XBT (a short story)

            “Twenty shots until the next XBT.”
            It was nearing time to launch the next expendable bathythermograph probe, or XBT.  The software was readied and two scientists headed out of the lab, radio in hand.  They donned lifejackets that had once been bright orange but were now closer to a dull rust color from long and dirty use on the deck and selected a T-5 probe from the box.
            Out on the deck they were alone, perched partway up the stack of levels in the stern of the ship, the gun deck below them and the paravane deck above.  It seemed that the others working the graveyard shift were all inside, perhaps wrestling with some mechanical puzzle or else simply keeping watch to make sure all was well, sipping strong coffee, playing cards to pass the time.  The scientists snapped the probe into the gun-shaped launcher.  They removed the plastic end cap from the black cylinder that housed the probe and its spool of fine copper wire.
            “We’re in position.”
            There was a pause, then the radio crackled back, “Launch probe.”
            In a moment the probe was sliding down the long tube that extended out and downward from the starboard side.  With a small splash it plunged from the end of the tube into the inky deep.  Now to wait while it made its journey towards the bottom, more than 4000 meters below.  Despite the very late (or very early, depending on your point of view) hour, it was warm.  The air was muggy – not exactly a welcome change from the air-conditioned lab, although the tinge of diesel fumes was less out here in the relative open.  There was little wind and the seas were calm.  Standing on the moving island of light that was the ship the sea quickly disappeared into the surrounding void.  What surface that could be seen appeared to rise disturbingly close up alongside them, like a churning wall of water.  It was only visible at all by the few swirls of foam formed by the ship’s passage and a reflection here and there off the constantly moving face of the black oily-looking water.  They waited for the go ahead to terminate the probe.
            Down in the lab, there was a strange blip on the screen showing the multibeam bathymetry data, but no one noticed as they were too busy entering in location data for the XBT or scrutinizing the movement of the streamer birds that regulated the depth of the hydrophone streamer.  There were, after all, 36 other monitor screens to watch.
            Outside there was a louder than usual splash.  The two scientists peered into the gloom.
            “Dolphin?” one wondered out loud.
            “While we’re shooting?  I hope not,” the other replied, “We’ll end up having to interrupt the line.”
            Was there something just under the water surface?  A pale sinuous shape at the very edge of the ship’s halo of light?  No, it must be a trick of the light and the weird perspective engendered by the lack of any sense of distance.  Perhaps more coffee was in order when they got back inside.
            The radio crackled again, “Terminate probe.”
            The scientists broke the wire that was still spooling out to the probe that was now falling behind them.  “Probe terminated,” they reported.  They were just turning to leave when it emerged.
            At first it looked like a whale back, though pale milky green in color rather than the expected grey.  As it lifted free from the surface it became clear that it was much longer than an orca or even a grey whale, more like an ancient marble column turned soft and rubbery.  It tapered as more of its length was exposed until the tip broke free of the clinging water.  One side of the enormous snake-like shape was covered with round suckers the size of dinner plates in a poisonous green color.  The cyclopean tentacle towered out of the water, waving gently with a sickening sort of grace ten meters or more above the uppermost deck.  Here and there along its length were clots of a coppery tangled substance, almost like seaweed wrapped around it.  “The XBT wire,” one of the scientists realized from the midst of her fascinated horror.
            The tentacle hovered for another movement before swooping down with surprising swiftness.  The two scientists were neatly plucked from the ship in the blink of an eye.  With a clatter, the radio fell to the deck.  They were held above the water for a long moment, crushed together so tightly they couldn’t speak and could barely draw breath.  Then, slowly, the tentacle disappeared beneath the smoothly rolling waves.

            Two hundred and sixty-seven shots until the next XBT.


-by Tanya Blacic aboard the R/V Langseth (with a wink to H. P. Lovecraft)

Life at Sea

September 27, 2014

Our small ship is in a state of endless motion with pitch, roll, yaw, and heave. We continuously experience a feeling of fluctuating gravity at sea, as one minute we are several pounds heavier and the next we are several pounds less. We’re tossed about endlessly like riders at the fair. It’s a feeling that can turn the stomach of the saltiest of sailors, but more often disturbs the newbies the most. At sea there is also no such thing as silence. Out here the engines are always running, hydraulic pumps are always droning, and ships operations occur around the clock. From my bunk I can feel us lurch forward and lean into a turn to starboard, or port, and then they reverse the pitch of the propeller as if applying an emergency brake to slow the ships forward motion. This reverse pitch causes a shudder in the hull that shakes us like a cheap hotel vibrating bed and it chatters every moveable thing. From my bunk I can also hear the acoustic pings emanating from the hull-mounted transducers. Speaking to me in code, they tell me if OBS operations are going well.  Based on the ping styles I can also discern the acoustic techniques used by WHOI and Scripps, so that I know which instrument type is being talked to. All of this information creates a movie in my mind that plays out until I fall asleep. Life on a ship is a constant immersion in all that is going on and for 30-days there will be no escape.

-Ernie Aaron

And then there was data…..

It’s been a week since we deployed all of our gear and started steaming along our lines, so now we have amassed a lot of data!  Although we can only steam at very low speeds while towing the equipment (~4.5 nautical miles an hour or  ~5 mph), each time we fire the air gun array, the 636 channels on the seismic streamer listen for returning sound waves for 18 seconds and record a total ~25 Mb of data. Repeat that every 30 seconds for 7 days, and it begins to add up!  We now have 400 Gb of seismic data alone, not including all of the other types of data we collect while underway (bathymetry, magnetics, gravity).  We are a data-collecting machine.  

Matt, Jenna and Derek sit  back and watch the data roll in from the Main Lab

Not only are we collecting data, we are also doing some preliminary data analysis to get a first look at the geology hidden below the ocean, which is always exciting.

Kara and Matt are entranced by velocity analysis

Although we are only a week in, our data collection has already taken us through water depths as shallow as 20 m and as deep as 6000 m.  At the edge of the continental shelf, water depths change rapidly from ~500 to ~3000 m over just 20 km – a slope of 10%.  For perspective, that’s very similar in elevation change and slope to the course for the Pikes Peak marathon.

Perspective view of seafloor depth from MGDS across the continental slope overlain by a higher resolution swath of bathymetric data that we acquired along our transect, which is also shown projected onto the seafloor.

We have also traveled over widely variable geology – from 35-km-thick continental crust to ~7-km-thick oceanic crust, and from sediment thicknesses of 5 m to over 7 km.   Our data are also revealing cool structures in the sediments and crust – faults, sediment waves, and more.  Below is a picture of a salt diapir that we imaged at the edge of the continental margin.  The salt was probably first deposited at least 150 millions years ago in a flat layer, but as more sediments were deposited on top of it, it got squeezed up and out into dramatic diapirs.

Preliminary image of a salt diapir in seismic reflection data near the base of the continental slope. The y-axis shows the time it takes for a sound wave to travel down in the earth and back again. This images shows about ~5 km down into the earth below the seafloor.

 Donna Shillington aboard the R/V Langseth

First day of land deployment - showing kids how cool seismology is - by Kara Jones

(Originally posted on September 12)

Today was the first day of the onshore deployment of the RT130s through southern Virginia and North Carolina. My partner, Yanjun Hao, and I, were just one of five teams working to deploy instruments along the two survey lines. We deployed the first two instruments at West Harnett Middle School and South Hartnett Elementary School, both outside of Lillington, NC. In both case, the fifth and sixth graders were very interested in learning about what we were doing and eager to participate. I explained to them the basic concept of P and S-waves and then asked the children to jump so that we could test that each of the channels on the sensors was working correctly. They very much enjoyed getting to see on the clié exactly what the signal they generated looked like. At both schools, I was surprised how much the children, and the teachers, knew about earthquake seismology and the intelligent questions they asked. A teacher asked whether they would detect the explosives detonated at nearby Fort Bragg, and a sixth grader named Gauge blew me away when he asked if the sensors would be able to record the sound waves generated by the planes or nearby explosions! In total, we probably spoke to 100 kids about the project today. It was a very encouraging to see how excited and interested they all were in the science. When we first arrived and explained that we would be installing a seismometer, a 5th grade teacher looked at us with wide eyed and asked "Are you seismologists?!" I nodded yes and she was so excited she started jumping up and down. Despite some rain and GPS trouble later in the day, the excitement that the elementary and middle schoolers showed about seismology was enough to make it a great start to the deployment.

At South Hartnett Elementary School in Anderson Creek, NC. I am showing one fifth grade class what the seismic signal they just generated looks like on the clié.

The land instruments

As a volunteer on the GeoPRISMS experiment I was given the opportunity to learn about active seismic onshore survey deployment. The deployment took place from September 8 to 16. During that time the team deployed 80 RT130s along the two seismic profiles in North Carolina and Virginia.  We first prepared the instruments and learned how to deploy them. 

Synchronizing the GPS clocks of the RT130 before deployment in the courtyard of the instrument center. Each instrument is powered by a deep cycle battery.

Pnina Miller (IRIS/PASSCAL) shows how to correctly orient the GPS antenna during the volunteer training

After the preparation we independently deployed the seismometers along the two profiles. Ana Corbalán and I installed 16 RT130s along the southern profile.  

 

Ana and I on our way to our next seismic station

The insulation was tough but gratifying. The weather in North Carolina is unpredictable. At times it was hot and humid. I was drenched in sweat burying the sensors. Other times we were caught in torrential downpours working under a tarp; terrified by the sound of thunder. The sites were located on mostly private property, hosted by people who were eager to help with the experiment. The interaction with the local people enriched the experience. Many of them showed true southern hospitality. 

 

Station deployed!

From an academic prospective I learned about survey design, instrument deployment and the logistics. This provided a distinctly unique experience that is unavailable in the classroom environment. Beatrice and Dan were tremendously helpful and supportive. I learned a great deal about active seismic from my conversations with them. They’re passionate about nurturing future geophysicist. The GeoPRISMS is an altruistic endeavor for them. I am thankful to them for investing so much of their time and expertise into the project. 

The GeoPRISMS experiment has been an overwhelmingly positive experience. I am grateful to have been given the opportunity to help with the deployment and look forward to my involvement in the recovery of the instruments! A future workshop will be held for processing the data and the inversions. This pre to post educational approach is invaluable to me as a future geophysicist.

Posted by Christopher Novitsky 

The Land Deployment Team!

From L-R: Yanjun Hao, David Boyd, Dam Lan, Ana Corbalan, Christopher Novitsky, Pnina Miller, Jason Leiker, Kara Jones, Beatrice Magnani (front), James Farrel (back), Dan Lizarralde.

It took us a while, but here we are, the team that deployed the land seismometers on Sept 12-15. The instruments are now continuously recording the Langseth shots and will continue recording for few more weeks. The East Carolina University in Greenville, NC graciously allowed us to use one of the research facilities on their West Campus (a place with a fascinating story - blog on that coming soon!) as the headquarter for operations. We will be back to the field at the end of October to pick up the instruments, download/save the data and demob.

Posted by Beatrice Magnani

The Night Watch in Action!

We've captured the process of recovering and deconstructing a Scripps OBS thanks to Harm's nifty GoPro camera attached to the crane. This OBS was a little tricky to hook, but otherwise it was a smooth recovery!

Time series of the recovery after the OBS has been attached to the crane. Photo Credit: Ernie Aaron.

See ya'll later,
Jenny Harding
R/V Endeavor