Earth's Tectonic Plates
Lamont graduate student Natalie Accardo reports from the Pacific. Blog 4: Jan. 13, 2013
The NoMelt project is more than just a seismic experiment; it also has an important magnetotelluric (MT) component. MT instruments measure natural magnetic and electric fields on the seafloor, allowing scientists to estimate the electrical conductivity of the underlying rocks. Conductivity is highly sensitive to tiny amounts of water and molten rock within the upper mantle and thus can help distinguish whether the mantle is “wet” (and thus easy to deform) or “dry” (rigid and plate-like).
To obtain information concerning the conductivity of the mantle, six long-period MT instruments were deployed along with the seismographs from the R/V Langseth in 2011. These instruments, which appear more like sea spiders than scientific hardware, sit on the ocean floor and record electrical and magnetic fields approximately every minute. We recover these instruments in the same way that we retrieve the OBS (previous post), although they proved to be much more shy than the OBS in communicating with us. We welcomed back our first MT instrument on a dark and windy night, and over the course of two weeks we recovered five additional instruments without incident, displaying them in all of their neon-orange glory on the stern deck.
With the last instruments safely strapped down, we have put the NoMelt site in our rearview mirror and are steadily speeding to our final destination of Honolulu. Sunny skies and calm seas accompany the slowing pace of activity during our four-day transit to port. Behind the boat, we trail fishing lines with every color of bait in the hopes that a tuna or mahi mahi might take a bite. Deck chairs have snuck their way out from the shelter of the hangers and onto the sun-drenched back deck where we, like moths to a lantern, try to soak up every last ray of sun before we must head back to the chilly Northeast.
Today we passed close enough to the island of Hawaii to give us our first glimpse of dry land in almost a month. The crew poured onto the main deck to snap photos and hunt for the tiniest glimpse of cellphone reception. There may be no better way to be welcomed back to land than the awesome sight of Mauna Loa towering above the clouds. Overall, the trip has been a great success. Most of our instruments survived their year of solitude on the dark, cold seafloor and came back to us with a set of unique and priceless data. We consider ourselves lucky to have gotten the chance to visit this remote region of the world, which will likely not see comparable human activity for some time.
Until next time, Aloha!
Lamont graduate student Natalie Accardo reports from the Pacific. Blog 3: Jan. 1, 2013.
Christmas found the R/V Melville in the middle of the Pacific Ocean on the last day of a seven-day transit to the NoMelt Project site. In a coincidence that we hoped would be auspicious, we reached our first OBS site late that night. As much as we yearn to be home to do celebrate the holidays with our families, we also realize how fortunate we are to have the chance to do what we do. Many of us began Christmas day with phone calls home to offer holiday greetings to our families and loved ones. Then the entire crew mustered on the upper deck for the requisite group photo, with more than one Santa Claus in attendance. Sunshine abounded as the captain led a crew-wide gift exchange that produced enough chocolate candies to feed an army. The rest of the day was filled with a “coits” (a ring toss) tournament on the main deck, where two young female scientists (that is us!!) came from behind to win the championship and all the pride and glory that come with it. An epic feast topped off with homemade pies and cakes ended the day for most of the crew; for the science party our adventure was just beginning.
We arrived at the first OBS station late into the night of the 25th with apprehension abounding. Recovering OBS instruments from the ocean floor is always a tricky business, especially in our case; these instruments have been sitting beneath more than 3.5 miles of water for over a year. With cold, tired batteries powering the instruments’ acoustic transponders, communicating with them through miles of ocean currents amounts to a whispered conversation on a stormy night.
We initiate communication with an OBS by transmitting audible “chirps” from a communications box in the main science lab to a transducer on the ship’s hull. The transducer acts as a speaker to transmit the chirp through the ocean and down to the instrument. If the OBS is alive and well, it transmits seven chirps in response. Given the distance these signals have to travel, it takes about eight long, stressful seconds to hear the instruments reply. Sometimes there is no reply, and we try again, at different locations, from different angles, with alternate acoustic devices.
Once we know an instrument is up and running, we conduct an acoustic survey by cruising around and sending continuous chirps. We measure the time it takes for the instrument to chirp back to determine the distance to the OBS, providing a precise estimate of the instrument’s actual location on the seafloor. Once we have completed the survey, we are ready to bring the OBS up. We send another series of commands that tells the instrument to release itself from the seafloor and then monitor the distance to it as it rises through ocean. Once on the surface, the captain skillfully steers the ship very close to the OBS so that we can hook lines onto it and pull it safely on board.
Our Christmas Night OBS was successfully recovered, and by New Year’s Day we had retrieved 12 OBS and one magnetotelluric instrument (to be discussed in the next installment). Sadly, two instruments never responded and are assumed lost to the deep; we are likely to never know why. Our success can be seen in the growing army of instruments that stand at attention on the main deck.
We are completing the charge around the perimeter of the deployment, picking up instruments approximately every 10 hours. Soon we will make the turn and head onto the central line of the deployment, where interstation spacing is much shorter and the recoveries come hard and fast. From the Pacific we wish everyone a happy and healthy New Year!
Lamont graduate student Natalie Accardo reports from the NoMelt recovery cruise.
Blog 2: Dec. 23, 2012
Today marks our sixth day aboard the R/V Melville on a journey to a remote region of the Pacific to retrieve seismic instruments that have been quietly recording earthquake signals on the ocean floor for the past year. We have covered more than 2,600 km thus far but must cruise for another two and a half days before we reach the NoMelt project site. We have been making good time — the ship’s crew has been pushing the Melville to move at a quick pace, 12.3 knots or 14 miles per hour – and should be at the project site around midnight on the 25th of December.
The Melville initially met rough seas off the coast of California that forced most of the science party to remain horizontal in our bunks in an attempt to sleep off the affects of seasickness. We hastily tied down laptops, keyboards, and a glittering Christmas-themed snow globe so that they would not be chucked about by the rolling waves. Sticky mats and cup holders found their way into the mess hall so that the those of us who could stomach a meal would not find ourselves with a lap full of spaghetti or coca-cola.
However, calm seas found their way to us two days out of port and have stuck with us since. Hotter temperatures and increasingly sunny days remind us that we are steadily cruising toward our tropical destination. We fill our days at a leisurely pace acquiring bathymetric and magnetic data from the ship’s onboard instruments, deploying drifter instruments, and working on projects we’ve brought from home. As we near the project site, the pace will pick up, and the science party will commence 24-hour round-the-clock scientific operations.
The science party makes up only six of the total 30 people on board. The rest represent the talented, permanent crew of the Melville, who work tirelessly to keep her safe and operational in the open ocean. Their vocations span the gamut from the engineers that keep the huge diesel engines humming smoothly to the computer technicians that keep the Internet running and the onboard ship computers (and scientists!) happy. The crew is gregarious and inviting, welcoming any question or concern, no matter how banal. They may even invite you to join in their card games … though few of us are brave enough to test their skills.
Christmas and New Year’s are just around the corner and promise to be exciting, as they will mark our first days retrieving the OBS from the deep. Until then we wish everyone safe holiday travels and happy holidays!
The R/V Marcus G. Langseth completed the initial portion of the NoMelt experiment on Dec 29, 2011. In the subsequent year, scientists began analyzing the active-source seismic data collected on that cruise, constructing initial models of the oceanic plate. The full analysis awaits the so-called “passive source” data – the year-long recordings of earthquakes and natural electrical and magnetic signals on the instruments that remain on the seafloor.
On Dec. 18, 2012, the R/V Melville departed San Diego to recover remainder of the NoMelt instruments and data. The expedition includes two scientists from Columbia’s Lamont-Doherty Earth Observatory: Post-doctoral scientist Patty Lin and graduate student Natalie Accardo. Natalie is sending regular reports from the ship, and I will post them here.
Post 1: Natalie Accardo, Dec. 19, 2012.
In the early hours of Dec. 18, a team of scientists aboard R/V Melville set out from San Diego to a remote portion of the Pacific Ocean on a trip that will take 28 days and cover more than 8,500 kilometers. On this voyage, we aim to recover 27 ocean bottom seismographs (OBS) instruments that have been sitting silently on the ocean floor for nearly a year. Throughout their stay on the seafloor, the OBS have been continuously listening and recording the shaking caused by distant earthquakes all over the world. By recording ground motion, we can constrain seismic wave properties and in turn the geologic characteristics of the oceanic plate. With this information, we hope to answer the multilayered question of what defines a tectonic plate.
For decades, geologists have focused most of their attention on locations where tectonic plates come together (i.e. subduction zones like Japan) and break apart (i.e. rift settings like the East African Rift System). Yet to better understand the complex processes happening at those sites, we must first understand the fundamental characteristics of a tectonic plate. For further information concerning instrument deployment and other aspects of this project, please refer to previous blog entries.
It takes seven days to make the 4,300 km journey from San Diego to the NoMelt OBS sites. During the transit time, we use instruments aboard the Melville to map topography and gravity of the ocean floor. Additionally, at regular intervals we toss “drifter” instruments overboard. These so-call “instruments of opportunity” were designed by students at the University of California San Diego (UCSD) to be deployed by any research vessel traveling through an area of interest. They are completely autonomous and will record sea surface information (temperature, salinity, etc.) wherever the currents take them, data that will be of use to oceanographers at UCSD.
Today marks only our second day on board and has given us our first true glimpse of the open ocean. Rocky seas have confined most of the science party to their bunks in a group effort to retain what is left of our last meal. However, the promise of calmer weather in the coming days has brought some cheer to the entire crew.
Academic vessels operate throughout the year, and research cruises are scheduled during seasons when the weather is good enough for scientific operations. I am lucky to have research targets in tropical latitudes; the downside is that cruises to these regions are often scheduled when the weather is poor at higher latitude – in this case, smack during the holidays.
With round-the-clock shifts, there are precious opportunities for Santa to slip onto a research ship unseen. But slip in he did, leaving treats and gifts around the R.V. Langseth to brighten our day. In the main science lab, the midnight shift’s usual stash of Starbursts and Sour-Patch Kids expanded into a orgy of Pringles, summer sausage and pepperjack cheese, chocolate-covered Pretzels, and a six-pound bag of Gummy Bears. My Charlie-Brown Christmas tree suddenly blossomed with ornaments, from simple paper snowflake cut-outs to handmade fish spun from copper wire snagged from the electrician’s shop. Beautiful paper wreathes appeared on several cabin doors. Over in the OBS lab, Santa delivered a set of pop guns with rubber darts; after briefly considering storming the bridge, the boys entertained themselves with target practice while retrieving the last instrument.
Christmas dawn broke slightly cloudy as we steamed into our last task of the experiment, checking on the status of one of the instruments that we will leave in place for a year. As we finished this task, the sun burst through into a brilliant blue sky — certainly not a white Christmas, but welcome nonetheless. The scientific party and crew mustered outside for the requisite cruise photo, bedecked in holiday cheer. It was time to turn for home.
The festivities of the day were tempered by a degree of melancholy at the thought of loved ones back at home, enjoying their own holidays in our absence. The Lamont Marine Office opened up the satellite phone lines, giving everyone a 15-min phone call, and the chatter from those calls resonated through the ship. As scientists, we have been given a remarkable opportunity to explore our planet and unravel its mysteries. Making that happen requires hard work and sacrifice – from the talented crew around us, from our families back home, and from ourselves. We are truly thankful for that opportunity.
Over the first 22 days aboard the R/V Marcus G. Langseth, we’ve zigged and zagged our way over a 360×240 mile region of the Pacific plate, first dropping instruments to the seafloor, and then shooting airguns to them (see previous posts). The final step is to recover a subset of the instruments: 34 ocean-bottom seismometers (OBS) that recorded the shooting, and three of the MT instruments. Excitement abounds – we will see our new data for the first time when we pick up the instruments, and get enticing first peeks that may confirm our ideas about plate structure. But there is plenty of apprehension as well; many potential obstacles are associated with recovering our instruments from the seafloor, and it is possible that some of them (and the data that they contain) could be lost permanently to the deep.
Much of the apprehension is stimulated by uncertainty in the communication with the instrument. As we arrive at each site, we send acoustic “pings” from a transmitter on the ship, asking the instrument to wake up; if the instrument is alive, it will ping back. A second set of pings instructs the instrument to release its anchor, a process that typically takes several minutes. Subsequent pings to the instrument allow us to estimate the distance to it and thus whether or not it is rising to the surface. In shallow water, this system works great. But at 5 km water depth, transmitting acoustic signals is akin to operating a TV remote with a weak battery from across a large crowded room. Signals are faint, and echoes from the ship and other noise sources mask the instrument signals, such that we were often unsure if the instrument was responding or not. The uncertainty was maddening.
Despite these uncertainties, the instruments did indeed hear our distant acoustic calls. The first several recoveries went well, and the data look excellent. However, we then encountered a string of three consecutive instruments that refused to budge from the seafloor. All responded to our pings, but they could not lift off the bottom. Stuck in the mud? Flooded? After spending several hours attempting to recover them, we sadly moved on. Forensic analysis of some of the recovered instruments revealed a likely cause: bad AA battery cells in the release systems, such that they have power to communicate, but not quite enough to complete the release process. Recognizing this, we devised a power-saving routine for the remainder of the recoveries; sending release commands in brief bursts that used minimal power, and then waiting in near-silence until the OBS appeared at the surface. This proved successful, and in the end, we recovered 30 of 34 OBS, and 2 of 3 MT instruments. We are frustrated by the losses, but thankful for the data in hand. Next year we will return to recover the broadband OBS that are recording the earthquake data – in the meantime, we hope to engineer a new plan to coax the four missing OBS back to the surface.
The NoMelt experiment aims to image the structure of an oceanic plate, including its deepest reaches up to 70 km beneath the seafloor. One of our primary means to do so is to create sound (acoustic) waves in the ocean from the ship, and record those waves at receivers on the seafloor, after they have traveled 10’s-100’s of miles through the rocks that underlie the ocean basin. The R/V Langseth is equipped with a large airgun array, which is capable of producing such sounds.
Each airgun consists of a steel cylinder that can hold a large volume of compressed air. When fired, the gun forces the air into a bubble in the water, which quickly pops and collapses under the water pressure. The “bang” associated with the collapsing bubble travels efficiently through the water, and when it reaches the seafloor, much of the sound energy converts to seismic waves that travel through the crust and mantle. With a single airgun, the sound is loud enough to penetrate only into the upper layers of sediment beneath the ship. We can crank up the volume by firing multiple guns at once, allowing the energy to travel deeper into the Earth and to greater distances. During NoMelt, the Langseth tows an array of 36 airguns that produce sufficient energy to probe well into the oceanic plate and travel back up to receivers deployed several hundred km away on the seafloor (see my previous post).
But just being loud is not sufficient. Simply shooting all the guns at once will produce a loud but “ringy” sound, in much the same way that turning a stereo up to maximum volume (11!) will distort the music. The Langseth’s airgun array is “tuned” to produce the ideal sound for our purpose. In practice, this means firing the guns microseconds apart, such that they interfere to produce a sharp, clean sonic pulse, rich in the low frequencies (think bass, rather than treble) that penetrate most effectively into the Earth. The array is also oriented to direct these pulses downward into the seafloor, rather than in all directions into the surrounding ocean. Finally, we fire the guns only once every 4 to 5 minutes, much more slowly than most seismic surveys. This allows the noisy echoes within the water column to die away, even out at the most distant instruments. This is critical for detecting the subtle lower amplitude arrivals returning from deep in the plate.
Over 11 days, we traversed 1000 miles of the ocean floor, traveling at 4 mph, shooting every 4 to 5 minutes, 24 hours a day. Student watchstanders and technicians continuously monitored the computers controlling the shooting. Protected species observers also worked around the clock, searching for nearby marine mammals (whales, dolphins) and protected sea turtles that may be sensitive to the noise. This is only a concern within ~1000 meters of the ship, but to be safe we monitor and report any activity up to four times this distance. In 11 days, we only encountered one small pod of sperm whales; when they meandered too close, we shut down our operation until they left the area, and then circled around and restarted the survey.
If all goes well, the recordings of these shots on the ocean bottom seismometers (OBS) will provide a truly unique portrait of the deeper (mantle) portion of the oceanic plate. This structure has not been comprehensively explored since the pre-airgun 1970’s, when large explosives tossed off the ship (essentially scientific depth charges) served as the sound source. Instrumentation and analysis tools have improved immeasurably since then. We now need to retrieve our OBS…
Oceanic plates are born at mid-ocean ridges, where hot mantle rocks are brought very close to the surface, partially melt, and then cool and crystallize. The newly formed rocks move outwards from the mid-ocean ridge, making way for the next batch of hot rock rising from below. Inch by inch, over millions of years, oceanic plates progress through a life cycle of birth at the mid-ocean ridge, cooling and aging in the open ocean basins, and destruction at a subduction zone, where they dive back into the mantle.
Because rocks contract inward as they cool, oceanic plates deepen considerably with age: from approximately 2500 meters depth at mid-ocean ridges to as much as 8000 meters depth in subduction-zone trenches. The NoMelt study region has matured to a middle-aged 70 million years (a plate age roughly equivalent to 40 human years), and sits at a seafloor depth of just over 5000 meters. That’s 3 miles of seawater, with the temperature at the bottom just above freezing – a very inhospitable environment to deploy our seafloor equipment.
Four days after departing Honolulu, we began deploying ocean-bottom seismometers (OBS) and seafloor MT instruments, over a grid spanning 360 miles by 250 miles. The instruments come in four flavors, designed for different types of measurements, but they have several components in common. First, they all deploy via “free fall” – they are hoisted over the side of the ship using a crane, and dropped into the water. They weigh several hundred pounds each and sink to the bottom within a few hours. Each contains a sensor such as a seismometer or a magnetometer, a low-power computer to record the data, and acoustic transceivers capable of receiving and replying to simple commands, such as “turn on” or “reply to this ping.” All are stocked with a battery supply capable of running the instrument for the duration of the experiment – up to a year for some instruments. All of these electronic components are housed in precisely engineered aluminum tubes and glass and titanium spheres designed to withstand the crushing pressures at 6000 meters below the sea surface.
Our deployment strategy poses some risks. We cannot ensure they land nicely in good spots on the seafloor. The combination of pressure and corrosion continuously wears on the instrument over a year-long deployment, and it can be difficult to withstand. If a problem occurs, then the instrument and any data it contains may be lost. And problems do occur – glass spheres implode, aluminum cases corrode and leak, instruments can float prematurely to the surface because they accidentally release from their anchors.
Tiny “upgrades” in instrument design can prove catastrophic. In one legendary case, a new disk drive was just heavy enough to make the anchorless instruments neutrally buoyant; instead of floating to the surface at the end of the experiment, they hovered 10 meters above the seafloor, never to be seen again. But there is no affordable alternative for deploying equipment on the seafloor in the open ocean, and over the last 15 years, the seafloor geophysics community (see www.obsip.org) has learned many lessons for minimizing the risk.
Working around the clock for four days, our team of technicians (from Scripps Institute of Oceanography and Woods Hole Oceanographic Institution), students, and PI’s deployed 61 OBS and nine MT instruments. Our time is tight, so we dropped them over the side and moved quickly to the next site, never knowing whether they reach a safe resting place on the bottom.
In a little over a week, we will return to recover 34 of the OBS (short-period instruments designed specifically to record the airgun shots from the Langseth) and two of the MT instruments. Only at that point will we truly learn if the deployment has been successful. We will not know the fate of the remaining 27 OBS and seven MT for another year.