Ground Penetrating Imaging Radars (GPIRs) have been developed and used to obtain information on underground media since the 1970’s. The early versions of the technique were based on the use of short pulse transmission towards the earth and measurement of the reflected signals through sampling methods. These systems could provide limited information on the underground media: The horizontal spatial resolution could not exceed 30-50 cm and a one-dimensional image of the area could be provided. However, the penetration could reach a maximum of 2m.
The development of high-speed data acquisition in recent years has given the possibility for wave front measurement systems. In the WATERPIPE project, a combination of horizontal mechanical-electronic receiver antenna systems and time domain measurement ground penetration was used to improve radar performance in order to inspect and evaluate water pipe conditions. The radar system consists of:
- Transmitter unit, including antenna
- Front-end receiver
- Scanning receiver antenna
- Signal digitaliser, computer and image display
- Inversion algorithm
The system operation is based on a series of principles. First, a pulse is generated by the transmitter, sending radiation towards the earth through a wide band transmitting antenna. The output pulse is repeated in a frequency of several MHz, and each pulse lasts no longer than a nanosecond. Ultra-wideband signals are used since this technology can provide extensive information on the underground medium. The energy content of the pulse is peaked in the spectral region 500-1500 MHz depending on the selected pulse duration. Thanks to the high frequency, these underground images can be obtained in very short time.
The radiation of the pulses illuminates the ground. Being an inhomogeneous medium, the electromagnetic properties of the earth provides strong dispersion phenomena. The reflected signals are measured and recorded on a surface of 1x1m, parallel to the earth medium. A 3D image a good spatial resolution of 5cm in all dimensions is presented. Just like the first versions of the system, the maximum penetration depth of the radar is still 2m.
Source: WATERPIPE 2nd Newsletter, December 2008