2 GPR LIMITATIONSPenetration depth and ability to resolve targets at depth is strongly dependent upon the local soil properties. Highly conductive soils can render the GPR method ineffective.There must be a sufficient electrical contrast between the target and the host materialsInterpretation of GPR data can be subjective The experience of the interpreter is very important.
3 Penetration depths 25 25 40 Antenna (MHz) In soil (m) In rock (m) Average penetration depths of radar signals in high resistivity geological environment absent of low resistive layers.
4 DEPTH123456Deep utilities must have larger diameters than shallow utilities in order to be detected with GPRDeep utilities must have larger diameters than shallow utilities in order to be detected with GPR
5 The GPR techniqueGPR is an electromagnetic method that detects interfaces between subsurface materials with differing dielectric constants. Your Easy Locator GPR system basically consists of:An antenna, which houses a transmitter and receiver.A monitor, which processes the received signal and produces a graphic display of the data.The transmitter radiates repetitive short-duration electromagnetic signals into the earth from the antenna moving across the ground surface. Electromagnetic waves are reflected back to the receiver by interfaces between materials with differing dielectric constants.GPR wave propagation from transmitter (Tx) and reflection to the receiver (Rx).
6 How GPR works GPR is, in principal, similar to sonar equipment (fish finders) found in boatsThe transmitter emits a “train” ofelectromagnetic impulses whichpropagate through the mediaReflection (i.e. scattering) occurs wherethe electrical properties of subsurfacematerials changeThe receiver picks up the “back-scattered”signal and displays it on a monitor
7 ? GPR signatures Time [ns] Depth [m] Length [m] The basic principle of reflection measurements. While the GPR unit is moved along the surface, reflections are achieved from underground structures, in theis case a geological boundary and a pipe. The reflections from the pipe form a hyperbolic signature in the diagram.The GPR method detecs metallic as well as non-metallic objects.
9 GPR APPLICATIONS CIVIL/STRUCTURAL ENGINEERING Utilities (pipes, cables), rebar and voids. Pre-studies for Horizontal Directional Drilling (HDD)Transportation: Roadways and railroad tracks, ice thickness,bridge deck and bridge fundation studies.ENVIRONMENTALHazardous waste mapping, underground storage tanks (UST),Sedimentology studies, Bathymetry.GEOTECHNICALStratigraphic mapping, cavities and sinkholes, groundwater, mining hazards, fracture detection, Earth dam studies, foundation studies, tunnel investigations.MILITARYOrdinance detection, runway integrity, clearing of trenching routesARCHAEOLOGYsite mapping, grave detection, artifacts
10 The most important markets for radar… Utility detectionPipe and culvert inspectionsConcrete and NDTRoad and bridge deck investigationsGeological mappingIce, snow and glacierBorehole radar
11 The difference in radargram between good and bad soil conditions The GPR performanceGPR is primarily affected by the conductivity and dielectric permittivity of the mediums.GPR works best in resistive, sandy or gravely soil types. Difficult, conductive types are typically composed of silts and clays or contains salt water.Depth of investigation is limited by signal attenuation of conductive soil but also dependent on the antenna selected.The difference in radargram between good and bad soil conditions
12 Comparison between the SHALLOW and MID antenna. Electrical ConduitsShallow antennaUnknownComparison between the SHALLOW and MID antenna.Water MainForce MainElectrical ConduitsUnknownMid antennaWater MainForce Main
13 Interpretation:Metallic water pipes shows sharperSewer line is large enough to show both top/bottom reflectionNote radiuses of the signaturesTrench shows