Seismic processing applied to radar data to investigate melt-water drainage structures in the southern Greenland Ice Sheet Jamin S. Greenbaum Institute for Geophysics, UT Austin Principal Investigators: Dr. Ginny Catania, UTIG Dr. Thomas Neumann, U. of Vermont UTIG Gale White Fellowship Seminar 30 April, 2008

Arctic Climate impact Assessment Increased Surface Melt in Greenland Swiss Camp

Zwally et al 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

Reflection Amplitude (mV) Two-way Travel Time (ms) Surface Zone Bed Zone Interior Zone Siple Dome, Antarctica Ground-Based Radio-Echo-Sounding (RES)

GPS unit 32 m transmitter tow rope/antenna 100 m Ground-Based Radar Acquisition

“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 10 5 to ‘fool’ Focus –Mute the direct arrival –Deconvolution: Invert source wavelet, remove multiples –Filtering: Noise reduction –Static corrections using navigation data –Migration

Height above WGS84 (m) Horizontal Position (km) Case 1: 400 m Ice Thickness Basic 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 Height above WGS84 (m) Horizontal Position (km) Diffractors begin very close to the surface. Diving layers Multiples below the bed Structure masked by reverberations

Picking Velocities for Migration Before Normal Move-out

Picking Velocities for Migration After Normal Move-out (flatten diffractors)

Case 1: Before and After Processing ~2 km

Case 2: 1 km Ice Thickness Basic Processing 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 Depth (m) 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