Ground penetrating radar
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Transcript of Ground penetrating radar
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.