decks) (former Southern Pacific freighter El Cid carried 830 mines on 3 decks) (former Southern Pacific freighter El Rio carried 830 mines on 3 decks) (former Old Dominion steamship Hamilton carried 612 mines on 2 decks) (former Old Dominion steamship Jefferson carried 612 mines on 2 decks) The mine barrage consisted of 18 rows of mines laid in an east–west direction. Ten rows of mines were laid at a depth of to be detonated by ships traveling on the surface. Submerged submarines were targeted by four rows of mines at , and another four rows at . Since Utsira is slightly north of Orkney, alignment of minefields within the central area A was skewed east-northeasterly from Orkney. Where possible, longitude was determined from a calibrated taut-wire anchored near a landmark and unreeled from a spool of piano wire aboard one of the cruisers acting as the minelaying formation guide. Latitude was checked from the elevation of the sun when atmospheric conditions permitted. The mine barrage required multiple missions, called "excursions", laying parallel rows of mines partway across the North Sea between Norway and Orkney. Mine Squadron One made thirteen two-day minelaying excursions laying parallel rows of mines while steaming in columns apart, with the last ship in each column dropping mines at intervals. As a minelayer exhausted its supply of mines, another minelayer in that column would drop back to the last position to continue the minelaying sequence. The minelayers were preceded by Royal Navy destroyers sweeping for enemy mines and submarines. A covering force of battleships with Royal Navy cruiser squadrons maneuvered nearby to defend the minelaying formation, but no German surface warships attempted engagement. Buoys were dropped temporarily marking the end point of a mining excursion to avoid leaving an unmined gap when the next excursion started. These buoys were subject to potential movement by storms or enemy action. Three to five percent of the new mines dropped into the North Sea detonated as soon as the salt pellets dissolved; and hydrophones detected premature detonations continuing for a week after minelaying. These premature detonations were initially attributed to activation of the horn fuze detonation circuits by seawater leaking into the mines; and mine spacing was increased from on the first minelaying excursion to on subsequent excursions to minimize leakage caused by detonation of nearby mines. About one percent of the mines deployed during the first excursion broke free