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

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 d i is the thickness of layer i, t i the total travel time through that layer, C is the speed of light and ε r,i the dielectric constant of the layer

Principle The amount of reflected energy at an interface is governed by: where ρ 1,2 is the reflection coefficient and ε r1 and ε r2 are the dielectric constants MaterialDielectric Constant Air1 Water81 Concrete6-12 HMA4-7 Ice1.5 Metals∞ Typical Dielectric Constants

Instrumentation The typical instrumentation for GPR includes the following: –Antenna Air-couple Ground-coupled –Control Unit –Display device –Storage device

Instrumentation

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

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

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 $ –IRIS systems Penetradar –GSSI handy-scan $6.500 b&w $ color

Manufacturers and costs Antennas –Air-coupled –Ground-coupled Penetradar Corporation

Manufacturers and costs Vehicular GPR’s –Single antenna –Multiple antennas –Custom configurations Penetradar Corporation GSSI (GeographicalSurvey Systems, Inc.) $ (includes vehicle, antenna, software and training)

Links and References Al-Qadi, I.L. and Lahouar (2005). “Measuring layer thickness with GPR –Theory to practice”. Construction and Building Materials, 19, ACI Committee 228, (2003). “In-place methods to estimate concrete strength (ACI 228.1R-03).” Farmington Hills: American Concrete Institute.