Launching ROV Tiburon on board the R/V Western Flyer. Click image for larger view and image credit.
Equipment Used on the Davidson Seamount Cruise
ROV Tiburon
The most important piece of equipment on this cruise is the remotely operated vehicle (ROV) Tiburon. ROV Tiburon is a high-tech undersea robot that is connected to the surface using a very long tether. The tether carries electrical power and computer-driven instructions down to the ROV, and transmits data and high-resolution video back up to the surface. You can find out more about ROV Tiburon on the MBARI web site. ROV Tiburon does not carry people, but it does carry over a dozen different computers, several video cameras, and an array of scientific instruments. Some of the main instruments we'll use on this cruise are described below.
Niskin bottles on ROV Tiburon. Click image for larger view and image credit.
Sampling equipment carried by ROV Tiburon
Niskin Bottles
ROV Tiburon will carry Niskin bottles on this cruise to collect samples of seawater and particles within that water. The Niskin bottles carried by ROV Tiburon are gray PVC cylinders about half a meter (18 inches) tall. They have lids at both ends that are left open until the sample is taken, so that ambient seawater can flow freely through the bottle. When the scientist wants to take a water sample, the ROV pilot will send a command down to ROV, which will cause the lids on the Niskin bottles to close, trapping the local seawater inside. After the ROV returns to the surface, samples of this water will be filtered to separate out particles of material that might provide food for corals on Davidson Seamount. By weighing the amount of particles in different locations at different times, we can get an idea of how much food is available.
The Manipulator Arm
ROV Tiburon carries several different tools for collecting samples of coral, rock, or sediment. Coral samples are of course collected using the ROV's robot arm, which is called a manipulator. This arm is controlled by the ROV copilot up in the control room of the Western Flyer . With at least five different joints, it is almost as flexible as your arm, but much stronger. Advanced robotic computers allow the ROV pilots to 'feel' how hard they are gripping objects, so they can delicately tweak off the tip of a coral colony or break loose a two-kilo (five-pound) chunk of lava.
ROV Tiburon collecting a push core. Click image for larger view and image credit.
Push Cores
ROV Tiburon also carries two instruments that let us collect sediments. One is called a push-core, and it looks like a clear plastic tube with a rubber handle on one end. Just as its name implies, the push core is pushed down into loose sediment using ROV Tiburon's manipulator arm. As the sediment fills up the core, water exits out the top through one-way valves. When the core is pulled up again, these valves close, which (most of the time) keeps the sediment from sliding out of the core tube. When we bring these cores back to the surface, we typically look for living animals and organic material in the sediments.
Suction samples
The suction sampler (removed from ROV Tiburon) and its carousel of glass jars. Click image for larger view and image credit.
To collect small samples of very fine sediment, or sometimes small bottom-dwelling animals, ROV Tiburon also carries a suction sampler. Like an underwater vacuum cleaner, this consists of a long flexible tube with a nozzle on one end and a collection jar on the other. Actually, there is a carousel with 24 collection jars, which can be used for different samples. A small video camera points at the jar currently being used, so that we can see what's going inside.
Current meters
Our main goal on this cruise will be to learn why corals thrive in certain parts of the Davidson Seamount, but not in others. We suspect that the success of the corals is partly determined by local variations in currents, which carry the small organisms and nutrients that corals need to survive. For this reason, we are bringing several different types of current meters on this cruise. These current meters will help us understand current speed and direction close to the corals, in their general vicinity, and at larger distances above and around the Davidson Seamount.
Diagram of the Acoustic Doppler Velocimeter. Click image for larger view and image credit.
The Acoustic Doppler Velocimeter (ADV)
We will dive on Davidson Seamount using the remotely operated vehicle (ROV) Tiburon. ROV Tiburon will carry a small, portable current meter that will allow us to measure water flow between and around individual corals. This extremely sensitive device, called an Acoustic Doppler Velocimeter (ADV) looks like a metal rod with three prongs on the end. It measures water currents by bouncing sound waves off of particles carried within the moving seawater, and taking advantage of something called the 'Doppler effect.'
You may not have heard of the Doppler effect, but you hear the results of this effect all the time. For example, when you hear the whistle of a passing train, it starts out sounding high (as the train moves toward you) and then sounds lower when the train moves away from you. Of course, the whistle itself is not changing its pitch--the change you hear is caused by the movement of the train.
The ADV uses this same effect to measure the movement of particles in seawater. Unfortunately, most particles in seawater don't whistle like trains, so the ADV emits high-pitched clicks (much higher than humans can hear) that bounce off these particles. Three tiny microphones at the end of each prong on the ADV collect the reflected sound waves. Microprocessors within the sensor determine how much the pitch of the reflected clicks has changed, depending on whether the particles are moving toward or away from each microphone. From this information, the sensor can determine how fast and in what direction the particles (and the water around them) are moving.
The electromagnetic current meter. Click image for larger view and image credit.
The bottom-moored electromagnetic current meter
To measure near-bottom currents for a longer period, we will use a bottom-moored electromagnetic current meter (you may see this referred to as an 'S4 current meter'). This instrument measures the direction and strength of currents within about one meter (39 inches) of the seafloor. We plan to leave this current meter on the bottom for several days to find out how currents around Davidson seamounts vary, for example with the changing tides. This instrument measures water currents using four electrodes spaced evenly around the sides of a vertical cylinder. Inside the cylinder is an electromagnet. As seawater moves around the magnetic field on the outside of the cylinder, it acts like a generator, creating a voltage that is sensed by the two pairs of electrodes, which are aligned North-South and East-West. The faster the water flows, the more voltage is generated. Data about the currents is stored inside this current meter, and will be downloaded after the meter is brought back to the surface.
The Acoustic Doppler current profiler (ADCP)
The third type of current meter we will be using on Davidson Seamount is called an Acoustic Doppler Current Profiler (ADCP). We will place this instrument near the top of Davidson Seamount. It will point upward into the water column to measure currents in the water above the seamount. This instrument will send sound waves up into the water column, and can measure currents at a variety of distances from the bottom simultaneously. Like the Acoustic Doppler Velocimeter described above, it uses the Doppler effect to detect the motion of particles in the moving water.
