The OHA-GEODAMS Seafloor Observatory

Schematic illustration of the OHA-GEODAMS seafloor observatory. Credit: J.-A. Olive (LG-ENS) & J.-Y. Royer (Geo-Ocean)

If it all goes well, our now-complete observatory will be the first to document seafloor spreading and transform faulting events with geodesy, hydro-acoustics, and seismology! 🤞

Expected pressure variations solely due to tides at our study area for the entire year 2024, from the EOT20 model of https://essd.copernicus.org/articles/13/3869/2021/. Credit: J.-A. Olive, LG-ENS

But pressure at the seafloor can fluctuate for many reasons! Tides, ocean dynamics, etc... To avoid misinterpreting an oceanographic signal as a tectonic signal, we deployed a mooring that will help us assess how the weight of the water column changes through time.

The A-0-A pressure sensor in its frame, hanging from a cable on the starboard side of the ship at night, waiting to be deployed. Credit: J.-A. Olive, LG-ENS / FOF

The A-0-A lets us measure vertical displacements of the seafloor: if the ground swells, the instrument will go up, feel less water weight, and the pressure will decrease.

An A-0-A pressure sensor, encased in a frame attached to four buoys floating on the ocean surface, waiting to be picked up. Credit: J.-A. Olive, LG-ENS / FOF

Last, but not least, we also recovered and re-deployed an A-0-A pressure sensor, right in the middle of the ridge's axial valley. This state-of-the-art instrument corrects its own drift by regularly re-calibrating itself against an inner chamber where the pressure is known.

Water color drawing of an acoustic transponder at the seafloor. Credit: J.-A. Olive, LG-ENS

But last week we succeeded in recovering and re-deploying 1 beacon from the ridge network, and 1 from the transform network. The first year of data did not disappoint! #ToBeContinued

Jean-Yves eagerly waiting for the transponder to respond. Credit: J.-A. Olive, LG-ENS / FOF

The control center from where we follow the transponders' deployments. Credit: J.-A. Olive, LG-ENS / FOF

Some deployments can take up to 11 hours, as we go through trial-and-error on different target sites with the tripod hanging from the ship on a 2-km long cable.

Deployments are tricky because we can only communicate with the beacon through an acoustic modem, and sometimes they're in no mood to talk to us (or the ship is just too noisy).

When we redeploy a transponder+tripod, we must always make sure it's standing upright on the seafloor, and that it can still ping its colleagues.

Julie, Anne and Edgar carefully assembling a metal tripod to redeploy an acoustic beacon. Credit: S. Furst / FOF

Preparing new tripods for redeploying acoustic transponders. Credit: S. Furst / FOF

Lise, Julie and Pierre-Yves admiring a reconditioned acoustic transponder, awaiting its tripod for redeployment. Also visible on the left in the ship's hangar: a couple of OBSs! Credit: J.-A. Olive, LG-ENS / FOF

Once the beacon is on board, we download the data, and place it atop a new tripod, to be re-deployed from the back deck, by cable. @jbeesau.bsky.social @pyraumer.bsky.social @annebriais.bsky.social @sismolise.bsky.social

a yellow float containing an acoustic beacon just surfaced near the ship. Credit: J.-A. Olive, LG-ENS / FOF

A yellow buoy containing a acoustic beacon is being recovered on the starboard side of R/V Marion Dufresne. Credit: J.-A. Olive, LG-ENS / FOF

Spotting the beacon when it surfaces can be challenging! It's essentially a meter-long pink cylinder encased in a yellow buoy. Lucky for us, the bridge crew has sharp eyes!

Last week, we paid a visit to our beacons and were delighted to find them all alive and pinging! Downloading the data from the ship with an acoustic modem was possible, but excruciatingly slow. That's why we recovered and re-deployed a couple of beacons to get a year's worth of geodetic data.

Simulation of the path of an acoustic ray at the bottom of the ocean, from an emitter (green star) to a receiver (red star). Black curve shows the ocean floor topography. Colors indicate the attenuation of the acoustic signal. Credit: J.-A. Olive, LG-ENS / CNRS

By measuring acoustic travel times and the speed of sound in the ocean, we can infer how the mid-ocean ridge axis stretches, and the adjacent transform slips.

A schematic illustration of our acoustic ranging network across the Southeast Indian Ridge. Credit: J.-A. Olive & J.-Y. Royer, LG-ENS / Geo-Ocean / CNRS

As tectonic plates drift further and further apart on either side of the mid-ocean ridge, sound waves take a longer and longer time to travel from beacon to beacon.

2 iXblue/Exail Canopus acoustic transponders encased in yellow floats, atop 2-m tall metal tripods, waiting to be deployed on the back deck of R/V Marion Dufresne during the 2024 GEODAMS cruise. Credit: J.-A. Olive, LG-ENS / FOF

The core of the OHA-GEODAMS project is to measure active deformation for 3 years on the Southeast Indian Ridge and the Amsterdam transform fault. To this end, in February 2024, we deployed 15 acoustic beacons that have been pinging each other every few hours ever since.

They also record whale songs, and even the sound of icebergs grinding against each other! See for example this great explainer video from @noaa.gov : oceanexplorer.noaa.gov/explorations...

Once comfortably settled in the SOFAR channel, hydrophones record the sound made by earthquakes near and far, and let us relocate them with much better accuracy than distant seismometers on land.

We then release the weight, sinking the entire mooring down to the seafloor in a matter of minutes.

We start by deploying the float containing the hydrophone from the back deck, then unroll the ~2 km long mooring...

Hydrophones are basically big microphones moored ~1000 m below the sea surface, in the SOFAR channel, where acoustic waves can travel thousands of kilometers across an ocean basin with minimal attenuation.

a hydrophone encased in an orange float just surfaced right in front of the R/V Marion Dufresne, on a calm sea. Credit: J.-A. Olive, LG-ENS / FOF

We also recovered and redeployed five hydrophones that were first deployed last year during the #GEODAMS 2024 cruise.

We send an acoustic signal that releases the instrument from its anchor. The buoys bring it to the surface at a little under 1 m/s, and we find it with the help of flashing lights and radio signals (but that’s for next year!)

A broadband OBS being deployed from the side of the ship. The green sphere is the instrument, kept away from the noise of the batteries and data loggers by the white "arm". Credit: L. Retailleau, IPGP/FOF

Using broadband OBSs, we will also carry out compliance measurements: that's looking at how the oceanic crust responds to changes in water pressure at the seafloor, due to tides and ocean dynamics. This should tell us whether magma is present at depth in the crust.

Deployment of a short-period OBS, hanging on a cable over the ocean, from the side of the ship. Credit: L. Retailleau. IPGP/FOF

This local network will let us locate the events with high precision to characterize how mid-ocean ridge faults slip.

water color by D. Pacaud illustrating the deployment of an ocean bottom seismometer from the Marion Dufresne

We deployed 7 Ocean Bottom Seismometers (OBSs) to record earthquakes and other signals emitted by the Southeast Indian Ridge and the Amsterdam transform fault.

Regional map of the Southeast Indian Ridge near Amsterdam Island

For the last ~10 days, we've been hard at work on and around the Southeast Indian Ridge at 37ºS to recover and redeploy a bunch of geophysical instruments. Let's break it down 👇

We will recover it at the location of our last hydrophone, West of Amsterdam, next week.

Deploying the glider from the workboat, with the Marion Dufresne in the background

This glider is equipped with a hydrophone (48kHz) to record the underwater soundscape, and in particular the cetaceans (i.e., blue whales, fin whales, killer whales, sperm whales, etc.). The glider will navigate south and west of Amsterdam, diving down to 1000 m, driven by buoyancy changes.

In the meantime, Julie stayed on the Marion Dufresne, to manage communications between the pilot team in Brest and the team on the workboat.

Séverine, Anne and Diane all dressed up to deploy the glider from the workboat

Deploying the workboat, with the glider in the front

In order to prevent the glider from colliding with the Marion Dufresne, it had to be launched at a certain distance. To do so, 3 brave members of our GEODAMS team, Anne, Séverine and Diane, boarded a workboat with 2 LDA crew members.

Julie & Jean-Arthur bringing the glider to the workboat for deployment. Credit: E. Klein

A few days ago, we deployed a Sea Explorer glider developed by ALSEAMAR and owned by ENSTA Bretagne at the location of our first hydrophone, southeast of Amsterdam island.