The Role of Geophysics in 3D Mapping

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The Role of Geophysics in 3D Mapping
Workshop on Three-Dimensional Geological Mapping
for Groundwater Applications, Geological Society
of America, Salt Lake City, October 15, 2005

• Susan Pullan
• Geological Survey of Canada, Ottawa, Canada

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3D Mapping

• Need for three-dimensional (3D) geological
  information on shallow deposits to
• assess or protect critical water resources
• address other environmental issues (e.g. waste
  disposal)
• understand geological hazards

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Projection of surface mapping
??
Boreholes
Geophysics
Aim of this talk is to discuss the why, what,
where, when, how of geophysics to better
understand the role it can play in 3D mapping
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Why use geophysics?

• There is a need to both identify and map
  subsurface sediments
• Geophysics can provide some of the information
  required to map
• e.g., information on subsurface architecture,
  
• stratigraphic relationships,
• variation in materials

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Example Waterloo Moraine, southern Ontario
Borehole indicated thick fine-grained unit (15-55
m depth)
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Waterloo Moraine Interpreted seismic section
Buried bedrock valley - 30 m deep, 400 m
wide Borehole samples little of subsurface
stratigraphy
12-fold CMP section - horizontal trace spacing
1.25 m
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Musquodoboit, N.S. - Processed CMP section
(12-fold)
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What geophysics?

• Different geophysical surveys measure different
  physical properties of the subsurface

• Geophysics requires subsurface targets to
  contrast significantly with their surroundings
• choose geophysical method based on an
  understanding or estimate
• of the physical properties of the
  target in relation to its environment

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• Electromagnetic methods
• rapid means of identifying areas of high
• salinities (low resisitivities - blue)
• and shallow bedrock
• (high resistivities - red)
• Ground Probing Radar (GPR)
• (mapping depth to water table in sands)

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What can geophysics NOT do?

• All geophysical methods have an inherent
  limitation on resolution
• depends on method, instrumentation and
  survey parameters,
• as well as site conditions (geological
  and cultural setting)

• No single survey can map all depths and/or
  targets

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Vertical Resolution depends on wavelength of the
recorded signal (wavelength
velocity/frequency)
Resolution 10-15 m 5-7 m
2-3 m
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Where should geophysics be used?

• Where subsurface architecture/stratigraphy is
  complex
• Where the target produces a clear measureable
  signal
• Where data with a high signal-to-noise ratio can
  be obtained

• Test first!

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Test sites can be used to determine vertical and
lateral variations In a target horizon over a
regional area
Optimum surface conditions fine-grained,
water-saturated (clay belts, Champlain Sea)
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Lefaivre study - investigation of the seismic
properties of overburden associated with
earthquake-induced disturbed terrain areas.
Treadwell Area
Seismic surveys have mapped the existence of a
small deep bedrock basin underlying the disturbed
area.
Lefaivre Area
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When should geophysics be used?

• When it has been ascertained that the target
  horizons and/or
• structure are characterized by a significant
  contrast in physical
• properties that is measurable at the surface
  using the
• geophysical method chosen
• Usually most effective early in the investigation
  

25 MHz Ground Probing Radar (mapping thickness
of surface sands)
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How should geophysics be used?

• In conjunction with boreholes or other
  information that can be used to verify the
  results

• With strong collaboration between geologist,
  hydrogeologist, and geophysicist from planning
  through interpretation

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• For geophysics to play an effective role in 3D
  mapping,
• three basic conditions must be
  met
• The measured physical property of the target
  horizon(s) or units
• must contrast significantly with the
  surrounding material

Electrical resistivity provides detailed images
of subsurface conductivity structure.
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• Data from the target depth must be sufficient in
  amplitude and
• signal-to-noise ratio to be interpretable.

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• The resolution of the methodology used and of the
  recorded signal
• must allow definition of the target.

Fraser River Delta, B.C. - Topset and Foreset
Beds
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Recommendations

• Involve the geophysicist(s), geologist(s) and
  hydrogeologist(s) in identifying the
  targets of the geophysical surveys and planning
  the geophysical program.
• Be sure all participants understand the
  potential and limitations of the geophysical
  survey(s) chosen.

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2. Conduct tests to ensure that the planned
geophysical survey in capable of
mapping/identifying the target horizon/structure.
Use these tests to identify the optimum
areas/times/parameters for surveying so that data
with as high a resolution as possible can be
obtained.
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3. Plan to acquire borehole or other
information that can be used to verify the
geophysical results.
Distance from start of line (km)
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4. Involve the geophysict(s), geologist(s) and
hydrogeologist(s) in the final assessment
and interpretation of the results.
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Last word be prepared for geophysics to raise
new questions/insights that will challenge or
require refinements to any preconceived 3D model!