Metadata Report for BODC Series Reference Number 739776

• 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 supplied by Natural Environment Research Council (NERC)

You must always use the following attribution statement to acknowledge the source of the information: "Contains data supplied by Natural Environment Research Council."

Narrative Documents

Instrument Description

CTD unit and auxiliary sensors

The CTD system used on the JR94 was the BAS Sea-Bird 911 plus, with a 12 bottle frame and pylon. Only 10 Niskin bottles were fitted to the frame, bottles 2 and 3 were removed to accommodate the LADCP battery. The CTD was fitted with the following scientific sensors:

The SBE35 has two sets of calibration information. From time to time a bath calibration is performed, the so-called linearisation, in which many polynomial coefficients are determined. At intermediate times, single-point-calibrations are performed, e.g. Triple Point of Water and Melting Point of Gallium, providing a slope and offset correction to the data returned from the linearisation. Thus two calibration dates are reported.

Initially the salinity samples from the CTD were analysed using the BAS Guildline Autosal model 8400B(serial number 63360). This Autosal was found to give unstable readings. A second Autosal (serial number 65763) was set up whilst at Rothera and used for sample analysis for the rest of the cruise. The Autosals were standardised using batch P143 IAPSO Standard Seawater supplied by Ocean Scientific International Ltd.

Sea-Bird Electronics SBE 911 and SBE 917 series CTD profilers

The SBE 911 and SBE 917 series of conductivity-temperature-depth (CTD) units are used to collect hydrographic profiles, including temperature, conductivity and pressure as standard. Each profiler consists of an underwater unit and deck unit or SEARAM. Auxiliary sensors, such as fluorometers, dissolved oxygen sensors and transmissometers, and carousel water samplers are commonly added to the underwater unit.

Underwater unit

The CTD underwater unit (SBE 9 or SBE 9 plus) comprises a protective cage (usually with a carousel water sampler), including a main pressure housing containing power supplies, acquisition electronics, telemetry circuitry, and a suite of modular sensors. The original SBE 9 incorporated Sea-Bird's standard modular SBE 3 temperature sensor and SBE 4 conductivity sensor, and a Paroscientific Digiquartz pressure sensor. The conductivity cell was connected to a pump-fed plastic tubing circuit that could include auxiliary sensors. Each SBE 9 unit was custom built to individual specification. The SBE 9 was replaced in 1997 by an off-the-shelf version, termed the SBE 9 plus, that incorporated the SBE 3 plus (or SBE 3P) temperature sensor, SBE 4C conductivity sensor and a Paroscientific Digiquartz pressure sensor. Sensors could be connected to a pump-fed plastic tubing circuit or stand-alone.

Temperature, conductivity and pressure sensors

The conductivity, temperature, and pressure sensors supplied with Sea-Bird CTD systems have outputs in the form of variable frequencies, which are measured using high-speed parallel counters. The resulting count totals are converted to numeric representations of the original frequencies, which bear a direct relationship to temperature, conductivity or pressure. Sampling frequencies for these sensors are typically set at 24 Hz.

The temperature sensing element is a glass-coated thermistor bead, pressure-protected inside a stainless steel tube, while the conductivity sensing element is a cylindrical, flow-through, borosilicate glass cell with three internal platinum electrodes. Thermistor resistance or conductivity cell resistance, respectively, is the controlling element in an optimized Wien Bridge oscillator circuit, which produces a frequency output that can be converted to a temperature or conductivity reading. These sensors are available with depth ratings of 6800 m (aluminium housing) or 10500 m (titanium housing). The Paroscientific Digiquartz pressure sensor comprises a quartz crystal resonator that responds to pressure-induced stress, and temperature is measured for thermal compensation of the calculated pressure.

Additional sensors

Optional sensors for dissolved oxygen, pH, light transmission, fluorescence and others do not require the very high levels of resolution needed in the primary CTD channels, nor do these sensors generally offer variable frequency outputs. Accordingly, signals from the auxiliary sensors are acquired using a conventional voltage-input multiplexed A/D converter (optional). Some Sea-Bird CTDs use a strain gauge pressure sensor (Senso-Metrics) in which case their pressure output data is in the same form as that from the auxiliary sensors as described above.

Deck unit or SEARAM

Each underwater unit is connected to a power supply and data logging system: the SBE 11 (or SBE 11 plus) deck unit allows real-time interfacing between the deck and the underwater unit via a conductive wire, while the submersible SBE 17 (or SBE 17 plus) SEARAM plugs directly into the underwater unit and data are downloaded on recovery of the CTD. The combination of SBE 9 and SBE 17 or SBE 11 are termed SBE 917 or SBE 911, respectively, while the combinations of SBE 9 plus and SBE 17 plus or SBE 11 plus are termed SBE 917 plus or SBE 911 plus.

Specifications

Specifications for the SBE 9 plus underwater unit are listed below:

Further details can be found in the manufacturer's specification sheet.

BODC Processing

CTD data files for JR94 were provided by the originator in Pstar format. The 32 files selected for transfer had been bin averaged by pressure to 1db. Files averaged to 24Hz were also provided by the originator however the 1db data were considered to be of a sufficiently high resolution for processing. The headers of three files were checked for consistancy using the plisth command in Unix. Each parameter selected for transfer was assigned a parameter code as follows:

The Matlab transfer function 360 was run to convert the Pstar files into an internal NetCDF format (QXF). The transfer automatically converted the conductivity channels from mS/cm to S/m. The parameters time (s) and scanno (scans) were not selected for transfer as the data were bin averaged by pressure. It should be noted that the Pstar file from station 17 did not contain channel 15 (depth), subsequently only 12 channels are present in the transferred file (BODC ishref 739948). Metadata from file headers were separated off into tagged ASCII files which were later used in loading data to the database. The coordinates of the QXF files were checked using Google Earth to make sure that they matched the cast locations described in the cruise report.

Data from the QXF files were visualised using the in-house editor Edserplo. Obvious data spikes were flagged as 'M', null values were flagged as 'N'. Overall, the data are of good quality and there were no significant data spikes present.

Originator's Data Processing

The following information is adapted from the cruise report for JR94.

Sampling strategy

A Conductivity-Temperature-Depth (CTD) probe was used to vertically profile the temperature and salinity of the water column. 32 full depth stations were sampled, station 01 was a test station and station 02 was the Rothera Time Series (RaTS) station, occupied immediately before docking at Rothera. At each CTD station on JR94, all ten Niskin bottles were closed and sampled for salinity analysis and subsequent CTD conductivity calibration. A thermometer was set to record a measurement on each bottle firing for comparison with the CTD measurements.

The CTD was started on deck, and deployed to 10 metres wire-out. The instrument was allowed to soak for 30 seconds after the pumps came on, and was then hauled until the termination arrived at the surface. The package was then veered to a nominal 10 metres above the seabed. The CTD was kept powered on until it was on deck, after which data acquisition was stopped. Data were recorded on the PC attached to the deck unit to a local (D:) drive using utility seasave. After a station the raw files were immediately copied as backup to a networked (N:) drive.

Data Processing

The PC software SBEDataProc was used to run DatCnv, which converted a binary .dat file to an ASCII .cnv file. The effect of the thermal mass of the conductivity cells was removed from the data using the SeaBird software celltm using the standard coefficients (a = 0.03 and b = 1/7), which generated another ASCII file with suffix _ctm.cnv.

After the thermal mass correction had been applied, the data were transferred to a UNIX machine processing took place using using c-shell scripts. Accurate water depth was not available in real time, so water depths were entered early on as zero. At the end of the cruise, true water depth was ascertained from CTD depth plus altimeter height off the bottom. Together with GPS position at the bottom of the cast, which was extracted automatically from CTD stations and nav files, the position and true water depth in pstar headers of all ctd files were rewritten and made consistent.

To simplify any reprocessing and reduce the possibility of typo errors, the scan numbers for start_down, bottom, and end_of_cast were selected from the 24Hz file, and entered into a file called 94station_dcs. The scan number for the bottom of file was found automatically using refval. The start and end scan numbers were found from listings to the screen, judging the start of the downcast after hauling to near surface, and selecting a scan number shortly before the CTD broke surface at the end. The final unix files are a 1Hz time series for the whole station (for use in LADCP processing, for example) and a 1db file of the downcast.

Field Calibrations

An initial comparison was made at 114 bottle closure events at 2000 metres or deeper. Primary temperature (T1) read slightly higher than secondary temperature (T2), and primary conductivity (C1) read slightly higher than secondary conductivity (C2). These effects tend to compensate so that the mean offsets between sample salinity (S) and sensor salinities (S1 and S2) were small. Closer graphical investigation showed that T1-T2 varied with pressure, while C1-C2 was steady. If T1 was used as the independent variable, the relationship was noisy, suggesting it was a pressure effect rather than a simple temperature calibration slope error. Since C1-C2 was independent of pressure, there was a corresponding trend in S1-S2. A trend with depth in S-S1 was discernable, despite the generally much higher scatter in the upper 1000 metres of water column. T1 and T2 both read slightly higher than the SBE35 temperature (T35). When the full dataset was viewed with pressure as an independent variable, a trend was discernable in T35-T1, but not in T35-T2.

The observed small trends can be explained by a dependence of T1 on pressure, with a tendency for T1 to read cooler at the surface than at 4000 metres. The T1 offset appeared to change by about 0.0015 over the pressure range sampled on the cruise. Offsets in T2 and C2 were evidently less than 0.001, and C1 had a small positive offset. T2 and S2 were accepted as being in agreement with T35 and S to within 0.001 in temperature or salinity without further offset.

Project Information

World Ocean Circulation Experiment (WOCE)

The World Ocean Circulation Experiment (WOCE) was a major international experiment which made measurements and undertook modelling studies of the deep oceans in order to provide a much improved understanding of the role of ocean circulation in changing and ameliorating the Earth's climate.

WOCE had two major goals:

• Goal 1. To develop models to predict climate and to collect the data necessary to test them.

• Goal 2. To determine the representativeness of the Goal 1 observations and to deduce cost effective means of determining long-term changes in ocean circulation.

Data Activity or Cruise Information

Cruise

Complete Cruise Metadata Report is available here

Fixed Station Information

No Fixed Station Information held for the Series

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: