The gravimeter is the instrument on board that measures gravity (pictured left). We are not measuring absolute gravity, but subtle variations in the local gravity field. Differences in the volume and density of the earth below us create small, but measurable, changes in the acceleration of gravity.
The gravimeter has a mass in its center, surrounded by wire coils, with magnets above and below the mass (demonstrated in the figure to the left). Electricity is sent through the coils, altering the magnetic field in an effort to keep the mass in a constant position. Without this induced magnetic field the mass would move up and down through the coils with changes in gravity.

Measuring gravity on a ship is more complicated than on solid ground because the movement of the ship also affects the motion of the mass at the center of the gravimeter. The gravimeter has many stabilizers to keep it as still as possible, but there are still variations in gravity due to the movement of the ship, people walking by, etc. All of these small accelerations have short wavelengths, whereas the gravitational variations of interest have long wavelengths. Shorter wavelength variations are filtered out leaving only the desired changes to the gravity field.

Recording variations in gravity can help us determine the thermal variations under the Galapagos region, as plume material is hotter (and therefore less dense) than cooler older rocks on the sea floor. Gravity variations can also help us determine the volume of erupted material along the observed lineaments and how much mantle melting is required to achieve these volumes.


The magnetometer affectionately called “Maggie”, is towed about 350 meters behind the ship and about 5 meters below the surface of the water. In the picture to the left Drew and Brandi are deploying the magnetometer off the back of the ship.

There is a glass cylinder filled with alcohol in the magnetometer. The particles are charged so that they all align in the same orientation, then the charge is released, and a sensor detects how far the particles move and in what direction. This change in orientation is dependent on the magnetic field. The releases in charges occur about once a second.

The variables that affect the magnetic field are how close rocks are below the magnetometer, how much iron is in these rocks, and the orientation of the iron, which is dependent on the orientation of the magnetic field at the time the rocks cooled. We are currently in a positive magnetic field (where magnets point north), but periodically the magnetic field flips, and this can be seen in the rocks.

The strongest signal from the magnetometer occurs when rocks are closer to the surface, have lots of iron in them, and have iron oriented in the same direction as the current magnetic field. A weak signal, however, can occur when rocks are far from the surface, have very little iron in them, or have iron that is oriented in the opposite direction as the current magnetic field.
The data from the magnetometer will allow us to determine where the magnetic field shifts in the rocks of the seafloor. This will be especially interesting if we see magnetic highs or lows in areas we don’t expect them. This may indicate ridge jumps or offsets across pseudo faults.

The magnetometer is often attacked by sharks. It sends out a pulse every second, and sharks seem to think this sounds like a delicious heartbeat. The old magnetometer with several bite marks is pictured left.

3.5kHz Knudsen Echo Sounder – Sub Bottom Profiler

The sub bottom profiler is a hull-mounted system that sends a single 3.5kHz beam down to the sea floor about every 3.5 seconds. This beam penetrates about a meter below the sediment before bouncing back. This allows us to get a single line of data below the ship where we can see what the rocks look like beneath the sediment. This is especially useful in highly sedimented areas where we think faulting has occurred, because it allows us to see beneath the surface to determine if the rocks below the surface show signs of faulting.

The detector used for the 3.5kHz Echo Sounder is also what we use to monitor the depth of the 12kHz pinger we attach to the line when we dredge. When we dredge we attach a 12kHz pinger 300 meters above the dredge so that we can determine how far from the sea floor the dredge is.

Multibeam Bathymetry - The EM122

The EM122 is a sophisticated sonar system mounted under the hull of the ship that measures bathymetry, or water depth. Initially most of our study area only had bathymetric data derived from satellite altimetry. The maps based on these data are not very accurate. For example we found a 5 km wide seamount that was mapped over 10 km off from its actual position. Satellites can also only detect changes in depth over areas greater than several square kilometers. The bathymetric data from the EM122 is accurate to 10 meters vertical resolution, with a spatial resolution of 50 meters. Because of the greater accuracy and coverage of the EM122 system, we have found many features that had never been observed before. The upper left picture is a map of our most recent bathymety data, with dredge sites labled. In the picture at the lower left Drew and Will watch the bathemetric data from the EM122, as well as all of the other monitors

The EM122 works in a similar way to the MR1 (described below). An auditory signal (ping) is sent down to the sea floor and out at a 65 degree angle about every 8 seconds. These sound waves hit the bottom and bounce back. There are detectors on the ship that record these sound waves returning and convert this travel time to depth. The EM122 software plots these data on a GPS grid so that we can watch the swath of bathymetric data build in real time. Using this system we can map a swath 5-8 km wide on the sea floor as we pass over it. On the left is a picture of a swath of data as it appears on the screen.

The speed of sound through the ocean varies depending on the temperature (and to a lesser extent the salinity) of the water. To compensate for temperature changes with water depth we run an XBT (Expendable Bathymetric Thermograph) about once a day, or whenever currents change dramatically. Bud is pictured at the left, as he releases an XBT.

The XBT is a weighted probe connected to a long, thin coil of copper wire that is launched from the ship into the water. The copper wire is connected to electronics that detect changes in temperature with depth down to 1000 meters. Once the data are recorded, the copper wire is ripped off to release the probe (hence the word expendable). The temperature changes recorded are converted to sound velocity through water, then the graph of sound velocity with water depth is loaded in to the EM122 system, so that the speed of the acoustic rays of the multibeam can be more precisely determined, resulting in greater accuracy in the bathymetric measurements. On the left is a picture of one of these graphs.

The MR1 Sidescan Sonar

We have mentioned the MR1 briefly before, but thought it deserved its own feature. The MR1 Fish is towed approximately 300 meters behind the ship and about 100 meters under the surface of the water. A sonar signal (ping) is sent down to the sea floor and out to an angle of 65 degrees every 8 seconds. The MR1 then listens for the return of the signal, creating a swath of data about 10km wide below the ship. The first three pictures to the left show the MR1 Fish being put in to the water.


If these sound waves hit a hard surface (like fresh lava), more of the signal bounces back, showing up as a highly reflective white surface. If the sound waves hit a soft surface, like sediment, they travel through the sediment up to about a meter (depending on the composition and density of the sediment). If the sound waves do not hit a hard surface by that time, the signal is not returned, and lack of reflectivity is shown by a black image. If the sound waves hit a hard surface less than a meter under the sea floor, some of them may still bounce back, resulting in various shades of gray. To make this image look more like the sea floor we take a negative of the image, so that lava flows appear dark and sediment is lighter in color. The next three pictures to the left show examples of these images once they have been processed.


If the sound waves hit a hard cliff facing the MR1 they also bounce back strongly, resulting in a black area. However, if there is a cliff facing away from the MR1 it will show up as a shadow, since the signal cannot reach to the other side of the cliff. This is demonstrated in the figure to the left.


We monitor these computer screens 24/7, partially so that we can make sure the MR1 fish is staying at the right depth under the surface of the water and that everything is running smoothly, but also so that we can look at the images as they are created, so that we can start interpreting the data. On the left is a picture of Marques watching the sonar data as it appears on the screen.

We use this sonar data (in tandem with bathymetric data) to decide where we want to dredge for rock samples. Areas of high reflectivity, and therefore hard fresh lava, are the best areas to dredge for rocks.

The TowCam

The TowCam is a high tech underwater camera developed by Dan Fornari and his engineering colleagues at WHOI. The TowCam is designed to be towed behind a research vessel about 3-5 meters above the sea floor. It can be used in waters as deep as 6000 meters. A previous version of the TowCam was snagged on the seafloor in the Galapagos on the NW submarine rift zone of Fernandina volcano in 2001 and lost. The TowCam we are using for this cruise was built just last week using one of the CTD frames on the R/V Melville. Some of the features of the Tow Cam include:

1a. A Nikon 995 camera in a special pressure housing made by DeepSea Power & Light, San Diego, CA, which protects it from water and pressure at depth. The camera points straight down and takes pictures of the sea floor at 10 second intervals.
1b. The camera will be secured at the bottom of the TowCam.

2. Powerful strobe lights illuminate the sea floor when taking an image.

3. Light sensors detect the strobe lights and send a small current to the CTD confirming that a photograph has been taken.

4. A SeaBird Model 25 CTD measures conductivity, temperature, and depth and provides telemetry to tell us how far off the bottom the TowCam is while towing it and also if there are any obstacles in front of us that we have to pull up to avoid.

5a. Stainless steel wax balls coated in surfboard wax weighing 8 lbs. each can be dropped onto the sea floor to pick up volcanic glass shards to be analyzed back in the lab. As many as 4 wax balls can be dropped to the sea floor on command. They then bounce around on the sea floor picking up glass shards before they are winched back up and brought to the surface with the TowCam.

 5b. A wax ball without any wax on it yet.

6. Four deep-sea batteries housed in orange plastic cases are used to power the strobe lights, camera and winch. All of the empty spaces in the battery cases are filled with oil.  The oil compensates the batteries so that there is no air in them and protects them from the high pressures in the deep ocean.

7. Power distribution junction boxes distribute power and electrical connections to the different components on the TowCam.

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