glacier basal conditions inferred from seismic data

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Amundsen Coast are thinning rapidly. In particular, particular, the most dramatic changes have occurred on Pine Island. Glacier and Thwaites Glacier, where.
GLACIER BASAL CONDITIONS INFERRED FROM SEISMIC DATA S. Picotti Picotti,, F. Accaino Accaino,, F. Pettenati I i Istituto N i Nazionale l di Oceanografia O fi e di Geofisica G fi i Sperimentale S i l (OGS) Trieste - ITALY 1. INTRODUCTION

Fig.. 1 - West Antarctica map showing the Fig

location of Thwaites Glacier (THW, red triangle).. Black triangles indicate the main triangle) subglacial lakes in Antarctica. Antarctica.

3. RESULTS OF IMAGING

Thwaites Glacier, located in an overdeepened basin that extends far inland, is one of the fastest and largest glaciers draining the West Antarctic Ice Sheet. Sheet. Together with Pine Island Glacier, it is one of the main candidates for a potential catastrophic collapse of the marine ice sheet along the Amundsen Coast Coast.. Recent studies indicate that the glaciers along the Amundsen Coast are thinning rapidly. rapidly. In particular, the most dramatic changes have occurred on Pine Island Glacier and Thwaites Glacier,, where the speed near the grounding line increased more than 25 25% % between 1974 and 2008. 2008.

Joughin and others (2009) 2009) used models constrained by remotely sensed data to infer the basal properties of both glaciers glaciers.. The results indicate strong basal melting in areas upstream of the grounding line, where the ice flow is fast and the basal shear stress is large.. Farther inland, they found mixed bed conditions, alternating from regions of low large drag (i (i..e. deforming sediments), to regions providing greater basal resistance (i (i..e. nonnondeforming sediments or even crystalline bedrock) bedrock).. In particular, for Thwaites Glacier they reported that the areas characterized by strong bed are more extensive than the weak regions, explaining the higher degree of stability with respect to Pine Island Glacier. Glacier. The main purpose of this work is to verify these hypothesis using the seismic method and, more generally, to show that active singlesingle-component seismic data can be effectively used to image i the h internal i l and d subglacial b l i l structures off the h ice i sheets h andd to determine d i the h bed b d properties of the subglacial environments. environments. During the 20082008-2009 Antarctic field season, 60 km of reflection seismic data were collected ~200 km inland of the current grounding line of Thwaites Glacier, Glacier consisting of one 4040-km profile along flow and two 10 10--km transverse profiles.. The survey was profiles designed to target a transition between two zones with different inferred basal drag drag.. Fig.. 2 - The acquisition geometry scheme. Fig scheme.

2. PROCESSING AND IMAGING PROCEDURES The applied processing adopted the 'true'true-amplitude' approach, which preserves the real amplitudes of the reflected signals for the AVO (Yilmaz Yilmaz,, 2001), 2001), and included included:: •reconstruction of the firn velocity profile and refraction statics using diving waves; waves; •surface surface--consistent deconvolution for the ghost elimination and wavelet compression; compression; •surface surface--consistent scaling and residual statics using the crosscross-correlation method. method.

Fig.. 3 - The imaging technique adopted in this work consists in an iterative updating Fig procedure for refining and improving an initial model in depth (b), involving prepre-stack depth migration, residual movemove-out analysis (a) and seismic reflection tomography (Yilmaz Yilmaz,, 2001) 2001). At each iteration, both velocity and reflector geometries are updated, until the two set of parameters reach a good degree of stability stability..

Fig. 4 – Velocity gradient at surface (a) and velocity model at the bed (b). Fig. (b). The first is obtained using the HerglotzHerglotz-Wiechert inversion method, and the second is the result of the procedure described in Fig Fig.. 3. The final prepre-stack depth migration is shown in (c), where the vertical axis indicates the depth below sea level (b.l.s.). AGC is applied to enhance the weak reflections reflections.. The imaging procedure goes on until the quality of p pre--stack depth pre p migration g is not sufficient sufficient.. Generally, y, this p point is reached when the events on the Common Imaging g g Gathers (CIGs) ( ) become flat (see Fig Fig.. 3a) and the seismic energy is well focused. focused. The semblances in (d) show that the depth residuals are well aligned around zero, with a maximum error of about ±10 m. The imaging evidences a clear continuous bed bed--conformable englacial reflection (ER) throughout the whole profile, about 100m 100m above the bed. bed. As the basal topography becomes more pronounced, the englacial horizon becomes more complicated with crosscross-cutting structures. structures. There is also a correspondence between the bed topography and the englacial structure geometry. geometry.

4. RESULTS OF AVO

5. CONCLUSIONS Fig. 5 - Englacial Reflection AVO

Fig. 6 - Sediments AVO

1. Travel Travel--time reflection tomography and imaging have been effectively applied to

resolve the subglacial structures of Thwaites Glacier (West Antarctica) Antarctica).. 2. Imaging and AVO analysis evidenced alternation of low deformable sediments (type Fig.. 5 above) and A sediments sediments;; Vp Vp= =2850 m/s, Vs Vs= =1500 m/s, ρ=2150 kg/m3; see Fig moderate deformable sediments (type B sediments sediments;; Vp Vp= =2400 m/s, Vs Vs= =1100 m/s, ρ=2100 kg/m3), ) and some variability between these two types of sediments sediments.. The inversion also indicates, accordingly to Joughin et al. al. (2009), 2009), a prevalence of type B sediments in the upstream (left) part of the survey, and a prevalence of type A sediments in the downstream (right) part of the survey (see Fig Fig.. 4). The transition between the two subglacial regimes coincides with a change in bed topography topography.. 3. The AVO trend of the englacial reflection corresponds to that of a fracture (see Fig Fig.. 6)

Fig.. 4 – P-wave reflectivity section, results of the AVO inversion Fig inversion.. The amount of reflected energy at the ice bottom depends on the contrast in seismic parameters at the iceice-bed interface interface.. Zoeppritz equations relate the reflection coefficients to the angle of incidence , the change in Pwave velocity ΔVp Vp,, S-wave velocity ΔVs Vs,, and density Δρ at the interface. interface. In AVO inversion we take the seismic processed and balanced preprestack data (which represents the reflection coefficients) and, using the Aki & Richards approximation of the Zoeppritz equations, we try to convertt these th d t to data t reflectivities fl ti iti (i (i..e. relative l ti change h i seismic in i i parameters), t ) which hi h have h clear l physical h i l meanings meanings. i . The Th black bl k arrow indicate i di t a change in bed topography, which correspond with a transition between two different subglacial regimes (see point 2 of conclusions) conclusions).. BIBLIOGRAPHY Joughin, I. and 6 others. 2009. Basal conditions for Pine Island and Thwaites Glaciers, West Antarctica, determined using satellite and airborne data. J. Glaciol. Joughin, Glaciol., 55, 55, 245– 245–257. Yilmaz,, O., 2001. Seismic Data Analysis: Processing, Inversion and Interpretation of Seismic Data. SEG Series: Investigation in Geophy Yilmaz Geophysics, sics, Tulsa.