Download Mapping the subsurface

Mapping the subsurface
Mark Rattenbury
Programme Leader, Geological Map of New Zealand
GNS Science, Lower Hutt
Why do we need to know about the subsurface?
We build on it and into it.
We grow food and timber in it.
We extract resources from it
Dangers lurk within it and are
recorded by it
Photos by D.B. Townsend (Townsend & others 2008)
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Surface outcrop
Extrapolation of trends
Application of geological rules
Surface
landform
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QMAP: Geological Map
of New Zealand
• Designed and built around GIS
software (ESRI ArcGIS) since
1994
• Pragmatic design based around
map production
• High-quality printed maps using
ArcGIS
• National consistency at
1:250,000
• Rich data attributes
• 21 separate GIS datasets
• Building towards seamless GIS
coverage
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The traditional geological map informs about the near-surface
Edbrooke 2001
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Edbrooke 2001, 2006
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Edbrooke 2006
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Geological maps re-evaluated and reclassified
(a knowledge-based approach using GIS)
= Alluvial fan hazard analysis
Grindley et al. 2009, Barrell et al. 2009
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Geological maps reclassified using material property estimates
(a knowledge-based approach using GIS)
= Regional Earthquake Susceptibility
Begg & Mazengarb 1996 @ 1:50k
Earthquake shaking potential
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Geological maps for gauging
liquefaction susceptibility
Liquefaction potential
87% of September 2010 earthquake
liquefaction occurrences were in modelled
VERY HIGH to HIGH susceptibility zones
Low
Medium
High
Very high
Pre Canterbury earthquake liquefaction susceptibility modelling based on interpretation of geological map unit description
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But geology is 3D
and paper maps don’t do 3D well
Kermode 1992 Auckland 1:50k
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Cox & Barrell (2007)
Rattenbury & others (1998)
Swapping 2D axes
2½D drapes on DTMs
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3D volumes in GIS
0
conventional map view
-5
-10
multiple Z
values for
single X,Y
-15
-20
-25
-30
-35
consider a buried sphere
(with its top eroded away)
-40
Contoured depth to
lower surface
-20 -25
-15
-15
Contoured depth to
upper surface
Contoured thickness
(isopach)
-10
5
-5
0
10 15 20
25
30
-30
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A new era for geological maps
Blend conventional lithostratigraphic
mapping philosophy and geological
principles with new datasets and tools:
• 3D geological modelling software
visualisation
• Drill hole data mining
• LiDAR surface analysis
• Ambient noise and passive seismic
analysis
• Cone penetrometer test analysis
Modelled greywacke basement underlying the
Taupo Volcanic Zone (Alvarez & others)
Modelled geological units in
Wellington CBD using
GSI3D software
• Ground-penetrating radar imaging
• Aeromagnetic data
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3D geological modelling software
Hutt Valley “It’s our Fault” project using
GSI3D modelling software, Boon et al. 2010
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Drill hole data (we need them!)
Leapfrog 3D modelling software
GSI3D modelling software
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Lidar topography for
geomorphological mapping
Relict geomorphology visible
through human modification
What does it tell us about
Christchurch geology?
Surface extent of the
Springston Formation
Surface extent of the
Christchurch Formation
After Begg & Jones in prep.
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Cone Penetration Testing (CPT) data
Measuring geotechnical property proxies
Photo: http://www.rdcl.co.nz
“Ic” – the soil behavior type index
N60 – relative density
with water =
liquefaction
susceptibility
Christchurch urban geological mapping
after Begg & Jones in prep. using
Leapfrog 3D modelling software
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Sinosteel Australia Pty Ltd
aeromagnetic data (courtesy of Dr J.
Meyers) and QMAP Waikato geology
after Edbrooke (2005)
lava flows are well
delineated on the
flanks of Pirongia
volcano
outcropping Murihiku
Supergroup strata have
well defined magnetic
character…
…that is still visible
under Oligocene Te
Kuiti Group sedimentary rock cover up to
300 m thick
The Junction Magnetic
Anomaly is the
magnetic expression of
the buried Dun
Mountain Ultramafic
Group peridotite rocks
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Extract from Forsyth, Barrell & Jongens 2008
New geological maps of Christchurch
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Geology after Forsyth, Barrell & Jongens 2008
ECAN Drillhole Database
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Christchurch urban
geological mapping
after Begg & Jones in
prep. using Leapfrog
3D modelling software
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Christchurch urban
geological mapping
after Begg & Jones in
prep. using Leapfrog
3D modelling software
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3D geological modelling
While 3D geological modelling software is improving
rapidly, there are serious technical challenges around:
Hardware demands
Software complexity
Lack of standards and interoperability
Handling of geological faults (discontinuities)
Quantifying uncertainty of the interpretation
Excellent for conveying a geologist’s understanding of 3D
subsurface geometries
New era in mapping out geotechnical ground classification
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3D, GIS and local government
GIS data can be imported and input into models
Drillhole and other geotechnical data (commonly held by
councils) provide vital constraints on subsurface
interpretation
3D models can be imported via into GIS component
surfaces or 2D derivatives e.g. depth slices, depth to
surface top/base and isopachs
2D-based GIS will continue to be a vital tool at GNS
Science for managing, analysing and presenting
geological data
www.gns.cri.nz
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