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geophysical methods of underwater survey include (1) a proton magnetometer towed behind a survey vessel, so as to detect iron and steel objects which distort the earth's magnetic field; (2) sidescan sonar that transmits sound waves in a fan-shaped beam to produce a graphic image of surface features on the sea-bed; (3) a sub-bottom profiler that emits sound pulses which bounce back from features and objects buried beneath the sea floor.