Ground penetrating radar is a form of radar which is designed for subsurface imaging. In other words, it's radar which can be used to see things underground, in contrast with the radar used to identify features in the ocean or to analyze walls to determine what might be on the other side. There are a number of applications for this technology, and several firms make a variety of radar units designed for this purpose.
This type of specialized radar works a lot like regular radar. The radar unit has a sending antenna which sends out pulses of sound, and a receiving antenna, which picks up those pulses when they bounce off objects underground. The returned pulses can be used to construct an image of what is underground, because they will vary depending on the depth and composition of buried objects.
Several factors influence the usefulness of ground penetrating radar. The first is the nature of the ground being examined, and what people are looking for; subtle geological differences, for example, can make it hard to get an accurate picture. The strength and frequency of the signal also makes a difference, as does the conductivity of the ground. Some types of rock and soil conduct sound better than others, generating a clearer picture.
Ground penetrating radar is extremely useful in archeology, where it can be used to inspect archaeological sites without being invasive. It saves a lot of time by identifying potential excavation sites ahead of time, and allowing people to get an idea of the layout of the site before they start digging. It can also be used to gather information about culturally sensitive sites, or sites which might be damaged through excavation.
This technology is also used in forensics, to look for buried evidence at the sites of crime scenes or suspected burials. The earth sciences also relies heavily on ground penetrating radar to learn more about the composition of the Earth, and to study specific sites. It can also have more practical applications: it can be used to identify leaks from reservoirs, chemical storage, and other sites, for example.
Simple forms of ground penetrating radar units are also used by enthusiasts who want to find buried metals and other materials. These machines are sometimes more useful than metal detectors, for people who understand how to read the images, as the radar can identify specific objects, not just those which contain metals.