Geophysical Karst Studies for Civil Projects · Geophysical Surveys (2D Resistivity Profiling and...
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Geophysical Karst Studies for Civil Projects John A. Mundell, P.E., L.P.G. Mundell & Associates, Inc.
Transcript of Geophysical Karst Studies for Civil Projects · Geophysical Surveys (2D Resistivity Profiling and...
Geophysical Karst Studies
for Civil Projects
John A. Mundell, P.E., L.P.G. Mundell & Associates, Inc.
GENERAL BACKGROUND CORE COMPETENCIES ACHIEVEMENTS COMPANY UNIQUENESS PEOPLE CLIENTS
MISSION AND VISION PROJECTS QUESTIONS
Uncertainty and Unknowns
Geophysics - Environmental
Dramatic Karst Development
Dramatic Karst Development
Dramatic Karst Development
Dramatic Karst Development
Dramatic Karst Development
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
Standard Approach: Soil/Bedrock Borings Here are 150 randomly chosen sampling locations (borings), surely we will find and characterize our karst (void) development!
Oops! We missed!
Void Area
We also missed these 11!
Void Areas
And these 9!
Void Areas
And these 13 too!
Void Areas
And these 13 too!
Void Areas
building runway
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
Geophysical Methods
• Electromagnetic Metal Detection • Electromagnetic Conductivity • Two Dimensional Electrical Resistivity • Seismic • Microgravity • Ground Penetrating Radar (GPR) • Downhole Logging
GEO-IMAGING TECHNIQUES
Electromagnetic methods
GEO-IMAGING TECHNIQUES
Radar
Deep metal detection
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.
GEO-IMAGING TECHNIQUES
Electrical Resistivity
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
Resistivity for Bedrock Surface/Karst Mapping
Indiana Geology
SITE
KARST DESKTOP STUDY RESULTS
KARST FEATURE
KARST VALLEY
INDIANA MAP SINKHOLE INVENTORY
2D Resistivity Profile lines
PROPOSED KARST STUDY SCOPE
2D RESISTIVITY PROFILES ALONG RUNWAY AND TAXIWAY
TOP OF BEDROCK TOPOGRAPHY MAP
TOP OF BEDROCK TOPOGRAPHY MAP
CONSTANT ELEVATION SLICE MAPS
ELEVATION 725 ELEVATION 750
ELEVATION 775 ELEVATION 800
CONSTANT ELEVATION SLICE MAP
ELEVATION 775
CONSTANT ELEVATION SLICE MAP
CONSTANT ELEVATION SLICE MAP
ELEVATION 750
CONSTANT ELEVATION SLICE MAP
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
Microgravity
For mapping voids and severe fracture zones within bedrock. Very useful for locating mined areas within
the upper 200 feet of the subsurface.
Microgravity Mapping Locates Missing Mass – Such as From Mined Coal Seam
Or From a Fractured/Weathered Zone in the Bedrock
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
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
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
Thanks For Your Time!
Questions?
