Iran has obtained sophisticated acoustic naval mines from Libya in exchange for poison-gas artillery munitions, U.S. officials said. Until now, Iran has used only contact mines in the Persian Gulf. These explode when hit by a ship's hull while the new acoustic mines, made in Yugoslavia, are detonated by the sound of passing ships and are difficult to sweep up. In exchange, the U.S. officials said, the Iranians provided Libya with poison gas artillery shells, probably containing mixtures of nerve gas and mustard gas. Libyan is believed to want the shells for possible use in its war with Chad.