In most jungle-covered areas, the use of wheel or even tracked vehicles is nigh impossible and all the party's field equipment is portable, because it is necessary to hand carry it along the seismic lines. Rivers provide access to the seismic lines at various points and helicopter clearings are made in the jungle as the lines are being cut. The field personnel live in fly camps along the line for weeks at a time and return to base camp to rest, dry out the equipment, and undertake any necessary repairs. The base camp is generally located on the banks of a river and the bulk supplies are delivered by water transport.