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GPR methods were first investigated almost a hundred years ago with the work of Hulsmeyer in 1904 and Lowy in 1910. The first techniques implemented used continuous wave (CW) systems to measure the location of scatterers within the earth. This work used both buried and surface mounted dipole arrays to form crude images of the scattering centres. This work relied on the diffraction and shielding effects of conductivity variations within the soil medium and of objects buried therein.
Later work progressed on to pulsed systems (Hulsenbeck, 1926). For geophysical work there is an obvious parallel between pulsed EM systems and pulsed acoustic (seismic) techniques. EM methods provide some advantages over acoustic methods in this role. Firstly, electromagnetic waves have an impedance discontinuity at the air/soil boundary of a factor typically between 2 and 4. This is in contrast to the acoustic impedance mismatch at around 100 or so. This means that for EM systems significant coupling of energy to the ground can be achieved by transducer systems remote from the interface. Effective acoustic coupling, on the other hand, can only be achieved by ground contacting transducers. A second advantage of EM over acoustics is the relative ease of producing a directional source. An increase in gain of the transducer leads to improvements in the detection threshold of the system.
One of the main the application areas for GPR systems has proved to be environmental sensing, whether for military or civilian use. Pulsed systems have been developed and used successfully over the last 74 years to probe in various media such as ice, water, sand and rock formations, and even the lunar surface during the moon landings.
There are many commercial and humanitarian applications of remote sensing of buried objects. These objects may be man made such as pipes and cables, or natural such as changes in the parameters of the ground representing geological features such as rock strata or mineral deposits. A humanitarian example of GPR application is an issue near the top of the political agenda at the present time, that of the clearing of landmines and other unexploded ordnance. This has become a large problem throughout the world as a result of indiscriminate mining of large swathes of land.
No entirely successful solution to the landmine problem has yet been proposed. The difficulty for GPR is detecting low contrast non-metallic objects with high clutter levels from stones and tree roots etc. In addition for this application there is the contrasting requirements of an extremely high detection accuracy and low false alarm rate. GPR systems are currently undergoing extensive development to overcome these problems.
The range of applications is currently increasing rapidly and includes archeological exploration, road and rail bed quality assessment, location of voids, tunnels and mineshafts, pipe and cable detection. As the application domain becomes more exacting in its requirements on the radar system so has the technology developed. Excellent reviews on the subject of GPR can be found in [1] (intro to sub-surface radar), [2] (Nilsson) and [3] (peters).
image from Hulsmeyer or lowy
relevent pic
pulsed system
commercial
mine