The bottom of a body of water is known as the benthic zone, regardless of how deep it occurs. In coastal waters the sea floor sits upon the continental shelf and is generally less than 200m deep. The majority of the ocean floor lies upon the ocean crust and is between 4000m to 6000m deep. Working at such depths requires specialist tools and skills to obtain the data needed.
Scientists who study the benthic zone can come from many disciplines and each will be interested in different aspects. The reasons for study are just as varied but can include:
- Understanding – Exploration of areas, learning about ecosystems and habitats.
- Resources – Locations and quantities of minerals, oil, gases, and food supplies.
- Geohazards – Landslides such as the one that caused the 2004 Tsunami, turbidity currents, is it safe to dump waste at sea?
Some Geo-scientists are interested in the terrain of an area, is it flat or rocky? Are there any canyons, mountains or volcanoes? To find out scientists need to map the area. On dry land satellites can be used to measure vast areas, but in water satellite signals (microwaves) can be absorbed. The further a signal travels through water the less likely it is to bounce back, so another method is needed to map the ocean floor.
Sound waves use pressure to move through gases, liquids and solids. In air sound moves at around 340 meters per second but in seawater it zooms along at around 1500 meters per second! Light cannot be used as it is absorbed by water very quickly; usually lighting no further than 30 meters. So by using sound scientists can find out different properties about the sea floor.
|Method used||Property discovered||How it’s done|
|Single-beam echo sounder||Bathymetry – the measurement of depth to the bottom. Can also be used to gain information about the sub-surface i.e. deep or shallow sediment.||Measured using the time it takes for the sound to be sent and returned.|
|Multi-beam echo sounder||Swath bathymetry – by taking lots of depth measurements from a single place the shape of the sea bed is revealed.||Measured using the time it takes for the sound to be sent and returned.|
|Backscatter||Measures the reflectivity of the floor, this will show what the floor is made of e.g. rocks, sand, mud.||The change to the strength of the returned sound wave.|
|Sound Velocity Profilers||Measures the speed sound moves through the water, as it varies depending on the water properties at different depths. Important to know otherwise depth calculations will be wrong.||Measures the temperature, and sometimes the salinity, of the water to determine the speed.|
Towed Ocean Bottom Instrument (TOBI) is an instrumented vehicle which is towed close to the bottom of the deep ocean from a ship, and uses sound to form detailed images of the sea floor. TOBI was developed by NOC and has been in service since 1990, it is has become one of world’s best tools for underwater geological surveys using side scan sonar. Read more about TOBI →
Autonomous Underwater Vehicles (AUVs) are robot submarines, which are used to explore the word’s oceans without a pilot, or any tether. Before launch from the mother ship, the AUV’s computers are programmed with instructions of where to go, what to measure, and what depths to go to. Read more about autosubs →
While seismic surveys are often used by the oil and gas industries to find reserves, scientists can use the same techniques to look for air bubbles, which can signify the presence of hydrothermal vents. Surveys are a complex operation that require skilled specialists to operate the equipment. Read more about seismic exploration →
After mapping an area a scientist may want to choose a few areas of interest to take samples from. The equipment they will use depends upon whether they wish to take sediment or rock samples.
Sediment lies at the bottom of the ocean floor; it is made from many items, including:
- tiny particles of rock, sand, silt and clay
- marine snow – clumps of living and dead microscopic organisms, faecal pellets and dust
- materials vented out from the Earth’s surface
Over time the sediment forms layer upon layer building up a record of the ocean floor at that moment. Scientists examine these layers to find out about the past. For example the size of particles shows how close to the shore the ocean floor used to be, because large particles sink faster and therefore nearer to land.
Corers are used to collect sediment from the ocean floor, they work by pushing or grabbing sediment into containers. Within the National Marine Equipment Pool there are eight different types of corer with both tubular and box varities available. Read more about corers →
Cores are an expensive and unique resource of immense scientific value, analysis of which can provide clues about climate change, conditions in past oceans and sedimentary processes. The British Ocean Sediment Core Research Facility (BOSCORF), based at NOC Southampton, is the UK’s national deep-sea core repository. Read more about sediment cores →
Rock samples are collected because they can tell scientists about:
- What the rocks are made of;
- How the rock was formed;
- Mineralisation – minerals can be an important resource;
- The mantle – the layer underneath the Earth’s crust;
- Earth’s evolution – how tectonic plates used to be arranged; and
- Future geohazards – earthquakes, volcanoes, tsunamis.
Dredges gather loose rocks sitting on the ocean floor using a technique that has changed little in hundreds of years. They have a chain-link bag with large metal-jawed opening that scoops the contents into the bag. They are lowered to the seabed on a cable and dragged along the bottom for some distance before being brought to the surface. Read more about dredges →
A variety of equipment can be used to get samples of rock that are not loose. If the rock is soft like the mineral rich chimneys of a hydro-thermal vent it may be picked up using grabbers or pincers, as found on the video-guided robotic underwater vehicle HyBIS or the ROV Isis. Isis is part of the National Marine Equipment Pool (NMEP) but HyBIS is a specialist instrument operated by the Marine Geosciences Group at NOC.
A Wax Corer, or rock chipper, is used to collect fresh volcanic glass. This material is the gold standard for volcanic studies as it represents the magmatic liquid rock. The rock chipper will smash into the ground and capture shards of glass in wax containers. When the containers are returned the wax is melted in beakers of hot water and the contents simply fall to the bottom. For deeper rock samples, rock drills can be used to take core samples of bedrock in a similar fashion to sediment cores. The specialised marine drills aren’t part of the NMEP but are available from the British Geological Survey.
Unlike sediments there is no national collection of oceanographic rocks, so rocks samples are housed in various institutional collections.