Safety and Interference Concerns With Ground Penetrating Radar

During investigations, especially civil surveys, GPR surveys are often performed near sensitive electronic equipment or tenant occupied spaces. To address safety and interference concerns GPR technology is quite benign. The energy source is, as the name implies, radar, or radio frequency. It is relatively low power so there are no deleterious effects from destructive radiation and no need to do locates after hours. The antennas used in civil surveys are fully shielded to direct all the transmitted energy into the ground and to eliminate surface reflection artifacts and radio frequency interference common to an unshielded system. The radar signal reflects off of any objects with a difference in conductivity so materials such as plastics or air voids, as well as steel, can be resolved. Distinguishing between different materials can only be done in a relative sense, and because concrete varies a great deal, a direct calibration must be done to get accurate depth measurements. The reflected signal from 3 mm steel reinforcement mesh can be more pronounced than 30 mm PVC conduit. This can cause conduit identification problems when the conduit has been tied directly below the re-mesh. For a core location, a clear area between the steel can be found quite easily. It is only when there is a need for a larger cut in the concrete, such as air ducts or stairways, where a problem may be encountered. To try to mitigate this larger area is scanned than is to be cut because conduit tends not to follow the exact same grid as the re-bar and tends to show itself over a larger area. GPR can accurately resolve objects such as re-bar, stress cables and conduit in concrete to a depth of 450 mm depending on how many other there are in between.

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