Following is a brief outline of the major steps involved in taking a CTD cast, based on generally accepted practices. However, each ship, crew, and resident technicians have their own operating procedures. Each scientific group has their own goals. Therefore, observe local ship and scientific procedures, particularly in areas of safety. Before the cruise a discussion of the planned work is advisable between the ship’s crew, resident technicians, and scientific party. At this time discuss and clarify any specific ship’s procedures.
Note: The following procedure was written for an SBE 9plus CTD operating with an SBE 11plus Deck Unit. Modify the procedure as necessary for your CTD.
10 to 15 minutes before Station:
- Review the next cast’s plan, including proposed maximum cast depth, bottom depth, and number of bottles to close and depths. If the cast will be close to the bottom, familiarize yourself with the bottom topography.
- Verify that all water samples have been obtained from the bottles from the previous cast. If so, drain the bottles and cock them. Hand manipulate each Carousel latch as you cock the bottle to ensure it is free to release and is not stuck in some way.
- Remove the soaker tubes from the conductivity cells.
- Remove any other sensor covers.
- With permission from the deck crew, power up the CTD. Check the Deck Unit front panel display to verify communication. Perform a quick frequency check of the main sensors.
- Start Seasave. Set up a fixed display. Select Do not archive data for this cast. Start acquisition and view the data to verify the system is operational.
- Clean optical sensor windows, and perform any required air calibration.
- Stop acquisition. Do Not turn the CTD Deck Unit off. Select begin archiving data immediately. Set up the plot scales and status line.
5 minutes before Station:
- Start the ship's depth sounder and obtain a good depth reading. Be careful reading the depth sounder; if it is improperly configured the trace will wrap around the plot and be incorrect. The bottom depth should be close to the expected charted depth.
- Fill out any parts of the cast log that can be done at this time.
On Station, On Deck:
- Verify the position and the bottom depth.
- The computer operator should begin filling out the software header.
- After receiving word from the bridge that they are on station and ready to begin, untie the CTD and move it into position. If this requires hydraulics, ensure you have the appropriate people in place and permission.
- Position the CTD under the block. Have the winchman remove any slack from the wire.
- Notify the computer room that the CTD is ready for launch. The computer room should start acquiring data.
- Obtain a barometric pressure reading and note it on the cast sheet.
- When the bridge, computer room, and winchman are ready (and you have permission to proceed), put the CTD in the water.
- Have the winchman lower the CTD to 10 meters (his readout), hold for 1 minute, and then bring it back to the surface. One operator should remain on deck to help the winchman see when to stop the CTD. The CTD should be far enough below the surface so that the package does not break the surface in the swells.
CTD Soaking at the Surface:
- Finish filling out the cast log. Re-check the bottom depth.
- Fill out the computer software log.
- Hold the CTD at the surface for at least 3 minutes.
- Check the status line to verify that the CTD values are correct. The pressure should be the soaking depth of the CTD. Comparing the CTD temperature and salinity to the ship's thermosalinograph is helpful. Log the information (CTD and thermosalinograph) on the cast sheet.
Starting the Cast:
- Call the winchman and have him start the cast down. Typical lowering speed is 1 m/sec, modified for conditions as needed.
- Watch the computer output and verify that the system is working.
During the Cast:
- Closely monitor the CTD output for malfunctions. Sudden noise in a channel is often a sign of a leaking cable. A periodically flashing error light on the Deck Unit is a sign of a bad spot in the slip rings. The modulo error count (usually on the status line) provides an indication of telemetry integrity; on a properly functioning system, there will be no modulo errors.
- Note any odd behavior or problems on the cast sheet. Keeping good notes and records is of critical importance. While you may remember what happened an hour from now, in the months that follow, these notes will be a vital link to the cruise as you process the data.
- Monitor the bottom depth. This is especially critical if the cast will be close to the bottom, or you are working in an area with varying topography such as in a canyon. Running the CTD into the bottom can cause serious (and expensive) damage.
Approaching the Bottom:
- Take extra care if the cast will take the CTD close to the bottom. Monitor the bottom depth, pinger, and altimeter, if available. As you get within 30 meters of the bottom, slow down the cast to 0.5 m/sec. If you wish to get closer than 10 m above the bottom, slow down to 0.2 m/sec. Keep in mind that ship roll will cause the CTD depth to oscillate by several meters.
- If the CTD does touch bottom, it will be apparent from the sudden, low salinity spike. A transmissometer, if installed, will also show a sudden low spike.
- Adjust these numbers and procedures as conditions dictate to avoid crashing the CTD into the bottom.
- When the CTD reaches the maximum cast depth, call the winchman and stop the descent.
- Log a position on the cast sheet. If a bottle will be closed at the bottom, allow the CTD to soak for at least 1 minute (preferably several minutes) and then close the bottle. Verify that the software records the bottle closure confirmation.
- Start the CTD upcast. Stop the CTD ascent at any other bottle closure depths. For each bottle, soak for at least 1 minute (preferably several minutes) and then close the bottle.
End of the Cast:
- As the CTD approaches the surface, have someone help spot for the winchman. Stop the CTD below the surface. Close a bottle if desired.
- When ready, recover the CTD. Avoid banging the system against the ship.
CTD Back on Board:
- Stop data acquisition and power off the CTD.
- Move the CTD it into its holding area and secure it.
- See Application Note 2D: Instructions for Care and Cleaning of Conductivity Cells for details on rinsing, cleaning, and storing the conductivity cell. Fill the conductivity cell with clean DI (or 1% Triton-X) and secure the filler device to the CTD frame. Freezing water in a conductivity cell will break the cell.
- See Application Note 64: SBE 43 Dissolved Oxygen Sensor - Background Information, Deployment Recommendations, and Cleaning and Storage for details on rinsing, cleaning, and storing SBE 43 (membrane-type) dissolved oxygen sensors; see the SBE 63 manual for details on rinsing, cleaning, and storing SBE 63 optical dissolved oxygen sensors.
- Rinse any optical sensors.
- Rinse the water sampler latches with clean water.
- Draw water samples from the bottles.
After the Cast:
- Re-plot the data and look at any channels that were not displayed in real time.
- Perform diagnostics and take a first pass through processing.
- Verify that the data is good (at least on a first-order basis) at this point, when you can still re-do the cast. Many casts are lost because they are not analyzed until months later, when the problems are discovered.
- Final processing may need to wait until bottle salts and post-cruise lab calibrations are available.