The Use of Geophysical Methods to Aid in Horizontal ...
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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
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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
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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
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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
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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?
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Geophysical Methods
Resistivity Imaging
Seismic Refraction
Shear wave testing
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Principles of Resistivity Imaging
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Applied Current Measured Voltage
Bedrock
Soil
A B M N
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Applied Current Measured Voltage
Bedrock
Soil
A B M N
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Principles of Refraction
Geophones
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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.
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Example: Cemetery CrossingAppalachian Plateau Geology
Legend
= existing gas lines
= proposed gas line
= proposed resistivity lines
Entry
Entry
Exit
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Crossing Fracture Zones
Feet
Elevation Contours (feet)
Legend= existing gas lines= proposed gas line
Cemetery
= possible fracture trace
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HDD Profile and Resistivity Lines
Feet
Proposed HDD Profile
Resistivity Electrode Locations
Legend= existing gas lines= proposed gas line
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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
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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
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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)
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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)
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In Conclusion
Geophysics can offer valuable insights on HDD Projects
Providing information on Overburden thickness Stratigraphy Fractures and faults