽  
Lasse Rabenstein ◽  
Satyan Singh ◽  
...  
Keyword(s):  
Seismic Data ◽  
Water Pressure ◽  
Drainage System ◽  
Late Summer ◽  
Active Surface ◽  
Flow Through ◽  
Borehole Camera ◽  
Ground Penetrating ◽  
Amplitude Versus ◽  
Conduit Network

ABSTRACTEnglacial hydrology plays an important role in routing surface water to the glacier's bed and it consequently affects the glacier's dynamics. However, it is often difficult to observe englacial conduit conditions on temperate glaciers because of their short-lived nature. We acquired repeated active surface seismic data over the Rhone Glacier, Switzerland to monitor and characterise englacial conduit conditions. Amplitude-versus-angle analysis suggested that the englacial conduit is water filled and between 0.5 and 4 m thick. A grid of GPR profiles, acquired during the 2018 melt season, showed the englacial conduit network persisting and covering ~ 14,000 m2. In late summer 2018, several boreholes were drilled into the conduit network. We observed generally stable water pressure, but there were also short sudden increases. A borehole camera provided images of a fast flowing englacial stream transporting sediment through the conduit. From these observations, we infer that the englacial conduit network is fed by surface meltwater and morainal streams. The surface and morainal streams merge together, enter the glacier subglacially and flow through subglacial channels along the flank. These subglacial channels flow into highly efficient englacial conduits traversing the up-glacier section of the overdeepening before connecting with the subglacial drainage system.

scholarly journals Borehole water-level variations and the structure of the subglacial hydrological system of Haut Glacier d’Arolla, Valais, Switzerland

1995 ◽  
Vol 41 (139) ◽  
pp. 572-583 ◽  
Author(s):  
B. P. Hubbard ◽  
M. J. Sharp ◽  
I. C. Willis ◽  
M. K. Nielsen ◽  
C. C. Smart
Keyword(s):  
Distributed System ◽  
Water Pressure ◽  
Drainage System ◽  
Late Summer ◽  
Variable Pressure ◽  
Sediment Layer ◽  
Drainage Channels ◽  
Hydrological System ◽  
Borehole Water ◽  
Water Level Variations

AbstractLate-summer subglacial water pressures have been measured in a dense array of boreholes in the ablation area of Haut Glacier d’Arolla, Switzerland. Interpolated surfaces of minimum diurnal water pressure and diurnal water-pressure variation suggest the presence of a subglacial channel within a more widespread, distributed drainage system. The channel flows along the centre of a variable pressure axis (VPA), some tens of metres wide, that is characterized by low minimum diurnal water pressures (frequently atmospheric) and high diurnal water-pressure variations. These characteristics are transitional over a lateral distance of c. 70 m to higher and more stable subglacial water pressures in the adjacent distributed system. Water-pressure variations recorded in boreholes located close to the centre of the VPA reflect the delivery of surface-derived meltwater to the glacier bed and result in a diurnally reversing, transverse hydraulic gradient that drives water out from the channel into the distributed system during the afternoon and back to the channel overnight. Subglacial observations suggest that such flow occurs through a vertically confined sediment layer. Borehole turbidity records indicate that the resulting diurnal water flows are responsible for the mobilization and transport of fine debris in suspension. Analysis of the propagation velocity and amplitude attenuation cf the diurnal pressure waves suggests that the hydraulic conductivity of the sediment layer decreases exponentially with distance from the channel, falling from c. 10−4 m s−1 at the channel boundary to c. 10−7 m s−1 70 m away. These apparent hydraulic conductivities are consistent with Darcian flow through clean sand and typical glacial till, respectively.We suggest that fine material is systematically flushed from basal sediments located adjacent to large, melt-season drainage channels beneath warm-based glaciers. This process may have important implications for patterns of glacier erosion, hydro-chemistry and dynamics.

scholarly journals Repeated ground-penetrating radar measurements to detect seasonal and annual variations of an englacial conduit network

2020 ◽  
Author(s):  
Gregory Church ◽  
Andreas Bauder ◽  
Melchior Grab ◽  
Cédric Schmelzbach ◽  
Hansruedi Maurer
Keyword(s):  
Ground Penetrating Radar ◽  
Winter Season ◽  
Drainage System ◽  
Reflection Coefficients ◽  
Drainage Systems ◽  
Impedance Inversion ◽  
Ground Penetrating ◽  
Conduit Network ◽  
Subglacial Drainage ◽  
Englacial Drainage

<p>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.</p><p>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 GeoVISION<sup>TM</sup> Dual-Scan borehole camera.</p><p>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.</p><p>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.</p>

scholarly journals Borehole water-level variations and the structure of the subglacial hydrological system of Haut Glacier d’Arolla, Valais, Switzerland

1995 ◽  
Vol 41 (139) ◽  
pp. 572-583 ◽  
Author(s):  
B. P. Hubbard ◽  
M. J. Sharp ◽  
I. C. Willis ◽  
M. K. Nielsen ◽  
C. C. Smart
Keyword(s):  
Distributed System ◽  
Water Pressure ◽  
Drainage System ◽  
Late Summer ◽  
Variable Pressure ◽  
Sediment Layer ◽  
Drainage Channels ◽  
Hydrological System ◽  
Borehole Water ◽  
Water Level Variations

AbstractLate-summer subglacial water pressures have been measured in a dense array of boreholes in the ablation area of Haut Glacier d’Arolla, Switzerland. Interpolated surfaces of minimum diurnal water pressure and diurnal water-pressure variation suggest the presence of a subglacial channel within a more widespread, distributed drainage system. The channel flows along the centre of avariable pressure axis(VPA), some tens of metres wide, that is characterized by low minimum diurnal water pressures (frequently atmospheric) and high diurnal water-pressure variations. These characteristics are transitional over a lateral distance of c. 70 m to higher and more stable subglacial water pressures in the adjacent distributed system. Water-pressure variations recorded in boreholes located close to the centre of the VPA reflect the delivery of surface-derived meltwater to the glacier bed and result in a diurnally reversing, transverse hydraulic gradient that drives water out from the channel into the distributed system during the afternoon and back to the channel overnight. Subglacial observations suggest that such flow occurs through a vertically confined sediment layer. Borehole turbidity records indicate that the resulting diurnal water flows are responsible for the mobilization and transport of fine debris in suspension. Analysis of the propagation velocity and amplitude attenuation cf the diurnal pressure waves suggests that the hydraulic conductivity of the sediment layer decreases exponentially with distance from the channel, falling from c. 10−4m s−1at the channel boundary to c. 10−7m s−170 m away. These apparent hydraulic conductivities are consistent with Darcian flow through clean sand and typical glacial till, respectively.We suggest that fine material is systematically flushed from basal sediments located adjacent to large, melt-season drainage channels beneath warm-based glaciers. This process may have important implications for patterns of glacier erosion, hydro-chemistry and dynamics.

scholarly journals Monitoring the seasonal changes of an englacial conduit network using repeated ground-penetrating radar measurements

2020 ◽  
Vol 14 (10) ◽  
pp. 3269-3286 ◽  
Author(s):  
Gregory Church ◽  
Melchior Grab ◽  
Cédric Schmelzbach ◽  
Andreas Bauder ◽  
Hansruedi Maurer
Keyword(s):  
Ground Penetrating Radar ◽  
Drainage System ◽  
Reflection Coefficients ◽  
System A ◽  
Impedance Inversion ◽  
Waveform Modelling ◽  
Infill Material ◽  
Ground Penetrating ◽  
Conduit Network ◽  
Subglacial Drainage

Abstract. Englacial conduits act as water pathways to feed surface meltwater into the subglacial drainage system. A change of meltwater into the subglacial drainage system can alter the glacier's dynamics. Between 2012 and 2019, repeated 25 MHz ground-penetrating radar (GPR) surveys were carried out over an active englacial conduit network within the ablation area of the temperate Rhonegletscher, Switzerland. In 2012, 2016, and 2017 GPR measurements were carried out only once a year, and an englacial conduit was detected in 2017. In 2018 and 2019 the repetition survey rate was increased to monitor seasonal variations in the detected englacial conduit. The resulting GPR data were processed using an impedance inversion workflow to compute GPR reflection coefficients and layer impedances, which are indicative of the conduit's infill material. The spatial and temporal evolution of the reflection coefficients also provided insights into the morphology of the Rhonegletscher's englacial conduit network. During the summer melt seasons, we observed an active, water-filled, sediment-transporting englacial conduit network that yielded large negative GPR reflection coefficients (<-0.2). The GPR surveys conducted during the summer provided evidence that the englacial conduit was 15–20 m±6 m wide, ∼0.4m±0.35m thick, ∼250m±6m long with a shallow inclination (2∘), and having a sinusoidal shape from the GPR data. We speculate that extensional hydraulic fracturing is responsible for the formation of the conduit as a result of the conduit network geometry observed and from borehole observations. Synthetic GPR waveform modelling using a thin water-filled conduit showed that a conduit thickness larger than 0.4 m (0.3× minimum wavelength) thick can be correctly identified using 25 MHz GPR data. During the winter periods, the englacial conduit no longer transports water and either physically closed or became very thin (<0.1 m), thereby producing small negative reflection coefficients that are caused by either sediments lying within the closed conduit or water within the very thin conduit. Furthermore, the englacial conduit reactivated during the following melt season at an identical position as in the previous year.

scholarly journals A test of common assumptions used to infer subglacial water flow through overdeepenings

2014 ◽  
Vol 60 (222) ◽  
pp. 725-734 ◽  
Author(s):  
Christine F. Dow ◽  
Jeffrey L. Kavanaugh ◽  
Johnny W. Sanders ◽  
Kurt M. Cuffey
Keyword(s):  
Melting Point ◽  
Water Pressure ◽  
Drainage System ◽  
Variable Pressure ◽  
Local Pressure ◽  
Overburden Pressure ◽  
Temperature Regimes ◽  
Hydrological System ◽  
Flow Through ◽  
Pressure Melting

AbstractBorehole instrument records from a cirque glacier with an overdeepened bed are examined to assess the validity of widely held glacial hydrological assumptions. At this glacier, hydraulic-potential calculations suggest water below overburden pressure will flow into the overdeepening, where the steepness of the riegel causes water to pool in the basin and increase in pressure. Our subglacial water pressure data also show high consistent pressures in the overdeepening and the presence of an active, variable-pressure drainage system towards the margin of the cirque. Therefore, we find that although uniform hydraulic-potential calculations are not directly applicable, they can still be useful for interpretation of the subglacial hydrological system. We also examine supercooling assumptions under different pressure and temperature regimes for water flowing over a riegel, driven using our borehole records of subglacial water temperatures that are consistently above the pressure-melting point during the late melt season. Our results show that even a slight increase in basal temperatures relative to the local pressure-melting point is sufficient to prevent a reduction in basal hydraulic conductivity as a result of supercooling freeze-on.

scholarly journals Greenland subglacial drainage evolution regulated by weakly connected regions of the bed

2016 ◽  
Vol 7 (1) ◽  
Author(s):  
Matthew J. Hoffman ◽  
Lauren C. Andrews ◽  
Stephen F. Price ◽  
Ginny A. Catania ◽  
Thomas A. Neumann ◽  
...  
Keyword(s):  
Numerical Models ◽  
Water Pressure ◽  
Seasonal Pattern ◽  
Drainage System ◽  
Greenland Ice Sheet ◽  
Late Summer ◽  
Initial Increase ◽  
Ice Dynamics ◽  
Hydrodynamic Coupling ◽  
Subglacial Drainage

Abstract Penetration of surface meltwater to the bed of the Greenland Ice Sheet each summer causes an initial increase in ice speed due to elevated basal water pressure, followed by slowdown in late summer that continues into fall and winter. While this seasonal pattern is commonly explained by an evolution of the subglacial drainage system from an inefficient distributed to efficient channelized configuration, mounting evidence indicates that subglacial channels are unable to explain important aspects of hydrodynamic coupling in late summer and fall. Here we use numerical models of subglacial drainage and ice flow to show that limited, gradual leakage of water and lowering of water pressure in weakly connected regions of the bed can explain the dominant features in late and post melt season ice dynamics. These results suggest that a third weakly connected drainage component should be included in the conceptual model of subglacial hydrology.

Interaction between water pressure in the basal drainage system and discharge from an Alpine glacier before and during a rainfall-induced subglacial hydrological event

1997 ◽  
Vol 24 ◽  
pp. 288-292 ◽  
Author(s):  
Andrew P. Barrett ◽  
David N. Collins
Keyword(s):  
Water Level ◽  
Water Pressure ◽  
Drainage System ◽  
Water Levels ◽  
Drainage Network ◽  
Alpine Glacier ◽  
Hydraulic Conditions ◽  
Borehole Water ◽  
Progressive Growth ◽  
Resultant Increase

Combined measurements of meltwater discharge from the portal and of water level in a borehole drilled to the bed of Findelengletscher, Switzerland, were obtained during the later part of the 1993 ablation season. A severe storm, lasting from 22 through 24 September, produced at least 130 mm of precipitation over the glacier, largely as rain. The combined hydrological records indicate periods during which the basal drainage system became constricted and water storage in the glacier increased, as well as phases of channel growth. During the storm, water pressure generally increased as water backed up in the drainage network. Abrupt, temporary falls in borehole water level were accompanied by pulses in portal discharge. On 24 September, whilst borehole water level continued to rise, water started to escape under pressure with a resultant increase in discharge. As the drainage network expanded, a large amount of debris was flushed from a wide area of the bed. Progressive growth in channel capacity as discharge increased enabled stored water to drain and borehole water level to fall rapidly. Possible relationships between observed borehole water levels and water pressures in subglacial channels are influenced by hydraulic conditions at the base of the hole, distance between the hole and a channel, and the nature of the substrate.

The effect of pore-water pressure on NATM tunnel linings in decomposed granite soil

2005 ◽  
Vol 42 (6) ◽  
pp. 1585-1599 ◽  
Author(s):  
J H Shin ◽  
D M Potts ◽  
L Zdravkovic
Keyword(s):  
Pore Water ◽  
Pore Water Pressure ◽  
Water Pressure ◽  
Drainage System ◽  
Tunnel Lining ◽  
Natm Tunnel ◽  
Decomposed Granite ◽  
Secondary Lining ◽  
Granite Soil ◽  
Functional Components

Tunnelling in a water bearing soil often produces a long-term interaction between the tunnel lining and the surrounding soil. With respect to lining design, infiltration and external pore-water pressures are often one of the most important factors to be considered. Development of pore-water pressure may accelerate leakage and cause deterioration of the lining. This can be particularly troublesome to structural and functional components of the tunnel and can often lead to structural failure. However, as a result of the complicated hydraulic boundary conditions and the long times often required for pore pressure equilibration, research on this subject is scarce. Consequently, most design approaches deal with the effects of pore-water pressure on the tunnel lining in a qualitative manner. In this paper, the development of pore-water pressure and its potential effects on the tunnel lining are investigated using the finite element method. In particular, the deterioration of a drainage system caused by clogging is considered. It is shown that the development of pore-water pressure on the lining is dependent on the lining permeability and the deterioration of the drainage system, particularly for a tunnel with both a primary and a secondary lining. The magnitude of pore-water pressure on a new Austrian tunnelling method (NATM) tunnel constructed in decomposed granite soil and the effect of tunnel shape are investigated. Design curves for estimating pore-water pressure loads on a secondary lining are proposed.Key words: numerical analysis, tunnel lining, decomposed granite.

Geometrical spreading in a layered transversely isotropic medium with vertical symmetry axis

Geophysics ◽  
2003 ◽  
Vol 68 (6) ◽  
pp. 2082-2091 ◽  
Author(s):  
Bjørn Ursin ◽  
Ketil Hokstad
Keyword(s):  
Ray Tracing ◽  
Seismic Data ◽  
Symmetry Axis ◽  
Transversely Isotropic ◽  
Analytical Approximation ◽  
Geometrical Spreading ◽  
S Wave ◽  
Weak Anisotropy ◽  
P Waves ◽  
Amplitude Versus

Compensation for geometrical spreading is important in prestack Kirchhoff migration and in amplitude versus offset/amplitude versus angle (AVO/AVA) analysis of seismic data. We present equations for the relative geometrical spreading of reflected and transmitted P‐ and S‐wave in horizontally layered transversely isotropic media with vertical symmetry axis (VTI). We show that relatively simple expressions are obtained when the geometrical spreading is expressed in terms of group velocities. In weakly anisotropic media, we obtain simple expressions also in terms of phase velocities. Also, we derive analytical equations for geometrical spreading based on the nonhyperbolic traveltime formula of Tsvankin and Thomsen, such that the geometrical spreading can be expressed in terms of the parameters used in time processing of seismic data. Comparison with numerical ray tracing demonstrates that the weak anisotropy approximation to geometrical spreading is accurate for P‐waves. It is less accurate for SV‐waves, but has qualitatively the correct form. For P waves, the nonhyperbolic equation for geometrical spreading compares favorably with ray‐tracing results for offset‐depth ratios less than five. For SV‐waves, the analytical approximation is accurate only at small offsets, and breaks down at offset‐depth ratios less than unity. The numerical results are in agreement with the range of validity for the nonhyperbolic traveltime equations.

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