Underwater Pipe Detection using Passive Magnetic Sensing

The detection and tracking of underwater assets can be challenging in high turbidity waters. Solutions based on magnetic sensing are promising for the detection of metallic objects. These typically rely on a magnetic field source (permanent magnet or electromagnet) to magnetize surrounding ferromagnetic objects and increase their magnetic presence so that these objects can be detected by the sensing electronics. Electromagnets have the disadvantage of requiring large amounts of power. Permanent magnets can disrupt and/or oversaturate onboard sensors and are attracted by ferromagnetic objects, increasing the risk of collision. To solve this issue, this paper presents a new device that uses a passive method of magnetic sensing for detecting ferrous objects underwater. The device measures the minute distortions of the Earth’s magnetic field (geomagnetic field) caused by ferromagnetic objects while filtering out the overall effects of the much stronger geomagnetic field itself. A prototype was built using a linear array of off-the-shelf 3-axis magnetometers, mounted on an underwater Remotely Operated Vehicle (ROV), then used to detect and map a vertically-mounted steel pipe in a pool.