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