the use of geophysical methods to aid in horizontal

The Use of Geophysical Methods to Aid in Horizontal Directional Drilling Projects

Warren T. “Ted” Dean, P.GChristopher M. Printz, P.G.

Johanna M. VaughanATS International, Inc.Christiansburg, Virginia

www.ats-intl.com

What is Horizontal Direction Drilling (HDD?)

HDD is a trenchless technology whereby a steerable drill bit advances a boring along a shallow arc in the subsurface

When to Use HDD? Anytime Trenching is Undesirable or Impractical

Crossing Sensitive Areas Such as: Roads Bodies of Water Conservation Areas Buildings Cemeteries or Archaeological Areas

Why Use Geophysical Methods?

The same conditions that make trenching impractical usually make vertical borings impractical

Without vertical borings, geophysics allows the inference of geologic conditions that the HDD may encounter

Common Questions to Answer with Geophysics

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 or faults?

Geophysical Methods

Resistivity Imaging

Seismic Refraction

Shear wave testing

Principles of Resistivity Imaging

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

Applied Current Measured Voltage

Bedrock

Soil

A B M N

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

Applied Current Measured Voltage

Bedrock

Soil

A B M N

Principles of Refraction

Geophones

Advantages and Disadvantages of Each Method

Both methods produce a cross sectional image. Resistivity tends to have a deeper reach. Resistivity tends to resolve vertical boundaries better. Resistivity is not sensitive to velocity inversions. Seismic refraction is more directly related to

rippability. Seismic refraction is less sensitive to interference

from buried metal.

Example: Cemetery CrossingAppalachian Plateau Geology

Legend

= existing gas lines

= proposed gas line

= proposed resistivity lines

Entry

Entry

Exit

Crossing Fracture Zones

Feet

Elevation Contours (feet)

Legend= existing gas lines= proposed gas line

Cemetery

= possible fracture trace

HDD Profile and Resistivity Lines

Feet

Proposed HDD Profile

Resistivity Electrode Locations

Legend= existing gas lines= proposed gas line

Line 2 Results

West East

sandstone

sandstone

Shale, siltstone, coalsandstone

Possible bedrock fracture zones

B-1

B-2

sandstonesandstone

sandstone

010203040507510012515020025030035040050060080010002500500010000

Resistivity Scale (ohm-meters)

Legend

= proposed HDD

= estimated lithological boundary

Cemetery

Example: Road CrossingBlue Ridge Geology

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. (cs) 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

HDD Entry/Exit Point

Geologic MapProposed HDD

Entry Point

Proposed HDD Exit Point

Entry Point

BR-B3

Seismic Refraction Results and Interpretations Seismic Refraction Scale and Interpretations

feet

West

BR-B3

Resistivity Results and Interpretations

Possible limits of mapped alluvium deposits

East

Resistivity Scale (ohm-meters)

Exit Point

feet

West East

Interpreted Resistivity Boundary of Mapped Fault

Pedlar FormationCatoctin Formation

Likely Fracture Zones

Resistivity Results and InterpretationsLines 2 and 3

Possible alluvium deposits

Possible limits of mapped alluvium deposits

Seismic Refraction Results and InterpretationsLines 2 and 3

Possible limits of mapped alluvium deposits

Interpreted Seismic Refraction Boundary of Mapped Fault

Pedlar FormationCatoctin Formation

Possible alluvium deposits

Seismic Refraction Scale and Interpretations

Resistivity Scale (ohm-meters)

In Conclusion

Geophysics can offer valuable insights on HDD Projects

Providing information on Overburden thickness Stratigraphy Fractures and faults