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The nature of the GPR system to be used in an application depend primarily on the properties of the obscuring medium and the nature of the target. These factors will determine the system requirements and have a bearing on the three key elements of the system, coupling of radiation to the ground, transmission of the signal to the target and back and the nature of the returned signal (signal processing).
Design considerations fall into the following categories, target depth, target size, necessary resolution (vertical and horizontal) and ground properties. These factors will affect the system design parameters, frequency of operation, bandwidth, antenna design and signal processing. Subsidiary constraints, such as antenna weight, size and location, may also affect design decisions. Each factor must be considered and may lead to conflicting and mutually exclusive design goals. In such a multi-parametric system, optimisation can be difficult.
For high resolution, effective target discrimination and minimum antenna size it is desirable to use the maximum possible operating frequency. Backscatter from localised targets decreases with increasing wavelength; antenna size and weight increases with increasing wavelength; antenna gain decreases with increasing wavelength; horizontal discrimination between multiple targets decreases with increasing wavelength. The limit on the upper frequency is imposed by propagation losses. For example clay soils may cause losses of over 100dB per metre at 1GHz but a more reasonable 20dB per metre at 100MHz. The choice of operating frequency is a trade-off between these conflicting requirements.
Bandwidth is determined by the necessary depth resolution. Large bandwidth systems, or conversely when viewed in the time domain, short pulse width systems, have a better ability to discriminate between targets in depth. The limiting factor on bandwidth is again the propagation losses of the ground material. There is no point designing a low dispersion antenna system over an extremely wide bandwidth if the high frequency content of the signal cannot be transmitted over the required distances to the target.
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