Marine Instrumentation
Marine Instrumentation is required for:
- Marine Geology
- Marine Biology
- Physical Oceanography
- Chemical Oceanography
- Remote Sensing
Instruments can be used on
- Research and other vessels
- Fixed platforms
- Satellites
- Submersibles and towed vehicles
- Floats and drifters
Data collection instrument types
To measure hydrographic properties of the sea (temperature, salinity, oxygen, nutrients,
currents, etc), instruments are required that can be lowered into the water (to
the bottom, if required) to record and take sea-water samples for later analysis
in the lab. Typical instruments include:
- CTDs
- Rosette samplers (bottles)
- ADCPs
- XBTs
- Submerged drifters (ARGO floats)
- Surface drifters
- Moorings
CTD
What is it and why do we use it?
A CTD stands for Conductivity, Temperature, and Depth (profiler).
- Primary tool for determining essential physical properties of sea water.
- Can measure temperature to 1/1000 of a degree, and salinity to 1/1000 parts per thousand.
- Made up of a set of small probes attached to a metal body, and measurements can be done at up to 30 readings per secondCTD is lowered on a cable down to the seafloor collecting data on the way.
- A CTD cast, depending on water depth, requires 30 minutes to 2 hours.
- Water sampling is often done at specific depths so scientists can learn what the physical properties of the water column are at that particular place and time.
Rosette sampler
Some analysis need to be done in the lab, or just to calibrate the CTD readings. For these purposes, samples of sea-water are taken by fixing a rosette sampler to the CTD. Examples can be seen in die previous figures showing the CTD.
The rosette sampler consists of a number of bottles, each with a volume of 5 – 20 litres.
The samples can be triggered from the vessel at preselected depths.
When the CTD+ rosette is brought back on deck, samples can be tapped from the bottles (left), and taken to the lab for processing.
ADCP
What is it and why do we use it? An Acoustic Doppler Current Profiler (ADCP). Measures flow speed and direction of water. Can be mounted on a ship or anchored on/close to the sea bed
How does it work?
- The ADCP measures water currents with sound, using a principle of sound waves called the Doppler effect.
- The ADCP transmits "pings" of sound at a constant frequency into the water.
- As the sound waves travel, they encounter particles suspended in the moving water, and are reflected reflect back to the instrument.
- Due to the Doppler effect, sound waves bounced back from a particle moving away from the profiler have a slightly lowered frequency when they return.
- The difference in frequency between the waves the profiler sends out and the echoes it receives is called the Doppler shift.
- The instrument uses this shift to calculate how fast the particle and the water around it are moving.
ADCP Data: The image (below) shows vectors depicting current direction (arrows) and speed (relative length of vector), recorded on the RV Melville. Courtesy of Chief Scientist Dr Lisa Beal (Miami) in a study of the Agulhas Current undertaken during 2003.
XBT
How do they work?
- An XBT is a small probe that is dropped from a ship and measures the temperature as it falls through the water.
- Two very small wires unroll while the probe is falling, and transmit the temperature data to the ship where it is recorded for later analysis.
- The probe is designed to fall at a known rate, so that the depth of the probe can be inferred from the time since it was launched.
- The probe is not recovered
Advantages
- Can be deployed underway.
- Rapid transfer of temperature.
- Data can be obtained over 1000 m.
Disadvantages
- Only Temperature is recorded (conductivity later added).
- Accuracy not as good or as deep as a CTD.
Underway observations
- XBT observations are used to investigate the large-scale, low-frequency modes of climate variability
- XBT lines are ideally covered 12 times per year with an XBT drop every 150 to 200 km.
Submerged difters ARGO floats
What are they and why do we use them?
- Argo floats are designed to sink to a pre-programmed depth, normally 1000m, and remain (and drift) there for about 9 days.
- After this period, the instrument rises automatically (or descends to 2000 m first and then rises) to the surface, where it remains for about one day. During the rise a CTD records the temperature-conductivity profile of the water.
- While at the surface the data and position are transmitted to orbiting satellites.
- The float then sinks again, and repeats the cycle.
- A float lifetime is typically a few years.
- Argo floats collect the oceanographic data necessary to understand the processes that influence our global climate.
- They eliminate the need for a research vessel and team.
Global distribution of Argo floats:
“Argo is a global array of 3,000 free-drifting profiling floats that measures the temperature and salinity of the upper 2000 m of the ocean. This allows, for the first time, continuous monitoring of the temperature, salinity, and velocity of the upper ocean, with all data being relayed and made publicly available within hours after collection.” To learn how to access this data visit http://www.argo.ucsd.edu/Argo_.
The most recent picture of the Argo array.
Surface drifters
- Drifters are primarily deployed in support of meteorological data collection programmes. They collect data without the need for expensive vessels, and in areas where ships go only occasionally.
- They drift freely with the surface ocean currents.
- They have now become the backbone of a new observing system that covers the entire ocean.
- Surface drifters have a float at the surface and can therefore transmit data via satellite. Surface metrological data is collected with sensors on top of the float and a temperature and occasionally a salinity sensor below the float. They are fitted with a "sea anchor" or “holey sock”. The sea anchor is a 15 m long tube of 1 m diameter with a series of holes.
- Sensors include air pressure, solar radiation and sea surface temperature (SST) and a sensor to indicate whether the float is pushed under by strong waves.
The Global Drifter Program
Satellite-tracked surface drifting buoy observations of currents, sea surface temperature, atmospheric pressure, winds and salinity.
Image: http://www.aoml.noaa.gov/phod/dac/gdp.html
Moorings
- Moorings are used wherever measurements are required at one location over an extended time period.
- Their design depends on the water depth and on the type of instrumentation for which the mooring is deployed, but the basic elements comprise an anchor, a mooring line (wire or rope) and one or more buoyancy elements which hold the mooring upright and preferably as close to vertical as possible. Instruments are attached to the mooring line, and they record data internally.
- If the buoyancy element is located at the surface the mooring can be subject to severe wave and wind effects.
- A subsurface mooring is recovered by remotely uncoupling the mooring from the anchor weight, allowing the mooring to rise to the surface where it is picked up.
Although the mooring instruments can capture a time series of the temperature, salinity, nutrients, oxygen, etc, the limited spatial coverage is compensated by deploying a number of moorings (referred to as an array).
Extensive arrays have been deployed as part of large climate-related programmes to study El Nino, such as TOGA (Tropical Ocean Global Atmosphere (see large number of moorings along the equator in the Pacific Ocean ). Such programmes require an enormous investment in terms of equipment, ships’ time, operational expenses and manpower.
Most of the arrays deployed by organisations in Southern Africa are in depths of a few hundred metres on the shelf. Some deep arrays in this region have been deployed by overseas research organisations.