an overview of geophysics use in transportation projects ...€¦ · an overview of geophysics use...

An Overview of Geophysics Use in Transportation Projects

and Choices for a Karst or Mine Investigations

By: Rick HooverOf

www.quality-geophysics.com

By: Rick Hoover of Science Applications International Coporation

Use of Geophysics forTransportation Projects

• National Cooperative Highway Research Program (NCHRP) Synthesis 357

• Prepared by Phil Sirles of Lakewood CO. and NCHRP Committee for Project 20-5

• Conducted 2004-2006 period• Literature search, Survey from all 50 State

DOT’s, District of Columbia, Canadian Provinces, and select Federal Agencies.


Significant Findings50

6 62 2 1

0

10

20

30

40

50

60

Res

pond

ents

DOTs YesDOTs NoCanadians YesCanadians NoFederal Agencies YesFederal Agencies No

N=67

Agency Response to the Use of Geophysics

3 3

68

3

23

0

10

20

30

40

50

60

70

80

90

100

Perc

ent

PrimaryMajorMinorOccasionalNo Response

N=59

DOT Involvement with geophysical investigations


Good News…

138 5 8

59

38

30

102030405060708090

100

Perc

ent

YesNoNo Response<25%25 to 50%50 to 75%>75%

N=58

Res

pond

ents

N=34

Has there been an increase in last 5 years? If so, what percentage of increase in level of effort for geophysical projects over the past five years?

•Approximately 60% of the agencies indicate an increase efforts to implement geophysics, with approximately 25% indicating an increase of between 50% and 100%


5%9%

10%

22%

6%

22%

26% Seismic

GPRVibration Monitoring

Resistivity

NDT

Borehole Logging Others

N=130

91% Stated they understood the difference between Geophysics and NDT

Most Commonly Used Methods


1%10%11%

22%

24%

32%

Others Bedrock Mapping

NDTSubsidence

Investigations

MappingSoil

MappingMan-Made Features

N=113

Common Applications

•Is NDT an application or method?


Greatest Value Geophysics adds to Transportation projects

21%

19%

17%

15%

10%

10%7% 1% Data Acquisition

Speed

Cost Benefit

Better Subsurface Characterization

2D, 3D Subsurface Assessments

Other

Results Presentation

No ResponseOther

N=1621. Speed

2. Cost Benefit

3. Better Characterization


14 134

2328

7

3327

91

0102030405060

Res

pond

ents

Cost Acquisition IssuesTimeliness of Results More QuestionsNon-Uniqueness Results FormatLack of Understanding Lack of ConfidenceOther No Response

N=159

93% DO NOT offer ANYANY training or instruction for geophysics!

Greatest Deterrent to using Geophysics


47

2

47

2

38

9

34

8

32

7

29

12

4

0

10

20

30

40

50

60

Res

pond

ents

YesNoNo Response

N=271

Training Knowledge

Experience Standards EasySoftware

Easy Equipment

Database of Qualified Providers

Necessary to Increase Comfort in Using Geophysics


The Setting

Professional Thinking about a Karst or Mine Problem


The Problem-Spatial Sampling Estimate

The Site

The Target Area Site (As) to Target (At) Ratio= As/At

Number of Borings

Detection Probability

Boring Grid

160 100% 15’ 100 90% 20’ 80 75% 25’ 50 50% 30’

1 Acre Site (As) approx 100’ x 400’Sinkhole or Mine (At) 20’ x 20’As/At=40,000/400=100

Number of Borings to Detect

50 borings, 50% probability of finding!


Karst or Mine Issues

Suspect that Geophysics can help But…

Soil Borings may not answer all of the questions

What method?

What parameters?


The Traditional Geophysical Answer- A Paradox

• Lets ask for bids and see what we get…

WHERE DO I TURN FOR HELP ????

• Highly variable results• Awareness• Standards

• Experience – Be the old guy.

• Experience – Ask the old guy.


What are the Geophysical Choices

• What do I ask of a Geophysics?

• How do I select a method?

• What do I look for?• How do I set

parameters?A number of Solutions have arrived…..


Key Highway Reference

http://www.cflhd.gov/agm/index.htm

Chapters Include:

•Bridge Substructure*

•Bridge Superstructure

•Pavements*

•Roadway Subsidence*

•Subsurface Characterization*

•Vibration Measurements*Geotechnical applications


ASTM(D-6429-99 Selecting Surface Geophysical Methods)

21 Applications• Natural Geologic

Conditions• Natural Hydrogeologic

Conditions• Inorganic

Contaminants• Organic Contaminants• Manmade Objects

13 Different Methods• Seismic• Electrical• Electromagnetic• Pipe/Cable Locators• Radar• Magnetics• Gravity


Consensus Standard for Investigating Voids/Sinkholes

(D-6429-99 Standard Guide to Selecting Surface Geophysical Methods)

PrimaryGravity

PrimaryRadar

PrimaryFrequency Domain EM

Secondary (Primary)Electrical D.C.

SecondarySeismic Refraction

ChoiceMethod


Thinking AboutGeophysical Applications

1-Physical measurements applied to geologic problems

2-Physical measurements depend onphysical contrasts.

3-Planning Must include:•Target size, •Size and issues with investigation area and •Method resolution


Seismic MethodsMeasures Mechanical Energy Velocity (feet per second)

18,00012,000Limestone/Dolomite Bedrock

4,500500Soil

Typical Maximum

Typical Minimum

Material


Beneficial Settings for Seismic Application

Near power linesSaturated Clay Soils

Irregular Ground Surface


Limiting Settings for Seismic Refraction

Operation of Heavy Mechanical Equipment,Significant Frozen Ground


Seismic Equipment-and Lay-out

12-24 Geophones•Spacing determines resolution

Seismic Source• Number of source locations determines level of effort and resolution

Seismograph


Data Collection

•Depth of Investigation is ~1/3rd distance to far offset•2-person Crew can collect four to six spreads of data per day.

•Spread Length Determines Depth of Investigation


Data Presentation•Cross Section View

• Top of Bedrock identified

• Fissures/Fractures

difficult to ID

• Voids within bedrock cannot be seen with refraction


Electrical MethodsMeasures Resistance to Electricity (ohm meters)

4,000200Limestone/Dolomite Bedrock

InfiniteInfiniteVoid8,000100Mud-Filled Fracture

8,0005Soil

Typical Maximum

Typical Minimum

Material

•Changes or anomalous values are the features of interest


Common Factors That Influence Resistivity

• Coarser Structure and/orSmaller Porosity

• Increasing Clay Fraction

• Increasing Soil Moisture Content

• Increasing Soil/Groundwater Conductivity

Decreasing Resistivity


Beneficial Settings for Electrical Methods

Seismically noisy areas,Clay Soils,

Irregular Ground Surface,


Limiting Settings for Electrical Methods

Very Dry Soils or Soils with High Resistivity,

Utilities – Underground metallic pipelines,

– Electric lines


Imaging Equipment• 28 or 56 Electrodes

Common– Electrode Spacing

and array type determines resolution


Electrical Data Collection• Depth of Investigation

is ~1/4th greatest electrode spacing,

•2 person crew can collect 3-5 spreads per day.


Electrical Data PresentationShallow Bedrock

Mud FilledVoid

Mud FilledFractureTop of Bedrock•Cross Section

View•Identification of Fissures and Fractures

•Depths not as accurate as seismic

•Can identify voids within bedrock


Electrical Data Presentation

Void Top of RockPotential Collapse

FeaturePotential Collapse

Feature

Dep

th (m

eter

s)

FracturedBedrock Void

Resistivity (ohm-meters)

•Good “In-Rock” Information


EM MethodsMeasures Electrical Conductivity (milliSiemen/meter)

152Limestone/Dolomite Bedrock

00Void

10015Soil

Typical Maximum

Typical Minimum

Material


Beneficial Settings for EM Surveys

Large Sites Requiring Reconnaissance Survey

Cost-Conscious Projects


Limiting Settings for EM

Small Project Sites,Buildings/Cars/Power-lines


EM Data Collection

• 18-foot depth of investigation,

• No in-ground sensors,

• Two to five+ miles surveyed per day

• Traverse spacing determines resolution


EM Data Example

•Relative Interpretation

ThickSoils

ThickSoils

ThinSoils

ThinSoils

•Can Calibrate with Soil Borings, Geoprobe or CPT

•Presentation is map view


EM Data ExampleCan provide cost effective “reconnaissance”assessment before development

EM is not quantitative


Development Example

• Engineer has nicely layed out the property

• Layed out boring program

• Boring Results did not make sense…..

•Building

•Parking


GeophysicalInput

• Two Borings were “too deep”

• One day EM survey

• Linear features apparent

• Poor fitting borings in “thick soil” areas

• All “good” borings were in relatively consistent soil thickness areas


Correlation EM to SB’s


GPR MethodsMeasures travel time nS/ft.

•Velocity is controlled by a complex relationship of dielectric permittivity and magnetic permeability

•In real applications for karst and mine features, velocities et. al. don’t matter…..

•The geometry of subsurface features that reflect the energy pulse matter


Beneficial Settings for GPR Application

Detailed (Small/Shallow Feature) SurveysEASY Access Areas

– Residential Settings– Parking Lots

– Mowed grass (not plowed field)


Limiting Settings for GPR

Moist Clay SoilsDeep Soil/Rock interface


GPR Equipment

• Depth of Investigation is 0 to 20 feet.

• No in-ground sensors.

• One Person Crew• Surveys of 1-3

miles/day not Uncommon

• Measurements per foot and spacing between traverses determine resolution


GPR Data Presentation

Void

•Presentation as Cross-section

•Transit Time Vertical Scale


GPR Data Presentation• GPR has Excellent Resolution !

When it works…..


GPR Suitability Index for Soils


Gravity MethodsMeasures Density grams/cubic centimeter

2.72.5Limestone/Dolomite Bedrock

00Void2.01.6Mud-Filled Fracture

2.01.6Soil

Typical Maximum

Typical Minimum

Material


Beneficial Settings for Gravity Application

Flat or Dipping TopographyIn Buildings or Over Concrete

Large Sites Requiring Reconnaissance Survey


Limiting Settings for Gravity

Difficult to Identify Subtle FeaturesIrregular Topography Presents Challenges


Gravity Data CollectionVery Deep Investigation

30 to 75 Measurements per Day With One Man Crew (Elevations take two people…)

Station Spacing Determines resolution


Gravity Presentation• Map

Presentation Common

• Can depict mass excesses,

• or mass deficiencies.

Mass Deficiency

Mass Excess


Five are Consensus Methods for Karst and Mine Void Investigations

Conclusions•Numerous Geophysical Methods are Available

•Each Geophysical Method has Advantages and Disadvantages•Resolution•Productivity•Site Setting

Seismic,EI, EM, GPR and Gravity

•Selection of the “right” method depends on the site, the desired level of detail and the budget.


The Problem-Spatial Sampling Estimate

The Site

The Target

Area Site (As) to Target (At) Ratio= As/AtDetection

Probability As/At=

10 As/At=

100 100 16 160 90 10 100 75 8 80 50 5 50

1 Acre Site (As) approx 100’ x 400’Sinkhole or Mine (At) 20’ x 20’As/At=40,000/400=100

Boring Grid Size

15’ Grid20’ Grid25’ Grid30’ Grid

GEOPHYSICS CAN HELP !


Highway Considerations• Re-enforced Concrete precludes EM on the roadways. • Classical seismic and resistivity require traffic control

to place sensors and make measurements. • Gravity requires stationary measurements. • Traffic Control will be major consideration for traffic

impact and safety.• “Rolling surveys” better than “static surveys”• Remaining options - GPR, Rolling Resistivity,

Rolling Seismic.


GPR Options

• GPR surveys can be effective on Highways.

• High speed radar; 5-10 miles per hour.


GPR Highway Example

•Planar View

•Or Cut-away cube view

SR33 NB Lane, North Side, 0-250 ft


High Amplitude Anomalies in Cube


Resistivity Options Capacitively Coupled Resistivity

Traditional resistivity uses probes hammered into the ground

CCR uses antenna dragged along the ground

Data Collection speed 2-3 miles per hour


Multi-receiver OhmMapper


OhmMapper Data Processed with Same Software as other Resistivity Methods

Data taken by Jay Hanson using OhmMapper. Inversion by RES2DINV.


Seismic Options

• Land-streamers eliminate the need to spend time planting geophones.


Still need to pause for

SEISMIC SOURCE OPTION• Elastic Wave

Generator has fast cycle time.

• Powerful Source (more energy than sledge hammer and shotgun shell)


Still need to pause for

SEISMIC SOURCE OPTION

Mini-vibrator has the ability to produce shear waves


Highway Seismic Example


Seismic Example


Presented by:Rick Hoover, P.G. has spent over 28 years practicing geophysics, and is currently a Senior Geophysicist with Science Applications International Corporation in Harrisburg, PA, with responsibilities of major project management, and geophysical mentoring. Mr. Hoovers’ geophysical experience ranges from environmental and engineering applications, exploration for oil and gas production, to research and development for a major oil company. Technical responsibilities have included all aspects of the geophysical industry, ranging from data acquisition, processing, data interpretation, and reporting. Surface Geophysical experience includes seismic reflection and refraction, resistivity, electrical imaging (EI) electromagnetic (including EM-31, EM-34, EM-61 and VLF), magnetometer and gradiometer, gravity, ground penetrating radar (GPR) and a variety of utility locating tools. Borehole geophysical experience includes the use of resistance, resistivity, SP, gamma, neutron, caliper, temperature, sonic, density, dipmeter, and televiewer data and equipment.

Mr. Hoover is a Pennsylvania Licensed Professional Geologist, a member of American Geophysical Union (AGU), American Institute of Professional Geologists (AIPG), American Society of Civil Engineers (ASCE), European Association of Exploration Geophysicists (EAEG), Environmental and Engineering Geophysical Society (EEGS) and the Society of Exploration Geophysicists (SEG). Mr. Hoover serves on the Transportation Research Board (TRB) geophysics committee, and the GeoInstitute geophysics committee.

6310 Allentown BoulevardHarrisburg, PA 17112

Phone-(717) 901-8835 Fax-(717) 901-8103www.quality-geophysics.com

an overview of geophysics use in transportation projects ...€¦ · an overview of geophysics use...

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