seismic processing applied to radar data to investigate melt-water drainage structures in the...
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Seismic processing applied to radar data to investigate melt-water drainage structures in the southern Greenland Ice Sheet
Jamin S. GreenbaumInstitute for Geophysics, UT Austin
Principal Investigators:Dr. Ginny Catania, UTIGDr. Thomas Neumann, U. of Vermont
UTIG Gale White Fellowship Seminar
30 April, 2008
Arctic Climate impact Assessment
1992 2002
Increased Surface Melt in Greenland
Swiss Camp
Zwally et al. 2002
GPS at Swiss Camp:Surface melt is related to ice velocity
What is the mechanism?
Horizontal Ice Velocity“Additional Motion” over winter avg
From: Zwally et al. 2002
2007 Focused Study Sites
Single Profile Only (2006)
Survey Grid (2007)
Swiss Camp
Case 1
Case 2
Case 3
-80 -40 0 40 80
Reflection Amplitude (mV)
12
10
8
6
4
2
0
Tw
o-w
ay T
rave
l Tim
e (m
s)
Surface Zone
Bed Zone
Interior Zone
Siple Dome, Antarctica
Ground-Based Radio-Echo-Sounding (RES)
GPS unit
32 m
transmitter
tow rope/antenna100 m
Ground-Based Radar Acquisition
-49.33 -49.32 -49.31 -49.3 -49.29 -49.28
69.548
69.55
69.552
69.554
69.556
69.558
69.56
69.562
69.564
69.566
69.568
Longitude (DD)
La
titu
de
(D
D)
“Mini 3D Seismic”4 km square “Race Tracks” (x 2)
Seismic Processing Overview
• Navigation Data:– Solve for transmitter position from receiver position
– Convert GPS geodetic measurements to local grid
• ArcGIS: Projected to UTM zone 22N
• Convert radar/nav data to SEG-Y format• Paradigm FOCUS software
– Divide sample rate by 105 to ‘fool’ Focus
– Mute the direct arrival
– Deconvolution: Invert source wavelet, remove multiples
– Filtering: Noise reduction
– Static corrections using navigation data
– Migration
He
igh
t a
bo
ve
W
GS
84
(m
)
Horizontal Position (km)
Case 1: 400 m Ice ThicknessBasic Processing
• Diffractors begin very close to the surface.
• Diving layers
• Multiples below the bed
• Structure masked by reverberations
Case 1: 400 m Ice Thickness:Basic Processing
He
igh
t a
bo
ve
W
GS
84
(m
)
Horizontal Position (km)
• Diffractors begin very close to the surface.
• Diving layers
• Multiples below the bed
• Structure masked by reverberations
Picking Velocities for MigrationBefore Normal Move-out
Picking Velocities for MigrationAfter Normal Move-out (flatten diffractors)
Case 1: Before and After Processing
~2 km ~2 km
Case 2: 1 km Ice ThicknessBasic Processing
distance (m)
time
(s)
0 500 1000 1500 2000
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
x 10-5
• Diffractors are buried
• No obvious diving layers
• Multiples below the bed
1 km Ice Thickness:After Seismic Processing
• Identical processing does not process as well as 400m case– Faint image– Artificial structure beneath the
bed remains
• Complications in structure– Not a smooth structure as
before
0
200
400
600
800
1000
De
pth
(m
)
0 0.5 1 1.5 2
Horizontal Position (km)
Current Results & Ongoing Work
• Case 1: 400 m ice thickness: Data seem to indicate a recently-active moulin– Diving layers (focused basal melt)– Very close to the surface– Reaches the bed– Simple structure (migrates easily)
• Case 2: 1 km ice thickness: Data do not indicate recent activity– Lack of diving layers indicates no long term water supply– Identical processing with much different results– More complicated internal geometry (implies smoothing due to water flow)
• Orthogonal Survey lines verify vertical nature to features
• Ongoing work:– Two posters and two oral presentations– Methods Publication: 2008 International Glaciological Society– Case 3: Intermediate ice-thickness (800-m)– Results paper: GRL– Attempt 3-D Processing
Acknowledgements
• Gale White Fellowship
• UTIG Fellowship Committee
• Ginny Catania
• Nathan Bangs
• Thomas Hess
• Paul Stoffa and Steffen Saustrup
Nancy Pelosi
Lynne Cox – Arctic/Antarctic Swimmer
Jack Cain – Ambassador to Denmark
Springtime in Greenland
Questions?
Motivation
• Observations indicate melt-induced velocity increase near the ELA (mechanism?):– Active moulins have been directly observed in the marginal regions
of the GIS but not as far upstream as the ELA
• 2006 fieldwork: Broad radar survey of the ELA suggests the presence of many subsurface features:– Preliminary processing indicated the features could be englacial
drainage features (possibly moulins)
– Azimuth ambiguity from radar omnidirectional wave propagation
– Shape & size hidden by noise and multiples