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GPR

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Ground penetrating radar (GPR) is a high frequency electromagnetic method that uses radio pulses to image the subsurface. This non-destructive method produces a continuous cross- sectional profile or record of subsurface features that can be applied to a number of engineering and environmental problems. GPR can be used in a variety of materials, including rock, soil, pavements and structures. It can detect buried objects, changes in material, voids or fractures.

A GPR system radiates pulses of high- frequency EM energy into the ground from a transmitting antenna. This EM wave propagates into the ground at a velocity that is related to the electrical properties of the subsurface materials present. When this wave encounters the interface of two materials having different properties (i.e., soil and water, soil and steel), a portion of the energy is reflected back to the surface, where it is detected by a receiver antenna and transmitted to a control unit for processing and display.

The depth of penetration for ground penetrating radar is limited by the electrical conductivity of the ground, and the transmitting frequency. As conductivity increases, the depth of penetration decreases. This is because the electromagnetic energy is more quickly converted into heat, causing a loss in signal strength at depth. Higher frequencies do not penetrate as far as lower frequencies, but give better resolution. Optimal depth penetration is achieved in dry sandy soils or massive dry materials such as granite, limestone, and concrete. Misunderstanding the characteristics of the subsurface and the instrument response often leads to misinterpretation of GPR data by less experienced operators.