finding the needle in the hay stack - tracing seabed gas seepages … · 2020-06-11 · finding the...
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Finding the needle in the hay stack - tracing seabed gas seepages using hullborne multibeam echo sounder, and AUV based sonar and optical systems Terje Thorsnes1, Harald Brunstad2, Petter Lågstad3, Shyam Chand1, Aivo Lepland1 and Arnfinn Karlsen3 1 – Geological Survey of Norway; 2 – Lundin Petroleum; 3 – Norwegian Defence Research Establishment
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A better understanding of the shallow geological systems in the Barents Sea with particular attention to fluid flow
Scientific objective
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• Identify regional spatial distribution of gas flares within an area of 3000 km2 in the Barents Sea
• Identify and describe associated structures (i.e. pockmarks, carbonate reefs) using geomorphology
• Document associated structures with visual tools
• Determine present activity (gas bubbling, bacterial mats)
• Sample seep related material (crusts, gas, mats sediments)
Specific objectives
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Study area – Lopphøgda, Barents Sea
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Blocks Gas seepages Multibeam areas
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Gas seepages Pockmark areas
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Bathymetry and backscatter signature of pockmarks and depressions
Pockmarks – common expression of fluid flow
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Internal structure
of pockmarks and
depressions
Hi-res seismic of pockmarks and depressions
Hullborne TOPAS – H.U.Sverdrup II
Sub-bottom profiler – HUGIN HUSII AUV
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FFI Hugin HUS AUV - Autonomous Underwater Vehicle
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• Multibeam echosounder • Traditional sidescan sonar • Synthetic Aperture Sonar • Photo system – e.g. Tfish b&w photos • Methane sniffer • Other environmental sensors, such as
Temperature, Salinity, Turbidity
AUVs - platform for a range of instruments
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Synthetic Aperture Sonar- HiSAS
• Range-independent resolution
• 5x5 cm – 2x180 m swath • Max resolution – 2x2 cm • 60 Gb/hr • Coverage 2 km2/hr • Bathymetry
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Step 1 – multibeam bathymetry incl. water column data (WCD)
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Step 2 – AUV and SAS
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Step 2 – AUV and SAS
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Methane sensor on AUV
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AUV track lines with colour coded methane levels
Initial response is rapid (seconds), but peak levels may take minutes to reach (T90)
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Step 3 – Tfish images, showing the first direct documentation of gas bubbles in the Barents Sea (width 7 m)
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Who said fish do not care about geology?
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Step 4 – ROV with video and sampling gear
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Carbonate crust samples – on deck
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Test – separate area – linking gas flares to carbonate crusts again?
Outline – gas flares From WCD
Gas flares from WCD
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HiSAS imagery, with Tfish photo overlay
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Conclusions and experiences
• Multi-scale approach: Hullborne MBE – AUV with HiSAS and Tfish - ROV • Water column data indicates gas flares (beware of fish shoals!) • Note – gas flares are episodic events – on and off • Hullborne MBE data give too low resolution for identifying seep-related structures • HiSAS may, or may not, identify carbonate crust structures • Visual documentation is necessary to verify carbonate crust structures, and related phenomena like depressions with algal mats • Sampling carbonate crusts with grabs and box corers is challenging – ROV…
Combination of hullborne hi-res acoustic tools with AUVs fitted with dedicated sensors provides excellent opportunities to increase the scientific understanding of shallow geological processes, and for more applied investigations related to natural or anthropogenic gas seepages
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Thanks for the attention!