THE GEAR

A mooring is an instrument, or series of instruments, attached along a cable that extends over a portion of the water column. A flotation buoy at the top of the mooring keeps the cable taught, and a heavy anchor at the bottom keeps the mooring from moving around. A variety of instruments can be attached to the mooring line, such as temperature or salinity sensors and current meters, which measure how fast the water is moving. These instruments collect data at specific time intervals and store the data until the mooring is recovered at a later date.

CTD is an acronym for the parameters that this device measures: “Conductivity (salinity), Temperature, and Depth.” Water temperature and salinity are important to oceanographers because they affect the movement of the water column, and thus the overall ocean circulation.

The CTD itself is a set of small probes attached to a large metal frame. The frame is lowered on a cable down to the seafloor, and scientists observe the water properties in real time via a conducting cable connecting the CTD to a computer on the ship.

Other attachments on the CTD package include Niskin bottles (gray bottles in image) that collect water samples at discrete depths for measuring chemical properties, Acoustic Doppler Current Profilers that measure the horizontal velocity (yellow instrument on side), and oxygen sensors that measure the dissolved oxygen content of the water.

This instrument provides a measure of the heat content of the water column. Resting on the sea floor for a number of years, the inverted echo sounder (IES) sends low-frequency sound signals to the sea surface, where they are reflected back down to the instrument. The time it takes for the signal to return to the instrument is a function of (1) how much water is above the instrument and (2) how warm the water column is overall. It might be helpful to know that sound travels more rapidly in warm water than in cold water. Coupling this device with a pressure sensor allows for differentiation of the two variables.

An Acoustic Doppler Current Profiler (ADCP) is a current meter that measures ocean velocity using the physical principle of Doppler Shift.

What is Doppler Shift? — Have you ever noticed how a police car siren sounds higher pitch when it’s approaching than compared to when it’s receding? The speed of the vehicle changes the frequency of the siren’s sound by compacting or extending the sound waves.

An ADCP has three or four acoustic heads, or transducers, to emit acoustic beams in different directions in order to measure ocean velocities in all three directions: northward, eastward, and vertical. Each acoustic beam penetrates a few hundred metres through the water column, with sound reflecting back to the instrument from particles in the water column along the way. The Doppler shift of the return signal tells us how fast the particles are moving, and therefore, how fast the ocean is moving. On the Research Vessel Melville, one ADCP is mounted in the hull of the ship and continuously measures ocean velocity down to 800m deep, even while the ship is moving. During the ACT Cruise, ADCPs are deployed in big floats on the top of each of the moorings to profile the upper water column. Another ADCP will be mounted on the CTD frame to measure full-depth ocean velocities.