The OHA-GEODAMS Seafloor Observatory
@geodams.bsky.social
OHA-GEODAMS is a submarine geodetic, hydro-acoustic and seismological observatory aimed at measuring seafloor spreading as it happens across the Southeast Indian Ridge and Amsterdam transform fault (37ºS). Funded by https://anr.fr/Project-ANR-24-CE49-7271
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! 🤞
February 1, 2025 at 7:18 AM
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! 🤞
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.
February 1, 2025 at 7:18 AM
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 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.
February 1, 2025 at 7:18 AM
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.
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
January 31, 2025 at 4:18 AM
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
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.
January 31, 2025 at 4:18 AM
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).
January 31, 2025 at 4:18 AM
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.
January 31, 2025 at 4:18 AM
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.
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
January 31, 2025 at 4:18 AM
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
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!
January 31, 2025 at 4:18 AM
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.
January 31, 2025 at 4:18 AM
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.
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.
January 31, 2025 at 4:18 AM
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.
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.
January 31, 2025 at 4:18 AM
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.
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...
NOAA Ocean Explorer: Submarine Ring of Fire 2003: Hydroacoustics Animation
oceanexplorer.noaa.gov
January 30, 2025 at 6:17 PM
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.
January 30, 2025 at 6:17 PM
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.
January 30, 2025 at 6:17 PM
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...
January 30, 2025 at 6:17 PM
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.
January 30, 2025 at 6:17 PM
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.
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!)
January 30, 2025 at 3:03 PM
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!)
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.
January 30, 2025 at 9:17 AM
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.
This local network will let us locate the events with high precision to characterize how mid-ocean ridge faults slip.
January 30, 2025 at 9:17 AM
This local network will let us locate the events with high precision to characterize how mid-ocean ridge faults slip.
We deployed 7 Ocean Bottom Seismometers (OBSs) to record earthquakes and other signals emitted by the Southeast Indian Ridge and the Amsterdam transform fault.
January 30, 2025 at 9:17 AM
We deployed 7 Ocean Bottom Seismometers (OBSs) to record earthquakes and other signals emitted by the Southeast Indian Ridge and the Amsterdam transform fault.