An investigation of the subglacial environment, Briksdalsbreen

Transcription

NERC Geophysical Equipment Facility - View more reports on our website at http://gef.nerc.ac.uk/reports.php
Temporal englacial and subglacial water and till
variability revealed by GPR
Jane K. Hart1 and Kirk Martinez2
Schools of Geography1 and Electronics and Computer Science2,
University of Southampton, Southampton, SO17 1BJ, UK
This study uses GPR combined with
borehole,
video
and
wireless
multisensor probe data to record the
temporal changes in englacial and
subglacial water and till variability
associated with a rapidly retreating
glacier. Over a 13 day period, ice radarwave data varied between 0.129 m/ns
and 0.159 m/ns, and water content from
7.7% to 1.44% (using Parens formula).
Basal reflection power (BRP) was
calculated to categorise the bed into
high water content, deforming bed and
sticky spots. Then a site on the
deforming bed and sticky spots were
monitored for 4 days each to record
their stability. The results showed that
the BRP values for the deforming bed
were high and relatively constant, whilst
those from the sticky spot were lower
and more variable. It is suggested that
these changes in englacial storage were
caused by the enhanced crevassing
generated by the newly floating ice
margin which accelerated glacier
retreat.
Introduction
An understanding of water and sediment
behaviour within the englacial and
subglacial systems are fundamental to
understanding glacier dynamics. Ground
Penetrating Radar (GPR) has been
successfully applied to glaciers to
understand glacier depth (Björnsson et
al., 1996), thermal regime (Holmlund et
al., 1996), englacial channel location
(Stuart et al., 2003) and the nature of
the subglacial bed (Arcone et al., 1995).
When combined with ground truthing
(i.e. borehole data and / or in situ
instrument data) these are very powerful
techniques for the understanding of
glacial processes.
The aim of our project was to use GPR
to investigate temporal and spatial
changes in the water content in the ice
and sediment within the englacial and
subglacial system over a 13 day period,
associated with a rapidly melting glacier.
Field Site
The
study
was
undertaken
at
Briksdalsbreen,
Southern
Norway
(Figure 1). This is an outlet glacier of the
Jostedalsbreen icecap resting on
Precambrian gneiss bedrock. The
glacier advanced 600 m between 1955
– 1996 (including 390 m since 1987),
over its proglacial lake and into a birch
forest (NVE, Winkler, 1996).
The 1987-1996 advance is thought to
have been due to a positive phase of
the North Atlantic Oscillation (Nesje and
Dahl, 2003) However, between 19962006 the glacier has retreated
approximately 350 m associated with a
negative phase of the NAO (reduced
precipitation) and global temperature
rise.
Since 2004 an area on the northern side
of the glacier has been investigated
(Figure 1c). This was chosen because it
was the only place that was crevasse
free and without rockfalls from the
adjacent icefall. As the glacier has
1
Figure 1 – Location of the site
dramatically retreated, so the ice has
thinned. In the study area shown in
Figure 1c the average depth of ice was
68m in 2004, 51m in 2005 and 30m in
2006.
Ground Truth Data
Boreholes were drilled with a Kärcher
HDS1000DE hot water drill and videos
were taken with a custom made CCD
camera
using
infra-red
(900nm)
illumination. All the boreholes showed
evidence of englacial drainage (from the
video and/or drainage during drilling)
and after drilling the holes refilled to a
height of 21m below the glacier surface.
The depth of the boreholes were
measured with the drill hose and
camera cable. We assume that these
boreholes were relatively vertical (and
so reflect true depth) because they were
so shallow.
The glacier was repeatedly surveyed by
a TOPCON differential GPS and an
estimation of surface melting could be
made, which was 1.88m between 29th
July to the 10th August 2006,
approximately 0.145 m/day.
GPR Survey
The system used for the survey was a
Sensors and Software Pulse Ekko 100
with a 1000V transmitter system. This
was used in a number of ways. Initially a
common offset survey was performed
using 50 MHz antennas on a grid
pattern, with a 2m antenna spacing and
0.5 m sampling interval. A custom built
sledge was constructed to hold the
2
Figure 2 – Typical Radargram
antennas at the correct distance apart
and allow movement along the transect.
In addition, a second common offset
survey was undertaken using 200 MHz
antennas on parts of the same grid (with
a 0.5m antenna spacing and 0.1m
sampling interval). In addition a common
midpoint survey (CMP) was also made
using the 200 MHz antenna. The
location of the transects were recorded
using a TOPCON differential GPS.
These measurements were carried out
over two separate days.
Once the data was analysed, a static
survey using the 50 Mhz antenna was
undertaken at two specific locations.
The radar was fixed to the glacier
surface and recording were made every
30 minutes during the working day.
The data was analysed using the
software package ReflexW. For the
initial analysis of the common offset
surveys, the following processes
applied: topographic correction, the
elimination of low frequency noise (dewow filter), the application of a SEC
(spreading
and
exponential
compensation) gain to compensate for
signal loss with depth, and a diffraction
stack migration. Figure 2 shows a
typical radargram and although the
glacier was relatively thin, the glacier
bed and bedrock beneath can be clearly
seen.
Water Content
The 200 MHz CMP survey taken on the
29th July 2006 indicated that the radarwave velocity through ice was 0.165
m/ns, which was similar to CMP
analysis and comparison with measured
borehole depths from previous years at
this site. However, closer analysis of the
data showed differences in the radarwave velocity through ice on different
days of the survey. Figure 3 shows a
3
plot of two way travel times against the
horizontal length along the profile (down
glacier) projected along the Transect F
(taking account of the three dimensional
aspect of the glacier). It can be seen
that the values from 27th July (Profiles A,
B, C, D27) are very different from those
collected on the 29th July (Profiles D29
and F). It is particularly interesting to
note the difference between D27 and
D29 that were taken in the same place
but on different days
Table 1 Ice radar-wave velocities
Parabolas
-
0.132
A
29/7
200
-
-
B
27/7
50
-
0.132
C
27/7
50
0.124
0.132
D
27/7
50
0.136
D
29/7
50
E
27/7
50
F
29/7
50
0.124
0.131
0.156
0.177
0.144
0.147
0.135
0.159
0.12
0
0.15
2
0.12
5
0.11
6
0.12
6
0.16
2
0.11
8
F
29/7
200
0.166
-
27/7
50&
200
0.134
0.134
29/7
50&
200
0.165
0.162
0.162
-
0.162
0.15
2
0.15
6
0.12
1
0.15
6
Mean Velocity
(m/ns)
3D
reconstruction
50
Ave
Borehole depths
Date
27/7
Ave
Frequency
MHz
Transects
A
0.126
0.152
0.129
0.124
0.129
0.131
0.162
0.167
0.131
0.133
0.127
0.158
0.161
0.129
0.158
The radar-wave velocity through ice was
calculated by three separate methods
(shown in Table 1): a) from measured
borehole depths; b) from a 3D
reconstruction based on measured
depths; and c) by integrating the mean
values from the parabolas in the
radargrams. The mean on 27th July was
0.129 m/ns and on the 29th July was
0.158 m/ns.
From these velocity measurements it is
possible to calculate the water content
(W). Macheret et al. (1993) used the
Paren (1970) mixture formula:
[1]
W = 3(εr – εi) / εw
and:
εr = (C/v) 2
[2]
where εr is the permittivity of ice, εi is the
permittivity of solid dry ice (taken as
3.19) and εw is the permittivity of water
(taken as 86), C is the velocity of light
and v = radar-wave velocity. In addition,
Frolov and Macheret (1989) used the
Looyenga (1965) mixing formula which
can be simplified as (Moore et al.,
1999):
W = -0.0027 + 0.0432(εr – εi) + 0.0004 (εr – εi)2
[3]
The lower radar-wave velocity from 27th
July results in a 7.7% water content
using the Paren formula and 9.2% water
content using the Looyenga formula,
and the upper radar-wave velocity from
the 29th July results in a 1.44% to 1.48%
water content respectively. Although the
values associated with the 29th July are
typical of temperate glaciers, the values
from the 27th are higher.
Temporal changes at the glacier
bed
A number of researchers (e.g. Gades et
al., 2000; Copland and Sharp, 2001)
have used the strength of the basal
reflection power (BRP) (see Appendix 1)
from GPR to indicate the nature of the
bed based on its theoretical reflectivity
R (calculated as a ratio between BRPr
max and BRPr) (R > 0.6 = water body; R
approx. 0.35 = wet till; R approx. 0.18 =
dry till; R < 0.1 gniess bedrock).
The BRP was calculated for each
transect (using equation A1 - Appendix)
Because of high spatial variation in the
data associated with individual data
points, the data were filtered using a 5
point (2.5 m) moving average.
4
Figure 3.Two way travel times of radar
reflection from the glacier bed.
The Briksdalsbreen filtered BRPr data
were divided into the different classes of
R. Then two locations were chosen for
the stationary GPR study. Site A was
located over a low R (< 0.2) (‘sticky
spot’) value area whilst Site B was
positioned over a medium R (0.2-0.5)
(deforming bed) value area.
Stationary GPR Survey Results
The stationary surveys were made at
the two sites over four days each. The
BRPr and R of the subglacial bed were
examined, as well as the water content
of the glacier.
Initially, the glacier bed needs to be
identified from the stationary radar
traces. This is not necessarily
straightforward as both the water
content in the glacier (and thus ice
radar-wave velocity) and the reflectivity
of the bed may change. The depth of
the glacier is known at the two sites
(from comparisons of the original survey
to the measured boreholes). Thus,
assuming a constant surface melt rate,
it is possible to show a suggested depth
range for the bed of the glacier on each
radar trace using the measured radarwave velocity range in ice (from 0.129
to 0.165) (Figure 4).
Two methods were used to calculate
BRP (Figure 4). Firstly across the whole
bed reflection window (using equation
A1 – Appendix), and identifying the bed
peak (using equation A2 – Appendix).
All the radar traces from Site B
(deforming bed) have a distinctive peak
at a similar depth which must reflect the
bed (Figure 4). However, at Site A
(sticky spot), there was no distinctive
peak and so both the maximum and
minimum peaks were taken within the
glacier depth window, to provide a range
of possible reflection values (Figure 4).
5
In previous years, studies from
Briksdalsbreen have not observed
dramatic temporal changes in ice radarwave velocity, and there have been few
reports of such differences in the
literature. Rapidly changing water
content implies a large capacity for
water storage within the glacier, due to
interconnected voids and englacial
crevasses (observed on the video
footage at Briksdalsbreen and described
as hydraulically connected fractures by
Fountain et al. 2005).
Figure 4. Typical radar trace from Site B
(left) and Site A (right).
Using both techniques it is clear that the
BRP values from Site A were much
lower than those from Site B, with much
higher variability. This demonstrates that
the BRP is showing a real variation in
bed reflection as located in the common
offset surveys. All of the Site B values
remained within the intermediate range
of R, in comparison to only 43% of the
readings from Site A. In addition, there
seemed to be little evidence for a diurnal
pattern over the seven hour study period
at both sites.
It is also possible to calculate the ice
radar-velocity and infer water content
over the static survey study period
(Table 2).
Discussion
Numerous researchers (e.g. Macheret
et al., 1993; Moore et al. 1999; Murray
et al., 2000) have argued that high water
content values in ice are due to the
presence of water filled cavities,
crevasses and channels. Our study
demonstrated that water content in the
glacier varied dramatically over the 13
days of the study, and this variability
was greatest over the sticky spots.
We suggest that these interconnected
crevasses are a product of the dramatic
break up of the glacier. During 2006 the
study area became a grounded-floating
ice transition zone (Hughes, 1975)
which lead to increased fracturing in this
zone. This may be analogous to the
rapid break up of other glaciers with
aquatic
margins,
for
example,
Jakobshavn Isbrae, Greenland (Zwally
et al. 2002).
Table 2 – Ice radar-wave velocities and
water content calculated over the study
period*
Date
Ice
Water Content
RadarParen
Looyenga
wave
method
method
velocity
(%)
(%)
(m/ns)
27/7
0.129
7.73
9.24
29/7
0.158
1.45
1.48
1/8
0.144 1.37 1.38 - 4.65
0.159
4.04
2/8
0.143 1.13 1.09 - 4.63
0.160
4.02
4/8
0.151 1.13 1.09 - 2.87
0.160
2.59
5/8
0.150 2.17 2.36 - 3.06
0.154
2.75
6/8
0.145
3.87
4.44
7/8
0.160
1.12
1.07
8/8
0.159
1.25
1.25
9/8
0.157
1.62
1.69
* Examples from Site A have a range of values
calculated from the different ways to identify the
bed.
6
Figure 5. Average BRP over the full range of
possible bed depths (see text for details); b)
BRP over time from the stationary surveys.
By April 2007, the glacier had retreated
a further 133m. It was no longer calving
into the lake, but had become grounded
on its gneiss bedrock. This dramatic
crevassing increased the storage (and
thus content) of water in the glacier.
This resulted in an additional increase in
velocity which accelerated the break up
of the glacier.
Acknowledgements
The authors would like to thank the
Glasweb 2006 team for help with data
collection (Al Riddoch, Ahmed Elsaify,
Gang Zou, Paritosh Padhy, Celine
Ragault,
David
Vaughan-Hirsch,
Mathew Westoby) and Inge and Gro
Melkevol for help with logistics. Thanks
also go the Tim Aspden and his
colleagues in the Cartographic Unit for
figure preparation, and to Dr Jim Wright
for help with the GIS. This research was
funded by the Royal Society, Royal
Society Paul Instrument Fund and
ESPRC.
References
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Hagen JO, Liestel O, PaÂ lsson F,
Erlingsson B. 1996. The thermal regime of
7
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Glaciology, 42, 23-32.
Copland, L. and M. Sharp (2001). Mapping
thermal and hydrological conditions
beneath a polythermal glacier with radioecho sounding. Journal of Glaciology, 47
(157), 232-242.
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N.H., Crabtree, M.D., 2000, Englacial
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Appendix
To calculate BRP, unprocessed GPR
data is used as follows (after Gades et
al., 2000):
P≡
1
t2
A2
∑
i = t1 i
2(t 2 − t1 + 1)
[A1]
where P is returned power, A is the sum
of the squared amplitudes and t1-t2 is
the time window. Over short transects it
is more accurate to manually locate the
position of the bed and use the following
equation:
BRP =
1
A2 2
2(t 2 − t1 + 1)
[A2]
Gades et al. (2000) also showed that
increasing ice thickness will effect
reflection strength, and suggest that this
can be compensated for by calculating
residual BRP (BRPr).
BRPr =
BRP(measured )
−1
BRP( predicted )
[A3]
8
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An investigation of the subglacial environment, Briksdalsbreen

Transcription

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