Detecting the subglacial conditions at Store Glacier, West Greenland, using a combined seismic-radar survey Spring campaign RESPONDER April 26 – May 19, 2018 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, (coen.hofstede@awi.de) (2) Scott Polar Research Institute, Cambridge, (3) Uppsala University, Uppsala, (4) University of the Sunshine Coast, Sippy Downs, Australia.

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.

W 3.2 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 3.3 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

E 5.3 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 1.7 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

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

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

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

Long offset analysis => not enough velocity data => determine R Ranges of R at normal incidence: ice - bedrock R: 0.30 0.67 ice - cons seds. R: -0.05 0.18 ice - uncons seds. R: -0.13 -0.01 ice - dilatant till R: -0.16 -0.03 ice - water R: -0.42 -0.39 Not possible to distinguish between dilatant till and unconsolidated sediments.

Varied coupling of geophones and source: Amplitudes of the direct wave of channel 96 (offset 34m): A = 0.069 A = 0.630 A = 0.24 A = 0.039

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) 331 181 34 0.00 0.20 0.40 0.60 T (s) direct p-wave direct s-wave basal reflection A0 A1

Low Site, across-flow profiles: up to downstream Low Site, across-flow profiles: up to downstream. Upstream, base 750m, trench base 1050m N 2.1 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

S 3.2 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)

Low Site across flow profiles: downstream, base 900m, trench base 1220m N 3.3 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

Source Amplitude through direct p-wave: Amplitude direct p-wave vs offset A0 = 33.38 => R = 0.08 A0 = 23.55 => R = - 0.40 A0 = 22.50 => R = -0.07 A0 = 20.53 => R = 0.15 A0 = 6.88 => R = 1.98 Can a direct p-wave reliably be registered by a vertically orientated p-sensor (geophone) or is coupling the problem?

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 3.8 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)

E 3.6 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

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)

High Site along-flow profile: time migrated base between 1250m – 1310m E 5.3km W T(s) 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 ICE englacial reflection base

High Site across flow profile: time migrated High Site across flow profile: time migrated base between 1150m – 1370m S 1.7 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

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

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.