Metal detectors (handheld and towed) are used extensively in shipwreck salvage. There are several common technologies used in metal detectors, but that most often utilized in underwater metal detectors is called Pulse Induction. In simple terms, the pulse induction machine simply fires a pulse of electromagnetic energy into the area around the coil. In the absence of metal, the ‘spike’ of energy decays at a uniform rate, and the time it takes to fall to zero volts can be accurately measured. However, if metal is present when the machine fires the pulse, a small current will flow from the metal, and the time for the voltage to drop to zero will be increased. These time differences are minute, but modern electronics make it possible to measure them accurately and identify the presence of metal at a reasonable distance.
Pulse Induction detectors have one major advantage over other metal detector technologies: they are virtually impervious to the effects of mineralization, and small objects such as rings and coins can be located even under highly-mineralized ‘black sand’. They do have one disadvantage, which is that the currently available technology lacks the ability to incorporate much discrimination into a Pulse induction detector. So, they detect everything made of metal. This is usually not a problem when salvaging a shipwreck, since virtually everything is “treasure”. Metal detectors have an advantage over magnetometers in that they can detect virtually any metal object, whereas magnetometers can only detect ferrous metals.
Pulse induction detectors come in a variety of sizes and shapes. Several manufacturers make large coils (36 inches to 48 inches) which are towed behind a boat. The primary limitation of a Pulse Induction metal detector is the distance from the coil at which an object can be detected. With a normal 8-inch or 10-inch loop, a large silver coin can be detected between 12 and 18 inches under the sand. With a larger loop, objects such as cannons or anchors may be detected up to several meters under the sand or sediment. With these limitations, metal detectors are primarily used during the salvage phase of an operation, although the larger, towed units are used to pinpoint non-ferrous objects (bronze cannon, large silver bars, etc.) in an area where a shipwreck has been widely scattered by the elements.