Marine seismic survey is a method of determining geological features below the sea floor. It works by sending sound waves into the rock layers beneath the sea floor and then recording the time it takes for each wave to bounce back as well as measuring the strength of each returning wave. Drilling exploration offshore is undertaken by a variety of mobile rigs suited to operating in different water depths. These include: Jack-up rigs are used in water depths of up to approximately 120 metres water depth. Semisubmersibles are the most common type of floating drilling unit and are used in water depths of 100 meters to 3000 metres. Drillships are vessels fitted with drilling equipment. They are used for exploration drilling for petroleum and for scientific drilling. Modern drillships are able to drill in water depths of up to 3000 metres. In order to drill, a marine riser is lowered to the seabed with a blowout preventer (BOP) at the bottom that connects to the wellhead. The oil and gas is extracted from the field then either transferred by pipeline to shore for further processing and distribution or loaded directly on to oil tankers for export.