Metadata Report for BODC Series Reference Number 609755

• 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

Open 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.

If the Information Provider does not provide a specific attribution statement, or if you are using Information from several Information Providers and multiple attributions are not practical in your product or application, you may consider using the following:

"Contains public sector information licensed under the Open Government Licence v1.0."

Narrative Documents

Waverider Wave Measuring Buoy

A waverider buoy measures waves by floating on the surface of the sea and measuring its own vertical motion in response to passing waves. The buoy contains an accelerometer which is stabilised in the vertical plane for storm and swell wave frequencies by means of a short length of chain attached to the bottom of the buoy. A length of rubber cord links this chain to the rest of the mooring.

In general a waverider buoy will contour the sea surface and generate a heave signal to an accuracy of better than five percent. This signal is used to amplitude modulate a 27-30 MHz radio signal which is transmitted continuously; this signal can be received by the recording device at a range of up to 50 km depending on local conditions.

For more information on waverider buoys see Driver (1980) (from which the above text has been taken).

References

Driver, J.S. (1980).
A guide to sea wave recording. Institute of Oceanographic Sciences, Report No. 103, pp 91 (16 figs).

IOS Spectral Wave Data Processing (1976-1988)

The Institute of Oceanographic Sciences, Taunton (now closed) operated a series of Waverider bouys from 1976 to 1988.

Calibration

All IOS Waverider buoys were calibrated at the National Maritime Institute, Hythe before and after deployment. The buoys were placed in a rig which carries them in a 3m diameter vertical circle. Rotation speed was indicated by a tachometer, but actual rotation period was measured by IOS personnel using a stopwatch. Rotation period was originally variable between 3.5 and 18 seconds (0.28-0.053 Hz) and in 1975 was changed to 3.5-40 seconds (0.28-0.025 Hz). The slower speed gives a better test of the Waverider's response fall-off above 20 seconds period.

Prior to May 1978, recordings were only made on a chart recorder. The measured values were corrected for the nominal frequency response for the accelerometer-integrator system, and then expressed as a percentage above or below the calculated figure. An average percentage deviation was then taken over all rig rotation speeds with periods of less than 20 seconds, and this was given as the calibration figure for the buoy. The procedure was later refined and buoys and receivers were calibrated separately to high accuracy, and a combined figure in volts per metre of buoy displacement was derived. It is estimated that an overall accuracy of +/-2% is achieved when the buoy is in motion. For further information, see Humphrey (1982).

Preliminary processing and quality checks

Magnetic tapes from the logger were returned to the laboratory where they were translated and processed by a computer program designed to check for timing or tape formatting errors. In addition the program subjected each wave record to a number of tests to check for characteristics not normally associated with wave records of this type. The tests were based on the assumptions that a wave record should display certain simple properties consistent with the behavior of a random process with an approximately normal distribution, and that the water surface should conform to certain well established steepness criteria.

The following tests were carried out:

1. Test for lost data points due to the format errors or telemetry failure.
2. Check for the occurrence of ten consecutive points of equal value (instrument failure test).
3. Check for an interval between successive up-crossings of the record mean level of > 25 seconds (wandering mean test).
4. Comparison of the difference in magnitude between successive data points with a test value based on the maximum probable water slope.
5. Comparison of absolute magnitude of data points with a test value equal to 4 times the record standard deviation.

Actions taken by the program were:

• Single lost data points, up to a max of 15 per record were replaced by the average value of the two neighbouring points. Two or more lost points caused the record to be rejected.
• Failure of test (2) or (3) caused the record to be rejected.
• Faulty points, up to a max of five, identified by test (4), were replaced by the average value of the two neighbouring points. Two or more adjacent faulty points caused the record to be rejected.
• No alteration to the record were made on failures of test (5). Six failures, up to 3 of which could be consecutive, were allowed before rejecting the record.

Calculation of Spectra

Each wave elevation time series was multiplied by a window function of the form 1/2(1-cos(2pi t/T)) where T is the length of record analysed and t elapsed time, and a Fast Fourier Transform was performed on 2048 points of the series. The spectra calculated were smoothed by averaging over 10 adjacent harmonics to give a final resolution of 0.0098 Hz.

The smoothed spectral estimates were multiplied by a factor to restore the energy removed in the windowing process. The factor was calculated separately for each spectrum and ensured that the variance of the record, calculated as the zeroeth moment of the spectrum, was the same as that calculated from the wave elevation time series. The estimates were also adjusted to compensate for the overall frequency response of the buoy, receiving and recording systems.

While this compensation process yields a frequency response which approximately flat over the frequency range of interest, it should be noted that the natural frequency of oscillation of the accelerometer platform within the buoy is approximately 40 seconds, and this can give rise to large amplitude low frequency noise. For this reason it is recommended that the first two smoothed spectral estimates of each spectrum supplied be disregarded, and in this way it should be possible to avoid the accidental inclusion of any spurious instrumental noise. The lowest frequency for which a valid estimate of spectral density exists should therefore be regarded at 0.444 Hz.

Three other quantities are calculated during the analysis as an aid to quality assessment:

References

Humphery, J.D. (1982). Calibration of Waverider systems by the Institute of Oceanographic Sciences, Taunton. Institute of Oceanographic Sciences, Report No. 133.

Pitt, E.G. and Crabb, J.A. (1978). User guide to South Uist spectral wave data. Institute of Oceanographic Sciences, Internal Document No. 43.

General Data Screening carried out by BODC

BODC screen both the series header qualifying information and the parameter values in the data cycles themselves.

Header information is inspected for:

• Irregularities such as unfeasible values
• Inconsistencies between related information, for example:
  • Times for instrument deployment and for start/end of data series
  • Length of record and the number of data cycles/cycle interval
  • Parameters expected and the parameters actually present in the data cycles
• Originator's comments on meter/mooring performance and data quality

Documents are written by BODC highlighting irregularities which cannot be resolved.

Data cycles are inspected using time or depth series plots of all parameters. Currents are additionally inspected using vector scatter plots and time series plots of North and East velocity components. These presentations undergo intrinsic and extrinsic screening to detect infeasible values within the data cycles themselves and inconsistencies as seen when comparing characteristics of adjacent data sets displaced with respect to depth, position or time. Values suspected of being of non-oceanographic origin may be tagged with the BODC flag denoting suspect value; the data values will not be altered.

The following types of irregularity, each relying on visual detection in the plot, are amongst those which may be flagged as suspect:

• Spurious data at the start or end of the record.
• Obvious spikes occurring in periods free from meteorological disturbance.
• A sequence of constant values in consecutive data cycles.

If a large percentage of the data is affected by irregularities then a Problem Report will be written rather than flagging the individual suspect values. Problem Reports are also used to highlight irregularities seen in the graphical data presentations.

Inconsistencies between the characteristics of the data set and those of its neighbours are sought and, where necessary, documented. This covers inconsistencies such as the following:

• Maximum and minimum values of parameters (spikes excluded).
• The occurrence of meteorological events.

This intrinsic and extrinsic screening of the parameter values seeks to confirm the qualifying information and the source laboratory's comments on the series. In screening and collating information, every care is taken to ensure that errors of BODC making are not introduced.

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

West Bexington (1983-1993)

1-D spectral wave data were collected between 1983 and 1993. Two waverider deployments (sometimes referred to as West Bexington (a) and West Bexington (b)) covered the period between November 1983 and April 1988. A pressure wave recorder installation operated between December 1987 and May 1995. All instruments were located within 0.05 degrees of the nominal station position.

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: West Bexington (1983-1995)

Related series for this Fixed Station are presented in the table below. Further information can be found by following the appropriate links.

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