Clearing the Mines

A U.S. Navy task force of more than 20 vessels gathered in the Gulf of Tonkin last week for the start of one of the nation's first big postwar missions. Code-named "Endsweep," the operation will search out and destroy the thousands of mines that the U.S. laid down last May to cut off shipping to Haiphong and six other North Vietnamese ports.

Unlike the tethered globes that floated on or near the surface during World Wars I and II, most of these new weapons lie on the bottom, waiting to be exploded by ship noises, the magnetism of a ship's steel hull or even water-pressure changes caused by a ship's approach. The mines were designed to deactivate themselves automatically at some predetermined time, but that time has apparently not yet come. So the U.S. promised, as part of the final cease-fire agreement, to go and clear all of the ports.

Endsweep, commanded by Rear Admiral Brian McCauley, will use some new minesweeping vehicles, about 50 of the huge Sikorsky "Sea Stallion" helicopters. Normally equipped with two 2,850-h.p. engines, the eleven-ton Sea Stallion has the strength to haul the heavy and complicated electronic gear needed for locating and detonating mines. There are four methods:

> Tethered mines will be cut from their cables by a severing wire pulled along by a low-flying helicopter. The freed mines will then float to the surface, where they can be harmlessly detonated by gunfire.

> Acoustic mines will be detonated by an electronic device that accurately and effectively reproduces shiplike noises in varying frequencies.

> Magnetic mines will be exploded by magnetic cables mounted on "sleds" towed by a helicopter flying safely overhead, as much as 60 feet above the surface.

> The "Mark 106" system, considered one of the Navy's most sophisticated anti-mine devices will also be towed by helicopter, and it produces a "multiple output" of signals that can detonate acoustic, pressure or magnetic mines.

Getting the mines out could take several months, the Navy says, and will start only when the North Vietnamese government approves the final mine-sweeping plan. Estimated cost: more than $1.3 million a day.

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