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Master Thesis Presentation
Ao Kaidong
Supervisors: Prof. Dr. Manfred F.Buchroithner
Dr. –Ing. Holger Kumke
31.01.2018
International Master of Science in Cartography
Ice-Basement Mapping of Eisriesenwelt Cave Using Ground Penetrating Radar
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Contents
• Introduction
• Study Site
• Data Collection
• Data Preparation
• Methodology
• Ice-Basement Maps
• Outlooks
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Introduction - Motivation
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Pioneering
• First areal ice-basement mapping in an ice cave globally. Example of the applicationof ground penetrating radar (GPR) in Eisriesenwelt, the largest ice cave on Earth.
“Hot” research topic
• Cave-ice volume and its stability is a crucial indicator of climate changes.
Proxy
• Together with the 3D visualization model of Eisriesenwelt, a large-scale ice-thicknessmap is able to serve as a historical ice volume record.
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Introduction - Workflow
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Data Collecting
• Collecting raw GPR data of ice thickness
Data Processing
• Data enhancement, time-depth conversion, layer analysis
Ice-Basement Mapping
• Feature point extraction, feature point interpolation, map generation
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Study Site
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• Northern Austrian Alps• Ice-covered part of cave system• Approx. 24,400 square metres in
201X
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Study Site
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Classification of Eisriesenwelt:
• A dynamic cave with strong Chimney Effect
Origin of the cave ice in Eisriesenwelt:
• Freezing of infiltrating water
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Data Preparation
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Data Collection
• 20. 04. 2017 – 23.04.2017
• 143 datasets (survey profiles)
• Type: IDS GPR, RIS MF Hi-Mod (Dual
frequency: 200HMz/600HMz)
• Sampling rate: 32 traces per second
• Hand-towing scans: 0.5m/s in average
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Data Preparation
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Data Preparation
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Data Pre-Processing Steps:
• Dewow Filtering:Eliminating low-frequency signal bias caused by Wow Effect
• Time-Zero Correction:Removal of the wave echo from the ice surface and radar shell
• User-Defined Gain Function:Exponential gain to enhance the traces horizontally
• Trace Editing:Removing the traces recorded while the antenna was occasionally stopped incidentally
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Data Preparation – Dewow Filtering
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Origin of Wow Effect:• Caused by swamping or saturation of
the recorded signal by early arrival of radar wave echo.
Dewow Filter:• Quasi Mean Filtering
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Data Preparation: Time-Zero Correction
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Data Preparation – Gain Function
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Amplitude information is enhanced by changing linear gain to exponential
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Data Preparation – Trace Editing
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Methodology: Time–Depth Conversion
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Electromagnetic Wave Propagation Properties
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Methodology: Time–Depth Conversion
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Data Migration:
• Correction of the trace features for true spatial position in nature
• Enhance section resolution
Precondition of Data Migration:
• The velocity structure of the subsurface must be known (or accurately estimated),and the stratigraphy is constructed of laterally invariant, constant velocity layers.
• The source is spatially uniform and propagates spherically.
Migration method:
• Kirchhoff Migration (only vertical velocity variation accounted)
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Methodology: Time – Depth Conversion
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Unmigrated Migrated
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Methodology – Layer Analysis
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Manual Boundary Drawing:
• Frequently applied in mineral and petroleum prospection
• The most efficient way to extract ice–rock boundaries
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Methodology – Feature Points Sampling
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Feature Points:
• Semi–Automated extracted
• Sampling density on the profile direction is less than 2 meters
Feature Points manual picking criteria:
• Discontinuity
• Distinguishable hollow structures
• Local maximum-depth points
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Methodology – Accuracy Assessment
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Resolving Power of GPR:
• Horizontal resolution ∆𝑙: The width of the radar Fresnel Zone
• Vertical resolution ∆𝑟 : One-forth of the dominant wavelength
Diameter of Fresnel Zone on average ice depth: 6.1m
Vertical Resolution: 0.2m
N.B: The Fresnel Zones could cover the sampling intervals effectively
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Methodology – Depth Interpolation
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Empirical Bayesian Kriging:
• Multiple semivariogram models involved
• The errors from classical Kriging were taken into account
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Methodology – Map Symbols
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Depth contour with 1-meter interval Depth contour with 2-meter interval
Depth contour with 0.5-meter interval GPR Survey Lines
Point of Interest Walking Path
Icefall Breccia
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Ice Basement Maps
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Siegfried Kame
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Outlooks
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A 2D map with a profile view with a clear ice-rock boundary can be made in further researches.
Ice volume can be calculated from the ice-thickness data.
Possibilities offered for 3D modelling of ice thickness.
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Thank you for your attention