Abstract: A method of surveying a cathodically protected metallic structure at least partly immersed in a liquid, such as an undersea pipeline which has sacrificial anodes attached at regular intervals along its length. The anodes provide a known electrolytic potential relative to the surrounding water. The potential differences in the water between a point adjacent an anode, and test points adjacent the pipeline are measured, and the varying level of protection along the pipe is determined from the variations between the measured potential differences.