Instrumentation

CTD

This widely used instrument is called a CTD (Conductivity-Temperature-Depth). It provides continuous profiles of the temperature, conductivity (from which salinity can be derived) and pressure (or depth) in the ocean. These measuremennts are obtained by means of a set of small probes attached to a metal rosette wheel. The CTD package or rosette contains 24 Niskin bottles that are filled with seawater. The salinity of these samples is measured in the ship later, and used for the calibration of the CTD salinity data.


Vertical Microstructure Profiler (VMP)

The VMP is a free-fall instrument that measures temperature, salinity and velocity profiles with very high vertical resolution (down to 1 cm). From these, the properties of small-scale turbulence and internal waves (waves that travel along the interfaces between water masses of different density, similar to waves in the ocean surface) in the ocean can be studied, including the rate at which different water masses mix. The VMP is free-fall to avoid vibrations induced by ship motion, which in conventional CTD / LADCP measurements masks turbulent signals. Before deployment, the profiler is ballasted with weights and programmed to dive to the required depth. When that depth is reached, it releases the weights and ascends to the surface, where it can be recovered upon completion of the CTD / LADCP cast. The recovery operation can take up to an hour.






ISW profiler

The ISW profiler, produced by Sea and Sun Technology GmbH in co-operation with ISW Wassermesstechnik, is an instrument for simultaneous microstructure and precision measurements of physical parameters in marine waters. It is designed for rapid vertical profiling within the upper 300 m and is lightly tethered to the ship during deployment. Data from the profiler are transferred via the tether to a laptop PC. The ISW profiler is equipped with 2 velocity microstructure shear sensors (for turbulence measurements), a microstructure temperature sensor, standard CTD sensors for precision measurements, a turbidity (light scattering) sensor, a vibration control sensor, and a two component tilt sensor. All sensors are mounted at the measuring head of the profiler (sensor end) with the microstructure sensors at the tip of a slim shaft, about 150mm in front of the CTD sensors. During deployment, the ship moves with speed approx. 0.5-1.0 knots with respect to the water against the wind. Disturbing effects caused by cable tension (vibrations) and the ship’s movement are minimized by maintaining slack in the tether. Typically 10 profiles are taken per station.




Shipboard ADCP

ADCP is short for Acoustic Doppler Current Profiler. It is an acoustic instrument that measures the velocity of the water currents underneath the ship relative to the ship speed. To do this, it sends out sound pulses (aka 'pings') of a set frequency. The pings are reflected by particles in the water like plankton, that move essentially at the same speed as the water. The Doppler effect results in a frequency shift in the reflected ping, and using that and the speed of sound in water, we get the current velocities. The RRS James Cook has two ADCPs onboard: a 150 kHz and a 75 kHz instrument. The higher frequency does not penetrate very deep but can give higher vertical resolution, whereas the lower frequency can reach as far as 800 m below the ship. As the ADCPs measure continously throughout the cruise, they help to detect the positions of the ACC fronts while steaming.

LADCP

The L in LADCP stands for Lowered. Just like the shipboard ADCP, the LADCP uses acoustics to measure water current velocities by sending out sound pulses and recording their Doppler shift. The LADCPs, however, are mounted on the CTD frame and lowered with the CTD on every station. By this we get full depth current velocity profiles. The measured velocities can be used e.g. to calculate transports or as reference for geostrophic velocities.



Floats

Like many of the instruments on the ship, profiling floats measure the temperature and salinity of the ocean at various depths. The difference is that the profiling floats are set free to drift with the ocean currents. Like robots they follow the mission that was programmed into them, sinking deep into the ocean to take their measurements and returning to the surface to transmit the information to satellites and then back to shore. We will deploy 32 profiling floats during our voyage, contributed by float groups in the USA and Australia. Of these, 18 are standard Argo floats that return to the surface every 10 days (see http://www.argo.ucsd.edu for more information). Six are a more sophisticated type that use satellite phone communation to return their data and to receive updates to their mission. Initially these floats will return ocean profiles every 2 days. The remaining 8 are a brand new type of float that measures ocean velocity, in addition to temperature and salinity, every 1.5 days and also uses the 2-way satellite phone communication. With measurements of velocity, we can not only observe changes in the ocean but we can understand why those changes are taking place. In total, the floats will return around 600 profiles of the upper ocean across the northern Kerguelen Plateau, filling in the gaps between the ship tracks. Once the ship returns to port, the floats will continue transmitting data for several years on their journey across the Southern Ocean.




Current Meters

A variety of single point and profiling current meters, including Aanderaa RCM 8, and 11's and Doppler Aanderaa Seaguard, Sonitek and RDI Workhorse instruments, and Seabird micrcats will be deployed in a mooring array of five moorings to measure ocean velocity, temperature and salinity. The moorings will be deployed in approximately 2000 m and 2500 m water depth. Three mooring will be deployed for approximately 40 days and two moorings will be deployed for two years. Current meters on the short term (40 days) moorings will provide high temporal resolution of ocean velocity near a complex topographic feature on the northern flank of the Kerguelen Plateau. The current meters will sample at the frequency necessary to adequately sample the internal wave period. The long-term (two year) moorings will provide temporal velocity and property (temperture and salinity) records needed to estimate of the long-term eddy variability of the region.


McLane Moored Profiler (MMP)

The MMP is a moored instrument that uses a traction drive to crawl up and down a taut mooring wire. As the MMP crawls up the mooring wire a CTD and acoustic current instrument measures temperature, salinity and velocity a high vertical resolution. The egg-shaped cross-section, faired end caps, and smooth external shell of the MMP give the vehicle low hydrodynamic drag and profiling stability. At the same time the shape accommodates a cylindrical housing that has sufficient length for batteries and electronics. In SOFINE an MMP will be deployed in the lower 1000 m of the three short-term moorings. The MMP will sample at the frequency necessary to adequately sample the internal wave period.





Site maintained by Peggy Courtois; last modified: 14:20, 28 April 2014