Automated electrical impedance measuring systems to detect leaks from buried pipes

This paper describes two automated electrical impedance-rneasuring systems intended for detecting leaks from buried pipes. Each system includes current injection and voltage detection, a switching system based on reed relays and a personal computer (PC). Both systems allow the implementation of any desired electrode configuration under PC control. AC current injection overcomes electrode polarisation and reduces electrode impedance. The injector and detector of the first system designed are commercially available instruments connected by GPIB, and permit the injection of sine and square waves from 10 Hz to 100 kHz. The PC obtains the real and imaginary parts of the soil impedance by digital coherent demodulation. The signals detected are contaminated by common mode voltage and electromagnetic interference between current-injecting wires and detecting wires. The second system designed uses a custorn-built detector that reduces interference by synchronously sampling square waveforms. Using an additional electrode eliminates the common mode voltage at the detector input. This second system yields only the real part of the soil impedance but is very cost-effective. Both systems have been successfully applied to the detection of simulated leaks from buried pipes in laboratory phantoms.