Metadata Report for BODC Series Reference Number 29694

• Metadata Summary

• Problem Reports

• Data Access Policy

• Narrative Documents

• Project Information

• Data Activity or Cruise Information

• Fixed Station Information

• BODC Quality Flags

• SeaDataNet Quality Flags

Metadata Summary

Problem Reports

No Problem Report Found in the Database

Data Access Policy

Public domain data

These data have no specific confidentiality restrictions for users. However, users must acknowledge data sources as it is not ethical to publish data without proper attribution. Any publication or other output resulting from usage of the data should include an acknowledgment.

The recommended acknowledgment is

"This study uses data from the data source/organisation/programme, provided by the British Oceanographic Data Centre and funded by the funding body."

Narrative Documents

Institute of Oceanographic Sciences Shipborne Wave Recorder

The IOS shipborne wave recorder (SBWR) (1,2) consists of a pair of heave transducers (accelerometers), a pair of pressure transducers, a signal processing unit and a data recorder. One heave transducer and one pressure transducer are mounted together on each side of the vessel close to the centre of pitch and 1-3m below the water-line. This reduces the influence of pitch and roll to negligible proportions. The heave transducers measure the longer period waves, while the pressure transducers measure the short period waves. Waves of up to 30m height and 25 seconds period can be measured; the lower detectable limit depends on the installation depth of the pressure units.

The two heave transducer signals are added and integrated twice, while the two pressure transducer signals are added and filtered. The two resulting signals are added for output to a data logger and/or chart recorder.

Calibration is carried out at intervals of from one to three years. The three year period applies to Trinity House lightvessels which are on station for this period between major refits. The heave and pressure transducers are calibrated separately. Each heave transducer is mounted in gimbals on an arm which is then rotated vertically. The signal from the transducer is amplified and fed to the chart recorder. The instrument sensitivity can then be adjusted with reference to the chart record. Each pressure transducer is calibrated statically by subjecting it to a series of discrete pressures and comparing its output voltage to the original calibration.

In general a 17 minute record is taken every 3 hours; 12 minutes (sometimes less) of each record is then analysed. When readings are taken the vessel should be stationary, or nearly so (speed less than two knots), to avoid distorting the wave periods. The system is relatively accurate for long wave periods.

During data processing, significant wave height was corrected to compensate for both the frequency response of the electronics and the hydrodynamic attenuation of the pressure fluctuations (Derbyshire, 1961); this correction was considered unreliable for Tz values of less than five seconds. However, later comparisons of SBWR, Waverider buoy and Geostat altimeter measurements by Pitt (1991) enabled a more reliable empirically- based correction procedure to be developed. This latest correction factor varies with the length of the recording vessel. The data held by BODC have had the original correction removed and the Pitt correction applied, however there is still some doubt as to the accuracy with which short period waves have been measured (Pitt, pers. comm.).

References

Haine, R.A. (1980).
Second generation shipborne wave recorder. Transducer Technology, 2, 25-28.

Pitt, E.G. (1991).
A new empirically-based correction procedure for shipborne wave recorder data. Applied Ocean Research, 13(4), 162-174.

Tucker, M.J. (1956).
A shipborne wave recorder. Transactions of the Institution of Naval Architects, London, 98, 236-250.

Data Documentation for Boyle Station Shipborne Wave Recorder

Instrumentation

IOS Shipborne Wave Recorder. Paper chart records of 15 minutes duration taken every 3 hours starting 15 minutes before the hour of 00, 03, 06, 09, 12, 15, 18 and 21. From 29th June 1974 onwards, backup recordings were made on analogue magnetic tape using a Geotech F.M. tape recorder.

Calibration

Onboard SBWR tests, as recommended by IOS, were carried out by Marex staff at monthly intervals. In addition, the equipment was periodically calibrated by IOS.

Data Analysis

The chart records were analysed manually by the Tucker-Draper method (Draper L., The Analysis and Presentation of Wave Data - A Plea for Uniformity, Proc. 10th Coastal Engineering Conf., Tokyo, 1966, Vol. 1, pp 1-11, American Society of Civil Engineers, 1967) to derive values for significant wave height and mean zero upcrossing wave period. During data processing, significant wave height was corrected to compensate for both the frequency response of the electronics and the hydrodynamic attenuation of the pressure fluctuations (Derbyshire, 1961); this correction was considered unreliable for Tz values of less than five seconds.

However, later comparisons of SBWR, Waverider buoy and Geostat altimeter measurements by Pitt (1991) enabled a more reliable empirically-based correction procedure to be developed. This latest correction factor varies with the length of the recording vessel. Measurements at Boyle station were made from the vessel Skagerak which had an estimated waterline length of 36.2m (overall length 37.9m, length between perpendiculars 34.5m). The data held by BODC have had the original correction removed and the Pitt correction applied, however there is still some doubt as to the accuracy with which short period waves have been measured (Pitt, pers. comm.). Where available, the analogue tape backup data were used to fill gaps in the chart record data.

Instrument History

Quality Control Procedures

Marex Quality Control

Initial quality control was performed by Marex during the initial preparation and processing of the data. Dubious values identified at this stage were deleted, or replaced by more reliable data from the backup magnetic tape where available. No details of the quality control procedures applied by Marex are available.

MPS Quality Control

Additional quality control was performed by Meteorological and Plotting Services (MPS) Oceanic Limited, Bushey Heath. Records were identified in which:

1. Significant or 3 hour maximum wave height varied by more than 50 percent between successive records.
2. Significant wave height exceeded 5 metres in summer, 7 metres in winter.
3. Maximum wave height (3 hours) exceeded 10 metres in summer, 14 in winter.
4. Mean zero upcrossing wave period exceeded 10 seconds in summer, 12 seconds in winter.
5. Unusual variations in mean zero upcrossing wave period were identified by visual examination of the data values.

Values identified by these tests were checked against the corresponding waverider buoy values. Sample checks were also made back to the raw data. Where the analysis was found to be erroneous, replacement values were generated from the original record. If the original record was found to be suspect, the quality control flag has been set to L.

BODC Quality Control

Further quality control was performed by BODC when the data were banked. Records which failed the following tests were examined in detail in the context of the time series of data as a whole:

1. If significant wave height changed by more than 3 metres between successive records, then it must not change by more than 1.5 metres on the following record.
2. If mean zero upcrossing wave period changed by more than 3 seconds between successive records, then it must not change by more than 1.5 seconds on the following record.
3. The ratio of significant wave height recorded by shipborne wave recorder to that recorded by waverider buoy at a given time should be within the range 0.5 to 2.0, and the absolute difference should not exceed 2.5 metres.
4. The ratio of mean zero upcrossing wave period recorded by shipborne wave recorder to that recorded by waverider buoy should be within the range 0.5 to 2.0, and the absolute difference should not exceed 3 seconds.

Two dimensional frequency distributions were plotted of:

1. Significant wave height versus mean zero upcrossing wave period for shipborne wave recorder data and also for waverider buoy data.
2. Significant wave height recorded by the shipborne wave recorder versus that from the waverider buoy.
3. Mean zero upcrossing wave period from the shipborne wave recorder versus that from the waverider buoy.

Extreme values on the periphery of these plots were also examined in detail in the context of the time series of data as a whole. Values identified as dubious by BODC screening procedures are flagged M.

Reference

Pitt, E.G. (1991).
A new empirically-based correction procedure for shipborne wave recorder data. Applied Ocean Research, 13(4), 162-174.

Project Information

No Project Information held for the Series

Data Activity or Cruise Information

No Data Activity or Cruise Information held for the Series

Fixed Station Information

Fixed Station Information

Boyle Station Site History

Site

The Boyle Station (50.7° N, 007.5° W) is sited in the Celtic Sea approximately mid-way between Lands End in England and Cape Clear at the southernmost tip of Ireland, in a water depth of 107 metres. It was occupied from 24 May 1974 to 01 June 1977, initially on behalf of the Celtic Sea Oceanographic Study Group (CSOSG) and later (from mid 1975) for the Oceanographic Committee of the United Kingdom Offshore Operators Association (UKOOA). The main aim of the data collection programme was to study the environmental climate for purposes of offshore operations.

Data Collection and Processing

Data collection and processing was performed by Marine Exploration Limited (MAREX), Isle of Wight.

The data collection programme included:

1. Wave data collected at 3 hourly intervals by means of a shipborne wave recorder and a moored waverider buoy.
2. Instrumentally measured wind data collected at hourly intervals.
3. Meteorological synoptic observations made at 3 hourly intervals.
4. Current meter data collected at 10 minute intervals at three different depths.
5. Bathythermograph data collected several times a month (these data were sent to the Hydrographic Department MOD (Navy), Taunton).

The data collection programme was centred about a ship on station for periods of up to 1 month at a time. Gaps of up to a week appear in the data at the end of each on-station period. A total of 38 sea trips was made during the programme, the merchant vessel Skagerak being used throughout.

On Station History

Note: Because the waverider buoy and current meters were deployed just before the ship took station, and were recovered after the ship left station, the period covered by data from these instruments may extend up to 1 day beyond the start and end times of the on station periods given in the above table.

Related Fixed Station activities are detailed in Appendix 1

BODC Quality Control Flags

The following single character qualifying flags may be associated with one or more individual parameters with a data cycle:

SeaDataNet Quality Control Flags

The following single character qualifying flags may be associated with one or more individual parameters with a data cycle:

Appendix 1: Boyle

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