1. Detecting the subglacial conditions at Store Glacier, West Greenland, using a combined seismic-radar survey Spring campaign RESPONDER April 26 – May 19, Coen Hofstede (1), Poul Christoffersen (2), Rickard Pettersson (3), Adrian McCallum (4), T.J. Young (2), Olaf Eisen (1) and Emma Smith (1) (1) Alfred Wegener Institute, Bremerhaven, (2) Scott Polar Research Institute, Cambridge, (3) Uppsala University, Uppsala, (4) University of the Sunshine Coast, Sippy Downs, Australia.

2. Two sites at Store use same subglacial water route:
High Site
Water routing map
seismic profile
radar profile
Low Site
High Site
Low Site
Goal:
Determine drilling site to collect subglacial soil sample.

3. W km E
Low Site: seismic profiles along flow ice flow: blue arrow and across flow
T(s)
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
ICE
base
base
S km N
T(s)
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
ICE
base
slope
base
TRENCH

4. E km W
High Site: seismic profiles along flow ice flow: blue arrow and across flow
T(s)
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
ICE
englacial reflection
base
S km N
T(s)
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
ICE
englacial reflection
base
TRENCH

5. Investigation of the basal conditions of Store Glacier, West Greenland:
Velocity analysis at 4 long offset gathers
Determination of Reflection Coefficient R. R is determines the bed and ice interface.
Radar data needed for ice base identification

6. Recording equipment:
300 m streamer: 96 gimballed 30Hz p- geophones Sledge: 4 geodes in series computer in tent Source: 10 m (100g) detonating cord on surface 2 person operation: 50 – 80 shots(7-12km) / day

7. Velocities from 3 long offset gathers, 1 is usefull: (combining 4 shots with increasing offset in 1 long offset gather) SP61: faint, not flat SP78: faint, flat  OK SP95: strong, not flat
bed reflection

8. Long offset analysis => not enough velocity data => determine R
Ranges of R at normal incidence:
ice - bedrock R:
ice - cons seds. R:
ice - uncons seds. R:
ice - dilatant till R:
ice - water R:
Not possible to distinguish between dilatant till and unconsolidated sediments.

9. Varied coupling of geophones and source:
Amplitudes of the direct wave of channel 96 (offset 34m):
A = A = A = A = 0.039

10. Reflection coefficient R and R(q):
R= (r2v2-r1v1)/(r2v2+r1v1)
R(q) = A1(q)/A0 r(q)e ar(q)
A0 from direct wave vs offset in shot records.
poor coupling => weak direct wave
good coupling => strong direct wave
medium 1: top
medium 2: bottom
r: density
v: p-wave velocity
q: angle of incidence
A1: amplitude reflector
A0: source amplitude
r: travelled path
a: seismic attenuation
offset(m)
0.00
0.20
0.40
0.60
T (s)
direct p-wave
direct s-wave
basal reflection A0 A1

11. Low Site, across-flow profiles: up to downstream
Low Site, across-flow profiles: up to downstream. Upstream, base 750m, trench base 1050m
N km S
T(s)
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
R = -0.40
water at base
ICE
R = 0.15
consolidated
sediments
slope
TRENCH

12. S km N
T(s)
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Low Site across-flow profiles: middle, base 900m trench 1220m - seismic (top) and radar (bottom). - TRENCH hardly visible, migration problematic R=1.98 not possible!
ICE
R = 0.08
consolidated sediments
englacial reflection
slope
slope
R = 1.98
TRENCH
ICE
englacial reflection
bed
slope
crevasses
slope
TRENCH
Depth (m)

13. Low Site across flow profiles: downstream, base 900m, trench base 1220m
N km S
T(s)
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
ICE
englacial reflection
R = -0.07
unconsolidated sediments
base trench
TRENCH

14. Source Amplitude through direct p-wave:
Amplitude direct p-wave vs offset
A0 = => R =
A0 = => R =
A0 = => R =
A0 = => R =
A0 = => R =
Can a direct p-wave reliably be registered by a vertically orientated p-sensor (geophone) or is coupling the problem?

15. Low Site along-flow profiles: North, base 800 to 1050m - seismic (top) and radar (bottom). - arrows: R<0. Water present exclusively or in sediments.
E km W
T(s)
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
ICE
R<0:wet base
R<0:wet base
R<0:wet base
ICE
bed
crevasses
base
Depth (m)

16. E km W
Low Site along-flow profiles, South : base 850m base trench 1150m time migrated profile: by muting the noisy ice column, the basal structure becomes visible after migration.
T(s)
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
ICE
base
englacial reflection
base trench
T(s)
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
ICE
base
slope
base
TRENCH

17. Low Site: water presence based on polarity of R
Water (exclusively or in sediments) present at flanks of the subglacial trough.
Bed contour
Shot location
Shot with R < 0
(water present)

18. High Site along-flow profile: time migrated base between 1250m – 1310m
E km W
T(s)
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
ICE
englacial reflection
base

19. High Site across flow profile: time migrated
High Site across flow profile: time migrated base between 1150m – 1370m
S km N
T(s)
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
ICE
wet base
wet base
base

20. Identified water or water containing sediments at Store Glacier:
2 Sites at Store:
seismic profile
identified water exclusive or in sediments.
High Site
Low Site
High Site
Low Site

21. Temporary conclusions:
General:
The lower 15% of the ice column is probably anisotropic because of an englacial reflection. This reflection is most pronounced at the High Site.
The determination of source amplitude A0 and thus the Reflection Coefficient R need further investigation. At poorly coupled shots, A0 can not be determined with the direct p-wave.
Low Site:
Water is present exclusively or in sediments at the flanks of the trough.
Through the reflection coefficient we identify consolidated sediments at the onset of the base of the trough. In the deeper part of the through the subglacial material can not be determined as the signal is too weak. At the northern flank we identity unconsolidated water containing sediments. The southern flank needs further investigation.