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Seismic Imaging of Sub-Glacial Sediments at Jakobshavn Isbræ and NEEM Greenland Georgios Tsoflias 1,2 , Jose Vélez 1,2 , Ross Black 2 and Cornelis Van der Veen 1,3 1 Center for Remote Sensing of Ice Sheets (CReSIS), The University of Kansas, Lawrence, Kansas , USA, [email protected] 2 Departyment of Geology, The University of Kansas, Lawrence, Kansas, USA 3 Department of Geography, The University of Kansas, Lawrence, Kansas, USA Objective Sub-glacial sediments can have significant impact on glacier flow, yet their presence, extent and thickness is poorly constrained under ice sheets and glaciers. We used active source seismic reflection to image sub- glacial sections at Jakobshavn Isbræ, West Greenland and at the North Greenland Eemian Ice Drilling (NEEM) location. Background Processes at the bed, especially in areas where deformable layers are present, can dominate ice stream flow. Therefore, ice stream behavior may be different over deforming till than on a water-lubricated ice/bed interface (e.g. Budd et al., 1984; Alley et al., 1986). Given the potential importance that the bed character may have on the behavior of an ice sheet or a glacier, it is necessary to determine the type of material at the ice/bed interface and extract information on properties that can influence ice flow, such as bed topography, the presence of water at the bed, and sub-glacial sediment thickness. Seismic waves have the capability to penetrate below the ice, thus allowing the study of sub-glacier materials. Observations of seismic velocity can help differentiate sub-glacier sediments from bedrock. Seismic reflection amplitude and polarity at the bed can infer the presence of water or changes in sediment properties affecting the flow character of the ice sheet, e.g. from basal sliding to deforming bed (e.g. Anandakrishnan et al., 1998; Smith, 2007; Peters et al., 2008). a. Jakobshavn Isbræ Previous seismic studies focused on imaging the bed and determining ice-column properties (Horgan et al., 2008; Vélez, 2012). We re-process the 9.8 Km seismic profile presented in Horgan et al. (2008) with the objective of imaging sub-glacier sediments (Fig. 1). Seismic data acquisition Line length: 9.8 Km Source: 0.5 Kg PETN placed in 10 m-deep steam-drilled shot holes Receivers: 24 - 28-Hz vertical geophones, 20 m spacing, 10/470 m near/far offset range, 160 m source & receiver roll b. NEEM Conclusions Seismic imaging revealed sub-glacial sediments at Jakobshavn Isbræ and at NEEM. At Jakobshavn seismic facies identified distinct depositional processes that resulted in varying sediment thickness and properties along the 10 Km profile. At NEEM, seismic confirmed the presence of sub-glacial sediments that were also encountered by the ice core. References Alley, R.B., Blankenship, D.D., Bentley, C.R., Rooney, S.T., (1986), Deformation of till beneath Ice Stream B, West Antarctica. Nature 322, 57–59. Anandakrishnan, S., D. D. Blankenship, R. B. Alley, and P.L. Stoffa, (1998), Influence of subglacial geology on the position of a West Antarctic ice stream from seismic observations. Nature, Vol 394. Büker, F, Green, A. G., and H, Horstmeyer (2000), 3-D high-resolution reflection seismic imaging of unconsolidated glacial and glaciolacustrine sediments: processing and interpretation. Geophysics, Vol. 65, Pag. 18–34. Horgan, H. J., S. Anandakrishnan, R. B. Alley, L. E. Peters, G. P. Tsoflias, D. E. Voigt, and J. P. Winberry (2008), Complex fabric development revealed by englacial seismic reflectivity: Jakobshavn Isbræ, Greenland, Geophysical Research Letters. Vol. 35, L105. Joughin, I., B. Smith, I. Howat, and T. Scambos (2010) MEaSUREs Greenland Ice Sheet Velocity Map from InSAR Data, Version 1. [NSIDC-0478.001/2008.12.01]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. http://dx.doi.org/10.5067/MEASURES/CRYOSPHERE/nsidc-0478.001. [2012]. Peters, L. E., S., Anandakrishnan, C. W., Holland, H. J., Horgan, R. B., and D. E., Voigt, (2008), Seismic detection of a sub- glacial lake near the South Pole, Antarctica, Geophysical Research Letters. Vol. 35, L 23501 Smith, A. M., (2007), Subglacial Bed Properties from Normal-Incidence Seismic Reflection Data. Journal of Environmental and Engineering Geophysics, Volume 12, Issue 1, pp. 3–13. Vélez, A. J., (2012), Development of a Seismic Snow Streamer and Use of Multi-Offset Reflection for Determining Glacier Ice Properties. Master Thesis: Department of Geology, The University of Kansas. Acknowledgements This work was funded by the National Science Foundation and the Center for Remote Sensing of Ice Sheets (CReSIS) (Grant No. ANT-0424589) at the University of Kansas. We thank the Penn State Ice and Climate Exploration group (Sridhar Anandakrishnan, Huw Horgan, Leo Peters, Paul Winberry and Don Voigt) for providing the Jakobshavn seismic data and assistance in the field. Partial student funding for this investigation was provided by the Harriet Jenkins Pre-Doctoral NASA Fellowship program. WE thank Schlumberger (VISTA) and IHS (KINGDOM) for the generous software donation to The University of Kansas. Figure 1. Location of the seismic profile (solid line) approximately 100 km from the terminus of Jakobshavn Isbræ. Seismic data from Horgan et al. (2008). Contours represent ice surface elevation in meters (elevation data from CReSIS). Ice flow velocity shown in gray scale (from Joughin et al., 2010). Seismic data processing for sub-glacial imaging Initial processing included geometry definition, eliminating bad traces, muting the direct arrival and band- pass filtering (75-375 Hz). Ice velocity of 3745 m/s (Vélez, 2012) was used to generate a constant velocity stacked and migrated profile. Velocities below the bed reflection were varied iteratively from 2800 to 4000 m/s at 200 m/s increments (Fig. 2). Based on the continuity of the bed and shallow sub-glacier reflections, a velocity of 3600 m/s was selected as the sub-glacier sediment velocity. Bedrock velocity of 5800 m/s was determined iteratively using velocities consistent with well-lithified materials from 4800 to 6800 m/s at 200 m/s increments and examining the continuity of deeper reflections (Fig. 3). Results: Seismic facies interpretation of sub-glacial imaging Ice and bedrock layers are interpreted based on seismic velocity as shown in Fig. 3. The middle velocity layer of sub-glacial sediments is subdivided in to three distinct seismic facies (Fig. 4) and interpreted based on published examples (e.g. Büker et al., 2000): i) Continuous reflections interpreted as basal till (yellow); ii) Lapping reflections interpreted as accreted sediments (orange); and iii) Discontinuous reflections interpreted as re-worked sediments (purple) (Fig. 4 and Fig. 6). Bed reflection polarity reversals observed on seismic are consistent with modeled waveforms resulting from a body of liquid water below ice (Fig. 5). Figure 2. Migrated seismic panels using two-layer velocity models. Ice velocity is held constant to 3745 m/s. Sub-glacier section velocities are shown as inserts in each panel. Figure 3. Section of the processed seismic line with amplitude data shown as wiggle traces and the three-layer velocity model as color overlay. Layer velocities are marked on the section. Figure 4. Sections of the seismic line showing examples of the sub-glacial sediment seismic facies. Figure 5. Polarity reversal on seismic data at the ice bed interface. Insert: Waveforms modeled for varying water saturated porosity sediments and water underlying ice. Figure 6. Processed seismic line with seismic facies interpretation overplayed in color. Basal Till (continuous facies - yellow): approx. max. thickness 70 m Accreted Sediments (lapping facies - orange): approx. max. thickness 90 m Re-Worked Sediments (discontinuous facies – purple): approx. max. thickness 110 m Figure 7. Location of seismic experiment near the NEEM ice core and orientation of the ice divide. We acquired a long-offset, pseudo-CMP seismic gather 6.5 km north of the NEEM ice core location (Fig. 7). Seismic data acquisition & processing Gather offset: 5.8 Km Source: 0.5 Kg PETN placed in 10 m steam-drilled holes Receivers: 96 - 100-Hz vertical geophones, 20 m spacing Band-pass filter (80-350 Hz) Normal moveout correction velocity 3840 m/s Figure 8. Shot one of near-offset data. A) Raw filtered data. B) NMO corrected data using ice velocity of 3,840 m/s. Results: Seismic sub-glacial imaging The CMP gather revealed two prominent reflections at the base of the ice (Fig. 8). The upper reflection (1280 ms) is interpreted as the base of ice – top of till interface. Thee lower reflection (1310 ms) is interpreted as the base of till - top of bedrock. The thickness of the subglacial sediment section is estimated to approximately 50 m. C11C-0768
