How the Underwater Mines Work

MOST people envision underwater mines as the sort of studded black balls that Gary Grant dodged in Destination Tokyo. But the delayed-action mines used to seal off North Vietnamese ports last week are considerably more complex. Sown by low-flying Navy planes, some of them were dropped to the surface by parachute; others, equipped with tail fins, plunged straight to the water. Then they were programmed to settle at various depths in patterns designed to frustrate enemy minesweepers. Some were probably sent to the bottom while others were moored by cables. The mines used last week were not the most sophisticated the U.S. possesses--the risk was too great that one of them, packed with advanced technology, might be recovered and eventually wind up in a Russian ordnance laboratory.

A minefield is generally seeded with a variety of devices. Some explode on contact. Some detonate magnetically when they pick up the magnetic field of a passing ship. Others explode at an acoustical cue, such as a ship's propellers alongside or overhead. Still others go off when a ship's hull increases the water pressure. A mine's relatively simple computer can be programmed to react to combinations of signals. Thus some mines are equipped with "counters." They will allow, say, nine ships to pass by and then blow up the tenth. Such mines greatly increase the dangers of minesweeping, since the sweeper may be the fatal tenth vessel.

Some mines can sit on the bottom for a time, awaiting a coded signal to activate. On signal, they can propel themselves through the water to a different position, or can search for a target.

Those who doubt the effectiveness of the mining operation point out that incoming cargo ships might stop outside the minefield and then unload their supplies onto shallow-draft wooden boats that might pass over the field without being detected. As a countermeasure, the Navy might set its mines to go off at extremely faint signals. With such hair triggers, however, the mines could be detonated by a strong current or even by a large passing fish.

Eventually, at some "predetermined time" which the Pentagon of course will not discuss, the mines will deactivate themselves automatically. Meantime, the Soviets may employ some of their surface minesweepers --they have more than 350--to try to clear the ports. If they do so, the U.S. could send in more planes and sow fresh minefields.

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