Ground Penetrating Radar is a technology that allows the visualization of the earth beneath the surface without the disruption of excavation. Unlike when using the traditional alternatives to GPR: drilling, probing, and digging, a record of subsurface features (buried objects as well as natural strata interfaces) can be made quickly and cheaply.
Most commonly, the GPR device, while moving across the surface, spits out a blast of Ultra High Frequency microwave energy from an antenna into the ground. Places in which the electrical conductivities and dielectric constants of the earth-medium changes abruptly, reflect some of the UHF waves back to a second, usually stationary, antenna where the degree and timing of reflected energy are popped into a computer for recording, analysis and display. There is also the Reflection Profiling Method in which the radar signal is broadcast from several locations known to the receiving antenna/computer and a subsurface model is built from the aggregate results. A process using transillumination and tomography techniques can take place if the two antennae can be place so that the material to be examined is between them, such as in mines and caves.
Typically, GPR is used to penetrate low-conductivity materials (like sand and rock) up to 30 meters. The range of utility in a high-moisture substrate (like clay and shale) can be as little as a single meter. Frequency of the energy also affects range as well as resolution of information. Relatively high frequencies -- as great as 1 GHz, will return very high resolution scans, but also limits the depth of penetration to as little as 0-10 meters. Scans with frequencies in the 25-200 MHz range can penetrate beyond 30 meters in some cases but suffer from degraded resolution. Penetration and reflection of depths up to five kilometers have been taken through polar ice and immense deposits of salt.
The quality of scans in saturated soils is much higher when the ground is frozen and the radar energy has the opportunity to penetrate and reflect without attenuating as keenly. Similarly, very high quality scans can be obtained through surface asphalt or cement since the paving keep the soil dry and scanning from a flat surface produces cleaner results.
Some of the typical applications for Ground Penetrating Radar include the location of: sinkholes, caves, tunnels, pipes, tanks, etc in the earth as well as finding: rebar, conduit, faults, voids, etc in and under formed concrete. GPR is also used for determining water depth and content in soils, both for construction and ecological assessment. GPR has been a great time saver for archaeologists and anthropologists as well -- identifying dig locations from the surface.
But GPR has many more exotic uses too. GPR went to the moon with Apollo 17 for the Lunar Sounder Experiment. GPR is being used in Precision Viticulture to closely monitor soil moisture for grape production, as pioneered at University of California, Berkeley. At the time of this writing, US soldiers in Iraq are using Ground Penetrating Radar units in trailers and even on the noses of airplanes to detect buried items of interest.
This write-up was inspired by a fluff piece on
CNN or
Fox News that I happened upon while at work, but in researching the topic, I consulted:
http://www.groundpenetratingradar.com/
http://www.g-p-r.com/tutorial.htm/
http://www.du.edu/~lconyer/
http://fate.clu-in.org/gpr.asp?techtypeid=41
http://esd.lbl.gov/people/shubbard/vita/webpage/pubs.html
http://www.lpi.usra.edu/expmoon/Apollo17/A17_Orbital_sounder.html