All of the exploration for petroleum accumulations in sedimentary basins was done by the use of surface methods. Exploration employs mostly subsurface techniques. Surface methods include the observation of direct indications of the presence of underlying petroleum, such as oil and gas seeps, and geological and geochemical prospecting. The reflection method, which works on the echo sounding principal, is the most common modern tool for subsurface exploration. Geological exploration of the outer continental shelf is accomplished by bottom sampling, shallow core drilling, and deep stratigraphic test drilling. Seismic exploration provides additional information at depth by measuring the velocity of shock or seismic waves through the various rock formations beneath the seafloor. Exploration in Arctic waters creates additional problems such as icebergs and superstructure icing of the drill ship. The proper completion of an oil well is essential for the control of, and maximum recovery from, a reservoir.