Ground Penetrating Radar

Gonzalo Gallo

CEE 498KM

Outline

• Introduction

• Principle

• Instrumentation

• Data Analysis

• Advantages and Limitations

• Manufacturers and costs

Introduction

• RADAR → Radio Detection and Ranging– Detect target in free space– Determine the range

• Civil engineering applications– Probing into soil to detect pipelines and tanks– Cavities– Thickness determination– Locating reinforcement– Identifying deterioration

Principle

• An EM pulse is sent through an antenna, penetrating into the surveyed material

• A portion of the energy is reflected back to the antenna when an interface between materials of dissimilar dielectric constants is encountered

http://www.cait.rutgers.edu/finalreports/GPR-RU4474.pdf

Principle

• The reflected signal has information on:– how quickly the signal

traveled– how much was attenuated

• These quantities depend on spatial configuration and materials

• The thickness of a layer is given by:

where di is the thickness of layer i, ti the total travel time through that layer, C is the speed of light and εr,i the dielectric constant of the layer

ir

ii

Ctd

,2

Principle

• The amount of reflected energy at an interface is governed by:

where ρ1,2 is the reflection coefficient and εr1 and εr2 are the dielectric constants

Material Dielectric Constant

Air 1

Water 81

Concrete 6-12

HMA 4-7

Ice 1.5

Metals ∞

21

212,1

rr

rr

Typical Dielectric Constants

Instrumentation

• The typical instrumentation for GPR includes the following:– Antenna

• Air-couple• Ground-coupled

– Control Unit– Display device– Storage device

http://www.cait.rutgers.edu/finalreports/GPR-RU4474.pdf

Instrumentation

http://georadar.ids-spa.it/images/new/gpr_products/antennas.gif

Instrumentation

• Another configuration is a portable arrangement– Small, hand-held

dipole antenna

• Used for locating rebar, embedments, voids, and other abnormalities

• 2.4lbs – 10” penetration

http://www.ndtjames.com/catalog/rebarLocators/datascanMKII.html

Data Analysis

• Before using calibrate– Copper plate → Complete reflection– Air → Complete transmission

• Establish dielectric constant of test surface with reflected energy equation

Data Analysis

• Techniques for analysis– Cluster

analysis– Topographic

plotting– Quantitative

peak tracking– Peak plotting

http://www.cait.rutgers.edu/finalreports/GPR-RU4474.pdf

Advantages and Limitations

• Advantages– Fast speed scanning with

non-contact antennas– Very sensitive to presence

of embedded metal objects– Sensitive to the presence

of moisture

In general, fast scanning and saves money

• Limitations– Rebar reflects as arch

patterns – if dense, individual bars are no longer discerned

– Limitations if dielectric properties are similar

– Dielectric properties unknown

– Material “loss”– Difficult in thin layers– Extensive data– Operator dependant

Manufacturers and costs

• Hand-held– Datascan MKII

• James Instruments ND• $12.000

– IRIS systems• Penetradar

– GSSI handy-scan• $6.500 b&w• $10.000 color

http://ndtdirect.com/shop/products.asp?viewall=1&stritem=CN-JA-RB1,CN-JA-RB2,CN-JA-RB3&keyword=Concrete&id1=44&id2=136

http://www.penetradar.com/GPR_Vehicles.htm

Manufacturers and costs

• Antennas– Air-coupled– Ground-coupled

Penetradar Corporation

http://www.penetradar.com/GPR_Vehicles.htm

Manufacturers and costs

• Vehicular GPR’s– Single antenna– Multiple antennas– Custom configurations

Penetradar CorporationGSSI (GeographicalSurvey Systems, Inc.)$48.000 (includes vehicle, antenna, software and training)

http://www.penetradar.com/GPR_Vehicles.htm

Links and References

• http://www.geophysical.com/• http://www.penetradar.com/• http://ndtdirect.com/• http://www.ndtjames.com/• http://www.cait.rutgers.edu/finalreports/GPR-RU4474.pdf• Al-Qadi, I.L. and Lahouar (2005). “Measuring layer

thickness with GPR –Theory to practice”. Construction and Building Materials, 19, 763-772

• ACI Committee 228, (2003). “In-place methods to estimate concrete strength (ACI 228.1R-03).” Farmington Hills: American Concrete Institute.