Abstract: The invention relates to a method and apparatus for measuring the corrosion current of an elongated structure immersed in a fluid and provided with a cathodic protection device, the apparatus comprising at least one closed magnetic circuit immersed in the fluid and orientated so that a certain amount of electric current can pass therethrough, a coil on this magnetic circuit, measuring means connected to the terminals of this coil and modulation means (perforated rotating disc) for periodically varying, in time, the quantity of electric current circulating through the magnetic circuit. The invention is applied more particularly to the surveillance of underwater pipe-lines.

Abstract: A metal pipeline detector is described in which a generator generates a magnetic field. The magnetic field couples the metal pipeline when the detector is located in the vicinity thereof, and as a result eddy currents are induced in the pipeline. Detector coils are arranged to be coupled by the magnetic field produced by the eddy currents but not by the magnetic field generated by the generator. The detector coils are spaced apart so that the magnitude of the field coupled in each coil is representative of the distance between the pipeline and the coil. Electrical processing circuitry acts to detect the difference between the magnetic field coupled in each coil so as to determine the degree of which the detector is displaced from the optimum path for tracking the pipeline. In addition the processing means acts in response to the outputs of the detector coils to provide an indication of the distance of the pipeline to the detector.