text: Artillery To keep enemy action to a minimum during the assault, a creeping barrage was planned. This required gunners to create a curtain of high explosive and shrapnel shell explosions that crept across the battlefield in lines, about one hundred metres in advance of the assaulting troops. The Allies had previously used creeping barrages at the Battle of Neuve Chapelle and the Battle of the Somme but had encountered two technical problems. The first was accurately synchronising the movement of the troops to the fall of the barrage: for Arras, this was overcome by rehearsal and strict scheduling. The second was the barrage falling erratically as the barrels of heavy guns wore swiftly but at differing rates during fire: for Arras, the rate of wear of each gun barrel was calculated and calibrated accordingly. While there was a risk of friendly fire, the creeping barrage forced the Germans to remain in their shelters, allowing Allied soldiers to advance without fear of machine gun fire. The new instantaneous No. 106 Fuze had been adapted from a French design for high-explosive shells so that they detonated on the slightest impact, vaporising barbed wire. Poison gas shells were used for the final minutes of the barrage.
  The principal danger to assaulting troops came from enemy artillery fire as they crossed no man's land, accounting for over half the casualties at the first day of the Somme. A further complication was the location of German artillery, hidden as it was behind the ridges. In response, specialist artillery units were created to attack German artillery. Their targets were provided by 1st Field Survey Company, Royal Engineers, who collated data obtained from flash spotting and sound ranging. (Flash spotting required Royal Flying Corps observers to record the location of telltale flashes made by guns whilst firing.) On Zero-Day, 9 April, over 80 per cent of German heavy guns in the sector were neutralised (that is, "unable to bring effective fire to bear, the crews being disabled or driven off") by counter-battery fire. Gas shells were also used against the draught horses of the batteries and to disrupt ammunition supply columns.