A cable for detecting, in a stable and waterproof way, any leak of petroleum, petroleum product, or other fluid from an underground pipeline by means of a change in the electrostatic capacity between inner and outer conductors of the cable due to permeation by the leaking fluid. The cable offers constant, uninterrupted monitoring of such a pipeline with no limit to the watching range, and can accurately locate even a minor leak. The invention also involves a method of installing a detection system using such a detector cable, and a detection method which depends on electric pulses as means for detecting such a leak.