1 Geophysical Applications to Subsurface Investigations: Reducing Uncertainty and Minimizing Risk 17 th Annual Technical Forum for Geohazards Impacting Transportation in the Appalachian Region August 15 th - 17 th , 2017 Blacksburg, Virginia Presented By Warren T. “Ted” Dean, P.G. Draper Aden Associates
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Geophysical Applications to Subsurface Investigations ......$1 spent up-front on geophysics saved (on average) $7 in total project cost . ... connecting the sinkhole to the underlying
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Geophysical Applications to Subsurface Investigations:Reducing Uncertainty and Minimizing Risk
17th Annual Technical Forum for Geohazards Impacting Transportation in the Appalachian Region
August 15th-17th, 2017Blacksburg, Virginia
Presented By
Warren T. “Ted” Dean, P.G.Draper Aden Associates
Why Perform Geophysical Testing?
• Sometimes provides information that you cannot get through invasive sampling.
• Fills in the blanks: Invasive sampling gives detailed information at one location; geophysics provides less detailed information over a broader area.
• Value: Reduces uncertainty and allows a targeted approach to invasive sampling.
Sinha, Thomas, Wang & Jung (2007) Penn State Study--Meta-analysis of cost-benefit studies of PADOT and VADOT projects
$1 spent up-front on geophysics saved (on average) $7 in total project cost
Resistivity
• The property of a material to inhibit or resist the flow of electric current
• Resistivity versus Resistance
• Primary Factors that Affect Resistivity– Moisture Content– Material Grain Size
Digitize the Bedrock SurfaceTo Obtain an X, Y, Z Data Set
0 200 400Feet
0 200 400
Feet
Contour Map of Bedrock Surface
0 200 400
Feet
650
655
660
665
670
675
680
685
Elevation Scale (feet)
When Trenching is Undesirable or Impractical
Crossing Sensitive Areas Such as:
Roads
Bodies of Water
Conservation Areas
Buildings
Cemeteries or Archaeological Areas
Horizontal Directional Drilling (HDD)Projects
How thick is the overburden?
May effect casing estimates
If the drilling is to take place in the soil we may want to avoid the rock
What is the stratigraphy?
How competent is the rock?
Are there major fracture zones?
Common Questions to Answer on HDD Projects
Mountain Crossing
al
ЄpЄc
pЄp
Gravel, sand, silt, and clay with boulder and cobble deposits in steeply dipping stream beds
Dark green, massive metamorphosed basalt containing local metavolcanic breccia. Gray-purple fine grained siltstone/phyllite interbedded with metabasalt near the top
Dark greenish gray, massive to foliated, garnet bearing, fine to medium grained granodiorite
Geologic MapProposed HDD
Entry Point
Proposed HDD Exit Point
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Exit
Entry Point
Proposed HDD Entry Point
Gravel, sand, silt, and clay with boulder and cobble deposits in steeply dipping stream beds
Dark green, massive metamorphosed basalt containing local metavolcanic breccia. Gray-purple fine grained siltstone/phyllite interbedded with metabasalt near the top
feet
WestResistivity Results and Interpretations
Possible limits of mapped alluvium deposits
East
Resistivity Scale (ohm-meters)
BR-B1
Soil
Rock
Exit Point
ЄpЄc
Gravel, sand, silt, and clay with boulder and cobble deposits in steeply dipping stream beds
Dark green, massive metamorphosed basalt containing local metavolcanic breccia. Gray-purple fine grained siltstone/phyllite interbedded with metabasalt near the top
Dark greenish gray, massive to foliated, garnet bearing, fine to medium grained granodiorite
feet
West East
Resistivity Scale (ohm-meters)
Likely Fracture Zones
Possible limits of mapped alluvium deposits
HDD Path
Pedlar Formation
Catoctin Formation
The Karst Environment
Connecting the Surface to the Subsurface
0 20 40 60 80 100 120 140 160 180 200 220
-40
-20
0
sinkholes
Voids bedrock
soil
Solution channel connecting the sinkhole to the underlying void.
Resistivity (ohm-meters)
Nashville, Tennessee
Detention Basin No.2
0 30 60 90 120feet
Resistivity Lines
0 30 60 90 120feet
Resistivity Results
0 30 60 90 120feet
Boring Explanation:
rubble fill
soil
rockvoid
0 50 100 150 200 250 300400
450
5002-25 2-20 2-15 2-10 2-5 2-1
0 50 100 150 200 250 300 350400
450
5003-25 3-20 3-15 3-10 3-5
3-1
0 50 100 200 300 500 25000 80000
Resistivity Scale (ohm-meters)
Potential air-filled voids
road
roadrecent
subsidence
recent subsidenceAT-2
(~2m south of line)AT-5
(@3-8.5)
AT-3 (~2m south of line)
Line 3
Line 2
Axes are in feet
Blacksburg, VA
8-ft long 2x4
841 Crestwood Drive
sinkhole
subtle depressio
n
Offset Throat
Crestwood Drive
N side of 841sinkhole
S side of 830likely sinkhole
throat
Possible air-filled voids
Line 4
Electromagnetic Induction
• Also Called Terrain Conductivity: measure of how well the subsurface conducts an electric current.