Monterey Bay Aquarium Research Institute

 

Exploring Ancient Coral Gardens
Davidson Expedition, January 26- February 4, 2006
Equipment to be used on the Davidson Seamount Cruise:


ROV Tiburon

An 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, as well as transmitting data and high-resolution video back up to the surface. You can find out more about ROV Tiburon on the MBARI web site here: http://www.mbari.org/dmo/vessels_vehicles/tiburon/tiburon_legacy.html. 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.

Tiburon

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 kilogram chunk of lava.

Glass suction sampler

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 about 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.

Push cores on swing arm rack

ROV Tiburon also carries two instruments that let us sample sediments. One is called a push-core, and 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.

push-core rack
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 a half meter 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 a sample of seawater inside. After the ROV returns to the surface, small amounts of this water sample will be filtered to separate out particles of material that might provide food for corals on Davison Seamount. By weighing the amount of particles in different locations at different times, we can get an idea of how much food is available.

Current meters

Our main goal on this cruise will be to learn what makes Davidson Seamount (and other seamounts) such a bountiful spot for large deep-sea corals and sponges. We suspect that the corals are here because of 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 currents close to the corals, currents around the Davidson Seamount itself, and currents in the waters above the seamount.

 

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 at a frequency of 25 Hz with a very low intensity so that the volume of water measured is on the order of 1 cubic centimeter. This sound bounces off the particles in this small volume of water and back to the sensors enabling researchers to determine very detailed velocity profiles.

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.

Image courtesy of NortekUSA.

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 a day or two 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 on the seafloor near the top of Davidson Seamount. It will point upward into the water column to measure currents in the water above the seamount. These instruments emit sound at a variety of frequencies but at a relatively low intensity. Using this sound, researchers can measure currents from the instrument to as far away as 250 meters simultaneously. Like the Acoustic Doppler Velocimeter described above, it uses the Doppler effect to detect the motion of particles in the moving water.


Image courtesy of SonTek