Ground-penetrating-radar (GPR) systems based on ultrawideband (UWB) generation and reception of reflected signals are often associated with military applications such as land-mine detection. But because of aging waterpipe infrastructure in many major cities within the European Union (EU), a UWB-based GPR system also has application for detecting leaks in underground waterpipes. The cost of replacing these aging waterpipe systems, if left to deteriorate beyond a state of repair, is prohibitive for many EU communities. A more practical alternative is the detection of leaks in infrastructure before major damage has occurred, in order to effect repairs and rehabilitate the waterpipe system. For that reason, a UWB-based GPR system is attractive for monitoring and detecting problems in underground pipes.

The authors report on work that was part of their proposal/ contract (No. 036887) with the Waterpipe European Commission research project, “Integrated High Resolution Imaging Ground Penetrating Radar and Decision Support System for Waterpipe Line Rehabilitation.” The proposed system operates in the frequency range from 100 MHz to 10 GHz and includes active UWB antennas, low-noise, variablegain amplifiers (VGAs), high-speed analog-to-digital converters (ADCs), and high-output-power configurable pulsed UWB signals.

Because of the expense of replacing entire waterpipe systems, resources are being allocated throughout the EU for waterpipe rehabilitation. With this emphasis on sustainable management, it has been necessary to develop risk-based approaches for the rehabilitation management of the water-supply network. Rehabilitation decisions should be based on inspection and evaluation of pipeline conditions, but this implies the capability of performing such inspections in a cost-effective and timely manner. In some cases, utility companies cannot locate a number of the old water supply pipes and current inspection technologies typically do not provide the needed detailed information on pipeline damage.

The authors present an approach for the effective inspection of underground waterpipes by means of a high-resolution ground penetration imaging radar (GPIR) system. The system is capable of detecting water pipes of different dimensions, of detecting leaks and damage in the pipes, and providing high-resolution images of the damaged areas. The proposed GPIR system can detect waterpipes made of various different types of materials, and detect leaks and damage in waterpipes of those different types of materials. It has a ground penetration depth to 2 m, and the imaging resolution of the damaged pipe is 5 cm or better. The time to perform a survey, that is, complete the detection and imaging of a section of waterpipe, is 10 s/m along the pipe axis. Thus, the survey time for a 1-km section of waterpipe will be about 3 h.

For transportability and mobility, systems the GPIR is mounted on a four-wheel vehicle. For programmability and signal processing, a laptop computer will also be on the vehicle, while the antenna system and transceiveranalog/ digital signal processing units will be placed in the front side of the vehicle. The antenna system consists of 10 UWB bow-tie patch antennas with periodic reception. The main subunits of the proposed GPIR system are

• • a reconfigurable short-pulse transmitter;
• a fixed UWB transmit antenna;
• a scanning UWB receive antenna;
• a receiver front end;
• a digital correlator receiver; and a laptop personal computer to perform signal processing and image reconstruction.

A pulsed signal with zero baseband spectral components is generated by a digitally controlled generator. The generator is capable of creating a variety of UWB signals, such as one-period sinusoid or two opposite and inverted triangular pulses. The short-pulse generator is designed to produce output amplitudes on the order of 10 V and will drive a fixed-position UWB antenna (Fig. 1). In order to achieve synchronization between the receiver and transmitter units, a sample of the reference signal is fed to the correlator receiver.

The receiver chain consists of a mechanically scanned antenna measuring 1 x 1 m. The pulse repetition frequency (PRF) is selected as 10 MHz in order to achieve short scanning times for all 10 UWB antennas, including averaging of the received signals. Received signals are amplified and then correlated with the reference pulse signal to obtain the envelope of the signals reflected by the ground medium. The use of UWB signals provides extensive information for the imaging of a wide range of underground target media. Each receiver scanning antenna collects a number of reflected signal waveforms for the processing unit. The reconstruction algorithm for the complex dielectric constant of the underground medium (two real numbers for each frequency spectral component) is obtained by means of a time-domain version of the Method of Auxiliary Sources, developed previously.1 The complex dielectric permittivity of the underground target, the waterpipe, is reconstructed by means of solving an inverse scattering problem.

The GPIR system consists of several subsystems, including:

• a transmitter unit (including the transmit antenna);
• a receiver front end (analog) and the scanning receive antenna;
• a signal digitizer (ADC);
• a laptop computer and image display; and • a structure for mounting the GPIR on a vehicle for mobility.

The transmitter unit, including the transmit antenna, employs an UWB signal generator to produce very short pulse width signals (500 to 2000 ps). The peak pulse amplitude of these signals is on the order of 10 V. Pulsed signals are sent to the antenna, which is a transverse electromagnetic (TEM) flared horn antenna, for transmission. While the receiving antenna is a scanning type, the transmit antenna is fixed. The transmitted pulse rate is 10 MHz, driven by a temperature-compensated crystal oscillator (TCXO). The use of the TEM horn antenna ensures wideband impedance matching of the antenna to the solid-state pulse generator.

Table 1 shows the specifications for the high-voltage, short-pulse generator. It consists of a crystal oscillator at 10 MHz that sets the system PRF with minimum jitter. Clock signals are channeled through a single-pole, fourthrow (SP4T) switch to control four parallel step-recovery- diode-based subsystems to produce short pulses with pulse widths ranging from 0.5 to 2.0 ns. The pulses have amplitude of typically 1 V. Following this, a broadband, high-power amplifier is used to boost these short pulsed signals to an amplitude of at least 10 V. Inputs signals to the high-power amplifier are selected by means of a SP4T switch (Fig. 2).

Figure 3 shows the pulser system, fabricated on RO4350 printed-circuit-board (PCB) material with a thickness of 0.5 mm from Rogers Corp. (www.rogerscorp.com). The PCB material, which has a dielectric constant of 3.48, is a glass-reinforced hydrocarbon/ceramic laminate with loss. The pulse tested was constructed, simulated, and fully tested. ² The prototype unit featured the integrated high-power, output-stage amplifier.

Figure 4 shows the TEM horn transmit antenna.^3-7 Computer-aided-engineering (CAE) simulations were also conducted on the TEM horn antenna in order to model its return-loss performance and radiation pattern, and the results are shown in Fig. 5. The simulated results indicate that the antenna’s directivity is about 9 dBi and the mean return loss (S₁₁) is better than 8 dB.

The receiver front end consists of a broadband low-noise amplifier (LNA) capable of operating from 100 MHz to 2 GHz and an anti-aliasing filter. The LNA provides maximum gain of 40 dB, which can be throttled down to 0 dB by means of analog control. The function of the LNA is to boost received echo pulse signals reflected by the underground structures to a level sufficient for digitization by a highspeed ADC. An active type antenna is used for the receive antenna, in order to provide broadband coverage in a relatively small form factor (the antenna occupies less than 10 x 5 cm in size). The LNA is based on a high-electronmobility- transistor (HEMT) device that is directly matched to the receive antenna. A patch bow-tie antenna is used as the receive antenna. A total of 10 similar antennas are set parallel to the Earth’s surface and approximately 20 cm above the ground. They are scanned mechanically through the use of an electromechanical single-pole, ten-throw (SP10T) switch. Raster-type scanning is used to scan a 1 x 2 m horizontal area by means of the Cartesian coordinate system. Table 2 provides a summary of the receiver’s basic performance specifications.

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