Articles tagged with: Science

RHUM-RUM launched

Written by Guilhem Barruol, Karin Sigloch on Friday, 07 September 2012.

15 September 2012

Indian Ocean seen from Le Port de La Réunion

From Sept. to Nov. 2012, we will be installing most of our instruments: about 30 land stations in La Réunion, Mauritius, Madagascar, and the Seychelles, as well as 57 ocean-bottom seismometers during a month-long cruise aboard RV "Marion Dufresne". The cruise starts on September 22nd, we expect to be blogging regularly from the ship.

Le Volcan

Written by Chris Scheingraber on Thursday, 27 September 2012. Posted in La Réunion

26 September 2012

Many scientists who flew to La Réunion from continental Europe arrived some days in advance to have a look around this magnificent tropical island before embarking (embargoing?) on the Marion Dufresne. La Réunion's active volcanism is owed to its location on top of the mantle plume that our experiment plans to image by means of seismic tomography.

The "Piton de la Fournaise", the active volcano in the south-east of the island has erupted numerous times in the recent past, including an eruption lasting from August 2006 until January 2007.

Another big eruption, which occurred in 1986, increased the island size by as much as 25 hectares. The newly added area is called "Pointe de la table" and lies on the coast about 5 km north-east of Saint Philippe.

"Jardin volcanique - coastal shore line close to the famous 'Pointe de la table'

Departure!

Written by Maria Tsekhmistrenko on Saturday, 29 September 2012. Posted in Cruise 2012

28 September 2012

This is what we have been waiting for: 48 ocean bottom seismometers (Picture: Chris Scheingraber).

Anxious waiting, nervous faces all around. But suddenly the tension melts away, gives way to joy. There they are!

Our four containers are finally being released; the harbor workers are pulling up in two big trucks. They have lifted their strike after one week. To say that we are "relieved" would be a gross understatement. After days of forced inactivity, the ship becomes happily busy. Our bulky ocean bottom seismometers are being unloaded from the containers by our technicians, and the ship crew hoists them onto the helicopter deck with cranes. It takes us about three hours, and now only a cyclope could keep us from leaving.

20120928 01 container

At 3 p.m. everything is ready for departure. While the pilot is coming onboard and the crew is pulling in the ropes, the scientists are assembling on the superstructure in front, facing the harbor exit and the Indian Ocean. The sky is cloudy, it is drizzling, but nobody minds in the least – we are finally underway!

The harbor episode due to the strike already felt like an odyssey in itself, but now the real work is coming up. Still, we have used to time to get to know each other better and to bond. Our international group should be working together all the better for it.

First ocean-bottom seismometer deployed.

Written by Chris Scheingraber, Simon Stähler on Sunday, 30 September 2012. Posted in Cruise 2012

29. September 2012

First ocean-bottom seismometer deployed (Picture: Chris Scheingraber).

During the first day spent entirely on the sea, we set up the lab, tested the release mechanisms for the ocean-bottom seismometers, and successfully deployed the first seismometer.

To drop or not to drop?

Written by Karin Sigloch on Thursday, 04 October 2012. Posted in Cruise 2012

4 October 2012

Annotated map describing an OBS deployment (station RR17). (Read more bellow)

Among our most crucial decisions is where on the seafloor to drop our seismometers. Geometrically, the task is not unlike dropping them from an airplane at 3-6 km altitude, hoping that they will land in a suitable spot. Some luck is needed, because the seafloor is not mapped down to the meter scale, and because ocean currents can carry the OBS away laterally by several hundreds of meters before they reach the seafloor.

However, careful preparation can maximize the chances of a successful landing. The most basic requirement: the instrument should be recoverable when called to return to the surface – that may not be possible if it landed in a crevasse or tumbled over on a steep slope. Hence we seek out relatively flat spots of at least 1 km2 area. Such places can be found even in (underwater) mountain ranges, but dropping an OBS there requires more preparation than on a vast abyssal plain.

The other morning we had a challenging case, a targeted OBS site on the steep flank of the Mascarene plateau. The only prior bathymetry information came from "Sandwell", a jargon referring to maps extrapolated from measurements of gravity satellites – very smoothed and blurry sea-scapes compared to reality, but much better than nothing (see annotated map). "Sandwell" suggested a little flat embayment in the plateau flank, but as we approached, the highly-resolving ship bathymeter gradually revealed a steeply incised canyon, 2800 m beneath the surface. No sediments to flatten out even a small area. We collectively pondered the evolving scene, and imagined mud avalanches rushing down the canyon channel. Fact or fiction? No way to tell. It seemed unwise to drop the OBS there.

With not many options available, we targeted a small protruding ledge a few miles up-flank on Sandwell's map. The approach was steep and unpromising, but suddenly sediments started showing up on the screen of the sediment sounder. The saddle area turned out to be narrower and shifted compared to the Sandwell prediction, but 1.5 miles wide and reasonably flat seemed lucky under the circumstances. We dropped the OBS above the likely equivalent of an Alpine meadow, bounded by a steeply incised mountain stream, perhaps with bottom-dwelling fish instead of grazing cows. It had taken us two hours, but had also satisfied our curiosity. Expert bathymetrist Jérôme summed it up: "Interesting – so that's what a canyon may look like for Sandwell."

* Picture Caption : 

Annotated map describing an OBS deployment (station RR17). Before our arrival, we had only the blurry bathymetric information that fills most of the picture (“Sandwell”, extrapolated from gravity satellite measurements). In real time, we replaced a ~10 km wide swath beneath our ship track with the ship’s own, more highly resolving measurements (top left to bottom right). It revealed that the seafloor in this area was not smooth at all – it took some cruising to find even a small flat spot.

Floating seminar

Written by William Jason Morgan on Thursday, 04 October 2012. Posted in Cruise 2012

4 October 2012

Detailed bathymetry measured by our ship has been laid over a coarser, pre-existing deep-ocean map. Note the canyon that appears on the highly resolved swath. The swath is about 10 km wide, the canyon is located around 3000 m depth.

One of the more interesting aspects of a research cruise is the 'floating seminar' atmosphere. I don't mean the daily afternoon presentations in the conference room by one of the cruise participants, as interesting as they are, I mean the daily interactions of persons with various specialties from various institutions all focused on a single research project. Seeing how the ocean bottom seismometers (OBSs) are assembled, tested, deployed over the side (gently!); having discussions of what can go wrong, tales of past mistakes, improvements being made all give one a better understanding of how a seismic experiment is planned and executed far beyond what one can get from reading journal articles of OBS results.

Another center of activity and discussion is the monitor screen that displays each 'sweep' of the incoming bathymetric data. The 'multibeam' bathymetry of the Marion Dufresne has acoustic sensors that look not only straight down to the seafloor but also are aimed outward to receive echos from many angles away from straight down. Then the return echos of the many sensors are put together in the system's computer, and a 'swath' of bathymetry is produced. The moving ship results in a ≈5-km-wide ribbon of seafloor depths mapped, analogous to laying out a series of aerial photographs for land mapping.

The general locations of each OBS site were selected when the experiment was planned, but the precise location for a 'drop' is made while looking at the bathymetry collected at the site. At this time, many eyes gather in the lab around the bathymetric screen -- all looking for that perfect flat, not too rocky, not too steep spot on the ocean floor. Lots of murmur in the peanut gallery as Karin and Guilhem make the final choice -- sometimes the initial 'X' is abandoned and a 'better' place several kilometers away is selected instead.

The bathymetric screen showing the swath of bathymetry collected as we steam from drop-point to drop-point is a constant focus of interest. Many so-called "seafloor canyons" have been seen in the swaths. These features are tens of meters deep and hundreds of meters wide; they are commonly made by avalanches on the steep slopes of continental shelves which create a fast moving mixture of water and suspended sand which cuts away the flat mud on the seafloor much like a river cuts away land surface. Near continents, such 'canyons' can be traced for hundreds of kilometers; the source here mid-ocean would most likely be collapse of the slopes around the carbonate banks and islands of the Mascarene Plateau. Our cruise isn't mapping them -- following them from beginning to end -- but we have crossed and noted perhaps a half-dozen on our zig-zag path from site to site and wonder collectively where and how each may have originated.

And last night the sky was brilliant; a bright Milky Way and the bow of the ship pointing directly toward the Southern Cross.

New Diving Paradise Discovered

Written by Heiner Igel on Wednesday, 30 October 2013. Posted in Cruise 2013

30 October 2013, Heiner Igel

Looking for a nice spot to do some deep diving? Not too touristy? Well, here’s the place. Last night at around 2am (I admit I was off-shift and sleeping) we came across a spectacular feature of the Indian Ocean’s topography: a seamount.  The average water depth in the area here between Madagascar and La Réunion is about 5000m. Within a few miles this seabottom topography rises to only about 50m water depth! Picture something like Mt. Blanc under the water surface and you are not far off! It s about the same dimension.

Seamounts are a fascinating feature of our planet! They are mostly extinct volcanos that were eroded by the sea and no longer appear as islands. There is an estimated 100.000 (!) seamounts under the oceans and many  of them wait to be explored. The one we moved over last night is known from satellite based seafloor topography estimates (so, ok we did not really “discover” it) but we are the first to map it precisely with  sonar depth mapping techniques.
This mapping technique sends sonar signals into the water and produces images just like the ones we know from medical imaging. Yes, and sometimes people come into the monitoring room, look at the screen and say “oh, look it’s a boy!”.
Yann, our experienced diver tells us that  50m depth is something that still can be reached by scuba-diving. It is likely to be an extremely rich and interesting environment from a biological point of view similar to corral reefs. So consider this for your next diving vacation. But beware the next bar is about 500km away!
 
Heiner
 

 

Swell

Written by Karin Sigloch on Wednesday, 30 October 2013. Posted in Cruise 2013

29 October 2013, Karin Sigloch

 

Swell on the open ocean (from last year’s cruise).

 

The tropical storm to our north was downgraded to “a tropical depression without development potential” – uplifting news to most of us. We did experience substantial winds and especially waves over the past days. Especially during meal times in the bow of the ship, we felt the blows to our stomachs.

Interestingly, the waves originated mostly not from the nearby storm but were “swell”, i.e., far-travelled wave trains generated by distant storms, mainly in the Southern Ocean. Experiencing the swell first hand is interesting to a seismologist because it causes most of what we consider “noise” in seismograms: unwanted signal that obscures “useful” features, like earthquakes. Through a non-linear coupling process that still awaits full explanation, the shallow water waves of the swell couple into the solid earth along coastlines and rough seafloor topography. Thus a small amount of water wave energy is converted into seismic waves that travel through the earth’s interior to seismological stations worldwide.

This “microseismic noise” is present on any kind of seismological recording, even in the middle of continents, but it is particularly pronounced on ocean-bottom sensors, which are sitting in the middle of the action, so to speak. As we have perused through our newly acquired data, it has been slightly sobering to realize the signal-processing challenge that this noise will present. The swell is a big nuisance to as seismologist, but going to sea can teach something like acceptance: if the swell is so real on my stomach, no wonder it is real on my data…

 

Progress

Written by Karin Sigloch on Wednesday, 06 November 2013. Posted in Cruise 2013

6 November 2013, Karin Sigloch

 We have fallen behind on blogging during a very busy week. As we are nearing the end of Leg 1 of the cruise, we have recovered 26 ocean-bottom seismometers. So far, every instrument has responded to our calls and returned from the seafloor – the most basic measure of success. Most instruments have done their job, but not all. A few have returned without data, or only hydrophone data, and some have drained their batteries prematurely. We are sorting through large quantities of recorded time series and engineering log files, trying to understand what we have obtained in each case, and why.

Compared to the variety of complications encountered in data logging and seismometer hardware, OBS recovery operations have been quite smooth. We found that the rise time of an OBS, typically 50-70 minutes depending on depth, can be predicted to within one or two minutes. Even better, the Meteor was recently equipped with a directional radio receiver that has so far detected every surfaced OBS by its emitted radio signals – often long before our handheld radio device or eyes could have. Compared to tales of heroic search attempts in the dark ages, it has become almost too easy – but such is progress, and we love the goniometer’s beep that heralds the arrival of yet another seismometer.

While the Meteor’s officers and skilled deck crew have perfected their techniques for getting the instruments on board, some of us have perfected their documentation skills. The two photos are extracts from an underwater movie of a recent recovery.

 

 RR04 has been approached and is floating in on the starboard side of the Meteor, in an absolutely calm sea – “duck pond weather”.

 

Seconds before it is pulled out of the ocean by the ship’s crane, RR04 has to roll around – a moment captured by Guilhem’s underwater camera strapped to a long sailor’s rod.

 

News from La Pérouse

Written by Karin Sigloch on Saturday, 09 November 2013. Posted in Cruise 2013

8 November 2013, Karin Sigloch

Bathymetric data acquired on La Pérouse seamount, rendered by Carmen Gaina

Shortly before his beheading, Louis XIV is said to have asked for news from La Pérouse, the explorer whom he had sent on a famous expedition around the world, and who had gone missing the year before. It remains unclear who died first, the king in Paris or La Pérouse in the southwest Pacific, and we have no news on this matter.

But northeast of La Réunion, a very large seamount is named after La Pérouse. Satellites have long detected it thanks to its strong gravity signal, but during last year’s cruise, we were the first to map a fringe of it with high-resolution bathymetry. These are directed arrays of sound wave pings that the Meteor sends to the seafloor every 10 seconds, to map out its depth several kilometers to the left and right of the ship. This year we returned, and did two perpendicular passes across La Pérouse.

The first pass showed that it is flat-topped: a so-called guyot, probably topped and rimmed by drowned coral reefs – the first piece of news about La Pérouse. When we returned, the bathymetry in addition revealed huge submarine landslides along the flanks of the seamount.

On the bridge, it was amusing to watch colleagues excitedly crying “Only 60 meters deep! 55 meters!”, leaning over the railing to see if the shallow water would change color. Whereas the Meteor’s captain, watching his nautical chart made from low-resolution satellite data, was grumbling “It should be 2000 meters here…it should be 1500 meters…” In view of such large uncertainties about the seafloor, a point of practical importance occurs: the seamount’s corals did not rise quite as close to the surface as the reef that spelled the end of La Pérouse’s expedition 225 years ago.

 

Shark attack on the magnetometer

Written by Karin Sigloch on Sunday, 17 November 2013. Posted in Cruise 2013

16 November 2013, Karin Sigloch

 

 

Our magnetometer seems to have been bitten by a shark last night. We haul it on deck prior to every OBS recovery, and this morning it came in with significant damage. Its plastic casing bore scratch marks in numerous places and a couple of deep bite holes, from which the thick casing had cracked outward in irregular patterns.

 

 

Colleagues and the Meteor's crew were aware of similar assaults on towed equipment in the past. The magnetometer is 120 cm long and has a slender, fish-like shape. It is pulled along on a 250 meter long cable that transmits data to the ship in real time. Magnetometers could be particularly interesting to sharks because they generate weak electromagnetic fields, which the predators may be able to sense.

The interest is not mutual. The magnetometer measures neither biological signals nor properties of the water column. Rather its purpose is to record the magnetization that is "frozen" into oceanic crustal rocks, from the time each parcel of seafloor originated from molten lava along an oceanic spreading ridge. Magnetometers record spatially alternating patterns of seafloor magnetization as the ship passes over, and these patterns can be tied to the timescale of polarity flips of the earth's magnetic field. Together they tell the story of how ocean basins have originated, grown, and vanished over geological time. The part of the Indian Ocean we are currently passing through is particularly old, dating from times for which information about the configuration of continents and oceans is sparse. Unfortunately our magnetometer will have to stay on board for now, until we are convinced that its hull is still waterproof.

 

Holes in the crust

Written by Carmen Gaina on Tuesday, 26 November 2013. Posted in Cruise 2013

22 November, Carmen Gaina

 

Seafloor depths at three different scales of zoom. Arrow points at a deep depression of -5500 m, even though the area is nominally located on an underwater mountain chain, the Southwest Indian spreading ridge (3-D renderings by C. Gaina using newly acquired bathymetry data).

We’ve been cruising along the Southwest Indian Mid-Ocean Ridge for a couple of days now, and we get to see more and more its effort to break through old oceanic plates. The ridge is bending and splitting, trying hard to establish itself between the older neighbours. And then we see the “Holes”! It is like the Grand Canyon of mid-ocean ridges, but this abyss bottoms out more than 5000 meters below sea level. Cliffs rise up more than 2500 meters to either of its sides. They expose rocks that are only about a million years old, instead of billions of years on land -- just a blink in geological time. Here the Earth’s crust is ripped apart until its flesh is laid bare -- the mantle.

At mid-ocean ridges, new crust is being added every year, and this is how tectonic plates are growing and moving away from each other. However, in rare places such as this one, only a small amount of magmatic material is added to the new plate boundary, not enough to fill the gap left by the divergence of the two ridge flanks. Oceanic crust is stretched until the mantle underneath rises to the surface. The Southwest Indian Ridge and the Arctic Gakkel Ridge (the so-called ultra-slow spreading ridges) are the only places on Earth where these extreme processes produce deep “holes” where one can peer through the oceanic crust into the mantle, and find out its deep secrets.

 

All nine French OBS are back

Written by Guilhem Barruol, Karin Sigloch on Monday, 25 November 2013. Posted in Cruise 2013

25 November 2013

 

 

Under a gorgeous tropical sun, no wind, calm sea, we recovered the last of nine French OBS from the INSU instrument pool. Relief was written on the faces of our two engineers Romuald Daniel and Xuan Li, who had worked hard to ensure the good functioning of the instruments and their safe return to the surface. So the hardware recovery was 100%, and we estimate that the overall data recovery rate (duration of useful functioning) for these 9 OBS will be around 80% -- a very good result for this kind of experiment, especially since several of the instruments had been deployed for the first time. Bravo and thanks very much to Romuald and Xuan.
We need to recover 4 more of the 48 German OBS, and we are awaiting them impatiently.

 

Second Mermaid released into the sea

Written by Karin Sigloch on Tuesday, 26 November 2013. Posted in Cruise 2013

25 November 2013, Karin Sigloch

Launch of the second of two MERMAIDs, novel seismological sensors for the oceans.

Today just before leaving station RR52, we deployed a "MERMAID", a novel kind of seismological sensor for the oceans, developed by the University of Nice. A MERMAID is an Argo float that is adapted to deliver data for global-scale seismic tomography, i.e., studies that compute the 3-D structure of the earth's mantle, using signals generated by naturally occurring, distant earthquakes. Very few seismometers are deployed in oceanic areas: islands for land seismometers are sparse and unevenly distributed, and OBS are expensive and high in maintenance. The MERMAIDs, which float around passively in the oceans, are designed to fill this data gap.

Thousands of Argo floats have already been deployed by physical oceanographers, hence the "carrier" component of MERMAID technology is reliably in place. Inside sits a hydrophone to record sounds in the water column, and processing hardware and software to decide whether any of the sounds are due to earthquakes. Seismic waves traveling through the solid earth hit the ocean bottom from below, where some of their energy is converted to an acoustic wave. If the MERMAID, usually drifting at around 1000 m depth, detects this kind of acoustic signal, it rises to the surface and transmits its information to its owner via satellite communication link. Then it sinks back to the deep and waits for the next earthquake.

The two Nicean MERMAIDs were supposed to be deployed together with our RHUM-RUM ocean-bottom seismometers in 2012. They would have floated around inside our network, and signals from the two types of sensors could have been compared. Unfortunately the MERMAIDs were not finished in 2012, so that now their journey will be a lonelier one, though no less interesting. On 24/11 a sizeable earthquake (mb 5.1) occurred on the triple junction, just hours after we had recovered our station RR50 there -- no luck. But we had also released the first MERMAID there, and it has already sent home its first earthquake alert.

RHUM-RUM in Madagascar

Written by Karin Sigloch on Saturday, 30 November 2013. Posted in Cruise 2013

28 November 2013, Geneviève Roult, Karin Sigloch

As part of the terrestrial component of the RHUM-RUM experiment, we installed five broadband stations in Madagascar in 2012 together with professor Gérard Rambolamanana and his team from the University of Antananarivo. They were deployed along the southeast coast, more precisely in Manakara, Farafangana, Manambondro, Manantenina and Mahatalaky. The stations should be checked and maintained about twice a year, so this November 2013, while we were at sea, our colleagues Christophe and Richard from the University of La Réunion for the third time did the tour from Tanararive to Fort-Dauphin. And what a tour!

The rainy season had unfortunately started early this year, and the condition of the route had gone from bad to worse. The car broke down, so that halfway through, a new car had to be called in from the south. The rivers had flooded, and river ferries, many of them man-powered, had become dangerous. All the experience of our local drivers was also needed for crossing the numerous river fords. The colleagues almost got stuck but finally arrived in Fort-Dauphin safe and sound – tired, but with a good harvest of seismological data.

The 5 seismological stations will stay in place until at least May 2014, but ultimately need to be dismantled and repatriated. We are still welcoming volunteers to go on the road with us in Madagascar then…

 

[12  >>