A mooring in oceanography is a collection of devices connected to a wire and anchored on the sea floor. It is the ''Lagrangian and Eulerian specification of the flow field, Eulerian way'' of measuring ocean currents, since a mooring is stationary at a fixed location. In contrast to that, the ''Lagrangian way'' measures the motion of an oceanographic

drifter A drifter is a vagrant who moves from place to place without a fixed home or employment. Drifter(s) or The Drifter(s) may also refer to: Films and television Films * ''The Drifter'' (1917 film), an American film directed by Fred Kelsey * ''Th ...

, the Lagrangian drifter.

Construction principle

The mooring is held up in the water column with various forms of buoyancy such as glass balls and syntactic foam floats. The attached instrumentation is wide-ranging but often includes CTDs (conductivity, temperature depth sensors),

current meter A current meter is an oceanographic device for flow measurement by mechanical, tilt, acoustical or electrical means. Different reference frames In physics, one distinguishes different reference frames depending on where the observer is located ...

s (e.g. acoustic Doppler current profilers or deprecated rotor current meters), and biological sensors to measure various parameters. Long-term moorings can be deployed for durations of two years or more, powered with

alkaline In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a basic, ionic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of a ...

or lithium battery packs.

Components

Top buoy

Surface buoy with meteorological sensors

Surface buoys

Moorings often include surface buoys that transmit real time data back to shore. The traditional approach is to use the

Argos System Argos is a global satellite-based system that collects, processes, and disseminates environmental data from fixed and mobile platforms around the world. The worldwide tracking and environmental monitoring system results from Franco-America ...

. Alternatively, one may use the commercial Iridium satellites which allow higher data rates. Syntactic foam sphere

Submerged buoys

In deeper waters, areas covered by

sea ice Sea ice arises as seawater freezes. Because ice is less dense than water, it floats on the ocean's surface (as does fresh water ice, which has an even lower density). Sea ice covers about 7% of the Earth's surface and about 12% of the world's o ...

, areas within or near shipping lines or areas that are prone to theft or vandalism, moorings are often submerged with no surface markers. Submerged moorings typically use an acoustic release or a Timed Release that connects the mooring to an anchor weight on the sea floor. The weight is released by sending a coded acoustic command signal and stays on the ground. Deep water anchors are typically made from steel and may be as large as 100 kg. A common deep water anchor consists of a stack of 2–4 railroad wheels. In shallow waters anchors may consist of a

concrete block A concrete masonry unit (CMU) is a standard-size rectangular block used in building construction. CMUs are some of the most versatile building products available because of the wide variety of appearances that can be achieved using them. Tho ...

or small portable anchor. The buoyancy of the ''floats'', i.e. of the top buoy plus additional packs of glass bulbs of foam, is sufficient to carry the instruments back to the surface. In order to avoid entangled ropes, it has been practical to place additional floats directly above each instrument.

Instrument housing

Prawlers

Prawlers (profiling crawlers) are sensor bodies which climb and descend the cable, to observe multiple depths. The energy to move is "free," harnessed by ratcheting upward via

wave energy Wave power is the capture of energy of wind waves to do useful work – for example, electricity generation, water desalination, or pumping water. A machine that exploits wave power is a wave energy converter (WEC). Waves are generated by win ...

, then returning downward via gravity.

Depth correction

Similar to a

kite A kite is a tethered heavier-than-air or lighter-than-air craft with wing surfaces that react against the air to create lift and drag forces. A kite consists of wings, tethers and anchors. Kites often have a bridle and tail to guide the fac ...

in the wind, the mooring line will follow a so-called (half-)

catenary In physics and geometry, a catenary (, ) is the curve that an idealized hanging chain or cable assumes under its own weight when supported only at its ends in a uniform gravitational field. The catenary curve has a U-like shape, superfici ...

. The influence of

current Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (stre ...

s (and

wind Wind is the natural movement of air or other gases relative to a planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few ...

if the top buoy is above the sea surface) can be modeled and the shape of the mooring line can be determined by software. If the currents are strong (above 0.1 m/s) and the mooring lines are long (more than 1 km), the instrument position may vary up to 50 m.

References

{{physical oceanography, expanded=other Oceanography Physical oceanography Oceanographic instrumentation Ocean currents Biological oceanography de:Boje (Schifffahrt)