Basic Ground Penetrating Radar Theory

GPR LIMITATIONS Penetration 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 materials Interpretation of GPR data can be subjective. The experience of the interpreter is very important.

Penetration depths 25 25 40 Antenna (MHz) In soil (m) In rock (m) 25 25 40 50 20 30 100 12 20 200 8 15 500 3.5 5 1000 1.5 3 Average penetration depths of radar signals in high resistivity geological environment absent of low resistive layers.

D E P T H 1 2 3 4 5 6 Deep utilities must have larger diameters than shallow utilities in order to be detected with GPR Deep utilities must have larger diameters than shallow utilities in order to be detected with GPR

The GPR technique GPR 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).

How GPR works GPR is, in principal, similar to sonar equipment (fish finders) found in boats The transmitter emits a “train” of electromagnetic impulses which propagate through the media Reflection (i.e. scattering) occurs where the electrical properties of subsurface materials change The receiver picks up the “back-scattered” signal and displays it on a monitor

? 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.

Data Examples & Interpretation

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. ENVIRONMENTAL Hazardous waste mapping, underground storage tanks (UST), Sedimentology studies, Bathymetry. GEOTECHNICAL Stratigraphic mapping, cavities and sinkholes, groundwater, mining hazards, fracture detection, Earth dam studies, foundation studies, tunnel investigations. MILITARY Ordinance detection, runway integrity, clearing of trenching routes ARCHAEOLOGY site mapping, grave detection, artifacts

The most important markets for radar… Utility detection Pipe and culvert inspections Concrete and NDT Road and bridge deck investigations Geological mapping Ice, snow and glacier Borehole radar

The difference in radargram between good and bad soil conditions The GPR performance GPR 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

Comparison between the SHALLOW and MID antenna. Electrical Conduits Shallow antenna Unknown   Comparison between the SHALLOW and MID antenna. Water Main Force Main Electrical Conduits Unknown Mid antenna Water Main Force Main

Interpretation: Metallic water pipes shows sharper Sewer line is large enough to show both top/bottom reflection Note radiuses of the signatures Trench shows

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