| Direct Measurements | • | Moored Instruments | • | Remote Sensing |
Direct Measurements
Manual oceanographic data collection can be undertaken by anyone with the correct equipment and access to the sea. Investigation by individuals is, however, often limited to small areas over short periods. At the other end of the research spectrum, national-level marine research in the Republic and Northern Ireland benefits from dedicated research vessels, which provide fully equipped and stable platforms from which large-scale temporal and spatial oceanographic data collection can be undertaken.
The
RV Celtic Explorer
 The RV Celtic Explorer berthed at Cobh, Co. Cork. The RV Celtic Explorer is Ireland’s largest research vessel and was commissioned at a cost of € 31.6 million in 2003. It can remain at sea for up to 45 days therefore allowing deep sea exploration off the continental shelf. This vessel is over 65 m long and is acoustically silent, an important condition for oceanographic surveys. It carries a range of scientific equipment including a hull-mounted multi-beam sonar which has proved invaluable during the recent Irish National Seabed Survey. Scientists and researchers can book time aboard the vessel to carry out specific oceanographic investigations. |
Ireland’s position on the Atlantic margin of Europe means that oceanographic research is of prime importance on local, national and international levels. This validates both small-scale and regional investigations and has provided the impetus for investment in the island’s marine survey resources. The result of this investment, e.g. dedicated marine research vessels and staff, is that Irish marine research can be conducted to the highest international standards.
Water Sampling
 Water sampling apparatus. |
The measurement of salinity and oxygen, nutrients and tracer concentrations for water quality testing and pollution control, requires the collection of water samples from various water depths.
Considerations for water sampling include: sample containers, procedures and transport, processing and analysis. Sample containers must be suitable for sampling the water without affecting the compound. Sampling procedures should be rigorous, ensuring that a representative sample is collected and no contamination occurs. Samples may need to be filtered or stabilised before analysis, a process sometimes undertaken in the field following collection. Transport of samples should occur in appropriate conditions, often in a dark refrigerated cooler.
Conductivity-Temperature-Depth (CTD) Profiling
Conductivity and temperature data from the water column can be collected using a Conductivity-Temperature-Depth (CTD) Vertical Profiler, a self-powered, self-contained micro-processing unit capable of collecting temperature, conductivity, and depth data. CTD profiling involves the principle of electrical measurement: a platinum thermometer changes its electrical resistance with temperature and when incorporated in an electrical oscillator, a change in its resistance produces a measurable change in the oscillator frequency. The conductivity of seawater and pressure changes can be measured in a similar way by a frequency change in the instrument’s second and third electrical oscillators. The combined signal is sent up through the single conductor cable on which the CTD is lowered from the survey platform. This produces a continuous reading of temperature and conductivity as functions of depth at a rate of up to 30 samples per second, which are processed, and corrected and can be used to calculate salinity and density. Calibration of the instrument is required prior to sampling.
Water Transparency / Turbidity
The transparency of ocean water is dependent upon the level of suspended foreign material (e.g. particulate matter) present. Significant changes in transparency / turbidity may arise from:
- Erosion, e.g. where sediments from the coast are extracted and transported by rivers and currents;
- Plankton blooms, e.g. nutrients can cause increases in phytoplankton growth;
- Pollution;
- Water mixing influencing the seabed, which disturbs existing sediments, introducing them into the water column.
Measurement of water transparency / turbidity is carried out using the Secchi Disc, a white (or black and white) plate, 30cm in diameter, which hangs horizontally from a non-stretchable measured rope. During sampling, the Secchi Disc is lowered into the water until it can no longer be seen. At this point the depth of the Disc is measured from the submerged rope. The Disc is then raised slowly until it again reaches the point of visibility. The depth of the Secchi Disc is again noted. The depth of water transparency is the mean of these two measurements. As a general rule, light can penetrate to twice the depth of Secchi Disc-visibility.
Measurement of water transparency / turbidity ideally takes place from the shaded leeward side of a drifting vessel, where there are minimal reflections and the water is smooth. Recent meteorological details, which have the potential to affect water turbidity, in addition to sun-angle, are usually noted with the sampled depth figures. As transparency / turbidity varies tidally and seasonally, repeat observations in one position over an annual cycle provide more value than individual samples.
Seabed Sediments
 A 6m seabed corer is deployed in the Irish Sea from the Irish Lights Vessel Granuaile. |
- Provide an insight into ocean history: as sediments generally accumulate on the sea floor at a rate of 0.1 to 10mm per 1000 years, vertical cored samples can retrieve material of significant age (e.g. material deposited between 100,000 to 10 million years ago can be extracted from a 1m core);
- Provide information on the nature and composition of the seabed: seabed sediments may be investigated remotely (see section on acoustic sonar) or physical sampling. While sonar surveys provide acoustic images of large areas of seabed, sediment sampling is required to confirm the interpretation of sediment types. Knowledge of the nature and composition of the seabed can be used to determine anchoring conditions and the strength and direction of the bottom currents, for marine aggregate identification, and during the decision-making process for marine developments, e.g. cable and pipeline routing.
Samples are best obtained by a diver or by corer, anchor or dredge methods. The more significant in size the sample, the more accurately the composition of the seabed in that area can be described. Sediment composition may be described by, among others, its grain size, porosity, colour, texture, mineralogy and biogenic content, e.g. organic matter and calcium carbonate from shell material. Moored Instruments
Where the objectives of data collection require repeated sampling from the same point over long periods (e.g. seasonally), moored instrumentation with pre-set or automatic sensors for oceanographic data collection is a comprehensive, cost-effective and labour-saving alternative to manual sampling. For example, instead of the manual CTD recording and the Secchi Disc method for assessing water transparency / turbidity, a multi-parameter water quality data-logger with turbidity sensor, moored 1m above the seabed and sampling at a 15min interval would provide a more comprehensive dataset when longer-term information is required. Other benefits are:
- The use of one instrument mooring for all sensors maximizes the potential for data correlation between data sets, as all data are collected from the same point; and
- On-site time during instrument deployment, servicing, and recovery is minimised.
|
 Profile and top view of an ADCP showing the four transducer elements in red. |
The ADCP contains acoustic transducers that each produce an acoustic pulse. The transducers are angled off-centre and evenly spaced around the head of the instrument. The instrument transmits acoustic signals into the water column. When the frequency of the transmitted signals is compared with the frequency of backscatter signals reflected off particles in the water, the velocity of the particles, and hence the water, can be calculated.
The ADCP can also be configured with pressure sensors to include the collection of wave data. Parameters may include the significant wave height, peak and mean period, velocity and directional- and non-directional wave spectra. Tide Gauge
 Tide gauge connected through a tube to a data logger on a pier. |
Tidal information is essential for safe navigation, fishing and coastal and marine developments. However, although sea level recording has a relatively long history in Ireland (e.g. measurements in Dublin since 1938), a co-ordinated network of gauges around the coastline has only recently been initiated. This type of system now exists in most other European countries, e.g. in the UK, where the UK National Tide Gauge Network (forty-five gauges, two of which are in Northern Ireland at Portrush, Co. Antrim and Bangor, Co. Down) was set up as a result of severe flooding along the east coast of England in 1953 and is funded by the Environment Agency.
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Remote Sensing
 Artist's impression of the European remote sensing satellite ENVISAT. Source: ESA. |
Solid bedrock and overlying sediments on the seabed can be revealed by analysis of returns (backscatter) from sonar pulses (e.g. from towed side-scan sonar or multi-beam bathymetric instruments). The strength of backscatter of the sound wave varies depending on the type and hardness of the seabed, e.g. smooth bedrock will return a much stronger signal than fine sand or mud, which absorb the sonar pulse. However, while sonar surveys provide acoustic images of large areas of seabed, sediment sampling is required to confirm the interpretation of sediment types. Remote sensing is therefore ideally used in conjunction with a programme of ground-truthing.
To find out more about some of the oceanographic parameters which can be measured from satellites read the section on satellite imagery. Information on seabed parameters that can be measured with sonar and seismic sensors can be found in the section on underwater acoustics.
Spatial Data Sources
| MIDA: | Tide gauge network |
| Weather Stations | |
| Mean monthly rainfall | |
| Sea surface temperatures (2003) |
Marine Institute: The Republic of Ireland’s Marine Institute provides Marine Data Online, a web-mapping services for marine fisheries, oceanography, marine environment and the Irish Wave Energy Resource Atlas.
Proudman Oceanographic Laboratory (POL): Part of the National Environmental Research Council (NERC), POL is a gateway to the UK’s oceanographic data resources. This site hosts:
- Applications Group for UK tidal predictions.
- Coastal Observatory for real-time measurements and modelling forecasts for the Irish Sea.
- Permanent Service for Mean Sea Level for long term sea level change information from tide gauges.
Use the following
links to find more information on the topic:
Ocean
Portal: The UNESCO directory of Ocean Data and Information related
web sites. Its objective is to help scientists and other ocean experts
in locating such data & information.
Proudman Oceanographic
Laboratory (POL): Information on hydrodynamic resources for Northern
Ireland and Great Britain may be found on this web site.
National Tidal
and Sea Level Facility : For UK tidal and sea level research.
BBC Weather
Online: This link provides information on the British and Northern
Ireland climate and sea-state.
Met
Éireann: This link provides information on marine weather and
sea-state .
Irish
Tides: Information of the Irish Tide Gauge Network.
Valeport: Information
on various measurement devices including CTDs and tide gauges.
Research Vessels: Information on the Celtic Explorer and the Celtic Voyager.
Australian Oceanographic Data Centre, January 2000, Instruction manual for observing and recording seabed samples, secchi disc, bioluminescence and sound velocity measurements. Third edition. AODC technical manual 1/2000.
Emery, W.J. and Thompson, R.E., 1998, Data Analysis Methods in Physical Oceanography. 1st edition, Elsevier Science Pub Co.
Murphy, J.; Sutton, G.; Mahony, C. and Woodworth P. , 2003, Scoping Study to assess the status of Ireland’s tide gauge infrastructure and outline current and future requirements, Dept. of Communications, Marine and Natural Resources, [Site visited 18/09/2006].
U.S. Army Corps of Engineers (USACE), 2002, Engineering and Design - Hydrographic Surveying (EM 1110-2-1003), [Site visited 18/09/2006].