geophysical karst studies for civil projects · geophysical surveys (2d resistivity profiling and...
TRANSCRIPT
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Geophysical Karst Studies
for Civil Projects
John A. Mundell, P.E., L.P.G. Mundell & Associates, Inc.
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GENERAL BACKGROUND CORE COMPETENCIES ACHIEVEMENTS COMPANY UNIQUENESS PEOPLE CLIENTS
MISSION AND VISION PROJECTS QUESTIONS
Uncertainty and Unknowns
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Geophysics - Environmental
Dramatic Karst Development
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Dramatic Karst Development
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Dramatic Karst Development
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Dramatic Karst Development
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Dramatic Karst Development
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Objective - find one 25 ft x 45 ft VOID within this 400 ft by 300 ft grid
Theoretical Project Site – 400 ft by 300 ft Area History indicates possible cave systems
and subsurface karst
0 100 300 200 400
200
300
100
0
Void Size
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Standard Approach: Soil/Bedrock Borings Here are 150 randomly chosen sampling locations (borings), surely we will find and characterize our karst (void) development!
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Oops! We missed!
Void Area
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We also missed these 11!
Void Areas
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And these 9!
Void Areas
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And these 13 too!
Void Areas
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And these 13 too!
Void Areas
building runway
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It would have taken a 20 ft by 20 ft geophysical survey grid to find the VOID AREA.
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Surgeon’s General Methods - pre-1977
What Is Applied Geophysics?
• The study and use of the naturally-occurring and induced properties of the earth for the purpose of characterizing objects of interest in the subsurface.
• Advances in technology and resolution are analogous to the medical imaging field
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TUMOR
Magnetic Resonance Imaging (MRI) - use of strong magnetic fields and radio frequency - 1977 first study of a human - observe continuous ‘structure’ of the body
Cross-Sections of a Head
Surgeon’s General Methods - post-1977
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Geophysical Methods
• Electromagnetic Metal Detection • Electromagnetic Conductivity • Two Dimensional Electrical Resistivity • Seismic • Microgravity • Ground Penetrating Radar (GPR) • Downhole Logging
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GEO-IMAGING TECHNIQUES
Electromagnetic methods
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GEO-IMAGING TECHNIQUES
Radar
Deep metal detection
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Resistivity Profiling
Highly detail method for characterizing subtle
variations in both shallow and deeper subsurface
resistivity
Often used in conjunction with more rapid
mapping techniques such as conductivity
Has been shown to be effective mapping tool for
karst features.
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GEO-IMAGING TECHNIQUES
Electrical Resistivity
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2D Electrical Resistivity Imaging System
Here’s an example of the subsurface coverage from a dipole-dipole array with one specific data point location shown:
Current (I) Flows between B & A while Voltage (V) is Measured between M & N
Apparent resistivity is calculated using ohm’s law (R=V/I) and the electrode geometries
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Resistivity for Bedrock Surface/Karst Mapping
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Indiana Geology
SITE
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KARST DESKTOP STUDY RESULTS
KARST FEATURE
KARST VALLEY
INDIANA MAP SINKHOLE INVENTORY
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2D Resistivity Profile lines
PROPOSED KARST STUDY SCOPE
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2D RESISTIVITY PROFILES ALONG RUNWAY AND TAXIWAY
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TOP OF BEDROCK TOPOGRAPHY MAP
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TOP OF BEDROCK TOPOGRAPHY MAP
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CONSTANT ELEVATION SLICE MAPS
ELEVATION 725 ELEVATION 750
ELEVATION 775 ELEVATION 800
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CONSTANT ELEVATION SLICE MAP
ELEVATION 775
CONSTANT ELEVATION SLICE MAP
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CONSTANT ELEVATION SLICE MAP
ELEVATION 750
CONSTANT ELEVATION SLICE MAP
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PROPOSED SCOPE – Phase II
Numerous geophysical techniques available to help on any civil project
Geophysical surveys yield increased confidence in finding unknown subsurface targets
Complimentary techniques allow for better overall characterization
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Microgravity
For mapping voids and severe fracture zones within bedrock. Very useful for locating mined areas within
the upper 200 feet of the subsurface.
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Microgravity Mapping Locates Missing Mass – Such as From Mined Coal Seam
Or From a Fractured/Weathered Zone in the Bedrock
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Subsurface Condition Mapping/Utilities/Pipelines
Buried Structure Locating
Structural Integrity Evaluation/Testing
Fill Material/Bedrock Characterization
Karst Determination/Voids Detection
Excavation or Boring Clearances
What Projects? Geophysics - Engineering
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Groundwater Investigations/Remediation Aquifer Mapping
Fracture/fault/karst Delineation
In-situ Aquifer Characterization
Landfill Projects Waste Limits and Characterization
Geologic Conditions
Leachate Migration
What Projects? Geophysics - Environmental
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Closing Remarks
Geophysical Surveys (2D Resistivity Profiling and Microgravity) are effective tools in the delineation of karst areas and can be helpful on manhy civil projects.
Geophysical surveys used in combination with standard drilling techniques yield increased confidence in finding unknown subsurface targets
Complimentary techniques can allow for better overall characterization
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Thanks For Your Time!
Questions?