Mapping the Galicia Rift off Spain

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Exploring ancient continental breakup west of Spain
Updated: 19 min 2 sec ago

Poseidon Visits (and Seismic Oceanography)

Mon, 06/10/2013 - 12:41
One of the secondary activities on the cruise has been the deployment of XBTs off the stern. XBTs are a standard oceanographic tool designed to measure the variation of water temperature with depth, providing information on mixing processes within the water column. As temperature is one of two primary controls on velocity of sound in water (the other being salinity), it is also of interest in the processing of our bathymetric data.

Poseidon's Zodiak on the way over to exchange supplies.
A few years ago, it was realised that seismic provides a method of directly observing the mixing processes, as the different water layers have sufficiently different seismic velocity and salinity for reflections to be generated at their boundaries: we have already seen reflections in the water column of our data, probably from boundaries between North Atlantic water and warmer, more saline Mediterranean water. However there have been relatively few studies of these processes using traditional oceanographic and seismic techniques, a deficiency being rectified by the deployment of XBTs at regular intervals during our cruise.

A successful exchange on medium-high seas!!
In addition to deploying ocean bottom seismometers to record our seismic shots, the German research vessel F.S. Poseidon has been carrying out oceanographic measurements, mainly using CTD casts (conductivity-temperature-depth), which provide more information than XBTs. As a result they had several XBTs left over. These they transferred to us this morning: Poseidon came within about 1 km of the Langseth and sent the XBTs over in a small boat. A real bumpy ride!

Goodbye, until we meet in Vigo!Tim Reston
University of Birmingham

Miracle workers of the Langseth overcome the curse of the Costa da Morte

Mon, 06/10/2013 - 00:32

After days of uneventful and productive data acquisition, a pale fell over the R/V Langseth. Early Sunday morning, one of the streamers began to report communication errors and soon failed to communicate at all.  A series of tests over the ensuing hours revealed that the problem was not on the ship but in the equipment out in the water.  Recovering and repairing seismic gear is not a quick task. To access this streamer, we had to undo many of the steps required to put it out to begin with: recover the port paravane, shift Streamer 3 starboard and out of the way, and then reel in part of Streamer 4. After hours of troubleshooting, the technical staff of the Langseth brought Streamer 4 back to life.  All of the equipment on the Langseth is… not new, and this certainly applies to the seismic streamers. The technical staff on the ship are pros at keeping this equipment alive (and many cases bringing it back from the dead). Twelve hours after the problems with Streamer 4 began, it was back in the water, and we were ready to start collecting data again. But no sooner had one problem been solved, another appeared. This time the trouble arose from the failure of a piece of equipment on the ship that is at the heart of our acquisition system – the real time navigation unit (or RTNU, for those in the know). This component gathers satellite and other navigational information from the seismic equipment and delivers it to the navigation software on the ship so that we can determine the positions of all of our equipment in the water, and where and when we need to be shooting.  Once again, the dedicated technical staff of the Langsethcame to the rescue.  Painstaking checking and double-checking of each component in the RTNU began last night and continued into the early hours of the morning. In the wee hours, it’s easy to get a little superstitious.  Did all these problems arise because Tim Reston and I each accidentally drew in lines on our chart indicating that we’d completed lines in our 3D box before we actually had? Or was it the curse of Costa de Morte (Coast of Death)? This part of the Galician coast is known for its shipwrecks and nicknamed accordingly. Of course, the real culprit was the non-newness of the gear in question. Once again, the Langseth’s miracle workers saved the day by assembling the working parts of various old RTNU’s into one working unit.  Thanks to their efforts, we are up and running again….
RTNU carnage on a table in the main labDonna Shillington10th June

Poseidon: OBS deployment update

Sat, 06/08/2013 - 07:13
On 5th of June, Poseidon deployed her last three OBH instruments. The crew then spent the next two days doing CTD ("conductivity, temperature, depth") measurements of the water column. They typically recovered good measurements of conductivity and temperature for depths down to 1000 m. These measurements can be used to monitor mixing of different water bodies (such as warmer Mediterranean waters with the cold Atlantic) and to calculate variations in velocity within the water column to compare with seismic reflections we observe within the water column. Rough seas for the last 1.5 days have made the CTD measurements challenging.

Today the Poseidon is recovering eight OBH to download the data they recorded and redeploy them elsewhere within the 3-D box. It will be exciting to see the first OBH data! We won't see the rest of the data until the remaining OBS and OBH are recovered in August and September.

Despite being in the same area, here on the Langseth the science party hasn't seen the Poseidon since our first day passing them on the way out to sea from Vigo. However, this may be because we are all busy below deck in the main lab (with no windows) processing data!

Marianne Karplus
8th June

Underway and beginning to collect data

Fri, 06/07/2013 - 23:43
For the last couple of days, we have been slowly (very slowly) steaming along at 4 knots (~4.6 miles an hour) towing all of the gear behind the ship and collecting seismic data. A lot of data! Each of the four seismic streamers behind the ship records returning sounds waves on 468 channels. Every time one of our air gun arrays fires, we collect 60 Mb of data.  Repeat that every 16 seconds for a few days, and it adds up.  Even though we have only been at it for a few days, we have already generated 405 Gb of raw seismic data, and that does not include all of the other types of marine geophysical data that we collect (bathymetry, magnetics, etc). Nonetheless, there are many reminders that we still have a long ways to go.  For example, a large map on a table in the main lab shows all 56 profiles that we plan to acquire during this cruise in our target area for 3D imaging (black horizontal lines in the image below). As we complete them, we draw a green line along the profile on the map. Four down, fifty-two to go! 

Donna Shillington8th June
Map in the main lab showing planned profiles. The ones we've already completed are in green
*Follow our progress on the "Survey Area" page as we update the sail lines every ~4 days.

The Source

Thu, 06/06/2013 - 06:07
Our fourth (and final) gun array was deployed last night!! This means that all of the hard work that the crew has performed (with our help, of course) will begin to pay off as the data streams in while we traverse east along the western most extension.

Marine reflection seismology involves actively generating soundwaves (rather than waiting for earthquakes as in many other types of seismology). The ideal seismic source is as close to a “spike” as possible. Sound waves from the source travel into the Earth, where they reflect off sedimentary layers as well as hard-rock surfaces. The returning reflections are recorded by over a thousand hydrophones (underwater microphones that gauge pressure changes created by the reflected seismic waves) in the streamers that we have been deploying for the last four days.

The source consists of a series of air guns of varying sizes, which are hung at a depth of 9m (~30 feet) below large inflatable tubes. The tubes are 60m (~200 feet) long and each has 9 active air guns (10 with one to spare). In our case there are two sets of air guns being towed 150m (~500 feet) behind the ship, that alternately fire. To create a strong source that is as spike-like as possible, the guns are carefully arranged and fire almost simultaneously. The air is released from the chamber of the air gun, creating a 3300 cubic inch bubble pulse, which collapses to create the sound waves.
Orientation of the streamer and gun arrays being towed by R/V Langseth.
The red circles indicate the location of the gun arrays.We are making sound in the ocean, where many mammals use sound to communicate and hunt for food. In order to ensure we are operating responsibly and minimizing our impact on mammals, we have five Protected Species Observers (PSO’s) onboard who both watch and listen for (from the observation deck in Donna’s previous post) any marine mammal that comes close to the ship. If any are spotted or heard within a specified radius around the ship, we power down the guns until they leave the area.
James Gibson

Langseth: The paravanes are out!

Mon, 06/03/2013 - 23:24
Most of the science team came out on deck this afternoon to watch the starboard-side paravane deployed in relatively calm waters under partly cloudy skies. The technical and engineering crew proceeded slowly and carefully through the deployment procedure, and after about a couple of hours the paravane and attached streamer were over 300 m off the starboard side of the Langseth.
The second paravane went in the water at 22:00 this evening, and streamer 2 is currently being uncoiled into the water behind the ship. Despite a few delays, we are making good progress!
Marianne Karplus4th June

Langseth: The birds

Mon, 06/03/2013 - 23:18
It's 6:30am ship time (Spanish time). The 12am-4am shift finished deploying streamer 4 at about 12:30am, and since then we've been waiting for the sun to rise and the seas to calm before putting out the paravanes and continuing to deploy streamers 2 and 3.

Most of the science group has been working in 4 hour shifts thus far - 4 hours of work and then 8 hours of time for other things each 12 hour period. The last day or two, I was using my 8 hour rest periods to eat a couple of saltines, lie down, and attempt to ignore the rocking of the ship, but I must be getting used to the seas (and they are calmer!) because I can now do other things like read papers and look at a computer screen.
We have been collecting data almost since we left port. We are mapping the bathymetry, collecting gravity data, recording ocean current directions, etc. Since we entered our 3-D box area yesterday, it's been exciting to identify fault scarps in the bathymetry.
Donna mentioned in her previous post that once a streamer is in the water, its location is monitored and it can be moved around using winged devices called "birds" that are attached to it. Imagine a number of actual birds holding a cable in their talons at an even spacing while flying. Our mechanical birds are not so different, except they are flying the streamer through the water. We can see where the birds are on a computer screen in the lab, and we can control the depths of the birds by remotely moving their wings. When a streamer goes into the water, it can take some time to get the weighting right and then for the birds to dive the streamer down to the desired depth (generally 8-12 meters below the sea surface).

Marianne Karplus
3rd June (posted late due to internet outage)

Assembing a bird to be attached to the streamer.Birds for streamers 2 and 3 waiting to be deployed.

Deploying seismic equipment on the high seas

Sun, 06/02/2013 - 10:16

After steaming for twelve hours out of port, we started the long process of putting out all of the seismic gear needed for this program. The weather worsened as we headed towards the field area, and we have been deploying equipment in 3-4 meter swells for the last 18 hours. This ship can roll by up to 10-15 degrees in these conditions (and more than a couple of people are feeling sea sick as a result).  On the deck, ropes and cables connected to equipment towed off the back of the ship lurch and clank rhythmically, and water commonly washes over the lower deck. Down in the main lab, stray items that aren’t properly stowed or strapped down start to roll back and forth, the ship creaks and groans, and office chairs swivel with each swell.
For this program, we will be towing an enormous amount of gear behind the ship to enable us to image faults involved in rifting, the exposure of mantle rocks and continental breakup in 3D.  Four 6-km-long seismic ‘streamers’ filled with pressure sensors (which can detect returning sound waves) will be towed 200 m apart, for a full spread of 600 meters.  As we deploy the streamers, we add weights to ballast the streamer, acoustic units to determine the locations of the streamers, and ‘birds’ that enable us to control the depth of the streamer remotely.  We also swap out broken pieces. The streamers are held apart by two gigantic paravanes, which are like large metal kites that fly out from either side of the ship. Each one weighs an astonishing 7.2 tons and is ~7.5x6 meters in size.   There are also myriad floats, cables and ropes to maintain the correct geometry of the entire array.  The streamers will record returning sound waves generated by two arrays of air guns, which will be towed 100 m apart and fired separately.  We expect that it will take us 3 days to deploy this complicated array of equipment behind the ship. The weather is expected to start improving tomorrow afternoon, which will help us greatly!

Donna Shillington
2nd June Looking forward on the Langseth as she takes a roll in the swell.A streamer with a 'bird' being deployed off the Langseth's stern into the waves.

The R/V Langseth departs Vigo and heads to sea

Sat, 06/01/2013 - 09:40

The R/V Marcus G. Langseth pushed away from the docks of Vigo at 8 am local time.  The sun was shining, and the views of the rugged cliffs, forests and Galician towns along the coastline were spectacular.  We will only be able to see land at the very beginning and very end of this 45-day cruise.  We steamed out of the protected waters around Vigo and out into the open Atlantic Ocean a few hours later, and happily were met by relatively calm seas (1-2 meter swells), although its quite brisk compared to summer weather back home.  We actually saw the F.S. Poseidon in the distance as she headed back to Vigo at the end of the first OBS cruise of this program. We only have a relatively short transit of ~10 hours before we begin to deploy the extensive suite of scientific equipment behind the ship needed to image the structures beneath the seafloor in 3D.  Putting out the seismic streamers and associated gear will take 3 days!

Donna Shillington
1st June The science party on deck as the ship departs Vigo.
Islas Cies

Poseidon: Leg 1 completed

Tue, 05/28/2013 - 08:29
The Poseidon successfully deployed 38 ocean bottom seismometers (OBS) on Leg 1!

Today they are reloading the ship with 18 more OBS to deploy on Leg 2. The personnel on board will also exchange two Ocean Bottom Instrument Consortium personnel for two GEOMAR personnel.

What is an OBS?

An OBS is an autonomous instrument that sits on the ocean floor and records waves (sound waves as well as other types) traveling through the earth and/or ocean water. All of our Galicia instruments have ocean bottom hydrophones (OBH) to record waves traveling through the ocean (including some types of whale calls!), and a subset of fifty also have geophones to record waves traveling through the sediments and rocks beneath the sea floor. 

The OBS record waves by measuring tiny motions of the earth and sea water, converting it into electrical signals, which are stored digitally. The geophones, data logger, and batteries are stored in a water tight, floating sphere, and the hydrophone is attached to the outside of the sphere. A heavy anchor attached to the sphere enables it to sink to the bottom when it is deployed (sent off into the ocean).

 To pick up the OBS, the ship goes to the location where it was deployed, and a sound signal with a particular frequency is sent out. The OBS replies acoustically, cuts its anchor, and resurfaces. Scientists can then download the data and begin to piece together a picture of the local Earth structure!

Marianne Karplus
28th May

Poseidon: Very good progress today!

Fri, 05/24/2013 - 18:32

We dropped 17 ocean bottom seismometers today, making 26 in all.  The weather is cloudy but bright.  There is 2-3 m of swell and this is not a very big ship, so when we were steaming eastwards into the weather there was plenty of water arriving on deck.  Later on we turned to the west, which was more comfortable and drier.  Scientist cabins are partially below the water-line and my port-hole gets a regular wash.

Tim Minshull
24th May 2013

Poseidon sails from Vigo!

Fri, 05/24/2013 - 08:21
On 21st May, F.S. Poseidon sailed from Vigo, Spain on the first of three expedition legs to deploy 78 ocean bottom seismometers (OBS) provided by OBIC and GEOMAR.

Seventy-two OBS will be deployed in a grid of 18 x 4 instruments across the 3-D seismic survey box. Six OBS will be deployed on a profile extending farther west, to try to locate the boundary between continental and oceanic crust.

An OBS is deployed. Photo by Dean Wilson.Gaye Bayrakci, a postdoctoral researcher at the University of Southampton, sent this email update today:

"Yesterday (23/05/2013) the weather was ok. We tested 24 acoustic units first. Then, we deployed 9 OBSs along the regional profile. Before the finish of the day, we tested 24 more acoustic units. Food is good. Thursday is the seaman's day, so we had some cake at 5pm at the coffee break.

"Today (24/05/2013) the weather is darker but its normal for this period of the year in this area. We started at 06am (utm). We just finished the deployment of 8 OBSs on the southernmost profile and reach the eastern end of regional profile. We deployed a 9th OBS here and now we are heading westward along the regional profile."

The international team of scientists aboard the Poseidon includes two GEOMAR geophysicists, two University of Southampton geophysicists, and five OBIC personnel from University of Southampton and Durham University.

The internet connection on board the Poseidon is not good enough at the moment to post to this blog or send photos, so the above photo is from a separate OBS deployment in the Indian Ocean.

Stay tuned for more updates soon!

Marianne Karplus
24th May 2013