From the Mudline to the Mantle: Investigating the Eastern North American Margin


Deployment of a SCRIPPS Ocean Bottom Seismometer from the R/V Endeavor

Deployment of a SCRIPPS Ocean Bottom Seismometer from the R/V Endeavor

Brandon Dugan (Rice University), Kathryn Volk (University of Michigan), Dylan Meyer (UT, Austin), Kristopher Darnell (UT, Austin), Afshin Aghayn (Oklahoma State), Pamela Moyer (University of New Hampshire), Gary Linkevitch (Rice University)

The NSF-GeoPRISMS-funded Eastern North America Margin (ENAM) Community Seismic Experiment (CSE) is a community-driven research project aimed to study continental breakup and the evolution of rifted margins. The ENAM CSE includes acquisition of passive and active-source data from broadband ocean bottom seismometers (OBSs), short-period OBSs, multi-channel seismics (MCS), and onshore seismometers (Fig.1). Data are augmented by the onshore EarthScope USArray seismometers. Together they provide coverage across the shoreline and over a range of length scales. Project data will facilitate detailed studies of the early rifting between eastern North America and northwest Africa in the Mesozoic including processes associated with the Central-Atlantic Magmatic Province (CAMP), the East Coast Magnetic Anomaly (ECMA), and the Blake Spur Magnetic Anomaly (BSMA), as well as high-resolution studies of shallow sedimentary and fluid-flow processes including Quaternary landslides and gas hydrate systems.
Another component of the ENAM CSE was engaging young scientists in the field geophysical program so they could study the eastern North America margin and be educated about the planning and implementation of a multi-investigator, multi-component research program. To accomplish this, we included young researchers (undergraduate and graduate students, post-docs, and assistant professors) in all onshore and offshore field programs. The final stage of training and education will be seismic processing workshops for the OBS and the MCS data in summer 2015. Information for applying will be distributed via GeoPRISMS and other community list-servers.
In this phase of the ENAM CSE we conducted onshore and offshore operations in September 2014. Onshore activities (led by Beatrice Magnani and Dan Lizarralde) included deploying 80 short-period seismic stations to record our offshore shots and recovering the instruments. Offshore activities included deploying and recovering 94 short-period OBSs from the R/V Endeavor (led by Harm Van Avendonk and Brandon Dugan) and shooting MCS seismic data and providing active sources for the short-period OBSs and land seismic stations from the R/V Marcus G. Langseth (led by Donna Shillington, Matt Hornbach, and Anne Becel). Together these activities yielded high quality seismic reflection and refraction data across the shoreline and down to the mantle.

Figure 1. Idealized instrument layout and transects of the ENAM Community Seismic Experiment.

Figure 1. Idealized instrument layout and transects of the ENAM Community Seismic Experiment.

When I first heard about the ENAM CSE, I was very excited by the available cruise opportunities. I have been on several cruises before, ranging from 5 days to 5 weeks, and had been aching to get back out to sea again. Considering my prior experience collecting, processing, and interpreting MCS data, I decided it would be a good idea to expose myself to an alternative data type so I applied for the OBS deployment cruise on the R/V Endeavor. From getting accepted to actually boarding the vessel was really a blur. The next thing I knew, we were casting off the deck lines and heading out into the wild blue yonder. We all settled into our daily routine during the first week and it was great getting to know the crew and research staff. Sadly, the 12-hour watch schedule made it difficult to cross over with those on the other watch, but we were still able to see them at some meals and during watch changes. As the cruise went on and days blurred together, morale and energy remained elevated. We enjoyed our primary task of deploying and recovering OBSs and we filled our free time with reading, card games, and mingling. I had read all the information available on the ENAM CSE website and had chatted with the chief scientists about the project, but lacked the tangible connection between the activities that controlled every day of our lives at sea and the research goals of the ENAM CSE. Then, approximately two weeks after starting the voyage, we started getting data back from the OBSs we had deployed. The link between the physical (data collection) and theoretical (objectives and hypotheses) composition of the ENAM CSE research goals began to take form. Kathryn Volk, Gary Linkevich, and I met with Dr. Harm Van Avendonk in the main lab soon after the first data from the deployment became available. As a result of my past experience with port-processed MCS data I found that I had difficulty readjusting my perspective to data showing migrated time once the velocity structure been applied to convert time into depth. Through careful explanation, it became apparent that the data could be used to identify structure marking large changes in seismic velocity – so large that material with a velocity of 7 km/s would display as a horizontal layer. The purpose of this was to confirm that the seismic source had penetrated to the crust-mantle boundary. These data helped us identified the direct arrival, along with the position and depth of the OBS, the seismic multiple, and additional arrivals with increasing seismic velocity (a more in-depth description of these interpretations can be found in the ENAM CSE blog post put up on 9/30/14). From this conversation, the theory behind the data we were collecting and the physics behind the instrumentation we were working with became clear to me: combining the data from each line together will produce a seismic velocity model down to the crust-mantle boundary beneath the ENAM CSE study area. This will allow us to infer information concerning the crustal structure within the study region. With this connection drawn, we continued our work with a better-informed sense of purpose and finished the cruise in high spirits knowing that we helped obtain a dataset that will prove to be very important for the scientific community. My experience aboard the R/V Endeavor was very rewarding. Beyond the excitement of being out on an adventure at sea, I had a unique experience, from learning the construction and operation of OBSs to the important interpretations that can derive from the data. I am looking forward to the data workshops that are being offered next year to continue my education in this area. Dylan Meyer, University of Texas at Austin

Six students from across the country came together to participate in the R/V Endeavor cruise, and I was one of them. I had never been out to sea before in my life, so I was both excited and nervous for what was to come as we pulled away from port. We started our shifts right away, three students – including me – working the noon to midnight shift, and three other students working from midnight to noon. It took a few days to get to our first line where we would start deploying ocean bottom seismometers. The first task we learned, and one we would repeat many times, was the ocean bottom seismometer assembly. We would work with our shift to attach the metal grate, the instrument box, the ratchet on the side floats, and finally we would secure the top float. The final touch to the assembly included a strobe light, a radio, and a reflective flag to detect the instrument once at the surface. When assembled, the OBS was ready to be deployed off the side of the ship, or as the Captain referred to it, ‘pick her up and put her in’. At night, we could distinguish the flashes of the strobe light before the instrument disappeared under the waves. We would repeat this task, moving from one site to the next until we finished a line. Once the R/V Langseth had shot active-source seismic across a line, we had to go back and recover the OBSs by fishing them back out of the Atlantic. We would first return to the drop site and send a remote command telling the OBS to start burning through the wire attaching the metal grate to the buoyant OBS. Fifteen minutes later, the metal grate would detach allowing the OBS to rise back up to the surface. In extra deep water (~5000 m depth) it could take an OBS over an hour to surface. Just before the instrument reached the surface, the students would head up to the bridge, grab a pair of binoculars, and start looking around to locate it, which was harder than expected! Sometimes, the OBS would surface far from the ship, the bright orange flag being no more than a small, orange dot on the horizon, bobbling in and out of view. Fortunately, the combination of radio, flag, and strobe light, along with a handful of eyes was helpful to spot the instrument. The task was then up to the Captain or the First and Second Mate to drive the boat right towards it and the OBS technicians or the students would retrieve the OBS using six feet long pools equipped with hooks at the end. It usually took a bit of strength and good hand eye coordination to snag the OBS with the hook. The knuckle boom would finally drag the instrument up out of the water and onto the deck. And then move onto the next site. One of the most valuable things I learned on this cruise was what it takes to collect data. We needed a team of people willing to spend a month together in the ocean, repeating a task over a hundred times in rain or shine, calm seas or stormy, to acquire large amounts of new data that will generate new research, publications, and discoveries, and that’s pretty cool. Kathryn Volk, University of Michigan

My first time at sea and I will never forget the sight of the vast ocean and endless sky – there were more colors, sounds, and motions than I ever imagined.Pamela Moyer, University of New Hampshire
Record sections of hydrophone (top) and geophone (bottom) of OBS207. This was an instrument from the WHOI OBSIP group.

Record sections of hydrophone (top) and geophone (bottom) of OBS207. This was an instrument from the WHOI OBSIP group.

My first few hours aboard the R/V Langseth were spent walking in circles trying to identify the rooms of the ship and trying to navigate from my bunk to the galley, then from the galley to the lab, then to the muster deck, and finally back to my bunk. It seemed that the combination of identical walls and floors, narrow stairwells, and tight turns created a maze. After a few days, the ship started to look more like a structured, intimate home. Once I began my midnight shift (12am-8am) a set routine developed. My primary job was to maintain watch—that is, stay awake during my shift and report data losses, animal interferences, equipment malfunctions, science-related decisions, and major changes. I performed this job in front of the ship’s 30 computer monitors alertly glancing between monitors at the continuously streaming data. The science mission was to collect seismic data on the ship’s 8 kilometer-long streamer, a cable containing hydrophones (Fig.2). We did this by generating a large source of pressure directed towards the seafloor. This pressure pulse travelled towards the seafloor and reflected some energy back towards the hydrophones at every significant sediment interface. However, the science team did little to alter the fundamental operation of the ship. Instead, we simply modified many small parameters. For instance, the streamer was sometimes 11 m deep, while other times it was 9 m deep. Sometimes, pressure pulses were fired every 90 sec and at other times were fired every 20 sec. These little tweaks kept the work interesting. But, much of what was happening aboard the ship was repetitive, and it was easy to sink into a lull. Yet, the cruise progressed and we processed more and more data, and built an increasingly complex image of the subsurface. I became interested in the Cape Fear Slide, and entered into intense discussions with Derek Sawyer, Matt Hornbach, and Ben Phrampus. While simultaneously looking at the processed seismic data, we started piecing together maps, background literature, pore-pressure model predictions, and BSR estimates. My experience became active and exciting with the inclusion of real-time data acquisition and interpretation. Suddenly, we were really focused on internal reflectors within the main portion of the slide and we kept asking if we were seeing faults or sediment waves. It was this basic science question that helped translate our terrabytes of data into a rewarding and focused experience. Back on land now, I’m helping to piece together the puzzle and seeing the value of the data that I helped collect. It’s this tangible portion of my experience that seems most important. The beauty, though, is that with such a large project and so much data across varied sedimentary structures, there are little nuggets of excitement for us all to find.Kristopher Darnell, University of Texas at Austin

You can learn so much from the PIs and the other students being in a such a stimulating research environment.Gary Linkevich, Rice University

“Report from the Field” was designed to inform the community of real-time, exciting GeoPRISMS -related research. Through this report, the authors expose the excitement, trials, and opportunities to conduct fieldwork, as well as the challenges they may have experienced by deploying research activities in unique geological settings. If you would like to contribute to this series and share your experience on the field, please contact the GeoPRISMS Office at info@geoprisms.org. This opportunity is open to anyone engaged in GeoPRISMS research, from senior researchers to undergraduate students.
We hope to hear from you!

 Reference information
From the Mudline to the Mantle: Investigating the Eastern North American Margin , Dugan B., Volk K., Meyer D., Darnell K., Aghayn A., Moyer P., Linkevich G.
GeoPRISMS Newsletter, Issue No. 33, Fall 2014. Retrieved from http://geoprisms.org
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