Geophysical Applications to Subsurface Investigations ......$1 spent up-front on geophysics saved...

1

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

Collecting 2-D Resistivity Data

4

4Multi-conductor resistivity cable

Electrode

Resistivity Meter

Data Acquisition

0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340

-5

0

5

10

15

20

25

30

35

40

45

50

AppliedCurrent

MeasuredVoltage

Bedrock

Soil

Resistivity Model

25

50

75

100

125

150

175

200

250

275

300

350

400

450

500

750

1000

Resistivity Scale(ohm-meters)

Distance (ft)

Ele

vatio

n (ft

)

Bedrock Surfaces

Resistivity Scale (ohm-meters)

0

25

50

75

100

150

200

250

300

350

400

450

500

1000

10000

100000

400000

0 100 ft

Mapping Bedrock

1-11-5

1-101-15

1-201-25

2-12-5

2-102-15

2-202-25

3-13-5

3-103-15

3-203-25

4-14-5

4-104-15

4-204-25

5-15-5

5-105-15

5-205-25

6-16-5

6-106-15

6-206-25

6-306-35

6-40

7-17-5

7-107-15

7-20

7-25

7-307-35

7-40

8-18-5

8-108-15

8-20

8-25

8-30

8-35

8-40

9-19-5

9-10

9-159-20

9-25

9-30

9-35

9-40

10-110-5

10-10

10-15

10-20

10-25

10-30

10-35

10-40

11-111-5

11-10

11-1511-20

11-2511-30

11-35

11-40

12-112-5

12-1012-15

12-2012-25

12-3012-35

12-40

13-113-5

13-10

13-1513-20

13-2513-30

13-3513-40

14-114-5

14-10

14-15

14-2014-25

14-30

14-35

14-40

15-115-5

15-1015-15

15-20

15-25

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

13

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.

• Applications:• Landfills and trenches• Buried drums, tanks, utilities• Buried structures

Structures and Utilities

-22-20-18-16-14-12-10-8-6-4-20246810121416182022

In-Phase(ppt)

Ground Penetrating Radar

Applications:• Location of Buried

drums or tanks• Location of Buried

Utilities• Archeological-forensic

studies• Reinforcement Steel• Subslab Voids

2.5-3 Feet Depth Slice

2.5-3 Feet Depth Slice with EM Overlay

Concrete Reinforcement and Utilities

0.25-0.5 Feet

1.0-1.25 Feet

GRID 1X

Y

Proposed drill hole

Sub-slab Voids

1.25 – 1.5 feet

Y-59

Bottom of BlocksVoid Joints

Thank You!

• Questions?