An air lift is a tool for excavating small holes on a sandy seabed. Because it is easily controlled, and works only on the loose soil it is very popular with subsea archaeologists.
It is built up of a large pipe with an air intake at the bottom:
Air Hose to
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The air is injected
into the pipe, and blends with the water ("aerating
" the water). The added air lowers the specific gravity of the water, and because it is lighter than the surrounding water, it is pushed upwards.
This creates suction from the bottom of the pipe where sand and small stones can be sucked in. The sand is then expelled at the top of the pipe where it is taken away by the current. Because it doesn't carry the sand far away it is mostly useful for smaller excavations.
It is quite popular to also fit the main suction pipe with a water jet. This jet
will then pierce and fluidise the seabed making it possible to also remove clay and anything more solid than sand that's on the seabed. Large rock (10-20 cm) is still impossible to move using this method.
The pipe is often manufactured from either steel or glass-fiber reinforced plastics (GRP). It is common to suspend the air lift in a winch or crane so that the diver or ROV only guides it horizontally. This is especially true if the air lift is manufactured in steel and fitted with a lot of accessories (double suction pipes, double air hoses, water jets, handles, pipeline guides - you name it, we weld it on). The air hose carries its own weight, though when in use it is often bouyant. For large air lifts, the air is often compressed on the vessel and then pumped down, while for smaller tools one can bring a pressure cannister with compressed air.
Since it relies on the compressed air to function, it decreases in efficiency with greater depth. This is because the amount of air required to fill the same volume increases with depth. It is a handy tool at depths up to about 130 m, after that other excavation methods become more cost effective.