Geophysics - Ground-penetrating radar (GPR)

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 is a geophysical method that uses radar pulses to image the subsurface. This nondestructive method uses electromagnetic radiation in the microwave band (UHF/VHF frequencies) of theradio spectrum, and detects the reflected signals from subsurface structures. GPR can have applications in a variety of media, including rock, soil, ice, fresh water, pavements and structures. In the right conditions, practitioners can use GPR to detect subsurface objects, changes in material properties, and voids and cracks. 
GPR uses high-frequency (usually polarized) radio waves, usually in the range 10 MHz to 1 GHz. A GPR transmitter emits electromagnetic energy into the ground. When the energy encounters a buried object or a boundary between materials having different permittivities, it may be reflected or refracted or scattered back to the surface. A receiving antenna can then record the variations in the return signal. The principles involved are similar to seismology, except GPR methods implement electromagnetic energy rather than acoustic energy, and energy may be reflected at boundaries where subsurface electrical properties change rather than subsurface mechanical properties as is the case with seismic energy.
The electrical conductivity of the ground, the transmitted center frequency, and the radiated power all may limit the effective depth range of GPR investigation. Increases in electrical conductivity attenuate the introduced electromagnetic wave, and thus the penetration depth decreases. Because of frequency-dependent attenuation mechanisms, higher frequencies do not penetrate as far as lower frequencies. However, higher frequencies may provide improved resolution. Thus operating frequency is always a trade-off between resolution and penetration. Optimal depth of subsurface penetration is achieved in ice where the depth of penetration can achieve several thousand metres (to bedrock in Greenland) at low GPR frequencies. Dry sandy soils or massive dry materials such as granite, limestone, and concrete tend to be resistive rather than conductive, and the depth of penetration could be up to 15-metre (49 ft). In moist and/or clay-laden soils and materials with high electrical conductivity, penetration may be as little as a few centimetres.
Ground-penetrating radar antennas are generally in contact with the ground for the strongest signal strength; however, GPR air-launched antennas can be used 
The GPR method uses high-frequency electromagnetic waves to provide detailed subsurface cross sections.
•microwave energy reflected back to the surface from different materials produces various electrical results
•Metal objects produce the strongest results, determining location, depth and size
•GPR can also identify excavations, for example where tanks were removed, and can delineate the boundaries of landfills and buried lagoons
•higher-frequency antennas (typically 900 or 1,500 MHz) are utilized for surveys on concrete slabs, to locate rebar, electrical conduits, or other pipes prior to core drilling or saw cutting.
Geo-Earth staffs skilled in digital signal-processing techniques that enhance GPR data interpretations. Signal processing can reduce background noise and identify weaker reflections (such as from fiberglass USTs, or subtle stratigraphic variations) that would remain undetected using analog GPR instrumentation. 
 


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