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  • Ice surface velocity is crucial for estimating the magnitude of ice flux and the mass balance of glaciers and ice sheets. These are fundamental factors to understand the dynamics of the East Antarctica fast glaciers that discharge into the Ross Ice Shelf and to predict climate change relevant parameters, such as sea level variations. In the lower left image you can see the ALOS PALSAR Mosaic of Antarctica: despite L band is good for preserving coherence a large portion of the ice sheet is not covered. C-band: 4-8 GHz (3.75 – 7.5 cm) X-band: 8-12 GHz (2.5 – 3.75 cm)
  • Imaging modes: Scansar, Stripmap, Spotlight, High Resolution Spotlight. These characteristics are very well suited to the estimation of ice surface velocity by means of amplitude tracking and DInSAR !!!!
  • This image shows the acquisition plan over the Ross Sea Sector The superimposed colors show the acquisition tracks on different glacier basins. Data are acquired moving the satellite to left looking position, in STRIPMAP mode which has a swath width of 30 km and spatial resolution lower than 3 m. The outlet glaciers flow from the East Antarctic Ice Sheet (EAIS) into the Ross Ice Shelf through rocky channels crossing the Transantarctic Mountains (TAM) . In this study we focus on the Nimrod and Byrd glaciers, in the lower part of the Ross Ice Shelf. ------------------- Q: ACQUISITION STATUS? A: missing only MacAyel, Bindschadler, Kamb
  • The investigated glaciers drain ice from the East Antarctic Ice Sheet (EAIS) through the Transantarctic Mountains into the Ross Embayment. Nimrod Glacier consist of two main tributaries which merge downwards the Kon-Tiki Nunatak (shown).
  • {Photo: view from Mt. Quackenbush towards Lonewolf Nunataks.} Catchment basin ~size of France and Spain together Changes in its flow dynamics would have an impact on the mass balance of East Antarctica and on the stability of the Ross Ice Shelf !!! The behavior of the TAM outlet glaciers, and especially of Byrd, is influenced by : The upstream catchment area The shearing on the rocky walls and on the bed (where lubrication by subglacial water is possible) of the glacier channel The reaction of the floating ice shelf to the ice input (buttressing) The dynamical behavior of these glaciers has been relatively steady over the past 1000 years. For Byrd some changes in the upstream flow have been recently linked to changes in the subglacial water flow. (discharge of subglacial lakes 200 km upstream of Byrd’s Grounding Line [Stearns, L., 2008])
  • Ice velocity is measured by means of Speckle Tracking: this consists in computing the ccf for a regular grid of patches on the 2 images and measuring the displacement of the peak of the ccf. We used a patch size of 256² with an overlap of 50%. Input images: Stripmap (SM) geocoded amplitude images (detected) (EEC) Pixel spacing = 1.25 m Spatial resolution ~ 3 m Looks ~ 1.3 Typical processed SM image size: 32000 x 50000 pixel Comment the images instead of last sentence. Left: CCF of slow and smooth area, and the RGB composition ot the master and the slave images. Coherence stems from the speckle pattern. The ccf shows a well defined peak with a small displacement, leading to high coherence and accuracy. Right: CCF of fast and crevassed area, coherence is given by the texture pattern. The ccf shape depends on the texture pattern with a larger and more displaced peak. This leads to lower coherence and accuracy.
  • TerraSAR-X Velocity map detail, the arrow size and color indicates the absolute velocity. The red squares show the patch extent and the patch overlap. Speckle tracking (vs. DInSAR): Absolute surface displacement is obtained Unambiguous measurements (no phase unwrapping) Robust to lower of coherence Lower accuracy.
  • Longitudinal profile was selected along the central line of the glacier and parallel to the flow direction. STRAIN RATE: dertivative of the STRAIN E=(l-l0)/l0. Gives an idea of the degree of compression/extension the ice is subject to. A positive value means the ice is expanding, while a negative value means that the ice is compressing. TENSILE STRENGTH: maximum stress that a material can withstand while being stretched before necking (-> crevasses onset). For glaciers: in an area of increasing expansion the tensile strength is reached when the ice „breaks“, i.e. starts to be crevassed (here is where we compute the TS), afther breaking the strain rate normally decreases since the material expands abruptly. FOR QUESTIONS: These are standard parameters to asses the state of a glacier (eg find GL) They help the interpretation and the combination of different measurements.
  • Constant velocity in the upper part (0.3 m/d) of the Nimrod and Marsh glaciers Approaching the Kon-Tiki Nunatak the flow becomes abruptly constrained because of the narrowing of the glacier width and sustains a rapid velocity increase , with the appearance of strong crevassing!! Maximum speeds of 2.4 m/d are reached downstream of the Nunatak where the 2 flows merge Ice velocity decreases abruptly as the fjord gets wider and possibly deeper It then stabilizes to a constant value of around 0.6 m/d crossing the grounding line and entering the Ross ice Shelf, which offers resistance (butressing) to the flow, avoiding further acceleration (this is in contrast to David Glacier which velocity increase after the grounding line into the floating Drygalski Ice Tongue) The transverse profile, extracted just above the GL, shows a plug like behavior, with abrupt velocity variation on the side bands and an almost constant value in the central part: this suggests that flow is mostly constrained by side drag. Technical info: BASELINE INTERVAL (Height of Ambiguity): -533.9 (-24.4) – 456.9 (17.78) m
  • The strain rate graph shows an equilibrium status in the upper slower part of Nimrod glacier, then increases (0.02 – 0.05 a -1 ) as the flow gets faster. In point (2) major crevassing appears, here we computed a value of 0.03 a -1 with a tensile strength of 487 – 562 kPa. In the fastest sector of Nimrod, the tensile strength follows an irregular behavior with a peak of 0.1 a -1 followed by a strong compression peak (-0.15) as the glacier slows down abruptly. It then maintains an equilibrated behavior as the speed remains constant. PROCESSING DETAILS: Velocity image (45m pixel spacing) was filtered with: Median filter, window size = 7x7 Low pass filter, window size = 3x3 Velocity profile (distance between values ~63m) was FILTERED with: Median filter, window size: 15 Smoothing filter, window size: 21
  • The velocity field and the longitudinal profile show a rather different pattern from Nimrod Gl. Ice velocity increases steadly in response to increased surface slope: the increase is initially moderate and then more consistent approaching the fjord. The maximum speed of 2.5 m/d is reached just after the GL. Ice velocity decreases slowly as it flows through the lower part of the fjord and into the Ross Ice Shelf, here flow from Byrd remains well distinguishable for its higher velocity. There are no abrupt variations of speed as in Nimrod. Two transverse profiles were extracted: The upper T1 shows a parabolic shape indicating strong basal drag In proximity of the GL the T2 profile shows a plug-like behavior indicating a prevalence of side drag over basal drag due to a well-lubricated bed. Technical info: BASELINE INTERVAL (Height of Ambiguity): -393.5 (-23.0) – +154.0 (52.9) m
  • Byrd glacier shows a much smoother longitududinal velocity profile than Nimrod, the Strain Rate graph is also much more regular. It shows an expansive behaviour till the velocity peak, with values oscillating between 0.002 and 0.15 After the GL the strain rate graph moves into the compression zone as the decelerates smoothly. Tensile strenghts have been computed for 3 points of crecasses onset, shown on the map. [read values 2,3]

TH4.TO4.1.ppt TH4.TO4.1.ppt Presentation Transcript

  • TERRASAR-X OBSERVATIONS OF ANTARCTIC OUTLET GLACIERS IN THE ROSS SEA SECTOR Kenneth Jezek 1 , Wael Abdel Jaber 2,3 , Dana Floricioiu 2 1 Byrd Polar Research Center, Ohio State University, Columbus, OH, USA 2 German Aerospace Center (DLR), Remote Sensing Technology Institute, Oberpfaffenhofen, Germany 3 Technical University of Munich, Remote Sensing Technology, Munich, Germany
  • Introduction
    • Ice surface velocity:
    • Crucial parameter for mass balance estimation of glaciers and ice sheets.
    • Fundamental factor to establish the state of glaciers and predict sea level variations.
    • Interferometric SAR data requirements for ice surface velocity:
    • Short repeat pass time span for high frequency SAR (C-, X-band) to avoid decorrelation
    • Left looking capabilities for areas South of 80°S. Available for: Radarsat-1/2 (1997/2008 campaigns), COSMO-SkyMed and TerraSAR-X.
    Antarctic ice velocity: ERS-1/2 (1996), RSAT-1 (1997, 2000) [Rignot et al, 2008]. Status 2000, update ongoing!
  • TerraSAR-X mission
    • TerraSAR-X mission: X-band (9.65 GHz) Synthetic Aperture Radar (SAR)
    • 1st satellite TSX-1 operational since Jan. 2008
    • 2nd satellite TDX-1 (launched for TanDEM-X mission) operational since Oct. 2010 (for TerraSAR-X mission)
    • Left and right imaging capabilities
    • Various imaging modes and incidence angles
    • Spatial resolution in stripmap mode, single look complex data: < 3 m in slant range and azimuth
    • Very accurate orbit estimation
    • 11 days repeat pass cycle
    • During the 2007-2008 International Polar Year (IPY) DLR initiated the acquisition and processing of interferometric TerraSAR-X data over important sectors of the polar ice sheets.
    TSX-1 TSX-1 & TDX-1
  • TerraSAR-X acquisition plan over the Ross Sea Sector of Antarctica
    • Left looking
    • Stripmap
    • Swath width: 30 km
    • Resolution: <3 m
    • Descending coverage
    • Background: RSAT-1
    Nimrod Beardmore 80°S Byrd Shackleton Amundsen Scott Van der Veen and Whillans Kamb Bindshadler Crary Reedy MacAyeal Ross Ice Shelf Courtesy: Katy Farness, OSU N
  • Nimrod Glacier (82.6°S, 160.7°E)
    • Two main tributaries: Nimrod and Marsh Glacier
    • Length: 135 km
    • Glacier in balanced state: mass balance 0.88 ± 0.39 Gt/a (after L. Stearns, 2010, in press)
    Ross Ice Shelf Marsh Gl. Kon-Tiki Nunatak TerraSAR-X image (2009) Nimrod Gl. flowing around the Kon-Tiki Nunatak TerraSAR-X mosaic (2009) N az rg
  • Byrd Glacier (80.5°S, 157.5°E)
    • One of the largest catchment basins in Antarctica (1 101 725 km 2 )
    • Fjord length: 136 km, width: 24 km
    • Ice export into the Ross Ice Shelf: 22.32 ± 1.72 Gt/a
    Byrd Gl. view upstream from Mt. Quackenbush TerraSAR-X mosaic (2010) Ross Ice Shelf az rg N
  • Ice velocity derivation by speckle tracking
    • Displacement for each patch is obtained from the position of the peak of the incoherent cross-correlation function (CCF).
    • A regular grid of patches with size of 256 2 pixels and 50% overlap is used.
    • Input: Stripmap geocoded images: 1.25 m pixel spacing, ~1.3 looks, <3 m sp. resolution.
    • Correlation between repeat pass images is given by the speckle pattern (smooth, highly coherent areas) or by the image texture (e.g. crevasses).
    NIMROD, Pair 000 Byte per pixel of input: 2 Master / Slave patch mean: 189.45242 186.07397 Patch size= 256 Crop size= 32 Oversampling factor=16 Central pixel on sras image (origin is LL), x: 9000, y: 32472 UTM Coords (easting - northing) [m]: 2324900.000 11137160.000 Displacement before interpolation: 1 -2 [pix] peakwidth x/y = 14 14 Corr. coeff: 0.52106100 Accuracy [pixel x/y] = 0.049388256 0.049388256 Corr. coeff: 0.52106100 Accuracy [meter x/y/abs] = 0.061735320 0.061735320 0.087306927 SNR1: 13.374758 ( 11.262859 dB) SNR2: 12.374988 ( 10.925448 dB) CC function - Mean: 269.38077 Min: 0.00500107 Max: 3602.90 Oversampled Max: 3613.71 Displacement after interpolation (+ is East and North): 0.812500 -2.06250 [pix] 1.01562 -2.57812 [m] displacement in 11 days [pix]: 2.21677 displacement in 11 days [m]: 2.77096 angle of displacement: 291.501 -68.4986 (Deg, from East, anti-ckw is positive) displacement /day [m]: 0.251906 displacement /year [m]: 91.9455 NIMROD, pair 000 Byte per pixel of input: 2 Master / Slave patch mean: 184.13252 183.36140 Patch size= 256 Crop size= 32 Oversampling factor=16 Central pixel on sras image (origin is LL), x: 8280, y: 12492 UTM Coords (easting - northing) [m]: 2324000.000 11112185.000 Displacement before interpolation: 0 -1 [pix] peakwidth x/y = 4 4 Corr. coeff: 0.38275356 Accuracy [pixel x/y] = 0.020792813 0.020792813 Corr. coeff: 0.38275356 Accuracy [meter x/y/abs] = 0.025991016 0.025991016 0.036756848 SNR1: 90.573555 ( 19.570014 dB) SNR2: 89.576592 ( 19.521945 dB) CC function - Mean: 10.541570 Min: 0.000357389 Max: 954.787 Oversampled Max: 958.024 Displacement after interpolation (+ is East and North): -0.125000 -1.06250 [pix] -0.156250 -1.32812 [m] displacement in 11 days [pix]: 1.06983 displacement in 11 days [m]: 1.33728 angle of displacement: 263.290 -96.7098 (Deg, from East, anti-ckw is positive) displacement /day [m]: 0.121571 displacement /year [m]: 44.3735 CCF: slow and smooth area NIMROD, Pair 020 Byte per pixel of input: 2 Master / Slave patch mean: 246.90729 245.42989 Patch size= 256 Crop size= 32 Oversampling factor=16 Central pixel on sras image (origin is LL), x: 13356, y: 21384 UTM Coords (easting - northing) [m]: 2274750.000 11211163.750 Displacement before interpolation: -13 -8 [pix] peakwidth x/y = 18 33 Corr. coeff: 0.12337598 Accuracy [pixel x/y] = 0.31180419 0.57164101 Corr. coeff: 0.12337598 Accuracy [meter x/y/abs] = 0.38975523 0.71455126 0.81393652 SNR1: 11.534828 ( 10.620111 dB) SNR2: 10.534860 ( 10.226288 dB) CC function - Mean: 268.90243 Min: 0.000812531 Max: 3101.74 Oversampled Max: 3113.25 Displacement after interpolation (+ is East and North): -12.7500 -7.87500 [pix] -15.9375 -9.84375 [m] displacement in 11 days [pix]: 14.9859 displacement in 11 days [m]: 18.7324 angle of displacement: 211.701 -148.299 (Deg, from East, anti-ckw is positive) displacement /day [m]: 1.70295 displacement /year [m]: 621.576 CCF: fast and crevassed area 11 days repeat pass 11 days repeat pass CCF: slow and crevassed area W-E cut N-S cut 0.156 m 1.328 m R: master (m) G: slave (s) B: (m+s)/2 W-E cut N-S cut 15.936 m 9.844 m R: master (m) G: slave (s) B: (m+s)/2
  • Speckle tracking: TerraSAR-X velocity map detail Patch size: 256 2 pixel (320 2 m 2 ) Patch distance: 128 pixel (160 m) 0 500 m 0.0 2.5 [m/d] 2 m/d
  • Speckle tracking (ST) accuracy with TerraSAR-X
    • Absolute accuracy of velocity estimate is affected by:
      • Atmospheric path delay
      • Solid earth tides
      • Orbit estimation and timing errors
      • Geocoding errors
    • Relative accuracy of an individual motion vector depends on:
      • Correlation coefficient γ
      • Patch size N
      • Shape of the cross-correlation function
      • Estimated empirically for highly coherent smooth areas [1] . ~10 times lower than absolute accuracy
    [1] R. Bamler, M. Eineder, “Accuracy of differential shift estimation by correlation and split-bandwidth interferometry for wideband and delta-k SAR systems,“ IEEE Geoscience and Remote Sensing Letters, Vol. 2, No. 2, April 2005 σ CR : Cramer-Rao bound More in presentation: „Glacier flow and topography measurements with TerraSAR-X And TanDEM-X “ by Michael Eineder (session TH4.T02.5, Room 1, 4:40 PM) σ rg = 0.2 m, σ az =0.1 m
  • Geophysical glacier properties derived from velocity profiles
    • Longitudinal strain rate
    • Obtained from the filtered longitudinal velocity profile applying [1]:
    • Tensile strength
    • Computed for points of crevasses initiation (according to amplitude image)
    • Assumption: only longitudinal component of strain is present on glacier‘s center line
    • Application of Glen‘s flow law of ice + „von Mises“ and „Griffith‘s“ failure criteria , leads to:
    [1] Forster, R., Rignot, E., Isacks, B., Jezek, K., „Interferometric radar observation of Glaciers Europa and Penguin, Hielo Patagónico Sur, Chile“, Journal of Glaciology, Vol. 45, No. 150, 1999 Δ v : velocity variation between two points Δ d : distance between two points ( Δ d ≈ 63 m ) A : flow parameter, depends on temperature, impurities and crystal orientation of ice ( A = 1.61 ∙10 -9 yr -1 kPa -3 )
    • 23 Stripmap pairs, coverage: ~35 000 km 2
    • Acquired: Oct. - Nov. 2009
    Nimrod Glacier: ice surface velocity from TerraSAR-X Ross Ice Shelf Plug like shape: strong side drag km 0 30 N Marsh Gl. Kon-Tiki Nunatak GL 0.0 2.5 [m/d] (L) (T) (T) SE NW 255.5 219.0 182.5 146.0 109.5 73.0 36.5 0.0 [m/a] (L) Ice flow GL 912.5 730.0 547.5 365.0 182.5 0.0 [m/a]
  • Nimrod Glacier: longitudinal strain rate (1) (2) (L) GL (1) (2) 912.5 730.0 547.5 365.0 182.5 0.0 [m/a] km 0 30 N Marsh Gl. Kon-Tiki Nunatak 0.0 2.5 [m/d] Ross Ice Shelf (L) (T) GL Ice flow (L) GL (1) (2) 487.0 - 562.4 204.1 - 235.7 TS M – TS G [kPa] 0.0356 0.0026 Strain rate [a -1 ] 0.72 0.27 Velocity [m/day] 119.0 46.2 Distance [km] (2) (1) Point
    • 29 Stripmap pairs, coverage: ~44 300 km 2
    • Acquired: Nov. 2010 – Feb. 2011
    Byrd Glacier: ice surface velocity from TerraSAR-X Ross Ice Shelf Plug like shape: side drag Parabolic shape: basal drag N GL km 0 30 (T2) (T1) (L) 0.0 2.5 [m/d] (L) Ice flow GL 912.5 730.0 547.5 365.0 182.5 0.0 [m/a] (T1) (T2) SE NW 912.5 730.0 547.5 365.0 182.5 0.0 [m/a]
  • Byrd Glacier: longitudinal strain rate (1) (2) (3) N GL km 0 30 Ross Ice Shelf (T2) (T1) (L) 0.0 2.5 [m/d] GL (L) (1) (2) (3) (L) Ice flow GL (1) (2) (3) 912.5 730.0 547.5 365.0 182.5 0.0 [m/a] 292.9 - 338.2 0.0077 1.48 161.0 (3) 226.68 - 261.7 171.1 - 197.6 TS M – TS G [kPa] 0.0036 0.0015 Strain rate [a -1 ] 0.40 0.20 Velocity [m/day] 70.0 3.0 Distance [km] (2) (1) Point
  • Summary and conclusions
    • Two large outlet glaciers of the Transantarctic Mountains (TAM), Nimrod and Byrd , have been studied with TerraSAR-X repeat pass images.
    • The TAM area south of 80°S is poorly known due to the limited availability of satellite data.
    • The acquisition plan in left looking mode was initiated as a major contribution of DLR to the 2007-2008 International Polar Year.
    • Ice surface velocity maps and profiles with very high resolution were obtained by means of speckle tracking : an accurate and robust method in these highly coherent areas.
    • Other important geophysical parameters, such as strain rate and tensile strength were also obtained.
    • The two glaciers show differences in ice flow behavior, influenced by:
      • Upstream processes
      • The topography of the glacier channel
      • (partly) the evolutionary stage of each glacier associated with the slow retreat of the Ross Ice Shelf since the last glacial maximum.
    Thank your for your attention!