Metadata Report for BODC Series Reference Number 18230

• 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 Quality Report

Current speed and direction values show no obvious faults. Hence, acceptable to IOS(T) validation checks

Data Quality Report

Time origin uncertainty unknown. The meter deployment time is used for the start time of the series. Field engineers did not log the time of meter switch on. No data processing note kept of either the data cycle number corresponding to meter immersion or the cycle number of the first good data cycle accepted; data prior to this being jettisoned; this cycle renumbered one starts the present series.

An inspection of series for which deployment time and meter switch on time were both logged, served to demonstrate that deployment times do not correspond to any particular stage in deployment operations.

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

NBA Recording Current Meter Model DNC2/DNC2B

Manufacturer's specifications: Meter (overall length 120cm or DNC2B 135cm, frontal section diameter 18cm, rear section diameter without fins 10.4cm, with fins fitted 73cm) is designed for depths down to 600m (2500m optional). It incorporates a spindle which is shackled into the mooring line. The meter is attached to the spindle through a gimbal mounting which permits a maximum 23.5deg deviation of the spindle from the vertical, the meter still remaining horizontal.

Meter comprises:-

1. Balanced impeller magnetically coupled to an electronic counter reed switch to give average velocity over an interrogation period, starting velocity less than 5cm/s (typically 3cm/s), system calibrations are provided up to 250cm/s (500cm/s optional) with an accuracy of 2 per cent; the readings are scaled for optimum resolution, the impeller count is reset to zero after each interrogation.

2. Stabilising tail plane.

3. Digital magnetic compass with 7 bit optical encoded disc, direction being recorded at instant of sampling only, range 0 to 360deg with 3deg accuracy and resolution; the allowable tilt for the compass i.e. the maximum deviation of the meter from the horizontal at which the compass still registers correctly, is 30deg in both pitch and roll axes.

4. Solid state crystal clock, timing accuracy 2sec/day over a temperature range of 0 - 20degC.

5. Thermistor, temperature sensor (optional), range 5 to 25degC, accuracy 0.25degC, resolution 0.25 per cent.

6. Pressure transducer (optional), range 0 to 100, 200, 500, or 1000psi, accuracy 1 per cent of range, resolution 0.25 per cent.

7. Inductive cell conductivity sensor (optional), range 0 to 85mmho/cm, accuracy 4 per cent, resolution 0.33mmho/cm.

8. Associated electronics.

Data are recorded in 8 bit binary words on magnetic tape, DNC2 uses 7 to 12 bit words. Interrogation rate adjustable in increments of 15sec, from 15sec to a maximum of 31min 45sec. Sample duration equals nominal interval between data cycles. Sample recording order: meter serial number, sample count (DNC2), speed, direction, and for DNC2B: sensor 1, sensor 2, sample count.

Manufacturer's calibration formulae:

speed = 0.978 + 32.1 * B * (M/T) cm/s
direction = 360 * N/128 deg magnetic

where B is the number of impeller revolutions per count, M is the binary equivalent of the count over the sampling interval T(sec) and N is the binary equivalent of the direction reading.

Note: Data collecting laboratories may calibrate their own meters and so not use the manufacturer's calibration equations.

BODC Current Meter Screening

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:
  • Depths of meter and sea bed.
  • Times for mooring deployment and for start/end of data series.
  • Length of record or number of data cycles, the cycle interval, the clock error and the period over which accrued.
  • Parameters stated as measured 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 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 following types of irregularity, each relying on visual detection in the time series 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, deemed abnormal, then instead of flagging the individual suspect values, a caution may be documented. Likewise documents will highlight irregularities seen in the current vector scatter plots such as incongruous centre holes, evidence of mooring 'knock-down', abnormal asymmetry in tidally dominated records or gaps as when a range of speeds or directions go unregistered due to meter malfunction.

The term 'knock-down' refers to the situation when the 'drag' exerted on a mooring at high current speeds may cause instruments to tilt beyond the angle at which they are intended to operate. At this point the efficiency of the current sensors to accurately record the flow is reduced.

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

• Maximum and minimum values of parameters (spikes excluded).
• The orientation and symmetry of the current vector scatter plot.
• The direction of rotation of the current vectors.
• The approximate amplitude and periodicity of the tidal currents.
• The occurrence of meteorological events and, finally, for series for which no time check was possible, the phase.

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.

Boyle CMD Data Processing

Data Processing carried out by IOS Taunton and by BODC.

Data processing carried out by IOS Taunton on paper tape data provided by Marex

1. Obvious spurious data are removed from the beginning and the end of records of direction (degrees magnetic) and impeller count.
2. Clock error is not ascertained hence time values are nominal.
3. Directions exceeding 360° and impeller counts exceeding instrument maximum are replaced by the previous good data value.
4. Impeller counts, summed for the sampling interval, are converted to speed using an approximation to the calibration equation.
5. Large differences (i.e. greater than X cm/s ) between consecutive speeds are taken as instrument malfunction and the equivalent count is replaced by the previous good value. Plateaux of high values are normally retained.
6. X is generally set at 20 cm/s; exceptions are as follows:
7. • X=15 used for BODC series ref. 4904 to 4928
   • X=25 used for BODC series ref. 18217, 18229 and 18309
   • X=30 used for BODC series ref. 4953
   • X=35 for BODC series ref. 4941
   • X = 80 used for BODC series ref. 18198
   • X not applied, hence spikes not removed, for BODC series ref. 4861 to 4897.

Data processing carried out by BODC, prior to data banking, on records of direction and impeller count received from IOS Taunton

1. Records of direction are calibrated and corrected for magnetic variation (10 °W).
2. Impeller count (ICOUNT) summed for the sampling interval is converted to speed in cm/s using the formula
   SPEED = (ICOUNT*B*G)/(SI*60)+K
   where
   B is the instrument scaling factor
   G, K the instrument calibration constants
   SI the sampling interval in minutes.
3. The time assigned to the parameters is the end of the period over which the impeller count was taken. The first record of a series contains direction in degrees true at start time ST and average speed in cm/s over the sampling interval SI terminating at ST. The nth record contains direction at time ST+(n-1)SI and average speed for the interval from ST+(n-2)SI to ST+(n-1)SI i.e. the interval terminating at ST+(n-1)SI The time for the nth record is given as ST+(n-1)SI.
4. The nominal sample interval is used in determining the time values i.e. they are not corrected for clock error.
5. Note the format of the originator's reference for the data when quoted by BODC will be BYMMP/ASn where B=AS=Boyle, 1970+Y=year, MM=month, P=position on mooring (eg top, upper, lower, bottom), ASn=nth meter deployed at Boyle.

Data Processing Notes

Marex meter number 331 also called 031. Calibrated DEC 1975 at British Hovercraft Corporation Ltd. to IOS specifications in range 0 - 150 cm/s. Starting speed for rotor 4.94 cm/s. Streaming velocity 12.66 cm/s

Data validation carried out at IOS Taunton

Processed data were examined via plots of north versus east velocity components. Faults were further confirmed by plotting time series of directions, north components and east components. Once a serious fault was found in a series, the data did not receive further investigation. Data without obvious errors were formed into half-hourly vector averages which were correlated with tidal data at a nearby reference site (Scilly for BODC series reference numbers 4861 to 5213 and 17816 to 18438, otherwise Lerwick) using the response method. These averages were then resolved into tidal and non-tidal (residual) components. The response method is generally more efficient with close to 29 days of data so short records have not been processed by this method.

Progressive vector plots of vector averaged currents and non-tidal components were examined. A residual containing a large proportion of tidal energy may point to either

1. Incorrectly recorded velocities which require further examination of the time series plots of north and east components.

2. An incorrect sampling interval which may require spectral analysis of the vector averaged data to check that the peak lies at the tidal frequency (based on the assumed sampling interval).

Data still without any outstanding error were now classed as acceptable. No check was made on current magnitudes. Quality control was essentially by visual inspection of data plotted in various ways. At each station, currents which have been recorded over 2 to 3 weeks show overall regularities which change very little from one month to the next e.g. current vectors when plotted show a characteristic elliptical form with a characteristic sense of rotation. These regularities have been used as a check on the self-consistency of the data.

Marex Standard Procedures for Current Meter Data

Calibration

The manufacturer's calibration formulae and the manufacturer's corrections for compass deviation were used for directions. Speed calibrations were performed either by IOS or by British Hovercraft Ltd. to IOS specifications, or else the manufacturer's speed calibrations were used. Comments given with each series identify the particular calibration used. IOS/BHC hydrodynamic calibration of the instrument was carried out in a towing tank. The run of water past the meter required to produce one revolution of the rotor and the starting speed of the rotor were determined.

Mooring system

U-shaped mooring i.e. pellet float to pellet line to sub-surface buoy to meter mooring line with in-line mounting of meters to anchor weight to ground line (over 800ft i.e. 244m) to anchor weight to marker buoy line to surface marker buoy. Marex practice was to mount the sub-surface buoy 6ft (1.8m) above the top current meter on a string. The site was chosen by echo sounding to find the required sea floor depth for the deployment. The site is positioned using Decca main chain. It is not possible to state categorically that the depth was measured below mean sea level but in classifying depths BODC/Marex have assumed that normal marine practice of adjusting echo sounder depths to allow for tidal variations was adopted.

Data sampling/processing

Speed was averaged instrumentally over the sampling interval. Direction was taken as a spot measurement at the end of the sampling interval. Direction was corrected for magnetic variation. Parameter values are given at times incremented by the nominal sample interval. The time assigned to each datacycle is that of the end of the sampling interval. Time values were not corrected for clock error and clock error was not determined. Data were processed and validated by IOS Taunton.

Additional Information

BODC note that the number of data cycles in the series is significantly less than the number expected for the coverage of the actual elapsed time from the deployment to the recovery of the mooring. This indicates that the series has been prematurely truncated.

Data Origin

Data collected by Marine Exploration Ltd. (Marex) for United Kingdom Offshore Operators Association Oceanographic Committee.

Project Information

No Project Information held for the Series

Data Activity or Cruise Information

Data Activity

Mooring Information

Mooring deployed 1100h. Recovered 1250h.

Related Data Activity activities are detailed in Appendix 1

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 2

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: NB609

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

If you are interested in these series, please be aware we offer a multiple file download service. Should your credentials be insufficient for automatic download, the service also offers a referral to our Enquiries Officer who may be able to negotiate access.

Appendix 2: Boyle

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

If you are interested in these series, please be aware we offer a multiple file download service. Should your credentials be insufficient for automatic download, the service also offers a referral to our Enquiries Officer who may be able to negotiate access.