Metadata Report for BODC Series Reference Number 365601

• 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

RRS Discovery 181 SeaSoar Data

Introduction

Documentation for the SeaSoar data collected on board RRS Discovery cruise 181 1/4-3/5 1989 by the Institute for Oceanographic Sciences Deacon Lab under Raymond T. Pollard.

Instrumentation

The IOSDL SeaSoar is designed to carry a Neil Brown Instrument Systems (NBIS) mark III shallow CTD and a fluorometer. IOSDL has two shallow CTDs; termed 'new' and 'old'. Initially the new shallow CTD was fitted, with platinum resistance thermometer probe (PRT) calibrated in-situ at the laboratory. The first few deployments were intended for performance trials and to measure the engineering parameters (pitch, roll, wing angle, bridle angle, propeller revolutions and vehicle strain) the fish was fitted with additional sensors and a telemetry pack took the place of the fluorometer.

SeaSoar Deployments

The first run was a test run and no scientific data were processed, the next five SeaSoar runs took place between CTD stations that were 75 km apart. Runs 2 and 3 were on the east-west section at about 41.5 °N while runs 4,5 and 6 were north-south BOFS line (Biogeochemical Ocean Flux Study) at about 20 °W.

After the telemetry pack was removed and the fluorometer installed. It was obvious that a front has been crossed between CTD stations 11841 and 11842 at the northern end of the BOFS line, so the SeaSoar was deployed and towed south until the front had been crossed, then back to the north. It was recovered and redeployed for the next three CTD stations to the end of the BOFS line.

The SeaSoar was deployed again on 23 April 1989 06:54 for a section south down 17 °W, but halfway through the deployment several sensors started giving bad readings. The SeaSoar was recovered and it was found that the fluorometer had flooded with sea water. The fluorometer was removed but after redeployment the temperature data were still noisy and 3 hours later at 10:20 the fish had to be recovered. The ship remained hove-to avoid a break in the section while the new shallow CTD was replaced with the old shallow CTD. The PRT on this instrument had also been calibrated in-situ at IOSDL.

Redeployment took place at about 14:50, however after only 7 hours the PRT failed on this CTD and the SeaSoar was recovered 21:25 to have the old shallow CTD replaced bye the new shallow CTD with a new uncalibrated sensor. While this change was being made the ship had to continue south due to time constrains. The SeaSoar was deployed again 24 April 02:00. During the next eighteen hours a spare fluorometer was located and the connections obtained from RVS. At 20:50 the SeaSoar was recovered to install the new fluorometer and deployed 22:45.

The SeaSoar was then towed south to 44 °N, 17 °W, north east to 47 °N, 14 °W and south east to 44 °N, 11°W over the next four days. After this the flight pattern became unstable and it was thought that the hydraulics systems might have failed. The SeaSoar was recovered at 15:20 on the 28 April 1989 and it was found that the ballast weight had come loose and that there was water in the hydraulics. After repairs had been made, the SeaSoar was deployed at 21:17 and towed back to the shelf edge where it was recovered on the 30 April 1989 14:21.

After recovering two moorings and deploying one, there was just time for a short SeaSoar run. The start was delayed because of failure of a graphite filled wire in the conductivity cell. This was repaired although it caused shift in the calibration, and the SeaSoar was deployed on 2 May 1989 00:05 to run up onto the shelf break and back out to sea before it was necessary to stop scientific work, recover the SeaSoar 09:00.

Data Capture and Reduction

CTD data were passed from the CTD deck unit to a Level A interface, and then to a SUN workstation for processing using the PSTAR suite of data processing programs. The CTD data were sampled at 8Hz, and reduced to one second averages by the level A.

Once the data were in PSTAR format the processing procedure followed the same route as described in Pollard et al. (1987). A new program (STRIPED) was developed for semi-interactive despiking of SeaSoar data.

CTD Calibration

The initial calibrations applied by CTDCAL were as follows:

After Failure of the two PRT sensors a best estimate of calibrations for the new probe was made:

Temperature T = -0.055 + T_raw * (0.0005)

An in-situ calibration of this probe was made after the cruise at the laboratory which gave:

Temperature T = -0.461 + T_raw * (0.0005*0.99799)

It was decided that this was sufficiently similar to the calibration used that it was not necessary to rework the raw data.

Absolute Salinity Calibration

The first six short runs were checked against the CTD casts at the beginning and end of each run and it was found that no further correction was necessary. During the longer runs a check was made by comparing the theta/S curves with suitable CTD casts and previous SeaSoar profiles and a fairly constant offset was applied from this. For an absolute calibration surface salinity samples were taken approximately every hour and were compared with near surface SeaSoar values by the procedure outline by Pollard et al (1987).

The corrections applied were as follows:

Fluorometer Calibration

Two different fluorometers were used, which required two different calibrations.

IOSDL Fluorometer

The IOSDL fluorometer only worked for the relatively short section (336km) along 20 °W and during this time very few chlorophyll "a" samples were taken (seven). They were spread over 1.3 days so were insufficient to determine the diurnal pattern of fluorescence. The initial calibration (provided by Dr. M. Fasham, from RRS Discovery cruise 175) used was:

Fluorescence(mg/m³) F = -2.325 + F_raw * (0.001*1.897)

The values given by this equation were high relative to the chlorophyll "a" samples, so a further correction was made of:

F = F*0.3

RVS/PML Fluorometer (Aquatracka SA240)

The PML fluorometer was calibrated against chlorophyll "a" samples taken between 113/2328 to 122/0759. The data were quite scattered but the best fit suggested that the response of the fluorometer varied with time and depth. The time response was approximately sinusoidal between day and night, while the depth response was linear between 0-30 dbar, bellow this it was assumed to be constant. To fit these observations, the following calibration (supplied by Dr R.D. Pingree) has been applied:

Chl'a'= [10^((V/1.59)-1)]/R

where

V is the fluorometer output in volts
R = response with time and depth = [(30-p)R_s+pR_n]/30 P<30
R = R_n P>30

where

p = pressure
R_s = surface response of sensor = 1.25 sin theta(t) + 2.25
R_n = night-time response of the sensor = 1.25 sin pi/2 + 2.25 = 3.5

Theta(t) is as follows:

RRS Discovery Cruise 181 CTD Data Documentation

Introduction

This data document covers the CTD data collected during RRS Discovery cruise 181 (1^st April to 1^st May 1989) under the direction of Dr. R.T. Pollard from the Institute of Oceanographic Sciences Deacon Laboratory.

Instrumentation

Two Neil Brown Instrument Systems (NBIS) Mark III CTDs were taken on the cruise, although only one was used. After an initial problem with the deck unit (a loose wire) the system worked well. An old Beckman oxygen sensor was used at the beginning of the cruise but by cast 11820 it had become obvious that this was not giving sensible output. It was replaced by a new sensor for cast 11821 and the data collected prior to this were abandoned. A SeaTech 1m transmissometer was incorporated into the CTD system and worked well throughout the cruise. The multisampler and rosette caused problems by consistently misfiring at depth. A full report of the difficulties encountered is given in the cruise report (Pollard et al., 1989).

Data Capture and Reduction

All CTD, oxygen and transmissometer data were logged on the NBIS deck unit and displayed on a BBC microcomputer in real time. Data were also recorded on digidata tapes as a back up to the main computing system. The data were logged via a level A interface to the level B Plessey computer for transfer to tape and to the level C system: three Sun 3/60 workstations. The level A despiked and averaged the data from 16 Hz to 1 Hz. The data were processed by the procedure outlined by Pollard, Read and Smithers (1987).

Calibration

The initial calibrations applied by CTDCAL were as follows:

Pressure (dbar) P = 0.0 + P(raw) * (0.1 * 0.99286144)
Temperature (deg C) T = 0.020909 + T(raw) * (0.0005 * 0.998965925)
Conductivity C = 0.0 + C(raw) * (0.001 * 1.00109073)

Temperature

The temperature calibration was obtained in the laboratory with the platinum resistance thermometer calibrated in-situ. Deep temperature values (at 3000 m or deeper) were compared with sample measurements taken by a digital reversing thermometer. Over 23 samples, the mean difference between temperature sensor and reversing thermometer was 0.0088 ±0.001 ° C. Very few adequate readings were obtained near the surface because of the failure of the second digital reversing thermometer. On this evidence no change was made to the temperature calibration.

Transmittance

Transmittance was also recorded although not shown in this report. It was calibrated with the equation:

Transmittance (%) T = 20 * V * 1.0032

where:

V (output voltage) = (4.355 / 4.096) * (T(raw) - 0.0) * 0.0001

Absolute Salinity

At the beginning of the cruise six full depth casts were made with twelve water samples each, analysed on the Guildline Autolab salinometer for calibration purposes. These gave a consistent offset over the entire cast which was comparable to the offset obtained by comparing the 0/S profiles to the curve obtained by Saunders (1986) for the deep, stable water of the North East Atlantic. A constant offset was therefore applied to salinity to match the Saunders curve.

After the first few casts the number of salinity samples drawn at each station was reduced to four. These were analysed during the cruise on the Guildline salinometer, but examination of the results after the cruise suggested that the salinometer was not working reliably, standardisations on successive sessions showing large offsets. All the CTD 0/S profiles were compared to the Saunders curve and adjustments were made during the cruise as the conductivity cell drifted slightly.

The bottle values were compared with the CTD data and gave poor statistics, but it is thought that this is most likely to be the result of problems with the salinometer rather than the conductivity sensor.

Oxygen

Calibration of the Beckman oxygen sensor caused more than the usual problems (Read, 1989). Data from the casts prior to station 11821 were discarded as the old sensor did not give sensible output.

Data from the replacement were given an initial calibration of:

oxygen current OXYC = 0.0 + OXYC(raw) * (0.001 * 1.05)
oxygen temperature OXYT = 0.0 + OXYT(raw) * (0.128 * 1.0)

However, OXYT was discarded in favour of CTD temperature (see Pollard, 1985).

The oxygen sensor was then calibrated with the formula:

oxygen O = OXYC * rho * exp (alpha* T + beta * P) * OXYSAT

using a least squares fit to calculate the coefficients rho, alpha and beta from bottle samples. With between ten and twelve samples for each cast, this should have given a good fit. However, it soon became apparent that many of the bottle sample values were imprecise (Read, 1989) and good values had to be carefully selected before the calibration coefficients could be calculated. This still gave widely varying results for rho, alpha and beta for each cast, so a best fit of the deep gradient over several selected casts was used to fix alpha and beta, and rho was then calculated individually for each cast. This correction improved the fit to the sample data but still caused the overall value of oxygen to vary up and down between casts. The problem was particularly pronounced when casts 11824 - 11845 were contoured. To minimise the cast to cast variations, the deepest data were fitted to the oxygen value given by Saunders (5.67 ml/l) and the offset added to the whole cast.

Project Information

No Project Information held for the Series

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

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SeaDataNet Quality Control Flags

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