A magnetometer detects changes in the Earths’ magnetic field that can vary naturally over a wide area or locally over metallic objects such as shipwrecks. The tesla (symbol T) is the unit of measurement of the strength of a magnetic field. The ‘Maggy’ is towed behind the vessel to avoid interference. Its cylindrical shape and buoyancy are designed to reduce risk of snags with moored gear. INFOMAR uses a Marine Magnetics Corporation SeaSPY towed Overhauser Magnetometer to acquire magnetic field data.