A magnetometer works on the same principle as a metal detector; however, it covers a larger area very quickly. Like a metal detector, a magnetometer picks up the interruption in the earth's magnetic field created by metal objects. As the magnetometer is being towed from a boat, it picks up these differences and signals the operator, usually by a beeping signal, when they are over a metal object. The tenders then mark the location with surface buoys and either divers search that location or some type of video is sent down. A magnetometer works well for finding iron and nickel objects. It can cost $3,000 to $20,000.
The Fishers Proton 4 Magnetometer is illustrated in these pictures. For more information, you can contact JW Fisher at: www.jwfishers.com