A number of archaeological groups and marine service companies are acquiring boat-towed metal detectors to assist in locating shipwrecks and to perform geophysical surveys. These devices can locate a variety of targets including the piles of magnetic ballast stones found on many old wrecks, gold and silver bars, cannons, anchors, pipelines, cables, and various metal debris which must be removed from an area before dredging. The two primary pieces of equipment used in these operations are a magnetometer and pulse induction (PI) metal detectors.