Overview of Ground Penetrating Radar (GPR) technique

GPR uses high-frequency-pulsed electromagnetic (EM) waves to acquire subsurface information. It is used for the measurement of subsurface conditions without drilling, probing or digging. The equipment consists of transmitting and receiving antennae, a radar control unit and data storage/display device.

Energy is propagated downward into the ground from a transmitting antenna and is reflected back to a receiving antenna from subsurface boundaries between media possessing different electromagnetic (EM) properties. The reflected signals are recorded to produce a scan or trace of radar data. Scans obtained as the antennae are moved over a surface are placed side by side to produce a radar profile.

The vertical scale of the radar profile is in units of two-way travel time, the time it takes for an EM wave to travel down to a reflector and back to the surface. The travel time may be converted to depth by relating it to on-site measurements or assumptions about the velocity of radar waves in the subsurface material under investigation.

GPR waves can reach depths up to 30 metres in low conductivity materials such as dry sand or granite. Clays, shale, and other high conductivity materials may attenuate or absorb GPR signals, decreasing the depth of penetration to 1 metre or less. The depth of penetration is also determined by the GPR antenna used.

The antennae used in a GPR survey are selected on the basis of the depth of interest and the size of the target. Penetration depth varies inversely with frequency and the higher the central frequency of the antenna, the smaller the size of object that can be resolved. Higher antenna frequencies give higher resolution, but less penetration, and vice versa. The lateral and vertical resolution of the results varies from 0.01 to 1.0 meters, depending on the choice of antenna frequency. Having a range of frequencies available makes the system ideal for locating objects of different sizes at various depths and in different soil conditions possible.

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