Ground-penetrating Radar (GPR) is a geophysical exploration technique using radar waves reflected back from subsurface discontinuities to depths of a few tens of metres.

This produces an output similar to a seismic reflection seismogram, but resulting from changes in the dielectric constant of subsurface layers rather than their acoustic impedance.

In other words, GPR uses radar waves: the way in which certain rocks reflect the waves back give an indication of what is down there.
GPR is used alongside seismic reflection to produce a slightly different image. The two images are combined to give a more thorough analysis of the underlying structure.

There are several reasons why ground-penetrating radar is hard to make work reliably in practice. Soil and dirt are generally moist and full of conductive elements, which act to scatter and absorb an extremely large portion of the radar energy. This seriously limits the depth the radar can reasonably operate. This is compounded by the fact that you want imaging radars to have a high operating frequency so that your range resolution (see range profile) is sufficiently small. Unfortunately, the higher the frequency, the more power is absorbed by the ground. Not good. Mines and other objects you wish to detect scatter the energy, however so do rocks, plant matter, and various other subterranean objects that aren't the target. This forces the engineer to design algorithms which can discriminate between a known types of scatterers such as mines and rocks. Dirt is a highly dispersive media, unlike air, in that different radio frequencies travel faster inside it than others. This acts to distort the radar waveform in an unknown way as it travels, again requiring the engineer to devise clever equalization techniques to un-mangle the recieved signal.

Oh, and engineers feel really wierd pointing radar antennas at the ground for some reason ..

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:
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