Leakage Detection in Buried Pipes by Electrical Resistance Imaging

Josep Jordana, Manel Gasulla and Ramón Pallás-Areny

Divisió d'Instrumentació i Bioenginyeria, Departament d'Enginyeria Electrònica Universitat Politècnica de Catalunya

Jordi Girona 1-3, Edifici C-4 08034 Barcelona, SPAIN

Phone 34-93-401-7483 Fax 34-93-401-6756

E-mail: jordana@eel.upc.es

Abstract: This work describes a non-invasive method to detect leaks in buried pipes that uses a surface linear electrode array perpendicular to the pipe axis. Two electrodes inject current and the remaining electrodes detect the drop in voltage. We have used both the dipole-dipole array and a modified Schlumberger array.

A single step reconstruction algorithm based on the sensitivity theorem yields 2D images of the cross section. The corresponding sensitivity matrices are ill- posed. Nevertheless, the reconstructed images allow us to detect leaks without needing any regularisation parameter because it is possible to replace the inverse of the sensitivity matrix by their transposed.

A personal computer controls current injection, electrode switching and voltage detection, which enables us to easily test different electrode arrays. The system has been first tested in the laboratory using a stainless steel tube immersed in water and covered by a rubber sleeve to simulate a non-conductive leak. By taking reference measurements with the immersed bare pipe, it is possible to reconstruct images showing the simulated leak using 16 electrodes and even 8 electrodes only, though with reduced resolution.

Field measurements have involved simulated water leaks from a 1 m plastic tube, 8 cm in radius buried at about 24 cm depth in a farm field. The system injected 1 kHz, 20 V peak-to-peak square waveforms, thus avoiding electrode polarisation effects. Images reconstructed from an eight?electrode array show the leak if the image before the leak is used as reference.

Keywords: Geoelectrics, resistance tomography, leak detection, synchronous sampling.