Below the seafloor


Geology below the seabed

For many marine geologists, looking at the sediments lying on the seafloor isn't enough. By looking at the layers of sediment below the surface, they can get an idea of the geological history of an area and see what processes have affected the seabed over time. This is particularly important for studying climate change, because sediment layers preserve a record of past climates.

Looking at the subsurface geology is also vitally important in the oil and gas industry. Profiles of the subsurface allow identification of geological features which can trap hydrocarbons, or cause them to escape. It can also tell scientists whether the area is likely to hold oil or not.

To look below the seafloor, we need specialised equipment. For a direct view, we can use sediment coring or drilling which reomves a cylinder of sediment from the seabed. This gives very detailed information on the sediments for a specific point on the seafloor. A broader view of the subseafloor can be gained by using seismic profiling.

Sediment coring

Sediment coring is a common way of getting very detailed information about the layers of sediment below the seafloor. Sites for coring are usually chosen based on results of an earlier sidescan survey, where target areas have been identified. There are several different types of corer, depending on what sort of sediment is on the seafloor, and what sort of sample needs to be taken.

A coring device comprises a steel cable attaching the equipment to the ship's winch, weights to help drive the corer into the seabed, and a steel tube known as the core barrel, which is the part of the corer which is pushed into the seafloor.

Gravity coring uses the weight of the coring equipment to push the core barrel into the soft mud at the bottom of the ocean. Weights may be added to the top of the corer to make sure that the core barrel is pushed far enough into the seafloor sediment. Piston corers are similar, but they have a piston inside the core barrel which provides suction to help pull to sediments up into the core barrel. An average core is between 5 and 20 metres long.

Once the corer has been brought back on board the ship, the core is removed. Plastic tubes (rather like drainpipes) are used to line the core barrel and keep the sediment core in one piece when it is removed. Cores are split in half lengthways to reveal the layers of sediment inside.

Left: Bringing up the coring apparatus

Above: How a corer works


Rock drilling

For areas where the seafloor is rocky, a different type of drilling kit is required. The ‘BRIDGE’ rock drill (shown on the right) was built by the British Geological Survey in the mid 1990s. The tripod rig is lowered to the seafloor on an electrically conducting cable, and takes metre-long cores by drilling vertically downward into the hard rocky seabed using a rotary diamond drill. It has so far been used in water depths of up to 4500m. An onboard camera is mounted on the drill frame so that the scientists can hit their targets accurately. A unique feature of the device is that the core samples are geographically orientated: they are marked as they enter the core barrel, and the marks refer to compasses on the drill frame. This is an extremely important feature, as it allows scientists to study the spatial relationships of properties of the cores – for example, their magnetisation directions, the preferred orientations of crystals, of fractures, and so on. To date the BRIDGE drill is the only tool available that can collect reliably orientated samples from hard-rock seafloor.

Special drilling ships, like the JOIDES Resolution (right) which is operated by the Ocean Drilling Program, have a special drilling rig on board the ship. The rig is 60m high, with a hole through the ship directly below, so that the drill string can be lowered through the centre of the ship and into the ocean. This set up allows drilling in very deep water and/or drilling of very deep holes into the seabed. Cores taken using this ship can be thousands of metres long, depending on the water depth. Drilling using this method requires the drill string to rotate, like a household drill, rather than relying on gravity like the more traditional coring methods.


Seismic Reflection Profiling

By using sound waves, we can create an image of the geology below the seafloor. A low frequency, high energy sonar pulse is emitted from an instrument towed behind the ship. These sound waves penetrate through the seabed and reflect off sediment layers and structures present in the subsurface. Reflected sound waves are detected and recorded by a sensor mounted on the seismic instrument. Once processed, these signals create a black and white image of a 2D section through the seafloor.

Left: Example of a seismic profile from the Rockall Trough, NW of Scotland. You can see that the seabed is composed of several layers of sediment which have a 'wavy' structure.

Some early seismic instruments relied on explosives to achieve sufficient sonar penetration into the seabed. Fortunately, today's technology is less hazardous!


Find out more about the marine environment:

Exploring below the seabed
Underwater landscapes
Ocean exploration: Part 1
Ocean exploration: Part 2
Manned exploration of the deep


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© NOCS
February 2007