Automatic  Extraction  of  Ice-­cap  Layers  from  Radar  Sounding  Data  over  Greenland  and  the  South  Polar  Residual  Cap  on  MarsSiting  Xiong,  Jan-­Peter  MullerImaging  Group,  Mullard  Space  Science  Laboratory  (MSSL),  University  College  London,  Department  of  Space  &  Climate  Physics,  Holmbury  St  Mary,  Dorking,  Surrey,  RH5  6NT,  UK,  siting.xiong.14@ucl.ac.uk,  j.muller@ucl.ac.uk

MethodsRadar depth sounding employs low frequency radar operating at several hundredsof KiloHz to MegaHz frequencies and has been applied to the field of subsurfaceinvestigations on both the Earth and Mars.

Over Antarctica and Greenland, the Multichannel Coherent Radar Depth Sounder(MCoRDS) onboard the NASA Operation IceBridge missions[1] has collected radarechograms since 2009 showing the subsurface ice layers caused by iceaccumulation and interrupted by subsurface ice flow. Over the Martian polarregions, subsurface layers are also detected by low frequency radar systems, i.e.MARSIS (Mars Advanced Radar for Subsurface Ionosphere Sounding on board ESA’sMars Express) and SHARAD (SHAllow subsurface RADar on board NASA’s MarsReconnaissance Orbiter) [2]–[5].

Although these subsurface layers are formed by different mechanisms, there is aneed for fast and automatic information extraction from these subsurface radarreflectors with the larger and larger coverage acquired nowadays. The detectionand automatic extraction of subsurface layers is very important preliminary work tofuture studies of surface evolution and past climate. This study presents a methodbased on the Radon Transform (RT) to automatically extract the subsurface layersover Greenland on Earth and South Polar Residual Cap on Mars.

Abstract

Ice  Layers  of  SPRC  on  MarsIce  Layers  of  Greenland  

Figure 4. Study area and the flight tracks of the IceBridge MCoRDS data used in this study.

Velocity Map (m/year)

Figure 5. Radar echogram along profile (a) AA’ and (b) BB’. Obvious folding structures can berecognised from these echograms.

Figure 7. Linear features extracted from two intersected radar echograms shown in 3-D format.

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References

Radar  Sounders

MCoRDS SHARADLaunch  year 2009 2006

Mission Operation  IceBridge NASA  Mars  Reconnaissance  Orbiter

Technique SAR SAR

Frequency 193.9  MHz 20  MHz

Bandwidth 10  MHz 10  MHzAntenna 𝜆/2  bow-­‐tie  dipole   10  m  dipoleTransmitted  Power 500 W 10  WPulse  Length 10-­‐30  microseconds 85  microsecondsPulse  repetition  frequency 9000  Hz 700  or  350  Hz

Platform  altitude -­‐-­‐-­‐-­‐ 255-­‐320  kmVertical  resolution -­‐-­‐-­‐-­‐ 15  m  (vacuum)Azimuth  resolution -­‐-­‐-­‐-­‐ 0.3-­‐1  kmAcross-­‐track  resolution -­‐-­‐-­‐-­‐ 3-­‐7  km

Penetration  depth -­‐-­‐-­‐-­‐ <  ~  1  km

Conclusions  and  Future  work

Figure 1. An example of RT to detect linear feature.

Figure 2. The calculation of correlation coefficient to connect and index peaks (The black dots on the line are detected peaks).

Figure 3. Flowchart of slice processing. There are three stages: (i). Slice the radar echogram; (ii). Extraction of curved linear features based on RT, (iii). Smoothing and (iv). 3-D visualisation. (green rectangles indicate the steps, white ones denote the intermediate results).

The  automated  extraction  of  icecap  layers  is  based  on  the  RadonTransform.  A  slice  processing  flow  is  proposed  and  utilised  in  this  study.  

The study site is located south of the North GreenlandEemian Ice Drilling (NEEM) ice core, where the MCoRDSacquired two intersecting radar echograms, as shown inFig. 4. The lower parts of the core, which contain ice fromthe Eemian interglacial, have been subject to folding.

SHARAD is a radar depth sounder onboard MarsReconnaissance Orbiter to study the subsurface ice water onMars. It is planned to obtain a 3D map with this data as thecoverage over the Martian poles is dense.

Greenland1.The slice processing based on Radon Transform caneffectively extract continuous ice sheet layers.2.Visualisation of ice sheet layers from intersecting radarechograms helps study the 3-­D structure of folded ice.The gaps along the extracted ice sheet layers indicatethe occurrence of anomalies in the ice sheet.3.In addition, effective interpolation for each ice layer isneeded to fill the gaps and smooth the layers.4.However, the effectiveness of the ice sheet layerdetection is qualitatively analysed at this moment,quantitative analyses are needed in future work.Moreover, the surface layer will be co-­registered tosurface elevation data in the future.

SPRC on Mars1.Due to the different mechanisms of theicecap sedimentation, the ice layers showdifferent appearance in radar sounder dataover the Martian south polar icecap, whichmakes the slice processing based on theRadon Transform not applicable.2.In this situation, a non-­orthogonal, complexvalued, log-­Gabor wavelets can be applied toSHARAD data to suppress the noise, andthen thresholded to extract the radarreflection boundaries.3.In the future it is planned to develop amethod to build a 3D block diagram of thedifferent layers.

Figure 6. Extraction of ice shet layers along profile AA’and BB’.

SHARAD  orbit  =  03261-­‐1Original  radar  sounder  data  

Denoised  image  after  application  of  a  non-­‐orthogonal,  complex  valued,  log-­‐Gabor  wavelets  

Extraction  reflectors  of  high  reflectance

The radar sounders which are used to detectsubsurface features of icecap over Greenlandand Martian polar regions, are nadir lookinglow frequency radars. The reflections arecaused by dielectric contrast in the icecapdeposits.

Figure 8. Imaging geometry of subsurface radar sounder.

This  study  is  jointly  sponsored  by  China  Scholarship  Council  (CSC)  and  University  College  London  (UCL).

The  authors  would  also  like  to  express  thanks  to  European  Space  Agency  (ESA)  for  providing  the  travel  support  under  the  ESA-­‐MOST  China  Dragon  Cooperation  (Project  ID:  10665).  

Acknowledgements