High swath-overlap seafloor and water column backscatter survey
over the Calypso hydrothermal vents, Bay of Plenty, and Brothers
Volcano, Kermadec Arc,
New Zealand
Potential for habitat mapping
and angular dependence study
Geoffroy Lamarche,
Ben Higgs,
Richard Wysoczanski
NIWA, Wellington
Geohab 2016
enhancing the benefits of New Zealand’s natural resources
High swath overlaps
Two datasets
1. Brother Volcano’s deep caldera
2. Calypso shallow hydrothermal vents
Two types of EM302 data
1. Seafloor Backscatter
2. Water-Column Backscatter
Two Research Themes
1. Angular dependence study
2. Seep and bubble detection
Geohab 2016
enhancing the benefits of New Zealand’s natural resources
Acquisition geometry
Calypso Hydrothermal Vents
•
•
•
•
•
Geohab 2016
37 lines
Lines spacing <50 m
Lengthy data acquisition
Overlaps unevenly distributed,
max. theoretical ~700%
enhancing the benefits of New Zealand’s natural resources
Mosaic with
no overlap
Merging of mosaic
Individual
profiles
•
•
•
•
•
•
Geohab 2016
Overwrite
Min
Max
Mean
Median
Sum
enhancing the benefits of New Zealand’s natural resources
Mosaic
All data overlap
(Overwrite)
Speckle filters
Classical filter : many speckle filters are proposed in the
literature. They all involve a neigboorhood dependence,
the counterpart is a loss of resolution
Colocated pixels averaging : Thanks to the excellent data positioning,
up to 30 pixels may come from the same footprint.
The mean value is speckle free!
All overlapping pixels
except from specular
(i.e. Incidence Angles > 12°)
Pixels from single swaths
Geohab 2016
‘traditional’ Speckle filter (Wiener)
Pixels from averaged swaths
enhancing the benefits of New Zealand’s natural resources
Equi-angle backscatter mosaics
 Using multiple incidence angles to build mosaics where pixels come from the same incidence angle ranges
 Pixels for which incidence angles are in the selected range are averaged
0 15 deg
15 30 deg
30 45 deg
45 60 deg
• The incidence angle signatures are clearly visible (see the specular one, ie 0-15 deg)
• Gaps in the mosaics due to slopes as swaths were all in the same direction
Geohab 2016
enhancing the benefits of New Zealand’s natural resources
60 75 deg
Augustin and Lamarche, 2015
Supervised segmentation
Identify some facies : usually the
scientists define some regions of
interest. Here we defined two zones
using a single threshold
Whole mosaic
Low BS ≤ -35 dB
High BS > -35 dB
Compute backscatter
curves using SonarScope
SW from Ifremer
From Augustin and Lamarche, 2015
Geohab 2016
Histograms of pixels for a
given incidence angle
Angular backscatter for the two regions
enhancing the benefits of New Zealand’s natural resources
Supervised segmentation five classes
5
4
3
2
1
From Augustin and Lamarche, 2015
Geohab 2016
Reflectivity
Segmentation 5 Facies
(median value of all individual segmentations)
enhancing the benefits of New Zealand’s natural resources
Ship-borne Multibeam Water Column Imagery
Efforts focus on developing tools for the post-acquisition chain
• Display, replay, processing and analysis of dataset
• Combine use of amplitude and phase
• Identify and accurately localize events in the water column
• Enable multiple targets detection
• Improve signal-to-noise ratio
and target resolution
Flare
Sonarscope software
(Matlab based
and 3DViewer)
Geohab 2016
enhancing the benefits of New Zealand’s natural resources
Calypso hydrothermal vent field
• Hydrothermal vents are important indicators of geochemical,
biological and volcanic processes
• Gas bubble and fluid discharge generate acoustic flares that are
transient and dynamic but identifiable using water-column
backscatter – yet this is qualitative
• Plumes can be several 100 m in height above seafloor
• Quantitative (characterization) method is needed
100 m
• Automated and systematic recognition is challenging
50 m
Hikurangi Margin
From Higgs, 2016
Geohab 2016
2D along-track curtain view, generated by summarising acoustic data over the Calypso Vent Field
enhancing the benefits of New Zealand’s natural resources
Seafloor-seep relationships
Vertical echo integration: vertically sum
the amplitudes of echoes in the water
column.
 Seeps can be identified on an
integrated surface as high-amplitude
zones, where seep-bubble echoes
have summed constructively.
 Echo-integration surfaces allow us to
analyse spatial features of seeps.
Geohab 2016
enhancing the benefits of New Zealand’s natural resources
Seafloor-seep relationships
 Applying the echo integration to the imaged
water column over the Calypso Vents shows the
distribution of seeps.
 By adjusting the amplitude threshold of the
integrated surface, we display only the major
seeps.
 Overlaying the integrated-seep indicators over
the fully-compensated backscatter show that
seep locations correspond with high-backscatter
regions.
Geohab 2016
enhancing the benefits of New Zealand’s natural resources
Hydrothermal plumes
• suspected diffuse venting from the upper parts of cones
• Concentrated fluid escape from craters off Clark & Rumble III >200 m
• Colonies of fish were also observed (swimbladders )
Two distinct hydrothermal plumes on top of Rumble
Two distinct hydrothermal
plumes on top of Clark Volcano
• Fish colonies will appear as
smeared globules whereas
plumes are concentrated flame
structures.
Geohab 2016
enhancing the benefits of New Zealand’s natural resources
Conclusion - High swath overlap
1. Backscatter
i.
Importance of seafloor morphology correction for incidence angle studies
ii. Specular reflection disappear- but limited to oversampled areas (multi-look in radar imagery)
iii. No loss of apparent resolution (if anything increase resolution)
iv. Equi-angle backscatter mosaics - The incidence angle behaviour is seen on the whole images instead of
locally as in conventional mosaics  Potential to segment data on incidence angle
2. Water-column backscatter
i.
Time series
ii. Better integration on full water column
iii. Study potential of angular incidence
3. Recommendations for further similar surveys
i.
Acquire star-like or at least cross lines & Opposite headings for consecutive lines
ii.
Use the maximum width aperture
iii. Minimize the number of MBES configurations (modes, …)
Geohab 2016
enhancing the benefits of New Zealand’s natural resources
Special Issue :
Seafloor backscatter from swath echosounders: technology and applications
• Guest Editors Geoffroy Lamarche (NIWA) & Xavier Lurton (Ifremer)
[email protected] – [email protected]
Dead line for submission: 31 August 2016
• Follow on the Backscatter Working Group (BSWG) work and
published Guidelines and Recommendations (May 2015)
• Themes:
 Technological achievements in sonar measurement of
backscatter;
 Acquisition procedures, in particular sonar calibration;
 Data processing and interpretation;
 User requirement and novel applications;
 Applications (habitat mapping, monitoring, engineering)
• Submission of manuscripts
https://www.editorialmanager.com/mari/default.aspx
• Papers published online upon acceptance for publication following
journal procedure
Geohab 2016
enhancing the benefits of New Zealand’s natural resources