In GPR surveys electromagnetic wave pulses are transmitted into the ground from an antenna. These pulses propagate through the ground reflecting off sub-surface boundaries before detection by a receiving antenna. In general, an object that is harder than the surrounding soil will reflect a stronger signal. Utilities, tunnels and other buried objects
can therefore be located. The distance from the GPR unit to the buried object is then computed from the time taken for the pulse to pass from the transmitter to the receiver. When utilized for pipeline detection the transmitting unit is passed in a direction perpendicular to the line of the pipe and as the unit approaches the pipe the signal travel time reduces. Where the approximate line of the pipe is not known with any certainty beforehand a grid is laid out on the surface of the ground to ensure complete coverage. As reported by Tong (1997) a major advantage of using GPR for the location of underground utilities is its independence of the pipeline material in that it can locate non-metallic as well as metallic pipes.

Despite the merits of utilizing GPR a number of drawbacks still limit its use. Although the maximum depth of analysis is about 3m in favorable conditions the pulses lose strength very quickly in conductive materials such as clay and saturated soils, thereby affecting the depth of penetration and in some cases making the use of GPR totally unsuitable. Recent work, however, by Access Network Systems Laboratories in Japan (Hata et al., 1997) on antenna design has produced a deep ground-penetrating radar capable of surveying at depths up to 5m. In addition, GPR does not identify specific
utilities (i.e. water vs. gas, electrical vs. telephone) hence verification is necessary using other methods. Finally, data interpretation requires highly skilled operators because GPR output is very difficult to interpret and, as described by Olhoeft (1999), has therefore been open to misuse. Conventional 2D radar profiles detect objects primarily in the vertical plane below the scan line and if anything other than the horizontal location of a pipe is required significant post processing of the image is needed, see for example Olhoeft (2000). Similarly, pipes appear as individual "blips" in the radar image, making it difficult to distinguish them from rocks and other buried objects. However, an interesting development in this field is digital signal processing (DSP) of the information obtained from the GPR receiver. The basic idea is that a utility is a long thin object that should appear in a sequence of GPR scans of a particular area. The DSP software suppresses images of discrete objects such as boulders and enhances images of possible utilities. This evidence is accumulated to find the relative depth and direction of the pipe (Conway, 1995).