The Research Begins: Dropping Instruments Into the Abyss

Aboard the R/V Marcus G. Langseth, Expedition MGL2204’s science team has started deploying ocean-bottom seismometers.

Brandon Shuck
June 10, 2022

We began our expedition by heading full steam to the East Pacific Rise, a seafloor spreading center at the Cocos Ridge, where new oceanic crust of the Cocos plate is being formed. Here, we will begin collecting our geophysical data by deploying ocean-bottom seismometers (OBS).

Once we reached our study region, we activated the multibeam bathymetry equipment and sub-bottom profiler, and the splendid sounds of sonar pulses put a cheerful mood on everyone aboard the vessel. Together, these data produce a detailed map of the rugged seafloor marked by extinct volcanoes and deep scars. Since there are very few sediments blanketing the Cocos plate in this region, our maps expose this tectonic history of magmatism and formation of new oceanic crust at the mid-ocean ridge, and its ensuing deformation as it cools, cracks, twists, and bends during its journey towards the Mexico subduction zone.

bathymetry data
Multibeam bathymetry mosaic of the seafloor topography across the Orozco Fracture Zone. The colors represent the depth of the seafloor (warm=shallower, cool=deeper) in a swath beneath our ship track. The lineations represented faulted oceanic crust that changes orientation as the oceanic plate is deformed by tectonic stresses. Image: Brian Boston

The OBS program forms a major component of our scientific expedition, as these instruments will allow us to study the physical properties of the oceanic plate and determine how much it has cracked and become filled with seawater, which could lubricate its dive beneath the Mexico continental plate.

At its core, the OBS holds a seismometer to record acoustic signals and is protected inside a pressure vessel. A meticulous workflow is followed to assemble the instruments, program the seismometer, synchronize GPS and clock, and load them to the starboard deck of the Langseth. From there, we say “so long” for now, as each OBS is carefully tossed overboard, where a metal weight on the base of the instrument drags it several thousand meters down to the seafloor. Once in the deep dark abyss of the seafloor, the sensitive devices inside the OBSs record minuscule — but critical — motions of sediments on the seafloor and pressure changes in the water column due to sound waves generated by our ship, the Langseth.

photos of people preparing the obs instruments
Preparing ocean-bottom seismometer instruments before deployment. Top left: An unassembled ocean-bottom seismometer. Bottom left: Brian Boston (LDEO), Jorge Real-Pérez (UNAM), and Langseth science technician Cody Bahlau working to prepare the instrument. Right: A fully assembled ocean-bottom seismometer waiting to be deployed. Photos: Brandon Shuck
Blog2 fig3 OBS 478x637.png
Science team and the Langseth crew moments before deploying an instrument. The team safely attaches the instrument to the A-frame which uses hydraulics to lift the instrument over the side of the ship and then release it to begin its descent to the seafloor. Photo: Brandon Shuck

After nearly a day of deploying instruments, our first profile is completely instrumented and the OBSs are happily recording, awaiting our seismic signals. Our first profile contains 26 OBS instruments spaced about 10 kilometers apart, extending from the Cocos mid-ocean ridge where the oceanic plate is born to around 300 kilometers away, where it has aged to approximately 5 million years old.

Next, we will prepare and deploy seismic sources so that we can start collecting data. All of the science team is itching in anticipation to see what we discover!

Blog2 fig4 pano 637x199.jpg
Panorama of the Langseth. Ocean-bottom seismometer operations take place on the main deck on the starboard side of the vessel. Multichannel hydrosphere streamers are shown on the right, which we will use in the second half of our experiment to collect a different type of data along the same profiles. Photo: Brandon Shuck

Brandon Shuck is a postdoctoral research fellow at Columbia Climate School’s Lamont-Doherty Earth Observatory.