Ground Penetrating Radar (GPR) has come into use over the last 20 years in civil engineering, geology and archaeology, for the detection of buried objects and for soil study. The detection of buried landmines has also been a subject of considerable interest, in particular due to radar's potential for the detection of plastic-cased mines which contain little or no metal. Today, a large number of organisations, in Europe and world-wide, are working on different parts of GPR systems, and amongst all the sensors proposed for humanitarain demining GPR has had by far the greatest research funding and effort dedicated to it.
GPR works by emitting an electromagnetic wave into the ground, rather than into the air as in many radar applications, using an antenna which does not need direct ground contact (in other domains direct contact is often required, e.g. Non-Destructive Testing). GPR systems usually operate in the microwave region, from several hundred MHz to several GHz. Buried objects, as well as the air-ground interface, cause reflections of the emitted energy, which are detected by a receiver antenna and associated circuitry. The antenna is one of the most crucial parts of a GPR system. Most mine-detection GPR systems use very low power and do not present any radiation hazard to the operator.
GPR systems can be subdivided into four categories, depending on their operating principle. The first type is an impulse time domain GPR, where the emitted pulse has a carrier frequency, modulated by a nominally rectangular envelope. This type of device operates in a limited frequency range, and has in most cases a mono-cycle pulse. The second type of time domain GPR is the so-called Chirp Radar, which transmits a pulse-train waveform where the carrier frequency of each pulse is rapidly changed across the pulse width. Frequency domain GPR transmits a signal with a changing carrier frequency over a chosen frequency range. This carrier frequency is changed, either continuously for example in a linear sweep (Frequency Modulated Continuous Wave Radar, or FMCW), or with a fixed step (stepped frequency radar).
The term Ultra Wide Band (UWB) GPR is generally used for a system having a fractional bandwidth which is larger than 25%.
What particularly matters for the detection of objects in a background medium, e.g. mines buried in soil, is the difference between the electromagnetic properties of the target (in particular its dielectric constant) and those of the background. The amount of energy reflected, upon which reliable detection is based, also depends on the objects size and form. Spatial resolution depends on the frequency used, and the resolution needed to cope with the smaller antipersonnel landmines, requires the use of high frequency bands (up to a few GHz). These higher frequencies are particularly limited in penetration depth. Microwaves are strongly attenuated by certain types of conductive soils such as clay, and attenuation increases with frequency and the water content of the medium. Wet clay in particular provides an extremely challenging environment (penetration is very poor). GPR systems for landmine detection are either designed to provide detection warnings when a mine-like object is located (e.g. an audio signal as is used in metal detectors), or to produce image data. As yet hand-held radar-only systems have not been brought to market though the use of radar with metal-detectors in dual-sensor hand-held systems is becoming established with extensive trials of the USA h-stamids equipment and trials of UK equipment. In both cases the GPR is confirmatory after the metal detector has found a target. Vehicle mounted radar systems with a broad sweep have also been developed and field tested, including in the LOTUS multisensor demonstrator (q.v.). [Source: EUDEM Survey, 1999]
