Ground-penetrating radars (GPR) are remote sensing systems that use radio waves to develop a picture of underground geologic formations, soil content and buried objects or structures. GPRs create this image by receiving the reflections of radar signals transmitted into the ground, and then analyzing the reflected wave data to produce a human-readable image.
GPRs can be deployed on many platforms, and have a wide spectrum of civilian and military applications.
GPRs produce their images by transmitting radio waves into the ground and measuring the properties of their reflection. Depending on the goal, GPR radio wave frequencies can span anywhere from 10 MHz to 1 GHz. GPR can use a single antenna for transmitting and receiving, two separate antennas, or multiple receiving antennae separated by known distances.
GPR transmission and reception equipment can be hand-carried, deployed on a vehicle or towed rig, or flown on an aircraft or satellite. Image production can be conducted on-site, or data can be sent off-site for further analysis and computationally intensive processing.
GPR's Nondestructive Attraction
GPR is a member of a broad family of remote sensing and analysis methodologies known as "nondestructive testing" (NDT). Scientists and engineers like NDT methods because they enable the measurement and analysis of sensitive areas and equipment without needing to disturb, disrupt or harm them.
The quality and characteristics of a GPR image are determined by factors including the wavelength and frequency of the transmitted radar signals, the quality of the receiving antenna and equipment, and the quality of the mathematical analysis and processing that produces the final image.
Higher frequency and shorter wavelength radar signals will produce images of higher resolution. But such signals do not penetrate as far into the earth. Lower frequency signals penetrate farther, but cannot provide the same level of detail.
On average, low frequency GPR signals can penetrate up to about 140 feet through solid rock, providing an image with about three-foot resolution. Soil impedes GPR signals more than rock, and liquids rapidly degrade GPR signals and dampen signal penetration.
Hand-pulled or vehicle-borne GPRs may be used at construction sites, mines or archaeological expeditions.
In excavation work, GPR is used to identify utility lines, underground conduits or unknown objects, and helps avoid causing damage. In mining and geology, GPR can help identify boundaries between different soil types, rock layers or mineral deposits. GPR can help direct an archaeological expedition to the best digging locations, and has been used to map interiors and hidden cavities in hard-to-examine structures like the Egyptian pyramids.
Air and Space-based GPR
Overhead GPR is used for many of the same applications, but on a larger scale. Satellite-based GPR has been used to find underground geologic formations and to map large archaeological sites and ruins.
GPR images can give construction teams a better idea of local soil structure and composition. GPR images can also reveal soil disruption that might indicate a recently built underground structure.
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